Components of male fitness in relation to body size in Epirrita autumnata (Lepidoptera, Geometridae)

Abstract. 1. The effect of body size on different components of male fitness was studied for Epirrita autumnata , a geometrid known for its eruptive population dynamics. Body size is the main determinant of female fecundity in this species.
2. Longevity of males was found to have a weak negative correlation with body size at low temperatures. No significant correlation was found at higher temperatures.
3. We found no correlation between male size and female fecundity or egg size which is consistent with the small size of spermatophores in this species.
4. Small and large males were equally successful when allowed to compete for females in laboratory conditions.
5. In one or two field collections, males found mating were larger than males found singly. Large males also had an advantage in finding of virgin females, offered experimentally. No size-assortative mating was recorded.
6. We conclude that size-dependent mate location ability is the factor accounting for most of the variance in male fitness in E.autumnata. The dependence of fitness on body size may well be equally strong in males and females.