COSTS OF REPRODUCTION IN THE WILD - PATH-ANALYSIS OF NATURAL-SELECTION AND EXPERIMENTAL TESTS OF CAUSATION

During 1991 through 1994, natural selection on reproductive effort in side-blotched lizards was indexed by measuring total clutch mass produ ced on the first clutch of the reproductive season and assessing how s uch effort in current reproduction affects subsequent survival and clu tch production. In addition, selection was also experimentally assesse d in free-ranging female side-blotched lizards by (1) surgically decre asing total clutch mass (direct ovarian manipulation) and enhancing cl utch mass using (2) exogenous gonadotropin, and (3) exogenous corticos terone. Surgical reduction of clutch mass uniformly enhanced survival. However, increasing clutch mass had more complex effects depending on year. Experimentally enhanced clutch mass enhanced survival in 1991, had no effect on survival in 1992, and decreased survival in 1993. Des pite the complexity of these experimental results, they are corroborat ed by our comparative data. It is important to note that local environ mental effects can obscure detection of costs arising from natural var iation in reproductive effort, and we removed such effects using path analysis. The striking shift in natural selection favoring females lay ing a large clutch mass (1991) to selection against females laying a l arge clutch mass (1993) is associated with an end of a severe multiyea r drought. Our natural-history observations suggest that the correlate d increase in predatory snake activity on our study site, coincident w ith the end of the drought, is the agent of natural selection. Althoug h the actual agents of selection (e.g., snake predation versus drought -related effects) are not resolved, the patterns of natural selection measured in our comparative and experimental data are also consistent with year-to-year changes in clutch mass and egg size that would be in dicative of rapid short-term evolution in these traits.