A pair of rare clouded leopard cubs have been born at the National Zoo research centre in northern Virginia, USA.

The National Zoo's Conservation and Research Center is celebrating the birth of two clouded leopard cubs -- the first births of their kind for the National Zoo in 16 years and the first in any North American zoo in six years.

Clouded leopards, an endangered species, present major challenges to those attempting to breed them in captivity -- one big reason for the long gap between captive births. Males are prone to aggression so severe that they often kill their potential mates. Females have a tendency to kill their offspring accidentally or intentionally shortly after giving birth.

The successful breeding that resulted in the two newborn cubs was much longer in the making than their three-month gestation period. Michael E. Ruane of the Washington Post explains:

Experimentation eventually suggested that if a male and female were raised together, the male would not kill the female once they reached adulthood and mated, Howard said. "You want to put the male in with the female, pair them up as early as possible," she said.

Hannibal and Jao Chu were such a pair. They were imported from Thailand last year, Howard said, and reached puberty together. Lang said experts believed the two mated several months ago at the center. About a week ago, curators realized Jao Chu looked as if she might be pregnant.

Several days ago, she was placed on a pregnancy watch, and when she turned down her usual snack of two dead mice Monday morning, Lang said they figured she was pregnant.

Early Tuesday, she vanished from the area of her enclosure that is monitored by video cameras, and about 1:30 a.m. Lang was summoned from his home on the center grounds. He unlocked the door of the leopard's enclosure, entered and spotted her in the corner with the two cubs.

The cubs' birth is especially important, the zoo says, because their genes come from outside the captive clouded leopard population. Their sex won't be known until their first veterinary exam, and they won't be on exhibit to the public.

Clouded leopards are native to southeast Asia, where they're in danger of extinction due to hunting and deforestation.