Eliot Spitzer recently joined the long line of male politicians—Bill Clinton, Jessie Jackson, Newt Gingrich, Gary Hart, Larry Craig—that can’t seem to keep a certain body part in their pants. The perennial question asked of these cuckolds is, why’d he do it? But for the biologist, a more apropos inquiry might be why are we surprised that he did?
Who’s Been Sleeping in My Bed?
After all, monogamy isn’t something normally observed in nature. Whether it’s polygny (more than one female at one time) or polyandry (more than one male) the animal kingdom is rife with examples of polygamy. Only about 3 percent of mammals mate for life, making monogamy the exception, not the rule.
Adultery is prevalent even in species once considered the stalwarts of monogamy. Many birds, for instance, practice social monogamy—they pair off and stay with one partner for extended periods, or even the course of a lifetime. This led observational scientists to believe that they did not have sex outside the couplet. However, the advent of genetic fingerprinting proved otherwise: the offspring’s DNA didn’t always match that of the male nest mate. Extra pair copulations (mating outside of the social pair) were unexpected and common. For birds and other animals, social monogamy does not necessarily mean sexual monogamy.
Mutual Philandering
So who’s cheating, and why? Originally, it was thought that in polygamous partnerships, it was the male who was doing all the philandering. In 1948, A.J. Bateman noted that the male fruit fly, Drosophila melanogaster, would mate whenever and with whomever possible; the female was discriminatory and bit less receptive to sex. This led him to conclude that because a male’s genetic lineage—sperm—was small, easy to make, and readily available, he could and would expend it whenever the mood struck. A female’s eggs, on the other hand, are large and biologically expensive to make; they needed to hold out for the best genetic partner possible. From an evolutionary standpoint, males benefited from numerous partners and were therefore naturally promiscuous; females did not benefit, making them naturally more sexually conservative.
This theory turned out to be wrong. Promiscuity is not limited to males, nor does it only benefit them. In her book Dr. Tatiana’s Sex Advice to All Creation, evolutionary biologist Olivia Judson writes, “From stick insects to chimpanzees, females are hardly ever faithful.” Rather, she asserts, females benefit from promiscuity. A greater number of partners mean higher rates of conception for rabbits and prairie dogs, a higher number of eggs for lizards, and more eggs fertilized for fish.
The point of all this bed jumping is, of course, reproductive fitness, making sure offspring get the best genes on the market. While both sexes may be promiscuous in an attempt to make this happen, there is a slight difference between males and females. Studies show that females mated with the most dominant or genetically successful male do not seek extra pair copulations—if they already have the best, why mess with rest? Males, however, will mate with females of varying status; they lean more toward spreading the seed than aiming high (note our politician’s choices, all of “lower” stature than their own: intern, aide, escort, bathroom stall mate, staffer).
If Mouse, Then Man?
But does this animal evidence tell us anything about human behavior? According to behavioral scientist David P. Barash and psychiatrist Judith Eve Lipton, it may give us clues to why so monogamy is hard. In their book Myth of Monogamy: Fidelity and Infidelity in Animals and People, the authors state that monogamy “goes against some of the deep-seated inclinations with which biology has endowed most creatures, including humans.”
In addition, the size difference between men and women indicates that monogamy isn’t necessarily what we’re programmed to do. In humans and other animals, if males are larger than females, it shows that men had to compete for women; the big and tall males won multiple mating opportunities. The degree of polygyny correlates with the size difference in sexes and since men are larger than women are, this indicates we have been mildly polygynous in our not too distant past.
