Male distribution and interactions at female oviposition sites as factors affecting mating success in the flyDryomyza anilis (Dryomyzidae)

Abstract

Dryomyza anilis males gather around carcasses where females arrive to lay their eggs and mate with them before oviposition. Distribution and mating success of males of different size at these sites were studied. The position of males of different size on the carcass and in ten circular zones around it was recorded every 10 min during 2–3 h observation periods. The position of mating pairs, mating initiations and the number of take-overs in different zones were also recorded. It was expected that males would distribute themselves according to the ideal free distribution, i.e. their gain rate in different zones in terms of number of mating initiations would be equalized. Males in different zones reached a stable distribution after 30 min from the beginning of observations. Although males, females and mating pairs were distributed over several zones, most matings were started and take-overs took place on the carcass or in the first zone (C+Z1) because arriving females usually reached the centre before being intercepted by a male. Therefore, males in C+Z1 attained a much higher mating rate than males further away: the percentage of matings started there was significantly higher than the percentage of males present there at the same time. The frequency of male-male fights was highest in C+Z1 which is likely to prevent more males from entering the central area. In this respect the observed male distribution bears resemblance to the ideal despotic distribution. However, the distribution of males of different size did not differ, and there were many small males in C+Z1. Small males in C+Z1 had a higher likelihood of starting a mating than small males further away but suffered from frequent take-overs like small males in all zones. For example, small males started 38% of all matings but there were only 8% of them among males guarding an ovipositing female. The corresponding figures for large males were 35% and 77%, respectively, suggesting that kleptoparasitic interactions and not aggressions between males were the main reason for differences in mating success between males of different size. Since small males do not have any alternative mating sites where large males would be absent, being the second last male to mate with a female is the best small males can achieve. Large males may even benefit from the presence of small males in the centre: they intercept females which large males can then easily take over.