Environmental constraints on fighting in flightless male fig wasps

Abstract

Two evolutionary models, one predicting reduced fighting costs with increased relatedness between rival males, and the other predicting fighting costs as a function of the interval between challenges for mating opportunities, were tested with data from Malaysian fig wasps. The cost of fighting was assessed by numerical rating of injuries, enabling an index of fighting severity to be calculated for each of 25 species. Male relatedness was determined indirectly through the degree of bias in the sex ratio. It was assumed that the rate of challenges for mating opportunities with a receptive female was related to male density. Contrary to prediction, fig wasp species with female-biased sex ratios, and therefore high relatedness, did not have reduced injury levels. The lack of an agreement was attributed to a difference in the way fighting costs are allocated in evolutionary models and the way they are obtained in real populations. When fighting costs are associated with an increased variance in male reproductive success without a decrease in average reproductive success, as they are in fig wasps, then high levels of relatedness may not lead to less intense fighting. A curvilinear relationship, with a peak in the cost of fighting at an intermediate density of males was predicted. Among fig wasp species, injury level declined with increasing density of males per fig-receptacle (syconium). Fig wasp species that entered the syconium to oviposit had high male densities and particularly low injury scores. It is suggested that species with this mode of life history were predisposed, by the associated high density of males, to the evolution of intense sperm competition and low injury levels. The injury level also declined when the density of males was low, as predicted, but the trend was apparent only between syconium-populations within species.