BIRDS DO IT, BEES DO IT Sex in the Animal Kingdom Sex is often thought of as an animal activity ("You''re a tiger!"), and the animal kingdom is as good a place as any to start a comparative sex education. Evolution has thrown up innumerable sexual variations in the living world. Some creatures, like aphids, manage to do without sex at all and reproduce using a cloning process, while others can''t seem to get enough of it, the humble single-celled paramecium evolving (somewhat extravagantly) eight sexes rather than the more usual two. Some creatures (many species of reef-living fish, for example) have distinctly shaky sexual identities and will change gender at the drop of a hat. Others like spiders and bristle worms combine sex with cannibalism. Indeed, compared to many animals, the sex lives of the most infamous human libertines seem positively lackluster. The Naked Ape As might be expected, when compared with animals, human sex lives most closely resemble those of our nearest relatives--the chimpanzees--rather than gorillas and our more distant cousins the monkeys. Gorillas have tiny penises compared to body size; on average they come in at 1 1"4 inches.

Orangutans aren''t much bigger at 1 1"2 inches, while the relatively small chimps are a whopping 23Ú4 inches. Humans come out best with an average of 5 1"2 inches, but there seems to be no reason for this relatively huge size; a 2-inch penis would do the job just as well. Gorillas don''t need to be well equipped in this respect because a dominant male keeps a harem of females. This means there''s very little day-to-day competition with other males. In contrast, chimps live in large groups where females mate with a number of different males every day. Chimps need a relatively large penis and a high volume of sperm production to compete. The sperm acts as a douche, washing out the sperm left by a female''s previous lovers. It''s the male chimp who produces the most sperm who will father the most children.

This is why chimp testicles weigh around 4 ounces each, while the more monogamous humans clock in at only an ounce and a half. Proportionally the largest testes in the primate world belong to the woolly spider monkey. Females of the species have been known to mate as many as eleven times a day when in heat and literally overflow with sperm. This excess is treated as a nutritious snack, and spider monkeys lick it up by the mouthful. Giving Him the Bone An interesting difference between humans and animals is the lack of a human "penis bone," also known as an os penis. The majority of mammals, including bears, cats, dogs, weasels, moles, shrews, and most primates, possess such a bone, and why humans don''t is a mystery. The size of these bones varies. Walruses have os bones two feet long, resembling ivory truncheons, while bats have tiny ones.

As a rule, fish lack an os bone (which would spoil their streamlined shape), as do most birds (where they''d be an unnecessary weight). Bird Penises Some birds have penises; these are usually found among flightless birds (where the extra weight isn''t a disadvantage) and aquatic birds like swans, ducks, and geese (where aquatic mating means there''s a chance of sperm being washed away). It''s probably a coincidence that penis-owning waterfowl seem to be the ones that most commonly indulge in rape. Male ducks have a reputation for this sort of thing and often take part in "gang bangs"--large numbers of drakes mobbing a female for sex and sometimes drowning her as a consequence. Bird penises tend to resemble mammalian penises but do not contain a urethra to drain the bladder. Some can be remarkably long; the Argentine lake duck commonly has an 8-inch penis, but the largest reach 16 inches. For those who''d like to see one, fresh ostrich penises can be bought over the Internet as an aphrodisiac. Big Penises The record for the world''s largest penis is held by the world''s largest mammal, the blue whale, which has an organ just over ten feet long and one foot in diameter.

The sperm whale (named after spermaceti, a mysterious white oily substance found in its head) comes a close second with a nine-footer. A whale''s penis is known as a dork, which is where the insult comes from. Among land mammals, giraffes have erections reaching four feet long, while elephant erections can reach five or six feet. One enterprising biologist has even gone as far as to calculate the average weight of an elephant penis--59.5 pounds (with testicles at 4.4 pounds each). Rhinos tend to be shorter in this department, their penises being around two feet long; however, they make up for it by being prodigious ejaculators, some coming as many as ten times during a half-hour session. These genitals have been put to some unusual uses in the past.

In India golf bags have been made from elephant penises, while in Africa rhino and hippo penises were often stretched and dried to make whips. Weird Penises Smaller animals may be less impressively equipped, but many make up for it by having bizarre penis shapes. Some animals, like marsupial opossums, have two--one for each of the female opossum''s paired vaginas. Pigs have corkscrew-shaped penises; the male pig makes semirotary motions to literally screw himself into his mate. Spider monkeys have barbed penises to help them hang on during mating. Tomcats also have barbed penises; the pain caused by withdrawal is the stimulus that causes the female to release her eggs. Like pigs, some bulls have corkscrew penises, though in this case it''s an undesirable medical condition that makes mating difficult. Insects have unusual penises.

For example, dragonflies have shovel-shaped organs designed to scoop any previously deposited rival semen out of a female. One of the most bizarre boners belongs to the European rabbit flea, which has a penis consisting of two coiled rods, with sperm wound around the smaller one like springs. Some earwigs have two penises--they are very brittle and a spare is needed in case the main one breaks off. Male bees (drones) have alarming-looking genitals. Even more alarming is the fact that they explode. Once the drone has mated with a queen bee in flight, his genitals burst with a pop that can be heard some distance away. The explosion blocks the queen''s genitals so she can''t receive sperm from any other male. Barnacles have developed a penis to help solve their immobility problem.

Once it''s glued itself to something solid, a male barnacle is stuck for life; however, its penis is commonly 150 percent longer than its body and is able to snake out and molest any female within striking distance. Super Sperm We''re used to thinking of sperm as numerous and tiny, but many creatures produce a small number of relatively enormous sperm cells. The house centipede produces a sperm cell that is as long as its body (commonly over 4.7 inches), and some beetle species go one better and produce sperm that are longer than their owners, coiling them up like springs inside the body. An unfortunate side effect is that after fertilization the female often has to waddle around with a sperm tail sticking out of her rear end. Proportionally, the record for the longest sperm belongs to the fruit fly, Drosophila bifurca; it has sperm 2.4 inches in length, twenty times longer than its entire body. Some creatures produce spermatophores, dense packets of sperm coated with protective gels.

Octopi use spermatophores that are three feet long and as thin as a pencil. Some octopus spermatophores have barbs to stop them from being withdrawn once they''re placed inside the female, a necessary precaution, as unbarbed spermatophores are often removed and eaten. Katydids (relatives of the grasshopper) also make use of sperm as food; here females have to be bribed with sperm food parcels before they''ll allow their eggs to be fertilized. Perverts of the Animal Kingdom Bedbugs have extremely unsavory sexual habits. Female bedbugs are born without vaginas, so sperm is passed on during an act of "traumatic copulation." The bedbug''s penis is a swordlike rod stabbed into the female''s body, depositing sperm into her gut. The sperm then travels via the bloodstream to a special storage gland, where it is kept for further use. This system has a benefit for the female, as in hard times she can use this sperm as a free meal.

Some African bedbugs play a variation to this game by indulging in "male rape." They stab other males in the hope that, when the attacked male mates with a female, some of their sperm will be passed on as well. To obtain a free sperm meal, some males deliberately invite an attack by disguising themselves as females. Some bean weevil species are equally repellent, having spine-covered penises that deliberately cripple the female. The lacerations caused by these penises are often fatal: the female lives long enough to lay her eggs but not so long that she has a chance to mate with rival males. Coming in a close third in the pervert stakes are banana slugs. Slugs are hermaphroditic, and some pairs mate as equals, playing both male and female roles. Others use this moment of intimacy as an opportunity to gnaw off the penis of their sex partner, forcing them to become exclusively female.

These penises represent a hearty meal; banana slugs are commonly 5 inches long but may have penises as long as 30 inches. Another animal with kinky habits is the American slipper snail (Crepidula fornicata). Here a male snail attaches itself to a rock and becomes a female. Another male snail will then settle on the female and mate with her; should a t.