Conflict between sexes in the water strider, Gerris lacustris: a test of two hypotheses for male guarding behavior

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Abstract
We studied the effect of operational sex ratio on female reluctance and male persistence to mate as well as on the length of copulation and postcopulatory guarding in Gerris lacustris by adding five surplus males or females to the basin with a pair in tandem. In the control treatment, a pair alone was tested. According to the copulatory guarding hypothesis (CGH), males should prolong mating and guard females in the presence of surplus males. According to the convenience polyandry hypothesis (CPH), females should show lower levels of resistance to prolonged mating in the presence of surplus males because the mating male protects the female against harassment from other males. As expected on the basis of both the CGH and CPH, mating (copulation + guarding) averaged longer in the male-biased treatment. The behavior of males and females during mating suggested that both hypotheses hold true: females showed less resistance to prolonged mating (as predicted from CPH), and male behavior suggested stronger efforts to stay on the female when surplus males were present (as predicted from CGH). Comparisons of the treatment with surplus females with the results from the mating pair without surplus individuals suggested that the capabilities of water striders in tandem to assess the sex of nearby nonmating striders are limited.

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