The evolution of same-sex sexual behaviour in non-human animals

Same-sex sexual behaviour (SSB) is a diverse group of behaviours, both in the ways that animals can express them, and in their widespread taxonomic presence. There are several hypotheses surrounding the origins and maintenance of these behaviours, and in this thesis I investigate these using both a model system and broader phylogenetic comparisons. I explore the existence of female-female social and sexual behaviours in a tractable, laboratory-based field cricket system (Teleogryllus oceanicus) and find evidence for genotype-by-environment interactions. In eight different species of field cricket (Gryllus spp.), I then assess trade-offs between courtship and aggression in male-male interactions, finding that aggression is reduced by the expression of courtship and other same-sex sexual behaviours. This chapter calls into question the common usage of the “mistaken identity” hypothesis in insects, which assumes that insects that engage in SSB are doing so because they cannot discern male from female interacting partners. Then, I constructed, researched, populated, and curated a database of all reliable records (over 2,000 in total) of non-human animal SSB in the scientific literature using best-practice meta-analytic procedures. With this, I compare trends in the evolutionary dynamics of same-sex sex, courtship, pair-bonding, and parental care across taxa, with special attention to mammals and birds. This chapter shows that different component behaviours of SSB have experienced different evolutionary histories, as well as widespread taxonomic and intersexual variation. This showed that SSB is not a monolith: its evolutionary history across animals is as diverse as the behaviour itself, potentially reflecting independent evolutionary origins and maintenance under diverse functional pressures. Finally, I utilised the same database to assess the impact of paid access to research on meta-analytical, comparative, and systematic research, showing that less well-funded or well-resourced researchers are at risk of coming to the wrong conclusions due to a lack of data availability. The work in this thesis emphasises the relevance of the social environment in determining patterns of SSB in insects and creates a framework for future research into SSB that is based on its diversity and unique evolutionary histories.