Effect of size on fighting and mating in a brachyuran crab with female-biased size dimoprohism, Ilyograpsus nodulosus(Macrophthalmidae)

Abstract

The effect of body size on fighting and mating were examined in both sexes of the rare, Japan-endemic crab Ilyograpsus nodulosus (Sakai, 1983) (Macrophthalmidae), a species in which females are larger than males (female-biased size dimorphism). We found that male-male fights were more common than female-female fights; and that fights between males that were closely size-matched were more common than those between males that were disparate in size. When there was a small size difference between the fighting males, larger and smaller crabs were equally likely to win. When there was a large size difference between the fighting males, the larger crab was more likely to win. Neither male nor female body size predicted the occurrence of copulation. In copulating pairs, male size was positively correlated with female size. Copulation duration decreased with female size. In a female choice experiment, female more often copulated with the larger of the two presented males. In a male choice experiment, males had no preference for the larger or the smaller female. When a single female was placed in a tank with four different sized males, males of all sizes first approached the female and males of all sized copulated, indicating that there was no advantage to large male size in scramble competition. There appears to be a small overall benefit to being a large female and no benefit to being a large male in this species. Male mating success appears to be based on encountering females rather than being successful in male-male competitions or being preferred by females.