Alexander Weiss at the University of Edinburgh and his colleagues collected data on captive orangutans in parks around the world. At the beginning of the study period, employees at each zoo who were familiar with the orangutans there rated the apes on their apparent happiness. Questions included how often each orangutan seemed to be in a positive or negative mood, whether it enjoyed social interactions, how well it was able to achieve its goals, and "how happy [raters] would be if they were the orangutan for a short period of time."
Over the next seven years, the researchers kept track of which orangutans had died. Though orangutans in captivity rarely live past their 30s, their aging process is similar to humans'. And, as in humans, females tend to outlive males.
So it wasn't surprising that more male orangutans died during the course of the study. But the researchers also found that orangutans rated as happier at the beginning of the study were less likely to die over the seven years that followed.
One standard deviation in happiness, they found, was worth about five and a half added years of life. That means the difference between a pretty happy orangutan and a pretty unhappy orangutan is 11 years of living--no small change when you can only hope for 30 to 35 years to begin with.
What could cause unhappy apes to die younger? One possibility is that apes appearing less happy are already ill in some subtle, pre-symptomatic way. Another explanation is that a positive attitude evolved through sexual selection, like a set of showy tail feathers, as a signal to potential mates that Suzy or Sammy Sunshine has good genes. (Though being able to live into old age presumably isn't as important to potential mates as just living long enough to make some baby apes.)
A third possibility is that unhappy orangutans are experiencing more stress in their life, or have a poor ability to handle stress. Our bodies react to stressors by activating a hormonal system that gears us up to fight or flee whatever real or figurative predator is chasing us. It's a survival mode in the short term, but keeping that mode switched on in the long term is damaging to our bodies. Unhappy apes may have their lives shortened by stress.
The authors note that in orangutans, as in humans, happiness doesn't rely on outside circumstances. Part of it is inherited: you're born with your personality. But genes aren't fate, and aiming for a positive attitude--or the fruit on the high branch--might keep you swinging around the jungle into old age.
Weiss, A., Adams, M., & King, J. (2011). Happy orang-utans live longer lives Biology Letters DOI: 10.1098/rsbl.2011.0543