Social Relationships
Yellow Cheeked Crested Gibbons
Courtesy of Thomas Geissmann; Gibbon
Research Lab (Permission Pending)
Gibbons are only one of two monogamous adult groups existing in Asia (none exist in Africa). Both are small arboreal animals (Hylabates and Presbytis potenziani). This (along with other factors) suggests that monogamy may hinders the male's optimum reproductive potential by limiting him to one mate (Fuentes, 2000). This relationship probably stemmed from strict mate guarding. Mate pairs are bonded for life unless they are replaced through aggression or separated by death. If a male dies, the female will take another mate for protection. The female doesn't need protection from other animals as much as she needs a male to defend the territory (since territories are limited and very important).
Female menstrual cycles average 28 days and gestation lasts 7-7.5 months. Conception occurs during the flowering-fruiting peak each year to insure plenty of resources for the female. Weaning of the young can begin as early as 3 months after birth and last up to 1 year. This also takes place during prime flowering seasons to allow maximum nourishment for the mother (Chivers et al., 1980). Copulation only occurs during a period of approximately 5 months per reproductive cycle (which last 2-2.5 yrs). There is only one offspring per reproductive cycle (Ellefson, 1974).
What Kind and Why Monogamy?
Drawbacks of Monogamy
- Inability to fully migrate to find resources (spatial inflexibility).
- Extra non mated breeding males are a hindrance
to group and resource availability and may be eliminated to equal out the sex
ratio (not seen in gibbons).
- Small group sizes make interspecific competition
difficult (Gibbons loose some of the best fruit to other larger species). This
is one reason monogamy
is only seen in specialized habitats.
- Monogamous territoriality becomes a trap. Interspecific
competition may increase intraspecific competition making courtship incredibly
important. This usually
puts non-territory holders and single individuals at a large disadvantage.
Benefits of Monogamy
-
The protection gained from this type of relationship minimizes reproductive
waste.
- Territoriality allows less intraspecific interference
and competition after
boundaries have been established.
- The efficiency of finding resources increases
with longevity in territory.
- Monogamy reduces sexual competition and energy
wasted on non-procreative sex.
(MacKinnon et al., 1984)
So Why Monogamy?
Females need a reason to want just one
male mate. Males do help forage for food. While this is minimal, multi-male care
could provide more help. Defense would be the largest factor.
Males defend the ever important territories more than females. Female defense
is secondary if the male needs assistance.
Males also need a reason for just one female. While
female same-sex aggression occurs, since males primarily defend the territory
(even
from females),
they
do have some opportunities for other female interaction. There are three suggested
reasons for male defense against other females. 1) The male could fool his mate,
chase a female away and then copulate with her.
2) There is a possibility that
bigamy would prove weaker than monogamy or hurt the chances of his current
offsprings' survival. 3) Additional female members could drain resources
(with monomorphism
of size, an adult female eats as much as an adult male- additional females
would increase needed resources by 1/3).
Males require monogamy because of the length
of
parental dependence of the young. The young take 6-9 years to reach
sexual maturity. Female gestation period lasts 7-8 months. Therefore, for a
male to ensure only
one successful offspring to reach adulthood, he must invest almost 10 years
of his life (through courting, mating, birth, to sexual maturity) (Brockelman
et
al., 1984).
Recap: Gibbons display facultative monogamy.
Females do not show complete dependence on males for reproductive success.
Males do help
forage for the young, but it is not completely necessary. Male territorial
defense is important. Polygyny is eliminated through same-sex aggression. Males
benefit
from
monogamy because of the long-term energy needed to ensure even one offspring
reaches sexual
maturity.
Male/Female Relationship and Roles
There is no true dominance between
male and female. However, the female will sometimes show occasional dominance.
She leads the family throughout the forest each day while the father takes
up the rear for possible defense of young. During pregnancy and early post
partem the female has shown distinct control (usually
over
food
sites) (Ellefson, 1974). All accounts of intra-pair aggression (rare threats
or violence) are initiated by the female. Males defend the
territory more than females. While males help care for young, females
spend
considerably
more
time raising
infants. Upon second conception, the male will take larger
care for the first young allowing the female to conserve more energy. Males
have to work much harder to mate with a female. He must present himself many
times
and
spend
large
amounts
of unreciprocated grooming of the female (Allogrooming)
(Palombit, 1996).
Monogamy Maintenance
Gibbons are also known for there long
loud wailings. These song bouts can last up to 20 minutes. Like birds, they
are very common in the mornings and decrease during the day.
While vocalizations used by males to maintain presence and
defense of a territory, between mated
pairs
duets can become very elaborate and intertwined. They serve
to attract, strengthen bonds, and keep mates. Some female species have special
throat sacs to enhance and increase the loudness of these songs. Gibbons also
have sternal and inguinal glands that serve for minor olfactory communication.
As
in most
primates, social grooming (allogrooming) serves
to attract and strengthen bonds between mates as well (Geissmann
B, 2001).
Attraction
There
are three variations of presenting oneself to another gibbon. One way is
demonstrated
by stationary posture and pointing of a
body part to
another. This is an invitation for grooming. Another behavior includes standing
quadrupedally with the hind quarters raised above the shoulders. He then
looks through his legs at the other individual. This is the genital present
and is
an invitation to copulate. The third variation entails rubbing the posterior
at or against the groin, ventral surface or face of another. This was for
reassurance or appeasement. While the first two behaviors are silent with
a neutral face,
this third variation includes a squeal and a grimace. All three demonstrate
social interaction between animals to elicit a response from the other. All
three can usually be followed by either self-grooming or allogrooming (Baldwin
et al., 1976).
Courtesy of the Melbourne
Zoo (Permission Pending)
Parent/Offspring Relationships
Both the mother and father help raise the young until they reach sexual maturity. The females take closer care of the infants while the males take care of the juveniles. Infants cling to the mother's ventral side while sleeping and juveniles sleep in the arms of the father (Geissmann, 2001). This is important since it takes 6-9 years for the young to reach sexual maturity and females only give birth every 2-2.5 years. Each family can have an infant, juvenile, and sub-adult at the same time. While the mother can take sole care of the young, the father's assistance with the juveniles allow the mother to save energy and produce another offspring before the first one reaches maturity (Fuentes, 2000).
Maturing Offspring
Young gibbons
are born with almost completely hairless ventral parts and look to their
mothers for warmth (Ellefson, 1974). Maturation then occurs somewhat ambiguously.
A young gibbon looks like the male but sings like the female. This contradiction
lowers
sexual attractiveness, and doesn't threaten the same sex adult. This
allows immature gibbons to stay in the group longer since they don't appear
as competitors (Geissmann, 2001). Sexual maturity in the wild is reached between
6-9 years (species dependent). However, in captivity several studies have shown
that maturity and hence breeding ability has been reached by 4 and 5 years of
age. This could be attributed to several factors. In these studies random gibbons
were put together in one habitat. This disruption of the family unit with the
removal of parental pressures or the appearance of a strange social stimulus
could contribute. Also, higher nutritional value and accessibility could be a
factor. It is also not certain as to whether wild maturation occurs later, or
earlier
than
expected
and just not noted correctly. If captive gibbons truly do reach maturity faster,
then it has definite long term benefits. Reproductive rate is also quicker (1-2
yrs
in captivity, 2-3 in wild). Together these factors would allow extra breeding
seasons and higher overall breeding success in captivity. This would greatly
benefit an animal that is so minimal in total population (Geissmann,
1991).
Dispersal
Upon reaching
sexual maturity, gibbons are forced from the family through aggression unless
they are helping in some way (Cooperation). Mature
males may roam around in the overlapping
home
ranges
or
try
to
claim
their own
territory in these areas (Chivers et al., 1980). Roaming gibbons are called floaters
(Tenaza, 1975). Some
have been observed to stay on the edge of a territory that holds a near-mature
female
that
will
soon
be
forced
out (Fuentes, 2000). Average dispersal rate is 710 m.
Several studies also show
higher intra- and inter-group aggression as newly mature males attempt to overtake
existing territories by replacing older males within families. This aggression
is seen more in small crowded areas where little room is available between territories.
This would lead to more non-nuclear families. In all replacements the young are
treated like direct relatives and no infanticide has been observed. The
only
benefit of applying energy to outside young would be for later group defense
upon maturation
to minimize his chances of being replaced (Brockelman
et al., 1998). Males may
also replace their own father either through aggression or submission from the
father. While
this
would
prove
inbreeding,
its
benefits
outweigh the costs (kin
selection) (Chivers et al.,
1980).
The sub-adult's choice depends on the availability of space and unmated individuals.
However, sub-adults without territories have a high mortality rate (Chivers
et
al., 1980).
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