Comparative ecogeographical variation in skull size and shape of two species of woolly opossums (genus Caluromys)

Environmental pressures are key mechanisms in the change of size or shape of an organism. In addition to external factors, the pressure of allometry (size-related shape changes) can also be present itself in the way that shape varies even in adulthood. In this study, we used a geometric morphometric approach to analyse skull size and shape variation in South-American Didelphidae, genus Caluromys, represented by two species, Caluromys philander and C. lanatus, in relation to allometry, geography and climate. We found significant allometric effects on these species, especially C. lanatus. The strong role of geographical space mainly on size of both species and on shape of C. lanatus suggests that they are spatially structured, supporting neutral processes, such as drift, in their evolution. Size of both species was related to latitudinal variation, the inverse pattern of Bergmann's rule. Temperature and the set of bioclimatic variables influenced both size and shape of species. The shape of C. philander was influenced by bioclimatic variables (e.g. rainfall), while the shape of C. lanatus showed such influence (mostly temperature) but just when under the presence of allometry. Our results indicate the importance of allometry and environment in the patterns of skull shape variation in the South-American Caluromys, with a concomitant influence of geography. Climatic factors, such as the temperature, have strong influence on cranial changes along populations, with the two species varying similarly in skull size and shape along a climatic gradient. The latitudinal distribution, although mostly parapatric, of these congeneric species in South America should be in part responsible for their similarities in the observed phenotypic variation.