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The distribution of individuals within a species is related to such factors as the distribution and abundance of resources, predation pressure, and genetically determined Behavioral propensities. In some animal species, individuals spend most of their time alone, whereas in others most or all of their time is spent in groups. With one exception, all diurnal primates live in social groups. In many animal taxa temporary aggregations form under certain conditions (e.g., schools of ﬁsh, ﬂocks of birds, herds of ungulates) but in most of these cases, aggregations are made up of diﬀerent individuals each time the animals congregate. One of the most common features of primate groups is the relative constancy of group membership. There are many theories attempting to explain why diurnal primates live in groups and the reasons for diﬀerent types of groups (see Janson 2000).
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1. Deﬁnition Of Terms
There is a great deal of debate concerning what factors may have led to group living among diurnal primates and the relationships that may exist between primate social structure and ecology. However, before discussing some of these issues, it is necessary to deﬁne some terms.
1.1 The Term ‘Social Group’
If one searches the primate literature for a deﬁnition of the term ‘social group,’ it is quite diﬃcult to ﬁnd. It seems that primatologists know a social group when they see one but are reluctant to deﬁne the term generally. In the literature, there are few attempts to formally deﬁne the primate ‘social group’ (e.g., Altmann 1962, Kummer 1967, Struhsaker 1969, Sussman 1999) and few critical evaluations of these deﬁnitions. Altmann (1962) deﬁnes a group as consisting of conspeciﬁc, intercommunicating individuals bounded by frontiers of far less frequent communication. Kummer (1967) considers that a group as a ‘spatial aggregation of primates that travel and rest together and at the same time avoid the proximity of other such aggregations is, at closer inspection, also a functional and reproductive unit’ (p. 359). These are both broad deﬁnitions that emphasize social proximity, social interaction, and synchrony of activity. However, all of these criteria are not equally useful in deﬁning the social group.
Reproduction was once thought to be a major raison d’etre of primate groups but a number of factors indicate that it is not a major common denominator of the primate group. For example, in many species breeding is seasonal but the group is stable throughout the year. In fact, in some species, Behavior during the breeding season actually disrupts the social group. In the Madagascar sifaka, Propithecus, groups break up during the short mating season and reformulate after breeding takes place. Most species of primates have mechanisms that ensure continuous gene ﬂow between groups. Groups exist within a breeding population (or deme) and genetic exchange is common between groups, at higher or lower frequencies, depending on the species.
Struhsaker (1969) oﬀered a more detailed deﬁnition of the social group. He included in his deﬁnition the following factors: (a) membership in the same social network; (b) the majority (at least 80 percent) of nonaggressive social interactions take place within this social network; (c) membership is temporally stable; (d) group members occupy the same home range; (e) group members have distinct social roles; (f ) progressions between members are synchronized; (g) group members exhibit diﬀerent Behavior toward nongroup conspeciﬁcs; and (h) social groups are relatively closed to new members.
Using some but not all of the above criteria, the following deﬁnition of a ‘social group’ in primates is proposed: a number of individuals who interact socially more frequently among themselves than with other individuals; group members exhibit diﬀerent Behavior towards nongroup members and occupy the same home range. Group membership is relatively stable over time and group members have distinct social roles but, although these two factors are characteristics of the group, the particulars of group stability and of social roles are extremely variable from species to species and are not necessary components of this deﬁnition.
It is also true that most social groups are relatively closed to new membership but this is also highly variable and would be diﬃcult to include in a general deﬁnition. In fact, the only social groups that were thought to be closed to new membership are monogamous pairs, and this does not appear to be as true as was previously believed (Fuentes 1998, Strier 2000); individual exchange between groups is a common occurrence in most diurnal primate species. Synchronized progressions, mentioned in the deﬁnitions of both Kummer and Strushaker, are not common to all primate groups. A number of groups divide into subgroups for various periods of time and these subgroups do not necessarily synchronize movements or activity while separated. Because of the dynamics of individual social relationships, Dunbar (1988, p. 12) preferred to view primate groups or social systems in terms of their constituent relationships. ‘Where the sets of relationships of several individuals coincide and are reciprocated, a formal group emerges as a stable cohesive unit. … These clusterings are suﬃciently distinctive in their cohesion in time and space that we can deﬁne them as levels of groupings.’
There are many diﬀerent types of social group found among the diurnal primates. At a very general level these include: monogamous pairs (i.e., one adult male, one adult female, and their immature oﬀspring); multimale groups (groups containing more than one adult male); one-male groups (groups containing only one adult male); and ﬁssion–fusion groups (groups that form dispersing and coalescing subgroups). Of course, each of these superﬁcial categories does not include groups of identical social structure or social organization. For example, multimale groups of lemurs are quite diﬀerent from those of baboons in structure, function, evolutionary history, and ecological advantage. This is even more obvious when we compare species with ‘ﬁssion–fusion’ group structures.
1.2 The Terms ‘Social Structure’ And ‘Social Organization’
The terms social structure and social organization have now been introduced. These terms are often used interchangeably; however, it is useful to distinguish between them. Following deﬁnitions similar to those proposed by Rowell (1972), the social structure of a species can best be divided into two components: group structure and population structure. Group structure is the size and composition (as to age and sex) of a primate group. Population structure is the demographic character of the population, population density, and the density and dispersal of groups within the breeding population or deme. One measures social structure using censusing techniques. Social organization is the pattern of social interactions that occurs between individuals within and between groups: it is a description of social Behavior.
Thus, social structure refers to the components of the social system, whereas social organization refers to the patterns of interaction between the components. Both patas monkeys and Hamadryas baboons live in one-male groups that can have essentially similar age and sexual composition. However, in Hamadryas onemale groups, the male is the center of attention of the group and directs group travel. In the patas monkey, the male takes a peripheral position in the group and the females initiate and direct group travel. Thus the patas monkey and the Hamadryas baboon have groups with similar structure but the social organization of the two species is very diﬀerent. Furthermore, diﬀerences in degrees of tolerance between adult males result in very diﬀerent distribution patterns of one-male groups. Hamadryas one-male groups often sleep and move together, whereas patas males are intolerant of each other and one-male groups are usually widely dispersed. Although the group structure is similar, the structure of the populations of these two species is radically diﬀerent.
1.3 Spacing Between And Within Groups
When dealing with animals living in groups, two levels of social spacing must be considered: between groups of the same species and between individuals living within the same group. Each species has developed complex communication patterns for both intergroup and intragroup spacing.
These patterns are part of the social organization of a species and they help to maintain the species-speciﬁc social structure. In group-living animals, such as diurnal primates, the terms home range, core area, area of exclusive use, and territory relate to intergroup spacing, whereas dominance hierarchies, roles, kinship lines, and peer groups are usually pertinent to spacing patterns within a group.
In the above deﬁnition of a ‘social group,’ group members were said to occupy the same home range. Home range can be deﬁned as the area over which a group normally travels in pursuit of its routine activities. Other terms such as day range and monthly range are used to denote the area over which the group travels in a given time period. If a group uses a particular part of its home range more frequently, or with greater regularity, than other parts, this zone is referred to as a core area. The core area often contains many of the major resources of the group. In some species of primates, the boundaries of diﬀerent groups overlap only slightly, whereas in others home ranges overlap a great deal and each group may have a very small area that is not shared by other conspeciﬁc groups (referred to as an area of exclusive use). In a number of primate species, home ranges of groups may overlap completely.
Mechanisms used for intergroup spacing also vary between species. In some species, spacing between groups involves the maintenance of exclusive use of a particular area, but in others it seems to involve the maintenance of intragroup integrity and a patterned use, by two or more groups, of the same space. However, a unitary deﬁnition of territoriality and a simple dichotomy between territorial and nonterritorial species might do more to confuse the issue than to aid in an understanding of ecological parameters of spatial relationships between individuals and groups of conspeciﬁcs (see Sussman 1999). For example, active, ritualized, agonistic defense Behaviors are used by many species of primates to maintain the integrity of home range boundaries or areas of exclusive use. However, in other species, spatial integrity is maintained between groups by exchange of vocalizations or with no apparent outward Behavioral spacing mechanisms. In fact, very few species of primates could be considered territorial in the strict sense of the term.
A large number of species that do not maintain exclusive areas employ ritualized postures and gestures, and/or vocalizations, to maintain group integrity and to regulate spatial relationships within a shared home range. Some of these species were thought to be highly territorial until recent studies have shown this not to be the case. Thus, there appears to be two separate phenomena: (a) the Behavioral mechanisms that control spacing and (b) the degree of isolation or overlap between groups. The use of the term territoriality often confounds these two factors and a more clear deﬁnition might be the active defense of individual or group home range boundaries by actual or ritualized agonistic encounters, thereby maintaining essentially exclusive use of the home range. This, then, describes a very speciﬁc set of Behaviors that control spacing and a particular pattern of spacing. However, even using this strict deﬁnition may conceal important variation in conditions of spacing between two species labeled as territorial.
2. Natural Selection And Primate Behavior
An important question relating to group-living among diurnal primates is why, in fact, do they live in groups? What advantage does this habit confer on individuals? The theory of natural selection predicts that evolution will act to increase the frequency of traits that improve an individual’s ability to survive and to reproduce more successfully than other members of its species. Traits that confer survival and reproductive advantages upon an individual are referred to as adaptations. There were, however, a number of characteristics that Darwin found diﬃcult to explain when he developed the theory of natural selection. These included morphological and Behavioral diﬀerences between the sexes that occur in many species, such as larger canines or body size, or greater aggressiveness in males than in females. In many cases it was not obvious how these secondary sexual characteristics gave any direct advantage to individuals in their attempt to obtain resources.
To explain these traits, Darwin formulated the theory of sexual selection. He proposed that many morphological and Behavioral diﬀerences between the sexes evolved because they contributed directly to an individual’s reproductive success, either by improving his or her competitive ability against members of the same sex, or by increasing attractiveness to the opposite sex. Some of the traits Darwin believed to be sexually selected were subsequently shown to proﬀer direct adaptive advantage to individuals. However, sexual selection is not an important mechanism in evolution.
A basic assumption of sexual selection theory is that there is a direct relationship between particular traits and genetics. If dominance in males is directly related to increased ﬁtness, then males must be able to pass on this trait to their oﬀspring. However, dominance is complex and involves a number of morphological and Behavioral characteristics as well as a social and ecological context. This is true of many other traits proposed to be sexually selected. It is often extremely diﬃcult, if not impossible, to explain the genetics of these complex traits.
Another phenomenon scientists found diﬃcult to explain was cooperative Behavior. Why would animals cooperate if this did not increase their evolutionary ﬁtness? If an animal avoids being eaten by a predator by joining forces with others, cooperation easily ﬁts within the rubric of individual selection. However, there are many cooperative Behaviors, especially among group-living animals, that appear to be of no beneﬁt or are even dangerous to the individual. To explain these Behaviors, the concepts of kin selection and reciprocal altruism were formulated.
Darwin was unaware of Mendelian genetics and thus genetic inheritance did not ﬁgure in his explanation of natural selection. Hamilton (1964) examined the potential genetic consequences of cooperative Behavior and developed the theory of kin selection to explain altruistic Behavior between related individuals. An individual’s traits, and the genes responsible for them, are more likely to be shared by close relatives than by unrelated individuals. Thus, by sacriﬁcing oneself for a number of one’s kin, an individual may be ensuring the survival of genetically inherited traits. Since more closely related individuals are more likely to share copies of the same genes, altruism is expected to be selectively directed toward kin, and more close kin will accrue more costly altruism (Silk 1987).
However, many altruistic acts are performed by unrelated individuals. Trivers (1971) formulated the theory of reciprocal altruism to explain cooperation and assistance in these cases. He believed that altruistic acts would only occur between unrelated individuals if they were likely to reciprocate in the future and had the opportunity to do so, if the beneﬁts to the recipient were greater than the costs to the altruist, and if there was little chance of cheating. Thus, both parties improve their individual ﬁtness in the long term. In practice both types of altruistic interactions are often diﬃcult to identify, and predictions derived from these theories are diﬃcult to evaluate empirically (see Silk 1987 for an excellent review).
These concepts formed the basis of the subdiscipline of Sociobiology (Wilson 1975), which was developed to explain the inﬂuences of biology on social Behavior. Sociobiology was very controversial when ﬁrst formulated because of the questionable genetic nature of many of the Behaviors it attempted to explain. Were Behaviors such as aggressiveness or dominance inherited or the result of a complex interaction between genetics and environment? However, many of the theories currently in vogue in socioecology are also
based on sexual selection, kin selection, and reciprocal altruism. These include theories attempting to interpret variation in social structure, social organization, and social Behavior in group-living primates, and the relationships between ecology and social systems. For example, one of the major theories attempting to relate individual spacing to that of resource distribution is based on sexual selection. In most vertebrates, females must invest a great deal of time and energy in successfully reproducing oﬀspring, whereas males invest a small amount. Because of this, the distribution of females in the environment is presumed to be more closely related to resources than is that of males. The distribution of males is related to the dynamics of male competition for females (Wrangham 1987). Thus, females compete for energy resources and males compete for females, and the social system of a primate species can be understood by studying these relationships. In fact, this theory was proposed when it was believed that, in most primates, females remained in the same home range throughout their lives and males dispersed. Now researchers have shown that, in the majority of primate taxa, either females or both sexes disperse (Strier 2000).
To date it still is not possible to relate patterns of resource distribution to patterns of social spacing among primates. Very little is known about the factors that underly the enormous amount of variability in primate social systems. These theories so far have not led to a better understanding of the relationships that might exist between ecology and social structure.
3. Ecology And Social Behavior Among Primates
Many of the theories currently attempting to explain the relationship between ecology and social structure among primates are one-dimensional (e.g., distribution of resources, predation pressure, infanticide avoidance). However, there are probably a multitude of factors interacting to cause the great variety of social groups among these animals.
We would expect that there will be some relationship between the spacing of resources in the environment and the spacing of those individuals using the re- sources. However, to explain the adaptive signiﬁcance of speciﬁc diﬀerences between the types of social structure and organization found among primates, diachronic as well as sychronic factors must be considered.
A complete understanding of the relationship between ecology and social Behavior among primates will most likely have to take into account at least the following factors: (a) the resources and periodicity of these resources within the total geographical range of the species, as well as those in any particular area; (b) the selectivity of the species—those resources chosen by the species vs. those available; (c) the evolutionary history of the species leading to its current phylogenetic (genetic) propensities (these should be related to the present and past geographic distribution of the species and the environments and resources which exist(ed) within these ranges); (d) the history of the particular population and group, since the conditions in a given area may select for population-speciﬁc or group-speciﬁc adaptations. This latter factor should take into account stochastic and demographic phenomena which can have a major inﬂuence on a population at any particular time (Altmann and Altmann 1979, Chan 1992, Rowell 1993).
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