RALEIGH, N.C. - Even hippos and whales, it turns out, can get fat. But how can you tell, let alone slim one down?

Obesity among zoo animals is such a complex problem that zoo nutritionists, scientists and others, from as far away as England, gathered at North Carolina State University last week for a symposium on such weighty matters as how to tell when an oyster's weight is about right.

The basic cause of chubbiness is no different for moray eels and wildebeests than for humans: "If the energy going in exceeds the energy going out, you're going to get fat," said Karen Lisi, a nutritionist at the Smithsonian National Zoological Park. "We don't like to hear that, but that's pretty much how it is for us, too."

With so much variation among creatures, though, nutritionists have to treat the diet of each species almost like an individual scientific study.

When your zoo has hundreds of creatures as different as tree frogs, fish, birds and elephants, the task can be overwhelming.

Even among birds, the variation in diet is huge, including hummingbirds that sip nectar, fruit-eating parrots and vultures that chow down on rotted meat. The diet for individual animals may have to be adjusted to compensate for changes such as pregnancy, lactation or simply aging, Lisi said.

Her zoo, with about 400 species and 2,000 individual animals, has its own nutrition lab.

Even simply determining whether an animal is overweight is so complex that part of the symposium was dedicated solely to that topic. Sometimes it's obvious when an animal is morbidly obese, Lisi said. Other times, though, a quirk of a given species, such as thick fur, makes it more difficult, and zoo staff might not be able to tell without tranquilizing it and checking by hand. *