A selfish strategy of social insect workers that promotes social cohesion

SOCIAL insect colonies are in many ways analogous to organisms¹, because kin-selected workers act selflessly and cohesively to promote the fitness of a few reproductive members^2,3. But workers can evolve selfish strategies which create reproductive conflict, reducing the functional integrity of colonies. For example, they can lay unfertilized (male) eggs ^3-6, compete directly with the queen to lay fertilized (female) eggs⁷, suppress the reproduction of other workers^8,9, choose among several queens¹⁰and generally favour closer over more distant kin¹¹. Conflicts over the sex ratio may be especially pervasive, even in highly eusocial insects. The unusually high relatedness (r = 3/4) of female hymenopteran workers to their full sisters means that workers should prefer more female-biased sex ratios than do queens⁴. The worker preference for females should be exerted most strongly on colonies where they are most likely to be full sisters, leaving male production to colonies where this advantage least applies^4,12-14; this prediction is supported by studies in ants and bees^12,15-17. Here we show that when colonies havezmultiple queens born in the same nest, the selfish worker sex-ratio strategy has a paradoxical side-effect which strongly promotes social cohesion. This strategy accounts for the peculiar colony cycle of epiponine wasps, and may be responsible for the maintenance of eusociality in this group.