THE SEASONAL TIMING OF OVIPOSITION IN SAND LIZARDS (LACERTA-AGILIS) -WHY EARLY CLUTCHES ARE BETTER

We studied a population of sand lizards (Lacerta agilis) near the nort hern edge of the species' range in coastal Sweden. We captured, marked , released and recaptured 98 adult female lizards over 5 years. Hatchl ings from 146 laboratory-incubated clutches (1279 eggs) from field-cau ght gravid females were measured, weighed, marked and released at the study site. Female sand lizards usually laid only a single clutch of 4 to 15 eggs each year, but varied considerably in the time of year at which they laid their eggs. Oviposition dates shifted between years de pending on weather (basking opportunities), but the relative timing of oviposition was consistent within a given female from year-to-year. T he first females to oviposit each year were large animals in good phys ical condition, that had grown rapidly in previous years. ''Early'' cl utches were larger than ''later'' clutches, had higher hatching succes s, and tended to have higher post-hatching survival rates. Offspring f rom early clutches were larger than ''later'' hatchlings, and differed in body proportions (probably because seasonal changes in maternal te mperatures directly modified offspring phenotypes). Overall, our study documents several strong correlates of the timing of oviposition, and suggests that variation in this trait among females has strong fitnes s consequences, perhaps related to maternal ''quality''. The correlati ons we observed between oviposition date and other traits that have be en invoked as determinants of hatchling survival in reptiles (e.g., ha tchling size, body shape, opportunities for multiple mating by the mot her) suggest that hypotheses advocating simple causal connections betw een these traits and hatchling success should be viewed with caution.