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It will come as no surprise to owners of peculiarly cantankerous cats or endearingly affectionate dogs, but scientists have now confirmed what pet lovers have always known: that each animal has its own personality.
More than 60 different species, from primates and rodents to fish, birds and even insects, have been scientifically documented to exhibit individual differences in characteristics such as aggressiveness or shyness.
Scientists believe there is an evolutionary basis to personal differences between members of the same species and that animal personality has to do with a trade-off between safe and risky behaviour.
The researchers say that their hypothesis can explain why even animals with relatively straight-forward and genetically innate behaviour such as snails and ants can be seen as having individual personalities.
"Farmers, dog-owners and others who spend time observing animals know that animals can differ strikingly in character and temperament," said Max Wolf, a biologist at the University of Groningen in the Netherlands.
"Yet only in recent years it has become evident that personalities are a widespread phenomenon in the animal kingdom. Indeed, animals as diverse as spiders, mice and squid appear to have personalities," Dr Wolf said.
"By 'personality' we usually mean some kind of consistency in behaviour. If an animal is bold, aggressive or extrovert they tend to be stable in these characteristics over time. They don't switch their behaviour abruptly," he said.
A young great tit who exhibits the personal trait of exploring a new environment in a superficial way will tend to continue this superficial exploration of its surroundings as it gets older, Dr Wolf said.
"Even more interestingly, it will also tend to be more aggressive than its thoroughly exploring conspecifics [members of the same species] and behave more boldly when confronted with novel objects, as well as have a different dominance position in the group hierarchy," Dr Wolf said.
The study, published in the journal Nature, points out that it is difficult to explain how such individual differences in behaviour have come about unless they can be seen as a response to two rival strategies in life - live fast and die young, or grow old gracefully.
"In many cases personalities are shaped by a simple underlying principle: the more an individual stands to lose in terms of reproduction, the more cautiously it should behave, in all kinds of situations and consistently over time," Dr Wolf said.
"Many personality traits have a risky component. According to our theory individuals who have little to lose should take risk, those who have much to lose should not.
"In effect, some individuals invest in future benefits and late reproduction, whereas other individuals put more emphasis on current benefits and reproduce early," he said.
The trade-off between the two strategies - risky and safe behaviour - results in the type of personality differences scientists and pet owners have observed in a wide range of species, the researchers say in their Nature study.
"It simultaneously explains the coexistence of behavioural types, the consistency of behaviour through time and the structure of behavioural correlations across contexts," the scientists say.
"Moreover, it explains the common finding that explorative behaviour and risk-related traits like boldness and aggressiveness are common characteristics of animal personalities," they say.
Dr Wolf said that whether the same approach can be used to explain differences in human personality is still open to question.
"At the very least, the results of this study could be taken to provide an evolutionary underpinning of the common saying 'He that has least to lose has least to fear'," he said.
- INDEPENDENT
