Effects of mismatched pre- and postnatal environments on offspring sex ratios in reintroduced populations.

Species reintroductions are becoming increasingly used as a conservation strategy to preserves threatened species and restore natural ecosystems. While species reintroductions can have significant positive impacts for threatened species and ecosystems, a large portion fail, despite our growing understanding of how they should be carried out. A potential contributor to this high rate of failure is offspring sex ratios in wild populations following release. A number of studies have reported unexpected biases in offspring sex ratios of reintroduced animals, and this pattern may be even more prominent than the literature suggests. Skewed sex ratios can slow the growth and recovery of reintroduced populations and subsequently limit the establishment of a viable wild population. Understanding the mechanisms and drivers of these unexpected sex ratios is an important first step in mitigating the damage they can do to reintroduction programmes. Here I explore the effect that a mismatch between pre- and postnatal environments has on sex allocation in reintroduced animals. Developmental experience in a captive environment can have lifelong effects on an individual’s physiology, such that they are unable to mount appropriate physiological responses to the current local conditions once reintroduced into the wild. As sex allocation acts through physiological mechanisms, this may impose constraints on their ability to adjust offspring sex ratios adaptively. I examined offspring sex ratios in captive and reintroduced populations of three species: Arabian oryx, California condor, and red wolf. Offspring sex ratios of individuals with matched pre- and postnatal environments were compared to those of individuals with mismatched pre- and postnatal environments (i.e., individuals that had been either captured or reintroduced). I then explored the relationships between several predictors of sex allocation (sire (father) and dam (mother) age, sire and dam parity, rainfall around the time of conception, and temperature around the time of conception) and offspring sex ratios, and examined how those relationship were affected by a mismatch between pre- and postnatal environments. Offspring sex ratios of mismatched Arabian oryx and red wolves did not differ from offspring sex ratios of matched individuals. However, captive born California condors that reproduced in the wild (mismatched environments) produced significantly more male offspring that those born in and reproducing in the wild (matched). A mismatch between pre- and postnatal environments affected the relationships that sex allocation had with rainfall, parental parity, and dam age (only in wild populations) in Arabian oryx. In California condors, the relationships that offspring sex ratios had with parental age, parental parity and temperature were all affected by a mismatch between pre- and postnatal environments. Finally, in red wolves, mismatched environments affected the relationships that offspring sex ratios had with sire age and temperature. These results support the hypothesis that the mismatched environments associated with reintroduction can impose physiological constraints on the ability of released animals to adjust offspring sex ratios adaptively. This may explain some of the unexpected sex ratio biases observed in reintroduced populations, and indicate that such constraints could be important in the success or failure of species reintroductions