EXPERIMENTAL STUDIES ON WATER STRIDER MATING DYNAMICS - SPATIAL VARIATION IN DENSITY AND SEX-RATIO

We used field surveys, field experiments and experiments in artificial pools to study the effects of variation in sex ratio and density on m ating dynamics of a stream water strider, Aquarius remigis. Our field survey documented the existence of ''hot spots'', sites of higher than average total gerrid density, a male-biased sex ratio, and higher tha n average female mating activity. Female gerrids frequently changed si tes, usually moving upstream, perhaps to spread their eggs among many sites. Male gerrids showed two movement strategies: some males frequen tly changed sites, while other males were stationary at hot spots. Sur prisingly, smaller males tended to be stationary at hot spots. A field manipulation of the availability of refuges for females to avoid hara ssment by males supported the notion that males prefer hot spots becau se they are sites where a scarcity of refuge for females makes it rela tively easy for males to intercept females. Experiments in plastic poo ls compared sites with 20 males:5 females (simulating hot spots) to po ols with 5 males:5 females. The rate of male harassment of females was higher in 20:5 pools as compared to 5:5 pools. In response to increas ed male harassment, females reduced their activity on the water and in creased their time spent out of the water and thus unable to forage. M ales showed a large male mating advantage (LMMA) in 5:5 pools, but, su rprisingly, not in 20:5 pools. This pattern can explain the field obse rvation that small males prefer hot spots. A behavioral mechanism that can explain the LMMA is as follows. Mating occurs when males overcome female resistance. Larger males have a mating advantage over smaller males if females resist heavily. Increased harassment (e.g., in 20:5 p ools as compared to 5:5 pools) might result in reduced female resistan ce to males and thus a reduced LMMA. Females also showed some non-rand om mating by size that might reflect an interplay between male prefere nce for large females and the avoidance of males by large females.