Biological Exuberance: Animal Homosexuality and Natural Diversity

Gray Heron (Ramo 1993:116-17). In some species (e.g., Little Egrets, Little Blue Herons) quantitative information is only available for the proportion of promiscuous copulations that are homosexual (the higher figure). Where both proportions are available (e.g., Cattle Egrets, Gray Herons), an average of the two is taken when calculating cross-species comparisons of frequency (see below). Different frequencies can also be obtained depending on whether a distinction between copulatory and noncopulatory (or “ritualized”) mounting is taken into account. (See chapter 3 for discussion of the—sometimes arbitrary—distinction between these two types of mounting.) In some cases, such as Pig-tailed and Crested Black Macaques, a sizable difference obtains. In Pigtails, 82 percent of mounting is same-sex if only noncopulatory mounts are considered (Oi 1990a:350-51 [table 4]), whereas 7-23 percent of all mounts are same-sex if “full” heterosexual copulations are included in the calculation (Bernstein 1967:226-7; Oi 1996:345). In Crested Blacks, roughly a third of noncopulatory mounts are between males, but overall this constitutes only about 8 percent of mounting activity when combined with copulatory (heterosexual) mounts (C. Reed, personal communication). In cases such as these, the latter (smaller) percentage is taken to be the overall rate of same-sex activity. For other species, however, the two rates are comparable. In Common Chimpanzees, for example, Nishida and Hosaka (1996:122, 129 [tables 9.7, 9.17a]) recorded 61 ritualized mounts between males compared to 123 male-female copulations (nonritualized), yielding a rate of 33 percent same-sex mountings. This is comparable to the exclusively noncopulatory figures of Bygott (1974; cited in Hanby 1974:845 [Japanese Macaque]), who found that 4 of 14 ritualized mounts (29 percent) occurred between males.

45

Of course, these cross-species comparisons refer only to those animals in which homosexual behavior has been observed and in which the appropriate quantitative information is available. In many species same-sex behavior is more or less common than the maximum and minimum figures obtained from these measures, but it has not been quantified and therefore cannot be compared to these examples. For these calculations, if multiple frequency proportions were available for the same species—either because of population, seasonal, or behavioral differences (as discussed above)—these were averaged prior to being combined with the figures for other species. Proportions for courtship behavior are based on quantitative information from 21 species (avg = 23 percent same-sex activity), for sexual behavior from 77 species (avg = 26 percent), for pairing behavior from 45 species (avg = 14 percent), and for population percentages from 56 species (avg = 27 percent). For the purposes of comparison, tallies of observed homosexual and heterosexual behaviors in each species are assumed to be representative of actually occurring frequencies. The statistics for pairing and sexual behavior do not include the many species in which the only form of pair-bonding, coparenting, or observed sexual behavior is between same-sex individuals, i.e., in which 100 percent of pairs, coparents, or observed sexual interactions are homosexual (were these to be included, the proportions would be considerably, and perhaps unrepresentatively, higher). In five of these cases, however—Northern Elephant Seals, Cheetahs, Grizzly Bears, Lesser Scaup Ducks, and Greater Rheas—the proportion of all families or nests that are tended by same-sex pairs or trios (as opposed to single individuals) is substituted for the proportion of same-sex pairs. For population calculations, figures represent only the sex or activity that has been quantified (e.g., for species in which only females form same-sex pairs, or in which only female pairs have been tallied, only the proportion of females in such pairs is included). Moreover, for many bird species-especially those in which only a small fraction of individuals participate in same-sex activity—population percentages are not available. However, in species with same-sex pairing, the proportion of homosexual pairs is roughly comparable to the proportion of individuals engaging in same-sex activity (the two differ, of course, if there are sizable numbers of nonbreeding birds that do not form pairs with either sex). In order not to bias the sample toward species with relatively high population percentages, therefore, pairing proportions for wild-bird populations have been substituted. In species where no other population data are available, these figures are taken as a rough estimate of the proportion of individuals involved in same-sex activity.

46