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NEW FINDINGS AND MODELS OF GIBBON SOCIAL ORGANIZATION, AND PARALLELS WITH HUMANS

Warren Y. Brockelman, Institute of Science and Technology for Research and Development and Faculty of Science, Mahidol University, Salaya, Putthamonthon, Nakhon Pathom 73170, Thailand


     The gibbons, or Lesser Apes, are the most distant relatives of humans among the apes, hence it is unclear what we might learn about the evolution of human social structure by studying them. In addition, many anthropologists would argue that human social structure is so highly variable and culturally controlled, unlike the more rigidly programmed social structure of gibbons, that we cannot even discuss its evolution. But human social structure can be viewed as a complex set of adaptive responses to a highly variable environment, and as such we can compare human social structure with that of other animals as a set of solutions to similar ecological and social problems. Surprisingly, the monogamous family unit in gibbons shows striking simiarities with the family unit seen in most human societies. Gibbons establish long term pair bonds for mating and rearing of offspring, parent-offspring relations extend into maturity of the offspring, and family units are not always nuclear families. In addition, gibbons have territories (analogous to private property) and a neighborhood structure. Neighboring gibbon groups vary in their genetic relationships to each other and social relations vary accordingly.
     Until the 1990's, the gibbons (Family Hylobatidae) were regarded as adhering invaryingly to a simple pattern of social organization consisting of monogamous, territorial, nuclear families. As such, their social system appeared to be rather rigid and in many ways a poor model for human evolution. Our view of gibbon society is now changing rapidly, for two main reasons: long term data from at least two sites which are starting to contradict the traditional view, and a somewhat belated application of neodarwinian natural selection theory toward explaining gibbon social structure and its evolution. Theories about gibbon social evolution are being set forth which can only be tested with more refined data than have been collected in the past. Recent empirical findings about dispersal, pair formation, and group stability indicate that gibbon group structure is more variable than has been noted in the past, and that delayed dispersal with subadults remaining in their family groups and helping their siblings and parents is common. Gibbons are also found to have a dual mating strategy consisting of investment in a long term mate and territory, and extra-pair copulation, as has been found in "monogamous" birds.
     In order to understand their similarities with humans, it is important to understand why gibbons are monogamous. The key to monogamy is how the two sexes contribute toward the raising of offspring together. If the male's investment in feeding, carrying, or otherwise caring for offspring is not essential to their survival, he should, according to darwinian logic, desert the family and search for another female that is sexually receptive. Male gibbons neither feed nor carry the offspring, which has made it difficult for social theorists to justify the male gibbon's fidelity to his mate. Moreover, some recent theories consider the male's territorial defense as a form of mate guarding or protection against potential infanticide from other males, rather than a form of resource defense. This merely weakens the justification for monogamy, because mate guarding is not a form of investment in offspring, and infanticide has never been seen in gibbons. It does not weaken the selective reasons for desertion when the female is not sexually receptive (and she is not for more than 2 years out of the minimum 3-year birth interval).
     We still lack a clear understanding of the benefits of territoriality to each sex in gibbons, and its relation to food resources. Empirical evidence suggests that territorial defense in male gibbons is not the same as mate defense, because when a male chases other males on the territorial border, he often leaves his mate vulnerable to other males. In one instance, such a territorial encounter with a subadult on the territorial border provided an opportunity for an extra-pair copulation by the neighboring adult male.
     A model of intrasexual competition proposed for primates by Wrangham (1979), based on earlier ideas of Trivers (1972) on parental investment and sexual selection, argues that, in many polygynous species, females should compete for resources and males should compete for mating opportunities with females. Recent theorists have inappropriately put this model to use in explaining the monogamous gibbon social system, arguing that male territoriality represents mate defense and not resource defense. The idea that males do not need to defend resources or are not limited by food resources is an erroneous overextension of the model and is explicitly contradicted by Trivers (1972). Resource limitation and mate limitation are two entirely different types of limitation that have little to do with one another, the former being dependent on the operational sex ratio and the latter on population density. Moreover, if female primates are limited by resource shortage, it is not possible that males are not also limited if they have the same dietary requirements.
     Thus, it is likely that male territorial defense represents a kind of paternal investment in the female and her offspring, and also represents a compromise strategy with mate defense.
     Observations on foraging and diet in gibbons in the Khao Yai Park study site in Thailand also suggest that the smaller ranges enabled by territoriality in both sexes permit a highly efficient exploitation of food resources in which detailed knowledge of food locations allows gibbons to outcompete all other potential competitors.
In summary, gibbons provide a useful model of human family dynamics in several ways. Future research on gibbons may contribute to our understanding of the biological family (e.g. Emlen, 1995) in a number of important general areas:
1. The economics of foraging and food limitation, and the value of territorial behavior as a recource exploitation strategy in each sex. The diet of gibbons is complex and the fruits, leaves, etc. consumed vary from month to month.
2. Pair bonding behavior. What is the value of the pair bond to each sex, and what is the role of grooming, duetting, reproductive behavior, and other behaviors in bonding.
3. Dispersal and neighborhood structure. What is the relationship of individuals in neighboring groups, and for how long are relations between parents and dispersed offspring maintined? What is the effect on inbreeding and population structure?


References:

Brockelman, W. Y., U. Reichard, U. Treesucon and J. J. Raemaekers. 1998. Dispersal, pair formation and social structure in gibbons (Hylobates lar). Behav. Ecol. Sociobiol. 42: 329-339.
Clutton-Brock, T. H. and P. H. Harvey. 1976. Evolutionary rules and primate societies. Pages 195-237 in P. P. G. Bateson and R. A. Hind, eds. Growing Points in Ethology. Cambridge University Press, Cambridge.
Emlen, S. T. 1995 An evolutionary theory of the family. Proc. Nat. Acad. Sci. USA 92: 8092-8099.
Palombit, R. 1994. Dynamic pair bonds in hylobatids: implications regarding monogamous social systems. Behaviour 128: 65-101.
Reichard, U. 1995. Extra-pair copulations in a monogamous gibbon (Hylobates lar). Ethology 100: 92-112.
Reichard, U. and V. Sommer. 1997. Group encounters in wild gibbons (Hylobates lar): agonism, affiliation, and the concept of infanticide. Behaviour 134: 1135-1174.
Trivers, R. L. 1972. Parental investment and sexual selection. Pages 136-179 in B. Campbell, ed. Sexual Selection and the Descent of Man. Aldine Press, Chicago.
Wrangham, R. W. 1979. On the evolution of ape social systems. Social Science Information 18(3): 335-368.
Wrangham, R. W. 1987. Evolution of social structure. Pages 282-298 in B. B. Smuts, D. L. Cheney, R. M. Seyfarth, R. W. Wrangham and T. T. Struhsaker, eds. Primate Societies. Univ. Chicago Press, Chicago.



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