Postmating sexual selection: the effects of male body size and recovery period on paternity and egg production rate in a water strider

The role of male body size in postmating sexual selection was explored in a semiaquatic insect, the water strider Gerris lateralis. To separate effects of male size per se from those due to numeric sperm competition, male recovery period (shown here to be proportional to ejaculate size) was manipulated independently of body size in a factorial experiment where virgin females were mated first with sterile males and then with focal males. Both relative male fertilization success and female reproductive rate were measured. The number of sperm transferred increased with male recovery period, an effect that was mediated by longer copulation duration, but there were no effects of body size on ejaculate size. Neither male size nor recovery period had any significant direct effects on male fertilization success. However, copulation duration influenced relative fertilization success, suggesting that males able to transfer more sperm also achieved higher fertilization success. Females exercised cryptic female choice by modulating their reproductive rate in a manner favoring large males and males that were successful in terms of achieving high relative fertilization success. Thus, successful males gained a twofold advantage in postmating sexual selection. This study has important implications for previous estimates of sexual selection in this group of insects because pre- and postmating sexual selection will be antagonistic due to limitations in male sperm production: males mating frequently (high mating success) will on average transfer fewer sperm in each mating and will hence tend to fertilize fewer eggs per mating (low fertilization success).