Inter-specific variation in Macropus crania: form, function and phylogeny

Abstract

Morphological variation was studied in the crania of seven species of kangaroos and wallabies (Macropus). How function and phylogeny interact to influence cranial form amongst adults of diverse species was considered. Thirty-six, three-dimensional landmarks were acquired on each of 65 adult Macropus specimens from Western Australia. The different species occupy a wide range of habitats and latitudes and there are well-known differences in diet among the species. Size and shape variability was examined using the methods of geometric morphometrics. Aspects of inter-specific variation explained by principal components were visualized and examined for correlations with size, latitude, and diet. The first principal component correlates strongly with inter-specific differences in latitude and it primarily contrasts species on the basis of the relative length of the snout anterior to the molar teeth, this being greater in species that live in cooler climates. Species scores on the second principal component correlate strongly with centroid size such that smaller crania have more flexed muzzles. Principal component three separates the short grass-eating, red kangaroo M. rufus, from all other species (most of which eat tough herbage from trees and shrubs). This principal component represents differences in the masticatory apparatus. Variation on the fourth principal component also seems to be functionally interpretable in terms of further differences in the masticatory apparatus of M. agilis. The relationship between phylogeny and cranial form was investigated by comparing a phenogram based on the shape distances between taxa with a consensus phylogeny drawn from the literature on Macropus genetics, ecology and morphology. The phenogram shows considerable similarity to current taxonomic views, but, our results also indicate differences that are related to function. Thus, this study serves to unravel the interaction of functional and phylogenetic influences on cranial form amongst Macropus species and raises further issues relating to ontogeny and phylogeny.