Unexpected sex ratio adjustment in a colonial cooperative bird: pairs with helpers produce more of the helping sex whereas pairs without helpers do not

摘要

For cooperatively breeding species, the most popular and intensely debated explanation for sex ratio variation is the helper repayment hypothesis. It predicts overall sex ratio bias towards the helping sex, and/or facultative adjustment towards the helping sex in absence of helpers. We tested these predictions in a colonial cooperative bird, the sociable weaver Philetairus socius. We quantified variation in overall sex ratio, and tested the effects of helping and local breeding conditions on individual sex ratio. To better understand the factors affecting sex ratio variation, we investigated sex-related differences in helping, dispersal behavior, and nestling morphology. Our results did not support the predictions of the helper repayment hypothesis. Indeed, although we found that males are the philopatric helping sex: (1) we did not detect a consistent bias in overall sex ratio; and (2), more importantly, it was pairs with helpers that produced additional males, not pairs without helpers. We propose and discuss different factors that could explain these results.