Consistent personality differences (e.g. shyness, sociability) among animals have been frequently reported by scientists. Such information used to be considered unscientific, even though it was widely used to describe animals and predict their behaviour (Hebb, 1946). Over the past three decades however animal personality has been studied systematically in various species (Gosling, 2001), using provided lists of descriptors, consistent with the five-factor model of personality (Costa & McCrea, 1992). Few researchers have allowed raters to produce their own list of descriptors (Dutton et al., 1997; Wemelsfelder et al., 2000), and none have investigated the potential influence of human individual differences on ratings. The three main goals of this thesis were (i) to develop in-depth personality profiles of domesticated horses, (ii) to investigate the effect of personal backgrounds and attitudes of observers in their construction of these profiles, and (iii) to provide insight into the reliability and validity of the provided assessments of horse personality. The thesis starts with a literature review of personality psychology, psychometrics, animal and horse personality; followed by the introduction of the concept of animal-as-a-scientist. As methodology is an important part of the project, the repertory grid technique (RGT) originally developed by Kelly (1955), was employed as a novel method to assess horse personality. This method gives observers the freedom to generate their own descriptors, which allows them to integrate the totality of their experience with animals into personality constructs which are meaningful to them. RGT has previously been used for personality assessment in chimpanzees (Dutton et al., 1997) and for the assessment of individual styles of interaction in pigs (Grajfoner et al., 2002). The experimental part of the thesis consists of two studies. In both, horse personality was assessed by groups of human participants, either familiar or unfamiliar with the horses. The familiar groups assessed the horses based on their past experiences. The unfamiliar groups watched short videos of horses interacting with a human. In the first study 44 female observers rated 21 horses from two stables. The results show a significant degree of agreement within the observer groups for 95% of horses. However, correlation of horse personality scores between the observer groups was not significant. The degree of agreement was higher when the observers were familiar not only with the horses but also with the descriptors (Adams-Weber, 1970). In the second study we further investigated the observed incongruity between familiar and unfamiliar observers. A novel object test was added to provide the unfamiliar observers with more information about the horses. Thirty four female observers rated 38 horses from three stables. Again, the degree of agreement was highly significant within the observer groups; however, correlation of the results between familiar and unfamiliar observers was only marginally improved. The degree of agreement was not consistently higher when the horses were rated on elicited constructs. Both studies showed that two personality dimensions, neuroticism and extroversion, are fairly robust in horses. The third dimension, agreeableness, was generated only by familiar observers. Contrary to expectations, observers’ personality, empathy or emotional intelligence did not significantly affect the degree of agreement or how they rated the horses’ personalities. Assessment of individual differences in horses using qualitative descriptors generated by the observers themselves is therefore not observers’ self projection. These results make a significant contribution to the debate on anthropomorphism. The overall degrees of agreement between the observers indicates consistent reliability of the RGT throughout the observer groups in both studies. Finally, the academic and practical implications of the study are discussed. On an academic level, individual differences in animals are of pivotal importance for understanding personality in the contexts of evolutionary, comparative and social psychology. On a practical level horses are, according to their individual differences, selectively used for different purposes: horse assisted therapy, racing and the police. Avenues for investigating the relationships between animal personality, performance and welfare should therefore be further explored.

This thesis focuses on individually specific differences in behavioural strategies, personalities, with two microtine rodents (Microtus arvalis and M. oeconomus) as study species. The work evaluates methodology necessary to reveal and measure consistent individual differences in behaviour, identifies possible proximate and ultimate mechanisms behind the existence of individual behavioural variability, and describes some ecological, evolutionary and behavioural consequences of personalities.

Interindividual differences in animal behaviour that are relatively consistent over time and context are referred to as animal personality. Personality has been recognized throughout the entire animal kingdom, in an array of species like molluscs, arthropods, fish, birds and mammals. The personality of non-human animals has been suggested to vary along five different axes, or continua; boldness-shyness, avoidanceexploration, activity, sociability and aggressiveness. Having a relatively fixed personality may seem nonadaptive compared to infinite behavioural plasticity so the individual would be able to respond adaptively to any changes in the environment. There can be physiological limitations to the phenotypic expressions of any trait, including behaviour. Variation in neuroendocrinology may thus explain why animals have personality. A candidate neurochemical that potentially proximately influences and forms personality, is serotonin (5- HT), one of the most omnipotent neurotransmitter of the animal body. In the many realms of the serotonergic system, there may arise individual differences which forms a proximate basis for differences in personality. In this review paper, I discuss the impact of the serotonergic system on a few different personality traits. Depending on the individual’s motivational state, serotonin can dampen or enhance aggression. Serotonin correlates negatively to anxious traits. Feeding behaviour is affected by serotonin in seemingly opposing directions. Overall, serotonin seem to underlie many behaviours that describe animal personality.

This thesis deals with two main issues: personality (stable individual differences in behavior) and behavioral endocrinology (or socioendocrinology) in nonhuman primates. The first part of the thesis comprises of two primate personality studies of two species: Hanuman langurs and Barbary macaques. Two basic methods of animal personality research (behavioral coding and trait rating) were compared. Stability of personality assessments has been demonstrated. Social rank of individuals was used to validate the questionnaire ratings as well as to illustrate the independence and stability of personality assessment. The second part of the thesis is based on two studies investigating hormone levels in female Barbary macaques. The relationship between cortisol, testosterone, dominance hierarchy and behavior was investigated. And the possible effect of maternal hormone levels around the time of conception on the sex of an infant was evaluated.

Social dominance influences many aspects of the life of animals living in social groups, including fitness. Recent work suggests that individuals occupying different positions in a dominance hierarchy may differ in their behavioural and cognitive traits. The first objective of this thesis was to determine whether personality (i.e. consistent behavioural differences between individuals) is correlated with dominance in natural groups of black-capped chickadees (Poecile atricapillus) in the wild. Dominance relationships within these groups were highly asymmetrical and transitive, which is typical of linear dominance hierarchies. None of the measured personality traits (i.e. aggressiveness in hand, exploration, activity and neophilia) were significantly correlated with dominance. These results suggest that personality does not contribute to the formation of black-capped chickadee hierarchies and add to the growing body of evidence that inherent attributes of individuals are not sufficient to explain the structure of linear dominance hierarchies observed in the wild. The second objective of this thesis was to investigate how dominance and correlates of competitive ability (i.e. sex, age and body condition) are related to novel problem-solving performance, which is a proxy for innovativeness, the invention of new behavioural patterns or the modification of an existing behaviour in a novel context. Problem-solving performance was not significantly associated with sex, age or body condition, but dominants were more efficient problem-solvers than subordinates. This finding suggests that efficiency when solving a novel problem might be driven by cognitive capacity instead of the necessity induced by the social position of an individual. Overall, results of this thesis help to better understand dominance hierarchies in wild groups of animals.

Individuals within a population can vary in the way that they acquire, store, and act on information from the environment. Researchers have commonly looked at differences in genetic architecture, physical environment, or personality as possible causes of individual variation in cognition. Though cognition is defined as a suite of mechanisms involving the processing of information, we have yet to asses information (i.e. a numerical measure of the uncertainty of an outcome) as a possible cause of individual variation in cognition. This thesis seeks to understand the causes of individual variation in cognition by using approaches that allow quantifying and/or manipulating information acquisition or its use. In Chapter 1, I look at the link between information gathering and exploratory personality by testing the correlation between activity in a novel environment and attraction to novelty in wild-caught black-capped chickadees (Poecile atricapillus). My results validate exploratory personality assessed in an open field test as a measure of information gathering. Fast exploration of a novel environment was positively correlated with novelty seeking, suggesting that exploration is an information gathering strategy. In Chapter 2, I test for experience with informative vs non-informative cues as a cause for individual differences in decision making and learning performance. Here, I manipulated the informational properties (i.e. presence and number of reliable cues) of the developmental environment of juvenile captive zebra finches (Taenopygia guttata). This rare longitudinal and experimental examination of the effect of informative versus non-informative cues during development suggests that experience with informative cues can cause increased discrimination learning accuracy and decision-making speed later in life. Finally, in Chapter 3 I looked into individual variation in information use and decision making using a game theoretic approach. Using a producer-scrounger game, groups of zebra finches were exposed to varying seed distributions. Individual strategy choice in a social-foraging game was not significantly correlated with an individual’s experience with informative cues or learning performance. Still, contrary to my predictions, fear response significantly predicted strategy choice where more fearful individuals were more likely to choose a producer strategy. By addressing information as a parameter, my results suggest that information can affect individual variation depending on context.

Many animal societies rely on highly influential keystone individuals for proper functioning. When information quality is important for group success, such keystone individuals have the potential to diminish group performance if they possess inaccurate information. Here, we test whether information quality (accurate or inaccurate) influences collective outcomes when keystone individuals are the first to acquire it. We trained keystone or generic individuals to attack or avoid novel stimuli and implanted these trained individuals within groups of naive colony-mates. We subsequently tracked how quickly groups learned about their environment in situations that matched (accurate information) or mismatched (inaccurate information) the training of the trained individual. We found that colonies with just one accurately informed individual were quicker to learn to attack a novel prey stimulus than colonies with no informed individuals. However, this effect was no more pronounced when the informed individual was a keystone individual. In contrast, keystones with inaccurate information had larger effects than generic individuals with identical information: groups containing keystones with inaccurate information took longer to learn to attack/avoid prey/predator stimuli and gained less weight than groups harbouring generic individuals with identical information. Our results convey that misinformed keystone individuals can become points of vulnerability for their societies.

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A recent theory within behavioural ecology suggests that some animal behaviour traits are consistent over time and over different contexts as part of an animals’ “personality”. Animal personality has generally been accepted within vertebrates, but the idea of invertebrates expressing personality has long been meet with scepticism. In this study, I investigate if red wood ants (Formica rufa) show exploratory and bold behaviours that fulfil the personality criteria. Moreover, I test if this species prefers dark surfaces instead of white surfaces (scototaxis). My experiment included tests of 30 ants and two different behaviour trials targeting exploration and boldness that repeated over three different time periods. My results shows that the exploratory behaviours were correlated between trials conducted at different times and therefore, that this behaviour was repeatable. It was also clear that the red wood ant consistently showed large between individuals, indicating difference in personality. However, the studied ants did not show preferences for dark surfaces and the performance in the scototaxis trial did not correlate with any other measurable behaviour. The result of this study suggests that Formica rufa show behaviours that match the animal personality criteria for exploratory behaviour, but my result could not validate that bold behaviours were repeatable across time and context.

Research in animal personality has been increasing over the last decade, as scientists realise its importance to a variety of health outcomes. In particular, personality has been shown to have an effect on immune function, stress, infant survival, overall well-being, morbidity, and mortality. Because of this, personality can play an important role in captive management, especially as stress is often a problem for captive animals. Research has already shown that personality affects captive breeding efforts, enclosure grouping, and stress regulation in some species. Only a few studies have focused on felids, but these have shown that there are possible applications for personality in that taxon. Because most felids are endangered, and because many of them face special challenges in captivity due to their size and biology, this work aimed to increase knowledge on felids, using personality as a framework, with implications for captive management as a target. Focusing on five species, I assessed the personality of domestic cats, Scottish wildcats, clouded and snow leopards, and African lions, and the well-being of the four latter species. With the exception of the domestic cat, there has been little to no personality work in these species, and none on well-being. I then compared the data within and among these species. I found three main personality factors among the species, including dimensions I labelled Neuroticism, Dominance, and Impulsiveness, with some differences, including an Agreeableness factor in some species, and elements of Openness. As in other species, well-being was negatively related to Neuroticism in most of the study species. Taking into consideration each species’ biology, natural history, and genetics, I discuss the implications and importance of using these species’ personality and well-being assessments in both captive management and conservation efforts. The results indicate that, like in humans, a targeted, individual approach to care is the best use of personality for captive animals.