Journal of Mammalogy, 88(4):1029–1039, 2007
ESCAPING INTRAGUILD PREDATION: MEXICAN KIT
FOXES SURVIVE WHILE COYOTES AND GOLDEN
EAGLES KILL CANADIAN SWIFT FOXES
AXEL MOEHRENSCHLAGER,* RURIK LIST,
AND
DAVID W. MACDONALD
Centre for Conservation Research, Calgary Zoological Society, 1300 Zoo Road, Calgary, Alberta T2E 7V6, Canada (AM)
Wildlife Conservation Research Unit, University of Oxford, Tubney House, Tubney OX13 5QL, United Kingdom (AM, RL, DWM)
Instituto de Ecologı́a, Universidad Nacional Autónoma de México, 04510 México, Distrito Federal, Mexico (RL)
Although interspecific killing among carnivores can drive populations toward extinction, it is generally unknown
how these intraguild interactions vary among populations, and whether the threat for vulnerable species can be
mitigated. We studied imperiled populations of swift foxes (Vulpes velox) in Canada and kit foxes (Vulpes
macrotis) in Mexico to determine potential differences in survival or predator-avoidance strategies. Survival rates
were significantly lower in Canada than in Mexico because of mortality caused by coyotes (Canis latrans) and
golden eagles (Aquila chrysaëtos), and the potential for population recovery is likely higher for the Mexican fox
population. Differences in body size between coyotes and foxes, diet, group sizes, intraspecific home-range
overlap, home-range sizes of coyotes, and movements of coyotes relative to foxes were similar among study
areas. However, Canadian foxes had home ranges that were approximately 3 times larger than those in Mexico,
and Canadian foxes were most frequently killed on their home-range peripheries. Home ranges of kit foxes
decreased in size as the availability of black-tailed prairie dog (Cynomys ludovicianus) colonies increased and
associated refuge holes, which foxes could use to escape predation, were significantly more abundant in Mexico
than in Canada. Small home ranges of foxes probably reduced encounters with coyotes in Mexico, and a high
availability of refuges likely allowed foxes to elude predators when such encounters did occur. Differences in
survival of foxes relative to mortality caused by coyotes demonstrate that interactions between carnivores can
vary greatly between populations and that, in some situations, vulnerable species may be able to coexist with
dominant carnivores despite a lack of large-scale habitat partitioning.
Key words:
carnivore, coexistence, conservation, home range, intraguild predation, refuge, reintroduction
Interspecific killing and, in cases where the victim is consumed, intraguild predation is increasingly recognized as widespread among carnivores (Arim and Marquet 2004). Palomares
and Caro (1999) identified 97 pairwise combinations of
interspecific killing in carnivores, involving 27 killing and 54
victim species. Interspecific killing has most frequently
involved canids and felids as the aggressors and canids, felids,
or mustelids as the victims. The body mass of interspecific
killers is positively correlated to that of their victims and groupliving carnivores kill larger victims than do solitary species
(Palomares and Caro 1999). The published literature likely
underestimates the true frequency of interspecific killing. In
Africa, for example, distribution, diet, and body size suggest
* Correspondent: [email protected]
Ó 2007 American Society of Mammalogists
www.mammalogy.org
1029
that carnivores may be killed by an average of 14.8 and eaten
by an average of 8.6 other species of carnivores, but relatively
few of these interactions have been documented (Caro and
Stoner 2003).
Habitat partitioning allows some populations of carnivores to
persist despite interspecific killing. For example, in southwestern Spain common genets (Genetta genetta) and Egyptian
mongooses (Herpestes ichneumon) avoided Matasgordas
habitat that was preferred by Iberian lynx (Lynx pardinus—
Palomares et al. 1996). Red foxes (Vulpes vulpes) avoided
coyotes (Canis latrans) in the Yukon Territory in Canada by
using brush habitats on the peripheries of the home ranges of
coyotes (Theberge and Wedeles 1989). Gray foxes (Urocyon
cinereoargenteus) that eluded predation by coyotes and bobcats (Lynx rufus) in the Santa Monica Mountains in California
also were restricted to brush areas (Fedriani et al. 2000).
The potential to elude aggressive carnivores through habitat
partitioning decreases in open, homogeneous environments and
subsequent interspecific killing can have dire consequences for
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JOURNAL OF MAMMALOGY
imperiled species. Spotted hyenas (Crocuta crocuta) and
lions (Panthera leo) frequently kill endangered African wild
dogs (Lycaon pictus—Creel et al. 2001). An increase in lions
also has caused a significant decrease in the number of
surviving young of cheetahs (Acinonyx jubatus) in the
Serengeti, and the sustainability of this population of cheetahs
has been in question (Kelly et al. 1998). Similarly, red foxes
are driving arctic foxes (Vulpes lagopus) to extinction in
Scandinavia through the assimilation of dens and killing of
pups (Hersteinsson and Macdonald 1982; Tannerfeldt et al.
2002, 2003). Although sympatry of such competing carnivores may lead to serious consequences for vulnerable
species, high rates of interspecific killing might not necessarily
occur in every population. It is generally unknown how
behavioral and demographic characteristics of dominant and
imperiled carnivores vary across populations, and how these
relative differences correspond to the persistence of vulnerable
species.
To address these gaps in knowledge, we chose to focus on
a system where an imperiled carnivore would have limited
opportunities for habitat partitioning, where mortality rates
could be high, and where comparisons could be made between
distinct populations. Coyotes killing swift foxes (Vulpes velox)
and kit foxes (Vulpes macrotis) on the arid grasslands of North
America is the strongest known example of interspecific killing
among carnivores (Palomares and Caro 1999). The eradication
of wolves (Canis lupus) from the great plains of North America
allowed for an increase in densities of coyotes, which are now
responsible for up to 100% of swift fox and 87% of kit fox
mortalities (Kamler et al. 2003c; White and Garrott 1997).
Increases in survival rates of swift foxes after intensive coyote
control in Texas suggest that coyotes may limit populations of
swift foxes (Kamler et al. 2003b).
Swift and kit foxes are genetically distinct species (Mercure
et al. 1993), but they interbreed regionally, display common
ecological traits, and share similar conservation needs (Moehrenschlager et al. 2004). These foxes have suffered widespread
population reductions primarily due to the loss or degradation
of native prairie habitat (List and Cypher 2004; Moehrenschlager and Sovada 2004). Although most swift and kit fox
populations in the United States are considered to be stable
within a core range that has been drastically reduced from
historic levels (Sovada and Scheick 2000), it is unclear what
the mortality causes and impacts of interspecific killing are on
vulnerable populations along the northern and southern
peripheries of the ranges of these foxes.
Kit foxes in Mexico are nationally listed as vulnerable and
swift foxes in Canada, which were extirpated from Canada by
1938, are now classified as endangered with a reintroduced
population that is small and isolated (Moehrenschlager and
Moehrenschlager 2001). The Mexican Janos–Casas Grandes
grasslands are rich with potential prey of canids because they
harbor the largest remaining population of black-tailed prairie
dogs (Cynomys ludovicianus) in the world. Comparatively,
swift foxes are on the northern limit of their range in Canada,
where a lack of prey availability may have contributed to their
extirpation by the 1930s (Herrero et al. 1991).
Our objective was to determine previously unknown characteristics of these populations such as the availability of refuges
for foxes, annual survival rates of foxes, prey use of foxes and
coyotes, and the interactive spatial dynamics of these canids.
We hypothesized that comparisons between Canada and
Mexico might show large interpopulation variation, because
these regions represent the maximum distance and potentially
the maximum habitat differentiation for swift and kit foxes
in North America. We used similar field protocols and standardized data analyses to compare the interaction of concurrently radiotracked foxes and coyotes in both regions.
MATERIALS AND METHODS
Study areas.— Locations of populations of swift foxes and
coyotes in the Canadian study area spanned the borders of
Alberta, Saskatchewan, and Montana (48853.29–49828.89N,
109844.29–110836.09W). The Mexican study area (30857.89–
30837.59N, 108812.59–108840.39W) was in the Chihuahuan
Desert ecoregion of northwestern Chihuahua. Elevations
ranged from 850 to 1,050 m above sea level in Canada and
1,400 to 1,600 m above sea level in Mexico. Native short-grass
prairie, primarily composed of grasses and forbs, characterized
both locations, but mesquite scrub (Prosopis glandulosa) also
was present on the edge of the Mexican study area. Annual
precipitation ranged from 280 to 465 mm in Canada and 193 to
252 mm in Mexico. Cattle ranching and limited crop
agriculture were the primary land uses in the Mexican and
Canadian study areas.
Captures of animals.— Swift foxes on our Canadian study
site were trapped from January 1995 until February 1998 using
109 39 39-cm Tomahawk double-door (Tomahawk Live
Trap Co., Tomahawk, Wisconsin), 83 31 31-cm Tomahawk single door, and 83 26 26-cm Havahart (Woodstream Corp., Lititz, Pennsylvania) traps that were modified to
reduce capture injuries (Moehrenschlager et al. 2003). Between
July 1994 and February 1996, kit foxes in Mexico were
captured with Victor soft-catch leg-hold traps (Woodstream
Corp.), Aldrich foot snares (Canadian Trading Post, Burlington, Ontario, Canada), and Tomahawk box traps (Tomahawk
Live Trap Co.). All foxes were manually restrained, measured,
and weighed, and their sex was determined. Age classifications
were based on recaptures of previously marked pups and on the
size, color, and wear of teeth. Captured foxes were classified as
juveniles (6–9.5 months of age) or adults (.15 months of age).
No trapping was conducted during the breeding, gestation, and
pup-rearing seasons. Foxes were fitted with mortality-sensor
radiocollars (Canada and Mexico: Lotek Inc., Newmarket,
Ontario, Canada; Mexico: Wildlife Materials Inc., Carbondale,
Illinois).
Five foxes on the Canadian site that were used in homerange analyses but not in survival comparisons had been
translocated from Wyoming to supplement the reintroduced
population. For these individuals, telemetry data were excluded
for the first 60 days postrelease, after which the behavior of
translocated animals was similar to that of resident foxes
(Moehrenschlager and Macdonald 2003), and body weights
August 2007
MOEHRENSCHLAGER ET AL.—ESCAPING INTRAGUILD PREDATION
from their 1st postrelease capture were used for comparisons
between species and countries.
In December 1994, Mexican coyotes were trapped with
Victor soft-catch leg-hold traps nos. 1.5 and 3 (Woodstream
Corp.) in sets baited with Hawbaker’s long distance call lure,
coyote food lure, and gray fox food lure (Hawbaker’s, Fort
Loudon, Pennsylvania). During February 1996, coyotes in
Canada were pursued with snowmobiles and captured with
a salmon landing net. One coyote in a culvert was net-captured
opportunistically by a landowner. Coyotes in both countries
were manually restrained, aged, and fitted with mortalitysensor radiocollars (Lotek Inc.). Coyotes and foxes in both
countries were released without injury after 10–35 min of
handling. The methods employed in Canada were compliant
with guidelines of the American Society of Mammalogists
(Animal Care and Use Committee 1998) and Canadian Council
for Animal Care. The Mexican study was conducted under an
animal care permit granted by the Instituto de Ecologı́a,
Universidad Nacional Autónoma de México.
Telemetry.— Daily relocations in Canada were obtained with
3-element Yagi null-peak telemetry antennas mounted on 4 4
trucks. Nocturnal dynamic-interaction radiotracking was conducted by taking simultaneous bearings from 2 vehicles at
15-min intervals (Doncaster 1990) to determine movement
speeds of foxes and coyotes. Aerial locations were obtained
every 2–3 weeks. In Mexico, radiolocations were obtained
through triangulation from mobile antenna stations using 4 4
trucks. Bearings also were obtained simultaneously from fixed
stations located on hilltops utilizing null-peak systems consisting of 2 parallel 4-element Yagi antennas. Relocations were
recorded for each individual 1–8 times per 6-h tracking sessions, with a 15-min minimum time interval. Similar to radiotracking in Canada, relocations were opportunistically obtained
during diurnal periods, but the primary emphasis was on nocturnal sessions when coyotes and foxes were most active.
Triangulation bearings were converted to location estimates
using LOCATE II (Pacer, Truro, Nova Scotia, Canada) in
Canada and Wildtrak (Todd 1993) in Mexico.
Dead foxes in Canada were briefly examined in the field for
external signs of trauma and kill-sites were searched for signs
of predators. Detailed necropsies were performed by a pathologist at the Calgary Zoo, Calgary, Alberta, Canada, where
causes of death were differentiated from scavenging by
assessing areas of hemorrhaging. In Mexico, the cause of
mortality of 1 uncollared kit fox that was killed by a predator
while caught in a leg-hold trap was determined by field staff.
Diet.— Fox and coyote scats were collected fresh monthly in
Canada and twice monthly in Mexico. Scats were oven-dried in
Canada, air-dried in Mexico, macerated, and partitioned into
prey components. Macroscopic samples were identified using
reference collections. Microscopic items such as invertebrate
remains, feathers, and hair were examined using a compound
microscope. Hair was identified to genus using the color,
medullar and cuticular patterns, length, and thickness (Teerink
1991). Relative and absolute percentages of occurrence (Ciucci
et al. 1996; White et al. 1996) were determined for coyotes and
foxes in each country. Given differences of prey species
1031
between countries, prey occurrences were classified into 5 broad
categories for comparative analyses: insects, rodents, birds,
lagomorphs, and large mammals (ungulates and livestock).
Refuges.— The density of holes that would be large enough
for foxes to use to escape larger predators was compared
between Canadian and Mexican habitats. Holes that foxes used
ranged from 10 cm to 30 cm in diameter and were too small for
entry by coyotes. We deemed sampled holes suitable as fox
refuges if the diameter of the hole was .10 cm and if
a continuous tunnel of at least 2 m led underground. Density of
escape holes in Canada was sampled at 54 random points using
three 200 10-m belt transects that radiated outwards and
were spaced 1208 apart. Ten systematic 1,000 3-m transects
were sampled in prairie dog towns in Mexico and 9 sites of
equivalent dimension were surveyed in Mexican grassland
regions that did not contain prairie dogs. Statistical comparisons of densities of holes between regions were made on
a transect level.
Data analyses.— Kaplan–Meier staggered-entry survival
rates were calculated using Egret, version 2.0.31 (Cytel
Software Corporation, Cambridge, Massachusetts). Survival
rates were compared using Z-tests (Pollock et al. 1989)
between foxes in Mexico and foxes in Canada incorporating
all mortalities, and between foxes in Mexico and foxes in
Canada using only coyote-caused mortalities.
Niche breadth of foxes and coyotes based on categories of
prey in the diet were compared using the Shannon–Wiener
function (Colwell and Futuyma 1971). Horn’s index (Horn
1966) was calculated to compare niche overlap between foxes
and coyotes between countries. Differences in prey items in the
diets were tested between species and countries using contingency table G-tests.
Comparisons of home-range data between studies are
frequently problematic because tracking regimes, autocorrelation among fixes, number of fixes, duration of study, homerange estimators, and calculations with differing software
packages are highly variable (Laundre and Keller 1984;
Lawson and Rodgers 1997; Swihart and Slade 1985). Independence of fixes can be achieved if the animal theoretically
has sufficient time to traverse its home range between sampling
intervals (Swihart and Slade 1985). Home ranges for foxes in
Mexico have been estimated previously (List and Macdonald
2003) but needed to be recalculated for consistency. We
calculated species- and country-specific times to independence
for subsequent comparisons of home-range size by determining
the time required for animals to traverse their ranges.
Maximum travel distances were calculated by determining
the longest trajectory of 95% minimum convex polygons using
Ranges 6 (Kenward et al. 2003). Speeds were calculated by
averaging the time and distance traveled of animals that were
relocated at least twice within 1 h. Necessary time intervals
for independence of fixes were calculated by dividing the
maximum travel distance by the average speed of coyotes
and foxes in Canada and Mexico, respectively. Fixed-kernel
and adaptive-kernel home-range sizes (Seaman and Powell
1996; Worton 1989) were determined using fixes separated by
at least the time thus calculated for independence, the Ranges 6
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JOURNAL OF MAMMALOGY
calculated for foxes and coyotes that had overlapping home
ranges. Simultaneous relocations of both species were
consistently obtained when separation distances were less than
2 km. The relative frequency of 0–1 km and 1–2 km fox–
coyote separation distances was compared between countries.
Group sizes of coyotes were compared for sightings in Canada
and Mexico. Variance values reported in the ‘‘Results’’ are
means 6 SE.
RESULTS
FIG. 1.—Kaplan–Meier staggered entry survival functions of 11 kit
foxes (Vulpes macrotis) in Mexico (from 15 December 1994 to 25
April 1996) and 47 swift foxes (Vulpes velox) in Canada (from 10
January 1995 to 9 February 1998).
default smoothing factor (Kenward et al. 2003), and only
animals that had a minimum of 20 independent fixes (representing generally .60 nonindependent fixes). To control
for potentially confounding differences in duration or frequency of tracking between countries, general linear model
comparisons of home-range sizes included the number of independent fixes and total radiotracking duration in the model.
Because of larger sample sizes for foxes, age and sex were
incorporated into models for foxes but not into those for
coyotes. In Mexico, prairie dog towns were mapped and the
percentage of the area of home ranges of kit foxes consisting
of prairie dog towns was calculated using ArcGIS8 (ESRI,
Redlands, California). Home-range overlap was determined
for conspecifics that were tracked concurrently for at least 1
month by averaging the reciprocal proportion of 95% fixedkernel home ranges. Differences in overlap were compared
between countries for coyotes, between pairs of mated foxes,
and between fox neighbors.
Fixed-kernel 50% and 99% utilization areas were determined
using all available fixes for foxes in Canada that were tracked
sufficiently for calculations of home-range size to determine
the relative location of mortalities caused by coyotes and eagles
in cores and peripheries of home ranges. To compare the relative attraction to or avoidance of fox dens by coyotes between
countries, 95% fixed-kernel utilization areas were constructed
for 10 coyotes in Canada and 7 coyotes in Mexico using all
radiofixes. Within these contours, numbers of fox dens and
movements of coyotes relative to den sites were compared
between countries. Within each area, the average distance of
500 random points to the closest fox den was determined to
yield the expected distance of coyotes to fox dens if coyotes
moved randomly. The observed distance was determined by
averaging the distances of individual coyote relocations to the
closest fox den within respective contours.
Distances between foxes and coyotes that were located
within 15 min during dynamic interaction tracking were
Captures and body weights of foxes and coyotes.— In
Canada, 47 foxes (14 adult males, 15 adult females, 6 juvenile
males, and 12 juvenile females) and 11 coyotes (5 adult males,
4 adult females, and 2 juvenile females), and in Mexico 11
foxes (7 adult males, 3 adult females, and 1 juvenile female)
and 8 coyotes (4 adult males, 2 adult females, and 2 juvenile
females) were captured and radiocollared. One juvenile female
coyote in Mexico was located for only 1 month and 1 juvenile
female coyote in Canada was never relocated, presumably
because of a failed radiocollar; these individuals were excluded
from subsequent analyses. Body weights of adult foxes did
not differ significantly between sexes (males: 2.3 6 0.1 kg,
females: 2.1 6 0.1 kg; F ¼ 1.8, d.f. ¼ 1, 39, P ¼ 0.19) or
countries (Canada: 2.2 6 0.1 kg, Mexico: 2.1 6 0.1 kg; F ¼
1.2, d.f. ¼ 1, 39, P ¼ 0.28). Body weights of adult coyotes
were greater for males than females (males: 15.4 6 0.8 kg,
females: 10.9 6 0.7 kg; F ¼ 14.4, d.f. ¼ 1, 15, P , 0.01) and
were similar between countries (Canada: 13.3 6 1.2 kg,
Mexico: 12.9 6 0.5 kg, F ¼ 2.6, d.f. ¼ 1, 15, P , 0.13). Body
weights of combined sexes of coyotes were 6.3 times greater in
Canada and 5.9 times greater in Mexico than those of foxes,
indicating that body-size parameters determining potential
interspecific resource competition were similar between
countries.
Survival of foxes in Canada and Mexico.— Survival rates of
foxes were lower in Canada than Mexico. After 548 days of
radiotracking (i.e., the total duration of the Mexican study),
none of the 11 kit foxes in Mexico had died, but 51.1% (24/47)
of foxes in Canada had died. At this point, survival of foxes
was significantly lower in Canada than Mexico for foxes dying
of any cause (Z ¼ 16.6, d.f. ¼ 1, P , 0.0001). Survival rates of
foxes were also lower in Canada than Mexico if only
mortalities due to coyotes were considered (Z ¼ 4.9, d.f. ¼
1, P , 0.0001; Fig. 1).
A coyote killed and consumed an uncollared kit fox that had
been caught in a leg-hold trap. Although the population of
golden eagles at Janos–Casas Grandes is the largest in Mexico
(Manzano-Fischer et al. 2006), no foxes were killed by this
predator. However, 1 eagle was observed on the ground,
attempting to reach a kit fox that had been caught in a leg-hold
trap within a den entrance. One year after radiotracking in
Mexico ended, 5 marked foxes were seen that had consequently survived for a minimum average of 694.6 days since
initial collaring (range: 591–1,051 days). Of 23 foxes in
Canada that were radiocollared in January 1995, 1 could not be
relocated after September 1997 and the remaining 22 were
August 2007
MOEHRENSCHLAGER ET AL.—ESCAPING INTRAGUILD PREDATION
TABLE 1.—Annual Kaplan–Meier survival rates of adult and
juvenile swift foxes (Vulpes velox) in Canada.
Year
Age
At
risk
Survival
rate
95% confidence
interval
11 January 1995
10 January 1996
11 January 1996
10 January 1997
11 January 1997
10 January 1998
Adult
Juvenile
Adult
Juvenile
Adult
Juvenile
18
8
21
8
19
3
0.38
0.50
0.52
0.63
0.43
0
0.150.60
0.110.80
0.290.72
0.230.86
0.200.64
0
known to be dead by February 1998. Over the duration of the
Canadian study, a staggered entry Cox proportional hazards
model incorporating age and sex of the 47 tracked foxes was
not significant (likelihood ratio v2 ¼ 1.0, d.f. ¼ 2, P ¼ 0.58),
with each of the single factors producing no significant effects.
Adult survival rates varied from 0.38 to 0.52 among years
(Table 1).
Of the 47 foxes in Canada, 8 died of unknown causes and 8
survived until the end of the study. Of the remaining 31 deaths,
the proportion of mortalities due to coyotes was 0.56 in 1995,
0.44 in 1996, and 0.17 in 1997. The proportion of mortalities of
foxes caused by golden eagles was 0.22 in 1995, 0.22 in 1996,
and then increased dramatically to 0.75 in 1997 (Table 2). With
1 possible exception, coyotes apparently did not consume the
swift foxes that they had killed. For coyote-caused mortalities,
the carcass was completely intact except for the area of injury
in 8 cases (6 retrieved within 7 days, and 2 retrieved within 22
and 19 days, respectively, since the last relocation when the
foxes were alive). The fox that may have been scavenged was
consumed less than 30% within 3 days of the relocation when it
was recorded alive, and 2 others had partially decomposed soft
tissues. For 4 eagle-caused mortalities, the carcass appeared
intact externally, but talon punctures were found during
necropsy (retrieved within 1, 1, 2, and 9 days since the last
relocation when the fox was alive). For 5 eagle-caused
mortalities, the body was primarily intact but the intestines
had been dragged out of the body cavity and partially eaten
(retrieved within 0, 1, 1, 9, and 17 days since the last relocation
when the fox was alive). For 4 eagle-killed foxes, consumption
of intestines and the rest of the body was evident (retrieved
after 7, 12, 15, and 17 days since the last relocation when the
fox was alive).
Diet of foxes and coyotes.— Diet assessments were based on
384 fox scats from Canada (538 prey occurrences), 174 coyote
scats from Canada (289 prey occurrences), 303 kit fox scats
from Mexico (345 prey occurrences), and 76 coyote scats from
Mexico (87 prey occurrences; Table 3). Dietary diversity
between foxes and coyotes was similar between countries
(Horn’s indexCan ¼ 1.08; Horn’s indexMex ¼ 0.97) and niche
breadth was similar for foxes (Shannon’s indexCan ¼ 0.67,
Shannon’s indexMex ¼ 0.76) and coyotes (Shannon’s
indexCan ¼ 0.77, Shannon’s indexMex ¼ 0.77). The country
species prey interaction was significant (v2 ¼ 24.2, d.f. ¼
4, P ¼ 0.0001) because occurrences of prey items differed
between countries and between species. Two-way interactions
1033
TABLE 2.—Causes of mortality of swift foxes (Vulpes velox) at our
study site in Canada, 1995–1998.
Cause of mortality
1995
1996
1997
Coyote
Eagle
Poison
Road
Starvation
Unknown predation
Unknown
Total
5
2
4
2
2
2
9
2
4
13
1998
Total
1
1
11
13
2
3
1
1
8
39
1
1
1
2
12
1
13
showed that occurrences of prey types differed between
countries for foxes (v2 ¼ 18.5, d.f. ¼ 4, P , 0.001) and
coyotes (v2 ¼ 30.0, d.f. ¼ 4, P , 0.0001). Rodents were the
most common prey item for foxes and coyotes in both
countries, making up more than 65% of prey occurrences, and
frequencies of rodent occurrence were similar between species
in Canada and Mexico. Insects were more common in fox than
coyote scats, whereas birds occurred more frequently in coyote
than fox scats of both countries. Foxes consumed lagomorphs
more frequently than did coyotes in Canada, but relative
occurrences of lagomorphs in scats were similar between
species in Mexico.
Movement speeds, home-range sizes, and overlaps.— The
movement speed of foxes averaged 31.1 6 1.7 m/min in
Canada and 28.6 6 1.1 m/min in Mexico. Coyotes in Canada
averaged 47.8 6 6.7 m/min compared to coyotes in Mexico,
which moved at 49.3 6 11.2 m/min. Maximum trajectories of
95% minimum convex home-range polygons differed between
species (F ¼ 39.8, d.f. ¼ 1, 64, P , 0.001) and countries (F ¼
9.2, d.f. ¼ 1, 64, P , 0.01), but not in the interaction term (F ¼
2.7, d.f. ¼ 1, 64, P ¼ 0.1). Trajectory distances for foxes
spanned 11.9 6 0.9 km in Canada (n ¼ 36) and 5.2 6 0.3 km
in Mexico (n ¼ 11; t ¼ 7.5, d.f. ¼ 40.7, P , 0.001), whereas
those of coyotes were similar between countries (Canada: 18.5
6 2.0 km, Mexico: 16.6 6 1.0; t ¼ 0.8, d.f. ¼ 15, P ¼ 0.46).
Using convex polygon trajectories and speed averages,
independent fix times for subsequent home-range size
comparisons were 6.4 h and 3.1 h for foxes in Canada and
Mexico, respectively, whereas those of coyotes in Canada and
Mexico were 6.5 h and 5.6 h, respectively. Thirty-six foxes in
Canada (121.9 6 17.7 independent fixes), 10 coyotes in
Canada (69.2 6 13.3 independent fixes), 10 foxes in Mexico
(83.7 6 17.5 independent fixes), and 4 coyotes in Mexico (39.8
6 8.9 independent fixes) were sampled sufficiently for
subsequent home-range size comparisons.
Foxes were unable to partition their habitats from coyotes in
either country. Home-range overlaps between foxes and
coyotes were extensive in Canada and Mexico, and home
ranges of foxes in the centers of respective study areas were
completely enveloped by home ranges of coyotes (Fig. 2).
Home-range sizes of foxes in Canada and Mexico were
influenced positively by duration of tracking (fixed kernel: F ¼
6.1, d.f. ¼ 1, 47, P ¼ 0.02; adaptive kernel: F ¼ 5.2, d.f. ¼ 1,
47, P ¼ 0.03) and number of fixes (fixed kernel: F ¼ 3.7, d.f. ¼
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TABLE 3.—Absolute frequency (number of times prey item occurred/total number of scats) and relative frequency (number of times prey item
occurred/total number of occurrences of all prey items) of prey items in fox and coyote scats in Mexico and Canada.
Canada
Swift Fox (n ¼ 384)
Mexico
Coyote (n ¼ 174)
Kit fox (n ¼ 303)
Coyote (n ¼ 76)
Food item
Absolute
frequency
Relative
frequency
Absolute
frequency
Relative
frequency
Absolute
frequency
Relative
frequency
Absolute
frequency
Relative
frequency
Insects
Rodents
Birds
Lagomorphs
Large mammals
33.3
83.3
11.2
9.8
2.3
23.8
59.5
8.0
7.1
1.7
17.8
86.2
27.5
1.1
33.3
10.7
51.9
16.6
0.7
20.1
21.4
65.3
6.9
14.2
5.9
18.9
57.3
6.1
12.5
5.2
7.9
68.4
9.2
14.5
14.5
6.9
59.8
8.0
12.6
12.6
1, 47, P ¼ 0.06; adaptive kernel: F ¼ 4.4, d.f. ¼ 1, 47, P ¼
0.04), but not sex (fixed kernel: F ¼ 0.2, d.f. ¼ 1, 47, P ¼ 0.66;
adaptive kernel: F ¼ 0.2, d.f. ¼ 1, 47, P ¼ 0.66) or age (fixed
kernel: F ¼ 0.32, d.f. ¼ 1, 47, P ¼ 0.6; adaptive kernel: F ¼
0.1, d.f. ¼ 1, 47, P ¼ 0.74). Home-range sizes were
significantly larger in Canada (fixed kernel: 31.9 6 4.8 km2,
adaptive kernel: 40.8 6 6.1 km2) than Mexico (fixed kernel:
10.4 6 1.0 km2, adaptive kernel: 11.9 6 1.0 km2) for foxes
(fixed kernel: F ¼ 4.2, d.f. ¼ 1, 47, P ¼ 0.048, adaptive kernel:
F ¼ 5.0, d.f. ¼ 1, 47, P ¼ 0.038; Fig. 3). Foxes in Canada were
twice as susceptible to predation in the 51–99% kernel utilization area, where 10 foxes were killed by eagles or coyotes, than
in the 50% fixed-kernel core area, where only 5 foxes were
killed by these predators. In Mexico, home-range size of foxes
was correlated to abundance of prairie dogs. As the percentage
of the area of home ranges composed of prairie dog towns
increased, home-range size estimated through fixed kernels
(F ¼ 13.6, d.f. ¼ 1, 9, P , 0.01, r2 ¼ 0.63) or adaptive kernels
(F ¼ 16.5, d.f. ¼ 1, 9, P , 0.01, r2 ¼ 0.67) decreased
significantly (Fig. 4).
Home-range overlap between foxes was similar between
countries (F ¼ 0.1, d.f. ¼ 1, 54, P ¼ 0.77), but differed
between mated pairs and neighbors (F ¼ 41.3, d.f. ¼ 1, 54, P ,
0.001); the interaction term was not significant (F ¼ 0.8, d.f. ¼
1, 54, P ¼ 0.38). Mated pairs shared larger proportions of
home ranges than neighbors in Canada (mated pairs: 74.3% 6
4.2%, n ¼ 12; neighbors: 28.8% 6 3.8%, n ¼ 31) and Mexico
(mated pairs: 84.0% 6 2.9%, n ¼ 2; neighbors: 23.9% 6 5.9%,
n ¼ 9).
Differences in home-range size among coyotes could not
be explained by duration of tracking (fixed kernel: F ¼ 0.02,
d.f. ¼ 1, 14, P ¼ 0.89; adaptive kernel: F ¼ 0.1, d.f. ¼ 1, 14,
P ¼ 0.79) or number of fixes (fixed kernel: F ¼ 1.2, d.f. ¼ 1,
14, P ¼ 0.3, adaptive kernel: F ¼ 1.0, d.f. ¼ 1, 14, P ¼ 0.34).
Home-range sizes were similar between coyotes in Canada
(fixed kernel: 88.9 6 25.8 km2, adaptive kernel: 109.7 6
29.9 km2) and coyotes in Mexico (fixed kernel: 80.2 6
17.3 km2, adaptive kernel: 99.1 6 18.2 km2; fixed kernel:
F ¼ 0.1, d.f. ¼ 1, 14, P ¼ 0.80; adaptive kernel: F ¼ 0.1,
d.f. ¼ 1, 14, P ¼ 0.78; Fig. 3). In Canada, 3 radiocollared
females and 1 radiocollared male coyote had litters, but their
mates were uncollared individuals. In both countries, collared
coyotes were not observed hunting together or sharing a den;
consequently documented home-range overlaps of coyotes
were not between mates. Home-range overlap between coyotes
was similar (t ¼ 0.3, d.f. ¼ 27, P ¼ 0.78) between Canada
(30.8% 6 4.2%, n ¼ 21) and Mexico (33.0% 6 5.3%, n ¼ 8).
Movements of coyotes relative to foxes and fox dens.— In
Canada and Mexico, all utilization areas of coyotes overlapped
fox dens. Although in both countries the number of fox dens
within utilization areas of coyotes depended upon the
proximity of the home ranges of coyotes to the areas in which
foxes were studied most intensively, the number of known fox
dens overlapped by individual coyotes did not differ between
countries (Canada: 38.3 6 9.3, Mexico: 42.6 6 11.3; t ¼ 0.29,
d.f. ¼ 15, P ¼ 0.77). Utilization-area sizes did not influence the
absolute (F ¼ 1.4, d.f. ¼ 1, 15, P ¼ 0.25) or proportional
differences (F ¼ 0.4, d.f. ¼ 1, 15, P ¼ 0.54) between the
observed and expected mean distances of locations of coyotes
from fox dens. Using paired Wilcoxon signed-rank tests,
observed and expected mean distances of locations of coyotes
from fox dens were similar in Canada (Z ¼ 1.5, n ¼ 10, P ¼
0.14 ) and in Mexico (Z ¼ 1.7, n ¼ 7, P ¼ 0.09), indicating that
coyotes in both countries did not seem to target or avoid fox
dens. Proportional differences between observed and expected
distances were similar between countries (Mann–Whitney U ¼
31, nCan ¼ 10, nMex ¼ 7; P ¼ 0.70), indicating that the
movements of coyotes relative to fox dens were similar in
Canada and Mexico.
Because foxes and coyotes with overlapping home ranges
were frequently several kilometers apart, the relative frequency
that foxes and coyotes were in close proximity could not be
compared between Canada and Mexico. However, when
coyotes and foxes were located within 15 min and within 2
km of one another, distances of less than 1 km for these paired
locations were more frequent in Mexico. In Mexico 65.0% of
60 paired locations were within 1 km compared to 38.5%
of 244 paired locations in Canada (likelihood-ratio v2 ¼ 13.7,
d.f. ¼ 1, P , 0.001). Foxes in Mexico likely faced a greater
number of coyotes during encounters than did foxes in Canada.
Coyotes in Mexico were more frequently observed in pairs
or groups of at least 3 than coyotes in Canada (Canadian
sightings, n ¼ 430): 1 coyote, 0.76; 2 coyotes, 0.19, 3
coyotes: 0.05; Mexican sightings, n ¼ 58: 1 coyote, 0.45; 2
coyotes, 0.31, 3 coyotes: 0.24; likelihood-ratio v2 ¼ 27.4,
d.f. ¼ 1, P , 0.001).
August 2007
MOEHRENSCHLAGER ET AL.—ESCAPING INTRAGUILD PREDATION
1035
FIG. 3.—Fixed-kernel home-range sizes of swift foxes (Vulpes
velox), kit foxes (Vulpes macrotis), and coyotes (Canis latrans) in
Canada and Mexico.
areas (0.9 6 0.3 holes/ha; Kruskal–Wallis v2 ¼ 35.8, d.f. ¼ 1,
P , 0.001). Although the density of holes in Canadian habitats
differed most extensively from prairie dog towns in Mexico,
densities of holes were also significantly different from
Mexican grassland sites that did not contain prairie dogs
(Mann–Whitney U ¼ 174, nCan ¼ 54, nMex ¼ 10, P ¼ 0.02).
DISCUSSION
Although 48–62% of swift foxes in Canada died annually,
no mortalities occurred among radiocollared foxes in Mexico
over 1.5 years. Coyotes accounted for up to 56% of annual
mortalities of foxes in Canada, whereas survival of foxes in
Mexico was not affected by coyotes. This finding illustrates
that the outcome of interactions between dominant and
FIG. 2.—Example of annual fixed-kernel home ranges of coyotes
(Canis latrans) relative to swift foxes (Vulpes velox) in Canada and to
kit foxes (Vulpes macrotis) in Mexico. Home ranges are shown for the
period of a) 1 March 1995–28 February 1996 in Mexico, and b) 1 June
1996–31 May 1997 in Canada. Within these 1-year intervals, the foxes
shown (black lines) had a minimum of 90 locations, coyotes (gray
lines) had a minimum of 40 locations, and individuals of both species
were tracked for a minimum of 4 months.
Foxes in Mexico had a greater density of potential escape
holes than did foxes in Canada. Densities of suitable escape
holes differed between prairie dog towns in Mexico (57.0 6
6.1 holes/ha), Mexican grassland areas that were dominated by
kangaroo rats (3.7 6 1.4 holes/ha), and Canadian grassland
FIG. 4.—Fixed-kernel home-range sizes of kit foxes (Vulpes
macrotis) in Mexico relative to the percentage of the home range
that contained prairie dog towns. The gray diamond represents 1 home
range that was excluded from the statistic because of a low number of
telemetry fixes (19 independent or 55 nonindependent fixes).
1036
JOURNAL OF MAMMALOGY
imperiled carnivores can vary greatly between populations.
Differences in survival rates suggest that kit fox populations in
Mexico may have a higher recovery potential than swift fox
populations in Canada. Understanding how such differences
can arise despite a lack of habitat partitioning may be crucial to
facilitate the persistence or recovery planning of other
vulnerable carnivore populations.
The frequency of mortalities of foxes caused by golden
eagles was far greater in the Canadian study area than in other
studies of swift or kit foxes where mortalities caused by golden
eagles have been rare (Moehrenschlager et al. 2004). Although
golden eagles can potentially drive populations of small canids
such as the endangered island fox (Urocyon littoralis) toward
extinction (Roemer et al. 2001), the magnitude of such impacts
was only evident in Canada in 1997, when eagles were
responsible for 75% of mortalities of swift foxes. The fact that
mortalities of foxes caused by coyotes were relatively rare
during this year may suggest that foxes were more susceptible
to eagle predation in 1 year, or that coyote and eagle predation
of swift foxes is largely compensatory. If the killing of swift
foxes is compensatory, coyote-caused mortalities would have
been more frequent in the absence of eagle predation in 1997.
Because foxes in Canada that were killed by coyotes were
almost never consumed, such killing likely reflects interference
competition instead of intraguild predation; in contrast, a greater
frequency of consumption of carcasses by golden eagles
suggests that such mortalities are primarily predatory.
Survival rates of adult swift foxes in Canada were similar
to the 0.40–0.75 range of annual survival rates of adult foxes
in the United States (Andersen et al. 2003; Kamler et al.
2003b; Kitchen et al. 1999; Olson and Lindzey 2002;
Schauster et al. 2002; Sovada et al. 1998), but the lack of
mortalities among radiotracked kit foxes in Mexico was
unique compared to 0.44–0.61 annual survival rates of
endangered San Joaquin kit foxes (V. macrotis mutica) in
California (Cypher et al. 2000; Ralls and White 1995; Spiegel
1996; Standley et al. 1992). Although we do not have density
estimates for golden eagles, they were likely more common in
the Mexican study area than in Canada because of the high
availability of prairie dogs (Ceballos et al. 1999). Two
breeding pairs and up to 16 different transient golden eagles
have been recorded in a single day within the ranges of kit
foxes in the Janos–Casas Grandes grasslands (National
Audubon Society 1999). Furthermore, golden eagles are
abundant in these areas throughout the year (Manzano-Fischer
et al. 1999), whereas they are rare on the Canadian prairies in
winter. Although kit foxes were primarily nocturnal, radiotracking data, observations of dens, the observation of an
attempt by an eagle to kill a captured kit fox, and a high
proportion of prairie dogs, which are diurnal, in the diet of kit
foxes indicate that kit foxes in Mexico were vulnerable to
golden eagles, which primarily hunt by day. Because coyotes
were consistently observed in the home ranges of kit foxes
during spotlight surveys (List and Macdonald 1998), coyotes
occasionally hunted within 1 km of simultaneously tracked
foxes, and a captured kit fox was killed by a coyote, kit foxes
also were vulnerable to being killed by coyotes. This
Vol. 88, No. 4
information raises the question of how kit foxes in Mexico
were able to elude golden eagles and coyotes.
Higher survival rates of foxes in Mexico than in Canada
could not be explained by differences in relative body size
compared to coyotes, prey use, intraspecific overlap in home
ranges, group sizes, home-range sizes of coyotes, hunting
strategies of coyotes, or movement rates of either canid.
Because a large proportion of the home ranges of foxes were
entirely overlapped by those of coyotes, foxes did not, and
probably could not, partition their habitat relative to coyotes in
either country. Within these overlap zones, coyotes in both
countries did not appear to divert their movements toward
simultaneously tracked foxes or their dens. In Colorado,
interspecific separation distances between swift foxes and
coyotes, which caused 80% of identifiable mortalities of swift
foxes, also did not differ from those expected by chance
(Kitchen et al. 1999), and the separation distances of San
Joaquin kit foxes from coyotes, which caused 65% of
mortalities of foxes (Ralls and White 1995), were random in
California. These results suggest that the killing of swift and kit
foxes may primarily depend on the likelihood of coyote
encounters.
Home-range sizes of swift foxes were approximately 3 times
larger than those of kit foxes. Home ranges of swift foxes in
Canada were larger than those documented elsewhere (Kamler
et al. 2003a; Kitchen et al. 1999; Pechacek et al. 2000; Sovada
et al. 2001; Zimmerman 1998), but home ranges of kit foxes in
Mexico were similar to those recorded in California and
Arizona (White and Ralls 1993; Zoellick and Smith 1992).
Home ranges of kit foxes in Mexico, which were approximately 8 times smaller than those of sympatric coyotes, were
likely overlapped by fewer home ranges of coyotes than those
of swift foxes in Canada, which were only 3 times smaller than
those of coyotes. Because intraspecific spacing patterns were
similar, the likelihood of foxes encountering coyotes was
probably higher in Canada than Mexico. Because Canadian
foxes were twice as likely to be preyed upon in the outer half of
their home ranges than in the home-range core, the wandering
associated with larger home ranges also may have placed foxes
in Canada at greater risk of coyote and eagle predation than kit
foxes in Mexico.
Larger home-range sizes of foxes in Canada suggest that
available prey for foxes may have been less abundant in
Canada than in Mexico. In a meta-analysis, home ranges of
swift and kit foxes were found to decrease with increasing
availability of lagomorph prey (White and Garrott 1997).
Similarly, in our study, home ranges of kit foxes in Mexico
decreased as the availability of prairie dog towns increased.
In the Mexican study area, there was an abundance of 15.6
prairie dogs per hectare, 4.4 banner-tailed kangaroo rats
(Dipodomys spectabilis) per hectare, 4.1 Merriam’s kangaroo
rats (Dipodomys merriami) per hectare, as well as tawnybellied cotton rats (Sigmodon fulviventer), white-throated
woodrats (Neotoma albigula), and 3 species of mice (Pacheco
et al. 2000). In the Canadian study area, prairie dogs, kangaroo
rats, woodrats, and cotton rats were not present, but
Richardson’s ground squirrels (Spermophilus richardsonii)
August 2007
MOEHRENSCHLAGER ET AL.—ESCAPING INTRAGUILD PREDATION
were potentially available at a greater abundance than the 0.7
spotted ground squirrels (Spermophilus spilosoma) per hectare
we documented in Chihuahua. However, Richardson’s ground
squirrels hibernate from late October to late February on the
Canadian prairies (Michener 1998), making them inaccessible
to swift foxes. Swift foxes consequently rely on arvicoline
rodents during the winter, but Klausz (1997) only caught 163
small mammals in 9,360 trap nights in our swift fox study
area from November 1995 to March 1996. This small abundance along with low species diversity (96% were North
American deermice [Peromyscus maniculatus] and 4% were
shrews [Sorex]), led Klausz (1997) to hypothesize that the latewinter period could lead to high mortalities of swift foxes. We
documented 1 starvation in late winter, and swift foxes were
likely more food stressed in Canada than Mexico, which
probably led to larger home ranges in Canada.
In addition to being a crucial prey item for kit foxes in
Mexico, prairie dogs create refuge holes that allow foxes to
potentially escape from other predators; in contrast, foxes in
Canada primarily rely on the diggings of American badgers
(Taxidea taxus—Herrero et al. 1991), a carnivore that is
increasingly imperiled in the region. Availability of refuges
from predation was significantly higher in Mexican sites that
were dominated by kangaroo rats or prairie dogs than in the
Canadian study area. This presents a further disadvantage for
foxes in Canada. First, foxes in Canada may have encountered
coyotes more frequently than foxes in Mexico, and 2nd, fewer
refuge holes were available for foxes in Canada to escape from
coyotes or golden eagles when they were encountered.
Although differences in fox home-range sizes and the
availability of refuge holes may have contributed to higher
survival rates of foxes in Mexico than in Canada, the relative
importance of these factors is difficult to tease apart. However,
it is clear that foxes in Chihuahua, Mexico, had higher survival
rates than swift or kit foxes that have been studied anywhere
else, and the protection provided by the prairie dog complex,
which has recently been threatened by expansions of cropland
agriculture (Ceballos et al. 2005), may be crucial to facilitate
the recovery of kit foxes in northern Mexico. Experiments that
alter prey availability to assess respective effects on homerange sizes of foxes and coyotes and subsequently test effects
of encounter probabilities on survival would be desirable, but
may be logistically difficult for these wide-ranging species.
Restricting access to existing refuge holes would be problematic because many populations of swift and kit foxes are
already imperiled, but experiments that increase availability of
refuges may be feasible to evaluate potential improvements in
survival of foxes. A potential link between the survival of
imperiled species and the abundance of animals that make
refuges could have profound conservation consequences,
because it might also lead to the protection of refuge-creating
species. Temporary refuges have been shown to benefit prey
species (Ylönen et al. 2003), but such advantages have not
been quantified previously for imperiled carnivores.
Further studies should aim to determine causal links between
prey abundance, home-range size, refuge availability, and the
survival of imperiled species that are otherwise killed by
1037
dominant carnivores. We speculate that the abundance and
differential exploitation of prey will determine relative homerange sizes of dominant and vulnerable intraguild competitors
(in this case, carnivores). The ratio of the home-range sizes of
the dominant carnivore to those of the vulnerable carnivores is
predicted to increase as the availability of prey that is best
exploited by the vulnerable competitor increases. Conversely,
this ratio is expected to decrease as the availability of prey best
exploited by the dominant competitor increases. We predict
that, by determining interspecific encounter frequencies, the
ratio of the home-range size of the dominant species to homerange size of the vulnerable species will be positively
correlated to the survival of the inferior competitor. We also
predict that residual variation from this relationship will be
explained by the group size of the dominant competitor, which
will be negatively correlated to survival of the vulnerable
competitor, and the abundance of potential refuge microhabitats, which will be positively correlated to the survival of
the vulnerable competitor. We encourage modeling approaches
to test these predictions, experimental manipulations of refuge
and prey availability, and further standardized field studies of
similar interspecific interactions between carnivores in different
environments.
ACKNOWLEDGMENTS
We thank M. Doughty, M. Eaton, E. Jiménez, G. Johnson, J.
Johnson, A. McKinney, J. Michie, C. Moehrenschlager, C. Philcox,
and J. Scharlemann for their dedicated fieldwork. D. Biggins, G.
Ceballos, P. Manzano-Fischer, B. Miller, and J. Pacheco provided
invaluable support on logistics, fund raising, and fieldwork. We thank
S. Black for her skillful necropsies, M. Percy for global information
system support, and A. South for software consultation. Preliminary
versions of the manuscript benefited from constructive comments by
D. Bell, L. Bonesi, P. Ferreras, K. Gibson, P. J. Johnson, P. Honess,
J. Kamler, A. Loveridge, G. Mason, and J. Murdoch. This project was
undertaken while R. List held scholarships from The British Council
and Consejo Nacional de Ciencia y Teconologı́a. Grant support was
provided by Alberta Environmental Protection, Alberta Sport
Recreation Parks and Wildlife Foundation, Canadian Wildlife Service,
Comisión Nacional para el Conocimiento y Uso de la Biodiversidad,
Dirección General de Apoyo al Personal Académico–Universidad
Nacional Autónoma de México, Express Pipelines, Green Plan
International, Husky Energy, Nature Saskatchewan, Parks Canada,
People’s Trust for Endangered Species, the Rocky Mountain Elk
Foundation, Saskatchewan Environment and Resource Management,
Swift Fox Conservation Society, University of Alberta Biodiversity
Fund, Wildlife Preservation Trust Canada, United States Agency for
International Development, and World Wildlife Fund Canada.
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Submitted 25 May 2006. Accepted 12 November 2006.
Associate Editor was Rodrigo A. Medellı́n.