Front Royal, Virginia (CNN) -- The young cheetahs chirped as a veterinarian gave them a quick but thorough checkup at the Smithsonian Conservation Biology Institute.
"People always expect them to sound like scary carnivores, but they sound more like little birds," Luis Padilla said.
The two cheetahs had been separated from their mother so they could be vaccinated, weighed and swabbed for DNA. While both cubs are attached to their mother, only one can truly call her mom.
Zoologists at the Smithsonian National Zoo have cross-fostered a cheetah cub, persuading a female cheetah to become a foster mom to an at-risk cub.
Cross-fostering cheetah cubs is rare, having been done six times within the North American captive cheetah population.
Single-cub cheetahs are usually at risk because their mothers don't produce enough milk. After two cubs were born to different mothers at the zoo, officials decided to have one mom tend to both to ensure their chance of survival.
The male cub was born first in December to first-time mother Amani.
"A cheetah female can't produce enough milk for just one cub because a single cub won't stimulate mom enough to keep producing milk," cheetah biologist Adrienne Crosier said. "We hand-raised the first cub for about 13 days."
When another singleton female cub was born to 9-year-old Zazi, the zoo biologists decided to sneak the first cub in with the second and see if Zazi would raise both as her own.
They took steps to transfer the smell of one cub to the other by doing things such as rubbing them together. Then they put the two cubs in a box together and let Zazi in. The big cat immediately went to the cub that wasn't naturally hers.
"She picked him up, she immediately started grooming him. And then laid down in the nest box with him, and it took about 20 to 30 minutes before she realized that there were two cubs in there with her," Crosier said.
Zazi has since proven to be a model mother. Within an hour, she was feeding and grooming both.
Cheetahs are considered to be vulnerable on the World Conservation Union's Red List of Threatened Animals. Researchers estimate that there are 7,000 to 12,000 cheetahs in the world, down from about 100,000 at the beginning of the 20th century.
"Hey buddy, nice teeth," Padilla told the young male as he took a look in his mouth and swabbed a sample for DNA testing.
The female let out a loud chirp as the veterinarian tried to swab her mouth.
"She's getting a little worked up," Padilla said.
Biologists are not sure who the real father of the little girl is, hence the DNA testing.
"Last year, we had our new young male coalition, and we wanted to literally teach them about female cheetahs and how to breed because they were very young and inexperienced," Crosier said. "It's always a good management tool to put experienced cats with inexperienced cats. So we were training our younger males with that older experienced female, and she actually bred two of the boys in one day. So we are not sure which of the boys fathered the second cub."
Crosier pointed out that genetics will determine the future of the cubs.
They'll stay with Zazi for another year, but they will likely get a breeding recommendation and go to other breeding centers.