Decision-making under risk in Barbary macaques (Macaca sylvanus)
by Tiffany Claire Bosshard
Date of Examination:2024-11-21
Date of issue:2025-02-24
Advisor:Prof. Dr. Julia Fischer
Referee:Prof. Dr. Dr. Peter Marschik
Referee:Dr. Igor Kagan
Persistent Address:
http://dx.doi.org/10.53846/goediss-11101
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Description:Doctoral Thesis
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Abstract
English
Risk is ubiquitous in the everyday lives of all animals. Individuals often face precarious situations, such as unpredictable resources and exposure to predators, in which their decisions may lead to a variety of possible outcomes. In the literature, the concept of risk has been investigated from two different perspectives. The first perspective refers to risk in its conventional sense, i.e., a chance of an adverse event occurring, and usually considers risk-taking in situations of predation risk, thus in an ecological framework. The second perspective refers to risk as the probabilistic variance in outcomes of a decision, and usually considers risk-taking in the framework of economic behaviour. Biological models of optimal decision-making formulate predictions relative to the ways in which rational organisms should behave in the face of risk, yet individuals often deviate from rational choice expectations. Therefore, it is still unclear the extent to which animal attitudes towards risk are governed by biological model predictions, and/or by other factors such as the task and context in which the risky choice is presented. The aim of this dissertation was to broaden existing knowledge on risk-taking in primates and test the predictive validity of biological principles of decision-making, using Barbary macaques (Macaca sylvanus) as a model. To this end, I assessed risk-taking within the framework of life-history theory, using a key life-history parameter, i.e., age. More specifically, I investigated age-related changes in risk-taking, relative to the prediction that risk propensity should decrease over time, due to a progressive shift in goals from gain acquisition to loss aversion. To do this, I implemented a naturalistic, ecological foraging task, in which the subjects assessed the cost and benefit trade-offs regarding high- and low-value food rewards paired with high- and low-risk predatory hazards (i.e., study 1). Against life-history theory predictions, I found no decline in propensity for risk across age. Rather, regardless of age, subjects were overall more willing to take a risk for a high-value reward, and overall less willing to take a risk when the high-risk stimulus was involved. However, taken as a measure of motivation, older individuals were in fact faster than their younger conspecifics to engage in a risky choice for a low-value reward. Further, to test the influence of task and context on animal attitudes towards risk, I conducted a second experiment, which explored the monkeys’ risk preferences in an economic framework. Namely, I presented the animals with a gambling task, in which they had to learn varying statistical patterns associated with ‘risky’ and ‘safe’ contingencies (i.e., study 2). The experimental apparatus consisted in a ‘risk box’, fitted with two levers, one corresponding to a constant but small reward delivery, i.e., safe, and the other corresponding to a pseudo-randomly absent or large reward delivery, i.e., risky. Although the sample size of three individuals in this study was very small, the animals showed a clear overall indifference to either the safe or risky contingency across sessions. However, within sessions, subjects seemed to rely on a heuristic technique by sticking to only one of the two levers, namely the first lever with which they started the session – arguably a cognitively simpler strategy than learning the probabilistic outcomes associated with different risk contingencies. Study 1 and study 2 varied in terms of ecological validity as well as cognitive demands, and concurrent findings suggest that Barbary macaques have a different approach to risk in each one. While study 1 demonstrated how the stakes involved in a risky choice, namely the value of the payoffs and the potential costs of a predatory threat, may influence risk propensities, study 2 showed how individuals may resort to simple decision rules when the process of evaluating probabilities of outcomes is cognitively too demanding or time-consuming. Taken together, the findings of these two studies do not accord with the notion that risk-taking behaviour is predicted, solely at least, by biological model expectations, but rather animals seem to have an adaptable set of behavioural and cognitive strategies to cope with risk, depending on the stakes, context, and cognitive requirements involved in the risky choice. Following the notion that certain behaviours may develop according to an individual’s intention or need in a given situation, I also described a case of a novel, idiosyncratic gesture in one individual Barbary macaque (i.e., study 3). Namely, one adult male was observed hand-clapping at conspecifics, and I was interested in determining whether the individual hand-clapped as a communicative gesture, and if so, which function the gesture had. Across observed hand-clap events, the subject combined hand-clapping with other behaviours and/or facial expressions, all of which agonistic in nature. In addition, the hand-clap mainly evoked either a submissive or counter-aggressive response from the recipients of the gesture. My observations therefore suggest that the individual intentionally used the hand-clap gesture for agonistic signalling, and present a relevant example of individual variation in behaviour. This led me to further reflect on individual differences in behaviour and how to assess them. As such, I implemented the Social Responsiveness Scale (SRS) – a survey-based behavioural assessment protocol – in Barbary macaques and, as a way to validate this methodology, I compared its resulting scores to actual behavioural observations (i.e., study 4). My findings from this study showed that SRS ratings did in fact not accurately reflect the animals’ true behavioural dispositions, and suggest that questionnaire-based assessments may lead to subjective interpretations of animals’ behaviours.
Keywords: Barbary macaques; Macaca sylvanus; risk-taking; decision-making; ecological risk; economic risk; life-history; gesture acquisition; hand-clapping; intentional communication; multimodal signalling; comparative cognition; social responsiveness scale; method validation
