The Evolution of Personality

"Evolutionary biologists have long been interested in individual differences (Darwin, 1859), but often regard individual variation within a population as the raw material on which selection acts rather than an end product of natural selection..."
-Réale et al., 2007

The heart of science lies in existential questions such as "Who am I?" and "Where did I come from?" Yet somehow, these are the very questions that scientists tend to shy away from. It's as if we're afraid that by unraveling the mysteries of our world and ourselves, we'll be left with nothing but a handful of yarn. But many of us see the quest for personal understanding differently - as a journey to gain appreciation for all the complexities and rare events that came together to weave the glorious tapestry that is life. It is in this push and pull of wanting to know more while still wanting to maintain mystery that the study of personality lies. And for this reason (and many others), the science of personality has been woefully understudied and underappreciated.

Among the primary challenges is that "personality" is extremely hard to define in a measurable way. With respect to an individual's personality, the Oxford dictionary defines it as "the combination of characteristics or qualities that form an individual's distinctive character". The Merriam-Webster dictionary defines it as "the complex of characteristics that distinguishes an individual or a nation or group; especially: the totality of an individual's behavioral and emotional characteristics". How in the heck is one to measure "an individual's distinctive character" or "the totality of an individual's behavioral and emotional characteristics"?! The task of developing a measurable definition for personality is less daunting when we consider the elements of it that we can agree upon: Personality involves: (1) differences in behavior across a population and (2) relative consistency of a type of behavior within an individual across time and in different contexts. Now if you pick a behavior you can measure, you can scientifically explore this one aspect of personality.

Denis Réale, Simon Reader, Daniel Sol, Peter McDougall, and Niels Dingemanse proposed five primary personality traits (they call them "temperament traits") that can be scientifically studied: (1) shyness-boldness in response to risky situations, (2) exploration or avoidance of new situations, (3) general activity levels, (4) aggressiveness, and (5) sociability. Each of these could be measured on a sliding scale and in countless ways, providing information on both how populations vary and how consistent each individual is across multiple contexts for a given personality trait.

Psychologists have been studying human personality for a long time. The question as to whether (other) animals also have personalities is loaded with ethical implications, although anyone with a pet will tell you the answer is obvious. This framework makes the study of personality accessible to animal behaviorists, ecologists, and evolutionary biologists. And this isn't just a breakthrough for some of our most whimsical curiosities ("Let's see exactly how social Fido is"), but it could provide us with important understandings of what makes some animals more adaptable to change, what neurological processes promote or restrain certain aspects of personalities, and how personalities evolve in response to their environments. However, researchers expanding these personality concepts across species would have to be extra-cautious to avoid anthropomorphism (the attribution of human characteristics to something non-human without scientific evidence).

The study of the ecology and evolution of personalities is young, but there are already many interesting examples of where it can take us. One of the first was a landmark study by Felicity Huntingford in 1976 on anti-predator behavior, exploratory behavior and aggressiveness in three-spined sticklebacks. Although she never used the words "personality" or "temperament", she showed that individual fish differed consistently in aggressiveness measures across multiple behavioral tests. She argues that this link between aggression towards your same species as well as towards predator species could serve an adaptive function, as both can be threats to offspring.

A series of studies on rhesus macaques by Markku Linnoila and his colleagues found that males that were more aggressive, less socially affiliative, more impulsive, more prone to taking risks, and had higher mortality rates also had lower levels of the neurotransmitter serotonin. Not only could these correlated personality traits be traced to a similar physiological attribute, but serotonin levels have a heritable component that can be influenced by evolutionary pressures.

However, it's important to remember that personality traits are not necessarily permanent: They can change with context and experience. In a study of shyness-boldness of the Southern dumpling squid, David Sinn, Natalie Moltschaniwskyj, Erik Wapstra and Sasha Dall found that squid were bolder when feeding competition was higher and when they were larger. However, an individual that was bold in a feeding test was not necessarily bold in a threat test.

Personality may seem like an enigma, but the study of personality could move us forward in important goals relating to conservation, mental health, evolutionary prediction, and genetics. But it will absolutely give us more to think about when we ponder "Who am I?"

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Further reading:
Réale, D., Reader, S.M., Sol, D., McDougall, P.T. and Dingemanse, N.J. Integrating animal temperament within ecology and evolution, Biological Reviews, 82, 291-318 (2007). DOI:10.1111/j.1469-185X.2007.00010.x.

Huntingford, F.A. The Relationship between anti-predator behavior and aggression among conspecifics in the three-spined stickleback, Gasterosteus aculeatus, Animal Behaviour, 24, 245-260 (1976). DOI:10.1016/S0003-3472(76)80034-6

Sinn, D.L., Moltschaniwsky, N.A., Wapstra, E., and Dall, S.R.X. Are behavioral syndromes invariant? Spatiotemporal variation in shy/bold behavior in squid, Behavioral Ecology and Sociobiology, 64, 693-702 (2010). DOI:10.1007/s00265-009-0887-2.

Image Credits:
Walking each other is by Thang Nguyen at Wikimedia Commons.

Can you keep a secret? is by Linda Jinterwas at flickr.