Or are they part of an evolutionary strategy developed by men to keep track of their women and keep them away from other men during fertile periods?

Research to be published in the upcoming issue of Proceedings B, a British Royal Society publication, indicates that women tend to be more interested in sex, in particular with men other than their primary romantic partners, when they are ovulating.

In response, their male partners appear to sense this and are more attentive, keeping a close eye on their women during this time.

The study by scientists in the departments of Psychology and Biology at the University of New Mexico in Albuquerque examined changes in women's sexual interests in men (primary partners and others) and men's efforts to track their partners' whereabouts as a function of women's fertility status.

Fifty one women, none using any form of hormone-based contraception, were asked to fill out questionnaires just prior to ovulation (when fertile) and during the luteal phase (when non-fertile). The fertility of the women was checked by using a commercially available ovulation detector. The women had a mean age of 19.6 years.

The questionnaire asked participants to rate how much they had engaged in each of a number of behaviors or feelings in the past two days. The initial questions concerned a variety of general mood states and interpersonal behaviors that would not change over the menstrual cycle. The later questions concerned sexual behavior, feelings, attractions and fantasies.

"The results showed that the women reported a notably greater sexual interest in and fantasy about non-primary partners when fertile than when non-fertile," comments Professor Steven Gangestad. "However, no change of sexual interest in primary partners was detected."

A second questionnaire asked the women to rate the extent to which their primary partner had engaged in behaviors related to enhanced male attention within the same two days, such as vigilance (e.g., unexpected phone call), monopolization (e.g., spending more free time with her) or spoiling (e.g., unexpected gifts etc.)

"It was clear from the results that the women's primary partners were more attentive and proprietary near ovulation," says Prof. Gangestad. "Results suggest a conflict of interest between the sexes when women are fertile."

"The study illustrates the subtlety of human adaptive behavior and reinforces the usefulness of looking at human romantic relationships as partly the outcome of genetic conflicts of interest," concludes Prof. Gangestad.

"The women's increased sexual interest in non-primary partners when fertile can be predicted via specific notions of intersexual selection," he added. Females may sometimes benefit reproductively from having males other than their partner sire offspring -- for example, to increase genetic quality or diversity in offspring. This would certainly be the case for ancestral women. Similar notions predict the men's counter strategy: greater vigilance -- a need to know their partner's whereabouts and activities. Male partners will enhance their reproductive interests by reducing the probability of investment in offspring not their own."

But this begs the question of what cues men use to detect the potential period of risk?

"There may be subtle clues in your partner's scent or visual signs," says Prof. Gangestad. "Or it may be a response to the partner's behavior, such as an increased interest in other men."

(Reference: Changes in women's sexual interests and their partners' mate retention tactics across the menstrual cycle: evidence for shifting conflicts of interest by Professor Steven W. Gangestad, Professor Randy Thornhill and Ms Christine E. Garver. PROCEEDINGS OF THE ROYAL SOCIETY: Biological Sciences - Vol. 269, No. 1494.)

30-Apr-2002