RALEIGH — 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 for a two-day symposium on such weighty matters as how to tell when an oyster's weight is about right.

"It's actually a huge problem, and a multifaceted one," said Michael Stoskopf, a professor at the college. "You have to look at not only diets themselves and the amount of calories delivered, but also things like exercise."

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, determining what it eats in the wild and how best to approximate it in captivity, said Richard Bergl, curator of conservation and research at the N.C. Zoological Park in Asheboro.

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