Project Details


Sexual size dimorphism (SSD) is a widespread biological phenomenon in which adults of one sex attain characteristically larger body sizes than the other sex. Depending on the species, males might be larger than females, or vice versa. It is often assumed that SSD evolves due to differential selection on body size, and several adaptive hypotheses invoking advantages for male- or female-biased SSD have been widely discussed. However, the genetic basis and thus the potential for evolution of SSD is almost completely undescribed, and relatively little is known about physiological and other proximate mechanisms that lead to sex differences in body size in the vast majority of species. It is therefore premature to focus on ultimate as opposed to proximate causation of SSD, as has been the general tilt of previous studies in this area. The present project will focus on physiological and ecological mechanisms of proximate causation of SSD in a lizard (Sceloporus undulatus) in which females grow faster as juveniles to become about 10% larger than males as adults. This project will address the following questions: 1) Is testosterone fundamentally responsible for sexually dimorphic growth rates and the development of female-biased SSD? 2) To what extent is SSD subject to environmental variation? 3) How do constraints imposed by growth/activity trade-offs affect the occurrence of SSD? To answer the questions posed in this project, two sets of studies will be done. 1) Descriptive studies will characterize growth rates, behavior, and sex steroid levels in juvenile lizards active in their native habitat. 2) Experimental studies involving hormone manipulations will resolve unambiguously the potential role of testosterone in leading to female-biased SSD. The work completed in this project will contribute important new information regarding the general question of how sex differences in growth lead to SSD. This work may open the door for future comparative studies on growth regulation in phrynosomatid lizards, a large family with a well-described phylogenetic history. Future comparative molecular studies in this group could provide a detailed understanding of the evolution of the regulation of growth-promoting gene expression and could help to resolve the contribution of genetic vs. environmental determinants of growth and SSD. As such, phrynosomatid lizards may prove to become a significant new model system for studies on growth regulation.

Effective start/end date1/1/0212/31/04


  • National Science Foundation: $230,337.00


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