Abstract
Evolutionary theory predicts that parents should invest equally in the two sexes. If one sex is more costly, a production bias is predicted in favour of the other. Two well‐studied causes of differential costs are size dimorphism, in which the larger sex should be more costly, and sex‐biased helping in cooperative breeders, in which the more helpful sex should be less costly because future helping “repays” some of its parents’ investment. We studied a bird species in which both processes should favor production of males. Female riflemen Acanthisitta chloris are larger than males, and we documented greater provisioning effort in more female‐biased broods indicating they are likely costlier to raise. Riflemen are also cooperative breeders, and males provide more help than females. Contrary to expectations, we observed no male bias in brood sex ratios, which did not differ significantly from parity. We tested whether the lack of a population‐wide pattern was a result of facultative sex allocation by individual females, but this hypothesis was not supported either. Our results show an absence of adaptive patterns despite a clear directional hypothesis derived from theory. This appears to be associated with a suboptimal female‐biased investment ratio. We conclude that predictions of adaptive sex allocation may falter because of mechanistic constraint, unrecognized costs and benefits, or weak selection.
Highlights
Theory predicts that parents should invest in sons and daughters (Fisher, 1930)
Where one sex is costlier to produce, the evolutionarily stable sex ratio is predicted to be biased against this costly sex, at a point where the fitness benefits it gains from rarity balance the extra cost of its production (Hamilton, 1967)
We found that female nestling riflemen were significantly heavier than male nestlings and that rifleman carers provisioned broods more frequently when they were more female-biased
Summary
Theory predicts that parents should invest in sons and daughters (Fisher, 1930). Sex allocation appears to have been finely tuned by natural selection, population sex ratios are not biased toward the apparently cheaper, more helpful sex (Koenig & Dickinson, 1996; Komdeur et al, 1997) These studies have provided support for the “local resource competition” hypothesis, in which philopatric offspring competing for their parents’ resources incur a cost that may mitigate the benefits they provide by helping (Clark, 1978). It is unlikely that the value of repayment is diminished by local resource competition The alignment of these factors means that riflemen provide a rare opportunity to test a strong directional prediction of biased offspring sex ratios in a natural population. We discuss the implications of our results for understanding variation in sex allocation within and between species
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