This chapter reviews various social and ecological circumstances linked to parental care that could drive variation in adaptive sex allocation in of vertebrates. The most extreme and precise sex ratio adjustments in relation to such social and environmental factors were found in species where the fitness benefits of facultative sex ratio adjustment are high and the apparent costs low. Furthermore, some studies report evidence for sex ratio bias, either at birth or at the end of the parental care period, while other studies do not, despite conditions predicted to favour adaptive sex ratio adjustments being satisfied. This discrepancy between studies conducted on different species, or even the same species, emphasizes the need for more robust predictions based on accurate estimates of potential fitness pay-offs. To this end, there is a need for better information on the long-term consequences of sex allocation on individual fitness. A major task for future work is to obtain more detailed information on the fitness functions of both parents and male and female offspring. Finally, theoretical models need to be developed that can predict the observed variation in the amount and precision of sex-ratio manipulation in response to the various factors that influence the selection pressure.
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