Sexual promiscuity is rampant throughout nature, and true faithfulness a fond fantasy. Oh, there are plenty of animals in which males and females team up to raise young, as we do, that form “pair bonds” of impressive endurance and apparent mutual affection, spending hours reaffirming their partnership by snuggling together like prairie voles or singing hooty, doo-wop love songs like gibbons, or dancing goofily like blue-footed boobies.

Yet as biologists have discovered through the application of DNA paternity tests to the offspring of these bonded pairs, social monogamy is very rarely accompanied by sexual, or genetic, monogamy. Assay the kids in a given brood, whether of birds, voles, lesser apes, foxes or any other pair-bonding species, and anywhere from 10 to 70 percent will prove to have been sired by somebody other than the resident male.

As David P. Barash, a professor of psychology at the University of Washington in Seattle, put it with Cole Porter flair: Infants have their infancy; adults, adultery. Dr. Barash, who wrote “The Myth of Monogamy” with his psychiatrist-wife, Judith Eve Lipton, cited a scene from the movie “Heartburn” in which a Nora Ephronesque character complains to her father about her husband’s philanderings and the father quips that if she’d wanted fidelity, she should have married a swan. Fat lot of good that would have done her, Dr. Barash said: we now know that swans can cheat, too. Instead, the heroine might have considered union with Diplozoon paradoxum, a flatworm that lives in gills of freshwater fish. “Males and females meet each other as adolescents, and their bodies literally fuse together, whereupon they remain faithful until death,” Dr. Barash said. “That’s the only species I know of in which there seems to be 100 percent monogamy.” And where the only hearts burned belong to the unlucky host fish.

Even the “oldest profession” that figured so prominently in Mr. Spitzer’s demise is old news. Nonhuman beings have been shown to pay for sex, too. Reporting in the journal Animal Behaviour, researchers from Adam Mickiewicz University and the University of South Bohemia described transactions among great grey shrikes, elegant raptorlike birds with silver capes, white bellies and black tails that, like 90 percent of bird species, form pair bonds to breed. A male shrike provisions his mate with so-called nuptial gifts: rodents, lizards, small birds or large insects that he impales on sticks. But when the male shrike hankers after extracurricular sex, he will offer a would-be mistress an even bigger kebab than the ones he gives to his wife — for the richer the offering, the researchers found, the greater the chance that the female will agree to a fly-by-night fling