NEW FINDINGS AND MODELS OF GIBBON SOCIAL ORGANIZATION, AND PARALLELS WITH
Warren Y. Brockelman, Institute of Science and Technology for Research and Development
and Faculty of Science, Mahidol University, Salaya, Putthamonthon, Nakhon Pathom 73170,
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
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
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?
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