COX, R. M.; SKELLY, S. L.; JOHN-ALDER, H. B.; Rutgers University: Ontogenetic and mechanistic studies of growth and sexual size dimorphism in lizards

Studies of sexual size dimorphism (SSD) have historically been dominated by an adaptive paradigm under which patterns of SSD are presumed to reflect differences in the selective forces acting on male versus female body size. However, comparative data at present provide only weak support for several primary adaptive hypotheses as explanations for SSD in lizards. Most adaptive hypotheses for SSD assume that body size is of central importance to the ecology, life history, and reproductive success of an organism, but the fact that adult body size is the result of an ontogenetic growth process subject to numerous proximate influences is often overlooked. We present a conceptual model in which SSD can be viewed as an incidental consequence of sexual dimorphism in growth, which may in turn result from fundamental sexual differences in physiology, behavior, ecology, and reproductive investment. This model is consistent with data from the lizard Sceloporus undulatus, where (1) the dimorphism in juvenile growth rate that produces female-larger SSD in the field can be eliminated under controlled lab conditions, (2) experimental testosterone (T) administration inhibits growth in juvenile males in the lab and in field enclosures, and (3) castration allows males to grow faster than control or T-implanted males in field enclosures and at a rate comparable to natural females. We also present data from several congeneric lizards to show how this model can be extended to species exhibiting different patterns of SSD, and argue that future studies of SSD will need to combine detailed demographic growth data with careful manipulations of the mechanisms potentially responsible for sexual dimorphisms in growth and body size. Supported by NSF IBN 0135167.