The second of two red panda cubs born at a Santa Barbara Zoo in California earlier this summer died over the weekend, the zoo said Monday.

Another cub born to a red panda named Ruby died earlier this month, the zoo said in a statement. That cub was found dead by zoo staff on July 21.

Red panda cubs die at a rate of around 40%, the zoo said. The pair, born on July 2, were Ruby’s first cubs.

The second cub had been “receiving around-the-clock care from our veterinary and animal care teams,” the zoo said in a statement.

“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 said.

The zoo had taken that cub from Ruby on July 19 after staff noticed it was not nursing properly, the zoo said.

“He remained in critical condition in a temperature-controlled incubator while receiving tube and bottle feedings, antibiotics and continuous veterinary monitoring,” the zoo said.

The cubs were born to the red pandas Ruby and Raj, and the next breeding opportunity won’t be until next year because its seasonal, the zoo said.

Red pandas, which are a little larger than a house cat, are considered endangered, according to the World Wildlife Fund. Over half of their territory in the eastern Himalayas and one reason for their endangered status is the loss of forests and other habitat, the organization says. They also get killed in traps meant for other animals.

Fewer than 10,000 mature red pandas are estimated to remain in the wild, according to the zoo.