Sperm-depleted males of the two-spotted spider mite can replenish sperm in a few hours.

In many animals, males increase their reproductive success by mating with as many females as possible. The number of females a male can fertilize is often limited by male competition for access to females, sperm competition, and the cost of sperm production. Especially, recent studies have shown that sperm production is more costly than previously expected. In the two-spotted spider mite, Tetranychus urticae Koch, the number of females a male can inseminate is limited mainly by male competition for access to females. However, in the absence of rivals, males mate with so many females that they can become sperm-depleted. Mating without sperm transfer does not produce any offspring, although it takes time and energy. Therefore, a question arises as to why males continue to mate even after sperm depletion. In this study, we hypothesized that males continue to mate because sperm is replenished after a short period. To test the hypothesis, we investigated how long it takes for sperm replenishment after sperm depletion. We found that in 3 h, sperm can be replenished enough to inseminate a few females. As 3 h is sufficiently short not to lose the next mating opportunity, the results support the hypothesis. However, copulation duration in the sperm-replenished males was significantly longer than in the sperm-depleted males but shorter than in males before sperm depletion. To explain the differences, further research would be necessary. In addition, anatomical physiology study in males is also required to confirm that sperm is indeed depleted and replenished.