Staff at Philadelphia Zoo are celebrating the birth of two adorable sloth bear cubs - an important milestone for a species listed as vulnerable by the International Union for Conservation of Nature (IUCN).

Born to mum Kayla and dad Bhalu on 2 January, the cubs are thriving, with Kayla taking incredible care of them behind the scenes in their den.

The zoo team are closely monitoring them using a streaming camera inside the den to ensure mom and cubs have a quiet space to bond and grow. Kayla is showing great maternal behaviour and has been in constant physical contact with the infants since birth. The sex of the cubs has not been determined and names have not been chosen.

"Philadelphia Zoo is proud to participate in the Association of Zoos and Aquarium's Species Survival Plan (SSP) for the threatened sloth bear," said Vice President of Animal Well-Being Rachel Metz. "We are looking forward to sharing the developmental milestones of these amazing bear cubs with our guests."

Curator of Carnivores and Ungulates Maggie Morse added: "We are thrilled to start off 2023 with these successful births. This was a collaborative effort from both our expert keepers and our veterinary team. Kayla has been trained by her keepers to do voluntary ultrasounds so we were able to keep the SSP updated with news of a successful pregnancy."

A typical gestation for sloth bears is 6-7 months. Like other bear species, sloth bears have delayed implantation, meaning that the fertilized egg does not attach to the uterine wall and starts developing right away, and that delay is variable. The cubs are tiny at birth, weighing only about a pound, and are essentially helpless, and rely completely on their mother for care.

Mum will typically care for its young for several months in a sheltered den before the cubs begin to emerge.