Testing optimal defence theory in a social insect: Increased risk is correlated with increased venom investment

Abstract

Abstract Investment in defence may not be uniformly beneficial across an organism's lifespan. Risk, and therefore potential payoff of defence, may change with age and behaviour, but for colony‐living organisms, it may also change with colony size and reproductive stage. The acorn ant Temnothorax longispinosus uses venom to defend against socially parasitic ants that raid their nests to steal brood. We investigated the idea that T. longispinosus adjust their venom allocation in accordance with raid risk. We tested the predictions that T. longispinosus ants should produce more venom when raids are most likely to occur and during the parts of the nest reproductive cycle when the potential fitness loss per raid is highest. We also asked whether venom volume varies between nurses and foragers within a colony, which have different potential risk levels, and whether this difference increases with colony size. We found that workers had more venom in the summer, both before and during the period when raids occur, than in the fall when pupae were no longer present in nests. Workers engaging in nursing behaviours had more venom as the pupa to worker ratio in the nest increased, indicating that nurses invest more in venom as the relative number of pupae requiring defence increases. In addition, the difference in venom volume between nurses and foragers grew with colony size. These results provide observational support for the hypothesis that individuals vary their investment in venom over their colony's development in conjunction with the risk of social parasitism.