Last November, the San Diego Zoo welcomed a litter of six adorable cheetah cubs.

The cheetah Addison had previously given birth to a litter of four cubs, but the San Diego Zoo says it wasn’t expecting to see six adorable baby cheetahs curling up next to their mamma cheetah.

According to the San Diego Zoo, six is an above average amount of cubs for one litter.

“A radiograph had shown she was definitely carrying three cubs, and possibly a fourth,” explains Jillian King, a senior keeper at the Cheetah Breeding Center.

The Zoo was happily surprised with half a dozen fuzzy bundles of  squirming spotted fluff.

The International Union for Conservation of Nature has classified Cheetahs as a vulnerable species. The San Diego Zoo says there’s only 10,000 cheetahs worldwide, down from 100,000 in 1900.

It can be difficult to encourage breeding in cheetahs,  and according to the San Diego Zoo, there is a complicated protocol to support mating. It involves placing female cheetahs in adjacent pens to male cheetahs leaving them time to mark their scent.

“After she’s had time to leave her scent in the habitat, we remove her and have one of the males go in. As he sniffs around, we watch and listen. His behavior tells us if she is in estrus,” King said. “If we see and hear the signs, we put the male and female in separate enclosures with a common fence line to see what signals she sends.”

The female cheetah sometimes flatly rejects her male suitor, despite his earnest efforts to court her. That is why, the San Diego Zoo says, the barrier between them is important. This fence is only lifted when all the signs are right.

“It’s been a really good prototype, as far as what we can do to get a self-sustaining population of cheetahs in managed care,” King said. “Only about a third of females will actually breed. Together, we’re trying to find out why.”

The San Diego Zoo says Addison’s cubs will spend a year with their mamma, eventually going on to other sites where they may further bolster the cheetah population.