South African cliff swallow [Earle 1985:46], band-tailed barbfhroat hummingbird [Harms and Ahumada 1992], calliope hummingbird [Armstrong 1988], ringed Parakeet [Hardy 1964]).
2. bird species in which supernormal clutches have been documented without any direct evidence of same-sex pairs (e.g., numerous gulls and other bird species; see note 70, chapter 4).
3. same-sex trios or joint parenting arrangements with little or no conclusive evidence of courtship, sex, or pair-bonding between the like-sexed coparenis (e.g., bobolink [Bollinger et al. 1986], various ducks. grouse [cf. note 11, this chapter] ).
4. bird species in which males associate in “pairs” or form “partnerships” with other males for joint displays during heterosexual courtships, but in which no overt courtship or sexual behavior occers between such partners or other same-sex individuals (e.g., several manakins of the genera Chiroxiphia, Pipra, Machaeropterus, and Masius—note however that males in these species often court “female-plumaged. birds, the sex of most of which has not been determined, while in two other species, some of these individuals have been determined to be males; wild turkey; king bird of paradise and possibly other birds of paradise. For further references, see McDonald 1989: (007 and Trainer and McDonald 1993:779).
5. species in which the only form of documented “same-sex” activity involves individuals mounting het-crosexual copulating pairs, such that the mounting activity is not necessarily limited to like-sexed individuals or the same-sex motivation/orientation is not clear (e.g., camel and Dagg 1981:92], Buller’s albatross [Warham 1967:129]).
6. species in which the only same-sex activity is mounting that appears to be exclusively aggressive in character with no sexual component (e.g., collared lemming, degu, ground squirrel; of. Dagg 1984 and sources cited therein; see also chapter 3 for further discussion of aggressive or “dominance” mounting and the difficulty of distinguishing this from sexual mounting); and species in which the only same-sex activities are “affectionate” behaviors or “platonic” companionships unaccompanied by either signs of sexual arousal or overt courtship or sexual behaviors.
7. other inconclusive cases, such as species reported in secondary sources as exhibiting homosexuality but whose original sources do not definitively document same-sex activity (e.g., avocets, reported in Terres [1980:813], with no mention of source, as engaging in homosexual mounting; Makkink [1936] and Hamilton [1975]—the most comprehensive primary field studies of this species and the most likely sources for this information—describe ritual mountings and masturbation “eruptive copulations”] but no homosexual mounting).
Armitage, K. B. (1962) “Social Behavior of a Colony of the Yellow-bellied Marmut (Marmota flaviventris).” Animal Behavior 10:319-31; Armstrong, D. P. (1988) “Persistent Attempts by a Male Calliope Hummingbird, Stellula calliope, to Copulate with Newly Fledged Conspecifics,” Canadian Field-Naturalist 102:259—60; Bollinger, E. K., T. A. Gavin, C. J. Hibbard, and J. T. Wootton (1986) “Two Male Bobolinks Feed Young at the Same Nest,” Wilson Bulletin 98:154—56; Dagg, A. I. (1984) “Homosexual Behavior and Female-Wale Mounting in Mammals—a First Survey,” Mammal Review 14:155—85; Earle, R. A. (1985) “A Description of the Social, Aggressive, and Maintenance Behavior of the South African Cliff Swallow Hirundo spilodera (Aves: Hirundinidae),” Navorsinge van die nasionale Museum, Bloemfontein 5:37—50; Gauthier-Pilters, H., and A. I. Dagg (1981) The Camel: Its Evolution, Ecology, Behavior, and Relationship to Man (Chicago: University of Chicago Press); Hardy, J.W. (1964) “Ringed Parakeets Nesting in Los Angeles, California,” Condor 65:445—47; Harms, K. E., and J.A. Ahumada (1992) “Observations of an Adult Hummingbird Provisioning an Incubating Adult,” Wilson Bulletin 104:369-70; Laurie, A. ( 1982) “Behavioral Ecology of the Greater One-horned Rhinoceros (Rhinoceros unicornis),” Journal of Zoology, London 196:307—41; Makkink, G. F. (1936) “An Attempt at an Ethogram of the European Avocet (Recurvirostra avosetta L.), With Ethological and Psychological Remarks,” Ardea 25:1-63; McDonald, D. B. (1989) “Correlates of Male Mating Success in a Lekking Bird with Male-Male Cooperation,” Animal Behavior 37:1007—22; Terres, J. K. (1980) The Audubon Society Encyclopedia of North American Birds (New York: Alfred A. Knopf); Trainer, J. M., and D. B. McDonald (1993) “Vocal Repertoire of the Long-tailed Manakin and Its Relation to Male- Male Cooperation,” Condor 95:769—81; Warham, J. (1967) “Snares Island Birds,” Notornis 14:122—39.
30
According to Wilson (1992), approximately 1,032,000 species of animals are currently known to science, although the number of actually occurring species is undoubtedly much higher—on the order of 10-100 million—and there are many complexities in estimating the total number of species. For further discussion, see Wilson, E. O. (1992) The Diversity of Life, pp. 131ff (Cambridge, Mass.: Belknap Press); Wilson, E. O. (1988) “The Current State of Biological Diversity,” in E. O. Wilson, ed., BioDiversity, pp. 3-18. (Washington, D.C.: National Academy Press); May, R. M. (1988) “How Many Species Are There on Earth?” Science 241:1441—49.
31
Le Boeuf and Mesnick 1991:155 (Elephant Seal); see also Wilson, E. O. (1975) Sociobiology: The New Synthesis (Cambridge and London: Belknap Press). This figure is borne out by the data on homosexuality: the average number of observation hours for scientific studies in which homosexual behavior has been observed is approximately 1,050 hours (based on data from 47 species in which the number of observation hours has been recorded).
32
Marten, M., J. May, and R. Taylor (1982) Weird and Wonderful Wildlife, p. 7. (San Francisco: Chronicle Books). A somewhat more precise estimate of the number of species that have been adequately studied can be obtained for a subset of animals by using the Zoological Record (a comprehensive electronic database that indexes more than a million zoological source documents, including articles from over 6,000 journals worldwide, over the past 20 years). The Zoological Record for the period 1978-97 lists 825 mammal species in which at least some aspects of courtship, sexual, pair-bonding, mating-system, and/or parenting behaviors have been studied (the behavior categories in which homosexuality, if present, is likely to be found). Homosexual behavior has been documented in 133 of these species, or approximately 16 percent—comparable to the lower range obtained using the estimate of Marten et al. (The following subject headings/behavior categories indexed by Zoological Record were used in compiling this estimate: Courtship, Lek, Sexual Display, Precopulatory Behavior, Copulation, Mating, Pair Formation, Monogamy, Polygamy, Cooperative Breeding, Breeding Habits, Parental Care, Care of Young, Homosexuality).
33
See chapter 4 for further discussion of these factors.
34
For species that do not engage in “heterosexual” mating at all, e.g., parthenogenetic or hermaphrodite animals, see the next section.
35
Clapham, P. J. (1996) “The Social and Reproductive Biology of Humpback Whales: An Ecological Perspective,” p. 37, Mammal Review 26:27-49.
36
Scott, P. E. (1994) “Lucifer Hummingbird (Calothorax lucifer),” in A. Poole and F. Gill, eds., The Birds of North America: Life Histories for the 21st Century, no. 134, p. 9. (Philadelphia: Academy of Natural Sciences; Washington, D.C.: American Ornithologists’ Union); Dejong, M. J. (1996) “Northern Rough-winged Swallow (Stelgidopteryx serripennis),” in Poole and Gill, The Birds of North America, no. 234, p. 9; Kricher, J. C. (1995) “Black-and-white Warbler (Pheucticus melanocephalus),” in Poole and Gill, The Birds of North America, no. 158, p. 9; O‘Brien, R. M. (1990) “Red-tailed Tropicbird (Phaethon rubricauda)”, in S. Marchant and P. J. Higgins, eds., Handbook of Australian, New
