Fission-fusion Behavior Research Articles

Long-term roosting data reveal a unimodular social network in large fission-fusion society of the colony-living Natterer’s bat (Myotis nattereri)

In many social animals, groups recurrently split into subgroups that regularly re-merge. Such fission-fusion behavior allows individuals to better balance the cost and benefits of group living. However, maintaining a large number of close social links in groups with fission-fusion dynamics may be difficult. It has been suggested that this is the reason why in several species, large groups show more subunits (higher modularity) than do small ones. Many bat species exhibit fission-fusion dynamics in their colonies. This makes them well suited to investigate the proposed link between group size, stability of social links, and group modularity. We studied the daily roosting associations of a Natterer’s bat colony (Myotis nattereri), where up to 80 members carried individual RFID-tags. Based on more than 10,000 individual recordings, we analyzed the influence of relatedness, age, sex, and breeding status on the colony’s social network structure during three breeding seasons. We found an almost fully connected social network with very low modularity and generally weak pairwise associations. Nevertheless, the relative strengths of associations between individuals remained stable across years. Sex, age, and breeding status significantly influenced the strength of an individual’s associations and determined the influence of individuals in the network. In general, associations between bats that were similar in all abovementioned traits were stronger than those between dissimilar individuals. Our results show that despite high fission-fusion dynamics, large colony sizes, and low modularity of their social network, Natterer’s bats were able to maintain stable long-term associations. For a variety of social and ecological reasons, large social groups often consist of several communities with stronger individual bonds within and weaker individual bonds between such social subunits. Unlike that predicted for its relatively large size, the studied Natterer’s bat colony that consisted of up to 80 individually marked bats was not subdivided into communities. Despite the fully connected network, the individual associations were not random. Instead, their strength was mainly driven by relatedness and similarity in age and breeding status of the colony members. Moreover, we found stable long-term pairwise relationships between individuals across several years. Our study shows that despite the strong fission-fusion behavior and large size of their colony, Natterer’s bats formed a fully connected, unimodal social network.

Social Network Analysis Reveals Potential Fission-Fusion Behavior in a Shark

Complex social networks and behaviors are difficult to observe for free-living marine species, especially those that move great distances. Using implanted acoustic transceivers to study the inter- and intraspecific interactions of sand tiger sharks Carcharias taurus, we observed group behavior that has historically been associated with higher order mammals. We found evidence strongly suggestive of fission-fusion behavior, or changes in group size and composition of sand tigers, related to five behavioral modes (summering, south migration, community bottleneck, dispersal, north migration). Our study shows sexually dimorphic behavior during migration, in addition to presenting evidence of a potential solitary phase for these typically gregarious sharks. Sand tigers spent up to 95 consecutive and 335 cumulative hours together, with the strongest relationships occurring between males. Species that exhibit fission-fusion group dynamics pose a particularly challenging issue for conservation and management because changes in group size and composition affect population estimates and amplify anthropogenic impacts.

Fission-fusion dynamics in black-and-white ruffed lemurs may facilitate both feeding strategies and communal care of infants in a spatially and temporally variable environment

Species with greater expression of fission-fusion dynamics show greater variability in the size, composition, and cohesion of subgroups over time. This may allow their adjustment to local environmental, social, and demographic conditions. We tested which environmental and social factors influenced subgroup size in one such species, the black-and-white ruffed lemur (Varecia variegata) at two sites within a fragmented forest landscape. Using instantaneous focal sampling at 15-min intervals, we collected data on subgroup size and composition, fruit and flower availability, and location. We also collected biweekly data on site-wide fruit and flower availability. We used these data to test the influence of season, food availability, reproduction, and predation risk on adult subgroup size in this species. Ruffed lemur adults consistently formed significantly larger subgroups in times of lower site-wide fruit availability, during the wet season, and when one or more infants were present. There were differences between sites in the relationship between subgroup size and tree size, fruit patch size, and site-wide flower availability. Variables measuring indirect predation risk showed no significant relationship with subgroup size. These results emphasize the strong relationship between fission-fusion dynamics and spatial and temporal variation in food availability, as well as the importance of sociality in a species with communal care of infants. Fission-fusion dynamics may serve as an evolutionary strategy to adapt to the unpredictable and sometimes harsh environmental conditions of Madagascar and may facilitate the persistence of communities of this critically endangered lemur despite widespread anthropogenic habitat modification. Fission-fusion dynamics (the joining and separating of subgroups within a larger group of animals) may allow species to adjust social benefits to changing environmental conditions. This study examines the underlying ecological and social factors that might influence fission-fusion dynamics in black-and-white ruffed lemurs. Subgroup size in this species was strongly related to food availability, though not always in the direction expected. This suggests that the relationship between food availability and sociality changes when food availability reaches very low levels. Additionally, infant presence was linked to larger adult subgroups, indicating a connection between fission-fusion behavior and the cooperative rearing of offspring seen in this species. Relationships between subgroup size and some environmental variables differed between sites, emphasizing the flexibility of grouping patterns in this species and indicating that local conditions may play a large role in the expression of fission-fusion dynamics.

Spatial cohesion of adult male chimpanzees (Pan troglodytes verus) in Taï National Park, Côte d'Ivoire.

Group living animals can exhibit fission-fusion behavior whereby individuals temporarily separate to reduce the costs of living in large groups. Primates living in groups with fission-fusion dynamics face numerous challenges in maintaining spatial cohesion, especially in environments with limited visibility. Here we investigated the spatial cohesion of adult male chimpanzees (Pan troglodytes verus) living in Taï National Park, Côte d'Ivoire, to better understand the mechanisms by which individuals maintain group cohesion during fission-fusion events. Over a 3-year period, we simultaneously tracked the movements of 2-4 males for 4-12 hr on up to 12 consecutive days using handheld GPS devices that recorded locations at one-minute intervals. Analyses of the male's inter-individual distance (IID) showed that the maximum, median, and mean IID values across all observations were 7.2 km, 73 m, and 483 m, respectively. These males (a) had maximum daily IID values below the limits of auditory communication (<1 km) for 63% of the observation time, (b) remained out of visual range (≥100 m) for 46% of observation time, and (c) remained within auditory range for 70% of the time when they were in different parties. We compared the observed distribution of IIDs with a random distribution obtained from permutations of the individuals' travel paths using Kolmogorov-Smirnov tests. Observation IID values were significantly smaller than those generated by the permutation procedure. We conclude that these male chimpanzees actively maintain cohesion when out of sight, and that auditory communication is one likely mechanism by which they do so. We discuss mechanisms by which chimpanzees may maintain the level of cohesion observed. This study provides a first analysis of spatial group cohesion over large distances in forest chimpanzees using high-resolution tracking, and illustrates the utility of such data for quantifying socio-ecological processes in primate ecology.

Short‐term separation from groups by male Japanese macaques: Costs and benefits in feeding behavior and social interaction

To expand our understanding of fission-fusion behavior and determine its variability among primates, studies of both individual-based and group-based fission-fusion are necessary. We conducted a parallel tracking study of male and female Japanese macaques (Macaca fuscata yakui) during the non-mating season to clarify the general features of separate ranging by males of this species, an example of fission-fusion behavior, and to reveal its associated costs and benefits. Males frequently engaged in short-term separate ranging, leaving the company of females and ranging on their own for periods averaging 68 min in duration. However, the males did not venture outside the group's home range. When ranging separately from the group, males spent more time feeding, particularly on fruit, stayed longer in each feeding tree, and fed at a lower rate than when ranging with the group. These behavioral changes suggest that males can avoid within-group feeding competition by ranging alone. However, this behavior was also associated with higher traveling costs, and these separated males were more vulnerable to intergroup competition and had fewer opportunities for social interaction. The frequency of separate ranging was lower when highly clumped food plant species were the main food source. Lower-ranked males, who received more aggression when ranging with the group, exhibited a higher frequency of separate ranging. This behavioral flexibility with respect to group cohesion may allow males to reduce the costs of group living without completely losing the benefits. Specifically, by ranging alone, males may acquire sufficient feeding time without being disturbed by other group members. Conversely, when ranging with the group, males can access grooming partners and advantages in intergroup competition.

Fission-fusion bat behavior as a strategy for balancing the conflicting needs of maximizing information accuracy and minimizing infection risk

Fission-fusion behavior, which is widely reported in social animals, has been considered as a mechanism for adapting to changing environmental conditions. Although several hypotheses have been proposed to explain the potential benefits of fission-fusion behavior, there are only a few theoretical studies that have systematically explored its mechanism or quantitatively examined the potential forces shaping its evolution. We developed a social learning model to investigate the mechanism and evolutionary forces that underlie a fission-fusion society. In particular, we focused on the day-roost choices of bat individuals because bat societies represent one of the most sophisticated fission-fusion systems. The assumptions of the study were as follows. Each individual selects a single day-roost to use, and forms a roosting group with roost mates. Bats randomly choose a roost to visit in order to inspect its quality. Inspection is not always accurate, i.e., it includes some error. After inspection, bats return to the current day-roost and share the new information with roost mates. Each bat estimates the quality of each potential roost by social learning and chooses which one to use based on the relative value of expected roost quality. The size distribution of sub-colonies is determined by this choice behavior. Three roost-switching behaviors (settlement, synchronized movement, and fission-fusion grouping) were predicted depending on two factors (the level of difficulty of evaluating roost quality and the capacity to remember roost quality information). Settlement behavior, in which most bats remain in the best roost, led to the highest fitness because the accuracy of estimating roost quality was improved when bats exchanged information with members in a large group. However, when disease transmission was combined with learning dynamics, the cost of infection significantly increased under both settlement and synchronized movement behaviors, and eventually fission-fusion behavior led to the highest fitness. These results highlight two conflicting factors: learning in a large group improves information accuracy, but living in a small group effectively reduces the risk of spreading disease. Dynamic change of group size by fission-fusion can resolve the dilemma between these two conflicting factors.

Ethogram and Natural History of Golden-backed Uakaris (Cacajao melanocephalus)

We present an ethogram for golden-backed uakaris (Cacajao melanocephalus), based on observations in the field and on a captive individual. We also provide additional observations on the ecology of the wild animals. We studied 3 free-living groups (maximum counts of 5, 15, and 26 individuals) during two wet-seasons (March–July 2007 and January–June 2008) in the flooded igapo forest of Jau National Park, Amazonas, Brazil. The groups lived in close proximity but never mixed, because river channels separated them. Groups showed fission-fusion behavior, subgroup sizes varied within groups, and we observed 13 different subgroup compositions. The areas used by the groups were ca. 0.82, 2.35, and 2.45 km2. We defined a total of 9 behavioral categories. In the wild, the amount of time allocated to the behaviors traveling and foraging/feeding differed between months, but we found no difference in the amount of time devoted to behavioral categories across 3 periods of the day, possibly as a result of the patchy and unpredictable distribution or availability of food patches. Further, the activity patterns varied among groups, perhaps as a reflection of the different group sizes and compositions and different range sizes. We recorded 34 feeding items for wild golden-backed uakaris between January and June 2008, mostly fruits and seeds. No significant variation in the number of different food types occurred across months. We recorded 6 primate species in the study areas. The uakaris neither mobbed nor fled from any other primate species, with the exception of white-fronted capuchin monkeys (Cebus albifrons). In addition, golden-backed uakaris sometimes fled when seeing giant otters (Pteroneura brasiliensis), perhaps indicating that the otters are potential predators of the monkeys. The ethogram and behavioral data provided here increase knowledge of the behavioral ecology of the elusive golden-backed uakari, and will facilitate future comparative studies.