Why do Anatolian ground squirrels exhibit a Bergmannian size pattern? A phylogenetic comparative analysis of geographic variation in body size

Abstract

A phylogenetic comparative analysis of geographic variation in body size of an obligately hibernating marmotine species (Anatolian ground squirrels, Spermophilus xanthoprymnus) is presented in relation to environmental variables that pertain to four principal hypotheses (heat conservation, heat dissipation, primary productivity, and seasonality hypotheses). Adult Anatolian ground squirrels (78 males and 90 females) were collected from ten geographic localities in Anatolia for use in morphometric analyses. First, the study tested whether significant variation in body size occurs over the geographic range of S. xanthoprymnus. Then, to understand the possible cause(s) of the observed pattern of geographic variation in body size of Anatolian ground squirrels, four hypotheses were tested, separately and simultaneously, using a phylogenetic comparative method. Overall, food availability (primary productivity hypothesis) and, especially in males, over-winter fasting endurance (seasonality hypothesis) are likely the primary underlying mechanisms generating the observed pattern of increasing body size towards colder, more seasonal environments, with higher summer precipitation and productivity (or a Bergmannian size pattern).