The Effect of Social Environment on Male–Male Competition in Guppies (Poecilia reticulata)

There are two general mechanisms through which sexual selection can operate: mate choice and competition for mates. In some cases, mate choice and competition can act in a complementary fashion, selecting for the elaboration of the same traits, but this is not always true; in some cases, male–male competition can subvert active female choice, hence changing the path of sexual selection (Qvarnstrom & Forsgren 1998; Moore & Moore 1999). Work in sticklebacks (Gasterosteus aculeatus) shows another potential interaction of male–male competition and female choice, where female assessment of male– male competition can reinforce a female’s initial choice (Candolin 1999). For this reason it is important to understand the interacting, and potentially opposing, roles played by the two processes of sexual selection. In general, there has not been much discussion about variation in relative roles of these two mechanisms. However, several studies have now documented how population-specific factors can alter mating systems. For example, in the sand goby (Pomatoschistus minutus), the importance of male–male competition increases relative to that of female choice in populations where nesting resources are rare (Forsgren et al. 1996; Svensson & Forsgren 2003). In the collared lizard (Crotaphytus collaris), the intensity of both intraand intersexual selection on size and colouration varies across populations based on the potential for females to gain access to and assess multiple males (Baird et al. 1997); females in populations that inhabit areas with a relatively high density of suitable areas for courtship displays are choosier, and males from these populations are more aggressive in intrasexual competition. The guppy (Poecilia reticulata) has become a model system for the study of sexual selection (Houde 1997). It is largely accepted that intersexual selection, where females select males on the basis of colouration, size and courtship displays, plays a very important role in Correspondence Anna C. Price, Department of Zoology, University of Toronto, 25 Harbord St., Toronto, ON, Canada M5S 3G5. E-mail: [email protected]