There has been extensive discussion of the key features of gibbon sociology—monogamy and territoriality. They confer both benefits and costs. In being monogamous, the male is reducing his potential reproductive success, and it is thought to be the available niche and distribution of food that leads to this sacrifice, and to the energetic costs of patrolling and defending this territory with its rich and predictable food supply. Females exclude other females, and males exclude strange males to maintain the system. The elaborate duets performed by most gibbons serve, to different extents, both to form and develop the pair bond and to establish and maintain the territory. These songs are reinforced by boundary patrols while seeking food, and by chases back and forth across the boundary. The complex interaction of all these factors defies simple explanations of such behaviors.
It is not clear whether the greater frequency and duration of disputes in hoolock gibbons, compared with other species,
is a function of greater tension in smaller forest patches, or some specific feature of hoolock socioecology. With the male tending to be promiscuous, even polygynous, it is important for the female to impose monogamy on the male to increase her reproductive success and to help in finding food, detecting predators, and excluding neighbors.
Given the stability of gibbon family groups for long periods, the details of dispersal of the maturing young and of the formation of new groups are of special interest because of their dynamism and relative rarity. The pattern that emerges is of young adults, recently excluded from the natal group, acquiring a territory with or without parental help, and thence a mate. Daughters tend to wander less far from the parental territory than sons and are more likely to receive parental help. A rare alternative is to take over the natal territory when one or both parents disappear; if one parent survives, the young may mate with him or her, but this incest is usually transient and/or reproductively ineffective. Parental-care strategy is to promote reproductive success in offspring, but the subadult emerges as a potential sexual competitor to the same-sexed parent; hence, it has to be excluded.
Gibbons are monogamous, because they are adapted to surviving on small fruit trees; it is almost impossible to get enough data to prove this. Reichard and Sommer (1997) echo the female resource/male mate defense argument, suggesting that extra-pair copulations (12% of those seen) help to confuse paternity and forestall infanticide; hence, kin relations extended into neighboring groups. They worked close to the H. lar/H. pileatus hybrid zone in Thailand, studying the isolated lar population, whose home ranges unusually overlapped by 64%. They found encounters between groups were common, occupying 9% of the active day.
Gibbons have been argued to be monogamous and territorial because of their adaptation to small, scattered sources of pulpy fruit. Since they have had to learn the availability of such resources, they cannot afford to share them with con-specifics. The adult male can only effectively defend an area adequate for one female and their offspring, thus they have overcome the basic polygynous drive that characterizes most mammals, including most primates. In exploiting a niche unoccupied by other primates, they have sacrificed the reproductive success that occurs in polygynous breeding systems. Infants are born singly every two to three years, depending on food availability for the parents; that is, mating occurs when there is a seasonal increase in food, usually fruit.
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