Behavioural Processes 51 (2000) 135 – 147 Sex-role reversal in vertebrates: behavioural and endocrinological accounts Marcel Eens *, Rianne Pinxten Department of Biology, Uniersity of Antwerp, U.I.A., Uniersiteitsplein 1, B-2610 Wilrijk Belgium Received 23 December 1999; received in revised form 20 May 2000; accepted 23 May 2000 Abstract Sex-role reversal occurs when females compete more intensely than males for access to mates. In this paper, we survey the occurrence of sex-role reversal in vertebrates: we focus on behavioural aspects of sex-role reversal and we examine possible endocrinological correlates of this phenomenon. The best documented cases among vertebrates of sex-role reversal occur in fish and birds. In nearly all sex-role reversed species or populations, females have higher potential reproductive rates than males. Some species in which females were previously thought to be the predominant competitors for mates (for instance seahorses and a dendrobatid frog), appear not to be sex-role reversed according to recent studies. The endocrinology of sex-role reversal has been studied in only a few species and therefore remains poorly understood. In birds, which probably have been studied the most in this respect, steroid hormones appear to follow the typical ancestral conditions (for instance no reversal of testosterone levels) in sex-role reversed species, whereas prolactin, a principal regulator of the onset and maintenance of incubation, departs from the usual avian pattern in that it is higher in males than in females. The study of sex-role reversed behaviour offers unique opportunities not only to test sexual selection theory, but also to enhance our understanding of the neuroendocrine mechanisms mediating behavioural sex differences. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Aggression; Female – female competition; Parental investment; Sex-role reversal; Sexual selection; Testosterone www.elsevier.com/locate/behavproc 1. Introduction Since Darwin (1871) proposed the concept of sexual selection to explain the evolution of sex differences, there have been impressive empirical and theoretical advances in this field (Andersson, 1994; Bateman, 1948; Clutton-Brock and Parker, 1992; Emlen and Oring, 1977; Trivers, 1972). Al- though some degree of competition for mates is common in both sexes of most species, the pre- dominant pattern in animals is that of males competing more intensely for mates than females (Andersson, 1994). In many animals, males also develop secondary sexual characters such as con- spicuous colours, exaggerated ornaments, or men- acing weapons. In general, female reproductive success is limited by gamete production, whereas * Corresponding author. Tel.: +32-3-8202284; fax: +32-3- 8202271. E-mail addresses: pintee@uia.ua.ac.be (M. Eens), pin- tex@uia.ua.ac.be (R. Pinxten). 0376-6357/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII:S0376-6357(00)00124-8