The cubs, born Jan. 2, won’t leave their den until mom Kayla is ready for them to venture out - likely in April.

In a victory for wildlife conservationists — as well as something for animals lovers to look forward to — the Philadelphia Zoo has announced the birth of two threatened sloth bear cubs.

The cubs were born Jan. 2 at 3:53 a.m. and 4:16 a.m. to mom Kayla and dad Bhalu, both 10 years old. This is the second successful birth of this species at the zoo in the last four years, but it is the first successful pair of sloth bear cubs born there in more than 30 years, according to zoo officials.

So far, mom and her cubs are doing well, with Kayla displaying stellar maternal behavior. She has been in constant physical contact with her babies since their birth. The zoo’s animal team has been monitoring them via camera in their behind-the-scenes den, but the sex of the cubs has not yet been determined. So, they haven’t been named yet.

Judging from the den-cam images these little cubs appear pretty playful and not slothful at all. But we will have to wait and see. They will not be visible to the public until sometime in April when Kayla decides it’s time for the kids to venture out.

Sloth bear cubs are pretty tiny at birth, weighing only about a pound and are basically helpless, relying on their mother for care. They begin walking at about one month. At three to four months, mother and cubs emerge from their den, and unlike any other bear species, the babies ride on their mother’s back for about six months. They nurse about a year, and remain with their mom for two to three years.

Dad’s work is already done; the fathers aren’t involved in infant care.

The zoo is pleased about its newest residents.

“Philadelphia Zoo is proud to participate in the Association of Zoos and Aquariums’ Species Survival Plan (SSP) for the threatened sloth bear,” said Rachel Metz, vice president of animal well-being. “We’re looking forward to sharing the developmental milestones of these amazing bear cubs with our guests.”

Maggie Morse, curator of carnivores and ungulates, said the births were a collaborative effort of the zoo’s expert keepers, veterinary team, and Kayla herself.

“Kayla has been trained to do voluntary ultrasounds, so we were able to keep the SSP updated with news of a successful pregnancy,” Morse said.

During her pregnancy, Kayla would stand and stretch out so the ultrasounds could be conducted on her stomach. While the tests were being done, she was given treats like honey water, melon, or peanut butter.

Sloth bears typically dwell in the lowland forests of India, Nepal, and Sri Lanka. They are listed as vulnerable by the International Union for Conservation of Nature, with their populations dramatically decreasing in recent decades because of habitat loss, poaching, and human-wildlife contact.