Monogamous for the Moment: Science of Sex

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.