Interspecific Contests Between Males of Two Japanese Stag Beetle
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Behaviour 150 (2013) 39–59 brill.com/beh Interspecific contests between males of two Japanese stag beetle species, Lucanus maculifemoratus and Prosopocoilus inclinatus: what overcomes a body size disadvantage? Yoshihito Hongo ∗ and Kohei Okamoto Department of Zoology, Graduate School of Science, Kyoto University, 606-8502, Sakyo, Kyoto, Japan *Corresponding author’s e-mail address: [email protected] Accepted 3 November 2012 Abstract In intra- and interspecific interactions, generally, body size differences between contestants are important, with larger animals being superior competitors. The males of two species of Japanese stag beetles, Lucanus maculifemoratus and Prosopocoilus inclinatus, have intrasexually selected large mandibles. Because these two species are sympatric and feed on the same food, winning in- terspecific competitions might have considerable effects on male fitness with regard to access to limited resources. If the outcome of interspecific competitions between these two species of beetles is determined by body size, the advantage would lie with the larger species, L. maculifemoratus. To test this prediction, we examined the behaviour and outcome of contests between these two species in detail. Contrary to our expectations, the larger size of the male L. maculifemoratus did not provide a competitive advantage against P. inclinatus. The higher winning rate of P. inclinatus was attributable to differences in mandible use: male L. maculifemoratus nipped the opponent’s dorsal side and threw it, whereas male P. inclinatus nipped both ventral and dorsal sides. The prox- imate cause of this difference was the interspecies difference in tactile stimulation to the mandible: a tactile stimulus on the underside of the mandible resulting in mandible-nipping behaviour was observed in both species, whereas a tactile stimulus to the upper side of the mandible resulting in mandible-nipping behaviour was observed only in P. inclinatus. The present study provides an im- portant counter-example to the general belief that larger species have an advantage in interspecific contests involving physical combat. Keywords interspecific competition, male contest behaviour, releaser for mandible nipping, stag bee- tle, Lucanus maculifemoratus, Prosopocoilus inclinatus. © 2013 Koninklijke Brill NV, Leiden DOI:10.1163/1568539X-00003036 40 Interspecific competition between two stag beetles 1. Introduction In nature, many animals compete for limited resources, such as food and mates. Most intraspecific contests for resources are ritualised, and often pro- ceed through several stages of visual displays before engaging in physical combat in order to reduce the possibility of physical injury to contestants (Clutton-Brock et al., 1982). To elucidate the evolutionary mechanism of animal conflict, Maynard Smith & Price (1973) developed several theoreti- cal models, collectively termed ‘game theory’. According to these models (Maynard Smith, 1974, 1982; Maynard Smith & Parker, 1976), the out- comes of contests are expected to be determined by the differences between opponents’ competitive abilities (resource-holding potential, RHP), owner- ship status, and by the value attached to the resource (RV) by contestants. When two contestants differ in these attributes, the asymmetries, if perceived by contestants, should be used to settle the contest. In consequences, esca- lated fighting should only occur when differences between the contestants are small (Parker, 1974; Enquist & Leimar, 1983). Empirical studies of many animals demonstrate that aggressive interac- tions influence access to habitats and resources (Greenberg & Ortiz, 1994; Downes & Bauwens, 2002). The outcomes of intraspecific competitions of- ten depend on factors such as body size and prior residency (Jackson & Cooper, 1991). In general, differences in body size play the most important role, with larger animals being superior competitors in situations involving interference competition (Dodson & Schwaab, 2001; Robinson-Wolrath & Owens, 2003). The same phenomenon could occur in interspecific competi- tion. Several studies have shown that in interspecific competition over similar limited resources, larger species often drive smaller ones out of the same niche (Bovbjerg, 1970; Langkilde & Shine, 2004). Therefore, it has been generally considered that outcomes of actual contests between heterospe- cific individuals are determined by body size (Losos, 1996; Righetti et al., 2000). However, body size difference is not always an important factor in in- terspecific contest resolution. For example, in several species of ungulates, such as fallow deer, roe deer and red deer, smaller species compete with larger species by using greater aggression and/or a larger group size (Alt- mann, 1956; Bartos et al., 1996). Similarly, smaller individuals of the cray- fish species Orconectes immunis beat larger individuals of a closely related species, O. limosus, by adopting aggression tactics in competition for shelter.