CARNIVORES PREFERRED HORSE OVER BISON IN PLEISTOCENE YUKON

How does predation pressure on different prey species change in response to significant ecological perturbations? Associated with Pleistocene climate cycles, mammal populations underwent repeated shifts in community structure. To examine how such community reorganization impacted predator-prey dynamics, we examined fossils from permafrost deposits in the Klondike goldfields of Yukon Territory, Canada. We used the frequencies of presences or absences of carnivore modifications on fossils of different species to evaluate changes in prey utilization between >50 kya BP and ~11 kya BP. Specifically, we tested whether the carnivore guild, dominated by Canis lupus, but including Panthera spelaea, and Arctodus simus, was biased towards specific prey species, or targeted prey as a function of their relative abundances. We focused on the region’s two dominant ungulates: steppe bison (Bison priscus) and caballine horses (Equus sp.). We selected mandibles (n_bison = 84, n_horse = 65) to taphonomically standardized our data. To estimate changes in relative population sizes of bison and horse through time, we synthesized AMS radiocarbon (¹⁴C) dated specimens using KDE_model (OxCal). Blinded to ¹⁴C results, we visually evaluated fossils for punctures, scores, and furrows consistent with carnivore modification. Tooth marks cannot differentiate predation from scavenging behaviors, though they provide useful metrics of prey utilization. We found that both bison and horses reached population maxima prior to and following the Last Glacial Maximum. However, the timing of peak abundances was offset, establishing two shifts in dominance. Horse fossils were modified by carnivores across their timeseries and regardless of bison population size. Bison fossils, by contrast, were modified only during periods with low horse abundance (prior to ~40 kyr BP and during the Terminal Pleistocene). Furthermore, although horse fossils showed similar rates of carnivore modification for sub-adults (~40% ) and adults (~30%), the clear majority of carnivore-modified bison fossils were subadult (nearly 50% of subadults vs. < 10% of adults; p < 0.05). Our results indicate a strong preference for horse among late Pleistocene Yukon carnivores, with bison (subadult) utilized when horse abundances were below an accessibility threshold.