AFTER THE MAMMOTHS: THE RESPONSE OF A SMALL GRAZER TO THE TERMINAL PLEISTOCENE MEGAFAUNA EXTINCTION

As recently as 14 ka, the Americas were home to a vastly different mammal community with large-bodied herbivores such as mammoth, mastodon, camels, llamas, and horses widespread across the continent. Shortly after the arrival of humans, about 80% of these megaherbivores were extinct, including all taxa over 600 kg. Studies of this event have generally focused on the causes of the terminal Pleistocene extinction rather than the ecosystem consequences of the loss of tens of millions of megaherbivores in North America. Here, we examine the response of a small gazing rodent, Sigmodon hispidus (cotton rat), to the extinction at a single location in the Great Plains of Texas. At Hall’s Cave, the megafauna extinction led to the catastrophic loss of 80% (12 of 15 species) of large-bodied herbivores and 20% (3 of 15 species) of apex predators. Using 16 tightly constrained temporal windows spanning the full glacial to the modern, and employing measurements of the lower M1, we examine population-level changes in body size of cotton rats before and after the extinction and during times of shifting climate. We find a dramatic and sustained increase in the mean body size of the population just after the extinction. This occurs despite a warming climate, which according to Bergmann’s rule would select for smaller, not larger body size. Moreover, the change in population body size is accompanied by a time transgressive shift in isotopic dietary niche and a greater overall isotopic dietary space. Our results suggest that cotton rats invaded new ecological niche space after the extinction of megaherbivores, and indicate that a combination of behavioral, physiological and morphological responses are likely for modern species in the wake of trophic downgrading.