The Phoenix Zoo has confirmed the birth of six Mexican gray wolf pups, the newest members of an endangered species that's teetering on the edge of extinction.

So far, the parents are raising the litter on their own, but the zoo's staff is watching closely. One of them is Carl Mohler, a senior keeper at the zoo.

“They are doing a really great job so far," Mohler said. "We haven’t had to have any kind of interference.

"The fact that they’re doing such a great job raising them is good news.”

The pups were born about four weeks ago, in early May, around the typical time for the species' breeding season. It's the zoo's first gray wolf litter in 20 years.

The zoo is housing the pups as a part of the cooperative breeding program with the Association of Zoos and Aquariums Mexican Gray Wolf Species Survival Plan and the U.S. Fish and Wildlife Service's Wolf Recovery plan.

The goal of the federal agency's program is to restore the endangered wolf's population and release more of them into their native territory of the southwestern U.S., including parts of Arizona and New Mexico.

Wild wolf population grows

The recovery efforts started after the government listed the predator as endangered in 1976, when the wolves were nearly extinct. The listing gave them protection and recovery funding under the Endangered Species Act.

According to the wildlife service, the number of wild wolves grew from 117 to 131 in its latest annual survey. This is the highest wolf count since 1998, when the U.S. government and Mexican agencies began releasing captive-bred wolves. Arizona is home to 64 of them; the other 67 were counted in New Mexico.

The breeding and reintroduction of the wolves remain controversial. Environmentalists consider human activities to be the most serious threat facing their reintroduction, but ranchers in areas where the wolves roam say their livelihood is at risk, arguing that the wolves kill cattle and trespass on their property.

There are measures in place to protect ranchers and better balance the coexistence between them and the wolves, which receive protections most animals do not. Ranchers say that legal gray area makes it hard for people who encounter them to defend themselves if needed.

MORE: Ranchers ask, how many wolves is too many?

At the Phoenix Zoo, for now

While Fish and Wildlife sorts out that problem, Mohler said he and the other 53 institutions in the U.S. and Mexico participating in the species survival program will focus on their jobs and hope to keep growing the captive population, which now tops 300.

The litter is being raised by the only two adult wolves in the Phoenix Zoo, Tazanna, the mother, and Tulio, the father, who are both 3 years old. Both adult wolves arrived at the zoo in 2017. The parents and pups rest and romp around in an enclosure on the zoo's Arizona Trail, where they can roam between a more open enclosure and a more private one.

Tazanna and Tulio have dug a burrow for the pups where they spend most of their days sleeping. The best chance to view them is early in the morning, when the zoo opens at 7 a.m. Other than that small window, they’ll be hard to spot until they grow older and become more social, Mohler said.

Just like human kids, the pups need to rest; any time they spend playing, Mohler said, they'll need to sleep for about triple that. Soon, the pups could start eating food regurgitated by the parents while still drinking mother’s milk and when they reach the eight to 10-week mark, they’ll start weaning away from the mother.

But, Mohler said, they likely won’t be moving away anytime soon.

“If they do leave us, they’ll go to other zoos, most likely on breeder recommendations or just to have a space for people around the United States to view them,” Mohler said, adding that the overall goal for the program is to breed a genetically diverse group of wolves in human care and in the wild.

As the pups grow older, the U.S. Fish and Wildlife will tell the zoo if and when the wolves can be released into the wild.

“We would love for them to be reintroduced into the natural habitat, but there’s all kinds of rules and requirements we need to follow,” Mohler said.

“Our responsibility is to house them, breed them if we’re approved for it and just care for them the best that we can.”

What do you want to know about Arizona's endangered species? Howl out to the reporter at andrew.nicla@gannett.com or at (602) 444-3821. Follow him on Twitter @AndrewNicla.

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Environmental journalism on azcentral.com and in the Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow the azcentral and Arizona Republic environmental reporting team at environment.azcentral.com and at OurGrandAZ on Facebook, Twitter and Instagram.