Personality in animals has been investigated across a variety of different scientific fields including agricultural science, animal behaviour, anthropology, psychology, veterinary medicine, and zoology. Thus, the definition for animal personality may vary according to the context and scope of study. However, there is recent consensus in the literature for a broad definition that describes animal personality as individual differences in behaviour that are consistent across time and ecological context. Here, consistency refers to the repeatability of behavioural differences between individuals and not a trait that presents itself the same way in varying environments. Animal personality traits are measurable and are described in over 100 species. Personality in non-human animals has also been referred to as animal disposition, coping style, and temperament. There are also personality norms through the species, often found between sexes. The diversity of animal personality can be compared in cross-species studies, demonstrating its pervasiveness in the evolutionary process of animals. Research on animal personality variation has been burgeoning since the mid 1990s. Recent studies have focused on its proximate causation and the ecological and evolutionary significance of personality in animals.
Animal personality vs. human personality The extent of personality phenomena considered when examining animal personality is significantly reduced compared to those studied in humans. Concepts such as personal objects, identity, attitudes and life stories are not considered relevant in animals. Similarly, any approach that requires the subject to explain motives, beliefs or feelings is not applicable to the study of animal behaviour. The study of animal personality is largely based on the observation and investigation of behavioural traits. In an ecological context, traits or 'characters' are attributes of an organism that are shared by members of a species. Traits can be shared by all or only a portion of individuals in a population. For example, studies in animal personality often examine traits such as aggressiveness, avoidance of novelty, boldness, exploration and sociality.
Background The initial framework used to study animal personality was comparative psychology. The descriptive language used by comparative psychologists in the late nineteenth century often attributed disposition and behavioural tendencies to individual animals in their studies. Many of these reports are the result of researchers anthropomorphizing the animal subjects and did not explicitly examine what is now considered animal personality. However, these studies do represent some of the first instances of scientists reporting individual differences in animal behaviour. Russian physiologist Ivan Pavlov was one of the first researchers to integrate personality into his research of animal behaviour. In his seminal studies on conditional reflexes, he categorized the behaviour of dogs as Excitable, Lively, Quiet or Inhibited. He linked these personalities to learning ability. The Excitable type, for example, showed signs of strong excitatory conditioning, but a limited ability for the acquisition of inhibitory connections. The Lively type was the most balanced and displayed rapid associative learning, while the Quiet type exhibited consistent but slow learning. The first study that empirically examined animal personality was done in 1938. Meredith Crawford quantified individual differences in the behaviour of young chimpanzees using a behaviour rating scale. Crawford conducted his research out of the Yale Laboratories of Primate Biology. Since then, psychologists have continued to investigate personality in animals across a wide range of taxa. Meanwhile, the incorporation of animal personality into the fields of ecology and evolution is a relatively new practice. Ecologists began to recognize the importance of individual differences in behaviour near the end of the twentieth century.
Methods
Rating of traits Rating traits involve a group of observers that scrutinize the behavioural traits of an individual animal. Observers are typically given a list of characters and/or descriptions to use as a reference for interpreting animal behaviour then use the list to rate the behaviours. The rating method uses observers and data recording instruments.
Coding behaviours Researchers will observe how individual animals respond to behavioural tests and code their reactions. This method is typically used in experimental studies in which environmental conditions are heavily manipulated.
Behavioural reaction norms As research in animal personality became more prevalent in ecological studies many behavioural ecologists were substituting behavioural plasticity for animal personality. That is, attributing variation in behaviour to diverse personality types rather than plasticity in a single behavioural trait. Thus, behavioural reaction norms were introduced as a means of incorporating personality and individual plasticity in the study of animal behaviour. Behavioural reaction norms measure the behaviour of an individual over an environmental gradient, therefore providing information on how an animal behaves on average and how their behaviour changes across an environmental gradient.
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