The Relative Importance of Intra- and Interspecific Competition in Scorpionfly Mating Systems

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Abstract
Little research has examined whether interspecific competition for resources may influence the evolution of resource-based social systems in animals. An experimental field study of adults of two species of co-occurring Panorpa scorpionflies determined the relative importance of intra- and interspecific interactions at food (dead arthropods) on the fecundity, weight changes, and mating system of each species. Panorpa species exhibit resource-defense polygyny that includes not only male-male competition for dead arthropods, which are offered to females in exchange for mating, but also female mate choice and condition-dependent alternative male mating tactics. The three mating tactics used by males are arthropod defense, salivary-mass defense, and forced copulation. P. latipennis and P. mirabilis were studied because their similar ecological requirements (diet, seasonal occurrence, etc.) result in a high degree of interspecific interaction in nature and because the latter species is strongly dominant over the former in fights for dead arthropods. I predicted that interspecific competition would occur, but that its effects would be strongly asymmetrical with P. mirabilis having a major effect on the fitness and social behavior of P. latipennis but not vice versa. This expectation was met. Intraspecific competition plays a much more important role in P. mirabilis than in P. latipennis, whereas P. latipennis individuals are dramatically affected by the presence of P. mirabilis. First, P. mirabilis had negative effects on female fecundity and on body weight of both sexes of P. latipennis. Second, P. mirabilis reduced the ability of P. latipennis males to adopt resourse-defense (arthropod and saliva) mating tactics, which led to males'' employing more forced copulation. As a result, the contribution to mating of forced copulation was higher than when P. latipennis was not in the presence of P. mirabilis. This is the first study to show that heterospecifics can influence the use of alternative male mating tactics and the relative contribution of the alternatives to mating success. Third, this is the first study to show that heterospecifics can influence female choice. P. latipennis females relax their mate-choice standards in the presence of P. mirabilis. Large body size was a major determination of male and female success in sexual competition, as well as of female success in egg production. Large body size was more important in P. latipennis when it was competing with P. mirabilis than when it was competing only with conspecifics. The reverse was the case in P. mirabilis. Exploitative competition plays little if any role in the asymmetrical effects. These effects stem from interference in the form of the greater aggressive dominance of P. mirabilis. I suggest that the use of a more upland habitat by P. latipennis evolved in the context of interspecific competition of P. mirabilis and that habitat separation is the major factor contributing to the coexistence of these two species.