Biological Exuberance: Animal Homosexuality and Natural Diversity

Moynihan (1990:19) states that homosexual pairing and/or mounting is found among nonbreeding Pied Kingfisher males, but does not further specify which categories of nonbreeders known to exist in this species (primary helpers, secondary helpers, or nonhelpers) are involved. However, the likelihood that they are nonhelpers can be deduced from independent descriptions of the behavior of each of these categories. Homosexuality probably does not take place between breeding males and secondary helpers, since the former are antagonistic to the latter, engaging in “intense and prolonged fights” with them (Reyer 1986:288). Likewise for primary and secondary helpers: the former often attack and fight the latter (Reyer 1986:291). Thus, homosexuality probably occurs largely among nonhelping nonbreeders, or among secondary helpers—the latter is less likely, though, since their attentions are usually focused on feeding females, often as potential mates for the next season (Reyer 1984:1170; Reyer 1980:222). Patterns of helping, breeding, and homosexual participation analogous to the bird examples also occur among mammals. In Red Foxes, for example, same-sex mounting occurs both among younger females (nonbreeders and/or helpers) and between them and older breeding females, but only a subset of each; in Bush Dogs, nonbreeders of both sexes act as helpers (Macdonald 1996:535), yet only males occasionally participate in same-sex mounting.

10

In fact, the only possible cases of adoption by homosexual pairs are in Hooded Warblers (where some male pairs may take over nests abandoned by females after they have been parasitized or robbed by predators), Black- headed Gulls (in which adoption of eggs by male pairs has been suggested [van Rhijn and Groothuis 1985:165-66] but not yet documented), and Cheetahs (in which paired males have occasionally been observed temporarily looking after lost cubs [Caro 1994:45, 91]). Coparenting of adopted pups by two females also occurs in Northern Elephant Seals, Gray Seals, and Spotted Seals, although the two females do not appear to have a “pair-bonded” or sexual relationship with each other.

11

For additional examples, see Squirrel Monkey, Common Murre, and Herring Gull. Another type of “helper” arrangement involves hierarchical societies in which only a small fraction of animals breed and the remainder assist them, often in a complex “caste” system in which each class of nonbreeders has its own specialized duties. This is typical of many social insects such as ants or honeybees, but is also found in some mammals such as naked mole- rats. Again, there is no particular association of homosexuality with these systems: homosexual behavior has been reported for perhaps only a handful of insect species with this type of social organization and is not specifically associated with helpers in these species. In fact, in most social insects helpers are asexual (and genetically sterile), and homosexual behavior is actually found among breeders, for example among fertile males participating in mating swarms (cf. O’Neill 1994 on Red Ants).

12

Hanuman Langur (Srivastava et al. 1991:506). For specific evidence or argumentation against the idea that homosexual relations are a form of “kin selection” (i.e., an association between indivduals who interact with or help one another primarily because they are related and will therefore potentially be “benefiting” their own genes, albeit indirectly), see Fernandez and Reboreda 1995:323 (Greater Rhea), Afton 1993:232 (Lesser Scaup Duck), Rose 1992:104, 112 (Killer Whale), Hashimoto et al. 1996:316 (Bonobo), Ens 1998:635h (Oystercatcher), as well as the numerous species with nonincestuous homosexual relations and/or incest taboos.

13

The general concept of a “population control” mechanism in animals would also be rejected by most biologists on theoretical grounds because it relies on the generally discredited notion of “group selection,” which maintains that an animal’s behavior sometimes benefits the population as a whole rather than the individual. This contradicts one of the most fundamental principles of evolutionary biology, that organisms act only in their self- interest. Some scientists, however, have strongly advocated the concept of group selection, and it remains an intriguing and controversial proposal. See, for example, Wynne-Edwards, V. C. (1986) Evolution through Group Selection (Oxford: Blackwell Scientific). For an overall critique of the notion of population regulation in humans, see Bates, D.G., and S. H. Lees (1979) “The Myth of Population Regulation,” in N. A. Chagnon and W. Irons, eds., Evolutionary Biology and Human Social Behavior: An Anthropological Perspective, pp. 273-89 (North Scituate, Mass.: Duxbury Press).

14

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.

15

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]).

16

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.

17

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: