Sumatran elephants are a critically endangered species and fewer than 700 remain on the island. The animal is protected under an Indonesian law on the conservation of biological natural resources and their ecosystems.

Local authorities are investigating the cause of death of the elephant, which was discovered in a joint patrol by local conservation groups on Wednesday. But they strongly suspect it was poisoned.

The authorities found pineapple in the elephant's stomach, even though pineapples are not grown in that area, further leading them to suspect poisoning, Zulhusni Syukri, program director of Rimba Satwa Foundation said.

The population of Sumatran elephants on the island shrank from 1,300 in 2014 to 693 last year, down nearly 50 per cent in seven years, Indonesian forestry and environment ministry data show.

The decline has occurred amid a loss of more than 69 per cent of the animal's potential habitat in the last 25 years -- the equivalent of one generation.

The expansion of palm plantations in Sumatra has led to conflicts between humans and animals over space and resources.

"The palm plantation workers near the elephants' roaming paths often have conflicts with the elephants because they eat the palm fruit," Syukri said.

At least seven Sumatran elephants have died in Riau province in the last three years, he added.

Sumatran elephants are a subspecies of the Asian elephant.

Australian Associated Press