Humans, young and old, are not the only species that get bored with their toys.

So do animals - and veterinary experts say that is unhealthy.

This week, about 25 staffers at the National Aquarium took up power drills, handsaws, PVC pipes and an assortment of hardware to build new toys.

Staffers strung together a rope ladder for South American monkeys, chained together Hula-Hoops for bottlenose dolphins and hacked PVC pipes into shapes that will challenge the eight arms of the giant Pacific octopus.

The toys are being placed this week inside the aquarium exhibits for a few hours every day - and then removed so the animals won't tire of them too quickly.

"The idea is to give them stimulation, which is something they need for a healthier life," said Jennifer Mead, a dolphin trainer who helped organize the effort. "Along with the stimulation, they're learning to adapt to their environments and be creative."

Dr. Ian Walker, the aquarium's associate veterinarian, said that studies have shown that animal enrichment - providing new toys along with occasionally altering an animal's diet or habitat - helps to keep animals healthy. It also has become a widely recognized practice among zoos and aquariums nationwide, he said.

"They have less illness, they don't show any of those negative signs - and mating occurs, which is a sign their environment is as close to the natural world as you're going to get," Walker said.

He says boredom is why horses will chew on the wooden doors to their stalls and tigers in a zoo will pace from side to side.

"It can be almost painful to watch," he said.

Jennifer Fiegl, an aquarium spokeswoman, said staffers have been building new toys for years and construct playthings three or four times a year.

Toys sometimes must be carefully introduced into an animal's habitat, Mead said, because the gadgets could seem threatening to the animals. But she said it's difficult to know in advance what may upset or appeal to an animal.

"It really depends on the animal and the object," she said.

When aquarium staffers made a toy for the bottlenose dolphins - a floating plastic barrel with scrubbing brushes attached to its sides - the dolphins were afraid of it.

"They swam at the other side of the pool because they didn't know what it was," she said. "It was about as large as they were and they were afraid of it."

Mead said she and fellow dolphin trainer, Sue Hunter, began placing the barrels alongside the dolphin pool, so the animals could get used to them for a few days.

The dolphins eventually warmed up to having the barrels in the water - after they learned that fish had been stocked in the barrels when they swam near to them, she said.

Fiegl said safety is a major factor in approval of toys. Their designs are reviewed by the aquarium's marine mammal, fish, rain forest and veterinary experts, she said.

Playthings cannot have sharp edges and must be designed with each animal's behavior in mind, she said.

The marmoset's rope ladder cannot contain artificial dyes, because the world's smallest monkeys could chew on it and get sick. The marmoset - a native of South America that is about the size of a chipmunk - seems to gnaw on just about everything, she said.

The toys also must be made of durable materials - no small factor when they are made for a dolphin with 100 teeth.

The Atlantic sea turtle's new back scrubber - a floating, rope-style doormat - is attached to a coating of hard plastic fencing material so that it can withstand chomps from the sea turtle's powerful jaws.

"It's not like we just say, 'Hey maybe this will work,' and we just throw it in," Mead said. "There's a lot more to it."