The Santa Barbara Zoo in California reported the death of a second red panda cub over the weekend. This cub was one of two born earlier this summer to the red panda named Ruby. The first cub had also died in July, discovered by zoo staff on July 21.

Red panda cubs face a mortality rate of about 40%. The pair of cubs, born on July 2, were Ruby’s first offspring. Despite the intensive care provided by veterinary and animal care teams, the second cub’s life could not be saved.

The zoo stated, “While the cause of death remains under investigation, this loss is heartbreaking for everyone who cared for these cubs and for all who have followed their journey.”

The zoo had removed the cub from Ruby on July 19 after observing improper nursing habits. Staff members placed the cub in a temperature-controlled incubator, administering tube and bottle feedings, antibiotics, and continuous veterinary monitoring.

Ruby and Raj, the parent red pandas, will have their next breeding opportunity next year, due to seasonal breeding constraints.

Red pandas are slightly larger than a house cat and are classified as endangered, according to the World Wildlife Fund. They face threats from habitat loss in the eastern Himalayas and are sometimes killed by traps set for other animals. Estimates suggest that fewer than 10,000 mature red pandas exist in the wild.