Tuyen, a 10-year-old female white-cheeked gibbon at the Smithsonian’s National Zoo, was humanely euthanized Oct. 12, after suffering from progressive neurologic disease and acute seizures. She had a long history of neurologic disease, which the Zoo’s animal care team had collaborated with consulting neurologists to treat. Despite treatment her prognosis was poor and her quality of life was rapidly declining.

Keepers described Tuyen as “bright and tenacious” despite struggling with a neurologic disease. Tuyen never gave birth to any offspring but was an ambassador for her species, which is critically endangered in the wild. White-cheeked gibbons are primarily threatened with habitat loss in their native range—which spans Vietnam, People’s Democratic Republic of Laos and China. Scientists estimate that the population has declined by 80 percent during the past 45 years. 

Visitors can still see white-cheeked gibbons Sydney and Muneca, and siamangs Bradley and Ronnie at Gibbon Ridge at the Zoo.