Lantar, the adult male tiger at the Philadelphia Zoo, eats about $6,000 worth of food a year. The keeper of this and other juicy informational morsels about animal feeding habits is Barbara Toddes, the zoo’s director of nutrition.

She's not eager to dispense lighthearted trivia, though. (Still, we did find out the zoo gets its meat from a national zoo food distribution store that provides horse meat -- yes, horse meat -- to Europe.) For Toddes, this is damn serious business. She has to figure out exactly what, how much and how often to feed every species from pygmy marmoset to giraffe.

Contrary to what you might think, Toddes' priority in formulating the zoo's menu isn't simulating the meals that the animals would eat in the wild. It's ensuring they get the right nutrients. "The food," she says, "is just a packaging."

Central to most animals' diets is a "complete feed" -- a formula animals would never encounter in their natural habitats. Toddes designs complete feeds by looking at what each animal's closest domestic relative eats, feeding the animal something similar and then closely monitoring its diet and behavior.

Then Toddes provides food encouraging "normal and natural" feeding behaviors. Fruit bats, for instance, have their food hung from the ceiling and walls so they can feed the way they would in the wild.

This principle only goes so far, of course. Large predators get their food served to them dead. End of discussion.

There are other menu considerations. For one thing, the animals have to like the food well enough to eat it (Primates, in particular, can be picky.). Toddes must also worry about practicality and affordability.

"I can't very well travel to New Zealand to collect a certain kind of termite," she says. That's not to say that the animals eat poorly when the zoo goes through hard times: In all Toddes' years at the zoo, the food budget (including an annual grocery bill of about $450,000) has never been cut.

Toddes regularly puts curious visitors to work preparing meals for the animals. They begin with hand washing -- "You have to scrub with soap for 20 seconds before rinsing" -- and end up in meticulously clean kitchens where visitors are asked to don latex gloves before they can "cook."

This is how you prepare a snack of fresh and dehydrated fruit for the fruit bats, whose complete feed is a nectar-based mush: Weigh out a piece of fruit and multiply the weight by the caloric density to get total calories. Repeat until the batch totals 53 calories. Do it again.

It's precise and painstaking work, but some zookeepers can count calories by sight.

When Toddes, who has been at this for 20 years, came to the zoo, there wasn't a nutritionist position. She began as a research assistant doing "digestion trials," which meant collecting and analyzing the stools of rhinos, kangaroos, elands and zebras.

"It was really fun," she says straight-facedly of the trials which eventually led to today's nutrition department. These days, nutrition is accepted as a very necessary part of the zoo's work but Toddes doesn't put visitors to work because she wants their respect; her real motive is more pragmatic.

"What I want you to take away is this: Don't you dare come to the zoo and feed my animals," she says. "You're screwing everything up."