Sex allocation, juvenile mortality and the costs imposed by offspring on parents and siblings

Generally, sex-specific mortality is not expected to affect optimal patterns of sex allocation. Several authors have, however, made verbal arguments that this is not true if juvenile mortality is sex specific during the period of parental care. Here, we provide formal mathematical models exploring the effect of such mortality on optimal sex allocation. We confirm the prediction that biased production of the sex with higher mortality during care is favoured. Crucially, however, this is only true when juvenile mortality in the period of parental care frees up resources for their current/future siblings (i.e. the saved investment is transferable). Furthermore, we show that although optimal sex allocation is consistent with the theory of equal investment (as asserted by previous authors), thinking in terms of equal investment is not readily feasible in some scenarios. We also show that differences in early mortality overcome biased sex allocation such that the sex ratio at independence is generally, but not always, biased in the opposite direction from that at birth. Our models should prove useful to empiricists investigating the effect of sex-specific juvenile mortality and antagonistic sibling interactions on sex allocation.