The Human Evolution Blog

Guest Blog post by Patrick Tkaczynski  (University of Roehampton, London, UK)

Do animals have personality? The last decade or so has seen behavioral and evolutionary biologists beginning to agree with centuries of pet owners and embrace the notion that animals have complicated personalities. In scientific terms, biologists have demonstrated variations in patterns of behavior that are consistent to an individual, but that vary from others. From the adorably named dumpling squid, to the less adorable (for some) tangle web spider, plus numerous examples among mammals and non-human primates, researchers have noted that individuals behave differently from their conspecifics, and that these differences are stable across time and context.

We can find stories in the literature of “bold” field crickets, “shy” hermit crabs, and “extrovert” orangutans, just to name a few examples.

For pet owners, this is unsurprising. Spend enough time with an animal and it is hard not to recognize (or project) personality characteristics. Indeed, this “specter of anthropomorphism” (Gosling & John, 1999) lurking on the shoulders of animal personality research may have suppressed interest in the field for a long time.

 

However, the turn of this century has seen an explosion of interest in animal personality. In fact, the ten most cited papers in this field have collectively been cited more than 4500 times despite being less than ten years old. This is principally because it provides a new framework for understanding behavior. That individuals behave differently from one another begs the question: why does no single optimal behavioral strategy appear to exist? That individuals behave consistently over time and context begs another question: why are individuals not completely behaviorally flexible?

The “state-dependent hypothesis” posits that variation in behavior is a reflection of variation in state. In other words, some trait or characteristic can influence certain behavioral strategies. For example, things like body size and strength, speed or other skill, quality of vision or other sensory perception, birth order, and even general health or physiological differences may alter the conditions enough that a different strategy (within a suite of possible behaviors) might be the best match for that particular animal. This would explain both the individual differences and the consistency for a particular individual.

Another, non-mutually exclusive theory, is called the “social-niche” hypothesis. This hypothesis holds that individuals occupying different social roles and dominance positions within a community have different factors determining their “fitness” (reproductive success), which can thus lead to different strategies. A powerful alpha animal may be better served by certain behaviors, while a lowly outcast might be able to pursue fitness in a different way.

For my PhD research, I am attempting to test elements of these hypotheses in a highly social animal: the Barbary macaque. These enigmatic primates live in multi-male, multi-female, female-bonded groups. They lead socially complex lives with long-term intra- and inter-sex relationships which are challenged during tense mating seasons, with males forming coalitions to compete for access to sexually receptive females.

Furthermore, Barbary Macaques live in the mountainous regions of Morocco and Algeria – habitats that experience massive fluctuation in climate throughout the year. In the summer, droughts are a very real survival threat with temperatures reaching 40˚C. In the winter, temperatures plummet to below 0˚C and snow can cover the ground for weeks at a time, severely limiting food resources. It has recently been demonstrated that social bonds predict survival through these difficult climatic stressors. Thus, these monkeys make ideal subjects for examining how personality may be associated with state variation, how personality is related to sociality, and ultimately how personality may be related to fitness outcomes.

The first challenge for my research was working out which particular personality phenotypes exist amongst the 30 adult macaques I chose to study. My team and I collected over 1200 hours of observational data on these monkeys, often working in the challenging conditions previously described (thanks team!). These data were divided into three different time periods based on the annual mating season cycle. I then looked to see whether certain behaviors were correlated over time (and context, given the shift between mating and non-mating season).

Using this approach, I identified three main behavioral phenotypes: “excitable,” characterized by high activity and frequency of brief affiliative or aggressive interactions; “sociable,” defined by preference for the company of others and high frequency of friendly interactions; and “tactile,” those spending long periods grooming self and others.

In addition, we performed an experiment involving the most commonly examined animal personality trait, “boldness,”which is a non-fearful reaction to a risky stimulus. Using audio playbacks, we attempted to simulate an inter-group encounter and record whether subjects responded in a “bold” or “shy” manner, quantified by the degree to which they demonstrated fearful or non-fearful responses.

To examine the state-dependent hypothesis, we looked at variation in physiology, specifically to see if any of the described phenotypes were associated with hormonal stress reactivity. One component of vertebrate stress response involves the production of glucocorticoid hormones to stimulate the metabolism of glucose to fuel a reaction to the stressor. As macaques experience a range of environmental and social stressors, we might expect a relationship between glucocorticoid levels and certain behavioral repertoires. Recent advances now make it possible to non-invasively monitor glucocorticoid levels in wild animals through the glamorous process of collecting fecal samples and extracting the metabolites of these hormones.

Having tested over 900 fecal samples, it appears that male and female Barbary macaques have a very different relationship between their personality phenotypes and glucocorticoid levels. In the males, more “excitable” males tend to have more reactive glucocorticoid profiles, while for the “tactile” phenotype, this relationship was inversed, with higher hormonal reactivity associated with lower expression of the “tactile” behaviors.

Given that the “excitable” phenotype is characterized by activity and the frequency of brief affiliative and aggression behaviors, it is not surprising that being excitable appears to be associated with wide swings in glucocorticoid levels. The tactile phenotype is characterized by sedentary behaviors and again, it is perhaps not surprising that higher expression of this personality is associated with more stable glucocorticoid levels.

For females, tactile females had higher glucocorticoid levels (as opposed to wider variation), which may reflect females using tactile behaviors as coping mechanisms to higher levels of stress. I am still working though the data, but it appears that the personalities of male and female Barbary macaques may be related to sex-dependent physiological responses to their social environments.

The next stage of my analysis uses the framework of the social-niche hypothesis to study the relationship between personality and sociality. This hypothesis predicts that individuals should assort themselves based on personality. Evidence for this has recently been demonstrated in great tits, with more shy individuals tending to associate more frequently and form stable social relationships with other shy individuals. As drought or snow can often limit food supply for Barbary macaques, I was particularly interested in who the macaques tolerated eating next to them. Based on initial results, it seems that Barbary macaques assort themselves in a similar way to the great tits, with bold eating with bold, and shy with shy. Macaques of a feather flock together.

The field of anthropology seeks to identify, understand, and explain the differences and similarities that exist between individuals, populations, and cultures. One of the major commonalities that exists in all human societies is that individuals are different, defined by our behavioral variation or personality. Like so many other domains once considered uniquely human, personality appears to be widespread in animal species as well. I hope my work on Barbary macaques helps us understand the evolutionary and behavioral ecology of personality more broadly and further highlight how complex and interesting these North African furballs really are!

 

-Post written by Patrick Tkaczynski, Ph.D. student at the University of Roehampton in London, UK.

Email: tkaczynp1@roehampton.ac.uk

Twitter: @P_Tkaczynski