Chantek, the orangutan renowned worldwide for his abilities to communicate in American Sign Language passed away at Zoo Atlanta on Monday at the age of 39.

Orangutan had been suffering symptoms of heart disease

According to Zoo Atlanta's website, their veterinarians started treating Chantek in September 2016, after he displayed symptoms of heart disease. However, until a necropsy is run on the orangutan, the zoo will not be able to officially announce the cause of his death.

After his heart condition was discovered, the Great Ape Heart Project regularly monitored his condition.

Heart disease is reportedly the main cause of death with great apes and the project was set up to treat the disease in orangutans, gorillas, bonobos, and chimpanzees. According to Zoo Atlanta, Chantek had the first EKG performed on a conscious orangutan as an aid to diagnose his condition. As noted on their website, from around 35 years of age, orangutans are considered to be geriatric. The zoo put Chantek on a low-sodium diet to treat his condition, just as would happen with humans.

Chantek learned to communicate first with humans, then with orangutans

Born in December 1977 at the Lawrenceville Yerkes Language Research Center, Chantek went on to live at the University of Tennessee-Chattanooga, where researchers taught him American Sign Language.

He went on to learn at least 150 signs. Lyn Miles, an anthropology professor at the university, said in a TED talk in 2014 that she considered Chantek to be her foster son, adding that he was the “first orangutan person.” A PBS documentary in 2014 featured Miles’ work with the orangutan. The BBC quoted Miles as saying she had little contact with the orangutan after he moved to the zoo, due to various rules, and the last time she saw Chantek, he asked her for a cheeseburger – which he signed as “meat cheese bread” – and told her to get the car to take him home.

Hayley Murphy, VP of Animal Divisions at Zoo Atlanta, said the orangutan had special methods of communicating with those he was closest to. In a statement on Monday, Murphy said it has been a privilege to have Chantek with them for the last 20 years and to give the orangutan a natural environment in which to live and mix with his orangutan family after having been alone in the research center.

Chantek lived with a female orangutan, Madu, 34, a male orangutan named Dumadi who is 10, six-year-old Remy and two-year-old Keju, a female orangutan.

The lead keeper of primates at the zoo, Lynn Yakubinis, talked of the orangutan’s patience with the younger primates. She also said she enjoyed it when Chantek would come to her, eyes sparkling, trying to engage her into a game, to get food or simply to get some attention. The zoo did say on their website that while Chantek regularly used sign language with his caregivers, he was a little shy using the method of communication with strangers and often used traditional orangutan methods instead.

As noted by the Atlanta Journal Constitution, Zoo Atlanta officials say employees who have worked closely with Chantek are now trying to decide how best to honor the orangutan’s memory.

Chantek’s remains are to be cremated and will be returned to the zoo.

The zoo reports that wild Bornean and Sumatran orangutans are critically endangered due to habitat loss, human encroachment, and deforestation for palm oil plantations. According to experts, Sumatran orangutans will likely be extinct within the next 10 years unless conservation efforts continue.