Evolutionary Links Between Skull Shape and Body Size Suggest Allometric Forces and Selection at Work in a Generalist Group of Lizards

Abstract

The vertebrate skull is a complex structure, and studies of skull shape have yielded considerable insight into the evolutionary forces shaping specialized phenotypes in organisms as diverse as bats, frogs, and fossorial animals. Here, we used phylogenetic comparative analyses of CT scans of male skulls from 57 species of Sceloporus lizards to explore patterns of skull evolution in a group of generalist taxa. We found that most interspecific variation is in terms of skull elongation such that some species have long, narrow skulls, whereas others exhibit more compact and robust skulls. We also found strong links to overall body size, with evolutionary shifts to larger bodies being associated with more compact skulls and slower evolutionary rates. This is the opposite of the pattern in most mammals in which larger bodied species have longer snouts, and more like the pattern in frogs in which function has played a more important evolutionary role. Also, unlike other vertebrates, the jaw, anterior, and posterior parts of the Sceloporus skull are largely integrated, having evolved independently of each other only to a limited, albeit significant, degree. Our results emphasize the importance of body size in the evolutionary shaping of the skull and suggest that additional studies of behavioral function in a generalist group are warranted.

Keywords: CT scans; Sceloporus; body size; skull shape.

FIGURE 1

Illustration of the 69 landmarks (red points) that we used to digitize the shape of male Sceloporus skulls.

FIGURE 2

(A) A regression of log‐transformed centroid size against the regression scores fitted with a trend line and (B) depicts the displacement in shape between small Sceloporus species (points) and large species (the vectors).

FIGURE 3

Results of Principal Components Analysis for male skull shape for measures of 57 Sceloporus lizard species. Panel (A) shows PC1 (snout length and jaw width) and PC2 (shortness of jaw length) while panel (B) illustrates interspecific differences in PC3 (snout height) and PC4 (shortness of center skull height). The size of the points is proportionate to the log‐transformed centroid size of the skulls.