Often a behavior is automatically assumed to involve courtship or sexuality when its participants are known to be of the opposite sex—and the criteria for a “sexual” interpretation are generally far less stringent than those applied to the corresponding interactions between like-sexed individuals. In other words, heterosexual interactions are given the benefit of the doubt as to their sexual content or motivation, even when there is little or no direct evidence for this or even overt evidence to the contrary. For example, simple genital nuzzling of a female Vicuna by a male—taking place outside of the breeding season, and without any mounting or copulation to accompany it—is classified as sexual behavior, while actual same-sex mounting in the same species is considered nonsexual or “play” behavior. In Musk-oxen, foreleg-kicking in heterosexual contexts is often much more aggressive than in homosexual contexts. The male’s blow to a female’s spine or pelvis is sometimes so forceful that it can be heard up to 150 feet away, yet this behavior is still classified as essentially courtship-oriented. If this level of aggression were exhibited in foreleg-kicking between males, the behavior would never be considered homosexual courtship (as it is, this classification is granted only grudgingly, accompanied by the obligatory reference to its “dominance” function between males).
When a male Giraffe sniffs a female’s rear end—without any mounting, erection, penetration, or ejaculation—he is described as being sexually interested in her and his behavior is classified as primarily, if not exclusively, sexual. Yet when a male Giraffe sniffs another male’s genitals, mounts him with an erect penis, and ejaculates—then he is engaging in “aggressive” or “dominance” behavior, and his actions are considered to be, at most, only secondarily or superficially sexual. In one study of Bank Swallows, all chases between males and females were assumed to be sexual even though they were rarely seen to result in copulation. Indeed, the majority of bird studies label dyads composed of a male and female as “[heterosexual] pairs” in spite of the fact that overt sexual (mounting) activity is rarely verified for all such couples. In contrast, most investigators will not even consider classifying same-sex interactions in birds to be courtship, sexual, or pair-bonding activity—even when they involve the same behavior patterns used in heterosexual contexts—unless mounting is observed. Certain associations between male and female Savanna Baboons and Rhesus Macaques are described as “sexual” relationships or “pair-bonds” even though they often do not include sexual activity. In contrast, bonds between same-sex individuals in these species are characterized as nonsexual “coalitions” or “alliances” even though they may involve sexual activities (as well as the same intensity and longevity found in heterosexual bonds). Finally, the “piping display” of the Oystercatcher described earlier was initially assumed to be a courtship behavior, largely because it is a common activity between males and females. Subsequent studies have shown that this is in fact a primarily nonsexual (territorial or dominance) interaction.106
Another strategy adopted by scientists when confronted with an apparently sexual behavior occurring between two males or two females is to deny its sexual content in both same-sex and opposite-sex contexts. For example, because female Crested Black Macaques show behavioral signs of orgasm during homosexual as well as heterosexual mounts, one scientist concluded that this behavior is not reliable evidence of female orgasm in either situation. The fact that intercourse and other sexual interactions occur between like-sexed individuals in Bottlenose and Spinner Dolphins is often taken to be “proof” that such behaviors have become largely divorced from their sexual content and are now forms of “greeting” or “social communication,” even in heterosexual contexts. Similarly, copulation in Common Murres has many nonreproductive features: in addition to occurring between males, in heterosexual pairs it frequently takes place before the female becomes fertile. Drawing an explicit analogy with “nonsexual” mounting in primates, one ornithologist suggested that in this bird heterosexual mounting must therefore serve an “appeasement” function rather than being principally a sexual behavior, i.e., females invite their male partners to mate in order to deflect aggression from them. Likewise, nonprocreative copulations in both heterosexual and homosexual contexts in Blue-bellied Rollers are categorized as a form of ritualized aggression or appeasement.107
A difference in form between homosexual and heterosexual behaviors is often interpreted as a difference in their sexual content. The reasoning is that if same-sex activity does not resemble opposite-sex activity, and only opposite-sex activity is by definition sexual, then same-sex activity cannot be sexual. For example, in Rhesus Macaques most heterosexual copulations involve a series of mounts by the male, only the last of which typically involves ejaculation. Because mounts between males are often single rather than series mounts, they are frequently classified as nonsexual, even when they include clear signs of sexual arousal such as erection, pelvic thrusting, penetration, and even ejaculation. A similar interpretation has also been suggested for mounting between male Japanese Macaques. In contrast, significant differences in form also exist between heterosexual copulation in Macaques (with series mounting) and male masturbatory patterns, yet both activities are clearly sexual and are typically classified as such.108
In other animals the very characteristics that are used to claim that same-sex activities are nonsexual—their briefness, “incompleteness,” or absence of signs of sexual arousal, for example—are as typical, if not more typical, of opposite-sex interactions that are classified as sexual behavior. Nearly a third of all mammals in which same-sex mounting occurs also have “symbolic” or “incomplete” heterosexual mounts in which erection, thrusting, penetration, and/or ejaculation do not occur; “ritual” heterosexual mountings are also typical of many bird species.109 In Kob antelopes, 52 percent of heterosexual copulations involve at least one mount by the male without an erection; in contrast, 56 percent of homosexual mountings between male Giraffes—sometimes classified as nonsexual—do involve erections. Likewise, only one in four to five heterosexual mounts among northern jacanas results in cloacal (genital) contact, and ejaculation probably occurs in less than three-quarters of Orang-utan heterosexual mounts.110 Evidence for sexual arousal or “completed” copulations is often entirely lacking in heterosexual contexts, yet such male-female mounts are still considered “sexual” behavior. In Walruses, Musk-oxen, Bighorn Sheep, Asiatic Mouflons, Grizzly Bears, and Olympic Marmots, for example, penetration and ejaculation are rarely, if ever, directly observable during heterosexual mounts, while male erections are routinely not visible during White-tailed Deer copulations, ejaculation can only be “assumed” to occur in observations of Orang-utan, White-faced Capuchin, and Northern Fur Seal heterosexual mating, and genital contact is difficult to verify during Ruff male-female mounts (among many other species).111
In fact, actual sperm transfer during heterosexual copulations in many species is so difficult to observe that biologists have had to develop a variety of special “ejaculation-verification” techniques. In birds such as Tree Swallows, for example, tiny glass beads or “microspheres” of various colors are inserted into males’ genital tracts. If the birds ejaculate during a heterosexual mating, these beads are transferred to the female’s genital tract, where they can be retrieved by scientists and checked for their color coding to determine which males have actually transferred sperm. For rodents and small marsupials, biologists actually inject several different radioactive substances into males’ prostate glands. During ejaculation, these are carried via semen into females, who are then monitored with a sort of “sperm Geiger counter” to determine which males, if any, have inseminated them.112 If such elaborate lengths are required to verify a fundamental and purportedly self-evident aspect of heterosexual mating, is it any wonder that homosexual matings should sometimes appear to be “incomplete”?