Abstract
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.
Highlights
An important challenge in evolutionary biology has been to understand strategies through which males invest in sperm production and allocation of sperm among mating opportunities to maximize reproductive success [1]
We found positive relationships between the amounts of sperm ejaculated per spawning bout and the relative size of the paired female, providing evidence that male salmon adjust sperm ejaculate allocation according to perceived female quality. This result is probably due to the strong relationship between female size and fecundity in salmonid species [25,39], including our population of chum salmon
We found a positive relationship between female size and number of eggs, egg diameter and egg
Summary
An important challenge in evolutionary biology has been to understand strategies through which males invest in sperm production and allocation of sperm among mating opportunities to maximize reproductive success [1]. Males should in theory tailor their sperm storage and expenditure in response to mating opportunities and female quality (e.g. fecundity) to maximize their reproductive success [4,5,6,7]. Males typically require time to replenish sperm and serum reserves between mating attempts, suggesting that there is a relationship between the duration of breeding seasons and sperm allocation in terms of the reproductive success of males. Sperm allocation between mating attempts in males is expected to affect reproductive success if the breeding season is very limited or only one season is available to reproduce. Previous empirical studies on sperm allocation have not focused on species possessing life histories with only one short breeding season with intensified reproductive competition among males, such as a semelparous life history (but see [11])
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