Sperm production is costly, and males are expected to strategically allocate this potentially limiting resource to maximize their fitness. Sperm allocation theory predicts that males should adjust their sperm expenditure in relation to female quality. However, the available empirical evidence is limited. In this study, we assessed whether wild male medaka (Oryzias latipes) would allocate their sperm depending on female quality under controlled conditions. Behavioral observations revealed that spawning behavior could be classified into four stages: male quivering, female quivering, sperm release, and stay. Of these behaviors, only the duration of sperm release was positively correlated with the number of sperm ejaculated in the aquarium, suggesting that males may adjust the sperm number expended for each mating by controlling the duration of sperm release. The estimated amount of sperm released per spawning was positively and significantly correlated with the body depth and weight of paired females. This result is consistent with the prediction arising from the sperm allocation hypothesis, and suggests that male medaka may allocate their sperm depending on paired female body depth and weight. However, the number of released sperm did not correlate with the number of spawned eggs, which was positively correlated with body depth and weight of females. These results imply that medaka females may have counter-strategies against sperm allocation by males, and consequently, males might not be able to adjust the number of sperm efficiently. We disscuss our results from the perspective of sexual conflict over sperm as a limited resource.
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Vol. 37 • No. 3