Large spatial variation and female bias in the estimated sex ratio of loggerhead sea turtle hatchlings of a Mediterranean rookery

Abstract

Adult sex ratio is a crucial demographic parameter affecting the viability of vertebrate populations. It figures amongst the determining factors in calculations of both demographic and genetic effective population sizes (Caughley & Gunn 1996). Theory predicts that the evolutionarily stable primary sex ratio should be unbiased except under certain conditions (for example, if parental investment is not equal for both sexes, if brothers compete for mates or if offspring of one sex help the parents; Fisher 1930). Sex determination in most vertebrate species is genotypic, leaving little scope for deviations from a balanced primary sex ratio. On the contrary, in many turtle, some lizard and all crocodilian species, gender is determined by the prevailing environmental conditions (mainly temperature) during embryonic development. Environmental sex determination leaves ample scope for variation in primary sex ratio, but should theoretically be subject to the same evolutionary constraint leading to balanced primary sex ratios as genotypic sex determination. However, although none of the conditions for predicted unbalanced primary sex ratios apply, consistently female-biased hatchling sex ratios (which can be assumed to be similar to primary sex ratios) are reported from different taxa with environmental sex determination, for example freshwater turtles