Biomedical Engineering Reference
In-Depth Information
them over long distances and supporting them when they
sleep. Although rhesus males may occasionally show some
of these patterns, their paternal role consists largely of
aggressive protection of distressed infants. Differences in
roles can be described in terms of frequency and direc-
tionality of particular behavior patterns, and social orga-
nizations can be thought of as the patterns of interactions
among roles.
phylogenetic constraints, these limitations shape the social
structures seen in different species.
Criteria for a Social Group
Although there is no definitive change in social organiza-
tion as one moves between prosimians and apes, various
authors have suggested that a nonhuman primate group
should meet the following five criteria to be considered
a society (
Eisenberg, 1966
). First, the group must show
some form of temporal stability. Second, it should show
some degree of spatial cohesion. The scales used to
measure these first two criteria should be species relative.
Chimpanzees typically express what has been termed
a fission
Evolution of Sociality
Sociality in nonhuman primates may have been selected
genetically based on several adaptive characteristics
(
Kleiman, 1977
). The first is a lower susceptibility to
predation. There is a statistical advantage to belonging to
a group if a predator is attacking; the odds that a particular
individual will be killed decrease as its group size
increases. Although living as a “clumped resource” may
make it easier for a predator to locate the group, it also
makes it harder for the predator to detect and isolate an
individual as prey. A group of animals may also be better
able to detect and defend against predators than an indi-
vidual can.
Improved acquisition of food is a second advantage.
Foraging activities that involve several cooperating indi-
viduals are more successful than solitary hunting is. This
trend is particularly evident when food appears in rich but
widely scattered clumps, such as fruit on a tree or insect
swarms. Since nonhuman primates are alert to the comings
and goings of their group members, individuals that spread
out in search of food but notify others upon the discovery of
food confer a definite advantage to social living. Such
notifications are common in laboratories, where food is
usually clumped and abundant.
Social living also allows nonhuman primates to share
resources that are difficult to find but locally abundant, and
this advantage extends beyond resources like food and
water. For example, safe and suitable sleeping sites for
hamadryas baboons are very limited. The ability to tolerate
other conspecific individuals at close range facilitates the
sharing of resources and aids in individual survival
(
Kummer, 1968
).
At the same time that the potential advantages of social
living promote group formation, environmental and other
factors limit the size of social groups. Resource distribution
may limit the size of animal groups. Feeding parties may be
limited by the number of animals that can feed on a clum-
ped food source. Large groups of animals are easier for
predators to track, and cryptic behavior is only as
successful as the least skillful group member. Disease
transmission can also limit the size of social groups
(
Freeland, 1976; Nunn et al., 2004
). These costs interact
with the advantages listed earlier to place constraints on the
size and composition of social groups. Taken together with
fusion style of social structure (
van Lawick-
Goodall, 1973; Hiraiwa-Hasegawa et al., 1984; Nishida,
1989
). For example, subgroups of animals that frequently
change members move about in a large, clearly defined area
and only exchange members with specific other subgroups
all living in the same area. Under these criteria, this group
can only be considered a stable social organization if the
time and space scales are much larger than those used for
other nonhuman primate species. For some galagos and
lorises (Lorisidae), the spatial pattern is even harder to
define. Although these species are rarely seen together,
evidence shows that they should be considered social
animals because they preferentially associate with partic-
ular individuals when they are social.
This evidence leads to a third criterion: members of
a social group must treat group members and nonmembers
differently, which represents a type of group member
recognition. In the laboratory, this behavior typically
manifests itself when animals are introduced into a stable
social unit or even into a room of individually housed
animals (
Southwick et al., 1976; Bernstein et al., 1983;
Williams and Abee, 1988
).
The fourth criterion pertains to the interactions among
the animals. A social group should have some form of
communication system that results in coordinated activity.
The fifth criterion is that the group should demonstrate
some form of division of labor, cooperation, or joint action
in the environment. This cooperation might be joint defense
against predators, a joint exploitation of resources, or
a division of labor, as seen in the complex differential
response patterns shown by individuals fulfilling role
functions. Ultimately, not every individual will show all of
the behavior patterns typical in a group, but all of these
behavior patterns will be seen in every group.
Most definitions of a society emphasize the communi-
cation network established within the group.
Altmann
(1965)
suggested that social groups could be recognized by
the high frequency of communication among group
members surrounded by a boundary of less frequent
communication with non-group members. The animals
e
