J Ethol (2006) 24:27–31 © Japan Ethological Society and Springer-Verlag Tokyo 2005 DOI 10.1007/s10164-005-0156-4 Springer-VerlagTokyohttp://www.springer.de101640289-07711439-5444Journal of EthologyJ EtholLifeSciences15610.1007/s10164-005-0156-4 Influence of male competition on male mating behaviour in the cave molly, Poecilia mexicana ARTICLE Received: November 1, 2004 / Accepted: February 9, 2005 / Published online: April 16, 2005 Rüdiger Riesch • Ingo Schlupp • Martin Plath R. Riesch (*) · M. Plath Biozentrum Grindel, Universität Hamburg, Martin-Luther-King Platz 3, 20146 Hamburg, Germany Tel. +49-40-428383874; Fax +49-40-428383937 e-mail: ruedigerriesch@hotmail.com I. Schlupp Department of Zoology, University of Oklahoma, 730 Van Vleet Oval, Norman, OK 73019, USA Abstract In many species, male mating behaviour is corre- lated with male body size, with large males often being preferred by females. Small surface-dwelling Poecilia mex- icana males compensate for this disadvantage by being more sexually active and using sneaky copulations. In a cave-dwelling population, however, small males do not show this behaviour. Do small males alter their behaviour in the presence of a large rival? Here, we investigated the influence of male competition on male mating behaviour in the cave form. Two males of different sizes were mated with a female either alone or together with the other male. No aggressive interactions were observed between either fish. There was no statistically significant difference in the fre- quency of sexual behaviours between the two treatments. In both treatments, large males were more sexually active than small males. Thus, small cave molly males do not switch to an alternative mating behaviour in the presence of a larger rival. Possibly, the extreme environmental conditions in the cave (e.g. low oxygen content and high levels of hydrogen sulphide) favour saving energetic costs, resulting in the absence of alternative mating behaviour in small males. Key words Cave fish · Male competition · Mating tactics · Poeciliidae Introduction Many live-bearing tooth carps (Poeciliidae) show a strong polymorphism in male body size (Snelson 1989). Such pro- nounced size polymorphism among males is also found in Atlantic mollies (Poecilia mexicana), where adult males range between 18 and 70 mm standard length (Menzel and Darnell 1973; Heubel, unpublished data). In some closely related species, e.g. the genus Xiphophorus, size polymor- phism has been shown to have a genetic basis (Kallman and Borkoski 1978; Ryan et al. 1992), but little is known about the heredity of male size in P. mexicana. In a variety of species, including many fishes, the mating behaviour of males co-varies considerably with body size, with large, dominant males often being preferred by females (Andersson 1994). To compensate for this, males that are not preferred by females often use “alternative” behaviour, such as sneaky copulations, sexual mimicry, or they may act as satellites (Dominey 1984; Gross 1984, 1996; Taborsky 1994). Also in poeciliids, male mating behaviour and body size are often strongly correlated, especially in swordtails (Xiphophorus spp.) and mollies (Poecilia: Mol- lienesia spp.; Parzefall 1969; Hughes 1985; Woodhead and Armstrong 1985; Farr et al. 1986; Farr 1989; Ryan and Causey 1989; Travis and Woodward 1989; Zimmerer and Kallman 1989; Ryan et al. 1992; Bisazza 1993; Travis 1994). In sailfin mollies (Poecilia latipinna), for example, large males use their enlarged sail-like dorsal fin for visual court- ship displays in front of females (Parzefall 1969; Ptacek 2002). Small males, in contrast, lack an enlarged dorsal fin and never court (Parzefall 1969). Instead, they rely on high numbers of coercive mating attempts to achieve mating success (Parzefall 1969; Schlupp et al. 2001). In the Atlantic molly (P. mexicana), large males are intensely coloured and aggressively defend territories comprising shoals of females against small males, which attempt sneak-like copulations (Parzefall 1969). All males show little or no courtship behaviour (Parzefall 1969; Ptacek 2002), and mating is exclusively initiated by males, leading to a male-biased, coercive mating system. So far, no data indicate that P. mexicana females advertise their readiness to mate, as is known from the related guppy, P. reticulata (Houde 1997). Nevertheless, Atlantic molly females readily choose between males of different sizes and strongly prefer to asso- ciate with large males (Plath et al. 2004a). Similarly to the sailfin molly, small Atlantic molly males show more sexual