How Costly Are Ejaculates for Japanese Macaques

Jul 7, 2026

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Sexual selection theory often assumes ejaculate is energetically cheap, yet energetic costs for primates have remained poorly quantified. This study measured the energy content of Japanese macaque (Macaca fuscata) ejaculates using standard bomb calorimetry and related ejaculate net energy to males’ daily basal metabolic rate (BMR). Fresh ejaculate contained about 3.0 kJ ml−1; with an assumed mean volume of 2.7 ml, an average ejaculate carried ~8.1 kJ. Calculations imply breeding-season investment of at least 0.8% and up to 6.0% of daily BMR, indicating ejaculate production incurs real costs.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages3

Reading Time8 min

Introduction

Background and motivation

Research aim

Methods

Non-invasive sample collection

Bomb calorimetry and calculations

Results

Primates (2006) 47:272–274 DOI 10.1007/s10329-005-0171-7 SHORT COMMUNICATION Ruth Thomsen Æ Joseph Soltis Æ Miki Matsubara Kiyoaki Matsubayashi Æ Manabu Onuma Osamu Takenaka How costly are ejaculates for Japanese macaques? Received: 20 December 2004 / Accepted: 28 October 2005 / Published online: 9 February 2006 􀀁 Japan Monkey Centre and Springer-Verlag 2006 Abstract 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. R. Thomsen (&) Institute of Zoo and Wildlife Research, Alfred-Kowalke-Str. 17, 10315 Berlin, Germany E-mail: [thomsen@izw-berlin.de](thomsen@izw-berlin.de) [Tel.:](Tel.:) + [49-30-5168701](49-30-5168701) J. Soltis Education and Science, Animal Programs Administration, Disney’s Animal Kingdom, Lake Buena Vista, FL 32830, USA M. Matsubara Laboratory of Human Evolution Studies, Department of Zoology, Kyoto-University, Sakyo-ku Kitashirakawa, Kyoto 606-8502, Japan K. Matsubayashi Æ M. Onuma Center for Human Evolution Modeling, Primate Research Institute, Inuyama 484-8506, Japan O. Takenaka Department of Cellular and Molecular Biology, Primate Research Institute, Inuyama 484-8506, Japan Keywords Ejaculate Æ Energy Æ Japanese macaques Æ kJ Æ Yakushima Introduction The current discussion of primate male reproductive strategies involves males’ investment in body size, secondary sexual traits, mate searching, mate guarding, male coalitions, ﬁghting against competitors and sperm competition (Kappeler and van Schaik 2004) . While many aspects of primate male reproductive strategies are expensive the production of sperm typically is assumed to be ‘‘cheap’’ in terms of lacking any costs. In other species than primates males are known to invest non-trivial amounts of energy for sperm production (Dewsbury 1982) . For example, grasshoppers (Isophya kraussi) use approximately 20% of their total body energy to produce one spermatophore (Voigt et al. 2005) . Adders (Vipera berus) lose as much body mass during the period of spermatogenesis as during the subsequent breeding season (Olsson et al. 1997), and lemon tetra males (Hyphessobrycon pulchripinnis) invest high energetic costs in sperm production (Nakatsuru and Kramer 1982) . In seasonally breeding primates males are known to reduce their testes sizes during the non-breeding season implying that testicular tissue, spermatogenesis and the production of ejaculates entails energy demands that are worth saving (Harcourt et al. 1995; Dixson 1998) . But nothing at all is known about the energetic costs of primate ejaculates. To estimate these costs we measured the energy content (kJ) of ejaculates from Japanese macaques (Macaca fuscata) . Then we related the net energy content of ejaculates to the daily basal metabolic rate (BMR) and calculated the relative amount of energy that individuals may require for ejaculate production. | Methods<br>Japanese macaques belong to those primate species in which captive as well as wild living

FAQs

What energy content was estimated for an average ejaculate?

Fresh ejaculate energy was about 3.0 kJ ml−1, and with a mean volume of 2.7 ml this yields approximately 8.1 kJ per average ejaculate.

Jul 7, 2026

Upload by Liam