Sports celebrities caught cheating on their partners may now resort to a new excuse. A recent study has shown that a game of soccer can elevate the levels of a hormone associated with sex drive by as much as one-third, albeit for a short time.

A team of American anthropologists studying an indigenous hunter-gatherer tribe in South America found that an hour of soccer is all that is required to boost the levels of testosterone in the tribesmen by up to 30 per cent.

The anthropologists, who were studying Tsimane, a red Indian tribe of 15,000 people in northwest Bolivia, weren’t trying to establish the link between testosterone levels and sexual behaviour but understand how the hormone levels can shoot up rapidly despite environmental stress.

People who live in resource limited environments are forced to make a trade-off between reproductive advantage and survival. Having high levels of testosterone tends to be immunologically and metabolically expensive. So men in such areas tend to have low testosterone levels.

Testosterone is known to be associated with sexual functioning and strong libido. The hormone is also linked to aggression, muscle formation and is required to prime the body against a potential injury.

A spike in testosterone, which is associated with seeking sex, for a short time may not completely explain why many sportsmen seek sexual gratification outside their relationships, but studies in the past have shown that sportsmen involved in competitive games have higher levels of circulating testosterone.

In his recent book Monogamy Gap: Men, Love and the Reality of Cheating, University of Winchester sociologist Eric Anderson touches upon the link between testosterone and cheating. “Because athletes and other socially esteemed individuals are more likely to develop higher levels of testosterone, their elevated testosterone further inhibits the bonding effect of being with their partners and insinuates a desire for more extradyadic sex,” Anderson says in his book.

Men in steady relationships or involved in parenting, on the contrary, have low testosterone levels.

The Tsimane men usually have very low levels of testosterone. The reason is not difficult to fathom, say the scientists. “Maintaining high levels of testosterone compromises the immune system, so it makes sense to keep it low in environments where parasites and pathogens are rampant,” says Ben Trumble, an anthropology graduate student at the University of Washington, who is the first author of the paper that appeared recently in the journal Proceedings of the Royal Society B. Besides, the tribe has been leading a peaceful life with no known territorial wars — another reason for an increase in testosterone levels — because of the isolated nature of their habitat.

Apart from those at the University of Washington, anthropologists from the University of California, Santa Barbara, and the University of New Mexico were also involved in the study.

The research has shown that Tsimane men have testosterone levels a third less than men living in the US, where life is less physically demanding. They are also known to have a stable amount of testosterone across their lifespans whereas men in the U.S. show a decline in testosterone as they age. Testosterone drops serve as a sentinel for age-related disease and Tsimane men show little incidence of obesity, heart disease and other illnesses linked with older age.

An earlier study by a team of Indian scientists, led by Milind Watve of the Indian Institute of Science Education and Research, Pune, indicated that people living in peaceful societies have in general low levels of circulating testosterone. “It is also known that testosterone levels increase with exercise and aggression,” says Watve.

The study bears out the fact that competition-linked bursts of testosterone are a fundamental aspect of human biology that persists even if it increases the risk of sickness or infection, the US anthropologists say.

According to Trumble, previous research on hormone-behaviour interactions largely focused on industrialised populations. “We wanted to extend this to populations that faced environmental stresses more representative of the conditions faced throughout most of human existence,” he says.

Trumble however, doesn’t agree with the idea that such spurts in testosterone alone can explain the promiscuous behaviour of many sports personalities, although he says that they did not study sexual behaviour or any behaviour other than football performance among men in the Tsimane population. And, he points out, levels of testosterone probably decrease to baseline within a few hours of a match.

“By the time a professional sportsman takes a shower and finishes talking to reporters and drives home, they are probably back to baseline levels of testosterone. That said, we did not study sexual behaviour, or professional soccer players, so we cannot comment on this issue,” says the US anthropologist.

But Manchester United footballer Ryan Giggs might find the results of this study rather handy.