Attractive males release fewer sperm per mating to increase the odds of producing offspring across a range of females, suggesting that couplings with attractive males may be less fruitful than those with unattractive ones. The new findings, published in American Naturalist, are based on mathematical models representing a range of male ejaculation strategies. The researchers, from University College London (UCL) and the University of Oxford, sought to identify the optimum “sperm load” per mating and how this might vary depending on mating patterns.
Previous studies have shown that in animals such as the domestic fowl, and fish such as the Arctic charr, males with privileged access to females produce ejaculations of lower fertilizing quality than subordinate males.
The paper suggested that future work should focus on males that are similarly attractive, but have different levels of resources to allocate to sperm production, to see how this alters their sperm number and quality. Additionally, the researchers say the model should also be expanded to include the effects of short-term sperm depletion and whether the lower fertility of attractive males causes females to start avoiding attractive males that mate too often.
The researchers caution that the results cannot be applied directly to humans. “How this work applies to humans and other primates is not yet known. Human attractiveness is complicated and influenced by a number of factors including cultural preferences. Nonetheless, ejaculate size and sperm quality are likely to have been molded by similar forces, like attractiveness and the number of sexual partners, that are important in other species,” concluded Tazzyman.
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