To even the most casual observer, it is clear that people are not homogenous in their behaviour, and that this goes beyond possible nurture influences such as cultural upbringing. Individuals vary in their behaviour in a consistent manner; some people are generally more aggressive, friendly and adventurous in every aspect of their lives. So obvious is this observation that we even have a word for it – personality. Likewise, anyone who has spent any significant amount of time in the company of animals will almost certainly acknowledge that they are not all the same. The extent to which this is apparent varies from species to species, of course, but the observation is not a revolutionary one. And yet, until relatively recently the concept of ‘personality’ in non-human animals was revolutionary. And it has had to work hard to shake off the criticism of anthropomorphism and pseudoscience.
It was long assumed that animals were infinitely plastic in their behaviour, being able to respond adaptively to all environments. When people actually started to look, however, it became apparent that this wasn’t the case. Individuals showed substantial variation in their responses to certain events and environments, and these responses were not always adaptive. There was a strong correlation however, in the responses of a single individual over time. Personality, you say?
One way in which the field of research into personality, that is “individual differences in behaviour that are contextually and temporally consistent”, has overcome it’s shaky start is to move away from the loaded term ‘personality’ (it does have the word ‘person’ in it, after all!) towards alternatives such as ‘temperament’ or ‘individuality’. Whatever you want to call the phenomenon, the evidence for personality in the animal kingdom is rife. Personality, as measured by aggression, exploration, neophobia (fear of new objects), courtship or boldness has now been identified in primates, rodents, birds, fish, reptiles, insects, spiders and cephalopods. And it’s consistency can be surprising, one study found that activeness was consistent between larva and adult in damselflies (Lestes congener), a transition which involves the destruction (apoptosis) of most of the cells in the body!
Not only is the existence of personality in animals indisputable, there appears to be complexity in animal personalities beyond variation along single isolated ‘personality’ axes. Certain suites of personality traits appear to be correlated into ‘behavioural syndromes’. For example, one of the main behavioural syndromes defined in the literature is the ‘proactive-reactive’ axis, which includes personality traits such as exploration, activity, aggression and boldness. Proactive individuals are aggressive, bold and highly exploratory, and tend to attempt to manipulate stressful situations. Conversely, reactive individuals are cautious, and respond passively to stressful situations. Empirical investigations into behavioural syndromes have yielded mixed results. Evidence for a correlation between some or all of the traits forming the proactive-reactive axis has now been documented in great tits (Parus major), chaffinches (Fringilla coelebs), zebra finches (Taeniopygia guttata), rats, chipmunks (Tarnias striatus), dumpling squid (Euprymna tasmanica), cockroaches (Gromphadorhina portentosa) and water striders (Aquarius remigis), to name a few.
Furthermore, as researchers increasingly begin to view the colonies of highly social species, such as bees and ants, at the super-organismal level, colony personality has been discovered in the form of inter-colony variation in traits such as defensiveness and foraging activity, traits forming part of the proactive-reactive axis, and there is evidence for the formation of behavioural syndromes in honeybees collective behaviour. Personality may play a key role in social behaviour in general, and in an unusual group of social spiders (Anelosimus sp), individual differences in behaviour (personality) influence their tendency to group up and live socially, as well as their response to prey. Reactive spiders tend to be more voracious predators, but are consequently less social with other spiders.
Personality traits encompassed in the proactive-reactive axis appear to be linked inherently to the way organisms deal with stress. In a stressful situation, increased heart rate, oxygen and glucose consumption are associated with an active behavioural response in proactive individuals. In contrast, reactive individuals have hormonal mechanisms which allow them to adjust to the situation passively. Several hormones have been associated with proactive and reactive personality types. High corticosterone levels in response to stress are associated with proactive personality in quail.
A link between personality and physiology may also be responsible for the apparent relationship between personality and disease and lifespan. In humans, personality is linked to psychological health; high neuroticism, and low conscientiousness, agreeableness and extraversion are linked to an increased risk of clinical disorders. Personality also affects physical health, with less resilient personality types being subjected to a 1.5 fold increase in their risk of premature death; a risk akin to high blood pressure. Furthermore, studies of exceptionally long-lived people have revealed a link between personality and longevity, with lifespan being strongly correlated with conscientiousness, and centenarians tending to be more extraverted and agreeable, and less neurotic than those with an average lifespan. A recent study identified one gene, SYNJ2, which appears to be related to agreeableness and longevity (and also depression).
Personality is likely to have wide-ranging impacts on animal populations, through its effects on dispersal, range expansion, response to environmental change and even speciation rates. The presence of different behavioural types in a population may also effect its propensity to become invasive, which could have important agricultural and biological control implications. Personality has been found to influence dispersal in mosquitofish (Gabusia affinis), mole rats (Heterocephalus glaber), lizards (Lacerta vivipara), house mice (Mus musculus musculus) and killifish (Rivulus hartii), among others. Understanding the evolution and maintenance of personality in animals may shed light onto our own personality. Personality in animals fundamentally affects the way they deal with stress, and strongly influences their social interactions. Whether we like it our not, in all animals (yup, that’s us too!), personality is linked to disease and life expectancy, a relationship we still understand relatively little about.
Articles in this Series:
- Animal Personality Part I: Individual Differences
- Animal Personality Part II: The Evolution of Personality
Want to Know More?
- Bell (2007) Animal Personalities. Nature (447), 539 – 540
- Gosling and John (1999) Personality dimensions in nonhuman animals: a cross-species review. Directory of Psychological Science (8), 69 – 75
- Herborn et al (2010) Personality in captivity represents personality in the wild. Animal Behaviour (79), 835 – 843
- Logue et al(2009) A behavioral syndrome linking courtship behavior towards males and females predicts reproductive success from a single mating in the hissing cockroach, Gromphadorhina portentosa. Behavioral Ecology (20) 781 – 788
- Brodin (2008) Behavioural syndrome over the boundaries of life: carryovers from larvae to adult damselfly. Behavioral Ecology (20), 30 – 37
- Pruitt et al (2012) Iterative evolution of increased behavioural variation characterizes the tranition to sociality in spiders and proves advantageous. The American Naturalist (180); 496 – 510
- Luciano et al (2012) Longevity candidate genes and their association with personality traits in the elderly. American Journal of Medical Genetics
- Cote et al (2010) Personality traits and dispersal tendency in the invasive mosquitofish (Gambusia affinis). Proceedings of the Royal Society B (277), 1571 – 1579
Featured image used under a creative commons licence from Wikimedia commons. Original image by Michael Maggs.