Evolutionary increases in body size are linked to female pelvis evolution in response to habitat rather than reproduction.

In female vertebrates, the pelvis plays a role in important biological processes including reproduction and locomotion, and its evolution is thus likely influenced by multiple selective pressures. Here, we used CT scans, 3D geometric morphometrics, and phylogenetic comparative methods to describe the evolution of the female pelvis of Sceloporus lizards, and to tease apart the relative importance of the evolution of live-bearing, arboreality, and allometry in altering pelvis shape. We found that the ancestral egg-laying species tended to exhibit dorsoventrally tall female pelvises, and that two of three clades of live-bearing Sceloporus evolved both larger body sizes and dorsoventrally flat, laterally wide female pelvises. Larger body sizes may have relaxed constraints on pelvis height, allowing these mostly terrestrial and rock-dwelling species to respond to selective forces that enhance crypsis or thermoregulation by evolving dorsoventrally flatter and laterally wider pelvises. In contrast, one large clade of live-bearing and arboreal species had dorsoventrally tall pelvises, like those of the ancestral egg-laying species. Again, evolutionary shifts to larger body sizes may have relaxed allometric constraints, allowing adaptive responses to arboreality that converge on those of terrestrial egg-layers. Further studies are needed in other taxa to determine the importance of larger body sizes and consequently relaxed allometric constraints in shaping other morphological features.