Robin Dunbar, Structural and Cognitive Solutions to Prevent Group Fragmentation in Group-Living Species
Abstract
Group-living is one of the six major evolutionary transitions. However, group-living creates stresses that naturally cause group fragmentation, and hence loss of the benefits that group-living provides. How species that live in large groups counteract these forces is not well understood. I analyse comparative data on grooming networks from a large sample of primate species and show that two different social grades can be differentiated in terms of network size and structure. I show that living in large, stable groups involves a combination of increased investment in bonding behaviours (made possible by a dietary adjustment) and the evolution of neuronally expensive cognitive skills of the kind known to underpin social relationships in humans. The first allows the stresses created by these relationships to be defused; the second allows large numbers of weak relationships to be managed, creating a form of multilevel sociality based on strong versus weak ties similar to that found in human social networks.
Paper
Group-living is one of the six major evolutionary transitions.
[FOOTNOTE: Maynard Smith, J. & Szathmáry, E. (1997), The Major Transitions in Evolution, Oxford University Press]
[MGH: I only count five, but Dunbar may logically be adding in ‘language’.]
However, living in groups creates ecological and psychological stresses that, all else equal, impose a limit on group size. In primates, the most intensely social of all mammals, these stresses are so severe that they would limit group size to ~15 individuals. Yet some primate species live in stable groups that number 40-100 individuals in size. In order to live in such large groups, animals need to find ways to mitigate these stresses. Herd-forming species avoid this problem by allowing individuals to join and leave on the basis of the momentary costs and benefits of being in the group, as specified by classic joiner-leaver models. In these species, individuals are effectively anonymous and personalised relationships are rare. In contrast, species that live in stable, bonded social groups exploit personalised (in many cases, life-long) relationships that buffer individuals against the stresses of group-living, thereby allowing them to live together in larger, more stable groups. How they manage this is not known.
In herd-forming species, groups are unstable principally because, at a proximate level, individual activity schedules get out of synchrony: groups fragment and disperse because some individuals stop to rest while others continue foraging. The stability of bonded social groups, in contrast, arises from individuals’ concerns not to become separated from their principal social partners. This means they constantly monitor social partners and continually reset their activity priorities so as to ensure that everyone’s schedules remain synchronised. Even so, bonded social groups can become dispersed and fragment when day journeys are long and/or group size is large.
Not only are anthropoid primates among the most intensely social of all mammals, they also have the largest stable groups, as well as some of the largest herd-like groupings. This makes primates a particularly useful taxon for understanding how animals manage the stresses of living in large groups. In some cases, group coordination is ensured by behavioural mechanisms explicitly designed to maintain group cohesion (e.g. ‘notifying’ rituals whereby a group agrees on the foraging route they will take during the day). In addition, social grooming is thought to play a key role, not least because some of the most social species devote as much as 20% of their day to it. In both monkeys and humans, the frequency of social interaction directly determines willingness to provide coalitionary support or other forms of altruistic aid, and creates the focus for the social monitoring that ensures that social partners stay together.
Grooming influences bond formation through its effect on the central endorphin system. The hand actions used in grooming, and ‘soft touch’ more generally, activate the highly specialised afferent c-tactile (CT) peripheral nerve system that triggers the release of b-endorphins in the brain. In humans, endorphin uptake has been shown to be explicitly involved in social bonding. This neural system works in tandem with a second cognitive process based on a separate neural pathway to create a dual process mechanism underpinning social bonding.
This second cognitive mechanism is not simply a matter of memory capacity and associative learning, but involves sophisticated forms of meta-cognition such as the capacity to infer abstract rules, analogical reasoning, mentalising and self-control that allow relationships to be managed. These cognitive capacities are unique to the anthropoid primates and depend on brain regions that are only found in this suborder.
If each bonded relationship requires, as seems to be the case, the investment of a minimum amount of time to ensure functionality and the time available for social grooming is ecologically limited, animals will be forced to choose between (a) dividing their limited available social time equally between all group members (at the cost of having increasingly poorly bonded relationships as group size increases) and (b) focussing their social effort on just a few individuals with whom they can maintain optimally bonded relationships (at the cost of sacrificing relationship quality with all other group members). Both strategies will result in unstable groups – the one because all bonds will be too weak to prevent individuals wandering away from the group during foraging, the other because the group will fragment into a set of disconnected subnetworks that are prone to becoming separated. Evidence from primates indicates that groups whose habitats limit the time they have available for social grooming are more likely to fragment during foraging than similar sized groups of the same species that can afford to devote more time to grooming.
There are three possible ways primates might solve this dilemma in order to live in larger groups.
These focus on structural, behavioural and cognitive solutions.
The latter two focus, respectively, on the grooming and cognitive components of the dual process mechanism that underpins social bonding.
The first of these solutions is structural (hypothesis H1). Primate grooming networks naturally partition into two layers: grooming cliques (in network terminology, their degree, defined by the number of grooming partners an individual has) and grooming chains (n-cliques, defined as the set of individuals linked together by a chain of such ties, even if they do not individually groom each other). When individuals cannot afford the time to groom every member of their group to the requisite level, these extended chains of virtual relationships might provide the basis for maintaining spatial cohesion during travel when groups get large. Each individual simply has to monitor its principal grooming partners, and a ‘friends-of-a-friend’ effect will keep the entire group together as an extended chain. In effect, grooming patterns form zones of declining gravitational attraction that radiate outwards around each individual to include some or all of the group, creating a form of gravitational drag that holds the set of animals together when their individual personal networks are mapped on top of each other. By reducing the number of fracture lines within the network across which no grooming (or social monitoring) takes place, the likelihood that individuals, or even subsets of individuals, will drift off on their own during foraging is reduced.
H1 [hypothesis 1] predicts that the size of these extended grooming chains will increase proportionately to group size, even if the number of individuals groomed remains constant; more importantly, grooming chains should encompass most (if not all) of the adults in a group so as to hold them in a tight structural web. If they do not, it implies that other mechanisms must play a role in bridging the gaps between the subnetworks.
The second possibility (hypothesis H2) is behavioural. The most pressing issue for group-living mammals is the need to mitigate the stresses incurred from living in close physical proximity with many other individuals. This is best done by devoting disproportionately more grooming time to core allies so as to ensure that these will always be nearby and willing to support each other in any conflicts that arise. In this way, the benefits of living in a group are retained but the costs are minimised, thereby tilting the cost/benefit ratio in favour of remaining in the group. It may not prevent fragmentation happening altogether, since other factors such as activity desynchrony may still result in groups dispersing when they become very large or travel very long distances. Nonetheless, it may be enough to make the problem manageable, thereby deferring the point at which groups naturally fragment. H2 thus focusses on the ‘strong ties’ at the centre of each individual’s social network. It predicts that animals will increase the amount of grooming directed to their core grooming partners in proportion to increasing group size in order to reinforce their key alliances, but that the size of both grooming cliques (degree) and grooming chains size will be unrelated to group size (or even reduce in size).
The third possibility is cognitive (hypothesis H3). Animals may be able to mitigate the stresses of group-living by using more sophisticated cognitive mechanisms that allow them to predict and manage others’ behaviour, especially those with whom their do not normally groom. This includes fine-tuning when and how to respond to others’ threats or spatial incursions, or knowing whom to avoid conflict with because they have more and/or higher rank allies (third party knowledge). This mechanism focusses not on the behavioural mechanisms involved in dyadic social interaction (the focus of hypothesis H2) but on the cognitive mechanisms that allow individuals to manage “weak ties” in the periphery of the network so as to defuse conflict and minimise the risk causing the group to fragment following escalated conflicts. These kinds of high order cognitive skills are associated with executive function and include causal reasoning, analogical reasoning, one trial learning, self-control (inhibition, the ability to defer reward) and mentalising (the capacity to understand others’ intentions), all of which are correlated (both within and between species) with the volume of the brain’s default mode neural network (and hence with brain size).
H3 [hypothesis 3] predicts that species in larger groups will score higher on these cognitive indices than those that live in smaller groups, but not necessarily groom with more individuals or devote more time to grooming.
These are not mutually exclusive alternatives. Rather, they are additive in the sense that they reflect ways of managing the stresses of group-living that differ in their level of cognitive demand. As such, they might represent progressively more sophisticated solutions to successive glass ceilings so as to allow progressively larger groups to be formed in a stepwise manner.
Discussion
… both hypotheses H2 and H3 are supported, this suggests that this is achieved by behavioural (bonding) and cognitive mechanisms, probably being introduced stepwise during the course of evolution. Genera of the upper grade disproportionately increase the amount of time devoted to grooming their core social partners as group size increases, providing they can effect change in diet to free of sufficient time to do so. This provides an effective buffer against the stresses created by the proximity of many other individuals, not least by the simple device of causing others to maintain their distance rather than risk being attacked by several individuals. Additional support for this is given by the fact that, in primates, rates of conflict within female dyads are negatively correlated with species group size …
… Grooming is the basis of coalition-formation in primates, and it is conspicuous that formal coalitions are universal in all the upper grade genera but rare (if not completely absent) among lower grade genera. The capacity to form coalitions is what makes it possible for the upper grade genera to defer the stress-induced infertility effects so as to live in larger groups.
However, while coalitions solve the stress problem, they don’t solve the associated coordination problem. Something else is needed to counteract the natural fragmentation process so that groups remain cohesive over time despite pressures favouring dispersal. That something seems to involve the capacity to make rapid inferences about high level rules (one-trial learning), understanding relationships beyond the confines of one’s own immediate grooming circle and what amount to the skills of diplomacy (knowing when to escalate a conflict and when not). These involve the ability to make rapid judgments about the meaning of signals and the nature of third party relationships so as to avoid escalating conflicts unnecessarily.
Understanding third party behaviour has been reported for many of the upper grade genera, but convincing evidence for such behaviour has not so far been reported for any of the lower grade genera. Similarly, many upper grade genera are able to evaluate the status of another individual simultaneously on two or more separate dimensions (e.g. kinship versus rank), whom another individual has alliances with, and, on the basis of observed reputation, how trustworthy they might be. Again, such competences have not, so far, been reported for any of the lower grade genera.
Reconciliation (repairing relationships destabilised by conflict), a behaviour that depends on the ability to recognise that a relationship has been weakened (thus implying some minimal capacity to mentalise), has been widely reported from upper grade genera but rarely (and usually with mixed results and only in the form of physical proximity without involving conciliatory signals or active grooming) in lower grade genera.
The finding that cognitive abilities and relative neocortex size seem to play a central role in managing relationships is reinforced by evidence from neuroimaging studies. In both monkeys and humans, the size of personal social networks correlates with the volume of the brain’s default mode network and its extensions down into the limbic system and the cerebellum. In primates, this very large connectome, with its massive white matter connections, takes up a substantial proportion of the non-visual cortex, contributing significantly to overall brain size. Moreover, it exhibits significant enlargement in anthropoid primates compared to prosimians.
In sum, it seems that the ecological need to live in large, stable social groups has necessitated finding ways to mitigate the escalating effects of stress so as to reduce the risk of groups fragmenting. This seems to have required a combination of investing more heavily both in additional grooming time for enhanced bonding of core alliances to mitigate the direct costs of stress and in social cognitive skills that are dependent on specialised neural circuits in the brain for managing relationships beyond this inner circle.
This distinction between close-bonded and peripheral relationships within a network is reminiscent of [the] concept of weak and strong ties in human social networks. These findings also provide a prima facie case for the claim that primate species that live in large social groups exploit more sophisticated forms of cognition, and hence why they should have evolved much larger brains than other orders. Nonetheless, although primates are, as a group, the most social of all the mammals, the same issues apply broadly to all social mammals. Understanding how other taxa that live in bonded social groups (equids, tylopods, elephants, delphinids, sciurids) solve these same coordination problems would add measurably to our understanding of the processes of social evolution.
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The Source of today’s exhibit has been:
Robin Dunbar, Structural and Cognitive Solutions to Prevent Group Fragmentation in Group-Living Species, Posted on bioRxiv, December 13, 2022
https://doi.org/10.1101/2022.12.13.520310
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