Skull shape and Bergmann's rule in mammals: hints from Old World porcupines

The genus Hystrix includes eight species of porcupines distributed in Eurasia and Africa, across a broad latitudinal gradient. Our aim was to assess whether porcupine skulls: (1) allow for a reliable interspecific distinction; (2) change in size proportionally with body size; (3) follow the Bergmann's rule. We measured 235 Hystrix skulls from museums and private collections. We tested for differences in skull size and we assessed whether variability in skull shape allows species recognition through a multivariate approach. All Hystrix species considered could be reliably identified by skull shape. Skull size was correlated with body size and species differed in skull shape and size, with skulls of Hystrix javanica and Hystrix africaeaustralis being respectively the smallest and the largest ones. Within Hystrix cristata, the Mediterranean and the sub-Saharan clades differed for both skull size and shape. Using skull size, we could distinguish among African, mainland Italian and Sicilian populations. Skull size of this species decreased in size for increasing latitude values, contrary to prediction by the Bergmann's rule. Such latitudinal pattern may depend on the adaption of H. cristata to Equatorial African conditions, where the species evolved. In Italy (where H. cristata was introduced in the VI Century AD) and in North Africa, a smaller body size may be due to the local climate, or to a 'founder effect'.