Tofu eaters have been the butt of carnivore’s jokes for years. But if you laughed at your vegetarian neighbors’ “tofurkey” this Thanksgiving, the last laugh may be on you. Their sex life may be better than yours.
That’s what scientists who studied a group of red colobus monkeys in Uganda’s Kibale National Park think, anyway. Their research, published in the journal Hormones and Behavior, studied the effects of phytoestrogens (plant-based estrogenic compounds similar to the human sex hormone estrogen) on the behavior of primates in the wild. Michael Wasserman and his colleagues from the University of California Berkeley’s Department of Environmental Science followed a group of the monkeys for 11 months, taking careful note of what they ate, and any effects their diet seemed to have on their behavior.
What they found was that the more that the monkeys ate the leaves of a tropical tree called Millettia dura that is high in phytoestrogens, the higher the levels of the hormones estradiol and cortisol were found in their collected fecal samples. This tree is a close relative of soy, also considered to be high in phytoestrogens. Observing the monkeys’ behavior after eating the leaves of this tree, the researchers watched the amount of aggression (as evidenced by the number of fights and chases), the time spent grooming, and the time spent having sex. The higher the levels of these estrogen-altered hormones in the monkeys, the less time they spent grooming, and the more time they spent having sex.
But isn’t estrogen a female sex hormone?
In one sense, yes. Women undergoing menopause often increase their intake of soy-based products because these products can relieve some of their adverse symptoms. So the researchers were interested to see what an increased diet high in phytoestrogens would do to male monkeys – would they become more effeminate? Contrary to expectations, they became more macho.
Co-author of the study Katherine Milton says about the study’s findings, “With all of the concern today about phytoestrogen intake by humans through soy products, it is very useful to find out more about the exposure to such compounds in living primates and, by analogy, human ancestors. This is particularly true when determining the influence of phytoestrogens on reproductive behavior, which is the whole keystone of natural selection.”
She and Wasserman point out that there may be multiple factors that influence primate hormone levels and their resulting behaviors. In other words, there could be other explanations for the male monkeys’ newfound enthusiasm for sex after eating the phytoestrogen-rich plants. They are following up their research on the red colobus monkeys with other studies on the behavior of chimpanzees, who primarily ate fruit. If they find high phytoestrogen levels in the chimpanzees (who, like apes, are very likely our distant ancestors), they’ll have a better idea of the effects of estrogenic plants like this and the role they played in human evolution.
As Wasserman says, “If phytoestrogens make up a significant proportion of a fruit-eating primate’s diet, and that consumption has similar physiological and behavioral effects as those observed in the red colobus, then estrogenic plants likely played an important role in human evolution. After studying the effects of phytoestrogens in apes and fruit-eating primates, we can then get a better sense of how these estrogenic compounds may influence human health and behavior.”
So if my husband eats soy, will he give up grooming and want to have more sex?
While many women might actually be able to put up with a little less grooming in their husbands if they made up for it by showing more interest in them sexually, the link between soy products and a greater enthusiasm for sex have not been proven. After all, the study was on monkeys, and husbands are humans. Most of them, anyway.