Anyone that spends lots of time with animals knows that no two are alike. And it is not just that they are different in size or color; they are different in how they behave and interact. For some people, "personality" is a uniquely human property. But is it?

Personality is defined as a consistent set of behaviors over time and in different situations. Scientists are finding that in many species, individual animals behave in consistently different ways and argue that these differences meet the scientific definition of personality. Some believe that personality is not the product of behavior, but that it’s the other way around. At least 60 different species, including spiders, birds, mice, fish, insects and primates, among others, have been found to have distinct personalities.

In an essay for Science magazine in 1998, Jane Goodall wrote, "When I began my study of wild chimpanzees in 1960 at Gombe Stream Research Center, it was not … quite proper to talk about animal personalities." The concept of animal personality is a hot topic in behavioral biology. And despite an increasing amount of research supporting this premise, some in the scientific community will still not use the word personality, preferring "behavior syndrome."

It is difficult to dispel concerns about anthropomorphism. Dr. Marcel van Aken, a Professor in Developmental Psychology at the University of Utrecht, says, "I'm not so concerned about it. You have to define clearly what you are going to measure and then let the data speak." He and fellow researchers plan to measure the personalities of birds and humans with a common set of tests, hoping to find clues to the evolution of human personality.

Psychologist Dr. Samuel Gosling heads up the Animal Personality Institute at the University of Texas in Austin. He is credited with sparking an avalanche of animal personality research over the last 10 years. During the mid-1990s, he and Dr. Oliver John performed research on a captive colony of 34 spotted hyenas at the University of California-Berkeley. They had caretakers rank the individual hyenas for human personality traits: assertiveness, excitability, "human-directed agreeableness", sociability, and curiosity. Without comparing notes, the results among the caretakers were amazingly consistent.

This year marks the 200^th anniversary of the birth of Charles Darwin, author of On The Origins of Species, a landmark and seminal work in evolutionary biology. In it, Darwin introduced the theory that populations evolve over the course of generations through a process of natural selection – of reproduction and survival.

"Darwin himself argued that emotions exist in nonhuman animals, and his evolutionary theory suggests that behavioral traits, including personality, can evolve in just the same way as fins, wings and arms," Dr. Gosling said. "We should realize that studying the personality of animals could help us understand a lot about human personality."

Some studies have shown that if animals reproduce more quickly, they exhibit personalities more reckless in character. Those that take longer to reproduce -- and thus must survive longer to procreate -- are more cautious. Dr. Piet Drent of the Netherlands Institute of Ecology and his colleagues are conducting what is considered the most ambitious investigation of personality in wild animals. In a New York Times article, Dr. Dent queries, "Why hasn't one personality become the standard in the population?" If being extroverted offers the best odds for a hyena to reproduce, you might expect that over time, all hyenas would wind up as extroverts.

But circumstances and needs change. A study of diminishing bluebird populations in western Montana tested for aggressiveness. It was determined that male birds needing to locate new nesting sites tended to be more aggressive; this aggressive behavior was required to seize territory. But as the population became established, the average level of aggression in the males went down. By contrast, more passive behavior was beneficial to the survival and rearing of offspring.

In December of 2007 the Science Daily reported, "Researchers at the Kluane Red Squirrel Project found that red squirrels have a range of personalities, from exploratory and aggressive to careful and passive. Both kinds of squirrels persist in the population because neither personality type offers an exclusive advantage for survival."

There are other questions surrounding personality development, such as, is it nature or nurture? A group of European scientists have evidence for a "curiosity gene" in the great tit, a tiny songbird.

Dr. Stephan J. Schoech of the University of Memphis and his students are studying the stress hormone corticosterone (CORT) in Florida scrub-jays. They have found that young birds whose mothers were not attentive had higher levels of CORT in their blood than those with mothers that spent more time brooding. Seven months later, those birds with the elevated CORT had more fearful temperaments.

Further research by Dr. Gosling (with Virginia S. Y. Kwan and Oliver P. John) was presented in the Journal of Personality and Social Psychology (2003); it was titled, "A Dog’s Got Personality: A Cross-Species Comparative Approach to Personality Judgments in Dogs and Humans Dog." Their study focused on four canine personality traits that had analogous human traits: Energy (human Extraversion), Affection (human Agreeableness), Emotional Reactivity (human Neuroticism), and Intelligence (human Openness/Intellect). The research was in part meant to show that personality traits exist in nonhuman animals, and that there must be a way to prove the assessments of animal personality as accurate. But independent of canine specific data results, this study functions as a blueprint for future research.

But does it matter that animals have personalities and is there a real need to study them?

In the conclusion of the Gosling/Kwan/John study, the authors wrote, " … animal studies provide unique opportunities to elucidate the dynamic interaction of biological, genetic, and environmental effects on personality, and to study personality change, links between personality and health, and even processes in personality perception."

An increased understanding of animal personalities helps with the breeding of endangered species and also helps scientists reintroducing endangered animals into habitats where they’ve been wiped out.

George Archibald, co-founder of the International Crane Foundation, has cultivated and revolutionized the preservation of rare crane species and their wetland habitats. In National Wildlife (February/March 2009) he indicates that personality is the key to a compatible couple. What an extremely aggressive female crane needs is a male who is even more aggressive. "Unless she is submissive to the male," says Archibald, "there is a possibility she won’t experience the hormonal changes that allow her to lay eggs."

The use of animal personality assessments is not restricted to wildlife purposes. These assessments are also being performed on domestic cats and dogs. The results are used to help place shelter animals with people having compatible personalities.

For those worried that we humans may be less special than we once thought – don’t worry. Kathy Streeter is curator of marine mammals at the New England Aquarium. She believes that we possess the ability to modify our behaviors, or even to change our overall characters, at least within limits. "You may wake up in a foul mood and wonder how you're going to get through the work day, but you can try to stay quiet and holed up in your office. We can consciously modify our behavior to very specific, conscious degrees, so that could be what makes us unique in terms of personality."

Besides, no animal does neurotic better.