Abstract
In the polyembryonic wasp Copidosoma floridanum, females commonly lay one male and one female egg in a lepidopteran host. Both sexes proliferate clonally within the growing host larva. Distinct larval castes develop from each wasp egg, the majority being ‘reproductives’ plus some ‘soldiers’ which sacrifice reproduction and attack competitors. Maturing mixed sex broods are usually female biased, as expected when intra-brood mating is common. Pre-mating dispersal followed by outbreeding is expected to increase sexual conflict over brood sex ratios and result in greater soldier attack rates. Owing to sexually asymmetric relatedness, intra-brood conflicts are expected to be resolved primarily via female soldier attack. We observed soldier behaviour in vitro to test whether lower intra-brood relatedness (siblings from either within-strain or between-strain crosses were presented) increased inter-sexual aggression by female as well as male soldiers. As found in prior studies, females were more aggressive than males but, contrary to expectations and previous empirical observations, soldiers of both sexes showed more aggression towards more closely related embryos. We speculate that lower intra-brood relatedness indicates maternal outbreeding and may suggest a rarity of mating opportunities for reproductives maturing from the current brood, which may enhance the value of opposite sex brood-mates, or that higher aggression towards relatives may be a side-effect of mechanisms to discriminate heterospecific competitors.
Highlights
The degree of relatedness between individuals underpins much of social evolution theory [1]
Our data concur with previous reports showing female C. floridanum soldiers are more aggressive than males [4], and are more likely to attack the polymorula of an intra-specific competitor than are males [10]
This is consistent with one function of C. floridanum soldiers being to mediate sex ratio conflict within mixed sex broods in favour of females [2]
Summary
The degree of relatedness between individuals underpins much of social evolution theory [1]. Aggression will more likely occur between more distantly (or negatively) related individuals, even when this behaviour incurs costs to the actor, i.e. spite [2] This is most likely to occur in outbred sub-populations in which relatedness between interacting individuals is low. With reduced dispersal between sub-populations and increased rates of inbreeding, relatedness increases within sub-populations, meaning that aggression rates are predicted to be reduced [2] These predictions are compounded because increased relatedness in sub-populations often increases the level of costs of aggression/altruism associated with competition for local resources [3,4]. This leads to difficulty in differentiating between the effects of these two important predictors of social behaviour
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