Biological Exuberance: Animal Homosexuality and Natural Diversity

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Mountain Sheep (Geist 1975:97—98).

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Vasey, P. L. (1997, August 19) “Summary: Homosexual or Dominance Behavior? (Discussion),” message posted to Primate Talk (on-line discussion list).

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Rhesus Macaque (Hamilton 1914). A standard and widely cited exposition of the dominance interpretation is Wickler, W. (1967) “Socio-sexual Signals and Their Intra-specific Imitation Among Primates,” in D. Morris, ed., Primate Ethology, pp. 69-147 (London: Weidenfield and Nicolson).

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On the occurrence of dominance hierarchies in various mammals and birds without homosexuality, and further references, see Wilson, E. O. (1975) Sociobiology: The New Synthesis, p. 283 (Cambridge and London: Belknap Press); Welty, J. C., and L. Baptista (1988) The Life of Birds, 4th ed., pp. 206-210 (New York: W. B. Saunders).

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For explicit statements on the absence, unimportance, or irrelevance of dominance hierarchies in these species or populations, see Gorilla (Yamagiwa 1987a:25; Robbins 1996:957); Savanna (Olive) Baboon (Rowell 1967b:507-8); Bottlenose Dolphin (Shane et al. 1986:42); Zebras (Penzhorn 1984:113; Schilder 1988:300); Musk-ox (Smith 1976:92-93); Koala (Smith 1980:187); Buff-breasted Sandpiper (Lanctot and Laredo 1992:7); Tree Swallow (Lombardo et al. 1994:556). In Gorilla all-male groups, dominance is not a central organizing feature of social interactions (including homosexual interactions) even though some semblance of a dominance “hierarchy” may exist and males clearly have different ranks. The same may also be true for Hanuman Langur all-male groups (Weber and Vogel 1970:75) and Collared Peccary mixed-sex groups (Sowls 1997:151-53) in which same-sex interactions occur. In Buff-breasted Sandpipers, although mounting between males may be accompanied by aggression and therefore superficially appears related to “dominance,” there is in fact no evidence that a dominance hierarchy exists in this species or constitutes an important aspect of its social organization. In some of these species (e.g., Zebras, Musk- oxen, Bottlenose Dolphins) dominance hierarchies are more prominent in captivity, although homosexual activity occurs in both wild and captive contexts. Finally, J. Steenberg (personal communication) suggests that mounting between female Matschie’s Tree Kangaroos is a dominance display, yet Hutchins et al. (1991:154-56, 161) found no clear-cut dominance hierarchy in the study population where this behavior was observed.

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Tasmanian Native Hen (Ridpath 1972:81) (in this species, hierarchies can be induced in wild birds by provisioning them with food, but dominance plays no role in their unprovisioned activities— including homosexual mounting, which is not associated in any way with induced dominance); Little Blue Heron (Werschkul 1982:383-84); white-browed sparrow weaver and other weavers (Collias, N. E., and E. C. Collias [1978] “Cooperative Breeding Behavior in the White-browed Sparrow Weaver,” Auk 95:472-84; Collias, N. E., and E. C. Collias [1978] “Group Territory, Dominance Hierarchy, Co-operative Breeding in Birds, and a New Factor,” Animal Behavior 26:308—9). Likewise, dominance systems occur in most Macaques, yet homosexual behavior is apparently absent in some species, e.g., the Barbary Macaque (Macaca sylvanus)—see Vasey, “Homosexual Behavior in Primates,” pp. 178-79; for an extensive summary of research on this species with no mention of same-sex mounting, see Fa, J. E., ed. (1984) The Barbary Macaque: A Case Study in Conservation (New York: Plenum Press). However, recent work seems to suggest that same-sex mounting may in fact occur: Di Trani, C. M. P. (1998) “Conflict Causes and Resolution in Semi-Free-Ranging Barbary Macaques (Macaca sylvanus ),” Folia Primatologica 69:47-48. Therefore, this example must be interpreted with caution, like many other instances involving an apparent “absence” of homosexual behavior (see chapter 4 for further discussion).

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Wolf (Zimen 1976, 1981); Spotted Hyena (Frank 1986); Squirrel Monkey (Baldwin and Baldwin 1981:294—95; Castell and Heinrich 1971:187-88); Bottlenose Dolphin (Samuels and Gifford 1997:82, 88—90). In Red Squirrels, both sexes have dominance systems yet same-sex mounting is much more prominent among males (Ferron 1980:135—36); in Bonobos, a dominance system is much more developed or important among males (de Waal 1997:72—74), yet homosexual activities occur in both sexes. A related observation is that in Bighorn Sheep, both sexes have well-defined dominance systems and exhibit same-sex mounting, yet only among males does it have some correlation with homosexual activity.

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For examples of animals that participate in interspecies homosexual mounting, see the profiles for Crabeating Macaque, Bottlenose Dolphin, Walrus, Greenshank, Orange Bishop Bird, and House Sparrow. On the occurrence of interspecies dominance hierarchies, see, for example, Fisler, G. F. (1977) “Interspecific Hierarchy at an Artificial Food Source,” Animal Behavior 25:240-44; Morse, D. H. (1974) “Niche Breadth as a Function of Social Dominance,” American Naturalist 108:818-30.

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Rhesus Macaque (Reinhardt et al. 1986:56); Japanese Macaque (Chapais and Mignault 1991:175-76; Vasey et al. 1998); Common Chimpanzee (Nishida and Hosaka 1996:122 [table 9.7]). See also Bygott 1974—cited in Hanby 1974:845 [Japanese Macaque]—who found that 59 percent of mounts between male Chimps were by subordinates on dominants or by equally ranked participants.

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Musk-ox (Reinhardt 1985:298). In Cattle Egrets, Fujioka and Yamagishi (1981:139) stated that males attempting homosexual copulations always rank higher than or equal to the males they mount. Yet two males in their study population who mounted other males were apparently not part of the dominance hierarchy (cf. their table 3), while the highest-ranking male did not participate in any same-sex mounts. M. Fujioka (personal communication) concedes that the rank of the males may not actually be an important factor in their homosexual mounting.

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Crested Black Macaque (Dixson 1977:77; Poirier 1964:96); American Bison (Reinhardt 1985:218, 222, 1987:8); Pig-tailed Macaque (Oi 1990a:350); Red Deer (Hall 1983:278); Pukeko (Jamieson and Craig 1987b:319- 22); Japanese Macaque (Chapais and Mignault 1991:175-76); Bighorn Sheep (Shackleton 1991:179-80).

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A further argument is provided by “pile-up mounts,” i.e., when three individuals are all mounted (stacked) on each other. In this case, the mounter-mountee relations rarely if ever follow dominance lines: either they occur in species without dominance hierarchies (e.g., Sage Grouse, Common Murre), or else it is not the case that the middle animal is both higher-ranking than the animal it is mounting but lower-ranking than the animal who is mounting it (e.g., Wolf, Bonobo). For more on pile-up mounts, see chapter 4.

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Common Chimpanzee (Nishida and Hosaka 1996:122; Bygott 1974 [cited in Hanby 1974:845 (Japanese Macaque)]); White-faced Capuchin (Manson et al. 1997:771, 780); Blackbuck (Dubost and Feer 1981:89— 90); Cavies (Rood 1972:36); Gray-capped Social Weaver (Collias and Collias 1980:218, 220). Although mounting between male Musk-oxen in captivity seems to follow dominance lines (Reinhardt 1985), in wild herds Smith (1976) found no dominance hierarchy within (as opposed to between) sex/age classes. Same-sex mounting in the wild occurs among age-mates (who are therefore essentially equal in rank, e.g., two-year-old males mount each other).

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See, for example, Bertrand 1969:191 (Stumptail Macaque); Simonds 1965:183, Sugiyama 1971:259 (Bonnet Macaque); Bernstein 1972:406 (Pig-tailed Macaque); Dixson et al. 1975:195-96 (Talapoin Monkey); Kaufmann 1974:309 (Whiptail Wallaby).

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A distinction between consensual and nonconsensual homosexual mounts is found in more than 30 different species of mammals and birds. Direct evidence of sexual arousal and stimulation on the part of animals being mounted is also available in many species, including orgasmic (and other) responses in female Japanese, Rhesus,