The last known male Sumatran rhino in Malaysia has died following a months-long battle with worsening health, National Geographic reports.

The death poses a further threat to the critically endangered species of which fewer than 80 remain in the world.

“It is with heavy hearts that we share the tragic news that Tam, Malaysia’s last male Sumatran rhino, has passed away,” non-profit organization Borneo Rhino Alliance wrote in a Facebook post Monday.

“We will share more details in due time, but right now we need some time to mourn his passing,” the organization added.

Caretakers noticed a decline in the rhino’s appetite and alertness in recent months, and subsequent tests revealed that his organs had begun to fail. Tam was estimated to have been in his thirties, and therefore of old age given that the species normally has an average life span in the wild of 30 to 40 years.

The rhino was captured after being discovered roaming in an oil palm plantation in 2008. Researchers hoped he would be able to contribute to efforts to breed the critically endangered species in captivity, but Tam died without reproducing.

Just one female Sumatran rhino, Iman, is left in Malaysia. Another female rhino, Puntung, was euthanized in 2017 due to cancer.

According to the International Rhino Foundation, Indonesia now holds the only remaining Sumatran rhinos — no more than 80 — in the world. Living mostly in the forests of Sumatra, the rhinos are threatened by poaching and deforestation; more than 70% of the population has been lost in recent decades.

Despite being widespread in the past, only five species of rhinos survive today and are all under threat of extinction, the International Union for Conservation of Nature (IUCN) says.

Write to Hillary Leung at hillary.leung@time.com.