Biological Exuberance: Animal Homosexuality and Natural Diversity

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PROBOSCIS MONKEY

IDENTIFICATION: A long-tailed monkey with a reddish orange to gray coat; males are bigger than females (weighing up to 50 pounds) and develop an enlarged, pendulous nose with age. DISTRIBUTION: Borneo; vulnerable. HABITAT: Coastal swamp forests. STUDY AREAS: Tanjung Puting National Park in Kalimantan Tengah, Indonesia.

GOLDEN MONKEY

IDENTIFICATION: A medium-sized, long-tailed monkey with a dark brown back and tail; golden orange chest, underparts, and elongated shoulder hairs; and a prominent white muzzle and blue face. DISTRIBUTION: South- central China; vulnerable. HABITAT: Mountain coniferous and bamboo forests. STUDY AREAS: Beijing Raising and Training Center for Endangered Animals, China; subspecies R.r. roxellana.

Social Organization

Both Proboscis and Golden Monkeys usually live in polygamous groups consisting of one male and several adult females (five on the average), along with several adolescent or juvenile females. Younger male Proboscis Monkeys sometimes join all-male troops before reaching adolescence—such groups contain males of all ages, including adults—while some male Golden Monkeys are solitary or peripheral. Cosexual groups of Proboscis Monkeys are female-centered, since female Proboscis Monkeys direct most of their behaviors—both friendly and aggressive —toward other females rather than males, and such female relationships hold the group together. In many cases females also take the initiative in directing the movement of the group, for example when leaving a sleeping tree or crossing a river. Although little else is known about the social organization of Golden Monkeys, it appears that they may also gather into huge troops—up to 600 animals—which are among the largest groupings found in any tree- living primate.

Description

Behavioral Expression: Both male and female Proboscis Monkeys participate in homosexual mounting. An adult female may mount another from behind, in a position similar to heterosexual mounting, and thrust against her. Such behavior sometimes occurs when two social groups meet each other. Unlike heterosexual copulation, lesbian mounting is usually not preceded by solicitation behavior on the part of the female being mounted (solicitation involves a special “pout” face with pursed lips, shaking of the head from side to side, and presentation of the hindquarters to the mounting animal). Male homosexual mounting—also in the front-to-back position, and with pelvic thrusting—occurs in younger animals (adolescents or juveniles) and is often a part of play-wrestling matches. It may also be interspersed with masturbation, in which the mounting male stimulates his penis with his hand. In some cases, the male being mounted may not be an entirely willing participant and will try to get away as the other male restrains him by biting gently on the neck; similar escape behavior is sometimes shown by females during heterosexual mounting. In Golden Monkeys, homosexual behavior has only been seen among females and again consists of one female mounting another.

Frequency: Same-sex mounting probably occurs only occasionally in Proboscis and Golden Monkeys. However, in Proboscis Monkeys at least, heterosexual mounting is also infrequent: in one study, only 12 mountings were observed over an entire year, and 2 of these (17 percent) were same-sex.

Orientation: Female Proboscis Monkeys that engage in same- sex mounting are probably bisexual, since they may alternate same-sex and opposite-sex activity during the same session. Golden Monkey females probably have a bisexual potential, since same-sex mounting has so far only been observed in captive situations when males are not present, and all such females engaging in same-sex mounting also mated heterosexually and raised young.

Nonreproductive and Alternative Heterosexualities

As noted above, female Proboscis Monkeys often refuse heterosexual copulation by breaking away from the male during mounting. A male may exhibit indifference to a female’s solicitations as well, either ignoring her completely or snarling at her to indicate his unwillingness. Male Golden Monkeys also frequently ignore females’ sexual invitations: nearly 50 percent of all female invitations fail to result in mounting by the male. Furthermore, many mounts by males do not result in ejaculation: for some Golden males, 18-97 percent of their copulations are nonejaculatory. Females, however, display an intense interest in sexual activity, frequently soliciting the male and copulating repeatedly—one female solicited a male 34 times, and was mounted 23 times, in one day. REVERSE mountings, in which the female mounts the male, are common in Golden Monkeys as well, accounting for anywhere from 3-40 percent of all heterosexual mounts. In such cases, the male usually adopts the prostrate solicitation posture typical of females inviting copulation. Heterosexual copulations in Proboscis Monkeys are sometimes also harassed or interrupted by younger animals, who try to disrupt the mating pair by climbing on the male, pulling at his nose, or making noises and distracting movements. Female Proboscis Monkeys often have sex when they are pregnant, in some cases soliciting copulations from males as late as two weeks before birth. In fact, heterosexual mating may occur more frequently during pregnancy than at any other time of the year. Golden Monkeys engage in sexual mounting outside of the mating season, and females often solicit sexual behavior while they are menstruating. As noted above, masturbation in Proboscis Monkeys may occur in same-sex contexts; similar behavior has been observed in heterosexual contexts among Golden Monkeys. In such cases, both males and females may eat the semen after ejaculation. A number of adult, sexually mature Proboscis males are nonbreeders, since they live in all-male groups: in one population, such individuals comprised 28 percent of all adult males. Solitary male Golden Monkeys are probably also nonbreeders.

Sources