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Sex differences in longitudinal personality stability in chimpanzees

Citation

Rawlings, Bruce et al. (2020), Sex differences in longitudinal personality stability in chimpanzees , Dryad, Dataset, https://doi.org/10.5061/dryad.xksn02vc0

Abstract

Personality factors analogous to the Big Five observed in humans are present in the great apes. However, few studies have examined the long-term stability of great ape personality, particularly using factor-based personality instruments. Here, we assessed overall group, and individual-level, stability of chimpanzee personality by collecting ratings for chimpanzees (N = 50) and comparing them to ratings collected approximately 10 years previously, using the same personality scale. The overall mean ratings of three of the six factors differed across the two time points. Sex differences in personality were also observed, with overall sex differences found for three traits, and males and females showing different trajectories for two further traits over the 10-year period. Regardless of sex, rank-order stability analysis revealed strong stability for dominance; individuals who were dominant at the first time point were also dominant 10 years later. The other personality factors exhibited moderate-to-weak rank-order stability indicating that individuals were variable in their rank-position consistency over time. As many studies assessing chimpanzee cognition rely on personality data collected several years prior to testing, these data highlight the importance of collecting current personality data when correlating them with cognitive performance.

Methods

Chimpanzees were rated on a seven-point Likert scale instrument measuring six overall traits; agreeableness, dominance, extraversion, methodical, openness and reactivity/undependability. The scale was generated from data collected on the NCCC chimpanzees across a two-stage process between April 2006 and December 2008 (Freeman et al. 2013). Ratings for T1 (2006-2008) and T2 (September 2015-December 2016), were collected during weekly staff meetings. Raters were either care-staff or supervisory staff, all of whom had worked daily with the chimpanzees for at least six months. At T1, the 17 raters had worked with the chimpanzees for 6 months-21 years, and rated 8 to 10 chimpanzees each week as part of a study investigating personality in a larger number of the NCCC chimpanzees (Freeman et al. 2013). At T2 the 8 raters had worked with the chimpanzees for 6 months-19 years and rated three to five chimpanzees each week. Four raters were present at both T1 and T2, providing some consistency in raters across time points. All raters were instructed to rate chimpanzees based on their overall experience of a chimpanzees’ typical behaviours and interactions, rather than specific and/or recent experiences, and were explicitly instructed not to discuss ratings with each other.


We analysed mean-rank stability, rank-order stability and a Reliable Change Index (RCI). Mean-rank stability was calculated by comparing overall mean scores for each of the six traits at T1 and T2. The mean ratings of personality were assessed at T1 and T2 such that if a mean rating of a trait changed from (for example) 4.1 to 4.6, this would represent an increase of 0.5 on the scale. Mixed effects ANOVAs were conducted; the two time points were the within-subjects independent variable, sex was the between-subjects independent variable, and personality rating was the dependent variable. To calculate rank-order stability, we examined correlations between individuals across the two rating periods. The RCI is used to distinguish individual change that is statistically significant from change that may have occurred due to measurement error. For each individual subject, the difference in ratings from T2-T1 were compared to the distribution of change scores expected solely by measurement error (RCI = (T2 score -T1 score)/standard error of the measurement of the difference. Using a 95% confidence interval, for each factor individuals were classified as having either ‘increased’, ‘decreased’, or stayed the ‘same’. 

Usage Notes

There are no missing values. The dataset provides the mean scores per factor, for all study subjects. For sex 1 = males, 2 = females. T1 refers to the intial data collection (April 2006-December 2008, by Freeman et al. 2013). T2 refers to the ratings collected between (September 2015-December 2016).

The T1 reference is:

Freeman, H., Brosnan, S. F., Hopper, L. M., Lambeth, S. P., Schapiro, S. J., & Gosling, S. D. (2013). Developing a comprehensive and comparative questionnaire for measuring personality in chimpanzees using a simultaneous top-down/bottom-up design. American Journal of Primatology, 75(10), 1042–1053. https://doi.org/10.1002/ajp.22168

Funding

Economic and Social Research Council, Award: 1449189