KeywordsSexual Orientation Sexual Arousal Penile Erection Fallow Deer Sexual Motivation
Courtship displays, mounting and/or genital contact, and stimulation between same-sex individuals.
Various authors use different definitions of homosexual behavior. The one used is after Sommer and Vasey (2006). Bagemihl (1999) characterizes homosexual behavior in terms of courtship, affection, interactions involving mounting and genital contact, pair bonding, and parenting activities between same-sex individuals. More recently, Poiani (2010) defines homosexual behavior as an interaction that is sexual or of sexual origin and that is performed between two or more individuals of the same sex. He distinguished homosexual behavior from homosexuality, which is a sexual orientation characterized by sexual attraction to individuals of the same sex.
Homosexual behavior either in males or females is common and widespread among social nonhuman species (Bagemihl 1999; Poiani 2010; Sommer and Vasey 2006). It is rarely associated with sexual orientation, however. Domestic sheep 0vis aries is so far the only mammalian species where spontaneous sexual orientation of up to 10% of rams to other males has been repeatedly reported (Perkins and Fitzgerald 1992). The proximate causes of same-sex preferences may be induced by specific situations. Steroids and other hormones, genetic and epigenetic mechanisms modified by stress during prenatal or perinatal period have been shown to have potential to induce homosexuality mainly, but not only, in mammalian males (Balthazart 2016). Also, early postnatal social environment such as removal of other sex from the rearing environment that affected sexual orientation in some mammals and birds has been reported (Poiani 2010).
In most of the cases, animal homosexual behavior is not associated with sexual orientation and the animals involved may and do reproduce. There are numerous situations when depriving individuals of members of the opposite sex causes them to engage in sexual interactions with members of the same sex. Though, when the deprivation is ceased, such formerly deprived animals immediately renew regular heterosexual behavior (Bailey and Zuk 2009). Still, most of the cases of homosexual behavior in animals are not primarily associated with clear sexual context. Understanding social relations requires correct interpretation of the significance of such behavior. While male-male mounting is a common behavior, female-female mounting is also widespread. For some farm ungulates, intra-female mounting has been understood to be a normal element of behavior associated with estrous aimed to signal to males the female is ready to mate. However, observations of female-female mounting outside the rut have been frequently reported as well. Female bonobos, Pan paniscus, for example, embrace frequently each other ventroventrally and rub their genital swellings against each other (Hohmann and Fruth 2000). Hohmann and Fruth (2000) observed bonobo females and investigated five hypotheses on the function of this behavior. During 5-year-lasting study, they found support for multifunctional origin of the behavior, most of all to reconciliation and social tension regulation. They concluded that genital contacts may express quality and dynamics of dyadic social relationships among female bonobos. Similar function can also be seen in many other primate species where homosexual behavior also often promotes alliance formation between sexual partners.
Same-sex mounting in socio-sexual context has been classically suggested to signal dominance. It was first suggested for primates, but later also for many other taxa. When reviewing literature, there are a number of papers supporting dominance signaling hypothesis. On the other hand, many other reports are not consistent with the view that actors are dominant and recipients subordinate (short review in Bartoš and Holečková 2006). The dominant individuals mount subordinates, solicit mounting by subordinate conspecifics, or prevent subordinate to mount them. Opposition against being mounted may but need not occur. As a result, there is a wide variation between as well as within species in who is mounting whom and how the mounted individual responds. Thus, who mounts whom may, in fact, reflect decision of the dominant individual involved rather than signaling the dominance per se.
What is misleading with dominance signaling hypothesis is that even when a dominant male mounts subordinate one, male-male mounting can be associated with penile erection, occasionally anal intromission, and ejaculation. In this context, erection has been regarded as indicative of sexual arousal and thus evidence of a sexual motivation underlying same-sex mounting (Bagemihl 1999). Moreover, mounting is apparently testosterone dependent, because bulls mounted others more frequently then steers, male-male mounting of fallow deer bucks was associated with the start of a seasonal increase in testosterone levels following seasonal minima (Bartoš and Holečková 2006), and testosterone treatment of cows increased their mounting behavior (Bouissou 1978). Here it should be emphasized that there is also a lot of obviously nonsexual situations such as serious fighting associated with penile erections (e.g., Bartoš and Holečková 2006). This seeming contradiction may most likely lie in a mix-up of motivations. For example, androgens, testosterone in particular, in males shape and modify sexual as well as agonistic behavior (Michael and Zumpe 1993). Therefore, it frequently happens that some elements of behavior typical for sexual behavior can occur in agonistic context and vice versa regardless if such a behavior was functional or even dangerous. Moreover, it is also known that disturbance and tension associated with a stressful situation can be linked to mounting. Thus, penile erections, anal intromission, and ejaculation during male-male mounting cannot be taken as real evidence of sexual motivation. Rather, it might be attributed to excitement associated with serious fighting (Bartoš and Holečková 2006).
In conclusion, same-sex orientation in nonhuman animals is extremely rare. In contrast, same-sex sexual behavior is quite common. It does not seem to have any special, exclusive function. In particular, it cannot be clearly linked to roles of a dominant actor and subordinate recipient. Instead, intra-sex behavior is likely to be a consequence of deprivation from the other sex or a side effect of excitement, not necessarily sexual. Hence, depending on circumstances, various proximate factors and ultimate functions may be involved.
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