Females would also indirectly benefit from a reduced takeover probability, as this would reduce the risk of infanticide.
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Increasing the number of coresiding OMUs appears to be functionally comparable to adding more males to a single group. Indeed, primate groups with more males have been shown to be less vulnerable to incursions by nonresident males Crockett and Janson ; Janson and van Schaik ; Newton ; cf. Borries and Koenig Rubenstein and Hack showed that as the chance of being harassed by bachelor males increases, stallions exhibit a higher propensity to associate with other stallions to thwart these extra group hazards in plains zebras in multilevel societies.
On the other hand, Fischhoff et al. That bachelors pose a significant threat to OMU leaders and females in modular colobines is supported by several lines of evidence: 1 AMUs of substantial size are more or less permanently on the outskirts of the reproductive group in most multilevel settings Grueter et al. Researchers have only rarely reported observations that can be interpreted as collective or collaborative aggression of unit males against intruders in modular colobines Grueter and van Schaik , but some circumstantial evidence has been presented Zhao and Li Krzton observed coordinated patrolling behavior among males of different breeding units in wild Rhinopithecus roxellana , vaguely reminiscent of chimpanzee boundary patrols Wilson and Wrangham In conclusion, males in modular societies may balance the costs and benefits of associating with potential allies and competitors, and the benefit of enhanced safety when associating with conspecifics may outweigh the costs and have prompted the formation of bands.
Even though the OMU males have to defend their females against takeovers from bachelor males, OMUs do not form a higher social level.
It could be that the local ecological conditions semidesert do not permit modularity in this case. However, data concerning genetic relationships among females in modular colobine societies are yet to be collected. Interunit ties may be further strengthened by infant handling involving females of different units Zhang et al. Modularity in papionins seems to have evolved from ancestral mm—mf groups of considerable size see earlier.
We outline two scenarios, the ecological and the social model, of how large groups might have become substructured. This model has been generally applied to hamadryas baboons. The alternative, the social model, which we favor here, posits that it was mainly social factors that triggered substructuring, but that ecology was a necessary precondition. An alternative to the ecological model is the time constraint model, and an alternative to the social model is the social brain model, both of which provide other possible explanations for substructuring in papionins.
It is generally thought that a combination of ecological and social factors promoted the emergence of modularity in papionins Bergman ; Dunbar ; Henzi and Barrett , ; Kummer , ; Swedell and Plummer ; Swedell and Saunders After having colonized a marginally productive semidesert habitat the habitat in which hamadryas baboons are thought to have evolved , ancestral mm—mf groups began to dissolve into smaller, widely separated foraging units in response to sparse dispersed food resources occurring in small patches and necessitating small group sizes dispersed resource hypothesis or subgroup hypothesis.
Single males could then monopolize these small groups of females and benefit from associating with these units on a long-term basis so as to be able to track female reproductive condition reliably and thereby ensure their access to receptive mates. Selection would have favored aggressive male herding and the emergence of exclusive OMUs to exclude reproductive competitors other males and prevent females from mating promiscuously mate guarding hypothesis, sensu Anderson OMU leaders were then guaranteed reproductive success and high paternity certainty owing to vigorous monopolization.
Females also profited from this social, spatial, and sexual constellation because they and their infants received protection from the male against dangers such as infanticide bodyguard hypothesis, sensu Smuts ; Wrangham a.
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Ironically, exclusive monopolization of subgroups of females would have led to an increased risk of infanticide by extragroup males, thereby strengthening the need for male defense of females Grueter and Zinner ; Henzi and Barrett ; Swedell and Plummer ; Swedell and Saunders At the same time there was a need for the small units to congregate regularly at certain scarce resources, such as safe sleeping sites and water places localized resource hypothesis. The distribution of localized resources explains the conglomeration aspect of the hamadryas system.
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A similar social feature to the male—female bonds in hamadryas also exists in savanna baboons, i. Recent tests, however, using data from hybrid baboons, failed to support the hypothesis that male—female friendships were the precursors to hamadryas male—female bonds. Although the ecological model has some explanatory power, several problems remain.
The model does not explain the gelada system, unless we assume that ancestral geladas were forced to split into independent OMUs by food constraints, for which there is no supportive evidence. Moreover, several baboon populations in southern Africa live in similarly marginal desert habitats to hamadryas baboons but do not show a hamadryas-like modular social organization Cowlishaw ; Jolly It is therefore obvious that the ecological model alone is not sufficient to explain the evolution of modularity in baboons.
According to the time constraint hypothesis Dunbar b , the maximum tolerable group size of savanna baboons in different habitats depends on environmental factors such as the rainfall pattern. If group size becomes too large for a particular habitat, groups exhibit signs of ecological stress, i. Intragroup competition and the concomitant need for coalitionary support among females increases, while the time a female can allocate to service the relationships on which this support is based is reduced.
When a female cohort grows too large, it is no longer possible for females to sustain alliances, causing substructuring of the group and finally group fission. Could a similar reasoning account for the formation of a multilevel social system in geladas and hamadryas baboons?
Whatever the ecological reason for the formation of large aggregations in these two species, e. However, geladas and hamadryas baboons seem to need to form large groups on a daily or permanent basis, and these large groups are not a result of an increase in population size, as in savanna baboons. Thus permanent fission does not seem an option for hamadryas baboons and geladas.
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In such small units females can maintain strong social relationships to ensure coalitionary support in case it is needed. An extended time constraint model would not explain the internal fission of the hamadryas system. In hamadryas baboons, contest competition for food seems to play a minor role and hence there is no need for coalitionary support among females and female—female bonds are weak Kummer ; Sigg ; Swedell The time constraint model also does not seem to fit geladas very well.
Gelada females maintain strong social bonds within small units and exhibit coalitionary support Dunbar , b , and it has been argued that they compete to gain access to small feeding sites that cannot accommodate many individuals, and that female—female competition intensifies when gelada OMUs coalesce into larger aggregations on open grasslands Barton However, given that their primary resource is abundantly available in their habitat Iwamoto ; Nguyen and Fashing, unpubl , female coalitionary bonds may instead be aimed at raising their rank in the hierarchy Dunbar ; cf.
Le Roux et al. The following hypothetical scenario can be envisaged for hamadryas baboons Grueter and Zinner ; Zinner et al. Large and relatively cohesive social groupings appear for ecological reasons, e. The resulting risk of not finding suitable, unrelated males in the natal group may then have promoted the development of very large social groups. In any case, these large social assemblages bear a cost in terms of enhanced aggression potential and begin to partition for social reasons.
The danger of being harassed or attacked by an unfamiliar individual might be greater than in a normal sized mm—mf group of savanna baboons. In particular, the presence of a large number of unfamiliar males may pose a threat of sexual coercion especially when females are in estrus and possibly infanticide for females. In large groups with a large number of unfamiliar individuals, social stress may reduce fecundity in females. In such a situation, other females may not be effective coalition partners and a female—female social network would not represent the best solution to this problem.
Female—female relationships may be weakened, and females fare better when they join a guarding male bodyguard and mate guarding hypothesis , thereby reducing coercion by unfamiliar group members, especially by males. This would trigger a shift from female bonding to cross-sex bonding sensu Byrne et al.
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An initially or ancestrally mixed mm—mf group becomes divided into modules OMUs with reproductive control for the male. The weakly bonded female network makes it easier for males to establish autonomous units Barton Ultimately, it is the demographic change, i.
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Above a certain size, baboon groups are apt to become substructured. The taxa Theropithecus gelada , Papio hamadryas , and most likely also P.
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The points indicate different groups. Data are from Swedell Dunbar , , envisaged a similar model for the evolution of present day gelada society out of a Papio -like mm—mf group.
However, contrary to the hamadryas pattern, in this model it was female bonds that were generated as a means to reduce stress in large gelada groups. Large social groupings or aggregations on a permanent or regular basis grasslands , which are determined by various ecological needs, such as predation avoidance or optimal habitat use and foraging localized resources and predation , may result in an insecure social environment. It is then conceivable that large groups split into OMUs because females started to form clusters with closely related females within a larger group, and later a male attached himself to them.
Dunbar argues that increased aggression and reduced reproductive output associated with growing group size resulted in females tending to bond together into small matrilineal groups for mutual protection coalitions to buffer themselves against the stresses and harassment imposed on them by living in large groups.
The alternative, that higher levels of sociality in geladas evolved via a merger of OMUs cannot be discounted, but does not seem to fit well with the phylogenetic relationships. This difference reflects the different allocation of social effort within these societies. However, what we need to explain is why strong male—female bonds have not become standard in geladas and why gelada females usually do not select a male as a coalition partner in the face of stress and harassment.
It may be that in the beginning the small number of hamadryas females clustering around a male were relatives, but that at a later stage in the evolution of the hamadryas system, males integrated unrelated females into these clusters, breaking up the female network within the unit Swedell and Plummer It is also worthy of mention that the gelada system is not exclusively founded on female bonds; some females also form close bonds with individual males Dunbar c.
Nevertheless, the disparate expression of social effort in hamadryas baboons and geladas remains a conundrum requiring further intellectual and empirical scrutiny. It has also been proposed that neocortical size may constrain group size in primates social brain hypothesis because it determines the ability to process complex information about social relationships Dunbar a , ; cf.
Large groups of several hundred members may therefore only persist on the condition that smaller closed subunits are formed in the case of hamadryas baboons and geladas OMUs in which information transfer is still feasible Fischer and Zinner Recent experimental evidence in a wild population of geladas supports this hypothesis. Bergman found that OMU leader males failed to recognize other males around them vocally, even males with which they associated nearly every day.
In sum, we give preferentiality to the social model for the evolution of multilevel societies in hamadryas baboons and possibly geladas, i. Ecology provides a logical explanation for the fission—fusion nature of the papionin social system. More predators cause more band cohesion in hamadryas Kummer et al.
However, the observed fission—fusion dynamic of a hamadryas band in Filoha, Ethiopia was not as pronounced as expected given the seasonal changes in resource availability Schreier and Swedell in press suggesting that other factors may also play a role. In geladas, a grass carpet on plateau tops in combination with greater exposure to predators causes units to congregate while the restricted availability of grass on the cliff faces causes the higher groupings to drift apart Dunbar Moreover, the presence of AMUs and the threats they cause have been shown to bring breeding individuals closer together Pappano et al.
Once a modular structure has evolved, multiple bonds among males seem to be the framework that holds a hamadryas band together Kummer , A lack of female transfer within bands and a lack of clan-based male bonds are likely reasons why gelada bands are not maintained as coherently as hamadryas bands.
The only closer spatial and social cohesion is found among gelada units that form teams of two units. Related females, although found in two different OMUs after a fission, would hold teams together by associating. When our definitions are applied to humans, they combine modularity with fission—fusion Aureli et al. The overwhelming majority of human societies are multilevel multifamily groups.
A multifamily group is composed of a number of OMUs, monogamous or polygynous polyandry is rare , interacting with each other on a regular basis. Modular communities of monogamous pairs, or mostly monogamous pairs, are a uniquely human phenomenon Chapais a. Another difference from other modular primate societies is that foraging units in humans are not necessarily OMUs. Human foraging parties are commonly sex-segregated owing to the division of labor, a fundamental and unique feature of human social organization. In other multilevel primate societies foraging units are OMUs and members of one sex do not cooperate in subsistence activities.
Multifamily groups always combine to form more inclusive social entities, which in turn combine to form still more inclusive entities. For example, in hunter-gatherer societies, families form bands ranging in size from 35 to 80 individuals, whose members cooperate in subsistence activities Gurven ; Hamilton et al. Human societies are thus both federated and nested, and the number of nested levels of organization is in principle unlimited when we consider the whole range of human societies Chapais b.
Like other primates, humans practice outbreeding through dispersal. However, humans have flexible residence patterns compared to other primates patrilocality, matrilocality, bilocality, and so on , with many societies exhibiting more than one pattern simultaneously. Thus although dispersal is often sex-biased, it may be bisexual, an uncommon feature in other primates Chapais Patrilocality is the majority pattern in human societies as a whole Murdoch In hunter-gatherer societies, however, bisexual dispersal is frequent Alvarez ; Hill et al.
Male bonding or fraternal interest groups are features of many human societies Rodseth and Novak ; Tiger , but females are also often gregarious and frequently form alliances with other females Rodseth and Novak ; cf. Wrangham b , or offer alloparental assistance Burkart et al.