DOI: 10.5061/dryad.xksn02vc0
title: Sex differences in longitudinal personality stability in chimpanzees

description: 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.
description: 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<i> </i>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. <br> 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’. 
description: 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.
 <i>American Journal of Primatology</i>,
 <i>75</i>(10), 1042–1053.
 https://doi.org/10.1002/ajp.22168