Dense habitats selecting for small body size: a comparative study on bovids

As a crucial adaptation impacting on most aspects of a species’ biology, body size is likely to be under multiple selective pressures. This study aims to determine whether the physical structure of dense habitats selects directly for small size in order to reduce resistance to movement from the vegetation and facilitate concealment (Manoeuvrability hypothesis). This is unclear because other selective forces could indirectly cause habitat to covary with body size. Focusing on bovids, I tested the explanatory power of the Manoeuvrability hypothesis in an interspecific comparative analysis controlling for phylogenetic relatedness. Bovids in dense habitats were indeed found to be reduced in size. Confirming the Food choice hypothesis, grazers, who predominantly feed on abundant food in open habitats, were found to be larger than browsers in a multivariate analysis. However, after controlling for diet there remained a residual variation in body size which correlated negatively with habitat density. No support was found for Bergmann's hypothesis as the explanation for this remaining correlation since body size did not correlate with latitude of the species’ range. Likewise, sexual selection on male body size in open habitats due to increased polygyny was an unlikely cause since body size did not correlate with sexual body size dimorphism in multivariate analysis. Rather, the finding that tail length was disproportionately shorter in closed habitats points to dense habitat structure selecting directly for compact body conformation and small size, as predicted by the Manoeuvrability hypothesis.