Humans aren’t the only lonely species. Monkeys may be lonely too.

Eliza Bliss-Moreau

More than two decades of research demonstrates that people who have more social connections do better—in terms of their general health, ability to recover from illness, and longevity (for a classic, oft cited study, see here; for a review, here; for popular press coverage and a lovely long read on human loneliness, here). Perhaps unsurprisingly, it’s not just the number of people you’re connected to that matters for your well-being. Whether people’s social relationships meet their social needs also has critical importance for health outcomes  regardless of how many social connections they have (for reviews of this literature see here, here, and here). That is, it’s possible to be well-connected socially and still feel totally alone in the world. It is also possible to have very few social relationships but not feel lonely at all.

Exciting new evidence illustrates that we humans might not be the only ones to experience loneliness—rhesus macaque monkeys may as well. As part of an interdisciplinary team, Dr. John Capitanio examined the social behavior of adult male macaques at the California National Primate Research Center and identified three different patterns. Some monkeys engaged in a lot of social interactions with other males, adult females, and younger monkeys. Other monkeys did not engage in a lot of social interactions at all. What’s critical some of these “lowly social” monkeys seemed fairly content with their lot in life—they went about their daily business without trying to build new relationships.

Housing&Enrichment©K.West-CNPRC,039The other lowly social monkeys did seem to care about their lot in life, however. They often physically approached adult females and younger animals, presumably in an effort to initiate an interaction. Similarly, the monkeys would walk by those animals to see what they were doing, even when trying not to engage with them. The fact that these behaviors were observed with adult females and younger monkeys suggests that these “lonely” adult male monkeys may have been looking for easy social relationships (because of how macaque societies are structured, relationships between adult males can be challenging). This heightened social interest persisted when evaluated 1.5 years later. In other words, there were monkeys who appeared to chronically desire social relationships but did not manage to make them happen—a potential monkey homologue of human loneliness. What’s more, as Capitanio points out, these lonely monkeys may be better models for human loneliness than previous animal models because the phenomenon emerged spontaneously in the context of large naturalistic social groups, rather than as a result of experimentally separating animals.

In and of itself, the finding that nonhuman animals might have the capacity to be lonely is an interesting one. It suggests that experiencing a mismatch between one’s social realities and one’s social desires is evolutionarily old, raising questions about what function loneliness might have served for our ancestors.

But perhaps more importantly, animal models of human psychological phenomena, such as a monkey model of loneliness, are critical to understanding the biological processes that contribute to them. Nonhuman primate (e.g., monkey) models are particularly important for understanding human function and dysfunction because we share so many biological and social features. Monkey models allow for precise experimental control (e.g., the ability to manipulate social environment, diet, sleep-cycles, etc.), intensive long-term longitudinal studies (i.e., the ability to track and evaluate many individual animals over the course of their entire lives), and the development of causal biological models. Understanding biological mechanisms is critical for developing effective early interventions and treatments for deleterious psychological experiences. Studying lonely monkeys may therefore unearth the biological and social processes that can be harnessed to help lonely humans in the future.

Photo: Adult male rhesus monkey at the CNPRC eating a zucchini. Photo Credit: Kathy West, CNPRC.