Late Pleistocene-Holocene mammalian body size change in Jordan's Azraq Basin: A case for climate driven species distribution shifts

Abstract

Mammalian body size diminution across the Late Pleistocene-Early Holocene transition in the southern Levant has been much researched, with special focus on gazelle in Levantine Mediterranean zones. Explanations of body size diminutions in those cases include temperature increase and anthropogenic factors. This study examines body size shifts in three mammalian taxa – Gazella (gazelle), Lepus (hare) and Vulpes (fox) – between 24,000–7500 cal BP, in the steppic Azraq Basin of northeast Jordan. Osteometrics derive from 19 archaeological sites through the Late Pleistocene-Early Holocene sequence. We use a ‘Z-score’ index of relative body size, and time-series analyses, to track body size shifts. All three taxa show relatively larger body size in the Late Pleistocene compared to the Early Holocene, with smallest Late Pleistocene sizes seen between 12,000–11,500 cal BP, during the Younger Dryas. While gazelles and hare recover size in our Early Holocene samples, they both show smaller sizes after 9000 cal BP. Similarity in size trends leads us to reject the influence of anthropogenic factors alone, in favour of ecological and climatic factors.

We attribute the sharp size decrease in gazelles in the Late Neolithic, 9000–7500 cal BP, to inter-specific size change, and the addition of a smaller Arabian species better adapted to the warmer arid post 8.2 kya conditions. Patterns for fox, albeit on small sample sizes, also hint at species turnover but in the Late Pleistocene. For hare, we track size decrease between the Late Pleistocene and Early Holocene as possible intra-specific diminution. Combined results argue for mammalian taxa experiencing complex responses to shifting ecological conditions in the Late Pleistocene-Early Holocene. Body size trends in the Azraq Basin appear counter to the expectations of Bergmann's rule of thermoregulation. We propose instead that forage conditions, and resource availability and limitations are better fit drivers of mammalian body size adaptations and turnover (including species distribution shifts) in this semi-arid case study region.