SHORTCOMMUNICATIONS 375 GRIZZLYBEAR PREDATIONON A BULLBISON INYELLOWSTONE NATIONAL PARK Bear P.O.Box TRAVISWYMAN, ManagementOffice, 168, I observed the incident from the road on the west side YellowstoneNational Park, WY 82190, USA, email: travis_wyman@ nps.gov of the Yellowstone River near the outlet of Yellowstone Lake (elevation 2,371 m) on the morningof 23 Septem- Ursus 13:375-377 (2002) ber 2000. I was observing a grizzly bear with 2 cubs-of- the-yearthat were digging for pocket gophers(Thomomys Key words: bison, Bison bison, grizzly bear, predationUrsus arctos, talpoides) in a meadow 300-400 m from the Lake Lodge. Yellowstone National Park This family grouphad been frequentingthe areathrough- out the summer. At approximately1200 hours, the bears began walking northeastalong the shore of Yellowstone Grizzly bears (Ursus arctos horribilis) will eat meat Lake toward the Yellowstone River outlet at Fishing whenever it is available to them (Mattson 1997). Ungu- Bridge. At approximately1300, the bears emerged onto late meat is more prominentin the diet of grizzly bears the road at Fishing Bridge Junction and crossed to the from the Yellowstone areathan other interior brown bear northside. The bearscontinued walking northin a direc- populations in North America (Craigheadand Mitchell tion that would have taken them past a young adult bull 1982, Mattsonet al. 1991, Mattson1997). Nitrogen(N,5) bison lying under a tree 15 meters from the road. The isotope analysisof haircollected from grizzly bearsin the bison stood up abruptlywhen the bears were approxi- GreaterYellowstone Area suggests that meat comprises mately5 metersaway. When the bison stood up, the bears almost half of the annualenergy intake of adult females appearedstartled. The adultfemale, then the cubs, stood and over half for adult males (Jacoby et al. 1999). Griz- up on their hind legs and looked at the bison. The bison zly bears obtain ungulate meat primarilythrough scav- stoodin an alertposture with his tail raisedand head down. enging winter-killed(Green et al. 1997) and wolf-killed After a few seconds, the adultbear lunged towardthe bi- (Canislupus; D. Smith,Yellowstone National Park [YNP], son. The bison immediatelyturned away and began trot- Wyoming, USA, personal communication, 2001) elk ting east, up slope along a bench directly above the road (Cervus elaphus) and bison (Bison bison) carcasses, and headingtoward Fishing Bridge. The adultbear loped af- by predationon elk calves (Guntherand Renkin 1990). ter the bison at less than full speed. Grizzlybear predation on adultelk andmoose (Alcesalces) I drove east along the road,observing the movementof has also been reported(Schleyer 1983). However,reports the bearsand bison approximately15 metersaway. After of predationon adultbison are rare. trottingabout 50 m, the bison broke into a full run. The In an extensive review of the historicalliterature on bi- adultbear then chasedthe bison at full speed. At the crest son, Roe (1951, 1970) found only one reference docu- of the hill above the Yellowstone River, the bear swiped mentinggrizzly bearpredation on bison. Review of more its paw acrossthe hindquartersof the bison, knockingthe recent bison literatureadds no additionalevidence that bison's back legs out from underit. The bison began to grizzly bears prey on adult bison (Meagher 1973, 1978; slide down the steep embankmentof the hill on its back. Reynolds et al. 1982; Shaw and Meagher 1998). Studies After strikinga tree with considerableforce on its front of grizzly bear behaviorand food habitsin YNP have not quarters,the inverted bison continued to slide toward a documentedgrizzly bear predation on adultbison (Mealey pedestrianboardwalk at the base of the hill. The grizzly 1975, Schleyer 1983, Guntherand Renkin 1990, Mattson leaped onto the stomach of the inverted bison and skid- et al. 1991, Schullery 1992, Mattson 1997, Knight et al. ded down the hill on top of it while attemptingto bite at 1999), except for the occasional killing of severely mal- the bison's neck. The bearand bison came to a stop at the nourishedbison in spring (Craigheadet al. 1995). base of the hill on the pedestrianboardwalk. The bear I observed a predatoryattack by an adult female griz- continued to bite and pull at the bison's neck while the zly bearon a young adultbull bison nearthe Yellowstone bison tried to get to its feet. The bison managedto stand River outlet in east-central YNP, Wyoming. The and struggledto remain standing,but the bear continued Yellowstone River outlet is surroundedby spruce-fir to pull the bison back down to the ground. When the (Picea spp.-Abies spp.) and lodgepole pine (Pinus bison did stand,its hindlegs buckledunder its own weight. contorta) forest types (Despain 1990). The area is used The beartook advantageof this andjumped onto the back extensivelyby grizzly bears(National Park Service 1984). of the bison, biting and clawing at its back, inflicting a Bison, mostly bulls, are active in the areafrom late April numberof bite andclaw woundsaround the bison's hump throughDecember, with small groupsof bison sometimes and lower back. Witha quick head motion, the bull man- over-winteringin the area. aged 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 snow at the new location. Three days later,I observed a leg was broken. This injurymay have occurredwhen the large adultmale grizzly scavenging on the carcass. I ob- bison slammedinto the tree while sliding down the steep served thatbear at the carcassfor 5 consecutive days and hill. The bison continuedattempts to standand fought off then did not see it again. In that time, the grizzly con- the bearwith its head and hornsfor several minutes. The sumed most of the bison. After the grizzly stopped re- bear stood up on its hind legs and swiped at the bull's turning to the carcass, a large male black bear (Ursus headwith its paws. The bison reactedby rearingup, which americanus)began frequentingthe carcassand scaveng- caused it to slide backwardinto a ditch adjacentto the ing the remains. The black bearreturned to the carcassto Fishing Bridge boardwalk. Being in the ditch appeared scavenge for several days until the carcass was entirely to put the bison in a better position to fend off the bear consumed. I did not see the female and her 2 cubs in the with its head and horns. area again for the remainderof the season. At this time the 2 cubs, which had been observingtheir Tootheruption and wear from the bison's mandiblein- motherfrom on top of the hill, came down and reunited dicated it was 31/2years old (M. Meagher, YNP, Wyo- with her near the bison. The bison continuedto struggle ming, USA, personalcommunication, 2001). Femurbone to keep up-rightand bled profuselyfrom its backand hind- marrowwas grayish-pinkin color, indicatingthat the bull quarters. The adultbear attackedthe bison several more may have been in the early stages of marrowfat deple- times, but the bison was able to use its head and horns to tion, although not yet severely malnourished(Cheatum repel the attacks. The cubs did not participatein these 1949). Visually, the bison had looked slightly thin, but attacksbut remainednearby. On 5 occasions the bears therewas no otherevidence of poor healthor injuryprior left the area and were no longer visible to me, then came to the attackor when I observed it fleeing from the bear. back andthe adultattacked the bull again,but was unable Based on the paucity of documentationin the litera- to kill it. The interval between attacks increased from ture, grizzly bear predationon adult bison is likely very approximately5 minutesto several hoursbetween return rare. However,the incidentI observedindicates that griz- visits. zly bears can opportunisticallypursue and kill adultbi- At approximately1800 the bears left and did not re- son underconditions where the bearhas an advantage. In turn, enabling me to investigate the bison in the ditch. this case, the bison was alone and lacked the group pro- The bison was startledupon my approachand attempted tection affordedby a herd; it was also young and likely to climb out of the ditch. It fell down and was unable to lacking in experience. The bison attemptedto flee rather pull itself out of the mud. Due to the proximity of the than stand its ground. Large ungulates such as bison bison to the main roadand concernsfor the safety of visi- (Smithet al. 2000), moose (Mech 1966),and elk (D. Smith, tors and a constructioncrew working on the road bridge YNP,personal communication, 2001), areoften moresuc- adjacentto the attack site, park managementdecided to cessful at deterringpredatory attacks by confrontingpreda- dispatchthe bison and move the carcass. After shooting tors ratherthan fleeing. Althoughhuman safety concerns the bison, the carcass was moved 0.9 km away to a loca- requiredeuthanizing the bison and moving its carcass, tion remote from public use areas. Managershoped that the extent of the injuries already inflicted on the bison the bearfamily groupwould follow the scenttrail to where lead me to conclude thatgiven more time and less human the carcass was disposed and scavenge the remains. interference,the grizzly would undoubtedlyhave killed The next morning(24 September),an adultfemale griz- the bull. zly with 2 cubs returnedto the area where the attackoc- curred. The 3 bearswere identicalin size andcolor to the bears that had encounteredthe bison the previous day, ACKNOWLEDGMENTS I the assistanceof K. who encour- and I believe they were the same family group. The adult appreciate Gunther, me to this accountand sci- female grizzly paced, circled, and sniffed the groundas aged publish providedinsight, she searchedthe site where she had attackedthe bison the entific knowledge andeditorial review of the manuscript. C. M. and R. Harris day before. Several visitors saw the bears from the main Schwartz, Haroldson,M, Meagher, also editorialcomments that made this a road and approachedthem in an attemptto get pictures.