Rank and colony defense against conspecifics in a facultatively eusocial hover wasp

An important benefit of social living is increased capacity for defense. Highly eusocial species have often evolved a fighting caste for this purpose, but many facultatively eusocial insects and cooperatively breeding vertebrates lack morphological castes and the decision to defend or not can depend on costs and benefits to each individual. Defense by subordinates in a social group can be regarded as a form of helping, and helping input often varies among subordinates of different age or size. Several hypotheses attempt to explain variation in helping effort, including the effects of relatedness and differences in the costs of helping. Evidence for these hypotheses is mixed and often lacks data on the rank of subordinates, an important determinant of expected future fitness. We examined individual variation in propensity to defend the nest against conspecifics in the tropical hairy-faced hover wasp Liostenogaster flavolineata. Prior to experimentation, we determined the positions of all wasps in the age-based queue to inherit the single egg-laying position in each L. flavolineata group. Two approaches were then used: observations of defense against natural intrusions by conspecifics and experimental trials where wasps were presented attached to a wire. Higher ranks were more likely to defend the nest than lower ranks, opposite to the pattern previously documented for another form of helping: foraging effort. Possible explanations for this result are that higher ranked females are better defenders and that they suffer a larger decrease in expected future fitness when an intruder usurps their position in the inheritance queue.