 Of the subjects, 100 were Sumatran ("Pongo abelii"), 54 Bornean ("Pongo pygmaeus") and 30 were hybrid orangutans.113 zoo employees, who were highly familiar with the typical behavior of the orangutans, used a four-item questionnaire to assess their subjective well-being.The results indicated that orangutans in higher subjective well-being were less likely to die during the follow-up period.