Biological Exuberance: Animal Homosexuality and Natural Diversity

Damaraland mole-rat (Bennett, N.C. [1994] “Reproductive Suppression in Social Cryptomys damarensis Colonies—a Lifetime of Socially-Induced Sterility in Males and Females,” Journal of Zoology, London 234:25-39); Killer Whale (Olesiuk et al. 1990:209). Long-term study of a stable Silver Gull population revealed that 93 percent of all eggs fail to produce birds that survive to breed, only 3 percent of the birds produce half of all surviving offspring, and 84-86 percent of the birds never produce any offspring who go on to breed themselves. In a number of other bird species, the proportion of “noncontributing” individuals is similarly high, ranging from 62-87 percent (Mills 1991:1525-26). Species with more than 50 percent nonbreeders in at least one sex, at any given time, include Bison (54 percent; based on figures in Lott 1981:98), Regent Bowerbirds (67 percent; based on figures in Lenz 1994:264, 267), Pronghorns (75 percent; based on figures in Kitchen 1974:11, 48, 50), and Grant’s Gazelles (92 percent; based on figures in table 2, Walther 1972:358). See pp. 196-99 for further examples.

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Mammals (Macdonald, D. W., ed. [1993] The Encyclopedia of Mammals, pp. 633, 646, 654, 656-57, 722-23 [New York: Facts on File]); Birds (Piersma, T. [1996] “Scolopacidae [Snipes, Sandpipers, and Phalaropes],” p. 476, in J. del Hoyo, A. Elliott, and J. Sargatal, eds., Handbook of the Birds of the World, vol. 3: Hoatzin to Auks, pp. 444—533 [Barcelona: Lynx Editions]); Grouse (Bergerud, A. T. [1988] “Population Ecology of North American Grouse,” in A. T. Bergerud and M. W. Gratson, eds., Adaptive Strategies and Population Ecology of Northern Grouse, pp. 578—685. [Minneapolis: University of Minnesota Press]).

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Nor does the occurrence of homosexual bonding in Oystercatchers fluctuate along with the environmentally induced population fluctuations that occur in this species (Heg and van Treuren 1998:689-90). On the other hand, the incidence of velvet-horn (transgendered) White-tailed Deer might be associated with overpopulation or drought cycles. Anecdotal reports from ranchers and longtime residents of some regions suggest that the occurrence of such Deer (who are infertile) is cyclic and related to the ending of drought periods (Thomas et al. 1970:3). Scientists studying one population found that, overall, the reproductive rate was not reduced by the presence of so many nonbreeding bucks (ibid., p. 19)—in fact, their data show that such populations actually had elevated reproductive rates. However, this skew might accord with a population regulation /fluctuation hypothesis. In populations with significant numbers of velvet-horns, there were higher ovulation rates, pregnancy rates, and numbers of does with fawns among both adult and yearling females (at least one of which—the ovulation rate for adult females in 1960—was statistically significant). Scientists were in fact puzzled over this apparently “opposite” finding: “the results are contrary to that expected if reproduction … was adversely affected” by the presence of velvet-horns in the herd (ibid., p. 17). In fact, we might expect a slightly delayed, rather than immediate, effect on the number of velvet-horns if their prevalence is a response to population pressure. In 1959-61 the population in this region was significantly elevated, and velvet-horn numbers actually peaked several years later in 1962 (at 9.4 percent). Taylor et al. do report periods of drought and overpopulation in the Deer herds of this region during this time (ibid., p. 25). In addition, if the overall reproductive rate is the same between populations with and without velvet-horns, the effect of the velvet-horns could still be to reduce population growth during times when the population is in fact increasing at a faster rate. Of course, much more systematic long-term investigation is required before any conclusions can be drawn about these possible connections.

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Tallies and designations of threatened species are based on the official roster of the World Conservation Union. The three categories (critically endangered, endangered, and vulnerable) represent points along a continuum, based on a set of five quantitative criteria that encompass the species’ rate of population decline, restricted geographic distribution, extent of population fluctuation, age distribution, effects of human disturbances (pollutants, introduced species, exploitation), and so on. See Baillie, J., and B. Groombridge, eds. (1996) 1996 IUCN Red List of Threatened Animals (Gland, Switzerland, and Cambridge, UK: IUCN-World Conservation Union).

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