The discussion of homosexuality and inter-gendered sexual activity covers mostly mammals and birds. A large number of zoological studies are cited to argue that such activity is very common, as revealed in controversially operationalised data.

The danger of annthropomorphism is ever present with the use of terms such as 'pleasure', 'courtsthip'. Sexually diverse behaviour in anaimals and humans is seen as driven by biological exuberance, an excess of sexual energy and, when combined with cultural elements, of ingenuity in discharging it. The case is well made that sexual behaviour has been neglected in animal studies, where old binaries and prejudices have prevailed, and that sexual diversity might have played a key role in the development of social and linguistic capacities.