Data from: Sperm allocation in relation to female size in a semelparous salmonid

To maximize reproductive success, males have to adaptively tailor their sperm expenditure in relation to the quality of potential mates because they require time to replenish their sperm supply for subsequent mating opportunities. Therefore, in mating contexts where males must choose among females in a short period of time, as is the case with semelparous species (which die after one intensely competitive short duration breeding season), selection on sperm allocation can be expected to be a powerful selective agent that shapes the male reproductive success. We quantitatively investigated sperm allocation patterns in chum salmon in relation to perceived female quality by developing a novel method for determining the amount of sperm allocated per ejaculate during spawning bouts. We examined the relationship between sperm expenditure and the body size of paired females (a proxy of egg number and egg quality) in the absence of male–male competition in an experimental channel. The estimated amount of sperm released per spawning event was positively correlated with the size of paired females. However, the number of spawning events a female participated in, which reduces the number of eggs she spawns in each subsequent bout, did not affect this relationship. These results provide support for predictions arising from the sperm allocation hypothesis, male salmon do economize their sperm expenditure in accordance with paired female body size as predicted for their first spawning event, but males overestimate or are unable to assess the quality of females beyond size and provide more sperm than they should in theory when paired with a female that spawned previously. Overall, the observed sperm allocation pattern in chum salmon appears to be adapted to maximize reproductive success assuming female size is an honest indicator of quality, although temporal changes in a female's quality during a reproductive season should be considered when examining sperm allocation strategies.

为最大化繁殖成功率，雄性需根据潜在配偶的品质适应性调整精子投入（sperm expenditure）——因为雄性需要时间补充精子储备，以应对后续交配机会。因此，在雄性需在短时间内于多个雌性间选择交配对象的情境中，比如终生一胎繁殖物种（semelparous species）：这类物种仅经历一次竞争激烈的短期繁殖季，繁殖后便死亡，精子分配（sperm allocation）策略所受的选择压力有望成为塑造雄性繁殖成功率的强大选择因子。本研究通过开发一种可测定产卵回合（spawning bout）中单次射精（ejaculate）的精子投入量的新方法，定量探究了大麻哈鱼（chum salmon）的精子分配模式与雄性感知到的雌性品质之间的关联。我们在实验水槽中开展了无雄性间竞争（male–male competition）的交配实验，检验了精子投入与配对雌性体型之间的关系——雌性体型可作为产卵数量与卵质的替代变量。实验估算得到的单次产卵事件释放的精子量，与配对雌性的体型呈正相关。不过，雌性参与的产卵回合数（该因素会降低雌性在后续每一轮产卵中的产卵量）并未对上述相关性产生影响。上述结果为精子分配假说（sperm allocation hypothesis）的相关预测提供了支持：雄性大麻哈鱼确实会如首次产卵事件的预测那样，根据配对雌性的体型调整精子投入；但雄性仅能通过体型评估雌性品质，无法或高估了雌性的实际品质，因此当与此前已完成产卵的雌性交配时，其精子投入量会高于理论最优值。总体而言，若假设雌性体型是其品质的诚实信号（honest indicator），那么大麻哈鱼所观测到的精子分配模式似乎是为最大化繁殖成功率而演化形成的；不过在探究精子分配策略时，仍需考虑雌性品质在繁殖季内的动态变化。