Avoidance of Sperm Competition in Males

Abstract

Although courtship displays differ between species, virgin females of any species seem to accept copulation easily whenever certain aspects of male performance are adequate. On the other hand, mated females frequently show the open-winged posture and raised abdomen stance to prevent access to courting males (Fig. 6.1). Many studies have clarified the significance of mate choice of mated females during mate-locating behaviour, with detection of male qualities such as age and size. After acceptance by the mated females, the males succeed in copulating but eventually encounter the severe problem that their mates have already stored enough sperm from previous males for fertilisation. Transferred sperm have to compete with the sperm of previous males inside the female reproductive organs. Because the sex ratio is around unity with female polyandry, the probability of encounter with already mated females must be high. Therefore, males might have to develop not a morphological but a behavioural or physiological device to avoid or to win the sperm competition. Sperm migration inside the female reproductive organs after copulation suggests that apyrene sperm may intercept the movement of the eupyrene sperm of the previous mated males. On the other hand, sperm always have a certain risk for competition with future males that will be accepted by the females. To inhibit insemination by the sperm of future males, a spermatophore mass is the only male tactic to delay copulation of the mated females. Consequently, strategic ejaculation by males might be evolved for rival and future males.