Just like humans, happier orangutans live longer, scientists find.

These findings might shed light on the evolution of happiness in humans.

Past research suggested that happy people live longer. To see if this also held true in our ape relatives, scientists asked zookeepers starting seven years ago to rate the happiness of 172 captive orangutans, including 89 Sumatran orangutans (Pongo abelii), 53 Bornean orangutans (Pongo pygmaeus) and 30 hybrid orangutans.

The keepers noted how often the apes were in good moods or bad moods, how much they enjoyed social interactions and whether the apes were effective at achieving their goals, such as getting certain items they want. The zookeepers were also asked to speculate how happy they would be if they were "in the shoes of" each of the orangutans in their care, a question meant to analyze "a more general sense of the individual orangutan's well-being," said researcher Alexander Weiss, a primatologist at the University of Edinburgh in Scotland.

The scientists found that orangutans who scored happier were significantly more likely to be alive up to seven years later. This effect remained even when factors such as sex, age, species and number of times they were transferred to new facilities were taken into account.

"Impressions of the well-being or happiness of orangutans — and likely other species — should not be dismissed as being wishful thinking or anthropomorphism," Weiss told LiveScience. "Instead, these impressions should be thought of as valid indicators of an animal's welfare and should be taken seriously."

Although researchers find it unlikely that good spirits cause longer life in orangutans, the happiness of captive orangutans might give indications as to the state of their health and welfare. As such, it could become a valuable tool for ensuring their future health, happiness and survival.

"One possibility is that lower happiness may be an early signal for underlying health problems or health problems that are not detected in ordinary screens. If this is the case, orangutans in captivity who exhibit lower subjective well-being could have their health more closely or more regularly screened," Weiss said. "Another possibility is that an orangutan's happiness reflects their being in a stressful physical or social environment and that this situation also leads to poorer health and a shorter life. If so, working to improve their environment could lead to better health and well-being."

This line of research could also yield insights into the role of happiness not just in orangutans but also in our distant ancestors.

"The findings suggest that measures of well-being or happiness such as these may be getting at the same phenomenon in humans and orangutans with whom we shared a common ancestor around 14 million years ago," Weiss said. "As such, it may be possible to study the evolution of something thought of as fairly subjective and ephemeral by some."

Such research "across a wide range of species could yield fascinating insights into the evolutionary basis of happiness, depression and a host of other psychological characteristics that impact the lives of humans and, most likely, a range of other species," Weiss added.

"Once the mechanisms and evolutionary bases of happiness are better understood, I would like to set up intervention studies to see whether happiness in orangutans or other species can be improved," he added. Weiss also hopes to figure out the mechanisms underlying the relationship between happiness and longevity.

The scientists detailed their findings online June 29 in the journal Biology Letters.