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Women who have sex at least once a week start the menopause later than those how make love less than once a month, according to a new study.
Scientists at University College London analysed data from the US Study of Women's Health Across the Nation (SWAN) - the "largest, most diverse and most representative longitudinal cohort study available to research aspects of the menopause transition" according to UCL.
Sexual activity was defined as sexual intercourse, oral sex, sexual touching and caressing or self-stimulation.
According to the study's first author, PhD candidate Megan Arnot: "The findings of our study suggest that if a woman is not having sex, and there is no chance of pregnancy, then the body 'chooses' not to invest in ovulation, as it would be pointless.
"There may be a biological energetic trade-off between investing energy into ovulation and investing elsewhere, such as keeping active by looking after grandchildren.
"The idea that women cease fertility in order to invest more time in their family is known as the Grandmother Hypothesis, which predicts that the menopause originally evolved in humans to reduce reproductive conflict between different generations of females, and allow women to increase their inclusive fitness through investing in their grandchildren."
Published in the journal Royal Society Open Science, the study also suggested that women's immune function is impaired during ovulation which makes them more susceptible to falling ill making it a costly process.
In evolutionary terms the scientists say that it would not be beneficial for the body to invest in ovulation when, due to the lack of sexual activity, it is unlikely to pay off with pregnancy.
Professor Ruth Mace added: "The menopause is, of course, an inevitability for women, and there is no behavioural intervention that will prevent reproductive cessation.
"Nonetheless, these results are an initial indication that menopause timing may be adaptive in response to the likelihood of becoming pregnant."
