Bruce Bagemihl - Biological Exuberance

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. 70Conversely, 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. 71Clearly, 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.

Attempts to determine the evolutionary “function” of homosexuality have sometimes led to even more obscure and implausible “explanations,” all revolving (predictably) around heterosexual breeding. For example, some scientists have suggested that homosexuality is a form of reproductive “competition”: females have sex (or form pair-bonds) with other females to monopolize their partner’s time and thereby prevent her from mating heterosexually, while males mount each other to reduce or redirect their rival’s sexual drive. 72However, in many species homosexual interactions are actively initiated by the animals who are mounted rather than by the mounters, and the participants often have a friendly rather than a competitive relationship with each other. 73Moreover, there is no evidence that participation in homosexual mounting reduces heterosexual activity—indeed, in some species the opposite is true, for the greatest amount of heterosexual mating is accomplished by precisely those individuals who are also the most active homosexually (as discussed previously). And, as already mentioned, in many animals homosexual activity does not even take place during the breeding season or is only exhibited by a small proportion of individuals.

Another version of this competition hypothesis is that homosexuality is a way of directly interfering with the heterosexual activity of a rival. In a number of birds—Pukeko, Guianan Cock-of-the-Rock, Ocher-bellied Flycatchers, and Buff-breasted Sandpipers—homosexual activity is claimed to be a form of “disruption” whereby one male prevents another from mating with a female, while possibly also “usurping” his partner and mating heterosexually himself. The specifics of same-sex courtship and mating in each case, however, do not support this interpretation. In Pukeko, for example, males do sometimes interrupt heterosexual mating attempts by inviting the other male to mount them, but they do not generally take advantage of the situation to mate with the female partner. Moreover, this occurs only infrequently, and males are more likely to ignore heterosexual matings by other males or watch them without interfering than they are to try to prevent them from occurring. In addition, even if a male were trying to use homosexuality as a way to disrupt a heterosexual mating, this strategy would not “work” unless the other male found the prospect of mounting him more appealing than completing his heterosexual copulation. Ironically, then, a “disruption” interpretation of same-sex mounting in this species—typically presented as an example of the primacy of heterosexual relations—actually entails the assumption that male Pukeko would prefer homosexual activity. In Ocher-bellied Flycatchers, the suggestion that males are trying to disrupt heterosexual matings, or to gain access to females, is entirely speculative. Males have never been seen mating with a female as a result of a homosexual interaction in this species, and in fact females are not even present during the majority of courtship pursuits between males. Homosexual activity is also classified as a form of courtship “disruption” in the Guianan Cock-of-the-Rock, yet there is little evidence in favor of this explanation. As much, if not more, homosexual activity takes place when females are not present on the male’s display territories, and males who initiate such “disruptions” almost never gain access to members of the opposite sex as a result and have rarely even been observed mating with females. Furthermore, visits by yearling males involving homosexual activity are distinct from true courtship disruptions, which are performed by rival adult males. Yearling visits are directed toward a wide variety of males, all of whom cooperate in the interaction. In contrast, rival males target only the most successful heterosexual breeders and are violently attacked by the males they try to disrupt. In addition, same-sex interactions are sometimes directed toward adult nonbreeders, who do not participate in heterosexual mating at all, so it is difficult to see how this could be a form of “disruption.” 74

One species in which at least some homosexual activity appears to be genuinely associated with disruption of heterosexual mating is the Buff-breasted Sandpiper, in which rival males often interrupt each other’s courtship attempts by mounting and pecking them. However, even in this case, the benefits of such activity are not clear-cut, since “disruption” does not always result in more favorable mating opportunities for the “disrupter” or reduced mating for the “disruptee.” Although a “disruptive” male is often able to lure females away from his rival, in other cases he may keep returning to mount the rival without trying to mate with any females. In addition, detailed studies have shown that a male’s success at copulating with females is not in fact related to his ability to repel disrupting males. Moreover, not all homosexual mounting is directly involved with disruption of heterosexual courtship, while many disruptions occur without any homosexual activity. 75This brings up a point that is also relevant for other species. Many animals—including ones that exhibit homosexuality in other contexts—use direct tactics to interrupt or harass heterosexual matings. These include threatening or physically attacking couples during copulation, and trying to pull or dislodge the partners from each other. Even if homosexual behavior were in some instances being used as a form of heterosexual disruption, it would still remain to be explained why some species—or only some individuals in a species—resort to this fairly unusual and indirect strategy, when more effective and efficient measures are available.