Numerous studies of sea turtle nesting ecology have revealed that females exhibit natal homing, whereby they imprint on the nesting area from which they hatch and subsequently return there to nest as adults. Because freshwater turtles comprise the majority of reptiles known to display environmental sex determination (ESD), the study of natal homing in this group may shed light on recent evolutionary models of sex allocation that are predicated on natal homing in reptiles with ESD. We examined natal homing in Graptemys kohnii, a freshwater turtle with ESD, using mitochondrial sequencing, microsatellite genotyping and mark and recapture of 290 nesting females. Females showed high fidelity to nesting areas, even after being transplanted several kilometres away. A Mantel test revealed significant genetic isolation by distance with respect to nesting locations (rZ0.147; p!0.05), suggesting that related females nest in close proximity to one another. The patterns of fidelity and genotype distributions are consistent with homing at a scale that may affect population sex ratios.

Although natural populations of most species exhibit a 1:1 sex ratio, biased sex ratios are known to be associated with non-Mendelian inheritance, as in sex-linked meiotic drive and cytoplasmic inheritance (Charnov 1982; Hurst 1993). We show how cultural inheritance, another type of non-Mendelian inheritance, can favor skewed primary sex ratios and propose that it may explain the female-biased sex ratios commonly observed in reptiles with environmental sex determination (ESD). Like cytoplasmic elements, cultural traits can be inherited through one sex. This, in turn, favors skewing the primary sex allocation in favor of the transmitting sex. Female nest-site philopatry is a sex-specific, culturally inherited trait in many reptiles with ESD and highly female-biased sex ratios. We propose that the association of nest-site selection with ESD facilitates the maternal manipulation of offspring sex ratios toward females.

Although natural populations of most species exhibit a 1:1 sex ratio, biased sex ratios are known to be associated with non-Mendelian inheritance, as in sex-linked meiotic drive and cytoplasmic inheritance (Charnov 1982;Hurst 1993). We show how cultural inheritance, another type of non-Mendelian inheritance, can favor skewed primary sex ratios and propose that it may explain the female-biased sex ratios commonly observed in reptiles with environmental sex determination (ESD). Like cytoplasmic elements, cultural traits can be inherited through one sex. This, in turn, favors skewing the primary sex allocation in favor of the transmitting sex. Female nest-site philopatry is a sexspecific, culturally inherited trait in many reptiles with ESD and highly female-biased sex ratios. We propose that the association of nest-site selection with ESD facilitates the maternal manipulation of offspring sex ratios toward females.

Although variation in population sex ratios is predicted to increase the extinction rate of clades with environmental sex determination (ESD), ESD is still seen in a wide array of natural systems. It is unclear how this common sex‐determining system has persisted despite this inherent disadvantage associated with ESD. We use simulation modelling to examine the effect of the sex ratio variance caused by ESD on population colonization and establishment. We find that an accelerating function of establishment success on initial population sex ratio favours a system that produces variance in sex ratios over one that consistently produces even sex ratios. This sex ratio variance causes ESD to be favoured over genetic sex determination, even when the mean global sex ratio under both sex‐determining systems is the same. Data from ESD populations suggest that the increase in population establishment can more than offset the increased risk of extinction associated with temporal fluctuations in the sex ratio. These findings demonstrate that selection in natural systems can favour increased variance in a trait, irrespective of the mean trait value. Our results indicate that sex ratio variation may provide an advantage to species with ESD, and may help explain the widespread existence of this sex‐determining system.

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