The evolutionary case for female promiscuity.

Science, technology, and life.
Nov. 3 2006 7:57 AM

Why Girls Sleep Around

The evolutionary case for female promiscuity.

(For the latest Human Nature columns on alcohol, cybersex, and Rush Limbaugh, click here.)

Evolution favors female promiscuity. In a study of mouse-like marsupials, "survival of babies with promiscuous mothers was almost three times as high as those in the monogamous group." Key reasons: 1) "The sperm of some males were far more successful than others." 2) "Babies fathered by these males were twice as likely to survive." Takeaway for women: "Polyandry improves female lifetime fitness." Takeaway for men: "Males with more competitive ejaculates sire more viable offspring." Fine print:  "Males usually died after a short and intense single mating season due to exhaustion and aggressive encounters with other males." (Did we mention that female promiscuity promotes big testicles and small brains in males? For Human Nature's case against promiscuity in humans, click here.)

William Saletan William Saletan

Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

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Lower body temperature increases longevity in mice. The mice were engineered to maintain temperatures one degree to half a degree lower than normal; the increase in longevity was 12 percent for males and 20 percent for females. Researchers' conclusion: Chilling your body could get you the life-extension benefits of calorie restriction without the nasty kale-and-tofu diet. Fine print: We don't know yet whether this is safe for humans. Cynical view: Hmmm … tofu, self-refrigeration, or death? I'll take death. (For Human Nature's previous updates on calorie restriction, see below and click here. For the life-extension benefits of exercise, click here.)

A substance in wine, grape skins, and peanuts might protect you from the harmful effects of a fatty diet. Mice that were fed the substance, resveratrol, with a fatty diet got just as fat as mice that ate the same diet without resveratrol. But they didn't get the same heart damage, liver damage, or pre-diabetic blood changes. They also lived 15 percent longer, just like mice that ate healthier food. Rash conclusions: 1) Now you can eat all the ice cream you want! 2) Drink more wine! 3) Buy resveratrol pills and live 10 years longer! Researchers' warnings: 1) You'd need about 1,000 bottles of red wine a day to get as much resveratrol as we gave the mice. 2) Wait till we find out whether it works in humans. 3) And wait till we make sure it's safe for you. 4) The safer course is to eat healthy food. 5) But, psssst, we're taking resveratrol ourselves. (For Human Nature's takes on global obesity and regulating fatty food, click here and here.)

A survey produced some surprises about global sex trends. Findings: 1) People are not losing their virginity earlier. 2) Promiscuity is more common in industrialized countries than in Africa. 3) Premarital sex is increasing only because people are getting married later. 4) Married people have more sex. 5) The richer the country, the lower the ratio of male-to-female promiscuity. Researchers' conclusions: 1) Education about sexual protection may be more important than promiscuity in preventing sexually transmitted diseases. 2) Single women may be better equipped to protect themselves than married women because they can use the availability of alternative men to negotiate condom use. (For Human Nature's take on polygamy and homosexuality, click here. For sodomy, click here. For contraception, click here. For cybersex, click here.)

Scientists claim to have grown "mini-livers" in the lab. Boasts: 1) Within a year, we'll use these livers (instead of animals or human volunteers) to test drugs. 2) Eventually we'll grow whole livers for transplants. 3) We grew the livers from cord-blood stem cells, so no embryos were destroyed. Rebuttals: 1) These aren't livers; they're the size of a quarter. 2) Growing a whole liver would require blood vessels and a "fibrous skeleton," which is a lot harder. 3) This research hasn't been peer-reviewed. 4) The scientists are announcing it anyway because they've formed a company to make money off it. (For Human Nature's take on growing organs from embryos, click here. For bladders grown in labs, click here. For meat grown in labs, click here.)

Another study suggests divorce harms women's long-term health. In the short term, women who got divorced reported seven percent more "psychological distress" than those who didn't. After a decade, the divorced group reported 37 percent more physical illness than the married group. Women who remarried fared better than those who didn't. Conservative spin: The illness is spiritual, since the differences persisted "even after the researchers controlled for … income." Liberal spin: The illness is economic, since, apart from income, divorce takes away your job options, housing security, insurance, transportation, co-parenting, social networks, and other support systems. (For Human Nature's previous update on divorce and women's health, click here.)

Scientists are developing life-extension drugs modeled on ultra low-calorie diets. Harsh "calorie restriction" dramatically extends longevity in many animals, possibly by triggering the body's "starvation response," which redirects cells from reproduction to maintenance. Skeptics' arguments: 1) Thinness increases your risk of death. 2) Models indicate CR might extend human life a year and a half at most. 3) For this you'd subsist on tofu and kale? 4) American can't even stick to moderate diets, let alone this. Optimists' arguments: 1) Drugs could deliver the same effect as CR without the misery. 2) One drug already shows promise in animals. 3) We could postpone diseases more cost-effectively by slowing aging than by spending money on each disease. 4) We could extend healthy life to 112 years, keeping "old" folks productive. (For Human Nature's take on aging as a conquerable disease, click here. For the economics of life extension, click here and here. For a previous update on CR, click here.)

A study of mice suggests depression can weaken your skeleton. Mice exposed to chronic mild stress "display behavioraldepression accompanied by impaired bone mass and structure." Antidepressants reverse this effect. Proposed conduit: the sympathetic nervous system. Old theory: Sticks and stones may break my bones, but words will never hurt me. New theory: Words may break your bones. (For Human Nature's previous update on spousal contact, stress relief, and blister healing, click here.)

Parents of "black and white" twins are going public. After a British newspaper featured a pair of Australian babies (one dark-skinned, the other light-skinned), two British couples showed off their own contrasting twins. All the twins are fraternal; all three couples include at least one mixed-race parent. Since skin color varies along a spectrum based on as many as seven genes, the odds of mixed-race parents producing a clearly black or white baby are extremely low. The odds of doing it twice in the same family, much less the same pregnancy, are that much lower. Tabloid spin: "One in a million!" Cynical View A: The "black" babies actually look quite mixed, and there are now roughly 1.3 million interracial couples in the United States alone, so there'll be more of these stories. Cynical View B: And then there'll be none, as the press realizes they're no longer newsworthy. (For Human Nature's take on the criminal propensities of black babies, click here.)

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