Evolutionary theory predicts that dioecious species should produce a balanced primary sex ratio maintained by frequency-dependent selection. Organisms with environmental sex determination, however, are vulnerable to maladaptive sex ratios, because environmental conditions vary spatio-temporally. For reptiles with temperature-dependent sex determination, nest-site choice is a behavioural maternal effect that could respond to sex-ratio selection, as mothers could adjust offspring sex ratios by choosing nest sites that will have particular thermal properties. This theoretical prediction has generated decades of empirical research, yet convincing evidence that sex-ratio selection is influencing nesting behaviours remains absent. Here, we provide the first experimental evidence from nature that sex-ratio selection, rather than only viability selection, is probably an important component of nest-site choice in a reptile with temperature-dependent sex determination. We compare painted turtle (Chrysemys picta) neonates from maternally selected nest sites with those from randomly selected nest sites, observing no substantive difference in hatching success or survival, but finding a profound difference in offspring sex ratio in the direction expected based on historical records. Additionally, we leverage long-term data to reconstruct our sex ratio results had the experiment been repeated in multiple years. As predicted by theory, our results suggest that sex-ratio selection has shaped nesting behaviour in ways likely to enhance maternal fitness.

Summary1. Offspring phenotypic variation can be substantially influenced by non-genetic factors such as maternal effects, which ultimately can influence organismal fitness. For oviparous organisms that lack parental care, oviposition-site choice and egg size are maternal effects that can greatly affect offspring traits. Yet, few studies examine the consequences of these traits in the wild. 2. We manipulated the contents of natural painted turtle nests such that offspring spent two life stages (incubation and hibernation) in either maternally selected nest sites or randomly selected nest sites and quantified treatment differences in environmental parameters and offspring phenotypes. Additionally, we tracked the fates of individual eggs and hatchlings, which allowed us to quantify the strength and form of selection acting on egg size during incubation and, for the first time, body size during hibernation. 3. Maternally selected nest sites were warmer and produced offspring that were longer and hatched earlier than their siblings emerging from cooler, randomly selected nests. Treatments did not affect any measured traits during hibernation. 4. We detected no selection on egg size during the incubation stage, but significant linear selection favouring larger hatchlings during hibernation. 5. Our results suggest that nest-site choice allows mothers to partially control the environment of their incubating eggs, but is less effective at controlling hatchling environments during hibernation. Additionally, we provide novel support for the 'bigger-is-better' hypothesis in turtles by showing a positive relationship between size and survival during the hibernation stage.