Reduction in last male sperm precedence caused by competitive assault on a mating male in Gastrophysa viridula, a highly polygamous leaf beetle

Abstract

A possibly novel aspect of direct male-male competition found in the dock beetle, Gatrophysa viridula is addressed. In this beetle, intra-sexual competitive tactics include persistent interruption and breaking of rival matings but with no subsequent takeover of the freed females. This situation creates a question about the ultimate value of male assaults, especially about their influence on the sperm precedence of both attacking and attacked males. Sperm precedence (progeny sired) of both undisturbed and attacked males was estimated by the irradiated male method. Sperm precedence of the last copulating, undisturbed male (Plast) was estimated as 0.45 (± 0.37 SD), provided ejaculation competitiveness of both irradiated and control males was equal. Comparison of sperm precedence resulting from undisturbed and assailant-disturbed copulation, revealed a 30% negative influence of competitor intrusion on the sperm priority of a mating male. This effect persists throughout the next 7 days with an average of 8.7 egg batches laid per female during this period. Two suspected genetic pay-offs of these energy and time-consuming assault tactics are considered. The first is related to the release of an extra individual to the pool of sexually available females. The second possibility is based on the promotion of the attacker's sperm already present within a female. According to a theoretical estimation, an attack on a mating male would contribute to the protection of the attacker's paternity from the next deposited batch in (1) 3.1, (2) 1.7 and (1) 1.0 eggs on average, if the attacking male had been, respectively, (1) the one who first copulated with that same female before the males attacked (2) the one before last (3) the second before last to copulate with the mating female (provided that there is stable sperm priority and limited migrations of beetles).