Seasonal variation in reproductive effort and its effect on offspring size in the lizard Sceloporus undulatus

Fence lizards, Sceloporus undulatus, reproduce multiple times per year. Surival of reproducing females drops dramatically between clutches from first to final clutch of the year. Also, offspring from later clutches emerge with less time available for growth and storage prior to hibernation. Considering these two facts, life history theory predicts that (1) females should exhibit greater reproductive effort (RE) in the second clutch of the active season than in the first, and (2) that the increase in RE should be mediated by a greater allocation of energy per offspring. To test these hypotheses, we compared reproductive traits (RE, clutch size, and egg mass) of lizards from a New Jersey population laying their first and second clutches. Two different estimates of RE (relative clutch mass, and clutch mass adjusted for condition) were significantly lower for the second clutch than for the first clutch. The difference in RE was manifested primarily as a decrease in clutch size. Clutch size was correlated with female body size and condition, but average egg mass was not correlated with either variable. Egg mass varied twice as much among clutches as it did within clutches. In fact, the variation in average egg mass observed among clutches was almost as great as that which exists among populations of S. undulatus. Variation in egg mass within and among clutches has consequences for offspring quality because egg mass was positively correlated with snout-vent length, body mass, and condition of hatchlings. Our data do not support the predictions of existing theories regarding the optimal allocation of RE among offspring.