Tales from the Sperm Wars

The observation of very low motility in naked mole rat sperm illustrates the power of evolutionary logic

One of the misconceptions about Evolution is that it is full of ‘just so’ stories tailored around facts that have been observed. Nothing could be farther from the truth, as the following story suggests. In 2007, Jaclyn Nascimento at University of California in San Diego and her colleagues tested sperm samples from humans, gorillas, chimpanzees and rhesus monkeys. In theory, the more promiscuous a species (where males may mate with several mates in quick succession), the more likely that conception would be a race among competing sperms. This suggests that sperm speed should be related to promiscuity. The results backed this expectation.  Sperm from chimpanzees and macaques—sexually promiscuous species—travels at a rate of 0.7 metres/hour. Sperm from gorillas—where a dominant male guards access to females—is very slow and travels at approximately 0.1 m/hr. Human sperm averages about 0.2 m/hr, indicating that human promiscuity is far lower than that of chimpanzees but still much higher than of the monogamous gorillas. 

It would then seem that sperm should be at about its slowest in gorillas, at least when compared to other mammal species. But even among gorillas, access to females is controlled by the male and an occasional female still evades the dragnet to mate with other males. Is there then a species where there is no sperm competition? Enter the naked mole rat. These animals live in colonies controlled by a queen. The queen chooses a single male to mate and ensures no other can mate with her; this means even the slim chance of cheating that exists for gorillas disappears. When Liana Maree of the University of the Western Cape, South Africa, studied the sperm of naked mole rats, she indeed found they may have the slowest sperm of any known mammal. Only 7 per cent of the sperm moved at all, and those that did clocked in at gorilla speed. Now, scientists have begun pondering the question of how naked mole rats are fertile at all, a question whose answer may impact studies of infertility in other species, including humans.

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Hartosh Singh Bal