No evidence for adaptive sex ratio adjustment in a cooperatively breeding bird with helpful helpers

Abstract

The repayment hypothesis for offspring sex allocation predicts that breeders in cooperatively breeding groups should overproduce the helping sex, particularly when they lack helpers. In birds that produce sexually size-dimorphic nestlings, sex allocation is further predicted to vary across hatching order to maximize brood survival. This could occur by biasing late-hatched nestlings toward either the energetically inexpensive sex (the intra-brood sharing-out hypothesis) or the more competitive sex (the intra-brood competitive equilibrium hypothesis). Here, we test these hypotheses using data from 553 nestlings in 109 broods of greater anis (Crotophaga major), a cooperatively breeding bird that breeds in groups composed of multiple reproductive pairs and non-breeding helpers. Helpers are predominantly males and increase the reproductive output of breeders; late-hatched nestlings are more vulnerable to starvation. Therefore, the repayment hypothesis predicts that groups without helpers should produce male-biased broods, whereas the intra-brood sharing-out and competitive equilibrium hypotheses predict that either females (the energetically inexpensive sex) or males (the more competitive sex), respectively, should be overproduced at the end of the laying sequence. Contrary to these predictions, population-wide sex ratios did not differ significantly from 50:50, and we found no evidence for facultative sex ratio adjustment within broods by helper presence or by hatch order. These results support a growing consensus that facultative sex allocation is less widespread in birds than once thought, even in cooperatively breeding species with sex-biased helping behavior.

Significance statement

Evolutionary biologists have long been interested in offspring sex ratios. Despite a rich body of literature, few clear trends have emerged among the many hypotheses proposed to explain offspring sex biases in birds, and relatively few studies have examined multiple hypotheses or long-term datasets. This study leverages an 11-year dataset on the cooperatively breeding greater ani to address three adaptive hypotheses for offspring sex allocation. However, consistent with recent meta-analyses, we find no support for adaptive sex allocation in line with the predictions of any of the three hypotheses. Facultative adjustment of sex ratios in birds may be more constrained than once thought, even in species in which it is predicted to be adaptive.