Introduction
Homosexual behavior, which has been documented extensively in non-human animals, has fascinated researchers for decades as it presents a paradox for evolutionary biologists. In a Darwinian view, the fittest organisms are those which can maximize reproductive success by producing the greatest number of viable offspring. From this perspective, opposite-sex mates should be preferred over same-sex sexual partners. Despite this, many species engage in homosexual behaviors which seems to undermine reproduction. Thus, it seems appropriate to ask
— as many researchers have — why animals engage in these behaviors at all.
The literature on this topic is vast with upwards of eighty proposed theories explaining the presence of homosexual behavior in animals. The observed same-sex sexual behaviors (SSBs) in non-human animals are highly variable and maintain different functions from species to species; thus, it is no surprise a plethora of theories have been established. A majority of these studies, using non-human animal models, have, however, attempted to generalize their findings to all animals, including humans. Animal models have been invaluable tools in research, but their role as applied to homosexual behavior in humans is problematic. This is because the spectrum of SSBs across species suggests that these theories — though valid for certain species — cannot be generalized. This review will analyze five key theories explained by animal models and three key human-based theories to demonstrate that the present strategy for understanding human homosexual behavior is flawed.
Theories as Applied to Non-Human Animals
A great deal of research has emphasized the paradox of selection which acts on non- reproductive animals. Not much is known regarding the evolutionary consequences of this type of behavior, whether it is a life-long pairing or part of other sexual interactions. The variety and ubiquity of SSB in animals is impressive (Bailey and Zuk, 2009). The question remains why animals continually engage in sexual behaviors that do not directly result in reproduction. The following theories aim to explain this phenomenon and will be further discussed in the present review: Same-Sex Pairing Theory, Social Glue Theory, Dominance Theory, Mate Choice Copying Theory, and Evolutionary Byproduct Theory.
Same-Sex Pairing Theory
Before exploring the same-sex pairing theory, it is important to define “pairing.” A term coined in the 1940s, pairing refers to the intense affinity and bonding observed between two individuals. This behavior is seen in a number of species. In general, pairing occurs as a mechanism to enhance the production of offspring through a lifelong bond in socially monogamous species. Monogamy is very prominent and especially effective in bird species: one parent is needed to defend or brood the offspring while the other parent collects and provides food. The duration of the monogamous pairings can range from one breeding season to life.
Though a majority of these pair-bonded birds are male-female pairs, this is not always the case.
For example, Nisbet and Hatch (1999) examined same-sex pairing in the Roseate Tern. They found that unbalanced sex-ratios among Roseate Tern colonies lead to an increase in the number of same-sex pairings. In this socially monogamous species, an unbalanced sex-ratio causes the less numerous sex to be the limiting factor in reproduction. In this instance, females were far more abundant, which resulted in a number of phenomena: first, the females bred less frequently than males; second, the females were older than males when they first produced offspring; lastly, females would breed with younger or less desirable males. As a result of this, Roseate Tern females have adopted an alternative strategy to increase their chances of reproductive success: they will pair with other females instead of, or in addition to, a male. These females will either share a male mate or reproduce via extra-pair copulations to achieve fertilization, and will then share a single nest. This behavior has been observed in other bird species as well.
Skewed sex ratios have resulted in same-sex pairings in other species as well. One example is a Laysan albatross colony that had a female-biased sex ratio (Young et al, 2008). This example is unique as Laysan albatross females only lay and incubate one egg per year, unlike other species that can maintain larger clutches. Thus, for the female-female pair-bonds to be successful and advantageous to both partners, the bonds must persist for more than one breeding season. This would allow each partner to reproduce in alternating seasons.
Female-female pair-bonds in both of these studies raised comparatively fewer offspring. Further, those offspring were frequently fledged in rather poor conditions. Nevertheless, these females were more successful than females that did not breed at all. These examples provide insight into how changes in the sex ratio of a population can alter the social structure. This flexibility in pair-bonding behavior offers an evolutionary advantage to — in this situation — females of a population that might not otherwise have the opportunity to reproduce. We see here examples of homosexual behavior serving as an adaptation: less fit females that could not acquire a male mate still have the chance to remain competitive.
Social Glue Theory
The Social Glue theory suggests that sexual behavior between members of the same sex can decrease conflict and improve social bonds. Through this, an individual’s fitness and ability to reproduce may be indirectly increased. Being socially bonded with members of a community can increase an individual’s access to resources such as food, protection, and assistance in raising young. A key point to this theory is that, despite engaging in non-reproductive sexual behaviors, individuals do mate with opposite-sex members when females are receptive.
Frans B.M. de Waal (1995) found that bonobos — a close relative of humans — are able to separate “sex for pleasure” from reproduction, much in the way humans do. Further, he observed that bonobos willingly engage in sexual behavior with both sexes. Sex, according to de Waal, was “the key to the social life of the bonobo”: anything that arouses the interest of more than one bonobo at a time resulted in sexual contact. The Social Glue theory comes in to play particularly when young female bonobos attempt to enter a new social group. Young females will try to establish relationships with senior female bonobos of a new group through sexual behaviors. If these advances are accepted by the senior, the younger female is gradually accepted into the group.
Further evidence for the Social Glue Theory in bonobos has been gathered by Clay and Zulberbuhler (2012). The researchers reasoned that lower-ranked females would solicit more sexual interactions than higher-ranked females in an attempt to develop bonds. Results supported this prediction; with low-ranked females, SSBs provided a means to develop alliances with more dominant females. The researchers believe this can subsequently strengthen an individual’s social position within a group, thus increasing both access to resources and reproductive success.
Examples of homosexual behavior serving as a way to strengthen social bonds have been studied in non-primate species as well. American bison males that self-assemble bachelor groups have demonstrated reciprocal SSB outside of the their breeding season. Research suggests this inter-male courtship neutralizes aggression and promotes low-risk coexistence. Further, the homosexual behavior observed in males seems to lead to the formation of “friendships” that were maintained for longer than half a year. It is also believed that the formation of these male herds, particularly among younger, smaller males, could reduce predation risks — a clear benefit for survival. (Vervaecke & Roden, 2006).
In these examples of Social Glue theory, sexual behavior (including but not limited to homosexual behavior) among members of a species results in stronger social bonds. These bonds, in turn, open doors which lead to a greater likelihood of survival and reproductive success. Such a trade-off suggests a benefit of sexual behavior that is not exclusively related to the opportunity to reproduce. According to the Social Glue Theory, homosexual interactions serve to increase the fitness of individuals, and these behaviors are separate from reproductive behaviors. Often, the Social Glue Theory goes hand-in-hand with the Dominance Theory.
Dominance Theory
In the Dominance theory, homosexual behavior is used as a form of intrasexual competition. Individuals use SSB either to establish and reinforce dominance hierarchies, or to reduce the reproductive success of competitors, thereby increasing their own. The former was seen among bonobos (de Waal, 1995; Clay & Zuberbuhler, 2012): lower-ranking females attempted to engage in SSB with dominant females, and the dominant females had the power to accept or reject these advances. The Social Glue and Dominance Theories do not always coincide, however. Vervaecke and Roden’s (2006) research with the American bison found no evidence that homosexual behavior was correlated with dominance. Conversely, the Dominance Theory may apply to a species that does not demonstrate Social Glue Theory interactions.
SSB plays an important role in establishing dominance among the broad-horned flour beetle. Lane and colleagues (2016) found that male flour beetles often rapidly established fixed active (mounter) and passive (mounted) SSB roles. Further, the stability of these roles significantly impacted whether or not aggression occurred between a pair. In this species, when males regularly switched roles during SSB, aggression was more likely to occur and occurred at a significantly higher rate than in pairs with fixed roles. If one male displays homosexual behavior and is not challenged by the other male, dominance is resolved and aggression is unlikely to occur, but if both males attempt to mount each other, the interaction may escalate to a physical contest.
If a male is more aggressive or has an alpha-male phenotype, he is likely to take the role of the active partner, demonstrating dominance via SSB. If another male is in some way inferior, it may be to his advantage to allow the dominant male to mount him. An inferior male would be more likely to lose in a physical contest and is thus spared when mounted. An inferior male has a better chance to reproduce with a female if he is not physically harmed by a dominant male.
Nevertheless, results show dominant males achieved higher mating success and were more likely to court females (Lane et al, 2016). Thus, SSB can be considered equivalent to ritualized fighting displays, acting as a non injurious way to resolve dominance. Additionally, it gives inferior males a chance to mate without being injured.
It is important to note, however, that other studies on flour beetle SSB found conflicting evidence. Evidence gathered by Levan and colleagues (2009) did not support the dominance theory among flour beetle males. Instead, their study found that paired males who frequently switched roles (active vs. passive) had no significant difference in reproductive performance with females. Further, their study showed there was no significant difference in body size between the males.
Mate Choice Copying Theory
Mate choice copying occurs when one individual becomes more or less likely to mate with another individual after observing that individual mating with a third individual. One hypothesis is that this occurs when the first individual changes its choice after seeing the second individual’s mating success. If the observed interaction was successful, there will be an increase in the likelihood of an attempted mating by the observer. This is because a successful mating demonstrates that the observed individual is a preferred mate by others. Likewise, there will be a decrease in the likelihood of mating if the interaction was unsuccessful and the observed individual was dismissed.
The mate choice copying strategy has been suggested as an explanation for the presence of homosexual behavior between male cave-dwelling tooth carp (Tobler et al, 2005). Results of this study showed that smaller satellite males frequently engaged in nipping, a sexual behavior, with both males and females. Evidence has shown that this behavior is not the result of an inability to discriminate between the sexes. Large males, conversely, engaged in nipping exclusively with females. These researchers propose that this is a form of mate choice copying, where small males’ exhibition of sexual behavior — even when directed at another male — may increase their attractiveness to females. This strategy may work because cave-dwelling tooth carp males tend to have lower aggression levels in general: large males do not attack small males that attempt SSBs. In other species, this is not always the case. Nevertheless, support for the mate choice copying theory has been observed in other species.
Homosexual behavior in female American bison can also be explained in part through the mate choice copying theory (Vervaecke & Roden, 2006). The researchers suggest that, in mimicking heterosexual behavior, female bison are “enhancing the perceptivity of reluctant heterosexual partners.” When a female is being mounted, the males always direct their attention to the act. Further, results showed that dominant males will immediately mount a female that had been mounted by a lower-ranked male. Female SSB could be explained as an overflow of the mate choice copying. Despite this theory, a confound was noted: if female SSB is at attempt to stimulate male mating, it is expected that there would be a correlation between the number of females tended and the number of homosexual dyads, but this was not the case.
Though the mate choice copying strategy explains some aspects of SSB, the end goal is to increase the reproductive success of the individuals involved. Thus, individuals across species that engage in these SSBs are not demonstrating exclusively homosexual behaviors, nor are they showing a preference for homosexual behavior over heterosexual behavior. If the SSBs are strategies to increase appeal, the persistence of the behavior is not evolutionarily paradoxical.
Evolutionary Byproduct Theory
The evolutionary byproduct theory is the last explanation of SSB in non-human animals that I will be discussing. This theory states that SSB persists because it is connected to a separate trait that is positively selected upon. The literature that provides evidence for this theory is vast. A select few examples will be included in this discussion.
First, I will return to the flour beetle. Observations of this species suggest that SSB allows males to expel older, lower quality sperm (Levan et al, 2008). Evolutionarily, high sperm production is a selective advantage: high-producing male flour beetles can successfully mate with up to seven females in fifteen minutes. These males, however, lack the physiological capability to down-regulate sperm production when the female population is less dense. Thus, the males will use SSB as a mechanism to discard old sperm before to mating with a female. By doing this, the males will expel higher quality, more successful sperm during heterosexual matings. This is another situation where SSB is observed when the end goal is to heterosexually copulate.
In Japanese macaques, SSB is believed to be a byproduct of high sex drive and genitalia development, both of which are positively selected for (Vasey et al, 2005). Evidence shows that female macaques frequently mount other females. Despite initially being described as “disoriented, clumsy, and awkward” attempts at mimicking male mounting, female SSB is distinctly different. Research has shown that macaque female-SSBs are performed to achieve pleasure that cannot be achieved in heterosexual pairings because of male anatomy constraints. The study suggested that some females do show a preference for female partners despite the presence of males. Unfortunately, these reports did not include data about the reproductive success of these individuals. Despite these exceptions, most female macaques go on to reproduce with males.
Another study suggests that SSB in male damselflies is a maladaptive consequence of strong selection on a male’s ability to alter his choice on varying female color morphs (Van Gossum et al, 2005). Reproductive decision flexibility is beneficial for one sex when a particular genotype of the opposite sex has high fitness in certain environments but low fitness in others. In a changing environment, the species’ ability to quickly adjust preferences for the more successful mate genotypes is advantageous. The increased plasticity of male damselflies allows them to adjust their preference to the more successful female genotype. This high plasticity may be over- exaggerated as SSB. An abundance of males in an environment would suggest that males are the more successful “morph.” Thus, other males would attempt to copulate with them. In this instance, a positively selected-for adaptation has overflowed to cause SSBs. The authors propose that this behavior has been maintained because the advantages of flexible frequency-dependent choice patterns outweigh the disadvantages of SSB.
Summary of Non-Human Theories
Each of the theories discussed above have presented evidence which explains the presence of various SSBs seen across many non-human animal species. All examples presented successfully defended the persistence of SSBs, despite the apparent Darwinian paradox.
Nevertheless, a majority of these theories cannot be applied to species that display exclusively homosexual behavior. In every instance discussed (with the exception of some Japanese macaques), the end goal was reproduction and individuals mated with the opposite-sex members.
With this in mind, it is challenging to use any of these explanations of SSB to describe homosexual behavior in humans. Though sexual behavior and preference among humans constitutes a broad spectrum, a significant portion of the population demonstrates exclusively homosexual behaviors and preferences. Given this information, I turn now to research studies that have focused exclusively on human homosexuality. These studies attempt to explain the continuous presence of SSB despite the apparent negative impact it has on reproductive fitness.
Same-Sex Sexual Behavior Theories as Applied to Humans
Having discussed non-human theories of homosexual behavior, I will now discuss three theories that have attempted to explain the persistence of human homosexuality: Balanced Polymorphism Theory, Increased Female Fecundity Theory, and Maternal Immunity Theory.
Research on these theories have looked exclusively at human subjects and focus largely on genetic and maternal factors. Further, these theories are not mutually exclusive and researchers admit that their findings are only small pieces of a much larger puzzle.
Balanced Polymorphism Theory
Balanced Polymorphism is an evolutionary theory, first proposed by Hutchinson in 1959, which works on the basis of a heterozygotic advantage — the advantaged acquired by combining two extreme genotypes. A common example of heterozygotic advantage is seen with sickle cell anemia. In 2000, E.M. Miller applied this theory to homosexuality, stating that alleles which partially prevent androgenization in male fetuses would be associated with a homosexual orientation. These alleles theoretically control for and prevent excessive aggression and psychopathy, features that make males unattractive to potential mates. Having none of these alleles would result in hyper-masculinization where as having all of these alleles would cause homosexuality. Therefore, heterosexual carriers that have some of these alleles have the reproductive advantage, making them more attractive to potential female mates.
Santtila and colleagues (2009) tested this theory in a study that compared the reproductive success of pairs of brothers. Maintaining that homosexuality resulted from androgenization-preventing alleles, the hypothesis was that heterosexual men with homosexual brothers (HeHo) would have a more of these alleles than heterosexual men with heterosexual brothers (HeHe). Thus, HeHo men would have a reproductive advantage that would outweigh the reproductive cost of homosexuality. The results of this study, however, did not provide support for Miller’s theory: no significant differences between HeHo and HeHe men were found in any of the fitness-linked variables. Despite these findings, the genetic component of human homosexuality is still widely researched and has brought forth a number of additional theories.
Increased Female Fecundity Theory
The Increase Female Fecundity Theory also attempts to explain the presence of human male homosexuality through genetics. This theory states that females related to homosexual probands along the maternal line show increased fecundity, which is a large reproductive advantage. This explanation suggests there are genetic factors linked to the X chromosome and that male homosexual orientation would not be not selected against.
Empirical evidence that supports the Increased Female Fecundity Theory has been collected (Camperio-Ciana et al, 2004). After surveying 98 homosexual and 100 heterosexual men, researchers found that homosexuals had a greater number of homosexual relatives on their maternal pedigree line than on their paternal pedigree line. The families of heterosexual men did not display this pattern. Further, results showed that the maternal relatives of homosexual men had higher fecundity than the maternal relatives of heterosexual men. Though this study has been criticized for its small sample size and the homogenous population, the results have been replicated by other researchers.
A follow-up study was conducted in which the family trees of 250 male probands, 152 of which were homosexual, were examined (Iemmola & Camperio-Ciani, 2009). The results of this study confirmed those of Camperio-Ciani and colleagues from 2004: researchers saw a significant fecundity increase among male homosexuals’ maternal relatives. Additionally, the results of this study found no evidence of increased paternal fecundity, supporting a partial connection of male homosexuality to the X chromosome. Despite these findings, a separate study identified larger family sizes among homosexual males’ paternal lines, though these results have not been replicated.
Maternal Immunity Theory
The final theory that I will discuss is the Maternal Immunity Theory. This theory states that, as a mother gives birth to more male offspring, she will progressively develop an immunity to an antigen present in male foetuses. This male-specific, Y-linked minor histocompatibility (Y-
H) antigen impacts the sexual differentiation in the brain of the feotus. After a number of pregnancies with males, the antigen has an increased effect on the mother’s immune system. This immunity, then, causes a reduction in the sexual differentiation of her male offspring.
One study found a correlation between the number of older brothers a man had and the likelihood that he identified as homosexual (Blanchard et al, 1997). Results showed that each additional older brother increased the odds of homosexuality by approximately 33%. The pattern observed in the fraternal birth order reflects the progressive immunization of mothers to the Y-H antigen. Additionally, this study found no connection to the number of older sisters on homosexuality in males, demonstrating probable involvement of the male-specific antigen. The results of this study were replicated by Camperio-Ciani and colleagues (2004). Homosexual men in the study had an excess of older male siblings compared to older female siblings, and that the birth order distribution of homosexuals was significantly different from that of heterosexuals in the sample.
Summary of Human Theories
The three theories presented above focus exclusively on homosexual behavior in human males and attempt to explain the persistence of SSB in the population. These theories rely largely on genetic evidence, which was not measured in most non-human animal studies. This genetic basis gives these theories a level of credibility not seen in the others, as genetic manipulation has the potential to establish causation, though it has not yet been done. Additionally, none of these theories have been applied to any non-human animal species, although the genetic information of other species may be insightful.
Though no single theory has yet explained the persistence of homosexual behavior in humans, pieces of evidence are beginning to come together to explain this true Darwinian paradox. While non-human SSB has been shown to frequently increase an individual’s ability to reproduce, no such benefit has been demonstrated in humans. Camperio-Ciani’s research has provided some evidence of an evolutionary advantage (increasing maternal fecundity), but in general this human behavior remains paradoxical.
Implications of the Research
The main goal of the research conducted on SSB in both human and non-human animal species is to understand why these behaviors have persisted in so many species, despite having no clear reproductive advantage. Evolutionary principles state that these behaviors must be advantageous, and many researchers have worked to uncover these advantages.
The theories under review in this paper have successfully demonstrated that there are advantages to SSB in many non-human animal species. While the act of copulating with a member of the same sex does not directly increase an individual’s mating success, numerous advantageous byproducts of SSBs have been identified. Engaging in SSBs can increase an individual’s access to resources such as food, protection, social support, and parental care.
Additionally, SSB can increase the likelihood or success of mating with an opposite sex member. It is unlikely, however, that engaging in SSB will bring a single species more than one or two of these benefits.
SSBs in non-human animals play different roles in the social environments of various species. Similarly, there is great variety in the physical performance of SSBs. The range is vast: some behaviors such as mate-pairings show no explicit sexual behavior, whereas the bonobos engage in female-female front-facing sexual behavior, not seen in other non-human species.
SSBs do not appear to be homologous across species: the presence of SSBs in distantly related species arose independently as unrelated events. Considering this, it is likely that the genetic basis of SSBs across species is highly varied. Therefore, one single theory cannot explain all SSBs in non-human animal species and theories are not generalizable to all non-human animal species.
For decades, researchers have also attempted to generalize SSB to humans through the knowledge acquired through animal model. One goal of this research was to attempt to “normalize” homosexual behavior in humans: if non-human animals engage in SSB, it must be a natural behavior and humans should not be criticized for it. Despite the good intentions of this research, my review has demonstrated that this generalization is not scientifically appropriate.
While there are differences in SSBs between many non-human animal species, these differences are even greater between non-human animal species and humans. As previously stated, human sexual behavior falls upon a spectrum ranging from “exclusively heterosexual” to “exclusively homosexual.” Such exclusive homosexuality has not been observed in other species — especially to the extent that it is seen in the human population. SSB in non-human animal species is largely a strategy used by an individual to increase their own reproductive success. As of now, no study has shown this to be true of human sexuality. The goals of SSBs observed in humans are not the same as those of non-human animal species. The theories that attempt to explain the behaviors, therefore, are not universal.
There is not yet adequate evidence to explain the persistence of homosexuality in humans. Thus, future studies are required in order to reach a conclusion. With this said, there are improvements that could be made to the studies in this review that would further our understanding of homosexuality in humans. One major criticism of all human homosexuality studies reviewed here is their explicit focus on males. In fact, an extreme majority of the human homosexuality studies focus only on males’ behavior. Certainly, homosexuality exists among human females. It is likely that the origin of male homosexuality is different from female homosexuality in humans, considering the Maternal Immunity Theory. Even if male homosexuality is fully explained by that theory, it does not explain female homosexuality.
It is important, however, that the motivation for continuing this research is conducted for ethical purposes. Many empirical studies have focused on identifying variation in the physiology of homosexual humans: differences in brain regions between homosexuals and heterosexuals are of particular interest in recent years. Such studies — though valuable for general knowledge acquisition — have the potential to “otherize” homosexuals. It is important that future studies proceed with the goal of explaining the paradox rather than demonstrating differences.
Future studies should also consider bisexuality a separate category from homosexuality. Most studies in this review combined them, and this could be a confounding factor. For example, the Maternal Immunity Theory may show bisexuality is prominent among the older brothers of exclusively homosexual males. Further, in the Increased Female Fecundity Theory, bisexual males’ maternal lines may have a differing degree of fecundity than homosexual males’ maternal lines. Lastly, bisexual individuals, in theory, are more likely than exclusively homosexual individuals to produce their own offspring. Some of the non-human animal studies may be applicable to bisexual humans. One hypothesis, then, may be that bisexuality is evolutionarily advantageous and that homosexuality is an over-exaggeration of this. Thus, the benefits of bisexuality would outweigh the reproductive costs of homosexuality.
A great deal of research must still be done to uncover the evolutionary advantages of human homosexuality. There is no trait in any organism that does not face selective pressures, homosexuality included. If scientific research is able to explain the evolutionary benefits associated with homosexuality in humans — as it has done with non-human animal species — there may be great societal benefits. This type of empirical evidence may be a small step in the direction of accepting homosexuality, particularly in societies that do not yet.
Glossary of Theories
Non-Human Theories
Same-Sex Pairing Theory: Socially monogamous species will engage in extra-pair copulations with opposite-sex members and then raise offspring with a same-sex partners.
Social Glue Theory: SSBs are performed to bring a social community closer together, diffuse tension and aggression, and increase individuals’ access to resources.
Dominance Theory: SSBs are used to establish dominance hierarchies and avoid physical conflict between same-sex competitors.
Mate Choice Copying Theory: Individuals will engage in SSB to make them more appealing to opposite-sex mates.
Evolutionary Byproduct Theory: SSBs have arisen as an exaggeration of an evolutionarily beneficial trait; or SSBs are connected to positively selected traits with benefits that outweigh disadvantages of SSB.
Human Theories
Balanced Polymorphism Theory: Heterozygosity of feminizing alleles makes males more appealing mates to females — an evolutionary advantage. An excess of these alleles causes homosexuality.
Increased Female Fecundity Theory: Homosexuality in males is correlated with increased fecundity in their females of their maternal lines. The disadvantage to the homosexual individual is negligible compared to the benefit of the maternal line females.
Maternal Immunity Theory: Additional male births by a mother increase her immunity to a male specific antigen that promotes androgenization of the male fetus brain. Lower androgenization causes homosexuality in males.

Note: Eukaryon is published by undergraduates at Lake Forest College, who are solely responsible for its content. The views expressed in Eukaryon do not necessarily reflect those of the College.

References

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Tobler, M., Wiedemann, K., & Plath, M. (2005). Homosexual behavior in a cavernicolous fish, Poecilia mexicana. Zeitschrift fur Fischkunde, 7 (2), 95-99.
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