Paleoecology and ecomorphology of the giant short-faced bear in Eastern Beringia

Abstract

The short-faced bear (Arctodus simus) was a widespread Tremarctine bear indigenous to North America until its extinction around 11,500 BP. Arctodus inhabited Pleistocene ice-free refugia in Eastern Beringia (the northwestern limit of its range) until at least 20,000 BP. Because of its gracile, long-legged build and extremely large size, most paleontologists believe this bear was a high-speed pursuit predator which had preyed on the largest herbivores of Pleistocene North America. Alternatively, energetic arguments have been used to suggest that Arctodus was too large to be carnivorous and evolved its large size within an herbivorous or omnivorous niche. To test these competing hypotheses, I reconstructed aspects of Arctodus' trophic position and paleodiet by analyzing stable isotope ratios ($\delta\ \sp{13}$C and $\delta\ \sp{15}$N) in bone collagen extracted from east Beringian fossils. Other bears and carnivores from Beringia were analyzed to help interpret the results. Isotopes reveal that Arctodus was highly carnivorous, it fed on herbivores which consumed C3 vegetation, and it did not eat salmon. The herbivore/omnivore hypothesis is thus rejected. Predatory hypotheses predict that we should find certain morphological features in a predatory bear which would enhance one or more of the following skills: top running speed, acceleration, or maneuverability at high speeds. I re-analyzed the postcranial morphology of Arctodus and used data on running speed and bone strength in other large mammals to show that a bear the size of Arctodus with long, gracile limbs would not have been able to endure the extreme dynamic forces incurred during predatory activities. Instead, Arctodus' morphology and body size indicate it had evolved to maximize locomotor efficiency using a pacing gait. I suggest that Arctodus evolved as a specialized scavenger adapted to cover an extremely large home range in order to seek out, procure, and defend large-mammal carcasses from other carnivores. By modeling herbivore populations and their mortality, I show that enough carcass biomass was being produced in Pleistocene Beringia to make this scavenging niche energetically feasible. The model helps show that Arctodus' extinction probably is best tied to a reduction of year-round carcasses on the landscape, a condition which arose in the Holocene when the herbivore fauna became less diverse and began to experience more seasonal mortality.