Biological Exuberance: Animal Homosexuality and Natural Diversity

Rate of occurrence is only one measure of frequency, however. It could be that sexual activity in general is much rarer in Hummingbirds than in Kobs, in which case comparing absolute numbers or rates gives a distorted or incomplete picture. A more meaningful comparison would be to look at how many heterosexual acts are performed during the same time period and express the frequency of homosexual activity as a proportion of all sexual activity. In fact, sexual activity is incredibly common among Kob and much rarer in Long-tailed Hermits: during the same study period, 1,032 heterosexual mounts among Kob were tabulated while only 6 heterosexual matings in Hummingbirds were observed. Thus, homosexual mounts constitute only 9 percent of all sexual activity among the Kob, whereas one-fourth of all copulations in Long-tailed Hermits are between males. This is diametrically opposed to the frequency rate or absolute count of homosexual activity in the same species.²⁸

These two cases offer a good example of the many complications that arise when attempting to answer the question “How common or frequent is homosexuality in animals?” The most valid answer—cliches aside—is, “It depends.” It depends not only on the measure of frequency being used, but also on the species, the behaviors being tabulated, the observation techniques that are employed, and many other factors. In this section we’ll explore some of these factors and try to arrive at some meaningful generalizations about the prevalence of homosexuality in the animal kingdom.

One broad measure of frequency is the total number of species in which homosexuality occurs. Same-sex behavior (comprising courtship, sexual, pair-bonding, and parental activities) has been documented in over 450 species of animals worldwide.²⁹ While this may seem like a lot of animals, it is in fact only a tiny fraction of the more than 1 million species that are known to exist.³⁰ Even considering the two animal groups that are the focus of this book—mammals and birds—homosexual behavior is known to occur in roughly 300 out of a total of about 13,000 species, or just over 2 percent. However, comparing the number of species that exhibit homosexuality against all known species is probably an inaccurate measure, since only a fraction of existing species have been studied in any depth—and detailed study is usually required to uncover behaviors such as homosexuality. Scientists have estimated that at least a thousand hours of field observation are required before more unusual but important activities will become apparent in a species’ behavior, and relatively few animals have received this level of scrutiny.³¹ Unfortunately, it is not known exactly how many species have been studied to this depth, although it has been estimated that perhaps only 1,000–2,000 have begun to be adequately described. Using these figures, the proportion of animal species exhibiting homosexual behavior comes in at 15–30 percent—a significant chunk.³²

In fact, the percentage is probably even higher than this, when we consider how easy it is for common behaviors to be missed during even the most detailed of study. A caveat of any scientific endeavor, particularly biology, is that much remains to be learned and observed, and many secrets await discovery—and this is especially true where sexual behavior is concerned. Nocturnal or tree-dwelling habits, elusiveness, habitat inaccessibility, small size, and problems in identifying individual animals are just some of the factors that make field observations of sexuality in many species exceedingly difficult.³³ Consider heterosexual mating, a behavior that is known to occur in all mammals and birds (and most other animals), usually with great regularity.³⁴ Yet in many species this activity has never been seen: “Despite literally thousands of hours of observations made by biologists over many years in the West Indies, Hawaii, and elsewhere, actual copulation in humpback whales has yet to be observed.”³⁵ Lucifer hummingbirds, northern rough- winged swallows, black-and-white warblers, red-tailed tropic birds, and several species of cranes (such as wattled and Siberian cranes) are just a handful of the birds in which heterosexual mating has never been recorded. In some cases, opposite-sex mating has been observed, but only a handful of times at most: in magnificent hummingbirds and black-headed grosbeaks, for example—the latter a common North American bird—copulation between males and females has only been seen once during the entire history of the scientific study of these species. Heterosexual copulation in Victoria’s Riflebirds was not documented until the mid-1990s (and then only several times), even though the species has been known to Western science for nearly a century and a half. During a ten-year study of Cheetahs, no opposite-sex matings were seen over the course of 5,000 hours of observation, and copulation has only been observed a total of five times in the wild during the entire scientific study of this animal. Similar patterns are characteristic of other species: in the akepa (a Hawaiian finch), only five copulations were witnessed during five years of study, only five heterosexual matings were seen in a four-year study of Spotted Hyenas, and only three matings in a three-year study of Agile Wallabies. Nests and eggs of many birds such as swallows and birds of paradise have yet to be discovered, while the first nest of the marbled murrelet was found in 1959, more than 170 years after discovery of the species by Western science.

And of course new revelations about heterosexual behavior are being made all the time: female initiation of mating activity in Orang-utans, for example, was not documented until 1980 in spite of nearly 22,000 hours of observation over the preceding 20 years (and prior extensive field studies often failed to report any heterosexual copulations). As recently as 1996, the existence of polygamous trios in the tanga’eo or Mangaia kingfisher (of the Cook Islands near New Zealand) were uncovered for the first time, and the full extent of heterosexual mating by Common Chimpanzees with animals outside their group was not understood until 1997. Multiple heterosexual matings by female Harbor Seals were not verified until 1998; even then, the behavior was never directly observed during three years of study (including continuous, 24-hour videotape surveillance of captive animals over an entire breeding season), and had to be verified indirectly through DNA testing.³⁶ If direct observation by scientists were used as the sole criterion for the existence of a behavior, we would have to conclude that many species never engage in heterosexuality (or in certain forms of heterosexuality)—yet we know this cannot be true. So the fact that homosexuality has not been seen in many animals does not necessarily mean that it is absent in those species—only that it has yet to be observed.

Ironically, many species in which heterosexuality has rarely or never been observed are ones in which homosexual activity has been recorded. No information on the heterosexual mating system of wild Emus was available prior to 1995, for example, although homosexual copulation in the same species had been observed in captivity more than 70 years earlier. Heterosexual mating has never been observed in Black- rumped Flameback Woodpeckers—although homosexual copulation has—while some studies of Nilgiri Langurs, Harbor Seals, Northern Quolls, and Gray-capped Social Weavers failed to record any instances of opposite-sex mounting, although same-sex mounting did occur. Similarly, documentation of sexual activity between male Walruses—including photographs—preceded by almost a decade comparable descriptions and photographic evidence of sexual activity between males and females. In Acorn Woodpeckers—a species that regularly engages in same-sex mounting—only 26 heterosexual copulations were recorded in over 1,400 hours of observation devoted specifically to recording opposite-sex mating. Likewise, heterosexual copulations in Australian Shelducks (a species in which females sometimes form homosexual pairs) were observed only nine times during nearly a decade of study, and on only three of these occasions was a complete behavioral sequence involved. Because of the difficulty of observing heterosexual copulation, the mating system of Killer Whales is still poorly understood and, according to one scientist, “may never be known with certainty.” Homosexual activity in the same species has already been documented, although its study is also still in its infancy.³⁷ Obviously, then, an activity can be part of the regular behavior of a species and still be completely missed by observers or documented only rarely, in spite of conscientious and in some cases exhaustive observational regimens (both in the wild and in captivity).

Scientists have often characterized homosexuality in animals as “extremely rare” or “quite common,” for example, or as occuring “regularly” or “infrequently” —often without any numerical or contextual information. Yet such statements are virtually meaningless without a common standard of measurement and an agreed-upon point of reference. In an attempt to standardize the evaluation of homosexual behavior, therefore, many scientists have collected quantitative information—usually tallies of particular behaviors (sexual, courtship, pairing, etc.). In a few cases, the difficulty of field observations has precluded the direct observation of both heterosexual and homosexual activity, and several innovative techniques have been developed to calculate the frequency of same-sex activity based on indirect measures. The sex of Gulls, for example, is often difficult to determine under field conditions, and in colonies that may contain tens of thousands of breeding pairs, the task of determining which couples are homosexual and which are heterosexual is a daunting one. However, once researchers discovered that lesbian pairs typically lay supernormal clutches, the frequency of same-sex pairs could be much more easily tallied by counting the number of nests with double the usual number of eggs. One ornithologist even developed a mathematical formula (see the beginning of this section) for estimating the total number of lesbian pairs in a population based on a sample of clutch sizes, taking into account same-sex pairs that lay smaller than supernormal clutches (or