One of the red panda cubs born at the Toronto Zoo in June has died.

The zoo announced the death of the cub in a post on social media on Thursday. The male red panda was brought to the facility's Wildlife Health Centre after staff observed that he was not drinking well, had a reduced appetite and felt cold to the touch, the Toronto Zoo said.

"The vet team moved him to an incubator in the intensive care unit, and despite all efforts, the littlest one passed away- he was just shy of six weeks," the zoo wrote in the post.

"While this outcome is not unexpected given their high mortality rate in their first year, we are truly saddened by this loss." The zoo noted that studies have shown that as few as 40 per cent of cubs reach their first birthday.

A post-mortem examination was conducted Wednesday night and found that the male cub had no fat reserve. The zoo said the cub was considered the runt and was at a disadvantage when his mom's milk production recently decreased.

The mom, Sakura, a 10-year-old female red panda, recently began experiencing significant health issues and showed "behaviours suggestive of abnormal neurological activity," according to the Toronto Zoo.

"Given her age and declining health, a precautionary decision was made by the Wildlife Care team to begin supplemental feedings for the cubs earlier this week," the zoo added. Staff were hoping to get Sakura healthy enough to raise her cubs until they could live independently.

In addition, the post-mortem also discovered that the male cub showed signs of aspiration affecting his lungs and had two intussusceptions in his intestines—a condition when one section of the intestine slides into the adjacent portion, which could obstruct or decrease the passage of food.

"This can also restrict blood flow to the affected piece of intestine and cause inflammation and adhesions. This condition can result in symptoms such as vomiting or diarrhea, but that was not experienced by this little cub," the zoo said.

Staff are now focused on the female cub, which the zoo said appears to be healthy, and her mother, Sakura.

"Sakura is currently stable and receiving medical treatment and we are balancing the risk of separating her from the remaining cub against the need to understand fully the cause of her seizures, which is why we have opted to have them remain together during this time," the zoo said, noting that while Sakura is no longer nursing her female cub, she is "caring for her in other ways."

The two red panda cubs were born on June 13. Sakura's pregnancy surprised zoo staff as she was past the typical breeding age. The 10-year-old red panda moved to the Toronto Zoo earlier this year for retirement.

The male cub's death comes almost a week after the zoo's two-year-old Masai giraffe passed away during a procedure.