Sex-ratio optimization with helpers at the nest

In many cooperatively breeding animals, offspring produced earlier in life assist their parents in raising subsequent broods. Such helping behaviour i s often confined to offspring of one sex. Sex-allocation theory predicts th at parents overproduce offspring of the helping sex, but the expected degre e of sex-ratio bias was thought to depend on specific details of female and male life histories, hampering empirical tests of the theory. Here we demo nstrate the following two theories. (i) If all parents produce the same sex ratio, the evolutionarily stable sex ratio obeys a very simple rule that i s valid for a general class of life histories. The rule predicts that the e xpected sex-ratio bias depends on the product of only two parameters which are relatively easily measured: the average number of helping offspring per nest and the relative contribution to offspring production per helper. (ii ) If the benefit of helping varies between parents, and parents facultative ly adjust the sex ratio accordingly, then the population sex ratio is not n ecessarily biased towards the helping sex. For example, in line with empiri cal evidence, if helpers are produced under favourable conditions and paren ts do not adjust their clutch size to the number of helpers, then a surplus of the non-helping sex is expected.