As reproductive seasons progress, females often shift from greater energetic investment in many small offspring towards investing less total energy into fewer, better provisioned offspring. Two primary hypotheses have been proposed as explanations. One is an adaptive hypothesis from life-history theory: early offspring have a survival advantage over those produced later. Accordingly, selection favors females that invest in offspring quantity early in the season and offspring quality later. The other suggests these patterns result from passive responses to seasonal changes in the environment experienced by reproducing females. To disentangle the causes underlying this pattern, we performed complementary field and laboratory studies with lizards (Anolis sagrei). The laboratory study controlled maternal environments and quantified reproductive patterns throughout the reproductive season for each female. The field study measured similar metrics from free ranging lizards across an entire reproductive season. In the laboratory, females increased relative effort per offspring as the reproductive season progressed; smaller eggs were laid earlier, larger eggs were laid later. Because these patterns consistently emerge under controlled laboratory conditions, they likely represent an intrinsic, potentially adaptive adjustment of reproductive effort as predicted by life-history theory. The field study revealed similar trends, suggesting that intrinsic patterns are strong enough to persist despite the environmental variability that characterizes natural habitats. The observed patterns are indicative of an adaptive seasonal shift in parental investment in response to a deteriorating offspring environment: allocating greater resources to late-produced offspring likely enhances maternal fitness.