Tanabe Ryohei, Akiyama Nobuhiko, Sato Noriyosi
Graduate School of Science and Technology, Tokai University, Shizuoka, Japan.
Institute of Oceanic Research and Development, Tokai University, Shizuoka, Japan.
J Evol Biol. 2025 Feb 1;38(2):284-288. doi: 10.1093/jeb/voae120.
When mating is promiscuous, the ejaculate volume allocated to each female is expected (intuitively) to be linked with the presence and number of rival males. Previous theories have indicated that, in the absence of rival males, males will allocate the minimum ejaculate volume sufficient for fertilization of all available oocytes. However, it is unclear if this ejaculation strategy is still effective where females have a mechanism to remove sperm after copulation ("female sperm rejection"). In the Japanese pygmy squid, Idiosepius paradoxus, female sperm rejection was observed to occur frequently, but males were able to increase the remaining sperm volume available for fertilization, suggesting that there is no significant impact of female sperm rejection on male ejaculation strategy. However, males decreased ejaculate volume in the presence of rival males and increased it in their absence, a pattern counterintuitive to predictions from previous theories. Females reject sperm at every copulation, so after copulation, the amount of a given male's sperm remaining with the female may decrease after each subsequent rival copulates with the female. Perhaps in this species, the presence of rivals signals the risk of further sperm rejection, so males choose to conserve their resources and move on.
当交配行为杂乱无章时，（直观地）可以预期分配给每只雌性的射精量与竞争对手雄性的存在和数量有关。以往的理论表明，在没有竞争对手雄性的情况下，雄性会分配足以使所有可用卵母细胞受精的最小射精量。然而，尚不清楚在雌性具有交配后清除精子的机制（“雌性精子排斥”）的情况下，这种射精策略是否仍然有效。在日本侏儒乌贼（Idiosepius paradoxus）中，经常观察到雌性精子排斥现象，但雄性能够增加可供受精的剩余精子量，这表明雌性精子排斥对雄性射精策略没有显著影响。然而，雄性在有竞争对手雄性在场时会减少射精量，在没有竞争对手雄性时会增加射精量，这一模式与以往理论的预测相悖。雌性每次交配都会排斥精子，因此交配后，在每只后续竞争对手与雌性交配后，雌性体内特定雄性剩余的精子量可能会减少。也许在这个物种中，竞争对手的存在预示着进一步精子排斥的风险，所以雄性选择保存资源并继续前进。
