25.5: There is unseen conflict among sperm cells.

In The Chimpanzees of Gombe: Patterns of Behavior, Jane Goodall reported female copulatory rates of “an average of between five and six copulations per female per hour in the early morning, after which the rate dropped gradually to about two per hour in the midmorning, rose very slightly during the afternoon, and tapered off to one per hour in the evening.” These observations were relevant to the realization reached by earlier researchers that if females (of any species, not just chimps) mate with more than one male, there may be competition among the males’ sperm.

The idea of sperm competition, or “sperm wars” as this has been called, gives rise to several testable predictions. One simple prediction is this: when a female is likely to mate with more than one male, the males that produce more sperm are more likely to be successful at fertilizing the female’s eggs. Some interesting observations support this prediction. For example, gorillas have golf-ball-sized testes, while chimpanzees’ testes are closer in size to baseballs. Gorilla testicles account for only 0.02% of body weight, whereas chimp testicles account for 0.30%, 15 times as much. Why the huge difference? Gorilla groups are relatively small, and all females within a group mate with just one male, the dominant silverback. In contrast, as Jane Goodall noted, fertile chimpanzee females may have sex dozens of times a day with many different males. Consequently, the tiny testes of the gorilla are perfectly adequate, but for a chimp male to win these sperm competitions, he must produce significantly more sperm (FIGURE 25-12). Similarly, among fruit bats, males of species that live in large social groups have significantly larger testes than males of species that live in smaller groups.

Q

Question 25.5

A chimp’s testicles are 15 times bigger, relative to body weight, than a gorilla’s! Why?

Sperm competition has given rise to several other adaptations.

Figure 25.12: The evolutionary consequences of sperm competition among males. The inset shows the size of a male chimp’s brain (top) relative to one of its testicles.
Figure 25.13: Sperm wars. The damselfly penis, which is described as a “shovel penis,” can scrape away the sperm deposited by a rival.

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This unseen conflict between males is an area of considerable investigation, as researchers try to understand the physical and behavioral consequences of such competition. And females are not passive players in these battles; research studies have documented how, in many cases, a female will eject the sperm from undesirable males.

TAKE-HOME MESSAGE 25.5

When females mate with more than one male, sperm competition occurs and can lead to a variety of adaptations, including increased sperm production and testis size, semen that can create a physical barrier to subsequent mating, toxic semen components, and penis morphology that aids in the displacement of rival males’ sperm.

What predictions can you make if the males of a species have testes that account for a relatively large percentage of their body weight?