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An animal’s distinguishing pattern of behaviors which remain consistent and stable across time and contexts.
Most dog owners would undoubtedly say that their pet has personality, and even a non-dog owner might ask someone if their dog is “friendly” before petting it. However, pets are not the only animals in which human personality characteristics are used for describing their behavior. For example, a mother to her child may describe a squirrel as “bold” when it approaches them at a park. Or at an oceanarium, one might overhear observers saying how a dolphin seems “reserved” as it swims alone. Is it possible that nonhuman animals have individual personalities? Interest in animal personality by lay persons as well as researchers is evident with even a casual perusal of the “pet” section of a bookstore. Some animal rescue organizations even use personality assessments to aid in matching potential owners with a suitable dog or cat personality. However, the study of animal personality has not always been accepted by the scientific community. Jane Goodall, renowned ethologist who studied wild chimpanzees for over 50 years, caused quite a bit of controversy when she first gave her subjects names such as “Fifi” or “David Greybeard” instead of the typical practice of assigning numbers. She was thought to be too anthropomorphic when describing their humanlike behaviors and giving them names. Despite criticism from “hard” scientists, she never doubted these wild chimpanzees had personalities. Fortunately, scientists have created ways to scientifically study animal personality and are now less apprehensive about describing animals in such ways. Since Goodall’s seminal work, research on animal personality has flourished. This entry will discuss how animal personality research has developed over the past few decades, how it is beneficial, and where it needs to go from here.
History of Animal Personality Research
One of the first arguments for the existence of animal personality came from Darwin. He believed that personality traits (consistent individual differences) were present in animals, and he also suggested that these behavioral traits could evolve in the same way as physical traits. However, during the mid-twentieth century, psychologists began to take a more normative approach rather than an idiosyncratic approach. The emergence of behaviorism, which thought of animals as simply vehicles on which general rules of behavior could be demonstrated, assumed that organisms were inherently similar and only differed as a result of environmental influences. Furthermore, during this time, an emphasis was placed on collecting large sample sizes and calculating an average of behaviors instead of examining behaviors independently. Large sample sizes were studied and individual variation was considered to be noise and unimportant for understanding the overall meaning of animal behavior. Later that century, though, the focus began to shift back to the study of consistent individual differences. Pioneering studies of animal personality appeared in the 1970s (e.g., Adamec 1975; Buirski et al. 1973; Stevenson-Hinde and Zunz 1978). These studies began a surge of interest in consistent individual characteristics among animals of various species, and research has begun to focus on animal personality more seriously. This line of research has resulted in a number of studies revealing individual differences in personality traits in such diverse species as primates, marine mammals, insects, fish, invertebrates, and birds (see Gosling 2001, for review).
Research Methods for Animal Personality
One reason for the surge in interest in animal personality was the development of methods for studying it scientifically. Obviously, an important limitation to the study of this topic is the inability of researchers and animal subjects to communicate with each other. This excludes the use of some of the most common methods for collecting data from human participants, such as self-report measures. As a result, animal studies often rely primarily on two methods for assessing animal personality which are referred to as coding and rating. The coding method involves observers judging the behavior of an animal within a standardized situation. The rating method, in contrast, involves one or more observers who are acquainted with an animal making judgments about its behavioral tendencies.
Coding an animal’s behavior during a naturally occurring activity such as eating or grooming is known as ethological coding (Vazire and Gosling 2004). This system was used in Capitanio’s (1999) study of sociability in rhesus macaques. Capitanio observed and coded the behaviors demonstrated by rhesus macaques, while they interacted with their social groups. From these observations, four main personality dimensions emerged: sociability, confidence, excitability, and equability. The coding method can also be used in experimental settings. In one such study, wild-caught cuttlefish were presented with three behavioral tests which aimed to assess individual responses to different contexts: alerting, threat, and feeding (Carere et al. 2015). Three factors were extracted that explained individual differences in behavioral responses to these tests, which were interpreted as being indicative of activity and deimatic behavior, reactivity in response to a threat, and the bold-shy axis. For example, individuals with high negative scores on the second factor showed high mobility and deimatic reactivity in response to the threat of being touched with a wooden stick.
The alternative to coding extends the rating method used in human research to animals. This involves a group of observers making judgments about an individual animal’s behavioral traits, based on their cumulative experience with the animal (Vazire and Gosling 2004). The validity of this method depends on the extent to which the observers know the animals and their specific experience with them (Highfill et al. 2010). Once selected, raters are typically provided with a list of adjectives or descriptions, which they use to rate each individual. For example, large numbers of chimpanzees have been rated on a number of traits from the human five-factor model (King and Figueredo 1997; Weiss et al. 2007). Using this method, these authors were able to replicate the factors of dominance, extraversion, conscientiousness, and agreeableness across multiple populations.
Some researchers believe that the best way to study personality in animals is to employ a combination of both coding and rating techniques. This generates more information and may increase the validity and reliability of the measures (Vazire and Gosling 2004). For example, one study examining personality in the black rhinoceros first used the rating method, by having keepers from 19 zoos rate their respective rhinoceros, then applied the coding method, by administering behavioral tests to the same individuals (Carlstead et al. 1999). Their findings indicated that each rated behavior trait was matched significantly with the animals’ observed behaviors. For example, rhinoceros rated as “fearful” by their keepers were also coded as “fearful” based on their behavior. Another study applied both methods to horses, via owner trait ratings and behavioral tests, resulting in four factors that demonstrated links between the two methods (Ijichi et al. 2013). For example, one factor identified proactive horses as those rated as highly extraverted, showing more resistance to handling, avoidance of a stressor, and exploration of a novel object. Another factor indicated that neurotic horses were more resistant to handling and showed greater startle responses and slower recovery from being startled, all of which were interpreted as indicating threat avoidance.
Methods for assessing animal personality can also be distinguished based on whether they predominantly incorporate top-down or bottom-up elements. The former describes methods that apply an adapted version of a previously established model to another species (Freeman et al. 2013), most commonly the human five-factor model (Goldberg 1990). This is usually implemented via the trait rating method (e.g., King and Figueredo 1997), and the primary advantage is that it facilitates cross-species comparisons (e.g., retrospective FFM application to 12 species: Gosling and John 1999). This being said, difficulties with comparisons can still arise, primarily as a result of differences in the specific traits that are measured between studies (Freeman and Gosling 2010). Crucially, however, it may result in the exclusion of species-specific traits and include ones that are not relevant to the target species (Freeman et al. 2011). In contrast, bottom-up measures use species-specific traits, ensuring inclusion of those of relevance to the target species, but decreasing the ease with which cross-species comparisons can be made (Freeman and Gosling 2010). The behavioral coding method tends to be primarily bottom-up, as it commonly utilizes ethograms derived from direct observations.
The incorporation of bottom-up and top-down elements into the trait rating and coding methods, respectively, has been suggested as a way to combine the benefits of both methods, but few studies have done so yet. One notable exception to this is a study of chimpanzee personality; trait items were identified from previous five-factor model-based assessments (top-down) and from those nominated by experts as relevant to chimpanzee personality and items from species-specific ethograms (bottom-up) (Freeman et al. 2013). The five factors found using this combined top-down and bottom-up design were strongly correlated with those elucidated in one previous bottom-up study and two top-down ones, therefore providing stronger evidence for the existence of these factors.
While there are many standardized measures for the study of human personality, the assessment techniques for animal personality have historically been viewed as subjective and anecdotal. More recently, animal personality research has gained greater viability with tests of reliability and validity. Another important element of personality is its relative consistency across time and contexts. For humans, the consistency of how an individual’s behavior is perceived by another is thought to be important evidence for human behavior as being structured within specific intentions and emotions (Morris et al. 2000). Therefore, when behavioral descriptions for animals are found to be consistent over time and across contexts, the same claim could be made. Consistency is assessed via test-retest procedures, which examine whether an individual exhibits similar behavioral traits during separate testing occasions. One such study examining rhesus monkeys employed the test-retest procedure by rating each individual every year for 4 years (Stevenson-Hinde et al. 1980). Their results indicated that some personality traits were stable year to year. For example, “confident” was the most stable trait despite age or sex, and its stability may be linked to dominance. Highfill and Kuczaj (2007) examined the stability of individual dolphin personality traits over time and across situations. Twelve out of 15 bottlenose dolphins demonstrated consistent personality traits after enduring changes in their environment and social group as a result of Hurricane Katrina. These findings parallel an earlier conclusion found in human personality research, which is that consistency over time among personality traits emerges when observers properly judge individual behaviors (Block 1977).
As it becomes more widely accepted to apply the term personality to nonhuman animal species, it raises questions about what it really means to have personality: when do individual differences in behavior become personality? Is demonstrating individual differences in a single behavior sufficient to conclude that a species has personality? There is currently no consensus on the answer to these questions, as demonstrated by differing viewpoints in the literature. For example, startle responses in beadlet anemones were found to show individual differences that were consistent over a 3-week period (Briffa et al. 2011); based on this, the authors concluded that these animals possess personality. In contrast, findings that male gray seals exhibited consistent individual differences in time spent alert across two breeding seasons did not lead the authors to unequivocally attribute personality to this species; instead, they conclude that while gray seals may possess personality, these results were only sufficient to demonstrate that they behave differently and consistently in this one behavior (Twiss and Franklin 2010). There are therefore no universally accepted standards describing what counts as personality and what does not.
Benefits of Studying Animal Personality
Animal personality research offers a number of both practical and theoretical benefits. In general, zoos and farms can more effectively manage animals and maintain their welfare if they can consider the specific characteristics of the individuals. Examining individual differences can allow predictions to be made about which animals may be at greatest risk for developing abnormal behaviors that may negatively impact welfare. Personality can also be used to individualize environmental enrichment and housing requirements (Coleman 2012). Furthermore, knowledge of personality types can aid in animal management techniques, such as breeding and reintroduction programs (Wielebnowski 1999). Finally, studies of animal personality can provide insights into that of humans, which includes links with significant topics such as health and immunity (Capitanio 1999; Maninger et al. 2003; Ironson and Hayward 2008).
Stereotypic behaviors have been estimated to occur in more than 85 million captive animals across the world (Mason and Latham 2004), which is of significance given that the occurrence of these behaviors is commonly used as an indicator of welfare problems; as a result, a variety of research has assessed whether specific personality traits can predict which individuals are at greatest risk of stereotypies. One study found that rhesus macaques were significantly more likely to perform stereotypies later in life if they had been reared in indoor cages and were highly nervous or gentle at 4 months old, but there was no such effect of personality for individuals reared outdoors in groups (Vandeleest et al. 2011). While much research has focused on nonhuman primates, some links between personality and stereotypies have been found in other species; parrots rated as more extraverted developed fewer stereotypies when deprived of enrichment and maintained lower levels than less extraverted individuals after enrichment was reintroduced (Cussen 2013). However, it is important not to assume that animals with few or no stereotypies experience good welfare. Individuals with a reactive personality tend to respond passively to stressors and so may not perform stereotypies; however, they also tend to have greater physiological stress responses and may therefore experience the worst welfare in poor environments, despite a lack of stereotypic behavior (Ijichi et al. 2013).
It is not news to animal care staff that individual animals find enrichment objects or activities differentially beneficial. However, the links between personality and interaction with enrichment are only just beginning to be elucidated in research. Several studies have focused specifically on the effects of novel enrichment on different personalities. The snow leopard personality dimensions of Active/Vigilant and Curious/Playful were significantly positively correlated with the number of visits made to novel enrichment objects (Gartner and Powell 2012). Negative responses to novel enrichments have also been assessed; chimpanzees categorized as stress sensitive exhibited increased self-directed behaviors after answering incorrectly in familiar cognitive testing tasks, even though participation was voluntary, with further individual variation in changes observed in specific behaviors (Yamanashi and Matsuzawa 2010). The effects of personality on engagement with enrichment and welfare indicators have been demonstrated across a range of taxa, despite generally sparse research in the area, indicating that personality is of great importance when designing enrichment programs to benefit all individuals.
Authors studying animal personality have often suggested individualization of the captive environment according to individual trait scores. Many of these proposals focus on animals’ reactions to visitors or ability to seclude themselves. For example, cheetahs with high scores on tense-fearful might benefit from greater access to hiding places in order to facilitate coping with their captive environment (Wielebnowski 1999), and cuttlefish falling toward the shy end of the bold-shy axis may require more protection from observers (Carere et al. 2015). Large crowd size may also have greater negative impacts on gorillas with high understanding scores (Stoinski et al. 2012). In general, further research is required to determine whether such suggestions based on personality actually do provide animals with improved well-being.
Another interesting area in which personality may be a useful tool is in research and training that occurs in zoos and other institutions with captive animals. Where participation in activities is voluntary, personality may affect which animals actually engage in them. For example, chimpanzees with higher openness to experience scores voluntarily spent more time present during research sessions (Herrelko et al. 2012). This has significance for researchers, as it is likely important to take into account the possible overrepresentation of certain personality types in data gathered from voluntary completion of research tasks. Personality-participation links may also have animal health consequences. Chimpanzees with higher openness to experience scores were more likely to complete the full glucose sampling procedure in their first training session; this sampling is necessary to manage their risk of developing type 2 diabetes (Reamer et al. 2014).
Using personality assessments to identify compatible partners and increase breeding success would have huge utility for captive facilities. Despite this, there is relatively little research on the topic, but what literature does exist is promising. For example, nonbreeding cheetahs of both sexes had higher scores on the tense-fearful dimension compared to breeding individuals (Wielebnowski 1999). Black rhino breeding pairs containing a female who was more dominant than the male were more likely to have higher breeding success (Carlstead et al. 1999). Giant pandas who were more fearful had lower mating success and offspring production, while aggressive females had increased mating success but lower offspring numbers than less aggressive mothers (Martin 2014).
Finally, animal personality studies can also provide useful comparisons and insights for the field of human personality development. For example, animal studies are helpful when investigating the impact of early environment on the ontogeny of individual behavioral differences (Gosling 2001). It has also been suggested that expanding the use of animal models to study how personality could influence disease susceptibility and resilience could provide a unique complement to human studies (Cavigelli 2005). For example, research with animal models can involve more experimental manipulation, unlike the predominantly correlational research performed with humans. Additionally, life span longitudinal studies can be performed with short-lived animal models, which would also help to address more collective effects of personality on health. Indeed, the utility of animal studies has been demonstrated multiple times; for example, relationships between sociability scores and immune responses (to SIV and tetanus) were demonstrated in rhesus macaques (Capitanio 1999; Maninger et al. 2003) before associations were found in humans between HIV progression and the personality dimensions of openness, extraversion, and conscientiousness (Ironson and Hayward 2008). Overall, animal models of human personality characteristics play an essential role in understanding the many different dimensions of human health and behavior.
The study of nonhuman animal personality has flourished in the last few decades, with research revealing consistent individual differences in behavior across diverse taxa (Gosling and John 1999). This body of work has confirmed the reliability of the two major methods, rating and coding, used in place of the self-reports commonly used in human research. Furthermore, using a combination of these methods, as well as incorporating both top-down and bottom-up elements, has proven useful for validating the personality factors found in different studies (Freeman et al. 2013). Finally, there are huge numbers of ways in which studying animal personality can be beneficial, from informing the management of captive populations, to providing insights into human health and disease progression. However, there is still much we are yet to discover about the personality of animals and how it relates to other aspects of their lives. While current findings demonstrate associations between personality scores and individuals’ differential responses to aspects of the captive environment, it will be interesting to see whether future research can actually use assessments of personality to successfully individualize management practices. Ultimately, such advances in our knowledge of the impact of animals’ personalities can help to improve not only their lives but also our own.
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