OF ALL THE WORLD'S CONTINENTS, Antarctica is the least spoiled. There are no malls, no fast food, no casinos—nothing to offend. Unless, of course, you're put off by prostitution, in which case steer clear of Ross Island, where the Adélie penguins turn icebergs into Heidi Fleiss bergs.

According to Dr. Fiona Hunter, a zoologist at Cambridge University, some of the island's female penguins—previously thought to be respectably monogamous—share their sexual favors with males other than their spouses. In return, the males give them stones, hundreds of which are needed to build the average nest. These encounters—the only play-for-pay ever documented in the animal kingdom—"take just a few minutes," says Hunter. "Then the females get up, take a stone and leave." It's the world's oldest profession, all right—but a kinder, gentler version. "The beauty of the penguin system is there are no pimps; the women choose to do it," says Hunter, whose findings appear in this month's issue of The Auk, a respected ornithological journal. "And there's no stigma attached."

Raised near Edinburgh, Hunter, 34, has spent 12 years studying birds of every stripe (to date she has observed more than 10,000 avian couplings). She found Antarctica's feathered floozies during a 1994 research trip to Ross Island, about 650 miles from the South Pole. Their behavior, she believes, has evolutionary benefits: "He stands the chance of fathering chicks and doesn't have to spend time raising them"; she gains access to a broader gene pool, not to mention a sturdier nest. And the cuckolded male? "He's clueless," says Hunter. "All he sees is his mate has come home with a stone." Husbands are always the last ones to know.