The scientist gasps and drops the binoculars. A notebook falls from astonished hands. Graduate students mutter in alarm. Nobody wants to be the one to tell the granting agency what they're seeing.

A female ape wraps her legs around another female, "rubbing her own clitoris against her partner's while emitting screams of enjoyment." The researcher explains: It's a form of greeting behavior. Or reconciliation. Possibly food-exchange behavior. It's certainly not sex. Not lesbian sex. Not hot lesbian sex.

Six bighorn rams cluster, rubbing, nuzzling and mounting each other. "Aggressosexual behavior ," the biologist explains. A way of establishing dominance.

A zoo penguin approaches another, bowing winsomely. The birds look identical and a zoogoer asks how to tell males and females apart. "We can tell by their behavior ," a researcher explains. "Eric is courting Dora." A keeper arrives with news: Eric has laid an egg.

They've been keeping it from us: There are homosexual and bisexual animals, ranging from charismatic megafauna like mountain gorillas to cats, dogs and guinea pigs. There are transgendered animals, transvestite animals (who adopt the behavior of the other gender but
don't have sex with their own), and animals who live in bisexual triads and quartets.

Bruce Bagemihl spent 10 years scouring the biological literature for data on alternative sexuality in animals to write "Biological Exuberance: Animal Homosexuality and Natural Diversity," 768 pages about exactly what goes on at "South Park's" Big Gay Al's Big Gay
Animal Sanctuary. The first section discusses animal sexuality in its many forms and the ways biologists have tried to explain it away. The second section, "A Wondrous Bestiary," describes unconventional sexuality in nearly 200 mammals and birds -- orangutans, whales, warthogs, fruit bats, chaffinches.

Bagemihl's dry style is obedient to the precepts of scientific writing. He explains why animals can be called homosexual or bisexual, but not gay, lesbian or queer, and he follows the rules -- though "homosexual" frightens some who prefer terms like male-only social interactions, multifemale associations, unisexuality, isosexuality or intrasexuality.
(Fortunately, as a book reviewer, I am not bound by this rule. We're talking gay animals!) Yet the book is thrillingly dense with new ideas, and with scandalous animal anecdotes. In other words, an ideal bedside read.

It's not just about hot sex. Bagemihl includes nonsexual bonds. Friendships. Female grizzlies sometimes form partnerships, traveling together, defending each other, raising cubs together and putting off hibernation in what seems to be an attempt to stay together longer.

Nor is it all cuddling and consensuality. Bagemihl chronicles homosexual incest (foxes), rape (albatrosses) and homophobia (white-tailed deer).

His favorites are beasts with "a special courtship pattern found only in homosexual interactions." Two percent of male ostriches ignore females and court males with a lively dance that involves running toward your chosen partner at 30 mph, skidding to a stop in front
of him, pirouetting madly, then "kantling," which includes crouching, rocking, fluffing your feathers, puffing your throat in and out and twisting your neck like a corkscrew. A male ostrich courting a female omits the speedy approach, shortens the display, adds a booming
song and may include symbolic feeding displays. Male ostriches have not been seen actually having sex, unlike male flamingo pairs, who mate, build nests and sometimes rear foster chicks.

Some homosexual animals have one-night stands and some have long marriages. Gay and lesbian geese stay together year after year. Bottlenose dolphins don't form male-female couples, but males often form lifelong pairs with other males. Some are interested only in
males, but others are bisexual and happily indulge in beak-genital propulsion and more with male or female alike.

Male black swans court and form stable pairs. With two males, they are able to defend huge territories from other swan couples, which sounds like a double-income-no-kids situation except that they often manage to wangle some eggs from somewhere -- all right, they steal
them -- and become model parents, twice as successful as straight parents.

There's a certain temptation to leaf through the book shouting "Caribou? Gay! Red-necked Wallaby? Gay! Golden Plover? GAY GAY GAY!" But of course it's not that simple.

All bonobos and 1 percent of ostriches participate in homosexual activities -- so within the animal kingdom there is tremendous diversity of sexualities. Moreover, the world is full of animals who are straight. But we know so little about the sex lives of most animals that we
must be cautious in our assumptions. Many creatures have never been seen having sex of any kind. The black-rumped flameback has been observed in male-male mating, but never male-female mating. Yet presumably they don't buy baby flamebacks at the corner store. ...

Surprisingly often, observers don't know what they're seeing. If males and females look alike, researchers assume that when they see animals mating, they are seeing a male and a female, and the one on top is the male. Thus, the penguin Eric, later renamed Erica. If they
switch positions, no doubt it's just confusion.

Often, it's plain that animals are engaging in homosexual behavior -- short of wearing gay pride T-shirts, there's no way those walruses could be clearer -- but the observer can't fathom it.

One unusually candid biologist wrestled with the realization that the bighorn rams he studied frequently had sex with each other, and weren't just showing nice wholesome aggression. "To state that the males had evolved a homosexual society was emotionally beyond me. To conceive of those magnificent beasts as 'queers' -- Oh God!"

Bagemihl ridicules ingenious explanations researchers have given for why animals might appear not to be straight arrows. It's dominance. It's a contest of stamina. It's barter for food. It's aggression. It's appeasement. They're confused and don't realize that they're both
the same sex. It's a way of reducing tension. They're just playing! And my favorite: It's a greeting.

Dominance is the most popular excuse, with animals portrayed as jockeying for status with the ferocity of assistant professors, when they're only fooling around. "At times, the very word dominance itself becomes simply code for 'homosexual mounting,' repeated mantralike
until it finally loses what little meaning it had to begin with," Bagemihl writes.

Captive animals are subjected to the prison comparison: They're like prisoners in an unnatural situation, so that's not real homosexual activity in that cage. While some captive animals adopt an "if you can't be with the one you love, love the one you're with" philosophy, others decline to have sex with animals they don't care for. When it comes to animals in the wild freely choosing to pirouette, or give the Really Big Greeting, this explanation collapses.

The idea that animals can't tell each other's gender and accidentally have sex or form homosexual pairs has the age-old appeal of making animals look really, really dumb, but doesn't hold up in the face of evidence that animals know quite well who they're hitting on.

Sometimes it just seems better not to bring it up. One researcher discovered homosexual mounting in white-tailed deer, yet when an 800-page book on white-tails was published, the researcher co-wrote the chapter on behavior with no mention of it.

A report on killer whale behavior that described homosexuality in male orcas was reissued as a government document for the U.S. Marine Mammal Commission with those passages -- and only those passages -- deleted.

Popular books by scientists often include material that doesn't make it into journals. The authors relax, drop the jargon, tell anecdotes, speculate. But, seeking sympathy for the animals they love, most scientists balk at describing bisexuality and homosexuality in the
animals. Will people be less likely to save the gorilla if the gorilla has a gay lifestyle? ...

Zoology adheres to a "folk model" of homosexuality as perverse, unnatural and bad, Bagemihl argues, and is far behind the humanities in recognizing it as a legitimate subject of inquiry. Bagemihl formulates the charmingly named theory of biological exuberance, of which
homosexuality is one manifestation. He wants to unlink biological analysis from the idea that reproduction -- and hence, heterosexuality -- is all. Biology must accept the apparent purposelessness of sexualities, he argues. Sexual pleasure is "inherently valuable" and "requires no further 'justification."' ...

So what if animals are gay? Are people vindicated in our diverse sex lives by diversity in animals? If they put us on trial, can we bring as character witnesses lions who make the Sign of the Great Tawny Beast with same-sex lions? (And they do. Unless that's just a
greeting. ) No, not unless we would bring those same lions to testify that killing your new significant other's children is a useful way to free up their time for you and your future children. Animals do all kinds of things that we frown on for ourselves.

But we can bring the lions to testify that there's nothing unnatural about human sex lives, that bisexuality and homosexuality are not among those twisted human inventions, like income tax, or graduate school, or step aerobics, that have no close analog in the wild.

As Bagemihl says of this widely expressed idea, "What is remarkable about the entire debate about the naturalness of homosexuality is the frequent absence of any reference to concrete facts or accurate, comprehensive information about animal homosexuality."

There's no longer any excuse. At more than 750 pages of profusely illustrated, carefully referenced information, this is the ideal book to slam down on the fingers of anyone who says homosexuality isn't natural.

--Susan McCarthy, Salon magazine