- Top of page
Personality assessment can be a valuable tool to aid in the captive management of gorillas, specifically with the formation and maintenance of bachelor groups, because it is a quick, low-cost alternative to protracted, multi-institutional behavioral studies [Kuhar et al., 2006]. Bachelor group formation is of concern because of the male-biased birth sex ratio in the 1980s [Faust et al., 2000], intense male–male competition for females, and the space constraints of captivity. Therefore, usually only one adult breeding male can be in any group with females. This leaves a “surplus” of male gorillas that must be housed in bachelor gorilla groups [Stoinski et al., 2004]. Personality factors that are predictive of high affiliative behavior rates and low aggressive behavior rates may be useful in predicting which males are better suited to living in all-male groups [Kuhar et al., 2006]. For example, silverbacks that scored high on Understanding (Agreeableness) were more accepting of blackbacks maturing into silverbacks within their bachelor group than those who scored low on Understanding [Kuhar et al., 2006]. Personality also has important health and life history implications for both humans [Chapman et al., 2011] and non-human primates [Capitanio, 2011]. Personality impacts longevity in captive gorillas with more extraverted gorillas living longer [Weiss et al., 2013], and must have played a role in the evolution of hominins [King et al., 1999] and other great apes.
Recent studies have shown that gorilla, chimpanzee, and orangutan personality shares many similarities to the human five-factor model, along with some individual species differences [Gold & Maple, 1994; King & Figueredo, 1997; Weiss et al., 2006]. The human five-factor model consists of Extraversion, Agreeableness, Neuroticism, Conscientiousness, and Openness [Goldberg, 1990]. The most significant difference in personality structure between humans, chimpanzees, gorillas, and orangutans is that humans and chimpanzees have been shown to have a Conscientiousness factor [King & Figueredo, 1997], while orangutans [Weiss et al., 2006] and gorillas [Eckardt et al., 2012; Gold, 1993] have not. The present study is the first to demonstrate the existence of a conscientiousness factor in captive gorillas.
Humans can reliably assess personality and subjective well-being in chimpanzees and orangutans with high levels of reliability and validity [Freeman & Gosling, 2010; King & Landau, 2003; Weiss et al., 2006]. Subjective well-being is an important expression of personality and has been shown to correlate with personality factors in humans [Diener et al., 2000; McCrae & Costa, 1991], chimpanzees [King & Landau, 2003], and orangutans [Weiss et al., 2006]. Therefore, it can also be used as a measure of the external validity of the personality factors [Weiss et al., 2006]. In addition, subjective well-being has been shown to have a strong positive effect on longevity of zoo-housed orangutans [Weiss et al., 2011a].
Personality assessment as an alternative to behavioral studies is only useful if personality factors accurately predict behavior [Kuhar et al., 2006]. Previous studies of captive gorillas have shown significant yet weak correlations between personality factors and behavior [Gold, 1993; Kuhar et al., 2006]. Specifically, Kuhar et al.  found that Extraversion was significantly positively related to affiliative behavior (initiate and receive) and Dominance was significantly positively related to displacement (initiate) and significantly negatively related to displacement (receive). Two recent studies that used trait adjectives instead of personality factors to correlate with behavior also found significant relationships [Eckardt et al., 2012; Murray, 2011].
The primary purpose of this study is to determine the personality structure of male gorillas in order to provide insight for the management of captive gorillas and specifically for the formation of bachelor groups. Knowledge of gorilla personality structure can also provide insight into the evolution of personality in hominoids. The secondary purpose of this study is to determine if personality predicts behavior and subjective well-being in gorillas. Both, if demonstrated, provide construct validity to the personality dimensions. If personality accurately predicts behavior in captive gorillas as it does in chimpanzees [Pederson et al., 2005], then zoos could use personality trait assessment as a quick, low-cost alternative to long-term multi-institutional behavioral studies in order to identify those males most suited to living in all-male groups [Gold, 1993; Gold & Maple, 1994; Kuhar et al., 2006].
Hypotheses and Predictions
Hypothesis 1: Personality predicts behavior in captive male gorillas living in bachelor groups. Predictions 1–4 are for convergent validity. Predictions 5 and 6 will provide discriminant validity. We used [Kuhar et al., 2006] as a model for our predicted significant and non-significant correlations.
Prediction 1: Dominance will be significantly positively correlated with displacement rates (initiated) and significantly negatively correlated with displacement rates (received) [Kuhar et al., 2006].
Prediction 2: Extraversion/Agreeableness will be significantly positively correlated with the affiliative (initiated) and affiliative (received) behavior categories [Kuhar et al., 2006].
Prediction 3: Extraversion/Agreeableness will be significantly positively correlated with play rates [Vazire et al., 2007].
Prediction 4: Extraversion/Agreeableness will be significantly positively correlated with approach rates (initiated) [Pederson et al., 2005].
Prediction 5: Extraversion/Agreeableness will not be significantly correlated with the displacement (initiated) or the displacement (received) rates [Kuhar et al., 2006].
Prediction 6: Dominance will not be significantly correlated with the affiliative (initiated) or affiliative (received) behavior categories [Kuhar et al., 2006].
Hypothesis 2: Personality predicts subjective well-being in captive male gorillas living in bachelor groups [King & Landau, 2003; Weiss et al., 2006]. Predictions 1 and 2 will provide convergent validity.
Prediction 1: There will be a significant positive relationship between Extraversion/Agreeableness and the subjective well-being summation measure [King & Landau, 2003; Weiss et al., 2006].
Prediction 2: There will be a significant positive relationship between Dominance and an ability to achieve goals [King & Landau, 2003; Weiss et al., 2006].
- Top of page
This study demonstrates that personality and subjective well-being in captive gorillas can be assessed by human raters with high levels of reliability and validity as it has been the case in other great apes [King & Figueredo, 1997; Weiss et al., 2006]. A principal components analysis showed that personality in captive male gorillas living in bachelor groups may be comprised of three components: Dominance, Extraversion/Agreeableness, and Conscientiousness. This is the first study to quantitatively demonstrate the existence of a Conscientiousness factor in gorillas, which is therefore a more ancient trait than previously thought. It had previously been shown to exist only in chimpanzees and humans.
The first factor, Dominance, was similar to the gorilla [Gold, 1993], chimpanzee [King & Figueredo, 1997], and orangutan [Weiss et al., 2006] Dominance factors (Table VI) and indicates the importance of this aspect of personality in these hominoids [Kuhar et al., 2006; Weiss et al., 2006]. In an earlier study of captive gorillas, Dominance was also the inverse of the gorilla Fearful factor [Gold, 1993] (Table VI). A Dominance factor is not present in humans but some of the traits that comprise the Dominance factor in the great apes are contained in the Extraversion factor in humans. Dominance may be more important in non-human animals than in humans because of the multiple social roles that individual humans assume in society [Gosling & John, 1999]. Dominance has been demonstrated in other non-human primate species [Weiss et al., 2011b] and many other animal species [Gosling & John, 1999].
Table VI. Correlations Between Factor Scores as Defined by Sedgwick County Zoo Gorillas', Captive Gorillas' [Gold, 1993], Captive Chimpanzees', and Captive Orangutans' Personality Structures
|Sedgwick County Zoo gorillas||Dominance||Extraversion/Agreeableness||Conscientiousness|
|Captive gorillas [Gold, 1993]|
The second factor, Extraversion/Agreeableness resembles the Extraversion factor in captive gorillas [Gold, 1993], chimpanzees [King & Figueredo, 1997], and orangutans [Weiss et al., 2006] and the Agreeableness (Understanding) factor in captive gorillas and orangutans (Table VI). It includes both social aspects of extraversion (e.g., sociable, friendly) and active (e.g., playful, active) aspects of extraversion [King & Figueredo, 1997]. It is also similar to the Extraversion and Agreeableness factors found in humans [McCrae & Costa, 1991]. Extraversion and Agreeableness have been demonstrated in other non-human primate species [Konečná et al., 2012; Weiss et al., 2011b] and many other non-human animals [Gosling & John, 1999].
The third factor, Conscientiousness, resembles the Conscientiousness factor found in humans [Goldberg, 1990] and chimpanzees [King & Figueredo, 1997] but with some differences. It includes three facets (aggressive, unpredictable, and emotional (Neuroticism)) [King, personal communication], is similar to the Dependability factor in chimpanzees, and is the inverse of the Emotionality factor in chimpanzees. It may be that the most ancestral form of Conscientiousness with all three facets appeared in the ancestor of gorillas, chimpanzees, and humans while the third facet, emotionality (Neuroticism), became part of a distinct Neuroticism factor in the last common ancestor of only humans and chimpanzees. Lastly, the aggressive (irritable) facet of the Conscientiousness factor in the common ancestor of chimpanzees and humans became part of the low pole of Agreeableness in humans, while chimpanzees retained the more ancestral form of Conscientiousness [Gosling & John, 1999; Weiss et al., 2006]. Conscientiousness was not found in orangutans [Weiss et al., 2006], wild gorillas [Eckardt et al., 2012], or captive gorillas [Gold, 1993], so its presence in captive gorillas needs to be confirmed with future studies because of our small sample size.
A Conscientiousness factor may not have been found in the previous captive gorilla study [Gold, 1993] because a different instrument (Maddingley Questionnaire) was used that did not contain adjectives that would lead to a distinct Conscientiousness factor [King, personal communication]. The wild gorilla study [Eckardt et al., 2012] that used the HPQ may not have found a Conscientiousness factor because the captive environment, that is higher interaction with humans, may highlight certain personality traits in captive gorillas [Freeman et al., 2013; Stoinski et al., 2012] that are not as evident in wild gorillas. In chimpanzees, two factors (Neuroticism and Openness) did not replicate across different types of captive environments [Weiss et al., 2009]. The captive environment also promotes certain behaviors in gorillas, such as regurgitation and reingestion [Gould & Bres, 1986], that are not present in wild gorillas. Lastly, the Big Five may not be as universal among all human cultures as previously thought [McCrae et al., 1996]. Among the Tsimane forager-farmers of Bolivia, Extraversion, Agreeableness, and Conscientiousness replicated while Neuroticism and Openness did not [Gurven et al., 2013]. If the Big Five is not universal among Homo sapiens or Pan troglodytes, then it is not surprising that it is not universal among two different species (gorilla, beringei) within the Gorilla genus.
The best way to demonstrate validity of an instrument is to have strong significant correlations between personality dimensions and behaviors that are expected to be associated with those personality dimensions (convergent validity) and non-significant correlations between personality factors and behaviors that are not expected to be associated with those personality dimensions (discriminant validity) [Freeman & Gosling, 2010; Pederson et al., 2005]. Based on the results of our predictions, we accept Hypothesis 1: Personality predicts behavior in captive male gorillas living in bachelor groups. The mean significant correlation between personality and behavior in this study was 0.79 (n = 13) (Table IV), which is higher than the mean in the chimpanzee study (0.35) [Pederson et al., 2005], all nonhuman primate studies (0.25) [Freeman & Gosling, 2010], and studies on humans (range: 0.1–0.3) [Meyer et al., 2001].
Based on the results of our predictions we can accept Hypothesis 2: Personality predicts subjective well-being in captive male gorillas living in bachelor groups. Our finding of a significant relationship between Extraversion/Agreeableness and all subjective well-being items except the ability to achieve goals is consistent with the findings for humans [McCrae & Costa, 1991], chimpanzees [King & Landau, 2003], and orangutans [Weiss et al., 2006]. Our finding of a significant positive relationship between Dominance and only one subjective well-being item, an ability to achieve goals, is consistent with the findings for orangutans [Weiss et al., 2006]. However in chimpanzees, in addition to being positively related to ability to achieve goals, Dominance was also positively related to all of the other subjective well-being items. Since male gorillas form close bonds with female gorillas [Harcourt & Stewart, 2007], dominance may not be as important to their well-being as it is for chimpanzees where males and females do not form long-term bonds. Additionally, at least for wild mountain gorillas, food is more abundant and evenly spaced [Harcourt & Stewart, 2007] so that the aggressive qualities used for hunting in chimpanzees [Goodall, 1986] are not as important to gorillas' well-being. Furthermore, in contrast to male chimpanzees, male gorillas in bachelor groups do not experience potential lethal aggression from other males [Goodall, 1986; Harcourt & Stewart, 2007]. Similarly, the solitary nature of adult male orangutans [Galdikas, 1985] may explain why Dominance is not linked to all of the subjective well-being items in orangutans as it is in chimpanzees. Subjective well-being is an important expression of personality and has been shown to correlate with personality factors in humans [Diener et al., 2000; McCrae & Costa, 1991], chimpanzees [King & Landau, 2003], and orangutans [Weiss et al., 2006]. The significant correlations between the personality factors and subjective well-being in this study provide external validity to the personality factors [Weiss et al., 2006].
This study is the first to suggest that a Conscientiousness factor exists in extant gorillas, and is therefore phylogenetically older than previously thought. This is not surprising considering humans and gorillas shared a last common ancestor around 10 million years ago and share 98% of their DNA [Scally et al., 2012]. Personality has been postulated to have an impact on human life history traits [Nettle, 2006] and could have had a similar impact on the survival and reproductive success of the common ancestor of humans and gorillas. There is a fundamental difference in the social structure of gorillas and orangutans, particularly in the relative importance of the adult male to the daily life of the adult females and their young. Adult male and female orangutans do not associate except during the time of mating [Galdikas, 1985], whereas silverbacks are the focus of the gorilla group, with male–female relationships providing the cohesive nature of the groups [Harcourt & Stewart, 2007]. In addition, adult male gorillas are unusual among primates in showing a high amount of paternal care and interaction with the young of the group [Harcourt & Stewart, 2007]. The social structure of extant gorillas indicates that the suite of correlated traits that comprises the Conscientiousness factor could be relevant to individual gorillas' reproductive success. Perhaps, a correlated combination of low aggression, low emotionality and high predictability in male gorillas would be favored by female gorillas to ensure that males will interact gently with females and their offspring. Individual variation between adult male gorillas in the suite of correlated traits that make up the Conscientiousness factor would be necessary for the emergence of this factor in the common ancestor of gorillas and humans and the maintenance of this factor in the gorilla lineage [Dingemanse & Réale, 2005; Réale et al., 2010].
This study has significant implications for the captive management and captive well-being of gorillas. The management of bachelor groups is an important issue currently in the captive management of gorillas. Presently, there are 28 males that reside in all-male groups in ten zoos in North America, and with 80 males under the age of 12 [Wharton, 2003], this number is expected to increase in the near future [Faust et al., 2000; Stoinski et al., 2004]. The results of this study could help to identify particular males on the basis of personality who are more suited to living in all-male groups (e.g., those males who score high on Conscientiousness; high on Extraversion/Agreeableness [Kuhar et al., 2006] and low on Dominance) [Gold, 1993], or those who insure varying personality profiles among group members (e.g., one gorilla can rank high on Dominance, but certainly not all).
The main limitation of this study was the small sample size which might be responsible for the smaller number of factors identified in this study—three factors in gorillas in comparison to six for chimpanzees and five for orangutans. Additionally, our findings are based on only male gorillas living in bachelor groups. Lastly, there were no species-specific items relevant only to gorillas and not to chimpanzees and orangutans added to the HPQ used in this study. It is possible that some relevant items from a gorilla (emic) perspective were missed [Uher & Asendorpf, 2008].
Future research should focus on determining the personality structure of both female and male captive gorillas living in family groups and bachelor groups, wild populations of western lowland gorillas, and bonobos (both captive and wild) to complete the picture of the evolution of personality in hominoids. Phylogenetic analyses and correlations that connect brain anatomy and genetic differences between hominoids to differences in personality structure will further validate the importance of the evolution of personality as a character trait in the biological sense in our primate lineage [Weiss et al., 2006]. In addition, more data on the relationship between personality factors and life history variables such as longevity [Weiss et al., 2013] will enhance our understanding of personality evolution in hominoids.