Most of us think that our pet cats and dogs have personalities, but is this term really appropriate? The most widely studied dimension of animal personality is shy/bold behaviour. It is assumed that `personality' should be constant across time in any given individual. However, organisms also develop,and it is quite possible that a shy child can grow into a bold and extrovert adult.
Studying simple animals with short life cycles makes it easy to investigate consistency in `personality' and in principle makes it possible to investigate the neural bases of the behaviours involved. David Sinn and his colleagues from the Universities of Texas and Tasmania used such an animal for their investigation of animal `personality' – the 7cm-long dumpling squid.
It is known that a dumpling squid that is shy in a predator-risk situation may be bold in a feeding situation. Sinn and his team repeatedly measured the behaviour of squid over a 16 week period, as the squid grew to maturity. They measured the personalities of juvenile and adult squid by poking them (a threat test) and by giving them shrimp to eat (a feeding test). They scored each animal on their personalities by grouping a range of behaviours,including speed of response and the frequency with which squid displayed certain behaviours in the test.
As in previous studies, squid could be bold in one test and shy in the other. This may be because the expression of any underlying `personality' is altered by the developmental and behavioural context: squid were found to be boldest in the feeding test shortly before sexual maturity, while boldness in the predation test increased after maturity.
This change associated with development was not consistent for shy and bold squid, and changed with the experimental situation. In the predation test, shy squid tended to remain shy, while squid that were initially classified as bold showed a wide range of behaviour as they grew from juveniles into adults.
Once sexual maturity had occurred, behavioural responses to a predation threat tended to be consistent: shy squid remained shy, while bold squid became bolder. The opposite effect was found in the feeding test: squid classed as `bold' as juveniles showed the greatest consistency in their personality, while individuals that were `shy' at sexual maturity tended to become shyer as they grew older.
These findings in squid support previous studies on great tits and sticklebacks, and suggest that animal personality is not fixed but is`plastic' – it changes depending on the environment and the developmental stage of the organism. In particular, it can be affected by key phases in the animal's physiology, such as sexual maturity.
However, the squid study also raises the question of whether the `shy/bold'dimension is really measuring something consistent in the animal. If this behaviour can show such variability across time, and can be altered by major developmental changes, it may be something that exists in experimental situations but has no consistent reality in natural conditions.
The solution probably lies in a richer description of behaviour: although it is tempting and convenient to reduce animal `personality' to a single dimension, this may obscure important parts of what the animals actually do. Humans are not simply `shy' or `bold' – why should other animals be?
