Here's the scene: A young woman who is feeling a hormone flush looks approvingly at someone other than her spouse and fantasizes about spending a weekend at the beach with a guy she doesn't even know.
Meanwhile, a dozen roses arrive on her doorstep, a very timely gift from her spouse.
What's going on here? The destruction of a marriage?
Nothing all that serious. Just a mating ritual that has been going on since the beginning of human history.
New research shows that a woman is most likely to fantasize about someone other than her spouse or current sex partner during the brief period each month when she is ovulating. The hormones that surge through her body, telling her that she has become fertile again, also cause her to look about and see if there's a better source of good genes for her offspring than the guy who just sent her roses.
And men somehow sense that change, so they pick that time to send flowers, or call the wife to see if she's really at work, or wherever she's supposed to be.
Old Rituals Still Kicking In
What it all comes down to is we may have moved up the evolutionary ladder past our chimp cousins, but when it comes to one aspect of our sexuality, we haven't moved all that far. When they are fertile, the gals are still checking out the gene pool, and the guys are still trying to hold on to their turf by trying to look like the best bull in the barn.
Research by biologists and psychologists at the University of New Mexico shows that the reproductive urges and rituals that guided our ancient ancestors are still very much alive in modern humans.
Women with an average age of 19.6 took part in the study. They filled out questionnaires twice a month, once when they were ovulating and once when they weren't. The women were tested using a standard ovulation detector to see if they were in fact fertile at the time they filled out one of the questionnaires. Only those who passed that test (51) were included in the study.
Few of the women were married, but most were in a serious relationship.
Dreaming of Someone Else
The women were asked to rate such things as how strongly they fantasized about someone other than their current sex partner, and how much they fantasized, or thought about, their main guy.
"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," says Steven Gangestad, professor of psychology and lead author of a report on the research in the current issue of Proceedings: Biological Sciences of the Royal Society of London.
But what about the guy back home, the bloke who sent the flowers?
He got zilch.
The women were asked if they also experienced an increase in sex fantasies with their current partner.
"That did not change," says Christine Garver, a doctoral candidate in evolutionary psychology at the university and one of the authors of the study.
"That's important," she says, because it shows that the women weren't just experiencing an overall increase in their sexual desire while they were fertile. Instead, they were looking around, "seeking the best genetic benefits for their offspring," Garver says.
These days, of course, shopping for a better mate has its complications, including a possible trip to divorce court, but that doesn't mean the urge to shop has subsided all that much.
Participants in the study revealed that their fantasies about someone other than their current partner was "65 to 80 percent higher during the high fertility phase," says Garver.
"That's an impressive statistic," she adds.
Mates Pay Attention
The women were also asked to fill out another questionnaire to see if they noted a change in the amount of attention they got from their main squeeze during the time when they were fertile. Was he vigilant (making that unexpected phone call)? Did he try to monopolize her time? Did he try to spoil her with flowers and a romantic dinner?
All of the above.
Or as Gangestad puts it: "It was clear from the results that the women's primary partners were more attentive and proprietary near ovulation."
So that brings us to this fundamental question: How do men know when their mate is fertile?
"We don't know the answer to that question," says Randy Thornhill, professor of biology at the university and the third author of the study. "But there's some suspect cues that men may use."
Clued In by Sight and Scent?
Thornhill and Gangestad have completed many studies in this general area over the years, and one study indicated that men like T-shirts worn by women who are "at high conception risk," or fertile.
"So there's a scent component, perhaps, that is influencing men's mate-guarding," Thornhill says.
Another clue, he adds, is the fact that women tend to act and dress differently when they are ovulating.
"Some studies indicate that women who are at high conception risk behave differently in terms of exposing more skin," he adds. One study used photos taken at a disco to detect differences in the exposure, so to speak, of ovulating women.
"The dressing difference would imply behavioral differences that potentially men pick up on," Thornhill says.
Hey, maybe we guys aren't quite as dense as we seem.
Still, it's not a bad idea to send those flowers.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.