Ancient DNA sequencing from Arctodus simus

Giant short-faced bears (Arctodus simus) were the largest carnivoran of Pleistocene North America and are one of the most extensively studied extinct megafaunal species from the continent. Smaller and larger forms of A. simus have previously been recognised and are sometimes considered subspecies (A. s. simus and A. s. yukonensis, respectively). However, researchers have also proposed that this size variation is primarily the result of sexual dimorphism. We sequenced 31 mitogenomes of A. simus from locations ranging from Alaska to New Mexico. Our results revealed a lack of phylogeographic structure in A. simus, as well as low genetic diversity and relatively recent mitochondrial diversification. These observations may either represent population bottlenecks during the Late Pleistocene or simply a naturally low effective population size resulting from a dispersed population and low population density. We found no evidence for genetic differences among our samples that were compatible with the previously proposed A. simus subspecies. In contrast, all large specimens to which we could assign a sex using genetic data were male, whereas the small specimens in our dataset were all female, supporting the hypothesis that A. simus size variation can be explained by sexual dimorphism.