Male density, female density and inter-sexual conflict in a stream-dwelling insect

Abstract

Male and female densities were experimentally manipulated to determine their effects on inter-sexual conflict over mating in the water strider, Aquarius remigis. Upon encounter, males and females engage in a pre-mating struggle. The two components of female mating behaviour were the rate of evasive behaviours displayed, termed the index of female resistance, and struggle duration. The behaviour patterns of individuals of each sex changes depending on their previous experience. Females held at the high male density performed fewer resistance behaviours during struggles and struggled longer than did females held at the low male density. Female mating frequency was positively related to male density. A measure of male behaviour (male insistence) was derived from a regression of struggle duration on the index of female resistance. Males held at the high male density had greater insistence than those held at the low male density. Female density had no effect on male insistence. Male mating frequency was not related to male or female density. Overall, male and female body length were related to neither behaviour pattern nor mating frequency, although mated males were significantly longer than unmated males at low male density. The results are consistent with the view that mating frequency depends on female resistance, which varies in response to the costs of struggling that depend upon previous male harassment. Male mating behaviour changed with male density. Males harass both males and females, but females generally stay at the edge or out of the water when males are present. Since encounter rate is consistently low, males assess their probability of mating via their encounter rate with other males. A high male density leads males to increase their effort when they do encounter a female. Female mating behaviour was not related to male phenotype, suggesting that female choice is not present in A. remigis.