Chapter 5: Short-faced Bear 2

Paul E. Matheus, Department of Biology and Wildlife, University of Alaska at Fairbanks, Alaska. “Diet and Co-ecology of Pleistocene Short-Faced Bears and Brown Bears in Eastern Beringia” in: Quaternary Research 44, 4476-453 (1995)

“Collagen was extracted for isotopic analysis from the well-preserved fossil bones of 16 short-faced bears and 14 brown bears from central Alaska (USA) and west-central Yukon Territory (Canada) In addition, 13 bears from two modern populations of Alaskan coastal brown bears were analyzed, six from Admirality Island and 7 from the Alaska Peninsula. Isotope data from the modern bears were used to help interpret fossil data.

Radiocarbon dates are available for two of the short-faced bears (NMC 7438; 26,04 ± 270 yr B.P, TO-2696; NMC 37577: 29,600 ± 1200 yr B.P., I-11037) Harington, 1977; C. R. Harington, personal communication, 1995. The rest are from sediments assigned a Rancholabrean age (probably mostly Wisconsinan). These sediments are mostly alluvium of somewhat uncertain age, and so possibly some of the samples, particularly of Arctodos, are older.” (1995:447, 448)

“δ¹⁵N levels indicate that Beringian short-faced bears were highly carnivorous, functioning primarily as secondary terrestrial consumers and residing trophically above the large majority of brown bears.

The isotopic data on Beringian brown bears suggests that individual diets were variable, but with a distinct clinal trend. One primary component of the diet was terrestrial vegetation, but in addition, there also is a marine influence to the diet of some of these brown bears, which I interpret as indicating salmon feeding. The conclusion that vegetation and salmon were the primary components of these brown bears’ diets is substantiated by the data from two coastal populations of modern Alaskan brown bears, which have a strikingly similar trend. Coastal bears are known to eat varying amounts of salmon and vegetation, with individual habits varying between individuals, often on the basis of age and sex.

Both Beringian bear species display a prominent geographic component in their isotopic ratios. In terms of brown bears, individuals from the Yukon are noticeably lighter in ¹³C than Alaskan brown bears. This could mean that Alaskan brown bears were foraging on occasional salmon, whereas the Yukon bears ate no salmon. The three short-faced bears from the Yukon had the highest δ¹⁵N levels of all short-faced bears analyzed, suggesting that they resided at even higher trophic levels (NMC 7438), which has the highest δ¹⁵N value, is a huge bear and the type specimen of A. simus yukonensis).

Considering the overall trend in the brown bear isotopic data, it seems more likely that the brown bears in the region of overlap were eating mostly salmon and vegetation and only occasionally terrestrial meat.” Paul E. Matheus Quaternary Research 44 (1995:449)

Arctodus as a Scavenging Specialist

“Stable isotopes reveal that Arctodus was a strict carnivore in eastern Beringia, but the isotope data do not reveal how short-faced bears forged, what specific food items they ate, or how they fit into Pleistocene mammal communities and the large carnivore guild. … I propose the following testable model:

Short-faced bears were primarily scavengers of widely dispersed large mammal carcasses and were simultaneously designed both for highly efficient locomotion and for intimidating other large carnivores. This allowed Arctodus to forage economically over a large home range and to seek out, procure, and defend carcasses from other large carnivores.

The model assumes that Arctodus’ morphology was driven by foraging style, and that Arctodus was foraging on meat. Under these assumptions, Arctodus must have been either an active predator, a scavenger of flesh, or a combination of both. … I suggest that Arctodus’ limbs were designed for a super-efficient gait, perhaps a pacing gait, but not for high speed. Furthermore, in terms of Kurtén’s model, it would seem less advantageous to have a slender build if Arctodus was trying to kill very large prey.” Paul E. Matheus Quaternary Research 44  (1995:451)

“Speth (1989) and Martin and Martin (1993) have pointed out that a carnivore can not obtain all of its calories from protein (lean meat) for a sustained period, primarily because the animal will eventually exceed the rate at which its liver can metabolize amino acids. Carnivores must obtain some calories from lipids (or carbohydrates), and lipids provide critical ingredients for metabolism, particularly fatty acids. During seasons when the flesh of prey animals is especially lean, carnivores must find supplemental sources of lipids or suffer severe deteriorations of their own body condition (Martin and Martin, 1993). Carnivores with the ability to open long bones and extract greasy marrow have a distinct advantage, especially if seasonal bottlenecks are severe or long, which was probably the case in ecosystems of the northern hemisphere during glacial phases of the late Pleistocene.

If short-faced bears were large, aggressive scavengers capable of stealing carcasses from other large carnivores, then it seems unlikely that brown bears could dominate them in direct interference competition. And while brown bears may have preferred to feed on animal carcasses, it seems more likely that they would have avoided direct confrontation with a dominant bear.

The ecological plasticity of brown bears and their ability to hibernate may have been the keys to their ultimate survival at the end of the Pleistocene, while Arctodus, the highly specialized forager, was not able to find a niche in Holocene ecosystems. Most likely, carcass densities on Holocene landscapes fell below levels necessary to sustain minimal viable populations of short-faced bears. Since many bears hibernate to survive poor food availability during winter, this may be an indirect indication that short-faced bears, and perhaps all New World bears, never evolved this strategy to survive seasonal dietary bottlenecks.” Paul E. Matheus Quaternary Research 44  (1995:452)