Interspecific contests between males of two Japanese stag beetle species, Lucanus maculifemoratus and Prosopocoilus inclinatus: what overcomes a body size disadvantage?
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 interspecific 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 proximate 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 important counter-example to the general belief that larger species have an advantage in interspecific contests involving physical combat.