Land hermit crabs spurn leftovers.

Hermit crabs wouldn't eat last night's pizza for breakfast unless there was nothing else in the fridge. Eating the same kind of food repeatedly just doesn't provide the nutritionally balanced diet they need to grow big and strong.

The land hermit crab Coenobita compressus prefers foods with a different odor from that of its last meal, reports Robert W. Thacker of the University of Hawaii in Honolulu. His studies, described in the February Animal Behaviour, also show that crabs on a mixed diet of flowers, crushed snails, and seeds grow faster than crabs fed only one type of food.

Every animal needs a balanced diet, Thacker says. He wanted to find out how the crabs achieved that goal, since they are generalists that scavenge whatever washes up or drops onto their home stretch of beach.

"Everyone said that hermit crabs just eat the first thing they walk up to," Thacker says. "But they're not just picking up anything on the beach, they're making active choices."

Even so, they won't hold out for new taste sensations indefinitely. Crabs that hadn't been fed for 9 hours were no longer picky eaters.