JOHN-ALDER, H.B.*; HAENEL, G.J.: Experimental and Demographic Analyses of Growth and Sexual Size Dimorphism in a Lizard, Sceloporus undulatus

Sexual size dimorphism (SSD) is a common phenomenon caused by a variety of environmental and genetic mechanisms in animals. In the current study, we investigate the demography of a population of eastern fence lizards (Sceloporus undulatus) to compare age structure and survivorship between the sexes, and we examine growth rates of juveniles under both natural and controlled laboratory conditions to elucidate causes of SSD in this species. Furthermore, using our laboratory growth data, we examine the heritability of juvenile growth rates. Our results show that SSD develops in the field before the end of the first year of age (before sexual maturity) because juvenile females grow more rapidly than juvenile males. In the laboratory environment, however, we observed no sexual difference in growth rates for lizards up to the size of maturity in the field. Thus, sexual differences in growth rate and subsequent development of SSD in this population are subject to strong proximal control. We found high levels of additive genetic variance for juvenile growth indicating a strong potential for selection to operate on juvenile growth rates. However, while selection on juvenile growth rate could explain differences in growth among populations, it would not necessarily contribute to SSD within our population due to the high plasticity in growth rate. Supported by NSF IBN9507529.