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. Repeatability Repeatability refers to the fraction of variation in a population that is owed to differences between individuals. Repeatability estimates are one of the most widely used statistical tools that can quantify consistent individual differences in behaviour. Formally, where, is variance among individuals and is the