One of two red panda cubs born to the Santa Barbara Zoo’s Ruby and Raj earlier this month died on Tuesday, the zoo announced.

Spokesperson Andrea Gruber said the cub was found deceased in its nest box with its mother during a routine check.

The cub’s cause of death is not yet known, Gruber said. Red panda cubs have an infant mortality rate of 40%.

The other cub was transferred to an incubator and is receiving special care, after staff found that it was not nursing properly and now requires bottle feeding, per Gruber.

“While it is always the goal for young animals to be raised by their parents, human intervention is sometimes necessary to provide supplemental care or hand-rearing to give each cub the best possible chance of survival,” Gruber said.

Dr. Julie Barnes, the zoo’s vice president of Animal Care & Health, said this is a “heartbreaking loss” for everyone involved.

“We were really hoping that Ruby would be successful in rearing her two cubs, but we also know that infant mortality is very high in this species, particularly for first time mothers,” she said. “Our animal care and veterinary teams are now focused on giving the surviving cub the best possible chance and taking care of Ruby.”

The red panda species is classified as endangered. Its population has declined by 50% over the last few decades, Gruber said, with only 2,500 to 10,000 red pandas estimated to remain in the wild.