COMPETITION
ENCOUNTER
BETWEEN
BEARSANDCOUGARS:
SOMEECOLOGICAL
IMPLICATIONS
KERRYM. MURPHY,HornockerWildlifeInstitute,P.O.Box 526, MammothHot Springs,WY 82190, USA, email:
[email protected]
GREGORYS. FELZIEN,'HornockerWildlifeInstitute,3246, UniversityStation,Moscow,ID83843, WA,USA
HornockerWildlifeInstitute,P.O.Box 3246, UniversityStation,Moscow,ID 83843, USA, email:
MAURICE
G. HORNOCKER,
[email protected]
TONIK. RUTH,HornockerWildlifeInstitute,3246, UniversityStation,Moscow,ID83843, WA,USA, email:
UIDAHO.EDU
RUTH8744@
NOVELL.
Abstract: Black bears (Ursus americanus)or grizzly bears (Ursus arctos) visited 8 of 55 cougar-killed(Felis concolor) ungulates in Glacier
NationalPark(GNP), Montana,from 1992 to 1995, and 19 of 58 cougarkills in Yellowstone NationalPark(YNP), Wyoming, from 1990 to 1995.
Bears displaced cougars from 4 of 8 carcasses they visited in GNP and 7 of 19 in YNP. Cougar predationprovided an average of 1.9 kg/day
(range= 0-6.8 kg/day) of biomass to bears that fed on cougar-killedungulates. This biomass was an importantpercent(up to 113%)of the daily
energy needs of bears when comparedto their caloric requirementsreportedin the literature. We suggest that ungulate carrionresulting from
cougar predationis importantnutritionallyto bears in some regions and seasons. Cougarsthat were displaced from their kills by bears lost an
average of 0.64 kg/day of ungulatebiomass, or 17-26% of their daily energy requirements. Biologists modelling or measuringcougarpredation
rates should be aware that losses to scavengersmay be significant.
Ursus 10:55-60
Key words: black bear, competition, cougar, Glacier National Park, grizzly bear, predation, Puma concolor, scavenging, Ursus americanus,
Ursus arctos, Yellowstone National Park.
Encountercompetitionis a form of interferencecompetitionthatoccurs when 1 or more mobile, nonspatiallyattachedindividualsare harmedas they directlypursuea
common resource(Schoener 1983). This harmincludes
injuriesresultingfromphysical interactions,loss of time,
or theft of food. For carnivores,the disputed resource
may be prey thata subordinatecarnivorehas killed and a
dominantcarnivoreintends to scavenge.
Interactionsbetweencougarsand scavengersthatwere
presentnearcougar-killedpreyhave been reported(Boyd
and O'Gara 1985, Spreadbury1989, White and Boyd
1989, Harrison 1990, Green 1994). Koehler and
Horocker (1991) found 4 bobcats (Lynx rufus) and 2
coyotes (Canislatrans)killed by cougarsnearprey cache
sites. Harrison(1990) found thata maternalfemale cougar residing where coyote abundancewas reduced by
humans killed ungulateprey less frequentlythan a maternalfemale living where coyote numberswere not manipulated.Coyotes displacedcougarsfromtheirkills less
frequentlywhere coyote numberswere controlled.
Althoughinteractionsbetweencougarsand scavengers
near cougar-killedprey have been documented,the ecological importanceof encountercompetitionremainsunclear. Our objective was to (1) documentthe frequency
thatblack bears and grizzly bearsused cougar-killedungulates, (2) estimate the ungulate biomass made available to bears as a result of cougar predation, and (3)
1Deceased.
documentthe frequencythatcougarswere displacedfrom
ungulateprey and calculate the quantityof biomass that
cougarslost to bears.
Ourresearchwas fundedby G.C.Hixon, M.J.Murdock
CharitableTrust, Montanaof Fish, Wildlife and Parks,
NathanielP. Reed, the P.W. Busch Family Foundation,
The NationalFish andWildlife Foundation,The National
GeographicSociety, The National Wildlife Federation,
The R.K. Mellon Foundation,the U.S. Fish and Wildlife
Service, and Yellowstone National Park. The Gallatin
National Forest, GardinerRanger District, and Glacier
National Parkprovided logistical support. We thankJ.
Cole, T. Fredrickson,B. Holmes, M. Johnson,J. Jonkel,
J. Murnane, B. O'Gara, T. Parker, S. Relyea, J.
Tischendorf, and B. Wiesner for their work in the field
or laboratory. B. Chapman and D. Stradley provided
aerial support on short notice. P. Munholland played
an important role in designing the cougar predation
studies. D. Pac graciously provided his unpublished
data on the weights of mule deer carcasses. We thank
agency personnel S. Frye, G. Green, T. Lemke, J.
Mack, J. Logan, C. Sime, T. Their, D. Tyers, J. Varley,
and R. Wuertz. We especially thank K. Aune and T.
Moore of the Montana and Wyoming wildlife laboratories, respectively, for their technical support and
guidance. We thank I. Ross and an anonymous person for reviewing this manuscript.
56
Ursus 10:1998
STUDYAREAS
GlacierNationalPark
Cougar-bearinteractionswere documentedconcurrent
with studies of cougar-wolf (Canis lupus) relationships
in a 790-km2area along the North Fork of the Flathead
River in GlacierNationalPark(GNP) in northwestMontana. Cougarspreyed on elk (Cervus elaphus), whitetailed deer (Odocoileus virginianus), moose (Alces
alces), and mule deer (Odocoileus hemionus). The valley was gently rolling terrainsupportinga montaneforest interspersedwith meadows. It was borderedby steep
mountains, 1700-3000 m in elevation. Snow accumulation promptedungulatesto winter at elevations <1600
m from late November to April. Thereafter,ungulates
used progressivelyhigherelevationsassociatedwith phenological changes in vegetation.
Black bears and grizzly bears were common. Some
grizzlies were active outside their dens for at least part
of the winter, particularlyin the Kintla Lakes area of
GNP, where carrionwas relatively abundant.
YellowstoneNationalPark
Cougarswere studied in the northernportionof Yellowstone NationalPark (YNP), Wyoming, and the contiguous ParadiseValley of south-centralMontana. The
winterdensity of adult-subadultsrangedfrom 2.1 to 3.5
per 100 km2 on a 662-km2core area where they spent
wintermonthson ungulaterangesbetween 1550-2500 m.
Elk and mule deer were far more abundantthan moose
and white-taileddeer. Winteringelk counts rangedfrom
9,400 to 19,000 from 1990 to 1995 (Mack and Singer
1992; J. Mack, YNP, unpubl.data). Mule deer counts
ranged from 1,600 to 2,500 between 1990 and 1994
(Lemke 1994). Duringlate spring,summer,and autumn,
cougarsused elevationsup to 2900 m and preyedon ungulatesthatwere dispersedrelativeto theirrestricteddistributionin winter. Ungulateranges supportedmontane
forest, sagebrushsteppe, and grasslandsteppe.
A minimumof 236 grizzly bears were present in the
20,000-km2GreaterYellowstone Ecosystem in autumn
1992 (U.S. Fish and Wildl. Serv. 1993, Eberhardtet al.
1994). Black bears were commonly observed. Bears
used dens from mid-Novemberto mid-April.
METHODS
DocumentingCougarPredation
Responses of bears to ungulates killed by
radiocollaredcougars were documented during winter,
1992-95, in GNP and during spring-autumn,1990-95,
in YNP. Using methodsreportedby Shaw (1977), cougars were located 1-3 times daily, between 0600 and
2300 hours from the ground or from airplanes,without
disturbing them. All location sites were intensively
searchedby walking the contours of slopes at 1-20 m
intervals to locate carcasses after cougars left. Tracking hounds were used to assist searches of locations
and to trail cougars between location sites. Intact ungulate carcasses were necropsied to distinguish cougar
predationfrom that of other carnivoresby looking for
gross pathology or feeding behaviors described in
O'Gara (1978), Wade and Bowns (1982), and Alberta
Agriculture(1990). Cougars were monitored on consecutive days until at least 2, but typically 4 ungulates
were killed. We attemptedto document all ungulates
killed by cougars duringthis period, called a predation
sequence.
BearVisitsto CougarPrey
Bear visits to cougar-killedungulates were detected
by observing bears or by noting the presence of bear
scats, hair, or tracks near carcasses. Hair samples of
carnivoreswere identified to genus or species by personnel at the Wyoming Game and Fish Laboratoryin
Laramie,following Moore et al. (1974). We could not
always discernthe species of bearthatused cougarprey.
Displacementof cougarsfrom cougarkills was recorded
as a special case of bear visitation. To identify displacement,we comparedthe weight of the ungulatecarcass that remained(afteruse by the bear) to the amount
present,on average, aftercougarsleft kills withoutbear
displacement. When possible, we interpretedtracks of
cougars and bears visible in the snow and radiotelemetry data to discern bear displacement.
Biomass Eaten by Bears
We estimated the wet ungulate biomass eaten by
bears (BEB) feeding on cougar kills only in YNP.
Biomass was considered available to bears only if
we detected bears at a cougar kill. BEB was the
difference between the live-weight of the ungulate
and the amount consumed by the cougar, multiplied
by the estimated fractional consumption of carcasses
by bears. To approximate the energy content of
BEB, we converted BEB to its caloric equivalent by
assuming dry weight was 32.4% of wet weight
(Robbins et al. 1974, Robbins 1993) and multiplying dry weight by its digestible energy of 4.6 kcal/g
(Mealey 1975). BEB for each cougar kill was
summed to estimate total BEB and divided by the
BEARS AND COUGARPREDATION*
length of the predation sequence in days to estimate
daily BEB.
Weightsof UngulatePrey.-The weights of ungulates
killed by cougars were estimatedusing age-specific and
sex-specific growth models constructedfrom empirical
data on live and field-dressedweights (Greerand Howe
1964; D. Pac, unpubl.data). Field-dressedweights were
correctedfor loss of viscera using coefficients obtained
from Quimby and Johnson (1951) or calculated from
Andersonet al. (1974). Birthweights and dates of parturition were estimatedfrom Johnson(1951), Robinetteet
al. (1973), Pac et al. (1991), or elk calf mortalitystudies
(J. Mack, YNP, unpubl.data). Weight maxima,minima,
rates of weight loss in winter,and ratesof weight gain in
spring-summer were obtained from Johnson (1951),
Greer(1965), Robinetteet al. (1973), andAndersonet al.
(1974). Ages were estimatedfor deer andelk using tooth
replacementand wear patterns(Quimbyand Gaab 1957,
Robinetteet al. 1957).
CougarConsumption.-The ungulatebiomasscougars
consumed from their kills was estimatedas the product
of the daily cougar consumption(specific to cougar social class) and the days the cougar remainedat the carcass. Daily consumptionwas estimated by subtracting
the weight of uneaten portions of ungulates from estimated live weights and dividing by the numberof days
(to the nearest0.5 day) thatthe cougarfed. Daily cougar
consumption was independently determined from instances when scavengersdid not use cougarkills.
Portionof UngulatesEatenby Bears.-Bear consumption was estimatedat 50% of cougar-killedelk based on
3 visual estimatesmadeby Green(1994). Bearconsumption of deer was estimatedat 93%,based on a visual estimate made by Green (1994), 2 estimates in this study,
and 2 estimatescalculatedby subtractingestimatedcougar consumption and weights of carcass remains from
live weights of deer fed on by bears. Our estimates of
bear consumptionwere imprecise and probablyover-estimatedindividualbear use because (1) they were based
Murphyet al. 57
on a small numberof observations,(2) coyotes, avifauna,
decomposers,tissueautolysisanddecay,andwaterevaporationreduced carrionbiomass, particularlyif bears intermittentlyfed overextendedperiods(>3 days)on cougar
kills, and(3) >1 bearmay have fed fromcarcasses. Therefore, BEB values were crudeestimatesof individualbear
consumption.
BiomassthatCougarsLost
The biomass cougars lost (BCL) was estimatedwhen
bearsdisplacedcougarsfromcougarkills. The fractional
consumptionby cougarswas multipliedby estimatedlive
weight of the ungulateto predictthe biomass that would
have been consumed if cougars had not been displaced.
The estimatedbiomassthe cougarconsumedbeforeit was
displaced(daily consumptionratemultipliedby the numberdays the cougarwas presentat the kill) was subtracted
from this predicted cougar consumption to determine
BCL. BCL for kills was summedfor predationsequences
to estimatetotal BCL and divided by the length of a sequence to estimatedaily BCL. Total BCL for sequences
was less than total BEB because bears often scavenged
cougarkills that cougarshad alreadyabandoned.
RESULTS
Fifty-five ungulateswere killed by cougarsin GNP and
58 in YNP (Table 1). Fifteen (27%) of 55 ungulatekills
in GNP occurredin the Kintla Lakes area where bears
were often active outside their dens during the winter.
Of these 15 kills, 5 were visited by grizzlies (3 cases of
displacement)and 2 by an unknownbear species (1 displacement). Bears visited 42% (n = 26) of cougar kills
we monitoredin YNP during spring, 25% (n = 24) in
summer,and 25% in autumn(n = 8). Visitation during
spring was greaterthan summerand autumncombined,
butnot significantlyso (X2= 1.95, P = 0.09, 1-sidedtest).
The mean total BEB for predationsequences in YNP
was 45 kg (SD = 55, n = 11). The range was from
Table 1. Frequencies bears visited or displaced (disp.) cougars from cougar-killed ungulates, Glacier National Park, 199295, and Yellowstone National Park, 1990-95.
GlacierNationalPark
Yellowstone
NationalPark
(winter, n = 55)
No. visitsa(%)
Black bear
Grizzly bear
Unknown bear
0
6
2
a Visits include instances when bears
sequences were included.
(0)
(11)
(4)
(spring-autumn,n = 58)
No. disp.(%)
0
3
1
(0)
(5)
(2)
No. visitsa(%)
8
1
10
(14)
(2)
(17)
No. disp.(%)
4
1
2
(7)
(2)
(3)
displaced cougars from cougar kills. Data for visitation by bears that were not associated with predation
58
Ursus 10:1998
zero kg during a 55-day sequence to 157 kg in 23 days.
The mean daily BEB was 1.9 kg/day (SD = 2.5, n = 11,
range = 0-6.8).
The mean total BCL for predationsequences was 16
kg (SD = 31, n = 11). The rangewas from zero kg during
a 55-day sequence to 94 kg in 20 days. The mean daily
BCL was 0.64 kg/day (SD = 1.4, n = 11, range = 0-4.7).
Daily consumptionrates of cougars averaged8.5 kg/
day for subadults(self-sufficient cougars <24 months)
andnonmaternalfemales combined(n = 7 cougars), 16.7
kg/day for family groups(n = 5 groups),and 13.8 kg/day
for an adult male. Prey consumptionby cougars averaged 82% by subadultsand 46% by adults (n = 12 cougars). Cougar predationrates averaged 1 ungulate/8.7
days (SD = 4.1, n = 11 sequences; 11 cougars).
DISCUSSION
Bears in our study areasused cougar kills frequently.
Although reportsof bear-cougarinteractionsare nearly
absentfrom the literature,we found about 1 in 7 cougarkilled ungulateswere scavenged by bears in GNP and 1
in 3 in YNP. Bears displacedcougarsfrom about 1 in 14
kills in GNP and 1 in 8 kills in YNP. Consideringonly
the KintlaLakes areaof GNP, bears visited about 1 in 2
cougarkills and displacedthem from 1 in 4 kills during
wintermonths. Green(1994) documentedthatbearsvisited 3 of 14 cougar-killedungulates(1 displacementby a
grizzly bear) in northernYNP.
We assumethatthe frequencywith which bearsdetect
cougar-killedprey is highest where ungulates, cougars,
andbearsexhibit strongspatialoverlapand occur at high
densities. Because bears and cougarsuse low elevation
habitatsduringthe springin the mountainousportionsof
the northwestern United States and western Canada
(Seidenstickeret al. 1973, Murphy1983, Servheen1983,
Logan and Irwin 1985, Mack 1988, Raine and Kansas
1990, this study), encountercompetitionbetween bears
and cougars may increase during spring in this region.
Our YNP data weakly supported(P < 0.09) that bears
visited cougarkills morefrequentlyin springthanduring
summerand autumn.
The absenceof elaboratecaching (e.g., in the trees)by
cougars supportsthe hypothesis that frequentresponses
by ursidsand canidsto cougar-killedprey arenot universal. Although large felids cache more extensively than
small ones, food cachingis not as commonor stereotyped
in felids as in canids(KleimanandEisenberg1973). Frequentdisplacementby scavengerscould providenatural
selection for more elaboratecaching of kills by cougars.
Alternatively,it may suggest that cougars can success-
fully defend at least some of theirkills from challengers
or cope with encountercompetitorsby feeding on their
prey between periodsof use by dominantspecies.
Infrequentbut windfallfeeding opportunitiesfor bears
on cougar-killedungulates were suggested by the high
standarddeviation(SD = 55) of the total BEB for predation sequences relative to its mean (45 kg). BEB was
zero kg for 27 of 38 cougar-killedungulatesfound during predation sequences. However, the ungulate
biomass availableto bearswas potentiallysignificantenergetically to them. The mean daily BEB (1.9 kg/day)
convertedto about2,830 kcalday. This valuerepresented
71% of the 4,000-kcal/day energy budget of a
nonhiberating103-kg captive black bear (David 1987),
and equaled 113%of the 2,500-kcal/daybudget of a hiberating, 204-kg captive grizzly bear (WattsandJonkel
1988).
The daily BEB was highest when cougarskilled large
(>80 kg) ungulates and bears visited a high numberof
cougar kills duringa sequence. The highest daily BEB
(6.8 kg/day) occurredwhen a female cougar accompanied by 2 large (42 and55 kg) kittenskilled 4 elk (2 large
calves and 2 cows) in 23 days. Bears visited 3 of these
kills and displacedthe cougarsonce. We estimatedthat
bears obtained 157 kg of carrionduringthis sequence.
BEB values were crude estimates because we lacked
adequate data concerning consumption of cougarkilled prey by bears. More than 1 bear may have fed
on individual cougar kills. We also did not evaluate
the direct and indirect energy costs and benefits, or
the risks of injury, associated with obtaining this food
relative to alternative foraging strategies available to
bears. Therefore, we can only hypothesize that cougar-killed prey provided significant energy gains to
bears.
Interactions between cougars and other carnivores
at cougar kills may directly influence the behavior and
survival of dominant competitors. Bears may develop
behaviors that, over long periods (>2 weeks) increase
their likelihood of detecting cougar kills. For example,
the habitat selections and activity schedules of bears
may be altered if they predictably and more efficiently
obtain food through encounter competition than
throughalternativeforaging strategies. Bears may also
temporarily change their behavior in direct response
to carrion they have discovered. Bear survival may
be reduced if the prospective food rewards provide
sufficient incentive for them to take risks. We found
5 coyotes that were killed by cougars near cougarkilled ungulates. Boyd and O'Gara(1985) and Koehler
and Hornocker (1991) made similar observations.
BEARS ANDCOUGARPREDATION*
Mean daily BCL (0.64 kg/day) was surprisinglyhigh,
about 26% of the 2.2-2.7 kg/day requiredfor a single
adult female cougar, or 17% of the 3.4-4.3 kg/day requiredfor a single adult male cougar as estimatedfrom
an energetics model (Ackermanet al. 1986). The high
standarddeviation (SD = 31) of total BCL relative to its
mean (16 kg) suggested that losses by cougars to bears
were sporadic.
The cougar's solitarynatureand predatoryspecialization on large vertebrateprey may predispose it to the
adverse effects of displacement. Cougars may lose a
considerable energy investment when displaced by an
aggressive scavengeror may incurinjurieswhen defending their kills. Cougar death due to interspecificfighting with scavengersat cougarkills has been documented
(White and Boyd 1989). The alternativeof killing additional prey to compensate for losses to scavengers,
however, also carriesenergetic costs and risk of injury.
Ross et al. (1995) noted that3 (27%)of 11 cougardeaths
in Alberta from naturalcauses were related to acquisition of prey. In response to scavenger pressures,cougars may restrict themselves to habitats and behavior
patternsin which interferenceis unprofitableto dominant species (Case and Gilpin 1974).
Differences in morphologyand behavioramong large
North American carnivorescontributeto asymmetryin
the effects of encounter competition. Morphological
characteristics(e.g., claws, long canines) or social hunting patterns allow some carnivores to take disproportionately large prey (Bekoff et al. 1984). Large felids
such as cougarssingly and consistentlykill prey as large
or largerthanthemselves(Kleimanand Eisenberg1973).
Unlike many large canids, solitary felids are not normally communalfeeders (Kleimanand Eisenberg 1973)
and usually do not consume kills as quickly as canids.
The small mass of cougars relative to North American
bears and their small group size relative to wolves and
coyotes may often preclude cougars from successfully
defendingtheir kills. These typical attributesof Felidae
(Kleimanand Eisenberg 1973, Eisenberg 1986) contribute to agonisticinteractionsbetween cougars,ursids,and
canids at cougar kills where these species are sympatric.
Asymmetryin encountercompetitionis also enhanced
by the cougar's poor olfaction and mobility relative to
othercarnivores. Cougarsmay not discovercarrionfrom
non-cougarsources as efficiently as canids, ursids, and
mustelids. Felids use their olfactory senses less than
canids for foraging (Kleiman and Eisenberg 1973).
Canids possess cursorialadaptations(Hildebrand1954)
that enhance travel speed and coverage of territories,
Murphyet al. 59
giving them a competitive edge over felids in detecting
carrion.
Although it appearsthat encounter competition confers effects that are negative on cougars and positive on
bears, some proximateand ultimatebenefits to cougars
may result from encounterand exploitationcompetition
between cougars and their co-evolved competitors. For
example, althoughspacing in cougar territoriesis probably driven indirectly by interspecific competition for
the resourcesthat space provides, displacementof cougars from their kills by other carnivores could further
enhance spacing and decrease the likelihood that individual cougars might deplete their local food supplies.
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