Costs of Reproduction in Water Skinks

Abstract

Cost of reproduction measured under laboratory conditions may have little bearing on actual cost observed in the field, because both environmental conditions and behavior can modify the manifestation of these costs. I studied survival, growth, and reproduction in a wild population of southern water skinks (Eulamprus tympanum) over 3 yr to measure costs of reproduction in females, to examine how the environment influences these costs, and to assess the extent to which females are able to reduce costs. The proportion of mature females that reproduced each year varied from 30—60%. Frequent reproduction appeared costly in terms of both survival rates and growth. Growth rates and survival rates were negatively correlated with reproduction during the 3—yr study. In the coolest and wettest year of the study, when food intake by females was relatively low, gravid females and females that had reproduced in the previous years survived well than nonproducing females. There was no evidence of a survival cost of reproduction in the other two years of the study. Reproduction also imposed a fecundity cost, as overall, mean growth rates of reproducing females were reduced relative to those of nonreproducing females of the same body size. Reduced growth rate translates to reduced litter size in these skinks, because fecundity increases with body size. These results suggest that reproduction can be costly in terms of both survival and future reproduction in these skinks, but that survival costs are not consistent from year to year and are mediated by environmental conditions. Females appear to adjust their frequency of reproduction to minimize these costs, as those females that skipped opportunities for reproduction were those expected to gain the greatest growth and fecundity benefit by skipping.