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How costly are ejaculates for Japanese macaques?
Ruth Thomsen1, Joseph Soltis, Miki Matsubara
1Institute of Zoo and Wildlife Research, Alfred-Kowalke-Str. 17, 10315, Berlin, Germany. thomsen@izw-berlin.de
Primates; Journal of Primatology
|February 10, 2006
Summary
Primate males invest significant energy in ejaculate production, challenging the "ejaculate is cheap" theory. Japanese macaque males use up to 6% of their daily basal metabolic rate for ejaculate production during breeding seasons.
Area of Science:
- Reproductive Biology
- Animal Energetics
- Primate Ecology
Background:
- Sexual selection theory often assumes low energetic costs for ejaculate production.
- The actual energy expenditure for ejaculate production in primates remains largely unquantified.
- Understanding male reproductive investment is crucial for evolutionary biology.
Purpose of the Study:
- To quantify the energy content of primate ejaculates.
- To estimate the energetic cost of ejaculate production relative to basal metabolic rate (BMR) in male primates.
- To test the hypothesis that ejaculate production incurs a significant energetic cost for males.
Main Methods:
- Collected ejaculates from Japanese macaques (Macaca fuscata).
- Measured ejaculate energy content using bomb calorimetry.
- Calculated the ratio of ejaculate energy to daily basal metabolic rate (BMR).
Main Results:
- Fresh macaque ejaculate contains 3.0 kJ/ml.
- An average ejaculate (2.7 ml) contains 8.1 kJ.
- Male macaques may expend 0.8%–6.0% of their daily BMR on ejaculate production.
Conclusions:
- Ejaculate production represents a non-negligible energetic cost for male primates.
- Findings challenge the long-held assumption of "cheap" ejaculates in sexual selection theory.
- This energy investment may have implications for male reproductive strategies and fitness.