Feeding time at the zoo.

Here's a challenge: Feed a healthful meal to thousands of hungry mouths, each with different likes and dislikes. No sweat, you say, thinking up ways to improve the typical lunchroom fare. The catch: In this job, each mouth you have to feed belongs to a different kind of animal species. From tiny leaf-cutting ants to huge whales, each species has different dietary needs.

Designing such meals is the job of zoo nutritionist Ellen Dierenfeld. She's one of only eight people nationwide who knows how to serve a healthful lunch to an egret, an aye-aye, and an elephant (and perhaps a human, too).

Dierenfeld's father was a veterinarian, so she grew up learning how to treat animals that were ill. Her most valued lesson: "Good nutrition is preventive medicine," she says. "I would rather keep animals healthy than fix them once they are sick."

Overseeing animal nutrition programs in four zoos and an aquarium for New York's Wildlife Conservation Society, Dierenfeld feeds 8,000 mouths a day, belonging to more than 1,000 different species. To satisfy all those finicky appetites, Dierenfeld spends about $1 million a year on mammoth-size meals: * 900 tons of hay for the elephants, giraffes, and other herbivores * 187 tons of fish to feed penguins, seals, and their seafaring friends * 36 tons of meat to toss to the lions and other carnivores * 4 tons of fruits and veggies for the birds, apes, and monkey gang * plus all the insects, worms, mice, and rats that reptiles and other critters can devour.

MENU FOR THE MASSES

Dierenfeld admits that menu-planning for the masses is no easy task. Keeping zoo animals (and humans) healthy means feeding each one just the right amount of nutrients, substances in food that every living thing needs to live, grow, and reproduce.

In addition to the big-three energizing, growth-promoting nutrients--carbohydrates, proteins, and fats--each animal needs certain amounts of vitamins and minerals. These nutrients help the body make and use other nutrients. For instance, B vitamins and the mineral manganese help the body use sugars.

Animals also need water, which transports all these other goodies to body cells. Inside cells, water enables the chemical reactions that release energy from food to take place.

But "it's hard to balance a diet unless you know what's in it," Dierenfeld says. And unlike the cereal boxes you yank off grocery shelves, zoo foods (e.g., insects) don't come with nutrition labels attached. So Dierenfeld must run bees through a blender and count the calories in crickets to analyze their nutritional content.

A typical zoo problem: What makes a better meal for snakes--mice or rats? By grinding up and extracting the nutrients from these rodents, Dierenfeld discovered that rats provide more vitamin E than mice do. In snakes (and people), vitamin E is necessary for building cell membranes. "Baby rats are like vitamin E pills for reptiles," Dierenfeld says. (Don't worry, you can get all the vitamin E you need from vegetable oils, eggs, and grains.

NUTRITION GONE WILD

Feeding zoo animals wasn't always such a scientific picnic. Some zoo keepers used to feed their animals whatever the critters would eat. For instance, some carnivores were fed exclusively muscle meat, a diet rich in protein, but low in some vitamins and minerals. Such feeding practices led to problems: A number of animals gained too much weight, others didn't reproduce, and some didn't live very long.

To end that trend, Dierenfeld and others who study animal nutrition "go wild." They travel to deserts, rain forests, oceans, and other natural habitats to see what wild animals eat. Then they try to re-create those diets for animals in captivity.

While keeping food tabs on wild aye-ayes, endangered primates from Madagascar, the "nutrient-detectives" found that in addition to eating seeds and insect larvae, the aye-ayes nibble on bits of a certain fungus. In a lab, the nutritionists discovered that the fungus was loaded with calcium, a mineral needed for strong bones, teeth, and muscles. The fungus is hard to get. So to keep aye-ayes healthy in zoos, Dierenfeld adds proper levels of calcium to their diet. (She suggests you do the same--with milk products, not fungus.

FRUIT FOR THOUGHT

Another challenge for food nutritionists: "Wild" diets can sometimes be very costly. So Dierenfeld often has to improvise. Instead of feeding monkeys native rain-forest fruits, for example, she feeds them fruits she can buy more easily.

Such substitutions can cause problems. These homegrown fruits are often sweeter and have thinner skins to suit human tastes. They may not supply the nutrients that animals need.

"We're analyzing 100 different Indonesian [wild] fruits and we're finding that they contain more fiber, less sugar, and more minerals than many cultivated fruits," Dierenfeld says. So Dierenfeld's lab must make adjustments in their cultivated fruit salad. If fruit-eating monkeys "don't get enough fiber, [they] can get digestive-tract blockages. Too much sugar, and their teeth rot," she says.

Keeping track of all these special requirements is tough, Dierenfeld says. But it's still easier than keeping kids away from candy.

Oh, some animals do like "junk food"--stuff like bananas and peanuts--she admits. But nature has a way of controlling the craving: One large, dominant chimp may "beat up" smaller chimps to get to the bananas. But when the big chimp eats too many sweets, its growth is stunted. The leaf-eating chimps grow bigger and stronger and push around the junk-food-loving chimp.

At the very least, says Dierenfel, she can limit animals' access to junk food. It's much harder to get people to give up their ice cream bars and start eating broccoli. "Once we understand an animal's diet," Dierenfeld says, "they eat better than we do."