Patterns of morphological evolution in Marmota (Rodentia, Sciuridae): geometric morphometrics of the cranium in the context of marmot phylogeny, ecology and conservation

Abstract

Marmots are of great interest for both sociobiologists studying the evolution of mammal societies and conservationists trying to protect them from extinction. In contrast, their phylogeny and morphological evolution are poorly understood and studied. Recently, a phylogenetic analysis using cytochrome b provided the first reconstruction of marmot evolutionary history and suggested that a high level of sociality evolved at least twice independently in the two proposed marmot subgenera. A morphological analysis of the marmot mandible supported this subgeneric classification and showed interesting, and unexpected, patterns in the evolution of marmot skeletal characters. In the present study we investigated a more complex, and potentially informative structure, the ventral cranium. Geometric morphometric techniques were applied in the first analysis of cranial morphology including all marmot species. Three main phenetic groups were found, which reflect phylogeny (subgenus Petromarmota, and Palaearctic subgenus Marmota) or geographical distribution (Palaearctic vs. Nearctic subgenus Marmota). Convergence in skeletal characters due to size similarities, a common finding in the sciurid skeleton according to traditional morphological analyses, did not occur in the marmot ventral cranium. Despite a genetic distance between Marmota vancouverensis and Marmota caligata similar to that among different populations of the latter species, the Vancouver Island marmot had the most atypical ventral cranium in the subgenus Petromarmota. This finding confirmed results obtained with the mandible, and emphasized the uniqueness of M. vancouverensis and the usefulness of complementing molecular analyses with morphological studies for a thorough characterization of population divergence, and a careful planning of conservation strategies.