Taking personality selection bias seriously in animal cognition research: a case study in capuchin monkeys (Sapajus apella)

Abstract

In most experimental work on animal cognition, researchers attempt to control for multiple interacting variables by training subjects prior to testing, allowing subjects to participate voluntarily, and providing subjects with food rewards. However, do such methods encourage selection bias from subjects' personalities? In this study, we trained eighteen zoo-housed capuchin monkeys (Sapajus apella) for two experiments, under conditions of positive reinforcement (i.e. food rewards) and free-choice participation. Using a combination of behavioral and rater-based methods, we identified and validated five personality dimensions in these capuchins (Assertiveness, Openness, Neuroticism, Sociability, and Attentiveness). Scores on Openness were positively related to individual differences in monkey task participation, reflecting previous work showing that such individuals are often more active, curious, and willing to engage in testing. We also found a negative relationship between scores on Assertiveness and performance on tasks, which may reflect the trade-offs between speed and accuracy in these animals' decision-making. Highly Assertive individuals (the most sociable within monkey groups) may also prioritize social interactions over engaging in research. Lastly, monkeys that consistently participated and performed well on both tasks showed significantly higher Openness and lower Assertiveness compared to others, mirroring relationships found between personality, participation, and performance among all participants. Participation and performance during training was clearly biased toward individuals with particular personalities (i.e. high Openness, low Assertiveness). Results are discussed in light of the need for careful interpretation of comparative data on animal cognition and the need for researchers to take personality selection bias more seriously.

Evaluating mood changes in response to anthropogenic noise with a response-slowing task in three species of zoo-housed primates

“…It is also likely that there are individual differences contributing to the results obtained here, and teasing out how personality or temperament influences responses to social stimuli is an important future direction. Related, there may have been selfselection among the subjects in that the subset of touchscreen trained animals who voluntarily participated during the Air and Water Show may have been willing to do so because they experienced less stress than groupmates (see Morton, Lee, & Buchanan-Smith, 2013;Polgar, Wood, & Haskell, 2017). However, that would have led us to underestimate the impact on mood for Japanese macaques.…”

Self-control assessments of capuchin monkeys with the rotating tray task and the accumulation task

“…There were consistencies in individual patterns of performance, though, with some monkeys generally performing better and others consistently responding impulsively. These individual differences might be particularly relevant for identifying potential interventions or behavioral correlates of impulsive behavior, and they highlight the importance of considering personality factors in behavior (for more discussion of this with regard to personality and selection bias in cognitive research with capuchin monkeys, see Morton, Lee, and Buchannan-Smith, 2013; Morton et al, 2013). Finally, we found that overall there were significant increases in performance on the current task compared to a previous assessment, perhaps indicating that continued exposure to these kinds of tasks plays a role in altering and improving self-control in nonhuman primates.…”

Lack of prosociality in great apes, capuchin monkeys and spider monkeys: convergent evidence from two different food distribution tasks

“…These results highlight the importance of implementing tasks that control for confounding variables and of using different experimental paradigms when previous results contradict one another. Although it is still possible that other populations perform differently when tested with the same task (because of differences in diet quality, housing or personality, for example [47]), administering multiple tasks (with control conditions) is necessary to demonstrate prosociality. Finally, it is important to stress that our conclusions are limited to prosocial behaviour in an active food distribution context and do not allow us to generalize our findings to other contexts, like helping behaviour [9][10][11][12][13] or passive food transfers [7,8], as prosocial behaviour might be restricted to specific contexts and not generalize to active food-sharing [10].…”