Amanda H. Korstjens

Fission-Fusion Dynamics: New Research Frameworks

Renewed interest in fission‐fusion dynamics is due to the recognition that such dynamics may create unique challenges for social interaction and distinctive selective pressures acting on underlying communicative and cognitive abilities. New frameworks for integrating current knowledge on fission‐fusion dynamics emerge from a fundamental rethinking of the term “fission‐fusion” away from its current general use as a label for a particular modal type of social system (i.e., “fission‐fusion societies”). Specifically, because the degree of spatial and temporal cohesion of group members varies both within and across taxa, any social system can be described in terms of the extent to which it expresses fission‐fusion dynamics. This perspective has implications for socioecology, communication, cognitive demands, and human social evolution.

Group size, grooming and social cohesion in primates

Most primates live in social groups in which affiliative bonds exist between individuals. Because these bonds need to be maintained through social interactions (grooming in most primates), sociality will be limited by time constraints. It has previously been shown that the time primates invest in grooming increases with group size. However, when groups become too large, individuals will not have enough time available to service all possible social relationships and group cohesion is expected to decrease. In this study, we used data from previously published studies to determine how large groups compromise on their grooming time and how ecological, phylogenetic and life history variables affect time invested in grooming (across species as well as within taxa). We used path analysis to analyse direct and indirect (via group size) effects on grooming. We showed that not only is grooming time determined by group size, but it is also affected by dispersal patterns and sex ratio. Furthermore, we found that grooming time is asymptotic when group size exceeds 40 individuals, indicating that time constraints resulting from ecological pressure force individuals to compromise on their grooming time. This was true across species, but a similar effect was also found within taxa. Cognitive constraints and predation pressure strongly affect group sizes and thereby have an indirect effect on primate grooming time. Primates that were found to live in groups larger than predicted by their neocortex size usually suffered from greater predation risk. However, most populations in our analysis were placed well within what we define as their eco-cognitive niche.

Time as an ecological constraint

Conventional approaches to population biology emphasise the roles of climatic conditions, nutrient flow and predation as constraints on population dynamics. We argue here that this focus has obscured the role of time as a crucial constraint on species’ abilities to survive in some habitats. Time constraints may be particularly intrusive both for species that live in intensely bonded groups (where the need to devote time to social interaction may ultimately limit the size of group that a species can maintain in a particular habitat) and for taxa that face constraints on the length of the active day. We use a linear programming approach that allows us to specify both how time allocations to different activities are influenced by local environmental and climatic variables and how these in turn limit group size and population density. The linear programming approach identifies the realizable niche space within which a species can maintain coherent groups that are larger than the minimum viable group size (or density). This approach thus allow us to understand better why a given taxon can survive in some habitats but not others, as well as the demographic stress that a population may face. In addition, they also allow us to evaluate the implications of both past and future climate change for a taxon’s ability to cope with particular habitats.

Fission-fusion social systems as a strategy for coping with ecological constraints: A primate case

Fission–fusion social systems, in which members of a social community form frequently changing subgroups, occur in a number of mammalian taxa. Such systems are assumed to be a response to the costs of grouping, but evidence to support this hypothesis is limited. We use a linear programming approach to build a time budget model that predicts the upper bound on group size in order to test the hypothesis that fission–fusion social systems are the outcome of time constraints. Comparative data from 14 wild chimpanzee (Pan spp.) populations are used to derive a set of equations defining the relationship between climatic variables and time budget components, which are then used to calculate the upper limits on group size that can be maintained in different habitats. We validate the model by showing that it correctly predicts the presence/absence of chimpanzees across sub-Saharan Africa and the group sizes observed in natural populations. The model suggests that the costs of travel are limiting for chimpanzees. Chimpanzees can reduce these costs dramatically by fissioning their bonded communities into small foraging parties. If they did not, they would be unable to live in any habitats where they currently occur.

Intragroup aggression, fission–fusion dynamics and feeding competition in spider monkeys

Scramble competition is related to animals depleting resources before others can use them, whereas contest competition is associated with the monopolization of resources and food-related aggression. One hypothesized major benefit of fission–fusion sociality is the reduction of scramble feeding competition between community members. We studied intragroup aggression in a community of spider monkeys. Despite seasonal variation in food availability, we found no seasonal difference in aggression rates, which, in combination with a basic lack of aggression among adult females, suggests that fission–fusion dynamics may reduce not only scramble, but also the intensity of contest competition. There were, however, numerous attacks from adult females towards subadult females, especially new immigrants. This aggression, although it occurred mainly during feeding, may not reflect contest competition. Instead, it may be used by resident females to limit scramble competition at the community level, which is ultimately affected by the number of community members. The aggressive harassment of subadult females by long-term resident females can be a means to encourage dispersal of natal subadult females and discourage immigration of new subadult females who are both potential future resource competitors.

Time as a constraint on group size in spider monkeys

An animal can only survive in a given habitat if it has enough time to find, process and digest food whilst avoiding predation. The time it has for food acquisition is affected by the vegetation and competition with conspecifics, which depends on aggregation tendencies. We used the relationships between time allocations, on the one hand, and climatic variables (as a proxy for habitat quality) and group size, on the other, to develop a model that predicts maximum ecologically tolerable group size at different locations for spider monkeys. Spider monkeys are particularly interesting because the social communities often split up into small units. Temperature variation and rainfall variation were the main determinants of time budgets. Community size and average annual rainfall determined party size. The model correctly predicted presence or absence of spider monkeys at 78–83% of 217 New World forest sites. Within the geographical range of the species, this time budget model predicted the presence of spider monkeys better than a model based directly on climate variables. Predicted community and party sizes were significantly larger at sites where spider monkeys are present than at sites where they are absent. As required by the model, predicted maximum community sizes were significantly larger than observed community sizes. Moving time showed a U-shaped relationship with party size, which suggests that moving time is the factor that keeps spider monkey communities from travelling together in a tight group.

HOW ADAPTIVE OR PHYLOGENETICALLY INERT IS PRIMATE SOCIAL BEHAVIOUR? A TEST WITH TWO SYMPATRIC COLOBINES

[Socio-ecological theories predict that females adapt their social behaviour to their environment. On the other hand, as a result of phylogenetic inertia, social behaviour may be slow to catch up when the environment changes. If social behaviour is adapted to the environment, competition and co-operation among females is predicted to reflect the characteristics of food sources. Contest competition both between and within groups is expected to result in alliances among related, philopatric, females. We compared social relationships and food characteristics of two sympatric and congeneric primate species, the red colobus and the black-and-white colobus of the Tai National Park, Ivory Coast. We found that affiliative interactions among females were comparable between the species. The differences in food characteristics could explain why black-and-white females competed more often than did red colobus females, both at the intra- and inter-group level. In contrast to socio-ecological theory, female inter-group aggression was not linked to female philopatry in black-and-white colobus. The species differed from each other and from other populations of the same or closely related species with respect to their inter-group behaviour which indicates that phylogenetic inertia did not constrain this aspect of social behaviour., Socio-ecological theories predict that females adapt their social behaviour to their environment. On the other hand, as a result of phylogenetic inertia, social behaviour may be slow to catch up when the environment changes. If social behaviour is adapted to the environment, competition and co-operation among females is predicted to reflect the characteristics of food sources. Contest competition both between and within groups is expected to result in alliances among related, philopatric, females. We compared social relationships and food characteristics of two sympatric and congeneric primate species, the red colobus and the black-and-white colobus of the Tai National Park, Ivory Coast. We found that affiliative interactions among females were comparable between the species. The differences in food characteristics could explain why black-and-white females competed more often than did red colobus females, both at the intra- and inter-group level. In contrast to socio-ecological theory, female inter-group aggression was not linked to female philopatry in black-and-white colobus. The species differed from each other and from other populations of the same or closely related species with respect to their inter-group behaviour which indicates that phylogenetic inertia did not constrain this aspect of social behaviour.]

Fissioning minimizes ranging costs in spider monkeys: a multiple-level approach

The adjustment to deal with intragroup food competition is probably the most plausible explanation of high levels of fission–fusion dynamics. However, studies did not always support expected relations between food availability, ranging costs, and subgroup size. We used several levels of analysis differing in the time and spatial scale in order to investigate this explanation in spider monkeys. In our study, subgroups were larger when food availability was higher across most levels of analyses used. We also found a fine-scale adjustment: compared to the food patch previously visited, spider monkeys traveled to larger patches just after fusions. This was not without an immediate travel cost: the interpatch distance and travel time after a fusion were longer than that before the fusion. This rapid adjustment shows the flexibility that fission–fusion dynamics can offer. Spider monkeys are in large subgroups only when food conditions are favorable, as evidenced by the fact that at all the other time-scale levels larger subgroups did not experience greater ranging costs than smaller subgroups. Our results indicate that on the whole spider monkeys successfully minimize ranging costs by fission and fusion of subgroups.

Time Constraints Limit Group Sizes and Distribution in Red and Black-and-White Colobus

Researchers have shown that, in frugivorous primates, a major constraint on group size is intra group feeding competition. The relationship is less obvious in folivorous primates. We investigated whether colobine group sizes are constrained by time limitations as a result of their low energy diet and ruminant-like digestive system. We used climate as an easy to obtain proxy for the productivity of a habitat. Using the relationships between climate, group size, and time budget components for populations of Colobus and Piliocolobus at different research sites, we created 2 taxon-specific models. In both genera, feeding time increased with group size or biomass. The models for Colobus and Piliocolobus correctly predicted the presence or absence of the genera at, respectively, 86% of 148 and 84% of 156 African primate sites. Median predicted group sizes where the respective genera were present are 19 for Colobus and 53 for Piliocolobus. We show that the differences between the 2 genera are due mainly to intrinsic differences in the way each taxon’s digestive physiology interacts with climatic variables to influence resting time requirements. The models may help us explore their responses to climatic change in both the past and the future.

Dispersal Patterns Among Olive Colobus in Taï National Park

In Primates, females are more likely to be philopatric than males. However, in some species like Procolobus verus, females or individuals of both sexes disperse. In Taï National Park, Ivory Coast, olive colobus groups are small, with one or two adult males and ≤6 females. Dispersal is common for juveniles and adults of both sexes. Adult male dispersal is less common than adult female dispersal. Adult females immigrated especially into small, one-male groups indicating that food competition played a role. Furthermore, unknown sexually receptive females visited resident groups and mated with the resident males for a few days before disappearing again. Adult males dispersed when this improved their mating opportunities. All juveniles left their natal groups. The dispersal of juveniles may be a strategy to prevent inbreeding with their parents. Dispersal by juvenile males furthermore seemed to be the result of mate competition. The high dispersal rates, visits by receptive females, and dispersal of all individuals in the population suggest that moving between groups is a strategy that can be used ad hoc in several situations more easily in the olive colobus than in most other primates. The predation risks related to moving between groups were reduced by dispersing in conspecific or allospecific groups and by dispersing to neighboring groups.