Social Brains and Behaviour in cooperative insect societies

Abstract

The inherent cognitive challenges faced by animals that live in cooperative groups have led to several hypotheses about how sociality may influence brain evolution and vice versa. The social brain hypothesis posits that the cognitive demands of living in cooperative societies have had a causal effect on the evolution of large and/or increasingly elaborate brains in certain animal groups. Evidence exists both for and against the social brain hypothesis as a valid mechanism in taxa that have been studied to date, including primates, ungulates, mammalian carnivores, birds, fish and even the tiny brains of social insects, historically thought to be lacking the capacity for higher level information processing. In many cooperative societies social living also necessitates specific cognitive abilities that influence behaviour, such as assessment of another’s physical condition in the initial formation of dominance relationships, and associative memory and recognition in established groups. In this thesis I present studies on both solitary and primitively eusocial wasps in which I investigate the relationship between sociality or cognitively demanding life histories and cognitive evolution. Firstly, in chapter 2 I investigate the hypothesis that a more cognitively demanding breeding strategy seen in several wasp species in the genus Ammophila leads to enlarged brain mushroom body structures relative to species which practice a simpler strategy, while also developing methods for using novel imaging techniques on such small brains. In chapter 3 I present evidence which directly contradicts the social brain hypothesis by showing that solitary foundresses in a wild population of the paper wasp Polistes dominulus display larger brains, larger mushroom body substructures and larger antennal lobes than social foundresses. In chapter 4 I show that subordinate P. dominulus females who are promoted to the dominant position become temporarily hyper aggressive towards lower ranking nestmates, and in chapter 5 I present evidence which rejects the hypothesis that Spanish P. dominulus populations use conspicuous facial markings as signals of physical quality. Each of these results are discussed in the context of the evolution of cognitive mechanisms for mediating behaviour in social insects. Overall, these findings suggest that social living does not present particularly high cognitive demands on insect brains in terms of production of significant amounts of neural tissues, but certain necessary abilities do likely place lower demands which are beyond the reach of the methods used in this work.