Dominance and reproductive success among slave-making worker ants

Abstract

Dominance hierarchies are of great theoretical interest because they result from a compromise between selfish and social behaviour. In a wide variety of vertebrates dominance promotes individual fitness¹, but often to the detriment of the group as a whole. For example, in gelada baboons, females at the top of the hierarchy produce more offspring because their ovulations are less frequently inhibited and they have fewer abortions than their more harassed subordinates². This type of selfish behaviour is perhaps least expected within ant colonies because their workers are exceptionally closely related³ and worker inclusive fitness is largely determined by the productivity of the colony as a whole⁴. For this reason our discovery of primate-like linear dominance hierarchies within colonies of slave-making ants uniquely demonstrates the importance of individual selection in social evolution. High ranking slave-maker workers receive more food from their slaves, have greater ovarian development and do less foraging than their subordinate sisters. This is also the first demonstration that ant queens can recognize the rank of their workers. Experiments show that the queen solicits food preferentially from the most dominant of her workers, possibly to limit their production of eggs. We explain these phenomena in terms of competition for the production of sons.