Details

Autor(en) / Beteiligte

• Khwaja, Nyil
• Preston, Stephanie A. J.
• Briskie, James V.
• Hatchwell, Ben J.

Titel

Testing the predictions of sex allocation hypotheses in dimorphic, cooperatively breeding riflemen

Ist Teil von

• Ecology and evolution, 2018-04, Vol.8 (7), p.3693-3701

Ort / Verlag

England: John Wiley & Sons, Inc

Erscheinungsjahr

2018

Link zum Volltext

Quelle

Wiley-Blackwell Journals

Beschreibungen/Notizen

• 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. Evolutionary theory predicts that if one sex is costlier to produce, offspring sex ratios should be biased in favour of the other. Here, we show that rifleman females are likely to be costlier to produce, because they are larger, require more provisioning effort and are less likely to help their parents raise future offspring. Despite this, we find that offspring sex ratios are not biased in favour of males and discuss possible reasons for this.