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04/10/2019 01:01

Showy Primates Have Smaller Testicles

Beat Müller Kommunikation
Universität Zürich

    Well-adorned or well-endowed – but not both. Evolutionary biologists at the University of Zurich have for the first time demonstrated that male primates either have large testicles or showy ornaments. Developing both at the same time may simply take too much energy.

    Male primates are highly competitive, especially about one thing: fathering offspring. To maximize their chances of passing on their genes, males of many primate species invest heavily in various sexual traits, such as a large body size, or long canines that can serve as weapons in direct contests over mates. What’s more, showy sexual ornaments such as manes, beards, fleshy swellings, and colorful skin patches can help them intimidate rivals and woo females. And if males can’t keep other males off their females, they will try to outcompete them at the level of sperm. By swamping the sperm of others, they can increase their chances of fertilization. But producing a lot of sperm requires large testicles.

    Showy body ornamentation leads to small testes

    All these male traits are energetically costly. So how do primates allocate their limited resources to the various sexual traits to maximize their reproductive success? This question is the focus of a new study by Stefan Lüpold, an evolutionary biologist at the University of Zurich (UZH), and his colleagues Leigh Simmons and Cyril Grueter from the University of Western Australia. These biologists compared the sexual traits of over 100 primate species, including humans. Individually, the expression of these traits increases with the intensity of male competition – as expected. But considering all traits jointly reveals an important trade-off: “Ornament elaboration comes at the expense of testicle size and sperm production. In a nutshell, the showiest males have the smallest testes,” says Lüpold.

    Limited resources determine degree of expression

    The new study is the first to examine all sexual traits simultaneously. It has brought to light the subtleties of how male primates invest in maximizing their reproductive success: “Big testicles come with large weapons but less ornamentation.” The researchers offer various explanations for their findings. But one of the key points may be the energy required to develop and maintain multiple sexual traits throughout a male’s sexual maturity. “It’s hard to have it all,” says Lüpold.


    Contact for scientific information:

    Professor Stefan Lüpold
    Department of Evolutionary Biology and Environmental Studies
    University of Zurich
    Tel. +41 44 635 47 77
    E-mail: stefan.luepold@ieu.uzh.ch


    Original publication:

    Stefan Lüpold, Leigh W. Simmons, and Cyril C. Grueter. Sexual ornaments but not weapons trade off against testes size in primates. Proceedings of the Royal Society B, 10 April 2019. DOI: 10.1098/rspb.2018.2542


    More information:

    http://www.media.uzh.ch


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    Criteria of this press release:
    Journalists
    Biology, Zoology / agricultural and forest sciences
    transregional, national
    Research results, Transfer of Science or Research
    English


     

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