Biological Exuberance: Animal Homosexuality and Natural Diversity

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).

34

It should also be reiterated that detailed longitudinal studies of breeding success and sexual orientation (comparble to that done on Silver Gulls) have not been conducted on any of these species to verify possible connections between bisexuality and reproduction. Moreover, all of these cases involve homosexuality among members of only one gender, which again is inconsistent with a “bisexual superiority” hypothesis.

35