Homosexuality as a Breeding Strategy

Some nonsexual interpretations of homosexual activity discussed in the previous chapters hinge on the indirect contribution of homosexuality to heterosexuality. For example, it has been suggested that homosexuality reinforces group cohesion and social bonds between individuals, thereby improving their well-being and allowing them, ultimately, to reproduce more successfully. It has also been claimed that homosexual “alliances” between animals improve their chances of gaining heterosexual copulations.⁵⁴ Some scientists have been even bolder in their view of the connection between homosexual and heterosexual activity, regarding the two to be directly related or even essentially continuous: same-sex activity is seen as simply an alternative breeding strategy adopted by some animals, or a way to attract or acquire partners of the opposite sex.⁵⁵ It has been proposed, for example, that female Rhesus Macaques sometimes form homosexual consortships to gain access to a male who is himself consorting with their female partner, or that male Bottlenose Dolphins form pairs with each other to seek out female partners.⁵⁶

Another standard “explanation” for homosexual activity among females, especially mammals, is that it attracts males and stimulates them to mate heterosexually. It has also been suggested that female mammals mount each other primarily when one or both partners are in heat, and hence homosexual activity acts as a signal to males of when females are ready to mate. A variation on the notion of homosexuality as a stimulant for heterosexuality is the speculation that males stimulate their own libidos by engaging in same-sex activity (rather than attracting female partners). For example, erotic fighting in African Elephants (during which both participants become sexually aroused) is claimed to stimulate the males so that they can then go out and seek female partners, while male homosexuality in Greenshanks and Golden Plovers is claimed to stimulate and strengthen the birds’ heterosexual drive.⁵⁷

Most of these rather fanciful speculations are not based on any systematic evidence, and in fact there are many arguments against such interpretations. To begin with, homosexual activity in many species is not restricted to the breeding season (i.e., the time when it could “stimulate” heterosexual mating) or to females who are in heat. In more than a third of the mammals and birds for which information is available concerning the chronology of homosexual activity, it occurs either year-round (i.e., both in the breeding and nonbreeding seasons or in females regardless of whether they are in heat or fertile), or else only during the nonbreeding season.⁵⁸ In some cases, the majority of homosexual activity occurs when females cannot conceive, e.g., when they are pregnant (some populations of Japanese Macaques) or during nonfertilizable stages of their cycle (Hanuman Langurs),⁵⁹ and therefore it cannot contribute to heterosexual mating.

Furthermore, even when homosexual activity does take place during the breeding season or at times when females can conceive, cases where it attracts members of the opposite sex or stimulates heterosexual mating activity are the exception, not the rule. In most species, other animals are entirely disinterested, nonchalant, or “underwhelmed” by any same-sex activity they may happen to observe (as discussed in chapter 2). Members of the opposite sex are often entirely absent from the vicinity (Hanuman Langurs) or may even stay away or leave when homosexual activity is taking place (Guianan Cock-of-the-Rock) or be chased away or ignored when they attempt to interact with animals engaging in homosexual activity (Japanese Macaques, Hanuman Langurs). Moreover, in many species homosexual alliances do not actually “improve” their participants’ chances at gaining opposite-sex partners, and the reproductive advantages of same-sex coalitions are often questionable. Male Calfbird companions who display together, for example, do not attract females, nor are they more successful at acquiring mating territories or overcoming rivals than “single” males. Male Cheetahs living in same-sex bonded coalitions (pairs or trios) are no more likely to encounter females than are single males (even though the standard interpretation of such bonding is that it enhances males’ reproductive opportunities and access to opposite-sex partners). They may in fact suffer reduced chances of heterosexual mating (and lowered reproductive output) because of competition or direct interference from their companions. Likewise, although coalitions of male Savanna Baboons sometimes cooperate in obtaining or defending female partners, one researcher points out that this is true for only one-quarter to one-third of all such alliances, concluding that most male partnerships serve many purposes besides obtaining mates and may even lack a recognizable “function.”⁶⁰

The case for male pair-bonding being solely a breeding strategy in Bottlenose Dolphins is also far from definitive. Pairing or “coalition” formation between males of this species is often interpreted—and widely cited—as a means whereby the animals obtain heterosexual mates. Although pairs (and trios) of Bottlenose males may cooperatively seek out and herd females for purposes of mating in some populations (e.g., Australia), this is not a uniform aspect of such partnerships, and in many cases it has yet to be documented. Heterosexual matings resulting from such associations have not in fact been observed in the Florida population where the most extensive study of male pairing has been conducted, nor in Ecuador, where it has recently been suggested that paired males may compete for females. In Australia, where herding and mounting of females by paired males have been observed, no “full” copulations involving penetration have actually been documented, so the reproductive status of this behavior is not clear. Moreover, nearly 38 percent of the animals herded by paired males in Australia were not definitvely sexed: researchers simply assumed that they were females. In fact, bonded males in other populations seek out male rather than female sexual partners in at least some contexts. In the Bahamas, pairs or coalitions of adult male Bottlenose Dolphins herd and chase Atlantic Spotted Dolphins; they typically pursue homosexual activities (including full penetrative copulation with other males) during these interspecies encounters. Finally, even if bonded males assist each other in obtaining heterosexual mates, this does not preclude a homosexual dimension to such partnerships—sequential and simultaneous bisexuality are, after all, prominent in this species. Same-sex pairs can form as long as ten to fifteen years before breeding activity commences, and homosexual activity may exist concurrently with heterosexual activity in such pairs once they do reach breeding age.⁶¹

One species in which same-sex activity among males sometimes does attract females is the Ruff. As already noted, however, homosexual behavior in this bird is not limited to contexts in which it might increase opportunities for heterosexual mating. It also occurs among nonbreeding males, when females are not present, and during the nonbreeding season. Similarly, homosexual activity in female Squirrel Monkeys sometimes does arouse the attentions of males. However, it is clear that the participating females engage in such behavior regardless of whether it draws males and even rebuff the advances of males who approach them during such activity. Sometimes heterosexual behavior serves as a stimulant for homosexual activity and not the other way around. Stumptail Macaques, aroused by the sight of heterosexual activity, often initiate same-sex interactions, while in Wolves, Savanna (Yellow) Baboons, and Mountain Sheep, animals watching heterosexual mating often become excited and engage in homosexual activities.⁶²

Even if males of some species are genuinely aroused by sexual activity between females, the evidence clearly shows that females are unconcerned with the effect of their behavior on males and do not structure their participation in homosexuality to maximize its impact on heterosexuality. Yet in spite of all this counterevidence, biologists still claim that a primary “function” of homosexual activity in females is to arouse males: “The sight of one female mounting another is said to excite males sexually in Squirrel monkeys and it may do so to males of other species too, for example men watching pornographic films of lesbian activity.”⁶³ By drawing an explicit parallel to human sexuality, the author of this statement hopes to argue for the evolutionary “usefulness” of homosexuality—but the analogy actually highlights the fundamental absurdity of this “explanation,” as well as its dependence on cultural rather than biological factors. True, many heterosexual men are aroused by the sight of two women having sex together, and lesbian sexuality is often packaged and trivialized as pornography to be consumed by straight men. But it would be ridiculous to conclude, on the basis of this, that lesbians have sex “in order” to arouse heterosexual men—yet this is exactly the type of reductionist thinking that is routinely applied to homosexual behavior in animals. It is also highly revealing that homosexual behavior among male animals is virtually never described as being stimulating for females.⁶⁴

Perhaps the most widespread version of the idea that homosexuality is really just a form of reproductive behavior concerns same-sex pairing in birds. It is frequently asserted that the “function” of such associations is to allow females to successfully raise young when they are unable to obtain a male mate. Not only is the initial premise of this explanation—that homosexual pairs result from the unavailability of members of the opposite sex—incorrect, but first and foremost, birds do not usually form same-sex pairs specifically to undertake parenting.⁶⁵ Species in which homosexual pairs never attempt to raise young are nearly as common as those in which same-sex parenting does occur. Even in those species where female pairs lay eggs, the proportion of their eggs that are actually fertile is usually low, indicating that the females do not mate with males or “try” to raise a family—fertil—ity rates as low as 0 percent for Kittiwakes, 0–15 percent for Western Gulls, 4–30 percent in Herring Gulls, 33 percent for Silver Gulls, and 8 percent in some populations of Ring-billed Gulls have been documented.⁶⁶ In addition, female pairs whose clutches are entirely infertile have been reported for Mute Swans, Black-winged Stilts, Roseate Terns, Blue Tits, Red-backed Shrikes, King and Gentoo Penguins, and Lovebirds (among others). Female Jackdaws who have lost their male partners sometimes pair up with nonbreeding females. However, these associations develop regardless of whether the widow has young, demonstrating that females do not form same-sex associations solely for the purpose of obtaining help in raising offspring. Moreover, only 10 percent of widowed females are involved in homosexual pairs, so even if such partnerships were “reproductively” motivated, it remains to be explained why only some females take advantage of such alternative parenting arrangements.

Nests belonging to homosexual pairs of Black-winged Stilts (left) and Red-backed Shrikes (right). Both females in the pair lay eggs, and therefore their nests contain “supernormal clutches” (double the usual number of eggs). Because neither female has mated with males, however, these clutches typically consist entirely of unfertilized eggs.

Furthermore, there are several different forms of same-sex parenting among birds (and other animals). In some cases, individuals develop full pair-bonds with their coparent, including courtship and sexual activity, and the partnership typically exists prior to and extends beyond the duration of parenting (e.g., Western Gulls, Black-winged Stilts). In other species, partners who already have offspring simply enter into a joint-parenting arrangement with no associated courtship or sexual activity between them, often lasting only until the young have been raised (e.g., Lesser Scaup Ducks). In still other cases, animals develop an intermediate arrangement, with “platonic” coparenting between individuals who may nevertheless continue to associate together even when not breeding (e.g., Acorn Woodpeckers, Squirrel Monkeys). And finally, in many species (e.g., Greylag Geese, Oystercatchers), individuals form bisexual trios that parent their offspring together (often contrasting with heterosexual trios and/or homosexual pairs within the same species).⁶⁷ All four types of arrangement could be interpreted as “strategies” to raise young, yet the differences between them remain unaddressed if homosexual associations are seen strictly as coparenting arrangements.

The putative benefits of same-sex breeding associations are also generally belied by the fact that not many individuals take advantage of them. The proportion of birds who participate in homosexual pairings or joint parenting arrangements is often relatively small—much smaller, in fact, than would be expected if this were simply an efficient or beneficial reproductive strategy. For example, most male Greater Rheas and Emus raise their young as single parents, but occasionally two males join forces, incubating their eggs in tandem and raising their chicks together. Single parenting can be taxing in these species—partnerless males, for instance, may fast during the entire incubation period, and single Greater Rhea fathers often lose eggs because they can’t keep large clutches warm—so it has been suggested that two males may be better equipped to handle the difficulties of parenting by helping each other. However, only a small fraction of nests are tended by two males (less than 3 percent in Greater Rheas): if this were truly a useful parenting strategy, why wouldn’t all males—or at least a larger proportion—be using it? Clearly something more—or something else—is involved in associations between males than simply the parenting benefits they may accrue. To further confound the picture, in Greater Rheas both same-sex coparenting and same-sex nest helpers occur. While some males jointly parent the same brood of young, a much higher percentage (about a quarter, still a minority of the population) are assisted by an adolescent male who separately parents one of their nests. Once again, this raises the question of why some males opt for joint parenting, others “choose” to have male helpers, while most do neither. And in many species the supposed advantages of coparenting as opposed to single parenting are in fact illusory. Most female Lesser Scaups raise their young with no help from males, but occasionally two or three females coparent. It is usually assumed that this strategy gives such females an advantage in parenting, but detailed studies of parental investment have shown that same-sex coparents are no more and no less successful than single parents. Moreover, each female in such an arrangement generally spends the same amount of time in parenting duties as do single females, i.e., she is not “relieved” of some of her responsibilities by her companion. In other words, there is essentially no reproductive advantage to joining forces with a parenting partner in this species.⁶⁸

Nor is the occurrence of homosexual pairing in other species correlated with the supposed advantages of having an opposite-sex partner to help with parenting. Even in birds where male-female coparenting is typical, there are often significant differences between species in how essential that biparental care is to successful chick-raising. In some birds, females can raise young without the assistance of a male partner, while in other species the male’s contribution is indispensable. If homosexual pairing were somehow related to the (in)ability of single birds to raise young on their own, one would expect same-sex associations to occur in species where biparental care is more important, i.e., where single birds cannot raise young on their own—yet the facts do not support this. Consider two parallel examples: Snow Geese and Black-billed Magpies. Homosexual pairing in female Snow Geese is claimed to allow otherwise single birds to raise young. However, biparental care is not essential for successful reproduction in this species: when females in heterosexual pairs have their male partners taken away from them, they are quite capable of raising their young as single parents. On the other hand, biparental care is essential in Black-billed Magpies, since females are unable to raise offspring on their own when they lose (or are deprived of) their male mates. Yet homosexual pairs of Magpies do not raise young together, nor do widowed females form same-sex pairs in this species (unlike the closely related Jackdaws). This is exactly the opposite of what would be expected if mateless birds were forming homosexual associations to enable them to parent.⁶⁹

In fact, most pair-bonding birds do not form same-sex couples when heterosexual mates desert them or are experimentally removed, indicating that homosexual pairing is not a widespread mechanism for achieving two-parent care (regardless of whether the latter is “indispensable” or simply preferred). Moreover, in those polygamous (non-pair-bonding) species such as the Superb Lyrebird where females could benefit significantly from male parental assistance (and appear to suffer detrimental effects in its absence, such as slowed growth of their offspring), female pairing and coparenting is noticeably absent.⁷⁰ Conversely, same-sex pairing and/or coparenting do occur in many species where single parents routinely raise young successfully. This is true for Hooded Warblers and Mallards (where heterosexual parents almost always separate and become single parents before the young are fledged), and Red Squirrels and Grizzlies (where heterosexual coparenting never occurs as part of these species’ polygamous mating systems). In none of these animals is a two-parent family (either heterosexual or homosexual) absolutely required for successful parenting.

To take this line of thinking a step further: in a few species homosexual associations may actually be detrimental to parenting. Besides providing no apparent parental benefits to each other, Calfbird female companions may in fact increase their risk of predator attacks by nesting so close to each other (thereby drawing attention to their location). Female Japanese Macaques in homosexual consortships also do not typically assist their partner with parenting and are often notably aggressive toward their consort’s offspring. Homosexual bonding is reproductively disadvantageous for both Oystercatchers and Jackdaws in bisexual trios, for slightly different reasons. Oystercatchers in such associations typically do not jointly incubate their supernormal clutches (only one bird sits on the nest at a time); because each incubator is unable to cover all the eggs simultaneously, the outsized clutch is often not kept adequately warm. As a result, bisexual trio parents hatch fewer eggs and produce significantly fewer fledglings than heterosexually paired Oystercatchers. Female Jackdaws in bisexual trios, on the other hand, do jointly incubate their supernormal clutches. However, because the two females are bonded to each other, both leave the nest together when their male partner arrives to relieve them, and he is unable to cover all their eggs and keep them warm. A parallel effect may occur in Lesser Scaup Ducks: although most female coparents exhibit remarkable cooperative defense of their joint broods, some pairs have been observed flying off together at the approach of a predator, temporarily abandoning their young in the face of danger. Finally, female Canada Geese in homosexual pairs sometimes roll eggs between their adjacent nests, breaking many of them in the process.⁷¹ Clearly, then, successful parenting—and, by extension, reproduction or “perpetuation of the species”—cannot be the whole story behind the formation of same-sex pair-bonds.