Shape analysis in a metacommunity context

The sigmodontine rodent genus Eligmodontia includes highly specialized desert-dwellers in the Southern Cone of South America. Species of gerbil mice have inflated auditory bullae, and are also capable of bipedal jumping and elevated urine concentration. Photo by Dario Podesta http://www.dariopodesta.com/en/ 

By Renan Maestri, L.R. Monteiro, R. Fornel, T.R.O. Freitas and Bruce D Patterson

Macroecological studies of functional traits often employ an assemblage-based approach to investigate trait variation. This simple approach summarizes traits of each assemblage, for example, by averaging the trait values of its constituent species. Commonly investigated questions with this method include ecogeographical rules, such as Bergmann’s rule, Rapoport’s rule, and others. Traits of interest are usually univariate, but they don’t need to be.

Shape variables, those obtained from geometric morphometric analyses, are quintessentially multivariate. So what if shape variables were used instead of univariate traits? What spatial patterns are reflected in geographical variation of shape across assemblages? Our study addresses these issues. We also examine the relative importance of environmental versus phylogenetic lineage effects in explaining shape variation. Do these two factors interact and produce phylogenetic niche conservatism at the metacommunity scale, or their effects are independent?

To answer these questions, we investigated skull and mandible shape variation of 228 species of sigmodontine rodents in the Neotropical region. This clade colonized South America during the Neogene, radiating both taxonomically and ecologically to a remarkable degree. These rats and mice are now widespread through the Neotropics, presenting considerable local richness and high spatial turnover across space. They are therefore an excellent group for such purposes.

The main pattern of shape variation emerging from our analyses stems from the geographically complementary distributions of the main clades of sigmodontines across the Neotropics. The dominance and diversity of members of the phylogenetic clade Oryzomyini in Amazonia, and of Phyllotini and Abrotrichini in the southern Andes, were reflected in overall patterns of skull and mandible shape variation. Nevertheless, environmental variables such as temperature seasonality and land cover were also highly associated with spatial variation in shape. Notably, assemblages of more arid and open environments were dominated by rats with big tympanic bullae, a well-known adaptation for increased sound reception, which presumably helps the mice avoid predation in open environments.

Variation-partitioning analyses demonstrated that the biogeographic distribution of lineages influenced the same proportion of spatial shape variation as did environmental variables, a pattern recognizable as phylogenetic niche conservatism at the metacommunity scale. 

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