Hani Freeman, a research fellow in animal behavior at Disney's Animal Kingdom in Florida, answers:

Evidence indicates that some nonhuman primates can be introverts or extraverts. In humans, introverts tend to spend more time alone focusing on their thoughts and less time engaging in group activities, whereas extraverts are often gregarious and enjoy interacting with their peers. Nonhuman primates also exhibit such qualities.

Recent studies have identified extraversion/introversion in great apes, including chimpanzees, gorillas and orangutans. Chimps exhibiting greater solitary and reserved behaviors are considered more introverted, whereas those that are more playful and interactive fall on the extraverted side of the spectrum. Extraversion/introversion behaviors, such as sociability, have also been identified in species of monkeys.

It is important to note that we are limited in our ability to interpret the behavior of nonhuman primates. Apes can learn to communicate with humans to some degree, but overall, they cannot tell us how they feel, at least not to the extent that humans can.

Caregivers who work regularly with apes offer the best impressions of where apes fall on the extraversion/introversion continuum. In one study conducted at zoological parks around the U.S., caregivers rated specific traits, such as friendliness, on a scale from 1 to 5. Gorillas that caregivers classified as introverts also interacted less with their peers than did their more outgoing counterparts. To eliminate the potential for bias, independent behavioral experts have studied these traits in gorillas and have confirmed these findings from caregivers.

Although current research suggests that human and nonhuman primates exhibit many similarities in introversion and extraversion, research on personality in nonhuman primates is still at its early stages. We have only recently started to understand the advantages of being introverted, for instance. In humans, introverts tend to be better observers and listeners compared with extraverts. In nonhuman primates, however, researchers have not yet identified the advantages of introversion. Future work, scientists hope, will delve more deeply into such personality traits in nonhuman primates—how they evolved and why they are important.