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Strategic decision making around social behaviours in wild vervet monkeys ("Chlorocebus pygerythrus")
Editeur(s)
Maison d'édition
Neuchâtel : Université de Neuchâtel
Date de parution
2021
Nombre de page
366
Résumé
Animals living in stable social groups find daily pressing challenges to feed, move and socialise although individuals make decisions that cannot just satisfy self-driven interests. Instead, individual decision-making needs to be tuned to a collective that may have different needs due to age, physiological state or rank status. As a result, individuals have evolved a capacity to integrate a wide and flexible set of options that take into consideration conflicts of interest that arise due to competition to make behavioural decisions. The same set of elements has most probably favoured the development of strategic cooperative interactions between individuals that help to ease tensions, building bonds, restoring conflicts, ultimately facilitating group cohesion and collective synchrony. Primate societies are exquisitely varied in their group structures, habitats, diets and sociality, but one key generality that stands out among female-bonded societies is the predominant role that females’ social behaviour play in the groups’ coexistence. In this thesis, I investigated how individual and group decision-making affects cooperative dynamics, and which factors influenced the emergence of such behaviours, and how social dilemmas could arise in some instances in wild vervet monkeys (Chlorocebus pygerythrus). For that, I conducted a mix of empirical and correlational studies focusing on adult females as subjects of study in some instances, or on group behaviour in others, trying to unveil which factors might spark the different social strategies manifested in concrete and observable behavioural outputs. I did so by combining cooperative and conflict scenarios using different time dimensions.
In Chapter I, I explored the mechanisms of direct reciprocity exchanges of grooming for tolerance in adult female vervet monkeys. Experiments with boxes that contain small food items permit testing tolerance/aggression dynamics and relate it to previous social interactions in the short term (minutes to hours). Further, vervets are eager to participate in such experiments and quickly discern personal colour-covers so they would only approach their boxes when testing female dyads. I found that the dominant female of the dyad did not base her reciprocity towards a previous grooming partner at the boxes set up based on the relationship quality that untied them (bond) nor on the difference in grooming contribution that individuals did. Instead, dominant females varied their tolerance disposition on the total time the grooming interaction lasted as a form of attitudinal reciprocity where the positive sensations of the grooming interaction reinforced the dominant’s benevolence by being tolerant. The dominants provided tolerance in a way that mimics an hourglass where the sand that falls is the tolerance decay and tolerance was exchanged for grooming for up to three hours. What is more, females did not lower their reciprocity despite having had additional partners, indicating that if this happened, it did not cause a sort of cognitive saturation. In Chapter II, I widened the time-dimension of grooming-based reciprocity dynamics. I experimentally fed eight subjects (four highest-ranking, four bottom-ranked) in four neighbouring groups. By feeding one subject at a time I intended to provide them with a “surplus time” for other activities and evaluated whether they changed their socialisation and how this would translate into their social network positioning. By studying grooming given as a source of strategic social investment, I assessed the fed females and found that they significantly increased their allocation of grooming to others while being fed, compared to two control phases around the treatment. They also increased their allocation of grooming (strength), gave more than what they received, and specifically targeted bonded, kin and adult females as preferential grooming partners. Interestingly, the high and low-ranking females reacted similarly to the feeding treatment evidencing akin grooming strategies by giving more grooming than they received. These results showed that grooming social networks are dynamic and hint at Machiavellian Intelligence abilities as a probable foundation for individuals to attempt social promotion in their groups through the provisioning of grooming services. In Chapter III, I studied the topic of intergroup encounters as a form of collective action problem between four group dyads. I created a blend of ecological and territorial variables and calculated them from the perspective of a focal group, but including the perspective of the group they encountered. This allowed a sense of whether the focal group valued more or less the area where the conflict lay. The ecological variables also reflected distinct timedimensions (here and now, year average, maximum in the year) The results showed that encounters concentrated more in core areas (highly used), in areas with higher current NDVI (greenness as a proxy of food availability), and with higher NDVI maximum year-values. As it appears, vervets’ willingness to fight rival groups is modulated upon perception values that integrate the present, future, and past information about the quality of a specific site. I found that among the possible ecological and territorial facets that could shape outcomes (winning/losing) neither appeared to be critical, even if aspects like relative distance to the core or relative intensity of use were considered. Strikingly, what seemed to increase the likelihood of winning was being smaller than your rival was. This is better understood using the volunteer’s dilemma, whereby bigger groups may be less successful in putting together enough volunteers to fight the rival group due to higher odds of cheating. This exciting finding could be better understood if one considered the importance of building consensus before initiating a fight against another group. If bigger groups have a significant portion of their individuals using alternative food sources than the one at stake during the intergroup encounter, that could explain that there is less agreement and therefore a lack of synchrony to outcompete rival groups of smaller size but higher group coordination.
In Chapter IV, I studied the phenomenon of group fissions in three neighbouring groups that took place amid a severe drought that affected Southern Africa (2014-2016). Here, I two full years before the groups fissioned using three months time windows to investigate the variation of activity budgets, grooming behaviours, some social network parameters at the individual and collective level (adult females), and group dispersion as a measure of group cohesion. I found that the different groups seemed to follow somehow different routes in their dealing with challenging ecological conditions. Contrary to expected, I did not find convergent progressive deterioration of the group cohesion and all groups adopted relatively flexible reactions. I assessed whether social network connectivity through grooming and being in proximity would be indicative of how the final composition of the two daughter groups was determined, but only one of the three groups followed that logic. Finally, I selected a suite of factors that could be informative of the females’ fitness to investigate whether they would play “stay” (risk-averse) or “leave” (risk-taking) strategies. I found that bottomranked females always left, top-ranking stayed, and middle-ranking could choose any option. Surprisingly, having experienced the death of a close family member was highly correlated with playing the “stay” strategy. In summary, these results bring forward evidence that vervet monkeys, and females, in particular, are capable of displaying highly flexible social repertoires in contexts of cooperation and conflict. As individuals use their evolutionary acquired abilities to cope with changing conditions (social, environmental), it is no surprise that they can adopt strategic decision-making processes that assist their social connectivity in the network. Beyond, the inevitable multi-layered nature of groups makes an ideal platform where conflicts of interest unfold. Such contexts represent interesting social dilemmas that require a certain level of coordination action of cooperators so to make up for the non-cooperative ones.
In Chapter I, I explored the mechanisms of direct reciprocity exchanges of grooming for tolerance in adult female vervet monkeys. Experiments with boxes that contain small food items permit testing tolerance/aggression dynamics and relate it to previous social interactions in the short term (minutes to hours). Further, vervets are eager to participate in such experiments and quickly discern personal colour-covers so they would only approach their boxes when testing female dyads. I found that the dominant female of the dyad did not base her reciprocity towards a previous grooming partner at the boxes set up based on the relationship quality that untied them (bond) nor on the difference in grooming contribution that individuals did. Instead, dominant females varied their tolerance disposition on the total time the grooming interaction lasted as a form of attitudinal reciprocity where the positive sensations of the grooming interaction reinforced the dominant’s benevolence by being tolerant. The dominants provided tolerance in a way that mimics an hourglass where the sand that falls is the tolerance decay and tolerance was exchanged for grooming for up to three hours. What is more, females did not lower their reciprocity despite having had additional partners, indicating that if this happened, it did not cause a sort of cognitive saturation. In Chapter II, I widened the time-dimension of grooming-based reciprocity dynamics. I experimentally fed eight subjects (four highest-ranking, four bottom-ranked) in four neighbouring groups. By feeding one subject at a time I intended to provide them with a “surplus time” for other activities and evaluated whether they changed their socialisation and how this would translate into their social network positioning. By studying grooming given as a source of strategic social investment, I assessed the fed females and found that they significantly increased their allocation of grooming to others while being fed, compared to two control phases around the treatment. They also increased their allocation of grooming (strength), gave more than what they received, and specifically targeted bonded, kin and adult females as preferential grooming partners. Interestingly, the high and low-ranking females reacted similarly to the feeding treatment evidencing akin grooming strategies by giving more grooming than they received. These results showed that grooming social networks are dynamic and hint at Machiavellian Intelligence abilities as a probable foundation for individuals to attempt social promotion in their groups through the provisioning of grooming services. In Chapter III, I studied the topic of intergroup encounters as a form of collective action problem between four group dyads. I created a blend of ecological and territorial variables and calculated them from the perspective of a focal group, but including the perspective of the group they encountered. This allowed a sense of whether the focal group valued more or less the area where the conflict lay. The ecological variables also reflected distinct timedimensions (here and now, year average, maximum in the year) The results showed that encounters concentrated more in core areas (highly used), in areas with higher current NDVI (greenness as a proxy of food availability), and with higher NDVI maximum year-values. As it appears, vervets’ willingness to fight rival groups is modulated upon perception values that integrate the present, future, and past information about the quality of a specific site. I found that among the possible ecological and territorial facets that could shape outcomes (winning/losing) neither appeared to be critical, even if aspects like relative distance to the core or relative intensity of use were considered. Strikingly, what seemed to increase the likelihood of winning was being smaller than your rival was. This is better understood using the volunteer’s dilemma, whereby bigger groups may be less successful in putting together enough volunteers to fight the rival group due to higher odds of cheating. This exciting finding could be better understood if one considered the importance of building consensus before initiating a fight against another group. If bigger groups have a significant portion of their individuals using alternative food sources than the one at stake during the intergroup encounter, that could explain that there is less agreement and therefore a lack of synchrony to outcompete rival groups of smaller size but higher group coordination.
In Chapter IV, I studied the phenomenon of group fissions in three neighbouring groups that took place amid a severe drought that affected Southern Africa (2014-2016). Here, I two full years before the groups fissioned using three months time windows to investigate the variation of activity budgets, grooming behaviours, some social network parameters at the individual and collective level (adult females), and group dispersion as a measure of group cohesion. I found that the different groups seemed to follow somehow different routes in their dealing with challenging ecological conditions. Contrary to expected, I did not find convergent progressive deterioration of the group cohesion and all groups adopted relatively flexible reactions. I assessed whether social network connectivity through grooming and being in proximity would be indicative of how the final composition of the two daughter groups was determined, but only one of the three groups followed that logic. Finally, I selected a suite of factors that could be informative of the females’ fitness to investigate whether they would play “stay” (risk-averse) or “leave” (risk-taking) strategies. I found that bottomranked females always left, top-ranking stayed, and middle-ranking could choose any option. Surprisingly, having experienced the death of a close family member was highly correlated with playing the “stay” strategy. In summary, these results bring forward evidence that vervet monkeys, and females, in particular, are capable of displaying highly flexible social repertoires in contexts of cooperation and conflict. As individuals use their evolutionary acquired abilities to cope with changing conditions (social, environmental), it is no surprise that they can adopt strategic decision-making processes that assist their social connectivity in the network. Beyond, the inevitable multi-layered nature of groups makes an ideal platform where conflicts of interest unfold. Such contexts represent interesting social dilemmas that require a certain level of coordination action of cooperators so to make up for the non-cooperative ones.
Notes
Committee Members:
Prof. Redouan Bshary, University of Neuchâtel – Supervisor
Prof. Klaus Zuberbühler, University of Neuchâtel – Internal examiner
Prof. Sarah F. Brosnan, Georgia State University – External examiner
Prof. Carel van Schaik, University of Zurich – External examiner
Thesis defence date: 14th July 2021
No thèse : 2912
Prof. Redouan Bshary, University of Neuchâtel – Supervisor
Prof. Klaus Zuberbühler, University of Neuchâtel – Internal examiner
Prof. Sarah F. Brosnan, Georgia State University – External examiner
Prof. Carel van Schaik, University of Zurich – External examiner
Thesis defence date: 14th July 2021
No thèse : 2912
Identifiants
Type de publication
doctoral thesis
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