Gibbon Mating System

Males and Females

Male and Female Roles

Gibbon males and females are considered codominant, but their family roles are slightly different (Carpenter 1940 in Leighton 1986).The female leads the group frequently, especially in initiating calls, directing the group along its route, and entering and leaving sleeping trees (Islam and Feeroz 1992). In a study by Reichard and Sommer (1997), females led 76 percent of the movements into and out of feeding trees, while the males simply followed. Males most often lead activities such as looking out for danger and entering territorial disputes (Islam and Feeroz 1992). Members of both sexes lead movements into food trees when the food supply is abundant (Islam and Feeroz 1992).

Paired gibbon males and females are not often aggressive toward each other, but the few times they are, the female is aggressive toward the male and the male initiates the “conciliatory-appeasement gestures” (Palombit 1996). Palombit (1996) suggests that this behavior indicates that the female is slightly dominant to the male gibbon.

Pair Bonds

Duet singing has been closely linked to pair bonds; a higher quality of song and synchronization reflects a stronger pair bond. Geissman and Orgeldinger (2000) found that gibbon pairs that performed synchronized duets spent more time grooming each other, performing the same activities, and being in close proximity to each other than did pairs whose duets were less synchronized. Thus, pairs that demonstrated strong attachment to each other performed duets that were more cohesive as well.
Much time and practice is required for a gibbon pair to perfect its duet, so that each can predict the call of the other (Haimoff 1981 in Geissman 1986).

In a study by Geissman (1986), when gibbon mates were changed, the number of calls and overall time spent singing increased between the newly mated pair. Being newly paired, gibbons want to perfect their duet as quickly as possible because unsynchronized and irregular duetting may attract solitary individuals (Geissman 1986). These individuals sense a weak and easily broken bond between the pair and may attempt to steal one member of the pair (Geissman 1986).

Photograph courtesy of David Pearce <http://www.primates.com/welcome.htm>

Monogamy

Gibbons are monogamous and are characterized by maintaining lifelong pair bonds. They are obligately monogamous, primarily because of the prolonged period of offspring dependence (see The Gibbon). The mated gibbons maximize their reproductive success through a large and specialized investment in their offspring (Trivers 1972 and Brown 1975 in Tilson 1981). While the mother must carry and feed the infant for at least the first year, she has very little time and energy left for other activities, so the male is necessary to defend the territory. Without the male, the female would have to sacrifice taking care of the young for defending the territory, which would result in low reproductive success of the pair (Leighton 1986).

Since the young develop so slowly, both parents are necessary for performing the parental and territorial roles in the family. According to Gittins and Raemaekers (1980), these lifetime pair bonds are “essential” for the rearing of such slowly developing young (in Palombit 1994). Without a male defending the territory, a female will probably conceive fewer offspring because of significant energy input required in defending the territory herself or because of her inability to compete on her own against other pairs for food (Leighton 1986).

While gibbons are termed obligately monogamous, two instances of females maintaining their territories without a paired male for ten months have been observed (Palombit 1994). In most cases, however, the male is crucial to the successful rearing of the offspring.

Some consider the gibbon mating system to be female-defense monogamy because of male behavior during group encounters, when he moves forward and performs chasing behaviors (Reichard and Sommer 1997). Others argue that it is still unclear whether the male defends his territory or his mate because he often performs the defense of both at the same time (Leighton 1986). I believe that since territories are partially determined by female distribution, the male defends the female because it is the area that she staked out that the male defends. The female’s primary concern is having enough food for herself and her offspring, which makes defending her food supply important. Thus, I believe that while the male primarily defends the female, in doing so he also defends her food supply, covering the concerns of both the male and the female.

Proximately Maintaining Monogamy

Intrasexual aggression between paired and solitary gibbons is one factor that helps maintain monogamy (Mitani 1984). Although she is not involved in most group encounters, a paired female will approach an invading solitary female in order to exclude her from the territory, preventing her from stealing her food supply and her mate (Mitani 1984). Without the presence of solitary females, a gibbon male is limited to his monogamous bond. While females will only make an effort to evict solitary females, males will force out both solitary males and other gibbon pairs (Raemaekers and Raemaekers 1985).

Intrasexual aggression between females may also contribute to maintaining monogamy in that aggression between two or more females would decrease the male’s overall reproductive success (Raemaekers and Raemaekers 1985). The area defensible by a single male is also typically only enough to maintain one female and her offspring (Raemaekers and Raemaekers 1985).
Females tend to distribute themselves across wide areas in order to secure food supplies for themselves and their offspring (Reichard and Sommer 1997). Males tend to follow the females and thus distribute themselves in the same way (Reichard and Sommer 1997). This distribution of females is another factor maintaining monogamy in gibbons because males are left with few opportunities to mate with nearby females (Reichard and Sommer 1997). Thus, males are restricted to one female as a result of the females’ “solitary nature, low density, and even dispersion,” as well as the need for paternal care of the offspring (Leighton 1986). While the female is tied up taking care of the slowly-developing offspring, the male is needed to defend the territory (and food resources), and sometimes to help care for the offspring as well (Leighton 1986).

The strong pair bond created through duetting helps maintain lifelong monogamous pairs in most gibbons. Since creating a synchronized duet requires so much time and energy put into practicing, males and females are not likely to want to start over forming a new pair bond. If pairs are constantly changed, much investment is required with each new pair bond formed, resulting in increased time and energy expenditure on the part of both individuals (Wicker 1980 in Geissman and Orgeldinger 2000). This energy could be used in other ways, like caring for young and foraging, which would help increase the reproductive success of the pair.

Exceptions to Monogamy

Because of the prolonged dependence of offspring, the male is tied to the female in order to have high reproductive success. His strategy, however, may be to defend his own female in order to have reproductive success, while also sneaking extra-pair copulations.Extra-pair copulations (copulations outside of a mated pair) occur quite frequently in gibbons, despite their monogamous classification. As many as 12 percent of copulations take place between unpaired individuals (Reichard 1995).

While extra-pair copulations follow the male strategy to mate with as many females as possible, females take part in EPCs as well. A female may mate with a neighboring or solitary male after sizing him up against her own mate during territorial encounters and after judging the intruding male to be of superior quality (Reichard 1995). This, then, fits with the female strategy that she wants the best possible genes for her offspring and encourages sperm competition (Reichard 1995). A female may engage in many intra-pair copulations in an attempt to deplete her mate’s sperm and prevent him from siring another female’s offspring through EPCs (Small 1988 and Sommer et al. 1992 in Reichard 1995).

While EPCs benefit the individual seeking them (usually the male), males must be careful not to be cuckolded themselves (Reichard 1995). Every time the male leaves his territory to mate with another female, he leaves his own mate unguarded against other males (Reichard 1995). Thus, he must choose at times to spread his genes elsewhere or to make sure that the offspring he helps raise are his own (Reichard 1995). To help prevent EPCs, a male will often stand between his mate and the neighboring male when groups encounter each other to avoid contact between his mate and other male (Reichard 1995).

Photograph courtesy of David Pearce <http://www.primates.com/welcome.htm>

It is also risky for a male to engage in EPCs away from his territory because in the case that his mate calls to him, he must answer, revealing that he is not in his territory guarding his mate; his lack of response or call from afar will signal to his mate that he is engaging in an extra-pair copulation, while also attracting other males to his unguarded female (Reichard and Sommer 1997). Little evidence is given of consequences of female EPCs, but Raemaekers and Raemaekers (1985) argue that a male may suspect that his mate’s offspring are not his and evict her from the territory.

Even though gibbon pairs are characterized by lifelong bonds, many pairs do not last until death. Palombit (1994) found that following a six year period only one out of five studied pair bonds was maintained. During this time, members of both sexes deserted their mates (Palombit 1994). The deserting mate immediately paired with another unmated individual, often one whose mate had died or deserted, in over half of the cases (Palombit 1994). In almost 70 percent of the cases the deserted individual was able to find a new mate without leaving the territory (Palombit 1994).

Although gibbons are known to be monogamous, polygyny has been observed in some black-crested gibbons (Hylobates concolor) (Jiang et al. 1999). Polygyny has been accounted for in this species by differing environmental and behavioral factors, such as limited space for suitable territories, a lack of suitable mates, and decreased female intrasexual aggression (Jiang et al. 1999).

Infanticide

Infanticide (killing of an infant) has never been observed among gibbons in the wild, but certain instances have been noted in which infants have disappeared and infanticide can be inferred (Reichard and Sommer 1997). Infanticide could occur following the replacement of a male mate or during a temporary invasion of another male, probably a neighbor (Reichard and Sommer 1997). A male will kill unrelated offspring in order to be the father to the female’s next infant, shorten the time until the female becomes receptive, or reduce competition for resources among other offspring (Reichard and Sommer 1997).

Male/Female Differences Between Gibbon Species

The strength of pair bonds can vary between gibbon species. Palombit (1996) found that siamang (Hylobates syndactylus) have a stronger pair bond (determined through amount of physical contact, close proximity, and sharing of sleep trees) than the white-handed gibbon (Hylobates lar). According to Palombit (1996), while the male and female contribute equally to maintaining the pair bond in the siamang, males contribute more to maintaining the pair bond in the white-handed gibbon. Unlike the siamang, the white-handed gibbon female seems to tolerate the male rather than seem to benefit from his presence, perhaps because of increased competition between mates and other group members for food sources that are more seasonally available than those of the siamang (Palombit 1996).