Sexually reproducing animals are faced with various challenges in order to find, approach and copulate with a mate in reproduction, achieve fertilisation and finally ensure that the offspring survive and in turn reproduce. In social species, this whole process includes additional dimensions involving sometimes intricate relationships of competition and cooperation; further contributing to the overall complexity. Reproduction is central to the survival of the lineage, its suppression leads to extinction. As a consequence of this basic tenet of evolutionary biology we are obviously bound to be puzzled by the emergence of behaviours such as same-sex sexual intercourse, and by the occurrence of exclusively homosexual individuals. Homosexuality is indeed an evolutionary paradox, but one that can be resolved within the broad framework of the theory of evolution itself, after we take into account the many variables and scenarios that make homosexuality likely to be expressed in the first place and then maintained across generations.

In this book the reader will discover that we humans are not the only mammal species that expresses exclusive homosexuality and that some of the evolutionary processes that may explain the emergence of homosexual attraction in humans may be common to other sexually reproducing species as well. More importantly, it will be also shown that across taxa same-sex sexual behaviour is pervasive in the context of bisexuality and its expression takes modalities that can blend in the same individual, whether male or female, characteristics that are both feminine and masculine.