THE PREDATORY SEQUENCE
AND
THE INFLUENCE OF INJURY RISK ON HUNTING BEHAVIOR
IN THE WOLF
A THESIS
SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL
OF THE UNIVERSITY OF MINNESOTA BY
DANIEL ROBERT MACNULTY
IN PARTIAL FULFILMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
MASTER OF SCIENCE
May 2002
© Daniel Robert MacNulty 2002
i
ACKNOWLEDGMENTS
The privilege of writing this thesis would not have been possible without the
commitment, encouragement, and patience of many individuals at many levels. In 1996
while I was working as a volunteer for the Yellowstone Wolf Project, Dr. David Mech
suggested I summarize the early observations of wolves hunting elk in the park's
Northern Range. Dr. Mech subsequently became my faculty advisor at the University of
Minnesota and continued to encourage me to study the hunting behavior of wolves in
YNP. Throughout my research Dr. Mech gave me the freedom to pursue my ideas, while
providing important guidance at critical junctures in the research process.
Dr. Douglas Smith, Yellowstone Wolf Project leader, was instrumental in
conceiving and supporting the idea of a study of wolf hunting behavior. Dr. Smith
handled administrative affairs to facilitate field research, and directed the Wolf Project’s
biannual winter study, which included the study on wolf hunting behavior. Mike Phillips
also encouraged the study and helped secure vital funding during his early tenure as Wolf
Project leader. Deborah Guernsey, administrative assistant, Yellowstone Wolf Project,
patiently endured my endless requests for information. Ms. Guernsey also tended to many
small but critical details that allowed field research to occur.
Wolf Project volunteers were the heart and soul of the data collection process.
Since 1995 many dedicated volunteers passed through the Wolf Project field program and
provided the bulk of the observations that comprise this thesis. Without their hard work
and dedication this study would not have been possible. I owe a special thanks to Wolf
Project volunteers Nathan Varley, Kevin Honness, Daniel Stahler, and Paul Frame for
going the extra mile (literally) to watch wolves hunt bison in Pelican Valley. Also,
without the interest and support of Lake District Rangers John Lounsbury and Lloyd
Kortge our research in Pelican Valley would not have been possible.
Robert Landis, Landis Wildlife Films, generously shared with me his film footage
of wolves hunting in Yellowstone. Bob’s films played a key role in clarifying my
understanding and interpretation of wolf hunting behavior. Dr. Thomas Drummer,
Michigan Technological University, provided important statistical expertise early in the
study, and generously provided statistical advice at various times throughout the study.
Dr. Lynn Eberly, University of Minnesota, instructed me in statistical methods for
analyzing correlated data, and thus opened my eyes to a new and indispensable area of
statistics. Dr. Eberly patiently responded to every email and every question without
exception. I am also indebted to Dr. James Halfpenny for initially introducing me to
Yellowstone National Park, and the Yellowstone Wolf Project, shortly after wolves were
restored to the park in 1995.
Funding for this project was provided by the National Geographic Society,
Yellowstone National Park Foundation, Dayton-Wilkie Natural History Fund, and the
Department of Ecology, Evolution, and Behavior at the University of Minnesota.
Housing and transportation in Yellowstone were provided by the Yellowstone Center for
Natural Resources, Yellowstone National Park.
Finally, Cory Counard provided valuable input and criticism throughout my
research, and provided essential moral support during preparation of the thesis.
ii
ABSTRACT
To study the hunting behavior of the wolf (Canis lupus) in Yellowstone National
Park (YNP), I first define the wolf predatory sequence as consisting of six distinct
behaviors: travel, approach, watch, attack, target, and capture. These behaviors are
organized into three nested groups: predation attempt, prey encounter, and hunting bout.
Using this framework, I first evaluate general patterns of wolf hunting behavior and
estimate success rates for wolves hunting various prey in YNP. I then compare reported
success rates for wolves hunting various prey species in North America to demonstrate
the general relationship between hunting success and prey size. Next I show that prey are
dangerous to wolves and that risk of prey-caused injury is related to prey size. Finally, I
evaluate the influence of injury risk on patterns of wolf hunting behavior.
From May 1995 to March 2000, 62 hunting bouts, 267 prey encounters, and 565
predation attempts were observed in their entirety. The typical hunting pattern involved a
brief hunting bout (< 60 min.) including at least one prey encounter (< 15 min.) and at
least one predation attempt (< 4 min.). Wolves encountered prey within 25 minutes of
hunting, and approximately once every 20 minutes thereafter (3.00 ± 0.42
encounters/hour/bout N = 62). Multiple prey encounters during hunting bouts were
neither a prominent nor important feature of hunting behavior patterns. Overall, the
estimated rate of success was 0.21 ± 0.03 kills per encounter, and bison (Bison bison)
were more difficult to kill (0.04 kills/encounter) than elk (Cervus elaphus) (0.24
kills/encounter). Comparisons with other studies indicate a broad association between
hunting success and prey size.
In general, prey that confronted wolves were more aggressive, and therefore less
likely to be killed than prey that fled. In YNP, bison confronted wolves more frequently
than elk (79% vs. 55% of encounters; χ2 = 8.60, d.f . = 1, P < 0.01) and charged wolves
more frequently than did elk (62% vs. 26% of encounters; χ2 = 22.20, d.f. = 1, P < 0.001),
suggesting that elk are less dangerous and more vulnerable to wolf predation than bison.
Wolf hunting behavior differed between encounters with bison and elk. During bison
encounters, wolves made fewer predation attempts (60% vs. 80%; χ2 = 8.50, d.f. = 1, P <
0.01) and shorter predation attempts (2.90 ± 0.51 min. vs. 4.00 ± 0.38 min.; t = 4.04, d.f.
= 165, P<0.001) than during elk encounters. Wolf encounters with bison also included
periods of watching from within 10 m.
Difference in wolf hunting behavior between bison and elk encounters suggest
that wolves assess their risk of injury and incorporate this information into their foraging
decisions. The tendency for wolves to attack elk more often than bison suggests that wolf
preference for vulnerable prey is an adaptive strategy to acquire food while minimizing
the risk of prey-caused injury.
iii
TABLE OF CONTENTS
ACKNOWLEDGMENTS……………………………………………………
i
ABSTRACT………………………………………………………………….
ii
TABLE OF CONTENTS…………………………………………………….
iii
LIST OF TABLES…………………………………………………………… iv
LIST OF FIGURES…………………………………………………………..
v
INTRODUCTION……………………………………………………………
1
Components of the Predatory Sequence…………………………………..
8
A Framework for the Predatory Sequence………………………………... 14
METHODS…………………………………………………………………...
16
Study Area………………………………………………………………...
16
Study Population………………………………………………………….. 17
Hunting Observations……………………………………………………..
18
Statistical Methods………………………………………………………... 21
RESULTS…………………………………………………………………….
23
Hunting Bouts……………………………………………………………..
23
Prey Encounters…………………………………………………………...
23
Predation Attempts………………………………………………………..
25
Hunting States…………………………………………………………….. 26
Hunting Success…………………………………………………………... 27
Anti-Predator Response and the Risk of Injury…………………………...
29
Hunting Behavior in Bison and Elk Encounters…………………………..
30
Success Rates for Wolves Hunting Various North American Prey………. 31
DISCUSSION………………………………………………………………... 31
The Predatory Sequence…………………………………………………..
31
General Patterns of Hunting Behavior……………………………………. 33
General Patterns of Hunting Success……………………………………... 37
Hunting Success and Prey Size…………………………………………… 41
Risk of Injury and Prey Size………………………………………………
41
Wolf Behavioral Response to the Risk of Injury………………………….
42
LITERATURE CITED………………………………………………………. 47
iv
LIST OF TABLES
Table 1.
Number of hunting bouts, prey encounters, and predation
attempts observed in their entirety and partially observed
(in parentheses) for various wolf packs in Yellowstone
National Park, May 1995 – March 2000.
53
Table 2.
Results of General Linear Mixed Model (GLMM) evaluating
the effects of hunting-state type and prey species on huntingstate duration (min.) in wolf encounters with elk and bison
herds in Yellowstone National Park, May 1995 – March 2000.
Predicted mean hunting-state duration with 95% confidence
intervals is shown in Figure 6.
54
Table 3.
Success rates for wolves hunting various prey species in
Yellowstone National Park, May 1995 - March 2000, based on
known outcomes from completely observed prey encounters
only, and on both complete and incompletely observed
encounters (in parentheses).
55
Table 4.
Age and sex of prey killed by wolves in Yellowstone National
Park, May 1995 – March 2000. The proportion killed in each
age/sex class for each species is shown in parentheses.
56
Table 5.
Results of General Linear Mixed Model (GLMM) evaluating
the effects of hunting-state outcome, hunting-state type, and
prey species on hunting-state duration (min.) in wolf
encounters with elk and bison herds in Yellowstone National
Park, May 1995 – March 2000. Predicted mean hunting-state
duration with 95% confidence intervals is shown in Figure 9.
57
Table 6.
Results of General Linear Mixed Model (GLMM) evaluating
the effects of hunting-state type on hunting-state duration
(min.) in wolf encounters with bison herds in Yellowstone
National Park, May 1995 – March 2000. Predicted mean
hunting-state duration with 95% confidence intervals is shown
in Figure 11.
58
Table 7.
Reported success rates for wolves hunting various
North American prey species.
59
v
LIST OF FIGURES
Figure 1.
The predatory sequence for wolves hunting herds of prey.
60
Figure 2.
Study area and general location of study wolf packs,
Yellowstone National Park, May 1995 - March 2000.
61
Figure 3.
Time of year wolf hunting bouts were observed in
Yellowstone National Park, May 1995 - March 2000.
62
Figure 4.
Time of day wolf hunting bouts were observed during intensive
winter study periods (mid-November to mid-December &
March) in Yellowstone National Park, 1995 - 2000.
63
Figure 5.
Number of prey present during wolf encounters with various
prey species in Yellowstone National Park, May 1995 - March
2000.
64
Figure 6.
Predicted mean duration (min.) of hunting states with 95%
confidence intervals in wolf encounters with elk and bison
herds in Yellowstone National Park, May 1995 – March 2000.
Fitted means and confidence intervals are derived from GLMM
results (Table 2).
65
Figure 7.
Results of completely observed wolf encounters with elk herds
and solitary elk in Yellowstone National Park, May 1995 March 2000.
66
Figure 8.
Results of completely observed wolf encounters with bison
herds and solitary bison in Yellowstone National Park, May
1995 - March 2000.
67
Figure 9.
Predicted mean duration (min.) of hunting states with 95%
confidence intervals in failed and successful wolf encounters
with elk and bison herds in Yellowstone National Park, May
1995 - March 2000. Fitted means and confidence intervals are
derived from GLMM results (Table 5).
68
Figure 10.
The association between mean wolf hunting success
(kills/encounter) and season (early winter: Nov 1 - Dec 31, midwinter: Jan 1 - Feb 28, late winter: Mar 1 - Apr 30, spring: May
1 - Jun 30) in wolf encounters with elk and bison in
Yellowstone National Park, May 1995 - March 2000.
69
vi
Figure 11.
Predicted mean duration (min.) of hunting states with 95%
confidence intervals in wolf encounters with bison herds in
Yellowstone National Park, May 1995 - March 2000. Fitted
means and confidence intervals are derived from GLMM
results (Table 6).
70
Figure 12.
The association between hunting success (kills/encounter) and
prey size for wolves hunting various North American prey.
71
1
INTRODUCTION
Previous attempts to observe the behavior of wolves hunting have been frustrated
by a number of factors including dense vegetation, rugged topography, and logistical
constraints such as access to remote study sites and fuel limitations during aerial
observation (Mech 1966a; Clark 1971; Haber 1977; Carbyn et al. 1993). As a result,
studies of wolf predation have depended largely on the examination of remains from
wolf-killed ungulates for insight into wolf hunting behavior. Most studies have found that
wolves kill mainly vulnerable prey (e.g., prey easily captured due to their circumstantial,
behavioral, or physical condition) (Murie 1944; Mech 1970; Carbyn 1974; Peterson
1977; Mech et al. 1998). In general, predators kill vulnerable prey when prey are difficult
to capture, and very difficult prey are aggressive and can injure a predator (Temple
1987). Therefore, for predators that rely on dangerous prey for food, vulnerable prey are
probably safer to kill.
Wolves feed mainly on ungulates that can injure (Mech 1966a; Rausch 1967;
Phillips 1984; Carbyn and Trottier 1988) or kill them (Ballard et al. 1987; PasitschniakArts et al. 1988; Mech and Nelson 1990; Weaver et al. 1992). Since failure to avoid
injury or death greatly decreases fitness, the risk of prey-caused injury may be a strong
selective force favoring wolf preference for vulnerable prey. If preference for vulnerable
prey is an adaptive strategy to acquire food while avoiding injury wolves should be able
to assess their risk of being injured or killed by prey and incorporate this information into
their foraging decisions.
2
In addition, if safer prey provide less food than dangerous prey, wolves cannot
simultaneously maximize food intake and minimize injury risk. Therefore, wolf
preference for vulnerable prey may represent a trade-off between food and safety. For
example, assuming prey size is related to injury risk (i.e. large prey are more dangerous
than small prey) (Weaver et al. 1992), wolves may prefer small prey at the expense of
food intake in order to minimize injury risk.
Trade-offs between food and safety have only been examined for foragers that
attempt to maximize food intake while minimizing the risk of predation (Krebs 1980;
Newman and Caraco 1987; Lima and Dill 1990). Predators that elicit a conspicuous fear
response from prey (e.g. trade food for safety) are considered “fierce” (Brown et al.
1999). In this model system, fierce predators freely pursue timid prey and contend only
with the risk of starvation. However, predators are known to respond to the risk of preycaused injury, either by avoiding dangerous prey or modifying their behavior while
foraging on dangerous prey. For instance, piscivorous birds attack fish with dangerous
spines less often, and handle them longer than fish without spines (Forbes 1989).
The dangerous-prey hypothesis proposes that predators respond to an increase in
injury risk by handling dangerous prey more carefully, leading to longer handling times
(Forbes 1989). In this case, predators manage the trade-off between food and safety
through adjustments in handling time that lower injury risk at the expense of prey
profitability, since handling time and prey profitability (e.g., net energetic return) are
inversely related (Stephens and Krebs 1986). Although the tendency for wolves to kill
vulnerable prey suggests that wolves trade between food and safety, the difficulty of
3
observing wolves hunt has hindered a close examination of the influence of injury risk on
their hunting behavior.
To examine wolf response to injury risk, one must first clearly characterize and
define their hunting behavior. In most studies, observations of wolves hunting are limited
in number, lacking in detail, and many are incomplete because the beginning or end of
the hunt was not observed. As a result, no single study has produced a definitive account
of wolf hunting behavior. Rather, current knowledge of wolf hunting behavior is the
result of information accumulated over several generations of field studies.
Initial studies provided the first general descriptions of wolf hunting behavior,
indicating that wolves engage in several different types of behavior while hunting (Murie
1944; Banfield 1954; Tener 1954; Crisler 1956; Kelsall 1957, 1960). However, these
early studies neither identified the individual behaviors explicitly, nor examined the
relationships among them.
A second generation of studies identified and described several types of wolf
hunting behavior, and recognized that the behaviors occur in a logical sequence (Mech
1966a, 1970; Gray 1983; Carbyn and Trottier 1987). Mech (1970) decomposed hunting
behavior into five “stages”: travel, stalk, encounter, rush, and chase. Gray (1983)
described the behavior of wolves hunting muskoxen (Ovibos moschatus) as a sequence of
six “events”: approach, circle herd, attack herd, cut off single individual, contact
individual, and kill individual. For hunts of bison Carbyn and Trottier (1987) described
five “categories”: watch, trail, trail and follow-up, harass, and rush.
4
The three predatory sequences differ in three main respects. First, the three
sequences do not share the same set of behaviors. Second, different definitions are
assigned to the same behavior. For example, Mech (1970) described the rush as an initial
charge toward prey when prey are first encountered, while Carbyn and Trottier (1987)
considered the rush to be the point during the encounter at which wolves grab prey.
Third, similar definitions are assigned to different behaviors. For instance, the initial
period of movement toward prey preceding attack has been described as ‘stalk’ (Mech
1970), ‘approach’ (Gray 1983), and ‘trail and follow-up’ (Carbyn and Trottier 1987).
Lack of consensus on the type and definition of behaviors that constitute the wolf
predatory sequence hinders further study of wolf hunting behavior, and highlights the
need for further clarification and explanation.
In addition, the exact relationship between behaviors in the predatory sequence
and more general types of wolf hunting behavior is uncertain. Specifically, studies that
describe sustained periods of hunting activity in which wolves travel from one prey to
another in succession, and make one or more attempts to kill prey during each encounter
(Murie 1944; Mech 1966a, b; Haber 1977; Carbyn and Trottier 1987; Carbyn et al. 1993;
Huggard 1993; Mech et al.1998), imply that behaviors from the predatory sequence are
components of more general types of hunting behavior (e.g., hunting bouts, prey
encounters, and predation attempts). These general types of hunting behavior and their
relationship with specific behaviors of the predatory sequence have not been explicitly
defined.
5
General patterns of wolf hunting behavior have been considered mainly in terms
of the behavior necessary to locate uncommon and widely dispersed vulnerable prey. For
instance, to increase the probability of locating vulnerable prey, wolves are believed to
encounter and attack several different sets of prey during a hunt (Murie 1944; Mech
1970; Mech et al. 1998), and to prefer prey herds to solitary prey (Huggard 1993;
Hebblewhite 2000). As a result, long hunts should have more prey encounters than short
hunts, and successful hunts should be marked by higher prey encounter rates than
unsuccessful hunts. Also, where prey herds are available, wolves should encounter prey
herds more frequently than solitary prey.
Multiple prey encounters during a hunt might occur simultaneously or
consecutively (Murie 1944; Mech 1966a, b; Clark 1971; Haber 1977; Gray 1983; Mech
et al. 1998). It is unknown whether the outcome of an encounter has any influence on the
occurrence, or outcome, of a consecutive encounter. Prey encounters can also involve
prey individuals that wolves encountered previously in the hunt (Clark 1971), or during a
different hunt (Fuller 1962; Mech 1966a; Carbyn et al. 1993). The various types of prey
encounter have not been defined, nor has the frequency of their occurrence been
measured.
Multiple attempts to kill prey during a single prey encounter have also been
observed. Multiple attempts can occur simultaneously (Murie 1944, Mech 1966a, b,
1988; Clark 1971; Carbyn 1974; Mech et al. 1998) or consecutively (Fuller 1957; Mech
1966a, b; Miller and Gunn 1977; Miller et al. 1985; Carbyn and Trottier 1987; Gray
1987; Carbyn et al. 1993; Mech et al. 1998). It is also unknown whether the outcome of a
6
predation attempt has any influence on the occurrence, or outcome, of a consecutive
predation attempt. The frequency of multiple predation attempts during prey encounters
is not known.
Overall, detailed information requiring extended and uninterrupted observations
of wolves hunting is scarce. For instance, no observational data are available describing
prey search time or prey encounter rate.
In the absence of a clearly defined predatory sequence, estimates of wolf hunting
success have involved a variety of measures defined differently for different studies. As a
result, comparisons of hunting success across studies can be confounded if the type of
measure and its definition are not taken into account. Previous measures of wolf hunting
success based on direct observation include: (1) number of kills per prey animal tested
(Mech 1966a; Haber 1977; Peterson 1977; Mech et al. 1998), (2) number of kills per
encounter (Carbyn et al. 1993; Mech et al. 1998), (3) number of kills per chase (Clark
1971; Carbyn et al. 1993; Nelson and Mech 1993), and (4) number of ‘successful’
behaviors per behavior of the same type (e.g., number of approaches resulting in an
attack per approach) (Mech 1970; Peterson 1977; Carbyn and Trottier 1987).
Confusion also arises over the term 'test'. A ‘test’ can be a prey encounter where
wolves pursue or hold an individual prey at bay (Mech 1966a; Peterson 1977; Mech et al.
1998), or a prey encounter where wolves simply move towards an individual prey
without necessarily pursuing or holding it at bay (Haber 1977). A 'test' can also apply to
entire herds of prey where hunting success is expressed as the number of kills per herd
7
tested, such that if a herd split into several smaller groups the encounter is still treated as
a single test (Haber 1977).
Similarly, estimates of success measured as kills per encounter score the outcome
of the overall encounter rather than the outcome of each predation attempt that might
comprise an encounter. In this case an encounter is considered to occur when wolves
watch prey (Carbyn et al. 1993), or at least approach prey (Mech et al. 1998). Where
success is estimated as the proportion of all chases that resulted in a kill, it is uncertain
whether ‘chases’ included predation attempts when prey were only held at bay (e.g.,
when pursuit did not occur).
Here I first review and clarify the predatory sequence for wolves to establish a
general framework within which to analyze their hunting behavior. I apply this
framework to a highly observable population of gray wolves recently restored to YNP to
(1) elucidate general patterns in wolf hunting behavior that have been difficult to quantify
previously, and (2) estimate rates of hunting success for various prey in YNP. I compare
reported rates of success among several different prey in North America to determine if
wolf hunting success is generally related to prey size.
Next I examine the influence of injury risk on patterns of wolf hunting behavior
during encounters with bison (350-1000 kg) and elk (75-340 kg). First, I demonstrate that
(1) prey in general are dangerous to wolves, and (2) bison are more dangerous than elk.
Second, I evaluate whether wolves (1) make foraging decisions based on injury risk, and
(2) trade food for safety. If elk are more vulnerable than bison, wolves should attempt to
8
kill elk more frequently than bison, and spend less time in elk encounters than bison
encounters.
Components of the Predatory Sequence
A review of all available published accounts of wolf hunting behavior indicates
that the predatory sequence in wolves is composed of six general hunting states which I
describe as: travel, watch, approach, attack, target, and capture. In general, hunting states
can be characterized according to the type of gait, or lack thereof, used by the wolves. All
hunting states are defined in terms of the behavior of the wolves, independent of prey
behavior.
Although wolves may engage in each hunting state in a predatory sequence, they
may also skip or repeat one or more hunting states. For instance, during encounters with
bison in Wood Buffalo National Park, wolves sometimes go directly from the watch state
to the attack state, or begin at the attack state, or even the capture state (Carbyn and
Trottier 1987). As a result, I refer to the components of the predatory sequence as
hunting states, where a state is considered to be any distinct behavior with a measurable
duration (Martin and Bateson 1993). I use the term ‘state’ to avoid the implication that
components necessarily follow a definite order as suggested by ‘stage’.
Travel State
The need to locate vulnerable prey requires that wolves travel widely (Murie
1944; Mech 1966a); therefore wolves are considered hunting anytime they are traveling
9
(Murie 1944; Kelsall 1960; Mech 1966a, 1970; Haber 1977; Mech et al. 1998). Traveling
involves wolves using any type of gait to move across the landscape without an obvious
intention to move toward a particular prey. While traveling, wolves locate prey either by
sight, direct scent, chance encounter, or tracking (Mech 1970). Although there are no
reports on the duration of travel necessary before encountering prey in general, Mech
(1966a) reported one case in which wolves traveled 100 km or more before encountering
vulnerable moose (Alces alces).
Watch State
Watching involves intent staring at prey (Clark 1971; Haber 1977; Nelson and
Mech 1994; Mech et al.1998), and has been described by some as surveillance (Clark
1971; Carbyn and Trottier 1988; Carbyn et al. 1993). Watching is believed to be a means
by which wolves assess the vulnerability of prey and thereby their chance for success
(Murie 1944; Mech et al. 1998). Wolves may assess prey by identifying a strategic
advantage (Mech et al. 1998). For example, wolves hunting bison have been observed to
wait until bison flee before they attempt to capture a calf (Carbyn and Trottier 1988), and
wolves hunting muskoxen are known to wait until a calf moves outside the protective
ring of the adults before attacking (Tener 1954). When wolves are seeking a strategic
advantage, they may appear to sleep while watching prey, but are quick to strike when an
opportunity presents itself (Carbyn and Trottier 1988; Mech 1988). In addition, Murie's
(1944) observation that wolves approach caribou (Rangifer tarandus) and then watch
them flee suggests that wolves may assess the physical condition of prey by observing
10
their locomotion, similar to the function of watching behavior described for spotted
hyena (Crocuta crocuta) (Kruuk 1972; Holekamp et. al 1997) and wild dog (Lycaon
pictus) (Fitzgibbon and Fanshawe 1988) hunting Thompson's gazelle (Gazella
thomsonii).
Wolves may watch when they first encounter prey (Haber 1968; Carbyn 1974;
Carbyn & Trottier 1987), after an initial approach (Murie 1944; Kelsall 1960; Haber
1968; Clark 1971; Carbyn 1974; Mech 1988; Nelson and Mech 1994), or following a
failed attack (Murie 1944; Tener 1954; Mech 1966a; Gray 1983, 1987; Carbyn and
Trottier 1988; Mech 1988). Except for an observation by Murie (1944) of a solitary wolf
watching Dall sheep (Ovis dalli) after a failed attack, all reported watches following an
attack have involved large, formidable prey, including moose (Mech 1966a), muskoxen
(Tener 1954; Gray 1983, 1987; Mech 1988), and bison (Carbyn and Trottier 1987, 1988;
Smith et al. 2000).
In general, the distance at which wolves watch prey tends to be greatest for
medium-size prey such as elk and caribou, and least for large prey such as moose, bison,
and muskoxen. Wolves have been reported to watch medium-size prey from 23 to 410 m
(Murie 1944; Kelsall 1960; Haber 1968; Clark 1971; Carbyn 1974) and large prey from 3
to 200 m (Tener 1954; Mech 1966a; Haber 1977; Carbyn and Trottier 1988; Mech 1988).
However, in one exceptional case three wolves watched an adult white-tailed deer from
25 m prior to an approach, and from 5 m following an approach (Nelson and Mech 1994).
The distance at which wolves watch prey also tends to be greatest prior to initial
attack. Prior to initial attack, wolves have been reported to watch prey from 3 to 410 m
11
(Murie 1944; Kelsall 1960; Haber 1968; Clark 1971; Carbyn 1974; Haber 1977; Mech
1988), and following an initial attack from 23 to 200 m (Tener 1954; Mech 1966a;
Carbyn and Trottier 1988).
Approach State
An approach involves wolves walking (Murie 1944; Banfield 1954; Crisler 1956;
Kelsall 1957, 1960; Haber 1968; Mech 1970, 1988; Clark 1971; Carbyn 1974; Gray
1983, 1987; Miller et al. 1985; Carbyn and Trottier 1988) or trotting (Murie 1944;
Banfield 1954; Haber 1968, 1977) toward prey. In one observation, Gray (1970)
characterized the approach of one wolf toward an adult male muskox as a gallop. When
wolves walk toward prey, they may do so casually with no attempt at concealment
(Murie 1944; Kelsall 1960; Carbyn 1974; Gray 1983, 1987; Miller et al. 1985; Carbyn &
Trottier 1988; Mech 1988; Nelson and Mech 1994) or they may stalk prey by walking
either upright, slowly and deliberately (Banfield 1954; Crisler 1956; Kelsall 1957, 1960;
Haber 1968; Mech 1970, 1988) or in a crouch, using topography (Clark 1971; Mech,
USGS, unpublished) or vegetation to conceal themselves (Haber 1977).
Reports of wolves stalking prey tend to be more common for encounters with
medium-size prey such as caribou (Banfield 1954; Crisler 1956; Kelsall 1957, 1960;
Haber 1968, 1977; Clark 1971) and Dall sheep (Murie 1944; cf. Haber 1968), than for
encounters with large prey such as moose (Mech 1966a) and muskoxen (Mech 1988),
although they do stalk the latter (Mech, USGS, unpublished). Wolves might stalk
medium-size prey to reduce the chance that they flee (Mech 1970). However, Murie
12
(1944) found that without stalking, wolves could approach to within a few hundred
meters of caribou.
If wolves do not leave prey, an approach can be followed by either a period of
watching (Murie 1944; Banfield 1954; Mech 1966b, 1988; Haber 1977; Carbyn and
Trottier 1988), or by some form of attack (Murie 1944; Banfield 1954; Crisler 1956;
Kelsall 1957; Mech 1966a, b, 1988; Haber 1977; Peterson 1977; Gray 1983, 1987; Miller
et al. 1985; Carbyn and Trottier 1988; Mech et al. 1998; Mech and Adams 1999).
Attack State
An attack involves wolves pursuing (Murie 1944; Banfield 1954; Crisler 1956;
Kelsall 1957, 1960; Mech 1966a, b, 1988; Haber 1977; Peterson 1977; Gray 1983, 1987;
Miller et al. 1985; Carbyn and Trottier 1987, 1988; Carbyn et al. 1993; Nelson and Mech
1993; Mech et al. 1998; Mech and Adams 1999) and/or holding prey at bay (Tener 1954;
Mech 1966a, 1988; Gray 1970; Miller and Gunn 1977; Peterson 1977; Gray 1983, 1987;
Carbyn and Trottier 1987, 1988; Carbyn et al. 1993; Mech et al. 1998; Mech and Adams
1999). Elsewhere, the term ‘attack’ has also been used to describe wolves and coyotes
biting and grabbing prey (Mech 1966a; Gese and Grothe 1995) or pursuing an individual
prey (Lingle 2002).
The gait of wolves pursuing prey is usually a gallop (Murie 1944; Crisler 1956;
Kelsall 1960; Miller et al. 1985; Carbyn and Trottier 1988) or trot (Murie 1944). If a prey
herd fragments into several smaller groups during a pursuit, wolves may move from one
group to another in succession (Murie 1944; Gray 1987), presumably to locate a
13
vulnerable individual (Murie 1944; Mech et al. 1998). Otherwise, if the herd continues to
flee as a single group, wolves may keep to the rear of the herd and wait for a vulnerable
individual to fall behind (Murie 1944; Crisler 1956; Kelsall 1960; Haber 1977; Carbyn
and Trottier 1988).
All reports of wolves holding prey at bay, except 2 involving white-tailed deer
(Odocoileus virginianus) (Mech 1984; Nelson and Mech 1994), involve large prey
including moose (Mech 1966a; Peterson 1977; Mech et al. 1998), muskoxen (Tener
1954; Gray 1970, 1983, 1987; Miller and Gunn 1977; Mech 1988; Mech and Adams
1999), and bison (Carbyn and Trottier 1987, 1988; Carbyn et al. 1993). If wolves attack
more than one prey and single out an individual, the next hunting state in the predatory
sequence would be target. If wolves attack a solitary prey and grab it, the next hunting
state would be capture.
Target State
The target state involves wolves pursuing and/or holding at bay a specific
individual from a prey group (i.e. 2 or more individuals) (Murie 1944; Banfield 1954;
Crisler 1956; Fuller 1957; Kelsall 1960, 1968; Mech 1966a, b, 1988; Haber 1977; Miller
and Gunn 1977; Gray 1983, 1987; Carbyn and Trottier 1987, 1988; Carbyn et al. 1993;
Mech et al. 1998; Mech and Adams 1999). If wolves are in pursuit, their running speed
typically increases from the attack state to the target state (Murie 1944; Crisler 1956;
Kelsall 1960; Carbyn and Trottier 1987). Prey targeted by wolves are often young (e.g., <
12 months) (Murie 1944; Kelsall 1968; Haber 1977; Miller and Gunn 1977;
14
Carbyn and Trottier 1987, 1988; Mech 1988; Mech et al. 1998) or crippled (Mech and
Frenzel 1971; Peterson 1977; Carbyn and Trottier 1987). Sometimes wolves target a prey
that falls behind the herd (Crisler 1956; Haber 1977; Mech 1988) or stumbles (Kelsall
1960). The target prey can be pursued or held at bay repeatedly if wolves fail to initially
subdue it (Fuller 1957; Mech 1966a; Carbyn et al. 1993). If wolves grab and restrain the
prey, the next hunting state in the predatory sequence would be capture.
Capture State
The capture state involves wolves grabbing and restraining an individual prey
(Murie 1944; Kesall 1960; Mech 1966a; Peterson 1977; Carbyn and Trottier 1987; Gray
1983, 1987; Carbyn and Trottier 1988; Carbyn et al. 1993; Nelson and Mech 1993; Mech
and Adams 1999). The capture state results in wolves killing prey (Murie 1944; Kelsall
1960; Mech 1966a; Peterson 1977; Smith 1980; Carbyn et al. 1993; Mech and Adams
1999) or prey escaping wolves (Mech 1966a; Peterson 1977; Gray 1983; Carbyn and
Trottier 1988; Nelson and Mech 1993; Mech and Adams 1999).
A Framework for the Predatory Sequence
To examine general patterns of wolf hunting behavior that involved one or more
hunting states, I developed a framework that defined the relationship between the
individual hunting states and three nested groups of hunting states: predation attempt,
prey encounter, and hunting bout (Fig. 1). A hunting bout was a discrete period beginning
when wolves began traveling or encountering live prey (whichever came first) and ending
15
when wolves stopped encountering live prey and rested (e.g., sat or laid down) or stopped
traveling (whichever came last). A hunting bout could contain at least one hunting state
(except watch, see below) or a continuous sequence of hunting states. Wolves often howl
or socialize while hunting (Banfield 1954; Mech 1966a; Haber 1977), and I considered
this to be part of the hunting bout, rather than marking its conclusion. One or more
hunting bouts could occur per day.
A prey encounter was a period during a hunting bout involving prey and
containing one or more of the following hunting states: approach, watch, attack, target,
capture. The unit of encounter during a hunting bout was either a solitary prey or herd of
prey. An individual prey was considered to be in a herd if its nearest neighbor was ≤ 20
m away. I considered a prey encounter to begin when wolves sighted prey and walked or
ran toward them. A prey encounter ended when wolves stopped staring at prey or
otherwise ceased to pay attention to prey (e.g., traveled away from prey). Situations that
involved wolves watching prey without initially walking or running toward them were
not considered encounters by my definition. One or more prey encounters could occur per
hunting bout.
Prey encounters were categorized as new, consecutive, repeat, return, or
simultaneous. A new encounter was the first encounter to occur in a hunting bout and
involved prey not previously encountered earlier in the day or in preceding days. A
consecutive encounter denoted the second, third, fourth, etc. encounter during a hunting
bout, and involved prey not previously encountered in the hunting bout or during a
different hunting bout earlier in the day or in preceding days. A repeat encounter
16
involved prey encountered earlier in the same hunting bout, while a return encounter
involved prey encountered in a different hunting bout earlier in the same day or in
preceding days. An encounter was simultaneous if it occurred at the same time as another
encounter in the same hunting bout.
A predation attempt occurred when wolves pursued, held at bay, or grabbed prey.
It was a period during a prey encounter that involved the sequential occurrence of one or
more of the following hunting states: attack, target (prey groups only), or capture. A
predation attempt failed if (1) the sequence leading from attack to capture was interrupted
or (2) a capture did not result in a kill. A subsequent attempt began when the sequence
restarted at one of the three hunting states. Multiple predation attempts could be
consecutive and/or simultaneous. A consecutive predation attempt was one that occurred
following a preceding attempt, and a simultaneous attempt was one that occurred at the
same time as another attempt in the same prey encounter.
METHODS
Study Area
Yellowstone National Park extends across 891,000 ha of a primarily forested
plateau in northwestern Wyoming (Fig. 2). Elevations range from 1,500 m to 3,300 m.
Several large montane grasslands punctuate the Yellowstone plateau and provide
unobstructed views of wildlife. However, continuous viewing can be inhibited by forests
on the periphery of grasslands, by occasional trees within grasslands and by varied
17
topography. Approximately 35,000 elk, 4,000 mule deer (Odocoileus hemionus), 3,000
bison, 700 moose, 200 pronghorn (Antilocapra americana), 200 bighorn (Ovis
canadensis) and scattered mountain goats (Oreamnos americanus) reside in YNP during
all or part of the year (D.W. Smith, National Park Service, unpublished data).
Observations of wolves hunting were made primarily in a 100,000 ha complex of
montane grasslands situated in the northern quarter of YNP referred to as the Northern
Range (Fig. 2). The Northern Range is a series of open valleys, ridges, and minor
plateaus linked by the Lamar and Yellowstone Rivers. Low elevations (1,500 m to 2,400
m) on the Northern Range create the warmest and driest conditions in YNP during
winter. As a result, the Northern Range serves as the principal winter range for nearly
12,000 elk and 700 bison (D.W. Smith, National Park Service, unpublished data). Elk
and bison occurred in singles or in herds of up to 800 and 75 animals, respectively. A
single paved road runs the length of the Northern Range and provides year-round access.
Wolves were routinely visible from observation points on or near the road. In winter,
wolves were also observed from a hilltop observation point in Pelican Valley in the
interior of YNP at approximately 2,500 m. Pelican Valley was accessed in winter by ski.
Study Population
A combined total of 31 radio-collared wolves were reintroduced to YNP in 1995
and 1996 (Bangs and Fritts 1996; Phillips and Smith 1996). Each subsequent year YNP
personnel radio-collared 30-50% of the pups born (Smith et al. 2000). Wolves observed
in the study were either members or descendents of the original reintroduced population.
18
From 1995 to 2000, 14-110 wolves comprised 2-7 packs of 2 to 27 wolves per pack (9.9
± 1.0 wolves/pack, N = 37 pack-years). Approximately 20-60 of the wolves studied were
radio-collared and 20-25 were individually recognizable by combination of color pattern,
radio-frequency, and body conformation.
Observations of wolves hunting were recorded from May 1995 to March 2000.
During the study the number and location of packs changed. In 1995, the study
population was limited to the three initial packs reintroduced to the Northern Range:
Crystal Creek, Rose Creek and Soda Butte. The Soda Butte pack eventually moved
outside YNP onto private lands, was returned to YNP, and was released in a remote
southern region of the park. In early 1996, a female and male dispersed from the Rose
Creek and Crystal Creek packs, respectively, and formed a third pack on the Northern
Range called Leopold. Shortly after their release in 1996, the Druid Peak pack replaced
the Crystal Creek pack on the Northern Range, and the Crystal Creek pack relocated to
the more remote Pelican Valley. For the remainder of the study, the Rose Creek,
Leopold, and Druid Peak packs were the focus of study because they inhabited the easily
accessed, and sparsely forested Northern Range (Fig. 2). In 1996, the Nez Perce and
Chief Joseph packs were also released, but they inhabited areas too forested and/or
inaccessible to allow observation from the ground.
Hunting Observations
Wolves were mainly observed hunting during two annual 30-day intensive study
periods in March and mid-November to mid-December (Fig. 3). In general, wolves on the
19
Northern Range were more easily observed during winter because they were attracted to
ungulates concentrating on low elevation winter range that was easily accessed by
observers. Observations were also made in April, May, and June during annual wolf-denmonitoring studies. Observations during other months were recorded opportunistically.
During each study period, teams of two observers were assigned to daily monitor
each Northern Range wolf pack from the ground from dawn to dusk. Observation effort
per hour was generally constant throughout the day. Nighttime observation was attempted
with night vision goggles but was ineffective due to long distances between wolves and
observers. In the eight study periods from May 1995 to March 2000, observers on the
Northern Range watched wolves for a total 1,901 hours. At least two observers monitored
the Crystal Creek pack in Pelican Valley during March 13-19, 1999 and March 23-31,
2000. Observers in Pelican Valley watched wolves for a total of 80 hours. All packs were
located daily from fixed-wing aircraft, weather permitting, during each study period.
Outside study periods, wolves were located from the air weekly. Hunting behavior was
observed from the aircraft as well as from the ground. Aerial observers recorded 24 wolfprey encounters, and ground observers recorded 560 wolf-prey encounters. To
standardize data collection, each observer was trained to record wolf hunting behavior
prior to each study period.
Observers on the ground first located wolf packs with radio-telemetry to obtain a
directional fix, and visually located and observed wolves with binoculars and spotting
scopes. Observers watched wolves at distances of 0.1 - 6.0 km for as long as they
remained in view and recorded hunting behavior using hand-held voice recorders and
20
digital stopwatches. Recorded observations were subsequently transcribed onto data
forms. Some observations were also recorded on video.
During each hunting state, observers recorded the following: duration of the
hunting state, number, age, and gender of wolves, number and age/sex class of prey, and
prey behavioral response. Wolf age and gender were determined during the annual effort
to capture and radio-collar wolves (Smith et al. 2000). Sex of wolves not captured was
determined by noting wolf body position while urinating (Mech 1970). Wolf age was also
determined during den monitoring by noting when individuals were born. Hunting states
that started or ended out of view were excluded from duration analyses. For this study
only watch states that occurred at close range (< 10 m) were noted.
Unless otherwise noted, all estimates of hunting success were based on prey
encounters, predation attempts, and hunting states that were observed in their entirety.
Estimates of hunting success included encounters involving both solitary and group
hunts. Hunting success was measured at the level of prey encounter (kills/encounter),
predation attempt (kills/attempt), and hunting state. For prey encounters and predation
attempts, a success was considered to be the occurrence of a single ungulate kill. At the
level of hunting state, success was measured according to whether the subsequent hunting
state in the predatory sequence occurred. For example, the success of wolves approaching
was measured as the proportion of approaches that resulted in an attack (i.e., number of
attacks per approach). Data on the association between hunting group size and hunting
success will be presented elsewhere.
21
Statistical Methods
Means are reported with standard errors throughout, and for all analyses, results
were considered significant at P < 0.05. P-values shown are for two-tailed tests.
Frequency data, such as kills/encounter and kills/attempt, were evaluated with Pearson's
chi-square test, or if more than one-fifth of fitted cells were sparse (e.g., frequency < 5),
with Fisher's exact test. All continuous data were checked for normality prior to analysis.
To satisfy normality assumptions, duration and count data (e.g., number of encounters
and attempts) were log and square-root transformed, respectively. However, results were
plotted in the original scale to aid interpretation. Prey encounter and predation attempt
duration were evaluated with Student's t-test. Continuous data were analyzed with a
Mann-Whitney U-test and Spearman’s rank correlation coefficient if sample sizes were
small (< 30), or transformations were not adequate. The relationship between prey mass
and anatomical point of capture was evaluated using ANOVA. Assumed weights (kg) for
elk were: cow, 226; yearling, 165; calf, 103 and bull, 266 (K. E. Murphy, National Park
Service, unpublished data). Assumed weights (kg) for bison were: calf, 271; cow, 430;
and bull, 679 (Meagher 1973). All the above tests assume independence of observations.
Analyses of hunting-state duration were performed with general linear mixed
models (GLMMs) (Verbeke and Molenberghs 2000) using the SAS 8.0 analysis package
(SAS Inc. 2000). A mixed linear model is a generalization of the standard linear model
(i.e. simple linear regression) which accounts for correlation and non-constant variance in
the data. Hunting states were correlated if they occurred during the same hunting bout or
prey encounter. In these models, an unstructured correlation matrix was used, which
22
allows for any level of correlation among hunting states within the same prey encounter
and within the same hunting bout. Model parameters were estimated using maximumlikelihood estimation, and significance of effects was determined by an approximate ttest. Model reduction was performed using the likelihood-ratio test. Results were robust
to other choices of correlation matrices. Predicted mean hunting-state duration was
derived from the GLMM analysis and plotted with confidence limits to illustrate the
significance of comparisons between different types of hunting state.
Sample sizes varied considerably among tests because not all observations
contained the same quality of information. For example, 267 prey encounters were
observed in their entirety, but accurate duration data were only recorded in 175 of those
encounters. Thus, analyses of encounter duration were restricted to those 175 prey
encounters.
The association between prey size and hunting success was examined by first
summarizing reported rates of success for various North American prey species. Where
more than one estimate was available for a particular prey species, estimates were
compared within prey species, using Pearson’s chi-square or Fisher’s exact test, to
identify if differences existed among studies. To quantify prey size, mean weights (kg)
were estimated for each prey species based on Nowak (1999). Spearman's rank
correlation coefficient (rs) was used to test the association between success rate and mean
prey weight.
23
RESULTS
Hunting Bouts
Observers watching the Rose Creek, Leopold, Druid Peak, and Crystal Creek
packs recorded 62 hunting bouts in their entirety and portions of an additional 400 (Table
1). I personally observed and recorded 78 (17%) of those 462 hunting bouts. During
intensive study periods, packs made on average 1.08 + 0.22 hunting bouts/observation
hour (N = 181 pack-observation days), and hunting bouts were observed mainly in the
morning and evening (Fig. 4). Wolf behavior immediately preceding a hunt was noted in
91 hunting bouts and included 36 (40%) resting, 21 (23%) sleeping, 12 (13%) feeding, 17
(19%) rallying (e.g., excited greeting), and 5 (5%) howling. The initial hunting state in
144 hunting bouts observed at the start included 101 (70%) travel, 36 (25%) approach,
and 7 (5%) attack states. In 65 hunting bouts that began with travel, wolves encountered
prey within 1 to 118 minutes (22.30 + 3.10 min., N = 65).
Duration of hunting bouts was 3-594 minutes (48.10 ± 9.80 min., N = 62), and
included 0-3 prey encounters (1.20 ± 0.09 encounters/bout, N = 62). Number of prey
encounters/bout was not significantly associated with duration of hunting bout (Spearman
rank correlation coefficient, rs = 0.22, N = 62, P=0.24).
Prey Encounters
Observers recorded 267 prey encounters in their entirety and portions of an
additional 317. The initial hunting state among 336 prey encounters observed at the start
included 291 (87%) approach, 44 (13%) attack, and 1 (0%) target states. Of the 584 total
24
prey encounters observed, 486 (83%) involved elk, 75 (13%) bison, 12 (2%) pronghorn,
6 (1%) bighorn sheep, 3 (0.5%) mule deer, and 2 (0.5%) moose encounters.
Overall, wolves encountered prey at a rate of 3.10 ± 0.42 encounters/hour/bout (N
= 62). Among 55 completely observed hunting bouts in the Northern Range where
wolves encountered elk or bison, wolves encountered elk at a slightly higher rate (3.28 +
0.48 encounters/hour/bout, N = 49) than they encountered bison (3.18 + 0.80
encounters/hour/bout, N = 6) but the difference was not significant (Mann-Whitney Utest, z = 0.16, P = 0.68). Among 529 encounters involving elk or bison on the Northern
Range, wolves did encounter elk more frequently 472 (89%) than bison 57 (11%). Data
were unavailable to adequately determine whether wolves were encountering prey
species proportionate to their occurrence in the study area.
Overall, wolves encountered herds of prey more often (85% of N = 584 prey
encounters) than solitary prey (15%; χ2 = 267, d.f. = 1, P < 0.001; Fig. 5). Again, data
were not available to determine whether wolves were encountering prey herds in
proportion to their occurrence in the study area. Among 175 prey encounters for which
duration data were available, duration of prey encounters ranged from < 1 to 553 minutes
(12.40 ± 3.30 min., N = 175).
Of 134 multiple encounters that occurred during hunting bouts, 118 (88%) were
consecutive, 8 (6%) simultaneous, 4 (3%) repeat, and 4 (3%) unknown. Among 178 prey
encounters observed at their finish, a consecutive encounter was more likely to occur
after an unsuccessful encounter (34.2%, N = 105) than after a successful encounter
(8.2%, N = 73; χ2 = 16.23, d.f. = 1, P < 0.001). Among 123 consecutive prey encounters
25
that followed a prey encounter with known outcome, a consecutive encounter was less
likely to be successful if it followed an unsuccessful encounter (15%, N = 115) than if it
followed a successful encounter (50%, N = 8; Fisher exact test, P = 0.03). For 40 known
intervals between consecutive encounters, time between consecutive encounters ranged
from 0 to 41 minutes (7.50 ± 1.50 min., N = 40). Only 12 return encounters were
observed, and 5 (42%) involved prey that were previously wounded.
In 267 prey encounters observed in their entirety, the number of predation
attempts per prey encounter was 0-45 (1.30 ± 0.18 attempts/encounter, N = 267). In the
175 prey encounters with known duration, the number of predation attempts was
positively related to duration of prey encounter (Spearman rank correlation coefficient, rs
= 0.39, N = 175, P < 0.001). During encounters, wolves attempted to kill prey at an
overall rate of 0.30 ± .04 attempts/min./encounter (N = 175).
Predation Attempts
Observers recorded 565 predation attempts in their entirety and portions of an
additional 171. In 320 predation attempts with known duration, predation attempt
duration was 0.07-41 minutes (3.70 ± 0.31 min., N = 320). The initial hunting state
among 564 predation attempts involving prey herds and observed from the start included
468 (83%) attack, 90 (16%) target, and 6 (1%) capture states. All 6 attempts beginning
with a capture involved newly born elk calves.
Among 260 multiple predation attempts that occurred during prey encounters, 196
(75%) were consecutive, 55 (21%) simultaneous, and 9 (4%) unknown. Among 443
26
predation attempts observed at their finish, a consecutive predation attempt tended to
occur more often after an unsuccessful predation attempt (29%, N = 377) than after a
successful predation attempt (17%, N = 66; χ2 = 3.60, d.f. = 1, P = 0.057). Among 92
consecutive predation attempts that followed a predation attempt with a known outcome,
a consecutive attempt was less likely to be successful if it followed an unsuccessful
predation attempt (5%, N = 83) than a successful predation attempt (22%, N = 9), but the
difference was not significant (Fisher exact test, P = 0.10). For 75 known intervals
between consecutive attempts, time between consecutive predation attempts was 0 - 88.5
minutes (6.60 ± 1.70 min., N = 75).
Hunting States
Observers recorded 1,472 complete hunting states and portions of an additional
391. Among the hunting states observed completely, 188 (13%) were travel, 83 (6%)
watch, 369 (25%) approach, 494 (33%) attack, 225 (15%) target, and 113 (8%) capture.
During elk and bison encounters, hunting-state duration remained the same
between approaches and attacks, decreased during target, and increased during capture
(Fig. 6). For both elk and bison encounters, target duration was significantly shorter than
other hunting states (Table 2.)
Wolves captured prey by grabbing the hindquarters, the neck, or both. Overall,
wolves grabbed prey heavier than 270 kg by the hind end (N = 32), prey between 200 and
270 kg by the hind end and neck (N = 35), and prey < 200 kg by the neck only (N = 31)
(ANOVA, F = 4.472, 95, P = 0.014).
27
Hunting Success
Overall, the estimated rate of success was 0.21 ± 0.03 kills per encounter and 0.16
± 0.02 kills per predation attempt (Table 3). Among prey encounters observed in their
entirety, wolves were more successful hunting elk (0.24 ± 0.03 kills/encounter, N = 211)
than bison (0.04 ± 0.03 kills/encounter, N =47; Mann-Whitney U-test, z = 8.11, P =
0.004), despite nearly twice as many predation attempts per bison encounter.
In encounters with prey herds, the sequence of hunting states leading to a kill
generally progressed from approach to attack, target and capture (Fig. 7a, 8a). Wolves
that approached elk herds attacked more frequently (63%, N = 177) than wolves that
approached bison herds (47%, N = 64; χ2 = 4.86, d.f. = 1, P = 0.03). During an attack on
a prey herd, wolves were more likely to target an individual from an elk herd (42%, N =
180) than an individual from a bison herd (17%, N = 53; χ2 = 10.82, d.f. = 1, P = 0.001).
Wolves were also more likely to capture a targeted elk (43%, N = 108) than a targeted
bison (23%, N = 35; χ2 = 4.38, d.f. = 1, P = 0.04), and more likely to kill a captured elk
(85%, N = 53) than a captured bison (20%, N = 10; Fisher exact test, P < 0.001). Wolves
were just as likely to target an individual bison following a watch (19%, N = 42) or attack
(17%, N = 53; χ2 = 0.07, d.f. = 1, P = 0.79).
In encounters with solitary prey, the sequence of hunting states generally
progressed from approach to attack, and to capture (Fig. 7b, 8b). Wolves that approached
solitary elk tended to attack more often (76%, N = 25) than wolves that approached
solitary bison (45%, N = 11), but the difference was not significant (Fisher exact test, P =
28
0.12). Wolves always killed captured solitary elk (N = 5), but never killed captured
solitary bison (N = 10). We did not observe wolves watching solitary bison.
Among elk, wolves killed adult females (48%), young-of-the-year (25%), adult
males (19%), yearlings (2%), and elk for which age and sex were unknown (6%) (Table
4). Among bison, wolves killed young-of-the-year (80%) and only 1 adult (20%).
Among 62 hunting bouts observed in their entirety, successful hunting bouts
tended to be longer (64.80 ± 23.50 min., N = 25) than unsuccessful hunting bouts (36.70
± 3.70 min., N = 37), but the difference was not significant (t = -0.77, d.f. = 45, P = 0.44).
The rate of encounter did not differ between successful (3.10 ± 0.47
encounters/hour/bout, N = 25) and unsuccessful hunting bouts (3.10 ± 0.63
encounters/hour/bout, N = 37). Among the 62 hunting bouts observed in their entirety,
only 2 (3%) contained multiple kills, all involving elk.
Among 175 prey encounters with known duration, successful prey encounters
were longer (27.68 ± 13.73 min., N = 40) than unsuccessful prey encounters (7.90 ± 1.24
min., N = 135; t = -4.39, d.f. = 66.8, P < 0.001). Among 134 prey encounters with at least
one predation attempt, successful prey encounters involved a lower attempt rate (0.20 ±
0.02 attempts/min./encounter, N = 40) than unsuccessful encounters (0.47 ± 0.07
attempts/min./encounter, N = 94; t = 3.17, d.f. = 94, P < 0.01).
Likewise, among 320 predation attempts with known duration, successful
predation attempts were longer (6.60 ± 1.20 min., N = 46) than unsuccessful predation
29
attempts (3.20 ± 0.29 min., N = 274; t = -6.32, d.f. = 74.1, P < 0.001). Also, successful
captures of elk and bison were significantly longer than failures (Fig. 9 and Table 5).
In wolf encounters with elk, hunting success varied significantly over season and
was highest during spring (Kruskal-Wallis one-way ANOVA, H = 9.24, P = 0.03; Fig.
10). In wolf encounters with bison, hunting success did not vary significantly over season
(Kruskal-Wallis one-way ANOVA, H = 2.99, P = 0.22; Fig. 10) despite high hunting
success during late winter.
Anti-predator Response and the Risk of Injury
During 258 encounters (211 with elk and 47 with bison) in which the behavioral
response of prey to wolves was noted, prey stood and confronted (60%, N = 154) wolves
more often than they fled (40%, N = 104; χ2 = 9.69, d.f. = 1, P = 0.02). Prey that
confronted wolves were more aggressive than prey that fled from wolves, being more
likely to charge wolves (48%, N = 154) than prey that fled (10%, N = 104; χ2 = 41.77,
d.f. = 1, P < 0.001), and more likely to kick at wolves (11%, N = 154) than prey that fled
(4%, N = 104; χ2 = 4.29, d.f. = 1, P = 0.04). Overall, prey that confronted wolves were
less likely to be killed (14%, N = 154) than prey that fled from wolves (26%, N = 104; χ2
= 4.29, d.f. = 1, P = 0.02).
Bison were more aggressive than elk during encounters with wolves. Bison stood
and confronted wolves more frequently (79%, N = 47) than elk (55%, N = 211; χ2 = 8.6,
d.f. = 1, P < 0.01), and charged wolves more frequently (62%, N = 47) than elk did (26%,
N = 211; χ2 = 22.2, d.f. = 1, P < 0.001).
30
Hunting Behavior in Bison and Elk Encounters
In general, wolf encounters with bison were longer (29.50 ± 17.50 min., N = 32)
than with elk (8.80 ± 1.00 min., N = 139), but not significantly (t = -0.65, d.f. = 39.5, P =
0.52). One 33.5-hour bison encounter, monitored by radio-telemetry during darkness, was
excluded from duration estimates, because it included the period from dusk to dawn when
direct observation was not possible.
Overall, a predation attempt was more likely during an elk encounter (80%, N =
211) than a bison encounter (60%, N = 47; χ2 = 8.5, d.f.=1, P < 0.01). The number of
predation attempts tended to be greater during encounters with bison (1.90 ± 0.95
attempts/encounter, N = 47) than with elk (1.20 ± 0.09 attempts/encounter, N = 211;
Mann-Whitney U-test, z = 3.12, P = 0.08). However, based on encounters with at least
one predation attempt, rate of predation attempt did not differ between bison (0.27 ± 0.06
attempts/min./encounter, N = 32) and elk (0.30 ± 0.05 attempts/min./encounter, N = 139;
t = 0.032, d.f. = 32, P = 0.98).
Predation attempts were shorter in encounters with bison (2.90 ± 0.51 min., N =
101) than with elk (4.00 ± 0.38 min., N = 215; t = 4.04, d.f. = 165, P < 0.001). Time
between consecutive predation attempts also tended to be longer for bison (8.80 ± 3.00
min., N = 41) than for elk (3.90 ± 0.88 min., N = 34), but not significantly (t = -0.493, d.f.
= 56.3, P = 0.62).
Among hunting states, an approach to a bison herd was more likely to result in a
target (17%, N = 64) than was an approach to an elk herd (8%, N = 177; χ2 = 4.32, d.f. =
31
1, P = 0.04). Successful bison captures tended to take longer than successful elk captures,
though the sample was small (Fig. 9). Wolves did not watch elk at close range (< 10 m).
In encounters with bison herds, watch duration was significantly greater than the duration
of other hunting states, except for the capture state (Fig. 11 and Table 6).
Success Rates for Wolves Hunting Various North American Prey
Wolves hunting bison tended to experience a lower rate of success in YNP than in
Wood Buffalo National Park (Table 7), but the difference was not significant for
kills/encounter (Fisher exact test, P = 0.69) or kills/attempt (Fisher exact test, P = 0.39).
Reported success rates for moose were not significantly different among studies
(Table 7) for kills/encounter (Fisher exact test, P = 0.13) or kills per animal (χ2 = 5.95,
d.f. = 1, P = 0.11). Estimates of hunting success among studies differed for caribou (χ2 =
17.90, d.f. = 1, P < 0.001), but not for Dall sheep (Fisher exact test, P = 0.39).
Wolves were more successful killing small prey than large prey. Overall, the rate
of success decreased as prey size increased (Spearman rank correlation coefficient, rs = 0.60, N = 11, P < 0.05; Fig. 12).
DISCUSSION
The Predatory Sequence
The predatory sequence presented for wolves is very similar to one reported for
spotted hyenas. Kruuk (1972) defined the predatory sequence for spotted hyena as (1)
32
search, (2) random dash, (3) chase, and (4) kill. Similar to the attack state in wolves,
hyenas gallop leisurely during the random dash and never at full speed. Kruuk (1972)
speculated that the main function of the random dash is to make prey run, which might
allow hyenas to identify any physically inferior individuals. Like the target state in
wolves, hyenas increase their running speed and focus on a single prey during the chase
(Kruuk 1972). The similarities in the predatory sequence between wolves and spotted
hyenas are further evidence of the evolutionary convergence in foraging behavior
between Canidae and Hyaenidae first noted by Kruuk (1972).
In wolves, the sequence of hunting states that led to a kill was determined by
several factors. First, whether wolves encountered herd prey or solitary prey determined
if a target state was, by definition, included in a sequence. Second, the outcome of a
hunting state also affected the actual sequence of hunting states comprising a hunting
bout since hunting states that failed to lead to the next state (e.g., approach to attack,
attack to target, target to capture) were often repeated. Third, some states were
occasionally skipped such as when wolves approached a prey herd and immediately
targeted an individual.
A hunting bout usually began with travel. It only started with an approach or
attack if wolves detected prey while resting. A prey encounter commonly started with an
approach, and sometimes with an attack. If an encounter involved a prey herd, a
predation attempt usually began with an attack, and rarely with a target. A predation
attempt might start with a capture if newborn prey were encountered, in which case it was
only necessary for wolves to approach and capture.
33
Brief target duration likely reflected constraints imposed by limited energetic
reserves or the risk of prey-caused injury. Several observers have noted that the speed at
which wolves pursue prey tends to increase when they transition from 'casual' pursuit of
an entire herd to focused pursuit (i.e. target) of an individual (Murie 1944; Crisler 1956;
Kelsall 1960; Carbyn and Trottier 1987). In a chase filmed in YNP (Landis,
unpublished), wolves increased their speed from approximately 14 km/hr during the
attack to 45 km/hr during the target state (calculated from the film [MacNulty,
unpublished]). Since speed is positively associated with energetic cost (Alexander et al.
1980; Heglund and Taylor 1988), wolves might minimize target duration to minimize
energy expenditures. When prey stand and confront wolves during a target, wolves may
limit target duration to reduce the risk of injury from aggressive prey.
Extended capture duration was likely due to the wolf’s small size and limited
ability to subdue prey. Unlike big cats (Hornocker 1970; Schaller 1967, 1972), or grizzly
bears (Ursus arctos) (MacNulty et al. 2002), wolves lack mass, muscular forelimbs, and
longer claws that enable other carnivores to quickly grab and overcome prey. Wolves
also cannot generally deliver a quick killing bite commonly used by the big cats. Instead,
wolves rely solely on their teeth to grab prey and tear into a vital area until the prey
weakens and falls.
General Patterns of Hunting Behavior
Previous studies indicate that wolves tend to socialize by howling, excited
greeting (e.g., rally), or both, prior to initiating a hunting bout (Murie 1944; Mech 1966a;
34
Haber 1977). However, the function of group socializing in the hunting behavior of
wolves is not clear. Group howling might serve to assemble the pack before setting out
on a hunt (Mech 1970). A rally could be a food-begging ritual initiated by young wolves
toward adults in anticipation of a kill, or a means for adults to motivate a pack leader to
initiate a hunt (Mech 1970). Wild dogs also rally prior to hunting (Estes and Goddard
1967; Schaller 1972; Creel and Creel 1995), and it may focus the attention of all pack
members on the hunt (Estes and Goddard 1967). In YNP, wolves were usually sleeping
or resting, and occasionally feeding, prior to a hunting bout, and group socializing then
was infrequent.
Though few comparable data for wolves elsewhere are available, the tendency for
YNP wolves to hunt more frequently during morning and evening than during mid-day
(Merrill 2002; Theuerkauf et al. in press) is consistent with the observed pattern of
hunting behavior for other carnivores including the African wild dog (Estes and Goddard
1967; Fuller and Kat 1990; Creel 2001), spotted hyena (Kruuk 1972), and lion (Schaller
1972). However, while the occurrence of a mid-day lull in hunting activity in YNP was
clear, the exact time of maximal hunting activity, and the magnitude of the observed
morning and evening peaks in hunting activity were likely affected by our inability to
observe wolves before dawn and after dusk. The crepuscular hunting activity is in accord
with the basic daily activity patterns of wolves (Mech and Merrill 1998; Merrill 2002).
In general, the typical hunting pattern for wolves in YNP involved a hunting bout
< 60 minutes long, containing at least one prey encounter (< 15 min.) and at least one
predation attempt (< 4 min.). I found no evidence to support the hypothesis that wolves
35
locate vulnerable prey by making multiple encounters during a hunt (Murie 1944; Mech
1970; Mech et al. 1998). Contrary to expectation, number of prey encounters in a hunting
bout did not differ between long and short hunting bouts, and encounter rates did not
differ between successful and unsuccessful hunting bouts. Multiple prey encounters
during hunting bouts were neither a prominent, nor important, feature of the overall
pattern of hunting behavior for wolves in YNP.
However, if locating vulnerable prey depends on making multiple prey
encounters, wolves in YNP might make several prey encounters during a succession of
short hunting bouts over the course of day rather than during a single hunting bout.
Unfortunately, data were not sufficient to test this hypothesis. Nonetheless, generally
short hunting bouts containing an average of 1 encounter per bout in YNP, and previous
reports that wolves rest following a prey encounter involving an attack (Mech 1970),
support the hypothesis that wolves make multiple prey encounters over several hunting
bouts.
If a hunting bout did include multiple encounters, they were more often
consecutive than simultaneous or repeat. A consecutive encounter was more likely to
occur, but less likely to succeed, if the previous encounter failed. Wolves that succeeded
during an encounter were probably preoccupied with feeding and less motivated to make
another encounter. My estimate of an approximate 8-minute interval between consecutive
encounters is likely biased low due to a tendency for wolves to go out of sight during
extended intervals between consecutive encounters.
36
Repeat encounters during a hunting bout may be infrequent due to increased prey
wariness following the initial encounter, which may make it more difficult for wolves to
relocate prey and/or kill prey once relocated. Also, wolves may not return to prey if they
have already determined that a vulnerable prey is not present. Return encounters
involving prey encountered in a previous hunting bout were also infrequent, perhaps for
the same reason. However, because not all prey encountered and abandoned were
wounded or otherwise identifiable, it was difficult to determine whether a prey had been
encountered in a previous hunting bout. Therefore, the frequency of return encounters
may have been underestimated.
Consistent with earlier expectations that wolves prefer prey herds to solitary prey
(Huggard 1993; Hebblewhite 2000), wolves in YNP encountered herds of prey more
frequently than solitary prey. However, without our having information on the abundance
of prey herds and solitary prey in the study area, we had no way of knowing whether
wolves simply encountered herds in proportion to their occurrence. Visibility bias could
also have influenced the tendency for wolves to encounter herds of prey more often than
solitary prey if solitary prey inhabit forests more frequently than grasslands. However,
evidence based on snow tracking in Banff National Park indicates that wolves encounter
prey herds more often than solitary prey in both open and forested habitats (Huggard
1993; Hebblewhite 2000).
Yellowstone wolves tended to encounter elk at a higher rate
(encounters/hour/bout) than bison but not significantly so, likely due to the small sample
of completely observed hunting bouts involving bison. Nonetheless, wolves did
37
encounter elk more frequently (proportion of the total number of prey encounters) than
bison, which was consistent with greater elk densities in YNP (Smith et al. 2000). For
other prey species found in or near forest such as moose and mule deer, visibility bias
may have underestimated prey encounter frequencies.
General Patterns of Hunting Success
Overall, the duration of wolf hunting behavior was more important to hunting
success in YNP than the frequency of hunting behavior. For example, while long prey
encounters and predation attempts were significantly more successful than short ones,
neither the rate at which wolves encountered prey, nor the rate at which wolves attempted
to kill prey had an affect on hunting success. Successful encounters were characterized by
significantly lower predation-attempt rates. Since long encounters were more successful
than short ones it is not surprising that long hunting bouts contained no more prey
encounters than short hunting bouts. Rather than make several encounters during a
hunting bout, it may be more advantageous for wolves to make a few extended
encounters during the same period.
A similar association between hunting success and encounter duration, found for
wolves hunting bison in Wood Buffalo National Park, Canada, was considered the result
of prey distribution patterns. Wolves there were thought to retain contact with bison herds
for long periods due to the difficulty in locating widely dispersed prey (Carbyn et al.
1993). Considering the high prey encounter rate in YNP, other factors may also be
involved.
38
Clearly, one would expect a hunting success to be longer than a failure for the
simple reason that it takes longer to kill than to give up. However, the exact factors that
cause successful encounters to be prolonged are unknown. The relationship between
hunting success and encounter duration may result from time expenditures required (1) to
kill an individual prey once it has been captured, (2) to identify vulnerable prey upon
encounter, and (3) to wait to capture a vulnerable prey once it has been identified (e.g., to
identify a strategic advantage). The time required to subdue prey once it has been
captured necessarily results in increased encounter duration, and in part explains the
difference in duration between successful and unsuccessful encounters. However, since a
successful capture only comprises an average 2.5 (elk) to 7 (bison) minutes of an
encounter that is on average nearly 28 minutes long, capture duration by itself does not
explain the relationship between hunting success and encounter duration.
Identifying a prey as vulnerable requires a period of information gathering (Mech
et al. 1998) that is likely not instantaneous. Failure to obtain adequate information may
lead wolves to attack when they should not or to not attack when they should, resulting in
unnecessary energetic expenditures or missed feeding opportunities, respectively (Hasson
1991). Wolves may also face unnecessary injury risk if they attack when they should not.
Therefore, accurate prey assessment may require extended contact with prey and may be
especially prolonged if prey mask their vulnerability (e.g., bluffing), and/or live in large
herds.
Prolonged encounter duration may also be related to an inability to identify
vulnerability in prey. Since hunting experience likely influences accurate prey
39
assessment, inexperienced wolves may retain contact with prey for longer periods than
experienced wolves. Haber (1977) believed that adult wolves (≥24 months) generally
identified a potential victim as not vulnerable in less than 2-3 minutes, while young
wolves (< 24 months) lingered near prey for hours or even a day longer making repeated
attempts to capture a prey, although his method for determining age of wolves was not
clear.
Once wolves identify a prey as vulnerable, additional time is often necessary to
grab the individual if other non-target prey are protecting it. For instance, female moose
will aggressively protect their offspring from attacking wolves (Mech 1966a; Peterson
1977). In herd prey, such as bison (Carbyn et al. 1993; MacNulty et al. 2002) and
muskoxen (Tener 1954; Gray 1987; Mech and Adams 1999), non-target individuals will
shield the target individual, and attempt to drive off attacking wolves. To minimize their
risk of injury from non-target prey, wolves must often wait and watch for an opportunity
to strike at the target.
Extended duration in successful encounters might also result from wolves
capturing and wounding prey, followed by a period in which wolves lie and wait for the
prey to weaken (Mech 1966a; Miller and Gunn 1977; Mech et al. 1998).
The negative relationship between predation-attempt rate and hunting success
may be explained in terms of wolf energetics, and the behavioral response of prey.
Because predation attempts are likely energetically expensive causing wolves to tire, each
additional attempt wolves make during an encounter may be less effective than the
previous attempt. Also, the effectiveness of high predation-attempt rate during an
40
encounter may be diminished by variation in the intensity of anti-predator behavior. For
instance, after an initial predation attempt, prey may become more alert or, if in a herd,
they could consolidate into a tight group, thereby decreasing the chance of targeting, and
thus that a subsequent predation attempt will succeed. Also, if the intensity of antipredator behavior decreases over a long period during an encounter, prey may be more
susceptible to a few predation attempts over an extended encounter rather than a rapid
series of predation attempts in a short encounter.
Hunting success in elk encounters was highest during spring, a period during
which elk in YNP experience nutritional stress due to low forage quality and abundance
(Houston 1982). Consequently, wolves probably enjoyed greater hunting success during
spring as a result of reduced elk condition (Mech 1977; Carbyn 1983). Hunting success
in bison encounters probably did not vary significantly over season due to the small
sample of bison encounters.
Overall, bison were more difficult for wolves to kill in YNP, confirming an earlier
result (Smith et al. 2000). In this study wolves infrequently killed bison (1 kill every 25
encounters) and did not make multiple kills. In contrast, wolves more frequently killed
elk (1 kill every 4 encounters) and sometimes more than one at a time. Differences in
success rate between wolves preying on bison and elk in YNP suggest that bison are
more difficult to capture and kill than elk, perhaps because of size differences.
41
Hunting Success and Prey Size
Analysis across several studies and prey species suggests that hunting success and
prey size are broadly related. Success was lowest among the large prey, including bison,
moose, and muskoxen, and higher among smaller prey including elk, caribou, and Dall
sheep. The negative relationship between prey size and hunting success is most likely due
to differences in injury risk among different prey sizes. Rates of success for wolves
hunting caribou likely differ due to different encounter definitions that were not explicit
for each study.
Lack of significant difference in success rate of wolves hunting bison in WBNP
and YNP is interesting considering that wolves in YNP have only recently learned to kill
bison (Smith et al. 2000), while wolves in WBNP have always killed bison (Carbyn et al.
1993).
Risk of Injury and Prey Size
Overall, prey encountered by wolves were aggressive and dangerous. Over 50%
of prey encounters involved prey that stood and confronted wolves. Prey that confronted
wolves were more likely to charge and kick at wolves than prey that fled. As a result,
prey that confronted wolves were less likely to be killed (Mech 1966a; Peterson 1977).
Prey that confront wolves are subject to intense scrutiny, including brief probing
attacks. The extent to which a prey represents a genuine injury risk will determine
whether wolves initiate an attack or escalate a probing attack. As a result, prey that
42
confront wolves are probably limited to those relatively few individuals that are
sufficiently fit and aggressive to intimidate and deter wolves.
Bison, which are generally larger than elk, were more dangerous to wolves than
elk. Bison stood and confronted wolves in nearly 80% of encounters, and charged wolves
in over 60% of encounters. In contrast, elk stood and confronted wolves in just over 50%
of encounters, and charged wolves in slightly more than 25% of encounters. These
differences in anti-predator behavior indicate that elk are less risky for wolves to kill than
are bison. As a result, elk are more vulnerable to wolf predation than are bison.
Observed differences in aggression toward wolves between elk and bison were
consistent with previous evidence that prey size and injury risk are positively related
(Weaver et al. 1992). In general, small prey are probably less risky than large prey
because they are less able to use their size to intimidate and physically threaten wolves.
Wolf Behavioral Response to the Risk of Injury
During this study, the mortality of 6 wolves was attributed to prey-caused injury.
Necropsy indicated that 4 wolves were definitely killed by elk, and 2 were probably
killed by elk and moose, respectively (D.W. Smith, National Park Service, unpublished
data). These wolf mortalities demonstrate that the risk of injury is significant during wolf
encounters with prey in YNP.
Comparisons of wolf hunting behavior between encounters with bison and elk
provide strong evidence that wolves assess their risk of injury and make foraging
decisions based on injury risk. Evidence that wolves modify their behavior to reduce
43
injury risk during bison encounters included (1) longer encounters, (2) infrequent
predation attempts, (3) brief predation attempts, (4) longer time between consecutive
attempts, and (5) frequent and extended watching behavior following an approach.
Longer bison encounters and infrequent predation attempts on bison also reflect apparent
trade-offs between food and safety.
Although the difference was not statistically significant, the tendency for bison
encounters to be longer than elk encounters is consistent with the hypothesis that
predators handle dangerous prey more carefully. However, prolonged encounters may
also result from the reluctance of bison to flee, thus allowing wolves to loiter near bison
for extended periods. Long bison encounters support the hypothesis that wolves accept
reduced prey profitability in exchange for safety. However, if wolves kill an adult bison it
is possible that large energetic returns compensate for large time expenditures during
encounters. Wolf encounters with large prey elsewhere also tend to be prolonged.
Encounter duration has lasted more than 36 hours with moose (Mech et al. 1998), 11
hours with bison (Carbyn and Trottier 1988), and 2.5 hours with muskoxen (Gray 1983).
In the Northwest Territories, Canada, wolves spent more time in encounters with
muskoxen than with Peary caribou, a much smaller prey (Gray 1983).
More frequent predation attempts during encounters with elk suggests that wolves
prefer attacking elk and avoid attacking bison. Since elk are less dangerous than bison,
preference for attacking elk suggests that wolves possess the ability to assess their risk of
injury and incorporate this information into their predation-attempt decisions. Wolves
likely minimized predation attempts on bison because (1) the probability of injury was
44
too high, and/or (2) long encounter duration (e.g., handling time) made bison
energetically unprofitable. Given that more biomass is generally available on a bison than
an elk, preference for elk is another indication that wolves trade food for safety. Contrary
to an earlier suggestion that prey encounter rates are the most important influence on wolf
diet (Huggard 1993), apparent differences in patterns of wolf hunting behavior between
bison and elk encounters in YNP suggests that risk of injury is an equally important
influence on the species of prey included in the wolf diet.
Wolves that pursued or held bison at bay also reduced their risk of injury by
minimizing the duration of the predation attempt. By minimizing the time they pursued or
held bison at bay, wolves minimized the amount of time they were exposed to a bison
counterattack. The tendency for extended periods between consecutive attempts also
likely reflected an effort to minimize injury risk. In such cases, wolves may have spent
extra time resting and/or waiting for a ‘safe’ opportunity to attack.
The tendency for YNP wolves to approach bison within close range and watch,
rather than immediately attack, is another indication that wolves use caution during bison
encounters. Periods of watching were prolonged, lasting an average of 13 minutes. While
the tendency of wolves to watch bison may be the result of fear, it also reflects a lack of
fear among bison towards wolves. For instance, targeting usually follows an approach
because generally bison do not flee from wolves (Smith et al. 2000), and attacks on
individuals located at the periphery of the herd often occur while non-target individuals
elsewhere in the herd continue to graze uninterrupted (MacNulty et al. 2002).
45
Sensitivity to the risk of prey-caused injury in YNP wolves was also apparent in
the wolves’ selection of capture point on prey. In YNP, wolves tended to grab the largest
prey by the hind end, medium-size prey by the hind end and neck, and small prey by the
neck. Grabbing the hind end of prey may lower the risk of injury in two ways. First, a
prey may have more difficulty delivering lethal blows to wolves it cannot keep in direct
sight (Mech 1966a). Second, wolves grabbing the hind end might be better positioned to
quickly retreat from an aggressive prey than wolves grabbing the neck. Other studies
confirm the association between capture point and prey size. Wolves tend to grab deer
almost anywhere (Mech 1970; Mech and Frenzel 1971) and caribou by the front end
(Murie 1944; Kelsall 1960; Smith 1980), and larger prey including bison (Carbyn et al.
1993), moose (Burkholder 1959; Mech 1966a; Haber 1977), and muskoxen (Gray 1983;
Mech and Adams 1999) by the hind end. Occasionally wolves will grab the nose of large
prey (Mech 1966a; Gray 1970; Nelson and Mech 1993), but this often appears to be an
effort to distract the prey while others are attacking the hind end (Mech 1966a).
Although many predators appear to make foraging decisions based on their risk of
injury from prey (Forbes 1989), predators are usually considered "fierce" (Brown et al.
1999) because they elicit a fear response from prey that involves trade-offs between food
and safety (Krebs 1980; Newman and Caraco 1987). Foragers must trade between food
and safety to survive and reproduce, and the fitness consequences related to this trade-off
result in the evolution of adaptive foraging strategies. Most foraging studies focus on the
46
behavior of a timid prey pursued by a fierce predator and assume that behaviorally
unresponsive predators hunt prey with no risk of prey-caused injury (Sih 1980; Caraco
1981; Dill 1987).
Comparisons of wolf hunting behavior between bison and elk encounters
demonstrate that predators hunting dangerous prey respond to the risk of injury and make
trade-offs between food and safety. In this case, safety represents the critical need to
avoid being killed or injured by a belligerent prey. However, since predators cannot
simultaneously maximize food intake and minimize injury risk, conflict arises in deciding
whether to attack or avoid dangerous prey. Wolves appear to have resolved this conflict
by evolving a preference for vulnerable prey. As a result, wolves are able to acquire food
while avoiding or minimizing the risk of injury (Mech 1970).
Since the fitness consequences of prey-caused injury are severe, the risk of injury
is likely a strong selective force over evolutionary time on the foraging behavior of
predators that rely on dangerous prey for food. Further insight to foraging behavior is
possible by moving beyond the traditional view of a timid prey pursued by a fierce
predator (Lima and Dill 1990; Brown et al. 1999) and considering more behaviorally
sophisticated systems where prey are not invariably timid, and predators are not
consistently fierce.
47
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53
Table 1. Number of hunting bouts, prey encounters, and predation attempts observed in
their entirety and partially observed (in parentheses) for various wolf packs in
Yellowstone National Park, May 1995 – March 2000.
Pack
No.
wolves
No.
hunting bouts
No.
prey encounters
No.
predation attempts
Rose Creek
10-22
23 (146)
90 (175)
160 (217)
Leopold
2-13
17 (125)
68 (156)
113 (165)
Druid Peak
5-9
18 (135)
75 (172)
154 (193)
Crystal Creek
2-16
4 (34)
26 (57)
125 (139)
Chief Joseph
2-11
0 (7)
1 (7)
3 (6)
Nez Perce
2-13
0 (4)
0 (4)
0 (3)
Soda Butte
3-8
0 (2)
1 (2)
1 (2)
0 (9)
6 (11)
9 (11)
62 (400)
267 (584)
565 (736)
Lone wolves
Total
54
Table 2. Results of General Linear Mixed Model (GLMM) evaluating the effects of
hunting-state type and prey species on hunting-state duration (min.) in wolf encounters
with elk and bison herds in Yellowstone National Park, May 1995 – March 2000.
Predicted mean hunting-state duration with 95% confidence intervals is shown in
Figure 6.
Parameter
Regression
coefficient1
S.E.
d.f.
Approximate t
P
-0.04
0.19
220
-0.24
0.8126
Approach
0.61
0.14
307
4.51
< 0.0001
Attack
0.63
0.13
307
4.77
< 0.0001
Capture
0.93
0.20
307
4.64
< 0.0001
0.04
0.17
307
0.22
0.8295
Constant
Hunting state
2
Prey species
Elk
1
A coefficient is interpreted as the typical difference in mean hunting-state duration relative to the reference
group for each covariate (i.e. target for hunting state, and bison for prey species) when all other covariates
are held constant. For example, the coefficient for the approach state indicates that the mean hunting-state
duration was on average 0.61 minutes more (because the coefficient is positive) during approach states than
during target states.
2
The watch state was excluded from this analysis because it only occurred during wolf encounters with
bison.
55
Table 3. Success rates for wolves hunting various prey species in Yellowstone National
Park, May 1995 - March 2000, based on known outcomes from completely observed prey
encounters only, and on both complete and incompletely observed encounters (in
parentheses) 1.
Prey species
No.
encounters
No.
attempts
No.
kills
Kills per
encounter
Kills per
attempt
211 (463)
246 (543)
50 (104)
0.24 (0.23)
0.20 (0.19)
Bison
47 (74)
91 (154)
2 (5)
0.04 (0.07)
0.02 (0.03)
Pronghorn
6 (10)
4 (7)
2 (2)
0.33 (0.20)
0.50 (0.29)
Bighorn
2 (6)
1 (4)
0 (0)
0 (0)
0 (0)
Mule Deer
1 (3)
1 (3)
1 (2)
1.00 (0.67)
1.00 (0.67)
Moose
0 (2)
0 (2)
0 (0)
0 (0)
0 (0)
Total
267 (558)
343 (713)
55 (113)
0.21 (0.21)
0.16 (0.16)
Elk
Weighted mean2
1
2
Encounters include both solitary and herd prey.
Mean weighted using number of prey encounters or predation attempts for each species.
56
Table 4. Age1 and sex of prey killed by wolves in Yellowstone National Park, May 1995
- March 2000. The proportion killed in each age/sex class for each prey species is shown
in parentheses.
Prey species
Adult
male
Adult
female
Yearling
Young-ofthe-year
Unknown
N
20 (0.19)
50 (0.48)
2 (0.02)
26 (0.25)
6 (0.06)
104
Bison2
0
1 (0.20)
0
4 (0.80)
0
5
Pronghorn3
0
0
0
2 (0.67)
1 (0.33)
3
Mule deer
0
1 (0.50)
0
1 (0.50)
0
2
20 (0.17)
52 (0.46)
2 (0.02)
33 (0.29)
7 (0.06)
114
Elk
Total
1
Age was classed as adult (>23 months), yearling (12-23 months), or young-of-the-year (<12 months).
Kills of the adult female and two young-of-the-year bison were reported earlier (Smith et al. 2001).
3
Includes 1 fawn killed in an encounter that was partially observed.
2
57
Table 5. Results of General Linear Mixed Model (GLMM) evaluating the effects of
hunting-state outcome, hunting-state type, and prey species on hunting-state duration
(min.) in wolf encounters with elk and bison herds in Yellowstone National Park, May
1995 – March 2000. Predicted mean hunting-state duration with 95% confidence
intervals is shown in Figure 9.
Parameter
Regression
coefficient1
S.E.
d.f.
Approximate
t
P
1.93
0.48
217
4.00
< 0.0001
Approach
-1.31
0.49
275
-2.70
0.0074
Attack
-1.37
0.50
275
-2.75
0.0063
Target
-2.27
0.51
275
-4.45
< 0.0001
-1.08
0.49
275
-2.18
0.0299
-0.97
0.46
275
-2.10
0.0371
Approach × Elk
1.08
0.50
275
2.16
0.0314
Attack × Elk
1.08
0.49
275
2.19
0.0294
Target × Elk
1.23
0.50
275
2.44
0.0155
Constant
Hunting state
2
Prey species
Elk
Hunting-state outcome
Failure
Hunting state × Prey species
Hunting state × Hunting-state outcome
1
Approach × Failure
1.02
0.50
275
2.05
0.0408
Attack × Failure
1.03
0.50
275
2.08
0.0382
Target × Failure
1.27
0.51
275
2.46
0.0145
A coefficient is interpreted as the typical difference in mean hunting-state duration relative to the reference
group for each covariate (i.e. capture for hunting state, bison for prey species, and success for outcome)
when all other covariates are held constant.
2
The watch state was excluded from this analysis because it only occurred during wolf encounters with
bison.
58
Table 6. Results of General Linear Mixed Model (GLMM) evaluating the effects of
hunting-state type on hunting-state duration (min.) in wolf encounters with bison herds in
Yellowstone National Park, May 1995 – March 2000. Predicted mean hunting-state
duration with 95% confidence intervals is shown in Figure 11.
Parameter
Regression
coefficient1
S.E.
d.f.
Approximate t
P
1.86
0.22
31
8.49
< 0.0001
Approach
-1.31
0.21
183
-6.31
< 0.0001
Attack
-1.19
0.21
183
-5.75
< 0.0001
Target
-1.85
0.23
183
-7.89
< 0.0001
Capture
-0.53
0.38
183
-1.39
0.1672
Constant
Hunting state
1
A coefficient is interpreted as the typical difference in mean hunting-state duration relative to the reference
group (i.e. watch). For example, the coefficient for the approach state indicates that the mean hunting-state
duration was on average 1.31 minutes less (because the coefficient is negative) during approach states than
during watch states.
59
Table 7. Reported success rates for wolves hunting various North American prey species.
Prey
species
No.
encounters
No.
attempts
No.
animals
No.
kills
Kills
per
encounter
Kills
per
attempt
Kills
per
animal
Moose
71
66
77
6
0.08
0.09
0.08
Mech 1966a
49
1
0.02
Peterson 1977
53
7
0.13
Mech et al.
1998
389
23
0.06
Haber 1977
Moose
Moose
37
Moose
0.19
Reference
Bison
31
46
3
0.10
0.07
Carbyn et al.
1993
Bison
74
154
5
0.07
0.03
This study
Muskoxen
21
3
0.141
Caribou
34
2
0.06
Caribou
16
9
0.56
Caribou
26
4
0.15
24
0.24
Dall
Sheep
Whitetailed
Deer
Dall
Sheep
Elk
1
44
303
100
60
18
463
12
186
543
0.05
Clark 1971
Haber 1977
0.01
0.33
104
0.23
Mech et al.
1998
Haber 1977
Nelson and
Mech 1993
0.20
6
Rate calculated from data reported in Grey (1983).
Grey 1983
0.03
0.20
Mech et al.
1998
This study
60
Figure 1. The predatory sequence for wolves hunting herds of prey.
Hunting Bout
Prey Encounter
Predation Attempt
Hunting State
1
Travel
Approach
Watch
Attack
The target state would not be included in wolf encounters with solitary prey.
Target1
Capture
61
Figure 2. Study area and general location of study wolf packs, Yellowstone National
Park, 19951-2000.
Chief Joseph
Rose Creek
Leopold
Druid Peak
Crystal Creek
Nez Perce
Soda Butte
Wolf Pack Territory
Park Boundary
N
Roads
MT
ID
1
2
WY
0
10
20 KM
Only Rose Creek, Crystal Creek, and Soda Butte packs were present in 1995.
Figures do not depict actual size or shape of wolf pack territories.
Northern Range
Lakes
2
62
Figure 3. Time of year wolf hunting bouts were observed in Yellowstone National Park,
May 1995 - March 2000.
160
No. hunting bouts observed a
140
120
100
80
60
40
20
0
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
Month
63
Figure 4. Time of day wolf hunting bouts were observed during intensive winter study
periods1 (mid-November to mid-December & March) in Yellowstone National Park,
1995 - 2000.
40
No. hunting bouts observed
a
35
30
25
20
15
10
5
00
19
00
18
00
17
00
16
00
15
00
14
00
13
00
12
00
11
00
0
10
90
0
80
0
70
0
60
50
0
0
Time of day (hrs.)
1
Daily monitoring was continuous during winter study and observation effort per hour was generally
constant.
64
No. prey encounters observed a
Figure 5. Number of prey present during wolf encounters with various prey species in
Yellowstone National Park, May 1995 - March 2000.
50
Elk
45
Bison
Mule deer
40
Pronghorn
35
Moose
30
Bighorn sheep
25
20
15
10
5
0
1
2-5
6-10
11-20
21-50
No. prey present
51-100
>100
65
Figure 6. Predicted mean duration (min.) of hunting states with 95% confidence intervals
in wolf encounters with elk (S) and bison (z) herds in Yellowstone National Park, May
1995 - March 2000. Fitted means and confidence intervals are derived from GLMM
results (Table 2). The number of hunting states is shown above each confidence interval.
4.5
10
4.0
29
3.5
Time (min.)
3.0
2.5
59
59
136
145
2.0
1.5
57
37
1.0
0.5
0.0
0.00
Approach 1.00
Attack
2.00
Hunting state
Target
3.00
Capture
4.00
66
Figure 7. Results of completely observed wolf encounters with (a) elk herds and
(b) solitary elk in Yellowstone National Park, May 1995 - March 2000. A single type
of state can occur multiple times during an encounter. Therefore, percents indicate the
proportion of the total number of hunting states (in parentheses) observed during an
encounter that resulted in a specific outcome (indicated by arrow). Percents in bold
indicate the first hunting state to occur in the encounter.
(a)
179 encounters with elk herds
85%
14%
1%
Approach (177)
63%
Attack (180)
42%
8%
Target (108)
43%
1%1
Capture (53)
85%
1
Kill (45)
Newborn elk calves
(b)
32 encounters with solitary elk
75%
25%
Approach (25)
76%
Attack (31)
16%
Capture (5)
100%
Kill (5)
67
Figure 8. Results of completely observed wolf encounters with (a) bison herds and (b)
solitary bison in Yellowstone National Park, May 1995 - March 2000. A single type of
state can occur multiple times during an encounter. Therefore, percents indicate the
proportion of the total number of hunting states (in parentheses) observed during an
encounter that resulted in a specific outcome (indicated by arrow). Percents in bold
indicate the first hunting state to occur in the encounter.
(a)
36 encounters with bison herds
97%
3%
Approach (64)
17%
43%
Watch (42)
19%
47%
Attack (53)
17%
19%
Target (35)
17%
23%
Capture (10)
20%
Kill (2)
(b)
11 encounters with solitary bison
91%
9%
Approach (11)
45%
Attack (10)
0%
Capture (0)
68
Figure 9. Predicted mean duration (min.) of hunting states with 95% confidence intervals
in failed (S) and successful (z) wolf encounters with (a) elk and (b) bison herds in
Yellowstone National Park, May 1995 - March 2000. Fitted means and confidence
intervals are derived from GLMM results (Table 5). The number of hunting states is
shown above each confidence interval.
4.0
(a) elk
26
3.5
Time (min.)
3.0
34
93 52
90
2.5
3
2.0
37
1.5
20
1.0
0.5
0.0
0
Approach
1
Attack
2
Target
3
Capture
4
Hunting state
20
(b) bison
18
3
16
Time (min.)
14
12
10
8
7
6
18 25
4
53 6
31 6
2
0
0
Approach
1
Attack
2
Target
Hunting state
3
Capture
4
69
Figure 10. The association between mean wolf hunting success (kills/encounter)1 and
season (early winter: Nov 1 - Dec 31, mid-winter: Jan 1 - Feb 28, late winter: Mar 1 - Apr
30, spring: May 1 - Jun 30) in wolf encounters with elk (S) and bison2 (z) in
Yellowstone National Park, May 1995 - March 2000. The number of encounters is shown
above each confidence interval.
0.5
Kills/encounter
0.4
84
0.3
209
105
51
0.2
46
0.1
12
14
Spring
Season
1
Includes known outcomes from both complete and incompletely observed prey encounters.
No bison encounters were observed during mid-winter.
2
4
Late Winter
3
Mid-winter
2
Early Winter
1
0
0
70
Figure 11. Predicted mean duration (min.) of hunting states with 95% confidence
intervals in wolf encounters with bison herds in Yellowstone National Park, May 1995 March 2000. Fitted means and confidence intervals are derived from GLMM results
(Table 6). The number of hunting states is shown above each confidence interval.
11
54
10
9
10
Time (min.) s
8
7
6
5
4
3
59
59
2
37
1
0
0
Approach 1
Watch
2
Attack
Hunting state
3
Target
4
Capture
5
71
Figure 12. The association between hunting success (kills/encounter)1 and prey size2 for
wolves hunting various North American prey (Spearman rank correlation coefficient,
rs = -0.60, N = 11, P < 0.05).
0.70
Dall Sheep
Caribou
0.60
Elk
Muskoxen
Kills/encounter a
0.50
Moose
Bison
0.40
0.30
0.20
0.10
0.00
0
100
200
300
400
500
Mean prey weight (kg)
1
Hunting success data are from Table 7.
Mean prey weights are estimated from Nowak (1999).
2
600
700