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How costly are ejaculates for Japanese macaques?

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Much sexual selection theory is based on the idea that ejaculate is cheap. Since further details are unknown our aim was to determine the energy that primate males require for ejaculate production. We addressed this problem by measuring the energy content (in kJ) of ejaculates from Japanese macaques (Macaca fuscata) using standard bomb calorimetry. Then, we estimated the relative amount of energy that individuals require for ejaculate production by relating the net energy content of ejaculates to males’ daily basal metabolic rate (BMR). Fresh macaque ejaculate contains 3.0 kJ ml−1. Assuming a mean volume of 2.7 ml an average macaque ejaculate contains 8.1 kJ. Depending on the individuals’ body mass (6–13 kg) and the number and volume of the ejaculates, macaque males are assumed to use between at least 0.8% and at most 6.0% of their BMR for ejaculate production per day during the breeding season. Even when regarding only the minimal energy investment of 0.8% of daily BMR for ejaculate production, clearly ejaculates come with some cost for primate males.

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We thank Prof. Dr. Heribert Hofer, Dr. Oliver Höner, Dr. Christian Voigt and Dr. Sylvia Ortmann from the Institute of Zoo and Wildlife Research and above all Prof. Dr. Alexander Harcourt for comments on the manuscript. Furthermore, we thank Prof. Dr. Hideyuki Ohsawa and Prof. Dr. Juichi Yamagiwa for support of our work in Japan. Ruth Thomsen was financed by the German Academic Exchange Service (HSP III, D/97/16290) and by the “Berliner Programm zur Förderung der Chancengleichheit für Frauen in Forschung und Lehre (N-16/04)”.

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Correspondence to Ruth Thomsen.

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Thomsen, R., Soltis, J., Matsubara, M. et al. How costly are ejaculates for Japanese macaques?. Primates 47, 272–274 (2006).

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