COX, Robert*; BARRETT, Michele; ZILBERMAN, Viktoriya; JOHN-ALDER, Henry; The Ohio State University; Rutgers University; Rutgers University; Rutgers University: Effects of sex and castration on growth of Yarrow�s Spiny Lizards (Sceloporus jarrovii) are reduced or absent in laboratory common-garden experiments

Adult males average about 10% larger (longer in snout-vent length) than adult females in natural populations of Yarrow�s Spiny Lizard (Sceloporus jarrovii). In two previous studies of free-living animals, we found that (1) sexual size dimorphism (SSD) develops because males grow more quickly than females as yearlings, and (2) growth of yearling males is inhibited by castration and restored to the rate of intact controls by exogenous testosterone (T). Here, we show that these effects of sex and castration on growth are reduced or absent under common-garden conditions. In a series of laboratory studies, growth was rapid under ad lib food availability, such that males grew only slightly more quickly than females. Food restriction reduced growth rate by about 50%, but this reduction was observed in both sexes, such that SSD was subdued (relative to natural expression) under either diet. At both high and low food levels, wet mass of abdominal fat pads was greater in females than in males, suggesting a sex difference in allocation to storage that could account for the minor sex difference in growth. Although castration inhibited the growth of free-living males, growth in captivity under ad lib food availability was rapid and did not differ among castrated males, intact controls, and castrated males treated with T. Collectively, our results show that sex differences in growth and effects of sex steroids on growth are subject to strong proximate environmental control in S. jarrovii. Our results also provide a cautionary example of the limitations of laboratory experiments in studies of endocrine growth regulation. Supported by NSF 0135167.