Interspecific territoriality in Calopteryx damselflies: the role of secondary sexual characters KATJA TYNKKYNEN, JANNE S. KOTIAHO, MARI LUOJUMA ¨ KI & JUKKA SUHONEN Department of Biological and Environmental Science, University of Jyva ¨skyla ¨, Finland (Received 27 July 2004; initial acceptance 15 September 2004; final acceptance 24 March 2005; published online 18 January 2006; MS. number: 8220R) Interspecific territoriality is usually interpreted to result from interspecific interference competition, al- though it may also originate from mistaken species recognition. In the latter case, it may be based on sim- ilarity of secondary sexual characters. In the damselfly Calopteryx splendens, males have pigmented wing spots as a sexual character, and males with the largest spots resemble males of another species, Calopteryx virgo. Probably because of this resemblance, C. virgo males are more aggressive towards large- than small- spotted C. splendens males. We examined whether wing spot size of C. splendens males affects territorial in- teractions between the species. In a removal experiment, the number of territorial C. splendens increased after C. virgo males were removed. However, interspecific territoriality was incomplete since before the re- moval the territories of the species partially overlapped. Wing spot size of C. splendens seemed to affect in- terspecific territoriality: before the removal territorial and nonterritorial C. splendens males had similar spot sizes and the distance to the nearest C. virgo territory increased with wing spot size of C. splendens. In ad- dition, after the removal of C. virgo, the relation between wing spot size and the distance to the nearest old C. virgo territory disappeared. Our results suggest that mistaken species recognition may account for the interspecific territorial behaviour in the two species. Furthermore, interspecific territoriality may cause neg- ative selection on wing spot size and thus may explain character displacement in wing spot size of C. splen- dens males. Ó 2005 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. In interspecific territoriality, which is commonly observed in nature (e.g. Murray 1971, 1981, 1988; Sorjonen 1986; Singer 1989; Robinson & Terborgh 1995; Nomakuchi & Higashi 1996; Griffis & Jaeger 1998; Marvin 1998; Genner et al. 1999), individuals are aggressive not only against conspecifics but also against heterospecifics, leading to a situation where individuals of two or more species have mutually exclusive territories. Such a pattern may re- sult from interspecific interference competition over re- sources, where individuals defend their territories against ecologically similar species (e.g. Reed 1982; Catchpole & Leisler 1988; Robinson & Terborgh 1995; Genner et al. 1999). However, interspecific territoriality may also be a consequence of interspecific aggression based on pheno- typic similarity of species (Murray 1971, 1981, 1988; see also Nishikawa 1987). If two species resemble each other, intraspecific aggressive behaviour related to territorial de- fence can mistakenly be directed towards heterospecific individuals, leading to interspecific territoriality (Murray 1971, 1981, 1988; Nishikawa 1987; Alatalo et al. 1994). Interspecific territoriality that has been related to in- terspecific interference competition has been observed in many taxa, such as birds (Reed 1982; Loyn et al. 1983; Sherry & Holmes 1988; Robinson & Terborgh 1995), mammals (Wolff et al. 1983), amphibians (Griffis & Jaeger 1998; Marvin 1998; see also Nishikawa 1987), fish (Kohda 1981; Genner et al. 1999) and insects (Nomakuchi & Higa- shi 1996). For example, in cichlid fish (Pseudotropheus spp.) males do not tolerate in their territories heterospe- cific intruders that use the same resources, leading to in- terspecific territoriality among ecological competitors (Genner et al. 1999). In contrast, there are only a few ex- amples of interspecific territoriality arising from mistaken species recognition. Possible cases are observed at least in birds (Murray 1971, 1981, 1988; Alatalo et al. 1994), am- phibians (Nishikawa 1987) and insects (Singer 1989). Different species can have similar secondary sexual characters (Alatalo et al. 1994; Sætre et al. 1997; Pfennig 1998, 2000). In such cases, interspecific territoriality and interspecific aggression arising from mistaken species rec- ognition may influence the evolution of sexual characters Correspondence: K. Tynkkynen, Department of Biological and Environ- mental Science, P.O. Box 35, FIN-40014, University of Jyva ¨skyla ¨, Fin- land (email: katynkky@bytl.jyu.fi). 299 0003–3472/05/$30.00/0 Ó 2005 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2006, 71, 299–306 doi:10.1016/j.anbehav.2005.03.042