general activity within the temperature gradient. Proactive fish (more aggressive, bold risk-takers, prone to routine formation) have a preference for higher temperature environments. Reactive fish (shy, less risk-prone, more flexible) favor medium colder temperatures. Evolutionary potential The degree of variation in a population has been determined to influence to direction and outcome of natural selection. Most scientific research has focused on genetic and phenotypic variation or differences in resource use; however, variation in consistent behaviours (i.e. personality) also has important evolutionary consequences. For example, personality in animals can affect the way individuals interact with their environment and with each other which can affect the relative fitness of individuals. Therefore, personality can influence selection. Also, behavioural traits are more dynamic which may allow an animal to adapt more quickly which, in turn, can speed up the rate of evolution. Further, natural or artificial selection cannot act on personality unless there is a mechanism for its inheritance. In rhesus macaques (Maccaca mulatta), the personality traits of Meek, Bold, Aggressive, Passive, Loner and Nervous have heritability values of 0.14 to 0.35, thus indicated that there is some genetic basis to the expression of personality traits in animals. In apes, including humans, heritability estimates of personality dimensions range from 0.07 to 0.63. In horses, heritability estimates range mostly between h2=0.15 and h2=0.40 for traits assessed in personality tests. Values at this level are considered as "promising" for artificial selection. Examples Personality in animals has been studied across a wide array of taxa. Some of these studies have investigated personality in mammals, elasmobranchs, reptiles, fish and birds. There have been several studies that have either been notable in that they have changed scientific understandings of animal personality or have applications in the field of human personality. For example, the study of personality in chimpanzees by King and Figueredo in 1997 was one of the first studies to apply the five-factor model in animal personality. It demonstrated the existence of personality traits in animals and provided a foundation for similar assessment strategies in future studies of personality in animals. Similarly, zebrafish have been used as a neurobehavioral model species for studying personality using the trait approach in non-human animals. These studies can then be translated to study personality development and personality disorders in humans. Another general example is the spider, Anelosimus studiosus. This spider forms groups in which some females show an aggressive personality type and