More Females Because Sisters Kill Their Brothers

That's a real sacrifice: a lot of work and no sex at all...

The social insects, like bees, wasps, ants or termites, offer some of the most interesting examples of altruism in the natural world: the sterile individuals from the workers' cast renounce to their own reproduction for the greater good of the colony (this sounds like a communist speech...).

But researches have proven that this sacrifice is not so voluntary: in most cases, the queen acts in this direction employing pheromones or physical activity.

A new research carried out in Canada and the United Kingdom adds another sinister side to the so-called altruism, in the case of a peculiar group of parasitic wasps. For a long time now, the role played by the sterile "soldier" larvae in polyembryonic parasitoid wasps has puzzled the researchers. They have been regarded as having a protective role for their broodmates as a whole, by guarding them against the attack by other species of parasitoids.

More recently, the soldiers were supposed to be mainly involved in a battle of the sexes, against their own siblings.

But the new research has rejected the brood-benefit hypothesis, favoring more the idea that their behavior is altogether spiteful. "We found that the bizarre genetics of these wasps means that brothers value their sisters more than sisters value their brothers, and so if sterile larvae function for the good of the group then it should be brothers who more willingly make the sacrifice. Alternatively, if the sterile larvae are used by each sex to wage war against the other sex, then it should be primarily females who are interested in killing their brothers. As it happens, most sterile larvae are female, suggesting a primary role in sex conflict", said Andy Gardner, from St John's College, Oxford University.

This discovery also explains the commonly unbalanced sex ratios in these insects, where females are much more numerous than males, due to sex differences in killing behavior.