The capybaras, which look like giant guinea pigs, arrived in October after an absence of nearly 40 years. The black howler monkeys, gone since 1972, are back. And the brand-new plum-headed parrots should take up residence this fall.

The National Zoo, intent on rebuilding its collection after years of decline, has those and several other new or returning animals on exhibit or in the acquisition pipeline. The goal is to invigorate the zoo's aging animal population by bringing in a younger, more diverse group of mammals and birds while enhancing the collection of reptiles, amphibians and invertebrates.

Deaths of older animals, transfers to other zoos and the halting of several breeding programs have severely impacted the zoo's population and variety. The numbers plummeted from more than 6,100 animals representing 485 species in 1992, according to annual zoo inventories, to a current estimate of 2,470 animals representing 435 species.

"If you don't keep track of the animals, you end up with a geriatric collection, and that's what happened here," said Bill Xanten, the zoo's general curator for animal programs, discussing the need to replenish exhibits. "The collection really fell off."

Xanten retired from the zoo in 1996 after a 40-year career that began as a keeper. He returned in February 2003 as the 163-acre animal park was facing its worst crisis in decades. The accidental poisoning of two red pandas the previous month had sparked a congressional inquiry into animal care under Lucy H. Spelman, the zoo's director since 2000. And the American Zoo and Aquarium Association was raising concerns about the stagnant animal collection and deteriorating facilities.

Armed with a mandate from Spelman to rebuild the collection, Xanten said he met with the curatorial staff and gave them carte blanche in acquiring more animals. He also worked with the American Zoo and Aquarium Association's Species Survival Program coordinators, who oversee animal placements for breeding.

Exhibits at the Elephant House were an immediate concern. The baby elephant was drawing crowds rivaling the celebrated giant pandas. But the zoo's sole pygmy hippo had just died, and the lone giraffe, offspring of two that died the previous year, was scheduled to be sent to another zoo to breed.

Over the next several months, the zoo brought in two giraffes and two pygmy hippos. The Elephant House also got three capybaras -- the largest rodent in the world -- which had not been exhibited at the zoo since 1965.

"They're a big hit with visitors," Xanten said.

Another popular attraction is the newly opened Kids' Farm, which offers visitors a chance to pet and help care for various farm animals.

The zoo is increasing its small-mammals collection, adding species and bringing in companion animals for existing exhibits. Besides two howler monkeys, which is the loudest land animal, visitors can view a tamandua, a tree-dwelling anteater from South America, not seen at the zoo since 1975, and two prehensile-tailed porcupines, not exhibited since 1987.

At the Bird House, curator Paul Tomassoni has boosted the collection and recently added a female tinamou from South America, a species not exhibited at the zoo since 1989.

Several new colorful and songbird species will arrive this year, including five plum-headed parrots, which the zoo will breed; a red-billed blue magpie, not exhibited since 1994; and two hooded pitas, last exhibited in 1992.

"We've got about 40 birds lined up to come in this year," Tomassoni said. Outdoor acquisitions are "winter hardy" and can be exhibited year-round.

A Northern-banded water snake made its zoo debut last summer in the Reptile House. An Eastern diamondback rattlesnake, not exhibited since 1973, also has joined the collection. Two Chinese alligators, not seen since 1983, will go on exhibit this month.

The collection's decline began during the tenure of Spelman's predecessor, Michael Robinson. He spent 16 years promoting the zoo as a "bio park," with a focus on animals and the plants native to their habitats.

Robinson showcased invertebrates and opened the popular Amazonia and Think Tank, with its "O-line" for free-ranging orangutans. But major breeding programs were halted or curtailed, and the hoofed-stock exhibits declined.

"We didn't have the facilities then to bring in new animals, and older animals were dying," Xanten said.

The National Academy of Sciences, which studied the zoo's operations for Congress, said the size of the collection fell 54 percent from 1993 to 2002. Acquisitions declined 67 percent from 1999 to 2002.

In rebuilding the collection, the zoo gets most of the animals as loans or gifts from other zoos or animal sanctuaries. Some animals, especially birds, are bought or traded. The zoo is also used as a holding facility for male zebras, giraffes and red pandas for future breeding elsewhere.

The zoo director gives final approval on acquisitions. If a species is new, the curator does a formal proposal on the impact of getting the animal, including cost and any special diet or exhibit requirements. Zoo veterinarians have to sign off and researchers can request a certain subspecies.

Sometimes, the animal collection changes because the zoo changes. Construction for the new Asia Trail has displaced some exhibits, for example, and the cheetahs and kangaroos will be moved from the area to make way for an expanded elephant facility in 2007.

And sometimes, public passions and curator interest dictate which animals come and go.

Pygmy hippos "are very hard to come by these days," said Elephant House curator Tony Barthel, because many zoos are building underwater viewing exhibits for the more aquatic Nile hippopotamus -- and have let their pygmy hippo breeding programs lapse.

The zoo, determined to keep pygmy hippos in its collection, got a male from Toronto last fall and a female from San Francisco in March, after her mate died.

"It's like a pendulum," Barthel said. "We'll continue looking at what animals are here now and what animals we want to have and what spaces will be available for the short term and the long term."

Zoo curator Bill Xanten feeds a recent acquisition: a tamandua, a South American anteater that the zoo had not had since 1975.

Elena Bell, a fourth-grader at D.C's Capital City Charter School, cleans Rose, a Polled Herford bull, at Kids' Farm.

A newcomer to the National Zoo's collection is the prehensile-tailed porcupine, a species the zoo had not exhibited since 1987.