GRIZZLY BEAR PREDATION ON MUSKOX

Ovibos moschatus is a formidable beast that is sometimes preyed upon by grizzly bears.Photo Credit:US Fish and Wildlife Service.

The muskox (Ovibos moschatus) is a large ungulate (the average male weight is from 273 to 364 kg [600 to 800 pounds]) equipped with curved horns and a shaggy pelage which can be up to 10 cm (4 inches) thick. It is a close relative of the sheep or goats (subfamily Caprinae) and is able to withstand incredibly frigid, arctic conditions (down to at least – 70 ºF). Muskox tend to live in herds and are famous for their defensive posturing – they often form a defensive circle with their heads (i.e., armament) facing outward toward the potential threat. Youngsters often hide amongst the adults for protection.

The barren-ground grizzly bears and muskox overlap in their distribution in northern Canada and Alaska. This shaggy beast would appear to be fairly impervious to grizzly attack. But, this is not the case. Grizzlies were originally reported feeding on muskox by early explorers and with recent reintroduction of these ungulates in parts of the Arctic, there have been a number of papers written on the predator-prey relationship of U. arctos and O. moschatus. Below I have reviewed what is known about the barren-ground grizzly predation on muskox.

Multiple Hunting Strategy

In the Thelon Game Sanctuary, grizzlies and muskox coexist, but the relationship is not always copasetic. Near the Thelon River, bears may use thick willow stands along the waterway to ambush muskox feeding on sedge in nearby clearings. Willows also attract muskox, as it is a preferred food of this beast. Gunn and Miller (1982) report finding a bear on a freshly killed, bull O. moschatus. They were able to scare the adult bear off and examine its kill and concluded that the bear had dispatched the big ungulate by first grasping its nose (crushing the nasal turbine bones and tearing off the nose in the process) and then inflicting a crippling bite to its skull. By grasping the nose, the bear may have prevented the muskox from bringing its horns to bear and also may have been more effective at throwing the animal to the ground.

In another study carried out in the northeastern Arctic slopes of Alaska, 92 grizzly-muskox interactions were observed (Reynolds et al. 2002). Fifty percent of these were known kills, 40 % were possible kills or scavenging events, and 10 % were incidents where a grizzly was seen chasing muskox. It was estimated that 16-39 % of muskox mortality was the result of bear predation. During the study period (1982-2001) the number of muskox killed by grizzly bears was zero to two deaths per year before 1993, one to four musk ox per year from 1994-1997 and five to ten deaths per year from 1998-2001. This increase in kill numbers was a function of an increase in the size of musk ox herds. An increase in kills may also be indicative of the bears learning how to better attack and take down these big, formidable animals. While solitary adult bears were most often seen attacking muskox (69 occasions), pairs or trios of adult bears were seen chasing, killing or eating these animals (three episodes). Sows with cubs or yearlings were seen interacting with muskox on three occasions.

Surplus Killing

Grizzly bears sometimes engage in surplus killing of muskox. In the study carried out by Reynolds et al. (2002) there were ten episodes where one to three bears killed from two to four adult muskox. On several occasions even more muskox were dispatched during a single hunting bout. For example, in one case five individuals (two adult females, a yearling and unsexed adult musk ox) were incapacitated by a single bear. In another case, a grizzly killed four calves and in another incident the victims were one adult female, one two-year old male and one yearling. In most cases, solitary bears were involved in these killing sprees, but in one case three grizzlies instigated the melee.

Clarkson et al. (1993) reported a fascinating case of surplus killing of muskox calves by a heterosexual pair of adult grizzlies. Within a distance of about two km, the two bears took down five young musk ox. By doing a little forensic work, the researchers were able to put together a likely picture of what had happened. Rather than form a defensive circle to try and parry the bear attacks, this herd of musk ox tried to out run the grizzlies. The researchers postulated that the calves trailed behind the adults and, therefore, were more vulnerable. The two bears chased the herd, which consisted of 40 to 50 muskox (with a minimum of eight calves). They killed the first calf and ate 90 % of the carcass. They then chased the herd down again and about 1.5-2.0 km from the first kill dispatched a second young musk ox. They ate 60 % of this second calve and began the hunt again. They killed the third calf about 300 m from the second. The third calf was about 30 % consumed by the bears and a wolverine (Gulo gulo) that was feeding on the carcass when the researchers arrived on the scene. The fourth calve was killed 400 m from the third. A golden eagle had just begun to feed on calf four when the researchers arrived. The final calf was killed about 200 m from the fourth – this last young muskox was not eaten either.

References:

Clarkson, P. L. and I. Sarma Liepins. 1993. Grizzly bear, Ursus arctos, predation on muskox, Ovibos moschatus, calves near the Horton River, Northwest Territories. Canadian Field Nat. 107:100-102.

Gunn, A. and F. L. Miller. 1982. Muskox bull killed by a barren-ground grizzly bear, Thelon Game Sanctuary, N.W.T. Arctic 35:545-546.

Reynolds, P. E., H. V. Reynolds and R. T. Shideler. 2002. Predation and multiple kills of muskoxen by grizzly bears. Ursus 13:79-84.

Copyright (2008) Scott W. Michael