Biological Exuberance: Animal Homosexuality and Natural Diversity

manage to break free. An unusual form of reproductive suppression also occurs among male Orangs. Although they become sexually mature at seven to ten years old, males generally fail to develop the full range of secondary sexual characteristics (such as the large cheek pads or “flanges,” a throat pouch, and a general weight increase) for another seven years, and sometimes this is delayed for as long as two decades. It is thought that this development is suppressed by the presence of a mature male, perhaps through social intimidation or stress, although the exact mechanism is not known. Nonbreeding males have been found to have higher estrogen levels than breeding males, so perhaps a physiological effect is also involved. Interestingly, nonflanged younger males have been observed copulating repeatedly with females without resulting in any pregnancies; perhaps this is related to their arrested sexual development, although it is also possible that they were simply mating during the nonovulatory phase of the female’s cycle. In addition, adolescent females experience ADOLESCENT STERILITY, lasting a year or longer, during which they can copulate without becoming pregnant. In fact, adolescent females have higher copulation rates than adult females, accounting for more than 60 percent of heterosexual mating. Adult females breed relatively infrequently, perhaps once every four to eight years. Because females in some populations tend to have synchronized reproductive cycles, there may be periods of up to two years when no adult females are available for mating.

Sources

*asterisked references discuss homosexuality/transgender

*Dutrillaux, B., M.-O. Rethore, and J. Lejeune (1975) “Comparaison du caryotype de l‘orang-outang (Pongo pygmaeus) a celui de l’homme, du chimpanze, et du gorille [Comparison of the Karyotype of the Orang-utan to Those of Man, Chimpanzee, and Gorilla].” Annales de Genetique 18:153-61.

Galdikas, B. M. F. (1995) “Social and Reproductive Behavior of Wild Adolescent Female Orangutans.” In R. D. Nadler, B. M. F. Galdikas, L. K. Sheeran, and N. Rosen, eds., The Neglected Ape, pp. 183 -90. New York: Plenum Press.

———(1985) “Orangutan Sociality at Tanjung Puting.” American Journal of Primatology 9:101-19.

———(1981) “Orangutan Reproduction in the Wild.” In C. E. Graham, ed., Reproductive Biology of the Great Apes, pp. 281-300. New York: Academic Press.

Harrisson, B. (1961) “A Study of Orang-utan Behavior in the Semi-Wild State.” International Zoo Yearbook 3:57-68.

Kaplan, G., and L. Rogers (1994) Orang-Utans in Borneo. Armidale, Australia: University of New England Press.

Kingsley, S. R. (1988) “Physiological Development of Male Orang-utans and Gorillas.” In J.H. Schwartz, ed., Orang-utan Biology, pp. 123-31. New York: Oxford University Press.

———(1982) “Causes of Nonbreeding and the Development of the Secondary Sexual Characteristics in the Male Orang Utan: A Hormonal Study.” In L. E. M. de Boer, ed., The Orang Utan: Its Biology and Conservation, pp. 215-29. The Hague: Dr W. Junk Publishers.

*MacKinnon, J. (1974) “The Behavior and Ecology of Wild Orang-utans (Pongo pygmaeus).” Animal Behavior 22:3-74.

*Maple, T. L. (1980) Orang-utan Behavior. New York: Van Nostrand Reinhold.

Mitani, J. C. (1985) “Mating Behavior of Male Orangutans in the Kutai Game Reserve, Indonesia.” Animal Behavior 33:392-402.

*Morris, D. (1964) “The Response of Animals to a Restricted Environment.” Symposia of the Zoological Society of London 13:99-118.

Nadler, R. D. (1988) “Sexual and Reproductive Behavior.” In J. H. Schwartz, ed., Orang-utan Biology, pp. 105-16. New York: Oxford University Press.

———(1982) “Reproductive Behavior and Endocrinology of Orang Utans.” In L. E. M. de Boer, ed., The Orang Utan: Its Biology and Conservation, pp. 231-48. The Hague: Dr W. Junk Publishers.

*Poole, T. B. (1987) “Social Behavior of a Group of Orangutans (Pongo pygmaeus) on an Artificial Island in Singapore Zoological Gardens.” Zoo Biology 6:315-30.

*Rijksen, H. D. (1978) A Fieldstudy on Sumatran Orang Utans (Pongo pygmaeus abelii Lesson 1827): Ecology, Behavior, and Conservation. Wageningen, Netherlands: H. Veenman & Zonen b.v.

Rodman, P. S. (1988) “Diversity and Consistency in Ecology and Behavior.” In J. H. Schwartz, ed., Orang-utan Biology, pp. 31-51. New York: Oxford University Press.

Schurmann, C. (1982) “Mating Behavior of Wild Orang Utans.” In L. E. M. de Boer, ed., The Orang Utan: Its Biology and Conservation, pp. 269-84. The Hague: Dr W. Junk Publishers.

Schurmann, C., and J. A. R. A. M. van Hooff (1986) “Reproductive Strategies of the Orang-Utan: New Data and a Reconsideration of Existing Sociosexual Models.” International Journal of Primatology 7:265- 87.

*Turleau, C., J. de Grouchy, F. Chavin-Colin, J. Mortelmans, and W. Van den Bergh (1975) “Inversion peri- centrique du 3, homozygote et heterozygote, et translation centromerique du 12 dans une famille d’orangs-outangs. Implications evolutives [Pericentric Inversion of Chromosome 3, Homozygous and Heterozygous, and Transposition of Centromere of Chromosome 12 in a Family of Orang-utans. Implications for Evolution].” Annales de Genetique 18:227-33.

Utani, S., and T. M. Setia (1995) “Behavioral Changes in Wild Male and Female Sumatran Orangutans (Pongo pygmaeus abelii) During and Following a Resident Male Take-over.” In R. D. Nadler, B.M.F. Galdikas, L.K. Sheeran, and N. Rosen, eds., The Neglected Ape, pp. 183- 90. New York: Plenum Press.

WHITE-HANDED GIBBON

IDENTIFICATION: A small ape (weighing up to 13 pounds) with a variable coat color (cream, black, brown, or reddish) and a white face ring, hands, and feet. DISTRIBUTION: China, Thailand, Laos, Burma, Malay Peninsula, Sumatra. HABITAT: Lowland and mountain deciduous and rain forests. STUDY AREAS: Huai Kha Khaeng Wildlife Sanctuary, Thailand.

SIAMANG

IDENTIFICATION: Similar to White-handed Gibbon, but larger (up to 24 pounds), with all-black fur and a prominent throat sac. DISTRIBUTION: Sumatra, Malay Peninsula. HABITAT: Lowland and mountain forests. STUDY AREA: Milwaukee County Zoo, Wisconsin.

Social Organization

Gibbons generally live in family groups consisting of a paired male and female and their offspring. Siamang heterosexual pairs may be more closely bonded than those of White-handed Gibbons. Both males and females perform complex vocal duets as part of bonding and territorial displays, although separate family groups have relatively little interaction with one another.

Description

Behavioral Expression: Within their nuclear family groups, male Gibbons sometimes engage in homosexual activities with each other. This incestuous activity often takes place between an adolescent or younger male and his father (or stepfather, if his parents have divorced and re-paired); in Siamangs it may also occur between brothers. A typical homosexual encounter between father and son in White- handed Gibbons occurs in the trees in the morning or early afternoon, while the family is resting or feeding. The mother is generally nonchalant about such encounters, ignoring the sexual activity even if she is close at hand. The two males often groom each other or engage in playful wrestling or chasing as part of their interaction. During such