Biological Exuberance: Animal Homosexuality and Natural Diversity

animal.

Frequency: Female homosexual mounting is a common and regular occurrence in Hanuman Langurs, comprising 37 percent of all mounting activity in some regions; each female participates in a homosexual mount roughly once every five days, usually in the morning or late afternoon. There is also considerable geographic variation in the frequency of mounting between females. Male homosexual mounting in both Hanuman and Nilgiri Langurs is less common, though it can occur frequently in Hanuman all-male bands (especially during periods of excitement). In one study, more than 40 homosexual interactions between male Hanumans were observed during three months. Among Nilgiri Langur males, nearly half of all grooming sessions are between males and over 10 percent of embracing is between males.

Orientation: All female Hanuman Langurs participate in homosexual activity, to varying degrees; most females are probably bisexual, also mating heterosexually to a greater or lesser extent. Male Hanuman Langurs are probably sequentially bisexual, since each male spends some part of his life in an all-male band, where homosexual activities typically occur—and at any given time, as much as 75-90 percent of the male population may be living in such bands. Some males are exclusively homosexual for long periods, since they may stay in these bands for more than five years—in some cases, decade-long residencies have been documented. In fact, some males never mate heterosexually, since they spend their entire adult lives in all- male bands. The highest-ranking male in a Nilgiri Langur troop engages in both homosexual and heterosexual mounting. The other males in the group, however, participate only in homosexual mounting during their stay in the troop, which can last for four or more years.

Nonreproductive and Alternative Heterosexualities

A large proportion of male Hanuman Langurs are nonbreeding: about a quarter of all males never reproduce during their lifetime, and (as noted above) the majority of the male population lives in all-male bands. Individual females occasionally go through nonbreeding periods in which menstruation may cease for months at a time. Furthermore, both males and females experience a postreproductive or “menopausal” period later in their lives: about 14 percent of the female population consists of nonbreeding older females who nevertheless are still sexually active. This period can last up to nine years, fully one-quarter of the average female’s life span. Males frequently rejoin all-male bands after they have bred, where they live out the remainder of their lives (six or more years). Langurs also participate in a variety of nonprocreative sexual behaviors during their breeding prime: in Hanumans, about 8 percent of copulations occur outside of the female’s fertilizable period, while sexual activity during pregnancy is common (especially during the second and third months of the six-to-seven-month gestation period). Females also occasionally mount males, while adult-juvenile heterosexual interactions also occur. Male Nilgiri Langurs often masturbate, and in some populations of this species, heterosexual mating is remarkably infrequent. In fact, the sexes often lead largely separate lives: adult males and females hardly ever interact with one another, and most social interactions take place within small subgroups consisting of monkeys of the same sex and age.

The breeding system of Hanuman Langurs is in many ways characterized by hostility and violence between the sexes and toward infants. As mentioned above, group members often harass and disrupt heterosexual matings, with the result that less than half of all copulations are completed (harassment also occurs in more than three- quarters of Nilgiri Langur copulations). Moreover, a systematic pattern of infanticide is prominent in Hanuman Langurs: males attempting to gain sexual access to females often brutally kill their infants. In some populations, infanticide accounts for as many as 30-60 percent of all infant deaths. The stress of male takeover attempts also sometimes results in abortion of fetuses, and in a few cases females even appear to induce the abortions themselves rather than have their babies subsequently killed by a male. For example, pregnant females may press and slide their bellies on the ground or allow other females to climb on or jump forcefully against them. During the raising of infants, abuse and neglect by females is also not uncommon, occurring in 12 percent of mother-infant interactions and 17 percent of “baby-sitter” interactions. Mistreatment includes abandonment; dangling, dropping, or dragging of the baby; shoving it against the ground; biting; and even kicking and throwing infants out of trees. Remarkably, infants are rarely seriously hurt as a result of such behaviors, although a few deaths (including choking) have been documented. In addition, young females from one group sometimes “kidnap” a baby from a neighboring group, keeping the infant for up to 33 hours before its mother is able to retrieve it. Occasionally the stolen baby dies as a result of the mishandling or neglect it experiences during a kidnapping.

Sources

*asterisked references discuss homosexuality/transgender

Agoramoorthy, G., and S. M. Mohnot (1988) “Infanticide and Juvenilicide in Hanuman Langurs (Presbytis entellus) Around Jodhpur, India.” Human Evolution 3:279-96.

Agoramoorthy, G., S. M. Mohnot, V. Sommer, and A. Srivastava (1988) “Abortions in Free-ranging Hanuman Langurs (Presbytis entellus)—a Male Induced Strategy?” Human Evolution 3:297-308.

Borries, C. (1997) “Infanticide in Seasonally Breeding Multimale Groups of Hanuman Langurs (Presbytis entellus) in Ramnagar (South Nepal).” Behavioral Ecology and Sociobiology 41:139-50.

*Dolhinow, P. (1978) “A Behavioral Repertoire for the Indian Langur (Presbytis entellus).” Primates 19:449- 72.

*Egozcue, J. (1972) “XX Male Presbytis entellus? A Retrospective Study.” Folia Primatologica 17:292-96.

*Hohmann, G. (1989) “Group Fission in Nilgiri Langurs (Presbytis johnii).” International Journal of Primatology 10:441-54.

Hohmann, G., and L. Vogl (1991) “Loud Calls of Male Nilgiri Langurs (Presbytis johnii): Age-, Individual-, and Population-Specific Differences.” International Journal of Primatology 12:503-24.

Hrdy, S. B. (1978) “Allomaternal Care and Abuse of Infants Among Hanuman Langurs.” In D. J. Chivers and J. Herbert, eds., Recent Advances in Primatology, vol. 1, pp. 169-72. London: Academic Press.

*———(1977) The Langurs of Abu: Male and Female Strategies of Reproduction. Cambridge, Mass.: Harvard University Press.

Johnson, J. M. (1984) “The Function of All-Male Trouping Structure in the Nilgiri Langur, Presbytis johnii.” In M. L. Roonwal, S. M. Mohnot, and N. S. Rathore, eds., Current Primate Researches, p. 397. Jodhpur, India: Jodhpur University.

*Mohnot, S. M. (1984) “Some Observations on All-Male Bands of the Hanuman Langur, Presbytis entellus.” In M. L. Roonwal, S. M. Mohnot, and N. S. Rathore, eds., Current Primate Researches, pp. 343-59. Jodhpur, India: Jodhpur University.

———(1980) “Intergroup Infant Kidnapping in Hanuman Langur.” Folia Primatologica 34:259-77.

*Poirier, F. E. (1970a) “The Nilgiri Langur (Presbytis johnii) of South India.” In L. A. Rosenblum, ed., Primate Behavior: Developments in Field and Laboratory Research, vol .1, pp. 251-383. New York: Academic Press.

*———(1970b) “The Communication Matrix of the Nilgiri Langur (Presbytis johnii) of South India.” Folia Primatologica 13:92–136.

———(1969) “Behavioral Flexibility and Intertroop Variation Among Nilgiri Langurs (Presbytis johnii) of South India.” Folia Primatologica 11:119–33.

Rajpurohit, L. S., V. Sommer, and S. M. Mohnot (1995) “Wanderers Between Harems and Bachelor Bands: Male Hanuman Langurs (Presbytis entellus) at Jodhpur in Rajasthan.” Behavior 132:255–99.

Sommer, V. (1989a) “Sexual Harassment in Langur Monkeys (Presbytis entellus): Competition for Nurture, Eggs, and Sperm?” Ethology 80:205–17.