"We were looking for a plant that was eaten only at that certain time of year by both of our study troops," says Higham, "and immediately the black plum, Vitex doniana, jumped out of the data. We googled it and the first thing we saw were adverts about fertility!" Tests showed that the fruit contained phyto-progestogens and it looked like they’d found their culprit.

It turns out that a closely related Mediterranean species, the "chasteberry" has been used by humans for gynaecological conditions for at least 2,500 years. Nowadays you can find it in health store remedies for PMT, which is thought to result from a high level of the hormone estrogen relative to that of progesterone. Since the berry, like the black plum, boosts progesterone levels, it is said to restore that ratio to reduce symptoms.

At much higher levels of consumption, the fruit raises progesterone levels beyond those during pregnancy, inhibiting ovulation and behaving very much like the human contraceptive pill. This seems to be what’s happening with the baboons. Eating the plums also seems to prevent the females from getting the swollen bottoms associated with fertility that male baboons find irresistible, so they seem to act as a social contraceptive, too.

This might not be a good thing, however, since a sexual swelling can afford protection from male aggression towards the female’s offspring. Higham thinks that the real value of the plums lie in their medicinal effects. At Gashaka-Gumti, baboons are living at the limits of their ecological range. Normally a savannah species, these baboons are living in forest with a relatively high rainfall, peaking at the time of year when the plums are eaten and coinciding with an increase in disease.

"It’s a difficult time for them ecologically," says Higham. We’ve got evidence that there’s higher infant mortality and adult female mortality at that time of year." The plums are reported to have antibacterial, antiviral and fungicidal properties and it seems that the disadvantages of temporary infertility are likely to be outweighed by the medicinal benefits.

Many researchers have made casual reports of a link between plant consumption and a possible effect on conception patterns in non-human primates, but until now nobody has linked physiological data with the animals’ feeding and reproductive behaviour. "This study is the first to clearly demonstrate in such a convincing way that diet does indeed directly affect the reproductive status of female primates in nature," says Professor Michael Huffman of the Primate Research Institute at Kyoto University in Japan.

These findings, just published in the journal Hormones and Behavior, emphasise the need to take the effects of diet into account when interpreting the behaviour of animals. For Dr Gillian Brown of St Andrews University, the way a youngster learns what to eat is the key. She says that if diet choice is learned through social interactions with the mother or other elders during early life, then fundamental aspects of primate reproduction, such as how often and at what time of year babies are born, could be affected by social learning.

Via the Scotsman