For 35 years, American zoologist Laurie Marker has been collecting and storing specimens in a cheetah sperm bank in Namibia, hoping conservationists never have to use them.

But she worries that the world's fastest land animal might be on the brink of extinction one day and need artificial reproduction to save it.

Marker says the sperm bank at the Cheetah Conservation Fund she founded in the southern African nation is a “frozen zoo” of cheetahs she's been building since 1990. It would be utilised in a worst-case scenario for the big cats, whose numbers have dropped alarmingly in the wild over the last 50 years.

“You don't do anything with it unless until it's needed,” Marker, one of the foremost experts on cheetahs, told The Associated Press from her research centre near the Namibian city of Otjiwarongo. “And we never want to get to that point.”

Conservationists mark World Cheetah Day on Thursday with less than 7,000 of them left in the wild, similar numbers to the critically endangered black rhino. There are only around 33 populations of cheetahs spread out in pockets mainly across Africa, with most of those populations having less than 100 animals, Marker said.

Like so many species, the sleek cats that can run at speeds of 70 miles per hour (112 km per hour) are in danger from habitat loss, human-wildlife conflict and the illegal animal trade.

Their shrinking, isolated groups mean their gene pool is shrinking also as small populations continuously breed among themselves, with repercussions for their reproduction rates.

Globally, cheetah numbers in the wild have dropped by 80% in the last half-century and they've been pushed out of 90% of their historical range.

Scientists believe that cheetahs already narrowly escaped extinction at the end of the last ice age around 10,000-12,000 years ago, which first reduced their gene pool.

Marker said the lack of genetic diversity, along with the fact that cheetahs have 70-80% abnormal sperm, mean they might need help in the future.

“And so, a sperm bank makes perfect sense, right?” Marker said.

Storing sperm is not unique to cheetahs in the wildlife world. It's a tactic that conservationists have developed for other species, including elephants, rhinos, antelopes, other big cats, birds and others.

The value of animal reproductive research, Marker said, is seen in the desperate battle to save the northern white rhino from extinction.