Whether sperm fly at high speed or laze their way towards an egg might depend on how much competition they face, suggests a new analysis of sperm samples. The study reveals that promiscuous primate species have faster sperm than their more monogamous counterparts.
Jaclyn Nascimento at the University of California in San Diego, US, and her colleagues received sperm samples from humans, gorillas, chimpanzees and rhesus monkeys for analysis. Researchers had collected sperm samples from the latter two species using artificial vaginas, while the gorillas were trained to give up sperm (with the helping hand of a researcher) in exchange for candy.
Nascimento, an electrical engineer, and her collaborators focused in on specific sperm within the diluted samples, and recorded the activity of these individual sperm on film. Next, they used a sophisticated computer algorithm specifically created to determine the speed of a given sperm by tracking its head.
After examining numerous sperm from the two men who provided sperm samples, the team calculated that human sperm travel at about 0.2 kilometres per hour, a finding within the same range as some previous studies.
By comparison, the sperm from chimpanzees and macaques – which are much more sexually promiscuous than humans – appears to travel at a rate of 0.7 km/h. Sperm from gorillas – a relatively monogamous species in which females tend to mate with just one male – is "ridiculously slow" and clocks in at just 0.1 km/h, according to Nascimento.
Then, scientists assessed the force with which the sperm move. This step involved using used a laser technology known as "optical tweezers" to try to hold sperm in place using light. If the sperm moved with enough force, they could break past this resistance field and go forward – otherwise, they continued to push against it without luck.
The experiment showed that not only do the sperm from chimps and macaques move faster, they also move with greater force, at about 50 piconewtons. By comparison, human sperm swim with a force of about 5 piconewtons, and gorillas with a measly 2 piconewtons. (Watch movies of the sperm caught in the optical tweezers.)
Nascimento says the findings suggest that sperm from promiscuous species such as chimps, where a female might mate with multiple males within an hour, have evolved to move faster as a result of competition. "The first ones to make it to the egg" succeed, she says.
Gorilla mating, on the other hand, follows a harem-like pattern: females are more likely to mate with only the dominant male. "You are the best by default," Nascimento says of the leisurely sperm in the less promiscuous primates.
The speed at which sperm travel is partly determined by a motor structure inside the cell that powers the movement of their tails. In humans, certain compounds such as caffeine and anti-impotence drugs can alter how fast sperm move.