Collective Decision-Making by Nonhumans: Cockroaches, Spiders, and Snub-nosed Monkeys
Social analysis of nonhumans is developing along a range of trajectories, and collective decision-making is one of the most generative. At stake are questions of group dynamics among animals and how powerfully they shape individual behaviors and unify conspecifics as a social unit. Here I’ll discuss a sampling of this line of inquiry using three recent studies: (1), Personality variation improves collective decision-making in cockroaches, (Planas-Sitjà); (2) Decision-making process during collective movement initiation in golden snub-nosed monkeys, (Wang et al); (3) Physical and social cues shape nest-site preference and prey capture behavior in social spiders, (Najm et al). They each showcase a range of interesting methods and analytical approaches, but taken together they also highlight the contours by which decision-making in nonhumans can be comprehended and further examined.
The first point is how pervasive this cultural dynamic is in the wild. As Planas-Sitjà notes: “Group living animals perform daily collective decisions, such as where to go or rest, and need to reach some form of agreement in order to keep the cohesion of the group.” His 2020 study, building on earlier research, models this phenomenon by analyzing how a collection of cockroaches choose one shelter over another. His subjects, domiciliary cockroaches, “are model organisms for studying the relationship between individual and collective behaviour, as they are a gregarious species yet spend part of their life alone. Thus, the behaviour of isolated individuals can be assessed with ecological realism and without inducing stress.”
Planas-Sitjà’s approach fits within a Durkheimian understanding of the social as modifying individual behavior but expands it to argue that “personality variation amplifies the effects of social interaction.” The social operates through an “aggregation process” that improves the “accuracy” of group decision-making—collectively, they choose the better quality shelter in an experimental setting: “In the case of cockroaches, this improvement is the result of a retention effect inside the shelter, leading to an increase in the population within the shelter because of the social inter-attraction and positive feedback between individuals.” But this group choice is shaped by the personalities of individual cockroaches, who display greater or lesser tendencies to visit a shelter. How these personalities combine (as “personality variation”) with social interactions opens onto an ongoing debate about self-organizing dynamics across a range of taxa.
In the case of golden snub-nosed monkeys, Wang et al consider the coordination and synchronization of daily activities. In contrast to choices about where to rest, they focus on group decisions about when and where to move. This is a matter of broad interest regarding eusocial insects, birds, and mammals, which can be divided into two types of groups: large ones (flocks of birds or schools fish) that result from “nearest neighbor effects” and smaller collectives (lions, wolves, and primates) in which several individuals communicate directly in reaching a decision to move. Wang et al focus on the latter; in these situations, “a successful collective movement requires several steps” that unfold across “a pre-departure period” during which individuals “display their motivations” about whether, when, or where to move. This is not a Durkheimian view of individuals being modified by the social but rather a question of how a “quorum threshold” is crossed—a more nebulous matter that suggests a protracted liminal state before a decision can be reached. But Wang et al too see greater accuracy—opting for better foraging settings—resulting from collective decision-making.
There are a host of questions here around leadership and how it’s constituted and the role of followers, which they find differentiated by sex and age categories: single males and females with infants are equally successful in initiating direction decisions, for instance. Thus, in the case of “partial leadership” commonly observed among mammals, they find collective movement is “governed by voting behavior and quorum fulfillment,” which combines two decision processes (timing and direction) “in a continuous procedure.” This involves acts of mimetism regarding pre-departure and departure behavior by which “votes” are indicated and a quorum achieved. In more general terms, Wang et al conclude, “Although a quorum is considered to be a simple and efficient way to achieve collective decisions, a threshold number for a positive decision outcome may vary substantially, and may be changed seasonally or [due to] resource abundance.” The ability to quantify the requirements for reaching a quorum is significant, but it leaves open the more nebulous matter of what occurs before this threshold is crossed as well as the duration of this period. Social interactions involve a great deal of both communicative and performative work, and this needs to be brought into view if the collective aspect of this decision making can be fully comprehended.
Najm et al focus on such “conspecific cueing” when social spiders (Stegodyphus dumicola) select a nesting site, with a particular attention to conflicts that arise during the decision-making process. S. dumicola (Araneae, Eresidae) live in colonies of tens to hundreds of females that cooperate in collective foraging, reproduction, and parental care (Wright et al. 2015). These colonies build 3-dimensional clusters of webs to form a nest, and 2-dimensioal webs for capturing prey. While the colony typically reside in a single retreat, groups can build multiple nests connected by capture webs. But, as Najm et al note, “Societies frequently contend with conflicting information, and groups can resolve this conflict in differing ways.” The conflict experimentally posed for the S. dumicola was a choice between suboptimal nest scaffolds. Would their decision turn on the presence of conspecifics (group sensibilities) or the relative qualities of the nests (physical structures)? The result: “Individuals strongly preferred to be with conspecifics, suggesting that they use conspecific cueing, which is common in social animals.” So this interpretive process of reading conspecific behavior should be identifiable across a range of taxa.
Thinking back to the Durkheimian matter of individual versus society, he would not be surprised at the finding that “reaching a consensus decision of where to settle improved prey capture success of groups,” or that “collective decision-making is more effective than an individual’s choices when negotiating tradeoffs.” Indeed, the resolution of conflict in favor of the collective was a hallmark of Durkheim’s thinking, and that social power is evident here: “While conflicts between individual choice and group choice can occur, we found that democratic decision-making repeatedly emerged in S. dumicola as group fragments always joined the majority over time.” But of course, at a cost; in the case of S. dumicola, the aversion to dispersal results in the “reduced genetic diversity characteristic of most social spiders.”
These studies demonstrate that the contours of collective decision-making can be analyzed and ascertained in experimental settings. Which leads to a larger set of questions concerning the social dynamics generating such instances of collective behavior. As well as, how do these forms of decision-making play a role in reproducing social organization within a species? Are decisions reached uniformly or do they vary by the modality of choice—whether to rest, move, or hunt?
Recommended Citation
Hartigan, John. “Collective Decision-Making by Nonhumans: Cockroaches, Spiders, and Snub-nosed Monkeys.” August 25, 2020. Commentary to Social Theory for Nonhumans. Manifold ed. Minneapolis: University of Minnesota Press, forthcoming. https://doi.org/10.5749/9781452958446.