Cooperative breeding, offspring packaging, and biased sex ratios in allodapine bees

It is not generally appreciated that positive kin interactions do not neces sarily result in an evolutionarily stable (ES) skewed sex ratio. Stability depends critically on the sex of both the helper and receiver. When help is given within one sex only no monomorphic ES strategy exists, and local res ource enhancement (LRE) between offspring of one sex does not predict a sex ratio bias toward that sex. I developed a model to clarify and examine the sex ratio biases that may be expected under cooperative breeding. I found that LRE between cooperatively breeding female allodapine bees cannot expla in their female-biased sex ratios. Allodapine females feed and protect brot hers, which may stabilize the female-biased sex ratio, but the model shows this is not the case because benevolence to males is likely to decrease rap idly as the number of females increases. For small broods this helping beha vior causes a female bias, but bigger broods could be sufficiently male bia sed to compensate the population sex ratio. Considering the fact that femal es need to be packaged into reproductive units (multifemale colonies), of w hich intermediate-sized units are the most productive, it is shown that fit ness returns from females are in fact a wavelike function. This results in a rugged fitness landscape, which could explain the female-biased populatio n sex ratios of allodapine bees as an adaptation to local fitness peaks rat her than a global optimum. In behaviors where organisms have to package lim ited resources into integer numbers of units, the possible solutions are li mited, and careful analysis is required. class structured groups, cooperati ve breeding, packaging, reproductive value, sex ratio.