Tacugama Chimpanzee sanctuary Sierra Leone

Bala Amarasekaran and his wife, Sharmila, spotted the baby chimpanzee tied to a tree in a village in a rural part of Sierra Leone.

He was for sale.

The couple bought the animal and pledged to care for him.

Little did they know that this chance encounter would change their lives. Three decades later, they run a sanctuary devoted to saving the critically endangered subspecies known as the Western chimpanzee.

"We didn't understand what we were getting into," Mr Amarasekaran says.

"We were newly married and we had this affection seeing this baby chimp and we thought: 'OK, we'd bring him home and nurse him.' That's all we thought about.

"But once he came into our lives, I think we got attached.

"We rescued another one and another until we had seven or eight chimps in our house and that is what drove us," he adds.

The Tacugama Chimpanzee Sanctuary the couple set up is now home to about 100 Western chimpanzees. It sits in a patch of pristine rainforest on the outskirts of the capital, Freetown.

These primates mostly live in the forests of Sierra Leone, Guinea and Liberia. They are also found in Ivory Coast, Ghana, Guinea-Bissau, Mali and Senegal.

Western chimpanzees have been found to use tools unknown in other chimpanzee populations, a 2016 paper in the American Journal of Primatology said. These include cracking nuts, hunting bush babies with spears and throwing stones.

But they are under threat.

The population is estimated to have declined by 80% between 1990 and 2014, to about 52,800.

They mostly live in the wild. Only 17% of Western chimpanzees are to be found in protected areas.

Read more about this photo story on BBC