1. ¹Behavioural Ecology Research Group, Department of Biological Sciences, Simon Fraser University, Burnaby, BC, Canada V5A 1S6
2. ²School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney NSW 2052, Australia

Correspondence: A Lindholm, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney NSW 2052, Australia. E-mail: anna.lindholm@unsw.edu.au

Received 13 January 2003; Accepted 10 June 2003; Published online 21 January 2004.

Abstract

Males of the livebearing fish, Poecilia parae, exhibit one of the most complex polymorphisms known to occur within populations, whereas females are monomorphic. We describe five distinct male colour morphs and an associated size dimorphism, and demonstrate through pedigree analysis that the locus or loci controlling the male colour polymorphism is linked to the Y-chromosome. Field surveys from 1999 to 2002 of nine populations in Guyana and Suriname, South America, indicate that some morphs are consistently abundant and others are rare, implying that the colour polymorphism has important fitness consequences. By rearing offspring of field-inseminated females, we showed that the common morph is also the most successful morph in terms of reproduction. However, dichotomous choice tests show that two rare morphs are preferred by females over the common morph. These results suggest that alternative male mating strategies, sperm competition, overt male–male competition, or other processes are overriding female preferences in these populations. Furthermore, Y-linkage of the colour polymorphism in P. parae supports the hypothesis that heterogametic sex chromosomes harbour sexually antagonistic traits beneficial to the heterogametic sex.