Fission-fusion Social System Research Articles

Factors underlying party size differences between chimpanzees and bonobos: a review and hypotheses for future study

Differences in party size and cohesiveness among females have been primary topics in socio-ecological comparisons of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). This paper aims to review previous studies that attempted to explain these differences and propose some hypotheses to be tested in future studies. Comparisons of recent data show that relative party size (expressed as a percentage of total group size) is significantly larger for bonobos than chimpanzees. Although the prolonged estrus of females, close association between mother and adult sons, female social relationships including unique homosexual behavior, and high female social status might be related to the increased party size and female cohesiveness of bonobos, these social and behavioral factors alone do not appear to explain the differences between the two species. Differences in ecological factors, including fruit-patch size, density of terrestrial herbs, and the availability of scattered foods that animals forage as they travel between large fruit patches could also contribute to the differences between chimpanzees and bonobos. However, these factors cannot fully account for the increased party size and female cohesiveness of bonobos. The higher female cohesiveness in bonobos may be explained by socio-ecological systems that reduce the cost in feeding efficiency incurred by attending mixed-sex parties. These systems may include female initiatives for party ranging movements as well as the factors mentioned above. Because of their geographical isolation, the two species probably evolved different social systems. Chimpanzees, whose habitats became very dry during some periods in the Pleistocene, likely evolved more flexible fission-fusion social systems to cope with seasonal and annual variation in food availability. On the other hand, bonobos had a large refugia forest in the middle of their range even during the driest periods in the Pleistocene. Therefore bonobos, whose habitats had more abundant food and smaller variation in food availability, probably evolved systems that help females stay in mixed parties without incurring large costs from contest and scramble competition.

Rapid vocal convergence in an Australian cockatoo, the galah Eolophus roseicapillus

There are several mechanisms for producing vocalizations that are similar to those of a conspecific, including song type matching during a vocal contest, group convergence on a signature call over time, or one-to-one matching of another individual's signature call. A novel category of conspecific matching has been described in a parrot, the orange-fronted conure, Aratinga canicularis, which shows rapid and directional modification of vocalizations over the course of a single vocal interaction. We examined whether a geographically and phylogenetically distant parrot, the galah, Eolophus roseicapillus, uses similar rapid call modifications during contact call exchanges, and sought to better define the conditions under which wild parrots directionally modify their contact calls. In each trial, we interactively broadcast a single call exemplar recorded from wild galahs and recorded the vocal responses of nearby galahs. Temporal parameters of response calls converged on the stimulus over the course of an interaction. Frequency measures also showed convergence, although this effect was less pronounced. Male galahs were more vocally responsive overall, and birds tended to converge more closely on male stimuli over the course of an interaction. This experiment confirms that other parrots, in this case galahs, are capable of rapid, directional call modification, and we suggest that this vocal technique may be an efficient mechanism for mediating the fission–fusion social system that is characteristic of many parrot species.

Structured association patterns and their energetic benefits in female eastern grey kangaroos, Macropus giganteus

Little is known about the determinants of females' social interactions in species with fission–fusion social organization, in which individuals associate with different individuals at different times. Even less is known about social organization in marsupials. We investigated the association patterns of wild female eastern grey kangaroos in southeast Queensland. Female kangaroos had a nonrandom social structure and we present a new methodology for analysing social structure that accounts for the amount of spatial overlap shared by pairs of female kangaroos. This simulation model showed that patterns of association among our female kangaroos were not simply those predicted by their patterns of spatial overlap. Females that associated frequently with certain associates were able to graze for longer because they were less vigilant, and thus were able to benefit from having frequent associates. Female kangaroo social structure was affected by the loss of key members of the society during a period of unusually high predation. Information-theoretical modelling showed that the factors affecting grazing behaviour changed after partial population turnover, with the strength of association between nearest neighbours having a positive effect on grazing behaviour before population turnover and group size having a positive effect on grazing behaviour after population turnover. We propose that social organization within this marsupial is more structured than previously thought.

The functions of the “Greeting Ceremony” among male mantled howlers (Alouatta palliata) on Agaltepec Island, Mexico

Nonhuman primates use greeting behaviors as nonaggressive communicatory signals in multiple social contexts. Adult male mantled howlers (Alouatta palliata) perform a ritual greeting that has been associated with bond-strengthening functions. The aim of this study is to explore the greeting patterns of male howlers living on Agaltepec Island, Mexico. Specifically, we analyzed the relationships between greetings and several individual, relational, and contextual variables, such as the expression of affiliation and agonism, dominance rank, age, kinship relationships, spatial organization, activity patterns, and subgrouping patterns. Greetings were more frequent between males with closer dominance ranks. Among those dyads that greeted at least once, dominant males initiated greetings more frequently than less-dominant males. On the other hand, more greetings were observed when one of the participants had recently returned to a subgroup and during locomotion. On the basis of these results, we propose that on Agaltepec greetings are a conflict management mechanism used between males of similar ranks. The fission-fusion social system of this group of howlers allows males with conflicting interests to remain separated, and greetings may reduce tension during fusion events.

Towing the party line: territoriality, risky boundaries and male group size in spider monkey fission–fusion societies

Spider monkey communities are classic fission-fusion primate societies. I present data suggesting that spider monkeys (Ateles chamek) at Lago Caiman are territorial; adult males traveled further and faster than adult females and subgroup size was significantly higher in boundary areas of the spider monkey territory where intercommunity disputes were observed than in non-boundary areas. I then go on to examine data from 20 Ateles communities distributed across 14 study sites and five species to investigate how a series of demographic, ecological and geographical parameters influence the number of males in a given spider monkey community. Analyses suggest that the number of males is not significantly related to the number of females in a community, the area of the community home range, or the total perimeter length of the community boundary. However, risky boundary perimeter length, or the length of perimeter that directly borders another spider monkey community, explains 88% of observed variations in the number of males in each community. I discuss the results in relation to spider monkey ecology and territoriality, as well as the potential of this relationship for explaining chimpanzee (Pan) behavior given their extremely similar fruit specialist, male philopatry, territoriality and fission-fusion social system.

Seasonal Changes in Male Associative Behavior and Subgrouping of Alouatta palliata on an Island

Howler groups are usually spatially cohesive and stable in composition; however, more flexible grouping patterns occur in some social groups. We analyzed the associative and subgrouping patterns of males living in a group with fission-fusion social organization. Based on information from previous studies on Alouatta palliata and other primates and in the current socioecological models, we established initial predictions on the variations in male behavior according to several socioecological factors. We studied associative behavior via scan sampling at 15-min intervals to register the identity of males in the subgroups and the presence and number of receptive females. We calculated an association index that was then transformed into a measure of association strength. We found individual association trends, as well as important seasonal differences in the subgrouping patterns of males. During the dry season the presence of many receptive females resulted in reduced levels of association, and therefore fewer males per subgroup. The scenario changed during the wet season, when males grouped together significantly more and kinship relationships were a major determinant for individual association preferences.

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.

The socioecology of elephants: analysis of the processes creating multitiered social structures

In this paper, we investigate the formation and function of the multilevelled, fission–fusion social structure in a free-ranging African elephant, Loxodonta africana, population. We quantitatively identified the existence of four social tiers by using cluster analysis on individual association data. We assessed the effects of season and study period on social structuring and levels of cohesion within and among social units. We found that second-tier units, potentially the equivalent of the ‘family’, were stable across seasonal periods but the number of units increased as the study progressed and the population grew. It appears that these units were sufficiently small not to be influenced by ecologically related factors, such as resource competition, that might otherwise lead to them splitting. On the other hand, third- and fourthtier units were significantly affected by season in a way that suggests a trade-off between ecological costs (e.g. from resource competition) and different social and ecological benefits (e.g. from predator defence, territoriality, knowledge sharing and rearing of young). Age structure also appeared to influence this multitiered social organization. The size of second-tier social units was significantly affected by the age of matriarchs: units lead by matriarchs likely to be grandmothers (i.e. females 35 years and older) were significantly larger than those lead by younger matriarchs. We present a conceptual framework for understanding the emergence of multiple-tier social structure from interactions driven by socioecological processes. This study is the first to use rigorous quantitative methods to statistically show the existence of four hierarchical tiers of social organization in a nonhuman animal. Additionally, our results elucidate the role that ecological processes play in producing complex social structures.

Violent coalitionary attacks and intraspecific killing in wild white-faced capuchin monkeys ( Cebus capucinus).

During 12 years of observation, we have observed three confirmed and two inferred lethal coalitionary attacks on adult male white-faced capuchins ( Cebus capucinus) by members of two habituated social groups at Lomas Barbudal Biological Reserve, Costa Rica. In one case, an alpha male was badly wounded and evicted from his group, and when later found by his former groupmates he was attacked by several of them and died less than 24 h later. In two other cases, lone extra-group males were mobbed by adult and immature males of a bisexual group. One victim's abdomen was torn open and he died less than 24 h later. A second victim was quite badly bitten but may have escaped. The fourth and fifth cases resulted from intergroup encounters. One victim lost the use of both arms but may have survived, whereas the other died of unknown causes within an hour of the attack. The observed death rate from coalitionary aggression at our site is approximately the same as that reported for eastern chimpanzees. Because at least three of the five observed incidents involved large coalitions attacking lone victims, they support the general hypothesis that imbalances of power contribute to intraspecific killing in primates. However, the occurrence of lethal coalitional attacks in a species lacking fission-fusion social organization poses a challenge to the more specific version of the imbalance-of-power hypothesis proposed by Manson and Wrangham in 1991 to explain chimpanzee and human intergroup aggression.

Chimpanzees cope with temporary reduction of escape opportunities

Many studies have focused on the responses of primates to changes in population density, but little is known about the responses to more subtle changes in the physical environment. Studies on primate responses to high density supported the use of various coping strategies depending on the circumstances. We investigated which strategy was adopted by a large group of captive chimpanzees in response to a special case of space restriction: the temporary reduction of escape opportunities. Due to work on the outdoor enclosure, the chimpanzees at Chester Zoological Gardens were confined to their indoor quarters for 1 mo. As it was winter, the chimpanzees spent most of their time indoors also during the 7-week control period, but had the option to go outside. Therefore, there was no major difference in the actual density between the two conditions, but by being confined indoors the chimpanzees had reduced escape opportunities. Under this condition they would be expected not to alter their behavior dramatically but to adjust it in ways to avoid conflict and to reduce tension. The chimpanzees did not change their overall aggression rates during the period with a reduction of escape opportunities versus the control period. The patterns of allogrooming and submissive greetings also did not differ between the two periods. However, they appeared to refrain from joining ongoing aggressive interactions during the period with reduced escape opportunities, whereas no change in counteraggression occurred between the two periods. Our findings suggest that the chimpanzees may adopt a selective inhibition strategy when escape opportunities are limited. Further support for such selectivity comes from the analysis of different types of aggressor-target dyads. During the period with reduced escape opportunities, aggression rates decreased in dyads characterized by high aggression rates at baseline, i.e., high aggressor-target dyads, whereas the same individuals did not change their aggressive tendencies in low aggressor-target dyads. Ours is among the few studies showing a decrease, albeit selective, in aggressive behavior during a situation of space restriction. The ability to rapidly adopt an appropriate strategy and to inhibit aggression during such restriction seems to confirm findings of chimpanzees under high density conditions. The inhibition of aggressive tendency may be especially developed in chimpanzees and may be related to the natural challenges posed by the routine changes in subgroup membership characteristic of their fission-fusion social organization.

Food supply and chimpanzee (Pan troglodytes schweinfurthii) party size in the Budongo Forest Reserve, Uganda.

A central issue in socioecology is the nature of the relationship between an organism's environment and its social structure. In chimpanzees, the fission-fusion social system is thought to minimize feeding competition for primary dietary components: ephemeral, dispersed patches of ripe fruit. Intragroup feeding competition is thought to force individuals into small parties. Informal observations in the Sonso region of the Budongo forest had suggested that in this habitat, food supply was such that feeding competition was less important in determining grouping patterns than elsewhere. We used data collected on food supply and party sizes over a 4-year period to investigate this suggestion. In accord with theoretical expectation, sizes of foraging parties fluctuated with the size of food patches. However, party sizes showed either negative or no relationship with habitat-wide measures of food abundance. Likewise party sizes showed little relationship to overall measures of food dispersion. For important dietary items, both fruit and leaves had patchy distributions, though the degree of clumping was not strong, and fruit was not more clumped than leaves. Generally, abundant food appeared to be less patchy, and chimpanzees appeared to use more patches as food became more abundant rather than forming larger parties. We suggest that both dispersal and abundance need to be considered when investigating the impact of food supply on grouping patterns, and that the importance of food as a factor in determining chimpanzee grouping patterns declines with increasing levels of abundance.

BEHAVIORAL DEVELOPMENT IN WILD BOTTLENOSE DOLPHIN NEWBORNS (TURSIOPS SP.)

AbstractNewborn characteristics, patterns of motoric and social behavioural development, and mother-infant relationships in free-ranging and semi-provisioned bottlenose dolphins (Tursiops sp.) are examined. Nine newborns were observed for 189 hours over the first 10 weeks of life. Newborn infants breathe more often than their mothers, and synchronize their breathing and swimming with her soon after birth, but show a gradual decline in synchrony as they age. Virtually all patterns of infant behaviour, mother-infant proximity, and spatial relationships with the mother changed as a function of infant age. Maternal activity, however, does not change over time, except that mothers decrease their role in maintaining proximity to their infants from the first month to the second month of infant life. Infants spend less time close to their mothers, less time echelon swimming (close, alongside the mother), and more time infant-position swimming (in contact under the mother) as they age. Infants spend more time traveling and socializing independently over time. They also separate from their mothers more often and for longer periods of time. Infants do not forage during the newborn period, but are observed 'practice foraging' by the end of the first month. Rubbing, petting, chasing, and displaying with other animals (including the mother) were common forms of socializing. Infants frequently initiate rubbing with their mothers, with particular focus on her head region. Infants typically associate with young females, adult females and other infants, but not with adult or subadult males. Developmental shifts and overall patterns are discussed in the context of the bottlenose dolphin's fission-fusion social organization and ecology. The effects of provisioning on dolphin behaviour are addressed.

ASSOCIATION BY MALE CHIMPANZEES: A SOCIAL TACTIC?

AbstractWhether and with whom to associate are possibly some of the most fundamental of choices faced by a chimpanzee, choices constrained by and dependent upon similar decisions made by potential association partners. Although much work has focused on the impact of ecological factors on the size of the temporary associations, or parties, it seems that chimpanzees, especially the males, form parties to gain social benefits, and that these benefits are dependent the particular composition of these parties. This paper examines the social determinates of the association patterns of male chimpanzees, members of the Sonso community in the Budongo Forest Reserve, western Uganda. Male chimpanzees showed clear preferences for association partners, and these partners had a strong tendency to associate in small parties. Strength of association within dyads accounted for a significant fraction of the variance in party size [all parties containing males: r2 = 0.30; male parties only: r2 = 0.24 (all dyads), r2 = 0 . 50 (dyads with positive associations only)]. These results supported the hypothesis that association was the result of tactical decisions, and refuted predictions of alternate hypotheses: random and passive (independent attraction to the same locations, such as fruiting trees) association. Further support for the tactical association hypothesis was provided by observations that association between males was flexible and that association patterns resolved into alternative association strategies, with some males shifting between strategies over time. The presence of cycling females influenced party size, which increased with the number present. The number of adult males in a party was also greater when cycling females were present, although remained similar whether one, two, three or four such females were in the party. Samples sizes for parties containing more than two cycling females were low, however. It is suggested that, for a Machiavellian primate in fission-fusion social system, the need to make decisions concerning association partners is cognitively demanding and may be an important, frequently used, function of the intelligence demonstrated by captive chimpanzees.

Could asynchrony in activity between the sexes cause intersexual social segregation in ruminants?

In many sexually dimorphic mammal species, the sexes live outside the mating season in separate social groups ('social segregation'). Social segregation occurs in a wide range of environmental conditions, but its cause in unknown. I suggest that social segregation is caused by a lower level of activity synchrony between individuals in mixed-sex groups than in single-sex groups, owing to sex differences in activity rhythm. As a consequence, mixed-sex groups are more likely to break up than single-sex groups, resulting in a predominance of single-sex groups at equilibrium. To test this hypothesis in red deer (Cervus elaphus L.), I developed an index of activity synchronization and showed that deer in mixed-sex groups were significantly less synchronized in their activity than deer in single-sex groups. Thus, low intersexual synchrony in activity can lead to social segregation. However, a lower level of intrasexual (female-female and male-male) activity synchrony within mixed-sex than within single-sex groups implies that additional factors (other than sex differences in foraging rhythm) contribute to the higher degree of instability of mixed-sex groups.

Communicative and other cognitive characteristics of bottlenose dolphins

Scientists have tried to capture the rich cognitive life of dolphins through field and laboratory studies of their brain anatomy, social lives, communication and perceptual abilities. Encopheliration quotient data sugest a level of intelligence or cognitive processing in the large-brained dolphin that is closer to the human range than are our nearest primate relatives. Field studies indicate a fission-fusion type of social structure, showing social complexity rivaling that found in chimpanzee societies. Notably, cetaceans are the only mammals other than humans that clearly demonstrate vocal learning and parallels in stages of vocal learning have been reported for humans, birds and dolphins. The dolphin's vocal plasticity from infancy through adulthood, in what is probably an 'open' communication system, is likely to be related to their fission-fusion social structure and, specifically, to the fluidity of their short-term associations. However, conflicting evidence exists on the composition and organization of the dolphins whistle repertoire. In general, the level of dolphin performance on complex auditory learning and memory tasks has been compared with that of primates on similar visual tasks; however, dolphins have also demonstrated sophistrcated visual processing abilities. Laboratory studies have also provided suggestive, evidence of minor self-recognition in the dolphin, an ability previously thought to be exclusive to humans humans and apes.

Fission-fusion social organization inAteles andPan

Recent research on the ecology and behavior of free-ranging spider monkeys (Ateles paniscus chamek) allows a more detailed comparison with the chimpanzee,Pan troglodytes, than has been possible previously. Despite their distant common ancestry, chimpanzees and spider monkeys share an unusual fission-fusion social system. In this paper, I compare subgroup size and composition, social unit structure, ranging behavior, patterns of philopatry and dispersal, and social relationships in the two taxa. It is proposed that spatial and temporal patchiness in food dispersion and abundance, resulting in a high-level of feeding competition between females within a group, has been the most important ecological selection pressure leading to the evolution of fission-fusion social organization in both species.

Party composition and dynamics inPan paniscus

The pygmy chimpanzee, or bonobo, Pan paniscus,diplays a fission-fusion social organization in which individuals associate in parties that vary in size and composition. Data from a 2-year field study of nonprovisioned P. paniscusshow that party composition varies with party size. Although females, on average, outnumber males, the proportion of males in the party increases in larger parties. This effect was not due to the greater number of known females. Both females and males will join and leave a party in the company of others, but only males appear frequently to join or leave as lone individuals. All-male parties were not observed, but all-female (nonnursery) parties were relatively common. These trends reflect greater cohesion among females than observed in P. troglodytes schweinfurthii.Cohesion between males and female P. paniscusmay increase with party size.

Food Competition and Foraging Party Size in the Black Spider Monkey (Ateles Paniscus Chamek)

AbstractIn Ateles, individuals forage for fruit in subgroups of unstable membership. The size and composition of these foraging parties are highly variable. Data from my long-term field study of a population of black spider monkeys in Peru show a strong positive correlation between the average size of these parties and the number and size of trees currently producing fruit in the environment. This correlation appears to be driven by intragroup feeding competition, since individuals in larger parties showed substantially lowered feeding efficiencies, as measured by increased daily ranges and decreased time spent feeding. Patch size and patch density, which jointly determine fruit abundance, interact to determine foraging party size, apparently by the following mechanism. The number of spider monkeys that can feed within a particular patch is limited by the size of that patch. When patches are close together (i. e. high patch density) individuals in excess of this limit are able to feed in nearby patches and thus maintain contact with the other members of their foraging party. When patch density is low, the increased travel costs associated with such a strategy become prohibitive and foraging parties are smaller than or equal to the number of individuals able to feed within a single patch. Females that are routinely excluded from the best feeding sites may have reduced reproductive success because of their low priority of access to resources. These data provide a quantitative link between two ecological variables (resource patch size and density) and fission-fusion social organization in the spider monkey.

Demography, ranging patterns, and activity budgets of black spider monkeys (Ateles paniscus chamek) in the Manu National Park, Peru.

Results of a long-term field study on the ecology and social organization of two groups of black spider monkeys (Ateles paniscus chamek) in Peru are reported. Demography, ranging patterns, and activity budgets provide insight into some of the proximate determinants of fission-fusion social organization in this species and illustrate the different strategies used by males and females to gain access to critical resources. Longitudinal data on known individuals provide evidence for male natal philopatry and female emigration at sexual maturity in this population. Interbirth intervals are long (mean = 34.5 months) in comparison with most other primate species, and 5 of 15 infants seen within a few days of birth died or disappeared before they were a year old. Home ranges are large (150-250 ha) and fairly discrete; overlap with neighboring groups is on the order of 10-15%. Males and females differed substantially in their ranging patterns; females, particularly those with infants, restricted much of their ranging to a "core area" 20-33% the size of the total group range, whereas males ranged more evenly over the entire area occupied by the group. Daily path length varied over almost an order of magnitude from 465 m to 4,070 m, with a mean of 1977 m. Males spent more time traveling and less time feeding than most females. These results are compared with those obtained in previous studies of Ateles and with similar data from other primate species to assess their implications for the evolution of fission-fusion sociality in spider monkeys. The ecological factors responsible for the evolution of very similar social organizations in spider monkeys and chimpanzees are discussed.