Morphological evolution in marmots (Rodentia, Sciuridae): size and shape of the dorsal and lateral surfaces of the cranium

Marmots are the largest ground squirrels and have been extensively studied by sociobiologists investigating the evolution of mammal societies. Being a member of the sciurid clade, traditionally considered inclined to convergence, they are also a group on which to test the hypothesis of sciurid propensity to homoplasy of osteological characters. In the present analysis, the dorsal and lateral surfaces of the cranium of all living marmot species are compared with geometric morphometric techniques. Phenetic groups are found which reflect the subgeneric classification of marmots and are consistent with previous morphometric analyses of the mandible and ventral cranium. Two species have distinctive morphologies and phenetic relationships not congruent with phylogeny. Marmota vancouverensis is highly divergent for osteological characters, fur colour and behaviour despite its young age and close genetic similarity to Marmota caligata. Its small population may represent a rare chance to study evolutionary processes during rapid allopatric speciation in mammals, but strong conservation efforts are required to preserve this unique component of the Vancouver Island biodiversity. Also, Marmota monax has distinctive cranial traits. These are possibly related to its long separate evolutionary history and unique ecology and behaviour. Size-related convergence is not evident in Marmota. When outgroup species are included, Spermophilus, Cynomys, Tamias, and Sciurus group together on one branch, Marmota on the other. This is best explained as a retention of the ancestral morphology in the smaller members of the Marmotini (Spermophilus, Cynomys, and Tamias) and the evolution of derived morphology in Marmota.