Convenience polyandry in a water strider: foraging conflicts and female control of copulation frequency and guarding duration

Abstract

The mating behaviour of the water strider Gerris buenoi Kirkaldy was investigated in the field and laboratory. The conflict between foraging demands and mating was examined and the convenience polyandry hypothesis for superfluous mating was tested directly. Mating is multiple, females are reluctant, males are persistent and mating consists of a period of copulation followed by a guarding phase in which the male and female remain in tandem without genital contact. Males make frequent attempts to mate with females that often lead to vigorous struggles initiated by the females. All matings were terminated with a struggle initiated by the female that continued until males were dislodged. Field data support the hypothesis that mating conflicts with foraging. Females occupied with prey offered little resistance to male suitors and guarding durations were significantly longer when females were consuming prey or had fed recently. To test the hypothesis that females were more willing to mate when foraging demands were reduced, mating behaviour of bugs under different feeding regimes were compared. Female hunger reduced mating frequency 50% in the laboratory and male hunger had no effect. These data demonstrate that mating conflicts with foraging by females and that females appear to vary mating behaviour according to this conflicting demand. The convenience polyandry hypothesis predicts that females will accept superfluous copulations when the costs of repelling male mating attempts (remaining single) are greater then the costs of mating. To test this hypothesis, mating behaviour was compared under two sex ratios (1:1 and 3:1), where harassment of females and thus the costs of repelling males was expected to be higher in the male-biased treatment. Female mating activity, frequency and duration all increased significantly under a sex ratio of 3:1. In the male-biased treatment, harassment of females by males increased significantly and both mating activity and frequency were a positive function of harassment rate. Increased mating frequency resulted from an increase in the proportion of mating attempts that led to mating. Pre-mating struggles that led to mating were about one-quarter the duration of those that did not. Therefore, increased mating frequency under high harassment rate resulted from a decrease in female reluctance to mate. There was no treatment effect on the duration of post-mating struggles, indicating that increased guarding duration observed under the male-biased sex ratio resulted from a female decision to postpone dislodging the male. These results demonstrate a strong degree of female control over most components of mating and support the convenience polyandry hypothesis for superfluous mating.