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

Abstract

It is not generally appreciated that positive kin interactions do not necessarily 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 resource 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 explain their female-biased sex ratios. Allodapine females feed and protect brothers, 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 rapidly as the number of females increases. For small broods this helping behavior causes a female bias, but bigger broods could be sufficiently male biased to compensate the population sex ratio. Considering the fact that females need to be packaged into reproductive units (multifemale colonies), of which intermediate-sized units are the most productive, it is shown that fitness returns from females are in fact a wavelike function. This results in a rugged fitness landscape, which could explain the female-biased population sex ratios of allodapine bees as an adaptation to local fitness peaks rather than a global optimum. In behaviors where organisms have to package limited resources into integer numbers of units, the possible solutions are limited, and careful analysis is required.