Human personality has fascinated us from early on. Aristotle, in his 'Nicomachean Ethics' identifies that humans "acquire a particular quality by constantly acting in a particular way... you become just by performing just actions, temperate by performing temperate actions, brave by performing brave actions.". Today personality in humans and animals is defined in an almost identical fashion, attributed by consistency in behavioral and emotional states. Our modern understanding is a resolution of evolved views corresponding to the parallel development of three fields: psychology, neuroscience and behavioural science. Findings by these fields share the understanding that temperament or personality is partly genetically inherited by next generations, but personality phenotypes develop during lifetime. Therefore, they are variable with conditions, experiences and changes in associated physiology.
The flexibility of personality, albeit more prominently supported in studies on humans, is now also at the forefront of animal behaviour studies. While the definitions applied in animal studies have been generally insistent on the consistency of phenotypes, animals require plasticity or flexibility in order to adapt to changes.
Peters' elephantnose fish can maintain sensing in light and dark conditions, perceiving shape, distance, size and electric properties of objects. They do this by producing their own weak electric signals which then interact with the environment and are sensed back, like sonar but with electric rather than sound waves. Their chosen environments in the Niger and Congo delta are dim or dark, and one adaptive explanation for this is that they can avoid visually orienting predators. In a recent study on this fascinating species we found that consistency in their level of boldness to approach and explore was only exhibited in the light, whereas in the dark all individuals, even those which were constantly timid in the light, behaved very boldly. As a consequence, the degree of change between light and dark was greater for those being more timid in the light. Conversely, those that were constantly bolder in the light simply maintained their level of boldness in the dark. This shows that bolder fish are at a disadvantage by remaining active in the risky bright conditions where they are conspicuous to predators, whereas timid fish prioritise safety when its risky and the location of resources only when its safe. To balance this disadvantage of bolder personalities, the flexibility of more timid animals is an energetically costly process. The trade-offs between the benefits and costs of each strategy, flexible or stable, can be resolved by the variable levels of flexibility/stability within populations. This process suggests that behavioral flexibility varies within populations and maintains trade-offs in selection.
This study demonstrates that personalities in animals, as in humans, can be flexible. However, individuals may indeed vary in this also, with some having more flexible personalities and others more constant ones. This opens new possibilities in how personality might be examined.