COMMUNICATIONS
375
SHORT
GRIZZLYBEAR PREDATIONON A BULLBISON INYELLOWSTONE
NATIONAL
PARK
TRAVISWYMAN,Bear ManagementOffice,P.O.Box 168,
YellowstoneNationalPark,WY82190, USA, email:
@nps.gov
travis_wyman
Ursus 13:375-377 (2002)
Key words: bison, Bison bison, grizzly bear, predationUrsus arctos,
Yellowstone NationalPark
Grizzly bears (Ursus arctos horribilis) will eat meat
wheneverit is availableto them (Mattson 1997). Ungulate meat is more prominentin the diet of grizzly bears
fromthe Yellowstoneareathanotherinteriorbrownbear
populationsin North America (Craigheadand Mitchell
1982, Mattsonet al. 1991, Mattson1997). Nitrogen(N,5)
isotope analysisof haircollected fromgrizzlybearsin the
GreaterYellowstone Area suggests that meat comprises
almost half of the annualenergy intake of adult females
and over half for adultmales (Jacobyet al. 1999). Grizzly bears obtain ungulate meat primarilythroughscavenging winter-killed(Greenet al. 1997) and wolf-killed
(Canislupus;D. Smith,YellowstoneNationalPark[YNP],
Wyoming, USA, personal communication, 2001) elk
(Cervuselaphus) and bison (Bison bison) carcasses, and
by predationon elk calves (Guntherand Renkin 1990).
Grizzlybearpredationon adultelk andmoose (Alcesalces)
has also been reported(Schleyer 1983). However,reports
of predationon adultbison are rare.
In an extensive review of the historicalliteratureon bison, Roe (1951, 1970) found only one reference documentinggrizzly bearpredationon bison. Review of more
recent bison literatureadds no additionalevidence that
grizzly bears prey on adult bison (Meagher 1973, 1978;
Reynolds et al. 1982; Shaw and Meagher 1998). Studies
of grizzly bearbehaviorand food habitsin YNP have not
documentedgrizzlybearpredationon adultbison (Mealey
1975, Schleyer 1983, GuntherandRenkin 1990, Mattson
et al. 1991, Schullery 1992, Mattson 1997, Knight et al.
1999), except for the occasional killing of severely malnourishedbison in spring(Craigheadet al. 1995).
I observed a predatoryattackby an adult female grizzly bearon a young adultbull bison nearthe Yellowstone
River outlet in east-central YNP, Wyoming. The
Yellowstone River outlet is surroundedby spruce-fir
(Picea spp.-Abies spp.) and lodgepole pine (Pinus
contorta) forest types (Despain 1990). The area is used
extensivelyby grizzlybears(NationalParkService 1984).
Bison, mostly bulls, are active in the areafrom late April
throughDecember,with smallgroupsof bison sometimes
over-winteringin the area.
I observedthe incident from the road on the west side
of the Yellowstone River near the outlet of Yellowstone
Lake (elevation 2,371 m) on the morningof 23 September 2000. I was observinga grizzly bearwith 2 cubs-ofthe-yearthatwere diggingfor pocketgophers(Thomomys
talpoides)in a meadow300-400 m fromthe LakeLodge.
This family grouphad been frequentingthe areathroughout the summer. At approximately1200 hours,the bears
began walking northeastalong the shore of Yellowstone
Lake toward the Yellowstone River outlet at Fishing
Bridge. At approximately1300, the bearsemergedonto
the road at Fishing Bridge Junction and crossed to the
northside. The bearscontinuedwalking northin a direction that would have taken them past a young adultbull
bison lying under a tree 15 meters from the road. The
bison stood up abruptlywhen the bears were approximately5 metersaway. Whenthe bison stoodup, the bears
appearedstartled. The adultfemale, then the cubs, stood
up on theirhind legs and looked at the bison. The bison
stoodin an alertposturewithhis tailraisedandheaddown.
After a few seconds, the adultbearlunged towardthe bison. The bison immediatelyturnedaway and began trotting east, up slope along a bench directly above the road
headingtowardFishing Bridge. The adultbearloped after the bison at less thanfull speed.
I droveeast along the road,observingthe movementof
the bearsandbison approximately15 metersaway. After
trottingabout50 m, the bison broke into a full run. The
adultbearthen chasedthe bison at full speed. At the crest
of the hill above the YellowstoneRiver, the bear swiped
its paw acrossthe hindquartersof the bison, knockingthe
bison's back legs out from underit. The bison began to
slide down the steep embankmentof the hill on its back.
After strikinga tree with considerableforce on its front
quarters,the invertedbison continued to slide toward a
pedestrianboardwalkat the base of the hill. The grizzly
leaped onto the stomach of the invertedbison and skidded down the hill on top of it while attemptingto bite at
the bison's neck. The bearandbison came to a stop at the
base of the hill on the pedestrianboardwalk. The bear
continuedto bite and pull at the bison's neck while the
bison triedto get to its feet. The bison managedto stand
and struggledto remainstanding,but the bearcontinued
to pull the bison back down to the ground. When the
bisondid stand,its hindlegs buckledunderits own weight.
The beartook advantageof this andjumpedonto the back
of the bison, biting and clawing at its back, inflicting a
numberof bite andclaw woundsaroundthe bison's hump
andlower back. Witha quickheadmotion, the bull managed to free itself from the bear and stand up a second
376
Ursus 13:2002
time. At this time, I observed that the bison's left front
leg was broken. This injurymay have occurredwhen the
bison slammedinto the tree while sliding down the steep
hill. The bison continuedattemptsto standandfoughtoff
the bearwith its head andhornsfor severalminutes. The
bear stood up on its hind legs and swiped at the bull's
headwithits paws. Thebisonreactedby rearingup,which
caused it to slide backwardinto a ditch adjacentto the
Fishing Bridge boardwalk. Being in the ditch appeared
to put the bison in a betterposition to fend off the bear
with its head and horns.
At this time the 2 cubs, which hadbeen observingtheir
motherfrom on top of the hill, came down and reunited
with her nearthe bison. The bison continuedto struggle
to keepup-rightandbledprofuselyfromits backandhindquarters.The adultbear attackedthe bison several more
times, but the bison was able to use its head and hornsto
repel the attacks. The cubs did not participatein these
attacksbut remainednearby. On 5 occasions the bears
left the areaand were no longer visible to me, then came
back andthe adultattackedthe bull again,but was unable
to kill it. The interval between attacks increased from
approximately5 minutesto severalhoursbetween return
visits.
At approximately1800 the bears left and did not return, enabling me to investigate the bison in the ditch.
The bison was startledupon my approachand attempted
to climb out of the ditch. It fell down and was unableto
pull itself out of the mud. Due to the proximity of the
bison to the mainroadandconcernsfor the safety of visitors and a constructioncrew workingon the roadbridge
adjacentto the attacksite, parkmanagementdecided to
dispatchthe bison and move the carcass. After shooting
the bison, the carcasswas moved 0.9 km away to a location remotefrom public use areas. Managershoped that
thebearfamilygroupwouldfollow the scenttrailto where
the carcasswas disposed and scavenge the remains.
The nextmorning(24 September),an adultfemale grizzly with 2 cubs returnedto the areawhere the attackoccurred.The 3 bearswere identicalin size andcolor to the
bears that had encounteredthe bison the previous day,
andI believe they were the same family group. The adult
female grizzly paced, circled, and sniffed the groundas
she searchedthe site whereshe had attackedthe bison the
day before. Several visitors saw the bearsfrom the main
road and approachedthem in an attemptto get pictures.
As they approached,the adult bear bluff chargedthem
andchasedthembacktowardthe road. Due to the danger
thatan adultfemale grizzly accompaniedby 2 cubs posed
to parkvisitorsandbridgeconstructionworkers,the bears
were hazed out of the area with crackershells. I monitoredthe areawhere the bison carcasswas moved to, but
never observedthe female with cubs or theirtracksin the
snow at the new location. Threedays later,I observeda
large adultmale grizzly scavengingon the carcass. I observedthatbearat the carcassfor 5 consecutive days and
then did not see it again. In that time, the grizzly consumed most of the bison. After the grizzly stopped returning to the carcass, a large male black bear (Ursus
americanus)began frequentingthe carcassand scavenging the remains. The blackbearreturnedto the carcassto
scavenge for several days until the carcass was entirely
consumed. I did not see the female and her 2 cubs in the
areaagain for the remainderof the season.
Tootheruptionand wear from the bison's mandibleindicated it was 31/2years old (M. Meagher,YNP, Wyoming,USA, personalcommunication,2001). Femurbone
marrowwas grayish-pinkin color, indicatingthatthe bull
may have been in the early stages of marrowfat depletion, althoughnot yet severely malnourished(Cheatum
1949). Visually, the bison had looked slightly thin, but
therewas no otherevidence of poorhealthor injuryprior
to the attackor when I observedit fleeing from the bear.
Based on the paucity of documentationin the literature, grizzly bear predationon adult bison is likely very
rare. However,the incidentI observedindicatesthatgrizzly bears can opportunisticallypursueand kill adultbison underconditionswherethe bearhas an advantage.In
this case, the bison was alone and lacked the groupprotection affordedby a herd; it was also young and likely
lackingin experience. The bison attemptedto flee rather
than stand its ground. Large ungulates such as bison
(Smithet al. 2000), moose (Mech 1966),andelk (D. Smith,
YNP,personalcommunication,2001), areoften moresuccessful atdeterringpredatoryattacksby confrontingpredatorsratherthanfleeing. Althoughhumansafetyconcerns
requiredeuthanizingthe bison and moving its carcass,
the extent of the injuries already inflicted on the bison
lead me to concludethatgiven moretime andless human
interference,the grizzly would undoubtedlyhave killed
the bull.
ACKNOWLEDGMENTS
I appreciatethe assistanceof K. Gunther,who encouraged me to publishthis accountandprovidedinsight,scientific knowledgeandeditorialreview of the manuscript.
C. Schwartz,M. Haroldson,M, Meagher,and R. Harris
also providedhelpfuleditorialcommentsthatmadethis a
bettermanuscript.
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GRIZZLYBEAR PREDATIONON A BISON CALFINYELLOWSTONE
NATIONAL
PARK
NATHAN
VARLEY,
SymbiosisConsulting,P.O.Box490, Gardiner,
MT59030, USA, email: [email protected]
KERRYA. GUNTHER,Bear ManagementOffice,P.O.Box 168,
YellowstoneNationalPark,WY82190, USA,email:
kerry_gunther@
nps.gov
Ursus 13:377-381 (2002)
Key words: bison, Bison bison, grizzly bear, predation,Ursus arctos
horribilis
Ungulate meat is one of the most concentrated sources
of digestible energy and protein available to grizzly bears
(Ursus arctos horribilis) in the Yellowstone ecosystem
(Mealey 1975, PritchardandRobbins 1990, Craigheadet
al. 1995). Grizzly bears obtain ungulatemeat primarily
throughscavengingwinter-killedelk (Cervuselaphus)and
bison(Bisonbison)carrion(Craigheadet al. 1995,Mattson
1997), usurping wolves (Canis lupus) from their ungulate kills (D. Smith, Yellowstone National Park, Wyoming, USA, unpublished data), and by predation on elk