Homosexual Behaviour in Animals: An Evolutionary Perspective

Volker Sommer & Paul L. Vasey, Eds. (2006)
Cambridge University Press
ISBN 0521864461 (hardback) £70.00 / $125.00

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Both field and captive behavioural observations indicate that same-sex sexual interactions are widespread throughout the animal kingdom, occurring in more than 450 different species and in every major animal group (Bagemihl 1999), including primates (Vasey 1995). While proximate studies have yielded important insights into the hormonal and neural mechanisms underlying same-sex behaviour (e.g. Adkins-Regan 1988; 1998, Adkins-Regan et al. 1997, Paredes & Baum 1997), research into homosexual behaviour from an evolutionary perspective is relatively scarce. Homosexual Behaviour in Animals seeks to redress this imbalance, and to address the problem that same-sex sexual interactions pose for Darwinian theory: If adaptiveness is measured by reproductive success and homosexual behaviour is not reproductive, then how did such behaviours come about?

The introduction by Vasey & Sommer begins with a history of observations and explanations of same-sex behaviour in animals, and possible reasons behind the lack of research into the subject. They examine definitions of “homosexual behaviour” and develop an analytical framework for the understanding of evolutionary causes of such behaviour. They then review hypotheses that have been proposed to explain the evolution of homosexual behaviour and use contributions to the volume to illustrate their critique of hypotheses that concern the ultimate causes, or function, of such behaviour. Part II presents data for a variety of non-primates that routinely engage in homosexual behaviour, including birds (Chapter 2, Kotrschal et al: greylag geese; Chapter 3, King: flamingos), cetaceans (Chapter 4, Mann: bottlenose dolphins), ungulates (Chapter 5, Vervaecke & Roden: American bison; Chapter 6, Bartoš & Holecková: fallow, white-tailed and red deer), and carnivores (Chapter 7, Yamane: feral cats). Part III investigates homosexual behaviour in non-human primates, including Old World monkeys (Chapter 8, Vasey: Japanese macaques; Chapter 9, Kapsalis & Johnson: rhesus macaques; Chapter 10, Sommer et al.: Indian langurs), and apes (Chapter 11, Yamagiwa: mountain gorillas; Chapter 12, Fruth & Hohmann: bonobos). This section concludes with an essay on the evolution of male homosexuality in humans (Werner, Chapter 13). Finally, in Part IV, Vasey (Chapter 14) provides a general discussion with recommendations for future directions in research, while Sommer (Chapter 15) discusses the sociopolitical implications of research into animal homosexuality.

The data included in this volume concern a broad array of species and provide support for a variety of hypotheses concerning the evolution of homosexual behaviour. The editors succeed in demonstrating that homosexual behaviour is an important component of the behavioural repertoire in some species, and that it has important evolutionary implications for social, sexual and reproductive strategies. Understandably, due to the lack of research on this topic, some chapters provide detailed descriptive data. Some data might have been better presented in figures, while case studies could have been illustrated in diagrams, or tabulated in some way. There were also a few places where sentences didn’t seem to make sense and a few typos seem to have crept in (e.g. androgene rather than androgen). I also found it odd that hypotheses and predictions were first mentioned in the discussion in several chapters, when they would have been better stated in the introduction, with the methods and results sections structured around the evolutionary hypotheses to be tested (after all, that’s what the book is hoping to encourage). Finally, as usual with CUP, the book is very expensive (£70), although perhaps there will be a paperback at some stage. However, these minor shortcomings don’t detract from the value of this book, which should stimulate interest in homosexual behaviour in animals, and encourage researchers to investigate the subject further.

JOANNA M. SETCHELL
Department of Anthropology, University College London

Adkins-Regan, E. (1988). Sex hormones and sexual orientation in animals. Psychobiology 16: 335-347.
Adkins-Regan, E. (1998). Hormonal mechanisms of mate choice. American Zoologist 38: 166-178.
Adkins-Regan, E., Mansukhani, V., Thompson, R., and Yang, S. (1997) Development of sexual partner preference in the zebra finch: a socially monogamous, pair-bonding animal. Archives of Sexual Behavior 31: 27-33.
Bagemihl, B. (1999). Biological Exuberance: Animal Homosexuality and Natural Diversity. New York: St Martin;s.
Paredes, R.G., and Baum, M. (1997). Role of the medial preoptic area/anterior hypothalamus in the control of masculine sexual behavior. Annual Reviews of Sexual Research 8: 68-101.
Vasey, P. (1995). Homosexual behavior in primates: a review of evidence and theory. International Journal of Primatology 16: 173-204.