SEX-RATIO REGULATION - THE ECONOMICS OF FRATRICIDE IN ANTS

In many insect societies, workers can manipulate the reproductive outp ut of their colony by killing kin of lesser value to them. For instanc e, workers of the mound-building For mica exsecta eliminate male brood in colonies headed by a single-mated queen. By combining an inclusive fitness model and empirical data, we investigated the selective cause s underlying these fratricides. Our model examines until which thresho ld stage in male brood development do the workers benefit from elimina ting males to rear extra females instead. We then determined the minim al developmental stage reached by male larvae before elimination in F. exsecta field colonies. Surprisingly, many male larvae were kept unti l they were close to pupation, and only then eliminated. According to our model, part of the eliminated males were so large that workers wou ld not benefit from replacing them with new females. Moreover, males w ere eliminated late in the season, so that new females could no longer be initiated, because matings take place synchronously during a short period. Together, these results indicate that workers did not replace male brood with new females, but rather reduced total brood size duri ng late larval development. Male destruction was probably triggered by resource limitation, and the timing of brood elimination suggests tha t males may have been fed to females when these start to grow exponent ially during the final larval stage. Hence, the evolution of fratricid es in ants is best explained by a combination of ecological, demograph ic and genetic parameters.