Bruce Bagemihl - Biological Exuberance

20

For various statements of this hypothesis, see Hutchinson, “A Speculative Consideration of Certain Possible Forms of Sexual Selection in Man”; Kirsch and Rodman, “Selection and Sexuality: The Darwinian View of Homosexuality”; for a refutation, see Futuyama and Risch, “Sexual Orientation, Sociobiology, and Evolution.” It is also possible that the homosexual gene would be recessive, i.e., not expressed when combined with the heterosexual gene—such individuals would therefore not be bisexual, but could still have a reproductive advantage. However, in the absence of any actual genetic information, there is no way to evaluate this version of the hypothesis, since individuals with a recessive homosexual gene would presumably be (superficially) indistinguishable from those with two heterosexual genes (for an alternate view and several other versions of this hypothesis, see McKnight, J. [1997] Straight Science? Homosexuality, Evolution, and Adaptation [London: Routledge]). Therefore, the following discussion is confined to assessing the version in which such individuals are actually behaviorally bisexual (e.g., Weinrich’s 1987 version). In the spirit of the “bisexual superiority” hypothesis, see also Caldwell and Caldwell’s (1967:15) suggestion that bisexuality in Bottlenose Dolphins represents a “more evolved” state because their sexuality is neither limited to reproductive activity nor confined to partners of only one sex. These scientists suggest that Dolphins may be more advanced than humans in this regard, based on the (erroneous) belief that Dolphins do not exhibit exclusive homosexuality, or (in their words) are not “fixated on a biologically inappropriate stimulus to the exclusion of the biologically appropriate one.” For more on the myth of human uniqueness with regard to exclusive homosexuality, see chapter 2.

21

Based on data in fig. 2, Braithwaite 1981:140; on heterosexual partitioning of incubation duties, and the possible advantages of greater male participation, see O‘Brien 1990:1186 and Brugger and Taborsky 1994. Another possible case of bisexual pairs being more successful at reproduction concerns the Snow Goose. Diamond (1989:101) had speculated that female pairs (in this and other species) that fertilize their eggs by mating with males might be able to produce more offspring than heterosexual pairs. However, this does not appear to be a genuine case: the initial suggestion was entirely conjectural and not based on actual long-term studies of the reproductive output of same-sex versus opposite-sex pairs. Furthermore, this idea was later shown to be based on faulty reasoning, since the critical factor for comparing reproductive advantage is the number of goslings produced by each female in the pair, not by the pair as a whole (as the females are usually not related to one another). See Conover (1989) and Grether and Weaver (1990) for further discussion.

22

Ruff (Hogan-Warburg 1966:179; van Rhijn 1973:197, 1991:76; Hugie and Lank 1997:220); Greylag Goose (Lorenz 1979:59-60); Pukeko (Jamieson and Craig 1987a:1251); Guianan Cock-of-the-Rock (Trail and Koutnik 1986:211, 215); Oystercatcher (Heg and van Treuren 1998:690; Ens 1998:635).

23

Sociable Weaver (based on data in Collias and Collias 1980:248[table 5] Bonnet Macaque (based on data in Sugiyama 1971:252, 259—60 [tables 2, 8, 9]); Asiatic Elephant (based on data in Poole et al. 1997:306-7 [fig.5]); Japanese Macaque (Hanby 1974:838; Vasey 1996 and personal communication).

24

Some “exclusively lesbian” females copulate with males to fertilize their eggs and thus are technically bisexual in their sexual behavior. However, in terms of pair-bonding these females only choose other females as partners, and therefore I follow Mills in not classifying these individuals as bisexual for the purpose of assessing their reproductive output. However, since exclusively homosexual females produce even fewer offspring than bisexual ones, including them in the bisexual category would not alter the overall conclusion that bisexual females are less prolific breeders.

25

Silver Gull (Mills 1991:1525).

26

Silver Gull (Mills 1989:397-98 [table 23.5]).

27

Kirsch and Rodman (1982:189) state that “it would be difficult to construct a crucial experiment” to test this hypothesis, while Futuyama and Risch (1984:158) note that “it is hard to see how some of these theories could ever be subjected to proper scientific testing.” They are primarily considering investigations on human homosexuality and bisexuality, yet (as we have seen) studies of homosexuality in wild animals can often provide exactly the type of information needed to evaluate these ideas.

28

Kirsch and Rodman, “Selection and Sexuality,” p. 189.

29

The few studies that have been conducted on bisexuality and reproductive output in humans also tend to agree with the Silver Gull (and other animal) findings. Two surveys of bisexual women (in Los Angeles and the UK) found that they had either less or statistically equivalent numbers of children over their lifetime than did exclusively heterosexual women (one study did find that before the age of 25, bisexual women generally have more children than heterosexual women, but this difference evens out once lifetime reproductive rates are considered). (Baker, R. R., and M. A. Bellis [1995] Human Sperm Competition: Copulation, Masturbation, and Infidelity , pp. 117-18 [London: Chapman and Hall]; Essock-Vitale, S. M., and M. T. McGuire [1985] “Women’s Lives Viewed from an Evolutionary Perspective: I. Sexual Histories, Reproductive Success, and Demographic Characteristics of a Random Subsample of American Women,” Ethology and Sociobiology 6:137-54.) This is one of the few examples of the relevant quantitative data in humans being available for testing the “bisexual superiority” hypothesis. Although Baker and Bellis (1995) address the question of how homosexuality affects reproductive output, their primary concern is in evaluating the hypothesis that bisexuality reduces rather than improves reproductive output, i.e., they are not specifically addressing the “bisexual superiority” hypothesis.

30

Jackdaw (Lorenz 1970:202–3); Canada Goose (Allen 1934:187–88); Oystercatcher (Heg and van Treuren 1998: 688-89; Ens 1998:635); Calfbird (Snow 1972:156; Snow 1976:108); Buff-breasted Sandpiper (Myers 1989:44—45); Cheetah (Caro and Collins 1987:59, 62; Caro 1993:25, 1994:252, 304).

31

Silver Gull (Mills 1991:1525 [table 1]); Black-headed Gull (based on table 3, van Rhijn and Groothuis 1985:161); Galah (based on figures in Rogers and McCulloch 1981:83-85). See also the discussion of sexual orientation profiles in chapter 2.

32

Kob (Buechner and Schloeth 1965:219 [based on table 2]); Bonobo (Idani 1991:90–91 [based on tables 5-6]); Japanese Macaque (Chapais and Mignault 1991:175 [based on table II]); Pig-tailed Macaque (Tokuda et al. 1968:288, 290 [based on tables 3 and 5]).

33

For example, an animal could participate in a large number of heterosexual copulations, only a few of which would actually lead to fertilization (not to mention successful birth or rearing of offspring), while an animal with fewer heterosexual encounters could have a higher proportion of fertilizations or successful pregnancies or could be a better parent. Moreover, females who mate repeatedly during one breeding season can only get pregnant or be fertilized once, effectively equalizing the difference between greater and lesser participation in heterosexual mating (unless promiscuity is positively correlated with parenting success). For further discussion of how copulation frequency does not necessarily reflect reproductive output, see Eberhard, W. G. (1996) Female Control: Sexual Selection by Cryptic Female Choice , especially pp. 418ff (Princeton: Princeton University Press).