SAN FRANCISCO — Giraffes like to neck. Lions can mate 50 times a day. Hornbills practice their own kind of bondage. And how do spiny African hedgehogs do it? Very carefully.

It's Valentine's Day at the San Francisco Zoo and the keepers are conducting their fifth annual Sex Tour. Sometimes romantic, often scientific, the tour is rated X--adults only, please.

"Animals do everything," says penguin keeper Jane Tollini, who first conceived of the tour for animal voyeurs. "Please don't go home and try some of these things. You may hurt yourself."

The Valentine tours, held over the weekend and today, are so popular they quickly sell out. Over the years, the idea has been copied by dozens of other zoos around the country, including the Los Angeles Zoo.

Riding around the San Francisco Zoo on the "Zebra Train," visitors learn about the reproductive behavior of various species, covering such themes as courtship, foreplay, domination, polygamy, monogamy and homosexuality.

By human standards, the penguin is the most romantic animal in the zoo, the tour guides say.

The male digs a burrow and decorates it with whatever he can find--even the bones of dead penguins. The female, not easily impressed, drags everything outside. The male brings items back in and the female takes them back out until the two are finally satisfied with the decor.

"They only breed once a year so they have to make the most of it," Tollini says. "It's always boy on top. And they may spend a while before they get it right."

At the other end of the spectrum are the lions, who can mate more than twice an hour, every hour in the day.

"Anybody raise your hand who can beat that record," quips zoo veterinarian Freeland Dunker, another tour guide.

The male bald eagle courts his mate by bringing her offerings of food. Once the female has agreed on a match, the endangered birds mate in flight. As the female soars overhead, the male flies underneath her upside down. The birds lock talons and plummet toward earth.

Orangutans also are anything but passive. "Their sex is quite imaginative," says Dunker. "They can do it swinging from trees. They can do it from all angles and directions."

Flamingos, on the other hand, like an audience--as many as 14 to 20 of the long-legged birds gathered around. "It's a group thing that gets them going," Dunker says.