As expected, across species testis weight generally tends to increase with body size. Their results clearly confirmed that the mating system is associated with testis size. In 1981, Alexander Harcourt and colleagues conducted a wide-ranging examination of relationships between testis weight, body weight, and mating systems across monkeys and apes. To be fully informative, comparisons across species require appropriate allowance for body size effects. Source: Adapted from a figure in Dixson 20. For instance, a 20-pound adult male macaque has testes weighing close to three ounces, whereas testes of a 7-fold heavier adult man weigh under two ounces! And some striking points are obvious even without intricate calculations. Yet both species are monogamous and owl monkey testes actually turn out to be particularly small when appropriate adjustment is made for body size. For instance, a two-pound owl monkey has a ratio of 0.12%, while the ratio for a 12-pound Javan gibbon is distinctly lower at only 0.08%.
In fact, the data clearly show an overall trend, with the testis-to-body size ratio decreasing at larger body sizes. His truly “seminal” paper was a mine of useful information but left non-linear scaling unmentioned. In 1938, Adolph Schultz, my illustrious predecessor as Director of the Anthropological Institute at the University of Zürich in Switzerland, published a pioneering study of testis size in 17 non-human primate species and humans. File licensed under the Creative Commons Attribution-Share Alike 4.0 International license.
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