Biological Exuberance: Animal Homosexuality and Natural Diversity

activities, either male may approach the other to initiate sex, which consists primarily of the two males rubbing their erect penises together, often leading to orgasm. This is done face-to-face (unlike heterosexual copulation, which is typically performed front-to-back). The father lifts up his knees and spreads his legs wide while sitting on a branch or hanging by his arms—this is an invitation to his son to have sex. The adolescent male embraces his father around the waist with his legs, then lowers his body until he is, in effect, sitting in his father’s lap, with his legs resting on top of the older male’s thighs. This allows their genitals to come into direct contact, and the younger male usually begins rapidly thrusting against his father; sometimes the older male will make pelvic thrusts as well. If his son ejaculates on him, the father may scoop up the semen and eat it. Genital contact can last for up to a minute, although the average is about 20 seconds; in comparison, heterosexual copulations in this species average only about 15 seconds.

A similar form of genital rubbing occurs between father and son in Siamangs. While both males are hanging by their arms, the younger grasps his father around the waist with his legs and both thrust against each other (in this species, heterosexual mating is occasionally also performed face-to-face). Unlike in White-handed Gibbons, this activity is sometimes accompanied by threats between the two males, and it appears that the younger male may on occasion want to terminate the activity before his father does. Sometimes two brothers—juveniles or adolescents, four to nine years old—thrust against each other face-to-face as well. Brothers are also generally affectionate with each other, touching and grooming one another, putting their arms over each other’s shoulders, and wrestling together. Fellatio also sometimes occurs in Siamangs: usually an older brother will lick and gently nibble on the penis and groin of his younger brother (who may be only one to three years old) while the latter dangles by his arms above him or sits with his legs spread. The older male may also masturbate the younger by pulling on his erect penis; if ejaculation occurs, the semen may be eaten. Occasionally, a son will lick and groom his father’s genital area, or the father might insert one of his fingers into his son’s anus.

Frequency: In those Gibbon families where homosexual activity takes place, it occurs quite frequently, and at rates that may equal or exceed heterosexual activity. In one White- handed Gibbon family, the father and son sometimes had sexual encounters as often as 8 times a day—although they averaged about twice a day—and homosexual activity took place on more than a third of the days that the family was observed. In fact, during one 18-day period, 44 homosexual interactions were recorded. In comparison, 23 heterosexual copulations were observed in another family over 18 (different) days, at a rate of 1-3 per day; other studies have found rates of 2 heterosexual matings per day (equivalent to heterosexual activity on about a third of the observation days). In a Siamang family observed in a zoo, about 30 percent of all sexual interactions were between males. It is not yet known, however, in what proportion of families homosexual activity occurs (in either of these species).

Orientation: Male Gibbon sexual life is probably sequentially bisexual, characterized by alternating periods of heterosexual and homosexual activity, with occasional long-term exclusive homosexuality. Younger males may experience entirely homosexual interactions with their fathers, or sexual activity with both parents (see below) while they are growing up, and then go on to mate heterosexually as adults. Once paired with a female, they may engage in incestuous encounters with their offspring of both sexes or have extended periods of exclusive homosexuality. In one White-handed Gibbon family, no heterosexual interactions were observed between the father and his female mate during the entire two years that homosexual activity was taking place between him and his son.

Nonreproductive and Alternative Heterosexualities

Like many other species where homosexual activity occurs between related individuals, heterosexual incest is also prominent among Gibbons. Siamang mothers and fathers both interact sexually with their offspring of the opposite sex, as do siblings. Adult males sometimes perform copulation-like thrusting with their daughters, as well as oral and manual stimulation of their genitals. In one case, a Siamang father was observed fondling his adolescent daughter’s vulva with his fingers while her younger brother licked her clitoris. Mothers may invite their juvenile sons—as young as four to five years—to lick and groom their genitals (usually with no hostile reaction from the father). When offspring grow up, mother-son pairs (and occasionally, brother-sister pairs) may sometimes develop in both White-handed Gibbons and Siamangs, often when a father dies and is replaced by his son. Nonreproductive sexual behaviors such as oral sex are also commonly performed in non-incestuous contexts, e.g., between a pair- bonded male and female. Cunnilingus (including direct clitoral licking), manual fondling of the vulva, and vaginal penetration with the fingers have all been observed in mated pairs. Females probably also experience orgasm during heterosexual encounters: in one episode in which a male and female were thrusting against each other, a shudder coursed through the female’s body, and she remained still for almost half a minute after a period of intense stimulation. Female White-handed Gibbons sometimes masturbate by rubbing their genitals against a surface, and they may experience orgasm this way; male Siamangs also masturbate, though not necessarily to orgasm.

In White-handed Gibbons, about 6-7 percent of heterosexual copulations occur when the female cannot conceive, e.g., during pregnancy or lactation. Some of these matings may be with males other than her mate. Although most Gibbon pairs are monogamous, it is estimated that 10-12 percent of White-handed Gibbon copulations are promiscuous. Nonmonogamous sexual activity also occurs in Siamangs and may be initiated by the female. Similarly, although many Gibbons (of both species) pair for life, divorce also occurs. In one study that followed 11 Gibbon heterosexual pairs over six years, 5 of them split up—usually when one partner left his or her mate to be with another individual. As a result, many White-handed Gibbon families—perhaps up to a third—involve step-parenting. Interestingly, even though there is a wide variety of possible sexual and pairing activities in these species, heterosexual activity is a relatively rare occurrence in wild Gibbons. For example, sexual behavior between male and female White-handed Gibbons generally occurs only once every two years or so, and for periods of only four or five months at a time when it does (females generally breed only every two to three years). In Siamangs, females go through regular periods of asexuality in which they delay breeding and turn over the care of their young to males. Females of this species look after their young only until they are 12-16 months old; at that time, males assume full responsibility for the offspring, but females do not reproduce again for another year. It is thought that this period of nonreproduction enables them to assume leadership roles in their group.

Sources

*asterisked references discuss homosexuality/transgender

Brockelman, W. Y., U. Reichard, U. Treesucon, and J. J. Raemaekers (1998) “Dispersal, Pair Formation, and Social Structure in Gibbons (Hylobates lar).” Behavioral Ecology and Sociobiology 42:329-39.

Chivers, D. J. (1974) The Siamang in Malaya: A Field Study of a Primate in Tropical Rain Forest. Contributions to Primatology, vol. 4. Basel: S. Karger.

———. (1972) “The Siamang and the Gibbon in the Malay Peninsula.” In D. M. Rumbaugh, ed., Gibbon and Siamang, vol.1, pp. 103-35. Basel: S. Karger.

Chivers, D. J., and J. J. Raemaekers (1980) “Long-term Changes in Behavior.” In D. J. Chivers, ed., Malayan Forest Primates: Ten Years’ Study in Tropical Rain Forest, pp. 209-60. New York: Plenum.

*Edwards, A.-M. A. R., and J. D. Todd (1991) “Homosexual Behavior in Wild White-handed Gibbons (Hylobates lar).” Primates 32:231-36.

Ellefson, J. O. (1974) “A Natural History of White-handed Gibbons in the Malayan Peninsula.” In D. M. Rumbaugh, ed., Gibbon and Siamang, vol. 3, pp. 1-136. Basel: S. Karger.

*Fox, G. J. (1977) “Social Dynamics in Siamang.” Ph.D. thesis, University of Wisconsin-Milwaukee.

———(1972) “Some Comparisons Between Siamang and Gibbon Behavior.” Folia Primatologica 18:122- 39.

Koyama, N. (1971) “Observations on Mating Behavior of Wild Siamang Gibbons at Fraser’s Hill, Malaysia.” Primates 12:183-89.

Leighton, D. R. (1987) “Gibbons: Territoriality and Monogamy.” In B. B. Smuts, D. L. Cheney, R. M. Seyfarth, R. W. Wrangham, and T. T. Struhsaker, eds., Primate Societies, pp. 135-45. Chicago: University of Chicago Press.