Article Summary - Meachen, J. A. & Samuels, J. X. (2012), Evolution in Coyotes (Canis Latrans) in Response to the Megafaunal Extinctions.

        The results that were garnered from Meachen and Samuels’ research showed a marked decrease in coyote body size throughout the Pleistocene and Holocene eras, up to the modern day. The coyote fossils from earlier history show larger cranial sizes, broader rostrums, and more prominent carnassial teeth. To determine the factors contributing to these evolutionary changes, Meachen and Samuels’ first sought to support the theory that they were not, in fact, due to climate change. Previous research had outlined what is called Bergmann’s law, which states that as temperatures decrease, mammal body size increases. Meachen and Samuels compared the bone sizes against mean annual temperatures of the areas in which they originated and found that there was no correlation, for coyotes and wolves, with the increase or decrease in temperature. This suggests that coyotes do not follow Bergmann’s law and that climate change was not the primary force behind the evolutionary changes. This complied with Meachen and Samuels’ initial postulate because it shows that the evolutionary changes were most likely attributable to biotic factors. For the purpose of the their research, the biotic factors that Meachen and Samuels examined were competition and predator/prey interactions.

There was a large morphological shift in the coyote that aligned with major megafaunal extinctions at the end of the Pleistocene era. The prey base for coyotes prior to these events consisted of larger mammals and ungulates, including horses, bison, camels, llama, and bison. This prey base required a larger more robust body size and stronger canine teeth, and craniums. As the populations of this prey dwindled, coyotes had to evolutionarily adapt to new species of prey. Extant coyotes hunt mainly rodents and lagomorphs.

In addition to the changing prey base, Meachen and Samuels theorized that the evolutionary changes were also attributable to the changing predator competition. In the Pleistocene era, coyotes had to compete not only with grey wolves, but more robust predators such as dire wolves, and Smilodon, the saber-toothed cat. While grey wolves competed mainly with the dire wolves, the larger coyote body size allowed them to fill the more wolf-like niche. During this time coyotes were also more likely to hunt in packs. There is also modern research that delineates the effects of changing predator competition on coyote hunting, diet, and social structure. In the 1980s, when grey wolves were absent from northern Montana, coyotes were largely solitary, or in pairs, and their diet consisted primarily of lagomorphs, such as rabbits. Following the reintroduction of grey wolves, coyote pack sized increase to primariy pairs and groups of three or more. Arjo et al. theorized that the changing pack structure was due to the need of defense against wolves, and that the change in diet was related to the possibility of scavenging carcasses from kills made by wolf packs. 

In addition to coyote body size, Meachen and Samuels also studied fossils from the grey wolf. However, unlike with the coyote, they found very minimal changes in the morphology of wolves from the Pleistocene and Holocene eras, versus the extant species.

Comments

        Meachen and Samuels have theorized that the evolutionary changes in the coyote, over time, are due to the changing prey base and predator competition based on the postulation that they are not, in fact, due to the abiotic climate change. This seems to be a negative conclusion, in that they do not explicitly present what is, but rather, what is not. Obviously, when it come to evolutionary development, it is near impossible to make definitive conlcusions due to the difficulty of archiving extensive, or complete, fossil records, and due to the fact that many factors are estimated, approximated, or hypothesized. For example, the effect of climate change was not supported due to the lack of correlation between extant coyote sizes and the temperatures of their habitats. However, predictions on the climate and habitats during the Pleistocene and Holocene eras cannot be completely accurate.