When Does Sexual Dimorphism Arise in Crown-Giant Anoles?

Size dimorphism among Anolis habitat specialists from Butler et al. 2003

In recent years the Anolis community has shown greater interest in understanding the developmental bases of anole diversity. As these data accumulate, we can start to synthetically understand the ecological (ultimate) and developmental (proximate) factors that regulated different aspects of anatomical diversification in anoles. Differences between males and females (i.e., sexual dimorphism) is one area that has received a considerable amount of attention among anole biologists interested in obtaining this integrative understanding of anatomical diversity. Over the last decade a number of papers have been published examining the evolutionary patterns of anatomical differences, ecological correlations, and the developmental/physiological processes underlying dimorphism in anoles.

Variation in body proportion (i.e., shape dimorphism) among Anolis habitat specialists from Butler et al 2003.

Compared to other anoles, crown-giant anoles have relatively low levels of size dimorphism, but vary greatly in body proportion. Males and females tend to vary in relative limb length, head proportions, and in the dimensions of their adhesive toe pads. In a recently published paper Vanhooydonck et al. examine the timing of divergence between male and female A. baracoae. They raised 23 individuals (9 males, 14 females) for 3.5 years, repeatedly measuring three anatomical traits and bite force 11 times over this time period. The authors found that bite force and dewlap size exhibit significant differences in growth between the sexes. Their analysis further suggests that these traits diverge at different times during ontogeny – bite force diverges during juvenile growth while dewlap size does not diverge until sexual maturity – illustrating the independent regulation of dimorphic traits during development. Head length and hindlimb length did not appear to have sexual differences, although it would be interesting to also perform a formal analysis of adult size and shape dimorphism on A. baracoae to see if this species has similar dimorphic trends compared to other crown-giant anoles.

The developmental bases of size dimorphism in A. sagrei from Cox et al. 2009

These results are consistent with other studies that show a mosaic pattern of male and female phenotypic divergence. Using a longitudinal study of male and female A. sagrei, Cox et al. 2009 showed us that body size dimorphism in this species begins early in juvenile life, only three weeks after hatching. Sanger et al. showed that dimorphism in facial length can emerge through two distinct developmental strategies, one early in ontogeny and one at the time of sexual maturity that appears to be clade-specific. Additional research on that compares across traits and among species will further elucidate the number of ways that dimorphism can arise in anoles. Further work that overlays an ecological perspective onto these patterns will also allow a more thorough understanding of whether natural or sexual selection is the primary driver of these differences in timing. As this future work progresses new insights into the evolutionary processes of anatomical diversification are sure to follow.

About Thomas Sanger

Thom Sanger is an Assistant Professor at Loyola University in Chicago. His lab specializes on understanding the developmental bases of Anolis lizard diversity.

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