Monkeys, too, can be lonely, even when surrounded by others in a social group.

What is loneliness? Is it wishing to be social but not having friends in your social group? More than just a socio-emotional condition, it can be a significant cause of poor health, and is of special concern in the elderly.

Social isolation, or loneliness, has been shown to impact brain and behavior and is recognized as a major risk factor for morbidity and mortality in humans for more than a quarter century.

Many are familiar with the proven health benefits of having long-term social relationships with friends and family. Researchers at the California National Primate Research Center (CNPRC) and elsewhere have demonstrated the scientific basis for these benefits, showing some physiological reasons for improved health and disease resistance.

However, not all people, nor animals, have the same level of desire to be social. Some prefer to be alone rather than engage with others. It is the choice of sociality, and the behavioral consequences when there is a disconnect between the desire to be social and the reality of social interaction, that was the focus of a research study by John Capitanio, Ph.D., Core Scientist in the Brain, Mind, and Behavior Unit at the CNPRC.

There is no fundamental reason that this perception of social disconnection, or loneliness, might be specific to humans. Rhesus monkeys, for example, are a highly social nonhuman primate species. Observing adult male rhesus monkeys in their large outdoor social groups at the CNPRC, Dr. Capitanio found a subset of animals with low social engagement that seem to want more interaction than they have. This study demonstrated that this species could serve as a useful animal model to understand the behavioral and biological consequences of loneliness.

“Importantly, our study describes a naturally occurring model of loneliness. Animal models of loneliness are usually induced models – for example an animal is physically separated from its companions. Our research suggests loneliness in monkeys can occur even in the presence of others – just like in humans” emphasizes Dr. Capitanio.

The research was published October 29, 2014 in the online journal PLoS ONE A Behavioral Taxonomy of Loneliness in Humans and Rhesus Monkeys (Macaca mulatta)and was co-authored with Drs. Louise Hawkley and John Cacioppo from the University of Chicago, and Dr. Steven Cole from UCLA. The research, which also includes studies with humans, was supported by the National Institute on Aging.

First using a sample of older adult humans, the psychologists examined how loneliness was influenced both by social network size and by the extent to which individuals believed that their daily social interactions reflected their own choice. As expected, the study showed that loneliness was highest among individuals that have low levels of social interaction, but who may be dissatisfied with those levels (low choice in determining those levels); in fact, individuals with comparably sized social networks, but who indicated that their amount of social interaction reflected their own choice (presumably reflecting satisfaction with their level of interaction), reported significantly less loneliness. Put another way, people who are lonely show a discrepancy between their social interest and social attainment.

As the second step in this study, Dr. Capitanio and colleagues sought to determine whether a similar classification might underlie known variability in adult male rhesus macaques’ tendency to affiliate, or “hang out” with others.

The research animals in this study live outdoors in large, half-acre field corrals, in a rich social environment consisting of 80–150 animals. Highly social monkeys show high levels of both simple social behaviors, like approaches, as well as more complex social behaviors, like grooming and contact. At the other end of the spectrum, however, the researchers identified two groups of animals. One group showed low levels of both simple and complex interaction; these animals seem to be relatively uninterested in social interaction, and may be similar to introverted humans. The other group, however, showed levels of simple social interaction that were comparable to those of high-sociable monkeys; however, their levels of complex social interaction were low. These animals appeared unable to convert their strong social interest into desirable social interaction — much like lonely humans.

Together, the results from these human and monkey studies suggest that nonhuman primates may provide a valuable animal model to better understand how chronic loneliness contributes to poor health as people age.

More about Dr. Capitanio’s research and publications.