Does the differential cost of sons and daughters lead to sex ratio adjustment in great frigatebirds Fregataminor?

Abstract

Sex allocation theory predicts that if benefits of producing sons and daughters differ and outweigh the costs of sex ratio adjustment, parents should produce more of the offspring that provide them with greater fitness. Potential benefits may be more likely to outweigh costs where sexual size dimorphism and, in birds, single‐egg clutches exist. Great frigatebirds Fregataminor are seabirds in which females are larger than males and clutch size is one egg. In our study population, sexual size dimorphism develops primarily during the period of complete juvenile dependence on parental care, consistent with a higher cost of producing daughters than sons. Over the course of the 1998 breeding season there was a shift from early season prevalence of daughters to late‐season prevalence of sons. Variation in food availability at time of egg laying, as indexed by sea surface temperature (SST), was a strong predictor of offspring sex in 1998. In contrast, SST in 2003 was not a predictor of offspring sex, nor was there a seasonal shift in the hatching sex ratio, despite a seasonal shift in SST. Besides food availability, we tested two additional factors in 2003 that could explain sex ratio adjustment in relation to the cost of reproduction. Offspring sex in 2003 was not related to natural or experimentally induced variation in maternal body condition; pre‐laying food supplements raised the body condition of females at the time of egg laying but did not affect offspring sex or egg mass. In addition, offspring sex was not predicted by the length of maternal telomere restriction fragments (TRFs), an index of age and possibly of reproductive experience. Broad confidence intervals on effect size suggest that undetected effects of maternal condition on offspring sex ratio could easily exist, but confidence intervals were narrower on the non‐significant effects of SST and TRF length on offspring sex ratio. The cause of different seasonal patterns of hatching sex ratio and different SST effects in 1998 and 2003 is unclear.