ANN ARBOR---Despite considerable evidence to the contrary, the cause of much of the conflict between the sexes isn't just that men are not monogamous.
According to a University of Michigan author, the underlying trouble is that we humans are anisogamous---we have sex cells of unequal size. Big eggs and little sperm. And what works to make eggs successful is very different from what works for sperm.
This fundamental inequality is the underlying reason that males and females of most mammalian species, from elephant seals and red deer to human beings, tend to engage in sex-specific types of mating behavior, says Bobbi S. Low, professor at the U-M School of Natural Resources and Environment and the author of "Why Sex Matters: A Darwinian Look at Human Behavior," forthcoming this year from Princeton University Press.
It's the reason guys act like guys whether they're lions or lambs or Yanomamo tribesman or the presidents of post-industrial nations.
In the book, Low draws on biology, anthropology, economics, and many other disciplines to analyze how the evolution of small male and large female sex cells, among many other factors including culture and environment, influences the way each sex behaves in the high-stakes evolutionary game of reproduction. Such an interdisciplinary approach avoids the flawed conclusion that biology is destiny.
Low also discusses various explanations for the unequal size of male and female gametes. That has been the recent subject of lively academic debate, as has the purpose of sex in the first place. Since sexual reproduction reduces our genetic contribution to the next generation by 50 percent and appears ridiculously inefficient compared to simply dividing in half like an amoeba, it's a mystery to many why we do it.
One explanation is that sex is a way to weed out surprisingly common adverse genetic mutations from the human gene pool. We profit from being sexual, as one scientist recently said, "to flush out these deleterious mutations." Another related explanation is that sex reduces fatal pathogen-stress. As Low points out, once there is sex, many other questions remain. "Once the purpose of sex is established," she notes, "you wonder why in so many species, there are two sexes, instead of 13 as there are in some slime molds." The reason, biologists conclude, relates to the survival value of anisogamy.
For small gametes, the best possible reproductive strategy is clearly to get there, as fast and as often as possible, according to Low, an expert in evolutionary biology and behavioral ecology. "But having little bitty gametes only helps if the gametes-carrier gets around and takes some risks," she says. "There's no advantage to sitting back and acting sweet and shy."
Males tend to be mating specialists, Low says, roamers who invest in getting their gametes into a warm, safe place as often as possible. Females are parenting specialists, homers who invest in nurturing and nest-building.
But the mating effort that males expend isn't all fun and games. It's risky business with a large, fixed cost that has to be paid up-front before you play. "At first, males have to invest a great deal of effort to get any return at all," Low says. "In many species, they have to grow large enough to engage in physical combat, and strong enough to range far. They may have to grow weapons, like antlers. Or decorations, like the tail of a peacock. All this takes time and effort, and it's dangerous. In all kinds of species, at all stages of life, males are more likely to die than females."
Once males have invested the effort, however, they stand to reap huge reproductive rewards with very little additional effort. Over 83 percent of all male elephant seals ever born die without reproducing, a 20-year study found. But a highly successful male may have over 90 offspring. A highly successful female in his harem will have about ten. Harems are common in many mammalian species, including humans. Still, it isn't all gravy for guys.
"Mating-specialist males have really a limited number of possible strategies to secure mates," Low observes. "They can try to control females. They can control and gather resources that are useful to females. Or they can display for females, independent of any resources."
The last technique---a risky, costly, and seemingly meaningless advertisement of one's quality as a mate---usually occurs when neither resources nor females can be controlled economically. As a mating strategy, Low observes, it's a little weak. You see it in forest-dwelling grouse drumming on a hollow log. And you also see it, she adds, in teen-age boys on skateboards.