The great Russian scientist Ivan Pavlov was one of the first to observe personality differences in the dogs he studied in his lab during the late 1800s. He could make all of them salivate at the sound of a bell in anticipation of food, but some of them just seemed to get into it more than others.
Down through the years a number of other scientists have discovered what seem to be personality traits in various animals, but the question of animal personalities has largely been ignored by science. Possibly because personality has long been considered distinctively human.
But that's changed.
"The study of animal personalities has become a hot topic in behavioral biology over the last five years or so," said Sander van Doorn, an evolutionary biologist at the Santa Fe Institute in New Mexico. He is a coauthor of a research paper published in the journal Nature that puts an evolutionary twist on the subject.
At least 60 different species, including spiders, birds, mice, fish, insects and primates, among others, have been found to have distinct personalities in the last few years.
What's particularly puzzling to biologists is similar traits, like aggression or shyness, are found in very dissimilar species, like fish and birds, but not in every member of the species, and those traits persist over a wide range of circumstances and over a long period of time. That is the very definition of personality— consistent behavior over time and in different situations.
The suggestion that animals have personalities won't come as a shock to pet lovers. Fido is a riot. Missy the cat is a bit aloof.
Yet researchers like Samuel Gosling of the University of Texas, Austin, still have trouble convincing fellow scientists.
"I get the most skepticism from scientists," he told a national science meeting a couple of years ago. "It's really the human behavior researchers who object the most."
A strong personality can be a liability in some situations. Personality is not the product of behavior. It's the other way around. Personality can lead to rigidity whereas flexibility might be more useful, especially in the wilds. So why did personality evolve among animals that would be better served by flexibility?
"That's something we're trying to explain because it's a bit puzzling," van Doorn said. "You would expect animals to be more flexible in their behavior."
The Santa Fe research is an attempt to find an evolutionary basis for the development of personalities among animals. By using computer modeling, and games designed to identify specific traits, the scientists believe they have hit on a primary principle that underlies the evolution of personality.
Animals that have a lot to lose, especially in terms of reproduction, are likely to be much more cautious in a wide range of situations than animals that have little to lose. The research shows, for example, that animals that reproduce as soon as they are able are much more aggressive and reckless than animals that spend some time, maybe even a year or so, surveying the territory first. The late bloomers will have more of a stake in the future, because they have not yet reproduced, and are thus much more cautious.
But they are also likely to be much more successful at breeding and raising their young, and thus more likely to propagate their genes.
Other scientists have found evidence in the wild that is compatible with that laboratory work. Max Wolf, a theoretical biologist at the University of Groningen in the Netherlands and lead author of the study in Nature notes that birds in his country, including oystercatchers, can either breed early or hold off until later, and that decision is a personality trait that is handed down from one generation to the next.
Van Doorn said researchers in the Netherlands have studied the personalities of a small bird, the great tit, over successive generations.
"They had the birds in aviaries and they tested individuals for their boldness or their shyness," van Doorn said. "The boldest were allowed to reproduce, and in the course of generations the difference (between the bold and the shy birds) increased, and this is usually an indication that there is a genetic component."
It's all a bit "puzzling," he added, because a risky personality leads to risky behavior in many unrelated activities. A hawk that will torment smaller birds is also likely to challenge an eagle.
"There are examples where you can see this rigidity in a personality is not always a very good thing," he said. "There are studies of spiders, for example, where some spiders are more aggressive than others when they defend their territories. But those spiders are also more aggressive to their own offspring. Sometimes, they eat them."
Other studies have shown that animals are more like people than was once thought, at least in terms of personalities. Members of the same species that are about the same age and size and sex can behave vary differently, even under similar circumstances.
That is counter intuitive, at least from an evolutionary standpoint. It indicates that even birds can be very rigid in their behavior, and that's not always the best policy. Even for an aggressive and fearless animal, sometimes its better to run.