Sexual dimorphism in body shape without sexual dimorphism in body size in water skinks (Eulamprus quoyii)

Sexual dimorphism in body size is often accompanied by shape dimorphism. Shape dimorphism may be selected directly, or may be an indirect result of selection for size dimorphism. To determine whether shape differences are likely to have been directly influenced by selection, the indirect effects of size dimorphism oil shape can be removed statistically, the approach of most previous studies, or, alternatively, we can examine the patterns of shape dimorphism in species that are not size dimorphic. Here I describe sexual shape dimorphism in adults and neonates of a lizard that is not dimorphic in body length, the eastern water skink (Enlamprus quoyii). In this species, snout-vent length is not significantly different between adult or neonate males and females, but there are significant shape differences between sexes in both adults and neonates. For a given body length, adult males have wider heads, longer limbs, shorter trunk length (the distance between the forelimb and the hindlimb), and greater mass than females. In addition, head width, forelimb length and mass increase significantly more rapidly with body length in adult males than in females, whereas the rate of change of hindlimb length and trunk length with length is not significantly different in males and females. The shape of the body is more similar in neonates than adults, but female neonates have significantly longer trunks than males. The intersexual shape difference in neonate E. quoyii suggests that. although growth rate differences anion, body parts are the main source of differences in shape between adults. the difference between male and female interlimb lengths is present initially. Shape differences present at birth that are preserved until adulthood are less common than growth differences as a source of adult shape differences. The intersexual shape differences among adult E. quoyii are similar to those reported for species that are sexually size dimorphic, suggesting that selective forces have influenced body shape in similar ways in both size dimorphic and nondimorphic species, and that allometric relationships alone may not be responsible for shape differences between males and females in size dimorphic species.