The more adventurous leopard cub edged his way out of a cave in his pen Thursday at Utah's Hogle Zoo, gingerly taking in the rest of his new surroundings.

Before he could get too far, mom stepped forward and put her front paws onto a boulder so she could survey the scene from a higher vantage point. Seeing her there, the cub wheeled and trotted under his mother's upraised torso toward a fallen tree, his previously unseen brother emerging suddenly from the cave and following in his footsteps.

Minutes later, the cubs were climbing and jumping, shinning down trees and batting one another with paws distinctive for their white and gray coloring.

Mom watched it all patiently, accepting cuddles every so often from one of her spotted boys, born at the zoo Feb. 17 as part of a breeding program to save the critically endangered Amur leopard species from east Asia.

Then the boys crashed, tuckered out by their rollicking public debut.

"They're definitely babies," chuckled Stephanie Natt, the lead zookeeper for Hogle's Asian Highland cat species, including lions, tigers, lynx, Pallas cats and snow leopards. "They're all play, play, play — then sleep."

Named Rafferty and Roman, the cubs represent successful efforts to rebuild populations of the Amur leopard, whose numbers have dwindled to roughly 60 in the wilds of southeastern Russia and northeastern China.

"This is a very significant birth," Natt said

The cubs' mom, Zeya, came from a British zoo to mate with Dimitri, who arrived at Hogle Zoo in 2013 from Minnesota with the express purpose of procreating.

Three months ago, their arranged courtship produced two healthy young males each weighing about a pound.

"Zeya was an excellent mom," Natt said, nursing her offspring to weight gains of a pound a week until recently, when zoo staffers started supplementing the cubs' diets with whole ground, raw horse or cow meat. Now both have close to 15 pounds on their already sinewy frames.

"These babies did great from the get-go," she said. "I'm curious now to see how much they're eating, how much they need to keep growing. Is it two pounds a day? Five?"

Dimitri was nowhere to be seen. He and Zeya were separated shortly after conception.

"Dad doesn't have anything to do with the kids. He did his job months ago," Natt said. "He's curious about them. But I don't know if he understands that they're his."

Zeya hasn't needed any help raising them. Even though this was her first surviving litter, she has been protective and come to the rescue on a couple of occasions when a cub climbed up something and couldn't get down.

"It's all based on instinct," Natt said, "and, fortunately, her instincts were excellent."

In the rough-and-tumble world of the pen, the cubs are learning how to climb and jump without getting hurt and building muscles strong enough that, out in the wilds of Russia, they could kill a deer or wild boar three times their weight and haul it high into trees, out of the reach of larger predators.

"They are climbers and like to be up on the rocks or in trees. They need to do that without injuring themselves," she said. "But they are floppy kittens so they will fall sometimes. But they are showing more agility."

And they seem ready to put a show on for zoo visitors this summer.