Happier orangutans are more likely to live for longer, according to a study.

一项研究显示，快乐的猩猩有可能活得时间更长。

A team of researchers in the UK and US devised a method to measure the happiness, or subjective¹ well-being², of captive orangutans（猩猩） .

In a follow-up study seven years later, the scientists found that happier primates⁴ were much more likely still to be alive.

The findings are published in the Royal Society journal Biology Letters.

The team, led by Dr Alex Weiss from the University of Edinburgh, asked the people who worked closely with each captive orangutan to participate in the study. He asked the keepers and carers to complete a questionnaire about individual animals they knew well; assessing the orangutans' personalities⁵ and attitude.

"The assessment⁶ was modelled on [established] methods of assessing human well-being," Dr Weiss explained to BBC Nature.

The questionnaire posed four key questions, including how much time the orangutan in question spent "happy, contented⁷ and enjoying itself". It also asked the human participants to imagine how happy they would be if they were that orangutan for a week.

By working out a happiness score for each of nearly 200 animals, the team was able to see how happiness influenced the orangutans' lives. Seven years later, when they revisited the study, they could see a clear association between happiness and longevity⁸（长寿） .

Fitter, happier

Professor Richard Byrne, a primate³ expert from the University of St Andrews who was not involved in this study said that "the findings were clear".

"[The team has] worked out that the difference between an orangutan being rated as very happy, compared to very unhappy, equated⁹ to 11 additional years of life-expectancy," said Professor Byrne.

But, he continued, "the authors rightly point out that the data don't tell us whether some subtle sign of health or illness makes an orangutan act more or less happy, or if its the reverse - that something intrinsic（本质的，固有的） to the individual orangutan, which shows up externally as happiness or sadness to us, predisposes the individual to be more likely to stay healthy or get ill."

But even without fully¹⁰ understanding whether happiness causes long life or if better health causes happiness, the researchers hope their results will be used to improve and extend the lives of endangered orangutans.

"[In captivity], we might be able to extend life by more closely monitoring the health of an animal that seems unhappy," explained Dr Weiss.

He also thinks the work could be usefully applied¹¹ to wild orangutans.

"There are lots of sanctuaries¹² that are temporary homes for animals that are rescued having been captured by traders and hunters," Dr Weiss told BBC Nature.

"These happiness or well-being measures could be used to work out if an animal is ready to be reintroduced into the wild.

"I'd love to see this questionnaire being used more broadly."