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How did comparisons of chimpanzees and gorillas test an adaptive explanation for testicle size?

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Marble bust of Aristotle with a beard and draped robe, showing detailed facial features and wavy hair, set against a dark background. — Philosophy of biology Detail of a Roman copy (2nd century bce) of a Greek alabaster portrait bust of Aristotle, c. 325 bce; in the collection of the Roman National Museum.

In the philosophy of biology, the comparative method was used to test an adaptive hypothesis regarding the relatively large testicle size of chimpanzees compared to gorillas.

The hypothesis posited that because female chimpanzees typically have multiple male sexual partners, males who produced more sperm—and thus possessed larger testicles—would gain an adaptive advantage in competing to fertilize the female. To test this, researchers compared the sexual habits and reproductive anatomy of chimpanzees with those of gorillas and other primates. The logic was that if testicle size was not correlated with the average number of male sexual partners in the expected manner, the hypothesis would be disproved. The study ultimately found that the evidence well supported the hypothesis, demonstrating that this comparative approach can provide a rigorous way to evaluate adaptive claims.1