Social Cooperation
Courtesy of Thomas Geissmann; Gibbon
Research Lab
Cooperation
With such small families and minimal intergroup interaction, most cooperation is between mates. Sub-adults contribute occasionally under crowded conditions when migration to a new territory is limited.
Grooming
Grooming consists of examination and
plucking at the hair or skin. This can be done with the fingernails for scratching
and
picking, or the lips or teeth. Usually, the hair is parted by the back of the
hand for closer examination of the skin. This can be done to oneself or after
presenting to another, allogrooming (grooming another) can take place (Baldwin
et al., 1976).
Allogrooming
Mate grooming occurs frequently and contributes
to
bond development and strengthening. Males usually groom females much more often
than females groom males. In fact, females rarely initiate grooming sessions.
They
will however, show signs of wanting to be groomed, coercing a male to initiate.
Males usually initiate grooming by presenting themselves to their mate (attraction).
Many times females reject the male. If she accepts she presents herself back
to the male. Extra attempts by the male help show the female the time and energy
willing to be invested for possible partnership. Sometimes
both
male
and
female
will
groom
each
other
simultaneously (Palombit, 1996).
Sub-Adult Cooperation
As aforementioned, smaller habitats
and larger population densities can cause increased aggression. This occurs
because of the increased importance
of territory and increased encroachment within them. When fewer territories
are available, larger groups and more competition for territories
becomes common. Sub-adults don't have anywhere to go and sometimes are allowed
to remain within their family if they contribute to the family in some
way. This can range from helping in foraging, defense, or alloparenting.
This cooperation helps
every member of the family. The father receives help in defending the territory,
minimizing the risk of being overtaken by another
male. The mother receives help in raising her young. Sub-adults receive resources
and protection. The young benefit from extra care and social play.
Social play among young and juveniles includes rapid brachiation or chasing,
diving and grasping of limbs, wrestling or mocking. These actions are described
as very acrobatic and significant in their development of locomotor skills
and coordination. This helps develop faster more dangerous offspring that
can contribute to defense upon maturation (Brockelman
et al., 1998).
If males are forced out of their family in crowded territories, they often attack other males and try to replace them. No infanticide is observed after aggressive male replacement in unknown families. By treating unrelated young as their own, they gain defensive help, minimizing their chances of replacement and therefore increase time to mate and raise blood offspring (Brockelman et al., 1998).
Rare Kin Selection
When sub-adult males seek their own territory,
they occasionally replace their father. Even though this would create inbreeding
with their mother, benefits overcome the costs. Normally, the father being replaced
is older and weaker, nearing death. Usually the mother would be in the same shape.
While inbreeding might occur for one or two cycles, the mother would soon die
and another mate would be found. The energy used in assuming this territory is
much less than that for locating and securing another one. Occasionally observed,
fathers will voluntarily let his male sub-adult take over his position. This kin
selection allows his offspring to mate, passing on half of his genes
and
increasing his overall inclusive fitness (Chivers et
al., 1980).
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