Male/female relationships: Gibbons are obligately monogamous (Palombit 1996), but have extra-pair copulations (EPCs) (Reichard 1995). This pair-bond is a long-term one; gibbon pairs may stay together for life, though "divorces" and re-pairings have been reported (Brockleman 1998). The pair bonds are also extremely important to gibbon social structure, as each group relies upon a female to lead the group in foraging and to care for the infants, and upon a male to maintain the territory (Islam & Feeroz 1992). Thus, when a female loses her mate due to death or divorce, he tends to be quickly replaced by a new mate (Brockleman 1998).
Pair-bonds are maintained in part by grooming and close physical proximity, and male gibbons have been observed to initiate these activities more than females; this suggests that males invest more in their bond than females do (Palombit 1996). The potential importance of these pair-maintenance activities, or at least of these behaviors as an indicator of pair-bond strength is illustrated by one set of gibbons in Palombit's 1996 study. One female began the study with a partner to whom she showed very little investment in pair maintenance, as measured by initiating close proximity, grooming, and similar behaviors. Her partner left her when another female became available, and this original female eventually gained a new partner to whom she demonstrated much more pair investment, and to whom she stayed mated.
Monogamy likely evolved in gibbons because of food resource distribution (Reichard & Sommer 1997). Early monogamy probably involved females dispersed due to these food resources, and males engaged in mate-guarding to ensure reproduction; monogamy was maintained as this mate-guarding became classic territoriality (Palombit 1996). The size of a territory which a male could defend is limited in size to one he could patrol, engage in behavior to keep intruders out, whose borders and resources could be remembered (Haimoff 1987). Because of gibbons' restrictive diet, the size of a territory which one male could defend would only have enough food resources to support that male, one adult female, and their offspring (Haimoff 1987).
The one exception to monogamy is the black-crested gibbon, a species believed to be polygynous. Black-crested gibbons live in environments previously believed to be inadequate gibbon territories, but have evolved digestive systems that allow them to eat much more leafy matter than the other gibbons. The territory which one male can defend would thus have the food resources to support more than one adult female, which presumably allowed polygyny to evolve (Haimoff 1987).
Extra-pair copulations, which have been estimated to make up around 12% of total copulations, also seem to be important aspects of male-female relationships (Brockleman 1998), and many social functions of gibbon behavior are now being evaluated in light of EPCs. Calling behavior, for example, has traditionally been seen as a way to advertise territoriality, and occasionally to broadcast reproductive availibility for a newly-mature subadult or a gibbon that has lost its mate (Marshall & Marshall 1976). In addition to these functions, calls may also advertise reproductive status to neighboring members of the opposite sex who might want to engage in EPCs with them. Gibbons also spend a high amount of time in intergroup interactions, and this may be a way to sum up other gibbons as potential mates (either in an EPC, or possibly through repairing), and for these EPCs to actually occur (a male and female sometimes have been seen to 'sneak off' as the female's mate is involved in territorial aggression). Gibbons thus may be constantly (and subconsciously) reviewing their reproductive options (Reichard 1995).
EPCs could allow males to impregnate more than one female and increase their fitness, and might allow a female to chose better genetic material for her offspring than her current mate offers. However, EPCs also have the risk for a male to lose significant investment if his female is impregnated with another male. If a male is gone because he is copulating with other females, he may leave his mate with the opportunity to have her own EPCs; the EPC strategy is thus one that must be carefully weighed (Reichard 1995). Recent research accordingly plays up the role of male-mate guarding to keep their female from being impregnated by another male. Males thus often position themselves between their mate and the neighboring male during group encounters (Reichard & Summer 1997).
Male/male and female/female relationships: Adult gibbons are hostile towards members of their own sex, and this intrasexual aggression is believed to most directly maintain gibbon monogamy and territoriality (Jiang & Wang 1999). Upon meeting, males threaten other males with calls and acrobatic displays, and this threatening behavior often progresses into a chase. Chases do not usually result in one gibbon being caught by the other, although fights with physical contact do occasionally ensue (Bramblett 1976). Gibbons have sharp canine teeth which they may use as weapons, and facial battle scars believed to be from intrasexual aggression can be seen on several adult males; one male even seems to have received a fatal blow from another male during an intrasexual territorial dispute (Palombit 1993). Females may also chase other females, though about 90% of chasing behavior is male-male (Reichard & Sommer 1996). Competition between members of the same sex, thus, seems to maintain gibbon social systems.
Adult gibbons were traditionally believed to force their same-sexed offspring out of the group upon sexual maturity (Wilson 1975, Bramblett 1976). Subadults are now understood to be tolerated within their groups for at least two years past sexual maturity, however (Brockleman 1998). Because gibbon habitat is often suited saturated with territories, and no good pairing opportunities may present themselves when subadults reach sexual maturity, remaining with the natal group could be advantageous for that newly mature individual since they would avoid the risk of solitary starvation. Their presence might also benefit the group by aiding with territorial defense; further, having extra subadults present might particularly be advantageous for the male gibbon, since they might 'guard' or discourage the female from EPCs (Brockleman 1998).
Parent/Offspring Relationships: Care for young infants is almost exclusively by the infant's mother in most gibbon species. In siamangs, males carry infants extensively in the offspring's second year, but this non-maternal care for a very young gibbon is rare in most species (Fisher & Geissmann 1990). If a mother dies, however, the father and subadults may join together to raise an infant (Geissmann & Braendle 1997). As well, fathers do play a role in playing and grooming their (older) infants once they are less dependant on their mothers (Islam & Feeroz 1992). * Females invest quite heavily in their young. Gibbons give birth to one offspring at a time, and the average minimal birth interval is about two to three years (Jiang & Wang 1999, Reichard 1995). The infant is quite dependant on its mother for most of that time (Reichard 1995).