“Do Birds of a Feather Flock Together or Do Opposites Attract?”

The CNPRC takes great effort and pride in the stewardship, welfare and care of its animals, and extensive teams of staff in animal care, enrichment and behavior management, and veterinarians work together to provide outstanding care to all of the animals. State-of-the-art research and scientific findings at the Center contribute to the understanding and treatment of human and animal disease, but also demonstrate leadership in the field of nonhuman primate care, enrichment and social housing. Novel research studies and progressive care increase knowledge of nonhuman primate behavior, nutrition, reproduction, development, health, and long-term social health, leading to further advancements in the level of care provided to captive nonhuman primates worldwide.

The CNPRC has model environmental enrichment and behavior management programs for the benefit of all its animals, the majority of which are rhesus monkeys (Macaca mulatta). Most of these animals live in large half-acre outdoor field corrals, in large natural social groups similar to those found in the wild. The remainder of the animals are housed as pairs in indoor housing, with a small percent housed singly as needed.

Programs provide daily enrichment and facilitate psychological well-being through provision of multiple forms of environmental enrichment and social housing. An important component of this program is to provide social experience for indoor-housed animals, that allows them to spend the daytime hours with another animal – either male or female pairs, or male / female pairs and offspring. The CNPRC Behavior Management staff use their extensive expertise and knowledge of the animals’ personalities to accomplish appropriate pairings.

Making significant progress in improving the social and psychological health of the animals, John Capitanio, PhD, Brain, Mind and Behavior Unit core scientist, has developed and led the BioBehavioral Assessment Program (BBA) for the past 14 years. The BBA has tested more than 4,000 infant rhesus macaques between 3–4 months of age, and categorized broad characteristics (i.e. patterns of emotionality, nervous temperament, activity level) and found that these personality factors are relatively stable throughout life.

Dr. Capitanio’s most recent publication reveals research findings using the BBA database that will make important contributions to captive nonhuman primate social welfare – Do “Birds of a Feather Flock Together” or Do “Opposites Attract”? Behavioral Responses and Temperament Predict Success in Pairings of Rhesus Monkeys in a Laboratory Setting (Capitanio et al, Am J Primatol. 2015 Aug 28. doi: 10.1002/ajp.22464.

[Epub ahead of print]). Also contributing to the research and publication were Shelley Blozis, UC Davis Psychology, Jessica Snarr, CNPRC Enrichment program, and Brenda McCowan, PhD, Brain, Mind and Behavior Unit core scientist.

Using data from the BBA program of standardized infant assessments, the research team found that they could identify potential predictors of success when animals were paired up to 10 years after the behavioral assessments.

Given that “ compatibility” is the ultimate goal of a pairing program, a critical issue is whether knowing something about the enduring behavioral tendencies of an animal (often referred to as temperament or personality) can increase the chance of a successful pairing attempt. This growing recognition that social needs of primates in captivity must be addressed can present challenges to staff at primate facilities charged with implementing pair-housing solutions for animals.

The limited research and success in the past of pair-housing nonhuman primates points to two questions: what are the traits that might be most influential in contributing to success (boldness/inhibition, emotionality, sociability), and what particular combination works best— should individuals be similar in their trait profile (“birds of a feather flock together”) or should individuals differ from each other (“opposites attract”)?

Understanding this issue in human terms, there is considerable information in psychology on the broad topics of interpersonal attraction and relationship quality. Much of this, however, is focused on romantic partner attraction and marital relationships, and may be of limited value for two reasons. First, “ marital” relationships (a better term might be relationships that involve a pair bond) are typically attachment relationships, which have different qualities than do relationships characterized as “ friendships”. One would expect that the criteria involved in evaluating a potential romantic partner would be different from the criteria used to establish a friendship. Second, social pairings of laboratory-housed monkeys are usually done within-sex to avoid pregnancy; the dynamics between a heterosexual pair and an isosexual pair are likely to be different.

Other human research has investigated same-sex relationships in the context of college roommates; a context which, one would argue, more closely approximates the situation of same-sex pair-housed laboratory animals. One such study of randomly assigned, same-sex, college dormitory roommates, for example, found that, despite the fact that female pairs overall exhibited greater complementarity than did male pairs, the degree of complementarity predicted positive friendships for both male roommate and female roommate pairs. In another study, friendships in college-age students were characterized by positive similarity in personality characteristics — suggesting that “ birds of a feather” may indeed flock together.

Finally, a handful of nonhuman primate studies have examined friendships (or positive social interaction) in social group (i.e., not pair-housing) settings. Dr. Tamara Weinstein, Brain, Mind, and Behavior Unit, and Dr. Capitanio studied yearling rhesus monkeys that were living in large outdoor enclosures, and found that these animals preferentially associated with animals that were similar to themselves on the characteristics of adaptability (flexible, gentle) and equability (calm, not active).11RhesusFemalesInfantsKW

In this current research by Dr. Capitanio, indoor-housed animals were identified that had experienced successful or unsuccessful pairing attempts, and all pairs in which both members had participated in the BioBehavioral Assessment program as infants were studied. The team examined patterns of behavioral responsiveness and temperament, and whether complementarity on measures was associated with greater success. While the BBA program generates a variety of outcome measures that might be relevant to pairing success (e.g., serotonin transporter genotype, cortisol concentrations), behavior managers at most facilities don’t have access to such information; consequently, the goal in the present analysis was to identify simple behavioral correlates of success that might be easily measured by individuals at other facilities who are charged with performing pairings.

The research team found that among females, success was higher when members of a pair were more similar in patterns of emotional responding during the infant assessments, regardless of the degree of emotionality – so, pairing a low-Nervous animal with another low-Nervous animal was more likely to lead to success than pairing a low- with a high-Nervous animal. In contrast, among males success was more likely when the average score for the two animals was as low as possible; in this case, a low-Nervous with a high-Nervous animal was likely to be a more successful pairing than two high-Nervous animals. Other significant influences for males’ (but not females’) success were age (younger pairs were more successful); weight (greater success when males had a greater weight difference); and rearing (more success when indoor-reared animals were paired together and when outdoor-reared animals were paired togethter).

Together, the results suggest that broad measures reflecting patterns of emotionality in infancy (and which remain relatively stable throughout life) can be usefully employed to increase the likelihood of success in pairing attempts, and ultimately improve psychological and physical health for indoor-housed animals at the CNPRC and other nonhuman primate facilities worldwide.

The authors would like to thank the CNPRC staff for all their help and hard work on this project, and excellent care of the animals. The research is supported by National Institutes of Health grants OD010962 and OD011107.

For more information on the CNPRC animal care oversight and regulations, see here.