The basis of cowardice in social defenders

Social defenders are the ultimate altruists. However, even among species made up of selfless clones, there is a broad range of defensive risk-taking attitudes, from brave soldiers that seek enemies away from the colony to cowards that defend only occasionally and after the colony has come under direct attack. We model the optimal defensive strategies available to social defenders, using the life history of galling aphid clones as a biological basis. Defence could be augmented by increasing the proportion of soldiers at birth, extending the duration of the soldier stage, and by increasing the risk-taking behaviour of soldiers to shift mortality away from reproductives. Cowardice, defined as a decrease in risk-taking behaviour, emerged when low reproductive rates and longer colony lifetimes made soldiers especially difficult to replace. Unexpectedly, high predation also favoured cowardice because disproportionate elimination of brave soldiers left defenceless colonies that were subsequently destroyed.