As another possible noble motive, I thought that widespread sharing of meat helps the hunter's whole tribe, which is likely to flourish or perish together. It's not enough to concentrate on nourishing your own family if the rest of your tribe is starving and can't fend off an attack by tribal enemies. This possible motive, though, returns us to the original paradox: the best way for the whole Ache tribe to become well nourished is for everybody to humble themselves by pounding good old reliable palm starch and collecting fruit or insect larvae. The men shouldn't waste their time gambling on the occasional peccary.

In a last effort to detect family values in men's hunting, I reflected on hunting's relevance to the role of men as protectors. The males of many territorial animal species, such as songbirds, lions, and chimpanzees, spend much time patrolling their territories. Such patrols serve multiple purposes: to detect and expel intruding rival males from adjacent territories; to observe whether adjacent territories are in turn ripe for intrusion; to detect predators that could endanger the male's mate and offspring; and to monitor seasonal changes in abundance of foods and other resources. Similarly, at the same time as human hunters are looking for game, they too are attentive to potential dangers and opportunities for the rest of the tribe. In addition, hunting provides a chance to practice the fighting skills that men employ in defending their tribe against enemies.

This role of hunting is undoubtedly an important one. Nevertheless, one has to ask what specific dangers the hunters are trying to detect, and whose interests they are thereby trying to advance. While lions and other big carnivores do pose dangers to people in some parts of the world, by far the greatest danger to traditional hunter-gatherer human societies everywhere has been posed by hunters from rival tribes. Men of such societies were involved in intermittent wars, the purpose of which was to kill men of other tribes. Captured women and children of defeated rival tribes were either killed or else spared and acquired as wives and slaves, respectively. At worst, patrolling groups of male hunters could thus be viewed as advancing their own genetic self-interest at the expense of rival groups of men. At best, they could be viewed as protecting their wives and children, but mainly against the dangers pound by other men. Even in the latter case, the harm and the good that adult men bring to the rest of society by their patrolling activities would be nearly equally balanced.

Thus, all five of my efforts to rescue Ache big-game hunting as a sensible way for men to contribute nobly to the best interests of their wives and children collapsed. Kris-ten Hawkes then reminded me of some painful truths about how an Ache man himself (as opposed to his wife and kids) gets big benefits from his kills besides the food entering his stomach.

To begin with, among the Ache, as among other peoples, extramarital sex is not uncommon. Dozens of Ache women, asked to name the potential fathers (their sex partners around the time of conception) of 66 of their children, named an average of 2.1 men per child. Among a sample of 28 Ache men, women named good hunters more often than poor hunters as their lovers, and they named good hunters as potential fathers of more children.

To understand the biological significance of adultery, recall that the facts of reproductive biology discussed in chapter 2 introduce a fundamental asymmetry into the interests of men and women. Having multiple sex partners contributes nothing directly to a woman's reproductive output. Once a woman has been fertilized by one man, having sex with another man cannot lead to another baby for at least nine months, and probably for at least several years under hunter-gatherer conditions of extended lacta-tional amenorrhea. In just a few minutes of adultery, though, an otherwise faithful man can double the number of his own offspring.

Now compare the reproductive outputs of men pursuing the two different hunting strategies that Hawkes terms the “provider” strategy and the “show-off” strategy. The provider hunts for foods yielding moderately high returns with high predictability, such as palm starch and rats. The show-off hunts for big animals; by scoring only occasional bonanzas amid many more days of empty bags, his mean return is lower. The provider brings home on the average the most food for his wife and kids, although he never acquires enough of a surplus to feed anyone else. The show-off on the average brings less food to his wife and kids but does occasionally have lots of meat to share with others.

Obviously, if a woman gauges her genetic interests by the number of children whom she can rear to maturity, that's a function of how much food she can provide them, so she is best off marrying a provider. But she is further well served by having show-offs as neighbors, with whom she can trade occasional adulterous sex for extra meat supplies for herself and her kids. The whole tribe also likes a show-off because of the occasional bonanzas that he brings home for sharing.

As for how a man can best advance his own genetic interests, the show-off enjoys advantages as well as disadvantages. One advantage is the extra kids he sires adultorously. The show-off also gains some advantages apart from adultery, such as prestige in his tribe's eyes. Others in the tribe want him as a neighbor because of his gifts of meat, and they may reward him with their daughters as mates. For the same reason, the tribe is likely to give favored treatment to the show-off's children. Among the disadvantages to the show-off are that he brings home on the average less food to his own wife and kids; this means that fewer of his legitimate children may survive to maturity. His wife may also philander while he is doing so, with the result that a lower percentage of her children are actually his. Is the show-off better off giving up the provider's certainty of paternity of a few kids, in return for the possibility of paternity of many kids?

The answer depends on several numbers, such as how many extra legitimate kids a provider's wife can rear, the percentage of a provider's wife's kids that are illegitimate), and how much a show-offs kids find their chances of survival increased by their favored status. The values of these numbers must differ among tribes, depending on the local ecology. When Hawkes estimated the values for the Ache, she concluded that, over a wide range of likely conditions, show-offs can expect to pass on their genes to more surviving children than can providers. This purpose, rather than the traditionally accepted purpose of bringing home the bacon to wife and kids, may be the real reason behind big-game hunting. Ache men thereby do good for themselves rather than for their families.

Thus, it is not the case that men hunters and women gatherers constitute a division of labor whereby the nuclear family as a unit most effectively promotes its joint interests, and whereby the work force is selectively deployed for the good of the group. Instead, the hunter-gatherer lifestyle involves a classic conflict of interest. As I discussed in chapter 2, what's best for a man's genetic interests isn't necessarily best for a woman's, and vice versa. Spouses share interests, but they also have divergent interests. A woman is best off married to a provider, but a man is not best off being a provider.

Biological studies of recent decades have demonstrated numerous such conflicts of interest in animals and humans-not only conflicts between husbands and wives (or between mated animals), but also between parents and children, between a pregnant woman and her fetus, and between siblings. Parents share genes with their offspring, and siblings share genes with each other. However, siblings are also potentially each other's closest competitors, and parents and offspring also potentially compete. Many animal studies have shown that rearing offspring reduces the parent's life expectancy because of the energy drain and risks that the parent incurs. To a parent, an offspring represents one opportunity to pass on genes, but the parent may have other such opportunities. The parent's interests may be better served by abandoning one offspring and devoting resources to other offspring, whereas the offspring's inter-ests may be best served by surviving at the expense) of its parents. In the animal world as in the human world, such conflicts not infrequently lead to infanticide, parricide (the murder of parents by an offspring), and siblicide (the murder of one sibling by another). While biologists explain the conflicts by theoretical calculations based on genetics and foraging ecology, all of us recognize them from experience, without doing any calculations. Conflicts of interest between people closely related by blood or marriage are the commonest, most gut-wrenching tragedies of our lives.