Fluctuating asymmetry and the mating system of the Japanese scorpionfly, Panorpa japonica

Abstract

Fluctuating asymmetry is defined as small, random deviations from perfect bilateral symmetry in a morphological trait. Although fluctuating asymmetry has been of interest to population and conservation biologists for decades because it is often negatively correlated with heterozygosity, growth, fecundity and survival, fluctuating asymmetry's role in sexual selection has only recently been investigated. In fights for nuptial food items between adult males of the Japanese scorpionfly, Panorpa japonica, under field conditions, fluctuating asymmetry of forewing length is significantly less in winners than in losers. Also, mating males in nature have relatively low fluctuating asymmetry compared with non-mating males. Males that lose competition for nuptial gifts become satellites of males with gifts, and the losers attempt forced copulation but have very low mating success relative to gift-holding males because females prefer to mate with gift-holders and actively avoid satellites. Under semi-natural cage conditions, fluctuating asymmetry and both male mating success and survival are significantly negatively correlated; the same correlation was found in caged females for survival but not for mating success. The fluctuating asymmetry and male body size relationship differs markedly between two populations. In one population, males show the relationship that is often found in studies of fluctuating asymmetry, that is, the largest and smallest individuals have maximum fluctuating asymmetry, and individuals of intermediate size have the least fluctuating asymmetry. In the second population, males of all body sizes show similar variation in fluctuating asymmetry. Inbreeding, parasites and/or physical conditions may play a causal role in variation in fluctuating asymmetry within and between populations of P. japonica.