"There may be different things going on at the surface, but underneath it all, there's something common in all three species that's leading to this," said study leader Alexander Weiss, a primate psychologist at the University of Edinburgh in Scotland.

The study team asked longtime caretakers of more than 500 chimpanzees and orangutans at zoos in five countries to fill out a questionnaire about the well-being of each animal they work with, including overall mood, how much the animals seemed to enjoy social interactions, and how successful they were in achieving goals (such as obtaining a desired item or spot within their enclosure).

The survey even asked the humans to imagine themselves as the animal and rate how happy they'd be.

When