Between-year variation in determinants of offspring survival in the Sand Lizard, Lacerta agilis

1. Life-history theory predicts that clutch size and offspring size should evolve to an optimal offspring size vs number (OSN) equilibrium. 2. Offspring size in Swedish Sand Lizards (Lacerta agilis) decreases up to 60% during a female's life with a concomitant increase in clutch size; why do female Sand Lizards not produce an offspring of approximately equal size through life, while adjusting clutch size in accordance with available res ources? 3. Our results show that there is year-to-year variation in what factors de termine female reproductive success, estimated by the number or proportion of recruits into the second year cohort. 4. In a year with relatively poor female growth rate, poor female condition and probably relatively low potential for resource acquisition, females ma ting with many partners did relatively better than females mating with fewe r partners. 5. In the poor year, relatively larger offspring survived better than small er ones. 6. In a year with relatively high resource levels, females producing both r elatively large young and large clutches were favoured by selection. 7. Depending on environmental conditions, female 'optimal tactics' may diff er between episodes of selection.