Homosexuality: A Paradox of Evolution
By Preston Hunter, 1994.
This paper examines homosexuality in the animal kingdom. The seeming paradox of the existence of homosexuality is delineated. The observed reality of animal homosexuality is described through examples as well as evidence for its biological causes. Possible theories of how homosexuality could have evolved and how it might be adaptive are examined.
Introduction: The Paradox
Sex is important but very inconvenient. Biologically speaking the word "sex" has two important and distinct meanings, one for each of the fundamental requirements of sexually reproducing species. Sex is a state: the separation of a species into two distinct genders, male and female (thus "hetero- sexuality"). Sex is also an action: the combined behavior of both a male and a female individual which results in the combination of genetic material from both.
While some plant and animal species produce both asexually and sexually, many species dispense with sex altogether and are strictly asexual. Through asexual reproduction organisms essentially clone themselves. All individuals in a population share an identical genetic makeup. This form of reproduction has a variety of advantages: It's behaviorally, physiolocally, and morphologically simpler; therefore there is less that can go wrong. Asexually reproducing organisms that are well-adapted to a particular environment need not dilute their adapted genes with from a potentially less-adapted other parent. Asexual reproducers don't have to expend the energy to search for, attract, and couple with a mate. Nor do they need to evolve specialized behaviors and physical features for doing so such as massive antlers, complex bird songs, or intimidating growls.
For reasons such as these Ridley (1993, p. vii) called the very existence of sex a "profound puzzle" which most creatures would be better off without. Why? Because sex makes possible the perpetual recombination of genetic codes that drives evolution. While evolution is the foundational force of biology, sex is the mortar that holds that foundation together. So advantageous is sex that despite the high biological cost of sex most macroscopic animals, including 99% of all vertebrates, reproduce sexually (Sites, 1993).
It is because sexual reproduction is so indispensable and so zealously selected for that the existence of homosexuality is a paradox. Homosexuality would seem to be the antithesis of sexual reproduction, the enemy of evolution. One must wonder: why wouldn't evolution quickly select against behavior which diverts an animal from sex and reproduction?
Yet despite seeming unlikelihood homosexuality does exist. Moreover, homosexuality is more than an insignificant quirk in one or two animal species and a social aberration in humans. It is widespread in animals. Why?
To answer why homosexuality exists we must first identify it. "Homosexuality" has two principle meanings which are distinct but related. Homosexuality is sexual activity between two individuals of the same gender. This includes same-sex coitus or genital contact, whether ventro-ventral, dorso-ventral, or otherwise and whether male-male or female-female.
Homosexuality is also the innate sexual preference for one's own gender or the biological urge for same-sex coitus. While much is said about the distinction between homosexual desires (or innate sex drive) and homosexual behavior in humans, this distinction is of limited or negligible significance in non-human populations in which we infer "desire" (be it instinctive or learned) primarily from behavior. The distinctive gulf between human desire/biological impulse and behavior (Marmor, 1980) is attributed to human social mores and cultural constraints (Dickemann, 1993), which epigenetic forces can also be accounted for with biological/evolutionary explanations (Alcock, 1984, pp. 522 & 524).
In summary, homosexuality is both the biological drive for same-sex coitus as well as the performance of same-sex coitus. Homosexuality (as defined by both definitions) has been observed in both males and females in both human and non-human populations. While it is difficult, if not impossible, to separate innate urge from action in non-human animals, the distinction between these two definitions will be important later as we discuss possible evolutionary explanations for homosexuality.
We will refer here to non-human fauna simply as "animals," in keeping with the human use of the word in this context. Human familiarity and understanding of the importance of heterosexuality renders the notion of animal homosexuality counterintuitive. Even today many who oppose human homosexuality on religious or other grounds call it unnatural, citing the animal kingdom as evidence. This is not a new argument. Some lines from Ovid's ninth book in the Metamorphosis (first century B. C.) contain these observations:
Cows do not burn with love for cows, nor mares for mares;
The ram is hot for the ewe, the doe follows the stag.
So also do birds mate, and among all the animals
No female is seized with desire for a female (Boswell, 1980).
Early Greek writers who defended homosexuality responded with pride in their supposed difference from the animal kingdom, saying, "All irrational animals merely copulate, but we rational ones are superior in this regard to all other animals. We discovered homosexual intercourse. Men under the sway of women are no better than dumb animals" (1980). Despite their differing philosophies, both sides of the debate were equally unaware of homosexuality among animals. This popular unawareness has continued to this day, (Paul, 1980; Weinrich, 1982) as characterized by a Thirteenth Century denunciation of homosexuality which stated flatly but falsely that, "No dumb animal is drawn to this evil" (Boswell, 1980).
In modern times, with science in high regard, some people use a variety of biological and evolutionary concepts to "prove" the unnaturalness (and, by implication, the invalidity) of homosexuality. The ultimate source of these arguments is social tradition and "religious precepts" (Weinrich,1982) rather than observable data. In truth, homosexuality occurs naturally and can be explained through evolutionary principles.
In modern times the scientific community has observed and thus become aware of widespread non-human homosexuality. Animals from "throughout the animal kingdom" have been observed engaged in same-sex courting and same-sex copulation (Alcock, 1989, p. 525) and "apparent homosexual behavior increases as we ascent [sic] the taxonomic tree toward mammals" (Denniston, 1980) and then towards primates.
To account for homosexuality (or any phenomenon) using evolution it is necessary that it be natural, i.e it must occur naturally without human influence. Thus far we have only used animal behavior to illustrate the naturalness of homosexuality. In essence: there are homosexual animals in nature; therefore homosexuality is natural.
That homosexuality is biological and natural in origin is further demonstrated by knowledge of the biological causes of homosexuality in humans. Human homosexuality was long ignored by biologists and relegated almost entirely to study by psychologists and social scientists because it was considered primarily a human and "unnatural" trait. Study has now spread to the realm of physical science and biology, as researchers probe the biological, developmental, and even genetic causes of human homosexuality.
The current openness and frequency with which homosexuality is discussed is a recent development (brought about by the discovery and rapid spread of Acquired Immune Deficiency Syndrome, or AIDS) (McWhirter, 1990), Human homosexuality is not so new. Human homosexuality has existed in every time and culture we have record of (Bullough, 1990) and people have long wondered about its causes.
Until recent decades homosexuality was widely considered a psychological illness. In the 1860's Karl Heinrich Ulrichs was one of the first scientists to formally postulate biological causes for homosexuality, positing the idea, based partially on his own firsthand experience, that homosexual instincts are inborn and therefore natural (1990). Ulrichs called homosexuals a " 'third sex'--a woman's mind in a man's body, and vice versa for women" (Rist, 1992). Sigmund Freud advanced theories of latent homosexuality (Bullough, 1990), but homosexuality was labeled an abnormality and mental illness by the American Psychiatric Association until 1973, when the APA dropped it from its official disease list (McWhirter, 1990). But despite the ideas of Ulrichs and a few similarly minded early theorists, homosexuality remained largely in the realm of psychology until only recently.
Early in this century biological causes for human homosexuality were speculated -- perhaps "some combination of body chemicals"(Broderick, 1977) -- but the idea that there could be biological causes was generally balked at. People wondered (as some still do), if a person's biology is tied to his genetic code, wouldn't this mean that homosexuality has genetic causes? The words of one skeptic are not unusual:"From a population-genetics viewpoint, the idea of a gay gene is a bit ridiculous. If there is a specific gene or set of genes that predisposes one to homosexuality then, assuming that homosexuals reproduce at a lower rate than the population in general, the gene would slowly disappear from the population. Since there are thousands of years of documented human homosexuality, a gay gene seems very unlikely" (Dave J727, 1993).
This opinion, though very popular, fails to take into account the following genetic principles and observed phenomena:
Having illustrated the existence of general animal homosexuality and the practicality of biological/hereditary explanations, it is now possible to discuss what evolutionary mechanisms might account for this seeming paradox.
From the existence of homosexuality in nature and from its frequent taxonomic clustering (i.e. more closely related organisms display increasingly similar homosexual behavior) we can infer that homosexuality has evolved. But because no one has been able to observe the evolution of homosexuality first hand we can only theorize as to how it happened. Based on behavioral studies of homosexuality in humans and animals a number of models and explanations have been proposed for how this seemingly gene-stopping practice has evolved.
There are two main types of explanations for how and why homosexuality has evolved in animals:
Through evolution males have acquired behavior compelling them to compete with other males for wider, more frequent copulation and thus greater reproductive success. The side effect model of homosexuality proposes that "hypersexuality" has consequently evolved in males which causes them to strive for a maximum number of copulations with a wide range of partners and that their imperfect sex recognition when in such a state of heat leads them to mount males as well as females.
This theory is well-supported by observations of sexual behavior in the animal kingdom. As Denniston points out (1980), animal "homosexuality" is a misnomer, and really refers to bisexuality or"ambisexuality," or the behavior of mounting with animals of both sexes. Only in a few isolated species are any animals even occasionally preferentially homosexual (as in some primates, including Japanese macaques; see Small, 1993, p. 145; West, 1977, p. 116). The side effect model of homosexuality can explain many same-sex sexual behaviors, such as male mallard ducks and stickleback fish courting each other when deprived of females (West, 1977, p. 43; Denniston, 1980) and the frequent homosexuality in all-male groups of mountain gorillas (Yamagiwa, 1987). This model, suggesting evolution has favored a greater sex drive in males, is supported by observed masturbation rates: There are no known human cultures in which females masturbate more frequently than males. The same is true for non-human primates. Rhesus monkeys raised in isolation from their parents (to prevent learned behavior) showed higher masturbation rates in males, suggesting an innate difference in sex drive (Green, 1978).
The side effect model also helps explain why animals can be observed attempting copulations outside of their species, such as the European toad photographed clasping a "human finger as it would a female of its species" and an Australian beetle observed attempting copulation with a beer bottle (Alcock, 1984, p. 352). In captivity animals deprived of their natural mates will seek to copulate with unusual partners, such as the male brown hyena observed to frequently copulate with its water bowl, the chimpanzee which formed "a sexual relationship with a cat, and an immature female gorilla [which] showed pseudomale behavior toward a dog (Grier, 1984, p. 626). Animals clearly have powerful sex drives which may cause their copulatory behavior to be somewhat indiscriminate, an understandable side effect because animals don't copulate having made a conscious decision to pass on their genes.
The side effect model is inadequate for explaining many other homosexual behaviors, however. This model predicts that "the value that males place on sexual variety for its own sake [to ensure multiple copulatory partners, should] be absent or much reduced in" females (Alcock, 1989, pp. 525-527). Female homosexuality in animals is certainly not "absent," as has been illustrated with many examples above, ranging from gulls and budgerigars to cows and primates. To what degree (if any) homosexuality is reduced in females is debatable.
The side effect model also has diminished applicability in non-mammal vertebrate species. Because these do not generally incubate offspring internally during pregnancy females can potentially increase reproductive success through multiple matings (as is the case with polyandrous phalaropes and Galapagos hawks). While it is true that biological cost is still associated with mothering (if only to produce eggs), it is evident from both non-male and non-mammal homosexuality that the side effect model does not fully explain general animal homosexuality.
Homosexuality the adaptive behavior: One reason the side effect model is inadequate by itself is that homosexual behavior is clearly intentional for adaptive reasons in some animals (not merely accidental during the fervent reproduction-maximizing pursuit of partners as the side effect model suggests). There are various reasons suggested for intentional homosexual behavior. Many have to do with an organism's reducing the copulations and gene dispersal of competitors, to ensure that a greater proportion of the next generation will be its own.
Male-male homosexuality in non-human primates is frequently "seen in the context of dominance rank and high social tension or stress than in a sexual context" (Yamagiwa, 1987; Yamagiwa, 1992). This has also been described as the reason behind male same-sex mounting and copulation in mountain sheep (Weinrich, 1982; Denniston, 1980). Dominating other males through homosexual coitus could be akin to the typical nonsexual aggression used against other males to decrease their share of the females.
"Surrogate sexual satisfaction" is another form of intentional adaptive homosexuality. This is the practice of copulating with an individual of one's own gender to cause that individual to be satisfied, so it won't seek other partners. This appears to be a motive for female-female copulation in langurs (Small, 1993, pp. 146-147) and male ten-spined sticklebacks fishes (Denniston, 1980).
Other possible explanations of how homosexuality could be advantageous include: Long term male-male and female-female homosexual relationships in primates such as Japanese macaques (Mehlman & Chapais, 1988), mountain gorillas (Yamagiwa, 1992), and gibbons (Edwards & Todd, 1991) result in lower stress and tension and/or better grooming for the individuals involved, all of which would positively influence an organism's health and prolong its chances for reproduction.
Homosexuality can also serve to make and maintain beneficial social alliances between individuals, such as in primates (Small, 1993, pg. 147) and gulls which typically mate for life but have lost a mate and need another one, even of the same gender, to safely raise their young (Davies, 1991).
In many species (such as tree shrews, squirrel monkeys, and many types of macaques; see Mitchell, 1979, p. 416) females engage in homosexual copulation when "they are in a new environment or undergoing stress." This could be an innate mechanism for limiting overpopulation or wasted energy on reproduction and fetus-production when environmental conditions make survivalship unlikely. One difficulty with applying the "density dependent population control" theory more broadly is that under "conditions of high density the frequency of any gene disposing toward homosexuality would be drastically reduced, since individuals bearing it would not breed. Consequently, the representation of such a gene would decline in subsequent generations," diminishing its effectiveness as a selected-for population control mechanism (Kirsch & Rodman, 1982).
The theory of kin selection has been cited by many. Kin selection theory explains why some animals will display "altruism," such as squirrels which give warning cries of predators to help their relatives even while endangering themselves or unmated birds which help rear their younger siblings. Close relatives such as nieces and nephews also carry on an individual's gene line. This theory has been applied to homosexuality and it has been argued that homosexuals can help tend and see to the survival of their close relatives such while not having to undergo the disadvantages of parenthood themselves, thus increasing their close relatives' survivalship and offsetting their own non-parentage (Ruse, 1981; Denniston, 1980; Kirsch & Rodman, 1982). One problem with this theory, however, is most animals which engage in homosexuality do quite well meeting their quota of heterosexual sex, and may even enjoy greater than average dissemination of their genes for some of the reasons discussed above.
Perhaps because of its history of being socially objectionable in human culture, homosexuality in the animal kingdom is poorly recognized and understood relative to its actual frequency. Relatively recent is the scientific understanding that homosexuality is (a) present -- even widespread -- in the animal kingdom, (b) it has biological causes, and (c) it can be accounted for by evolutionary theory.
Animal studies can do a great do much to illuminate the human condition and vice versa,but sweeping parallels cannot be drawn between man and animals. Our unique social complexitites create such an ocean of differences that direct comparisons and conclusions about human behavior are impossible. Whatever we learn about animal behavior or biological causes of human behavior, our possession of consciousness gives us a certain indemnity from behaving for purely biological reasons. The studies and conclusions discussed in this paper indicate nothing about the appropriateness of any behavior in humans.
Exactly how widespread homosexuality is in the animal kingdom is still unknown. Researchers have usually not looked for it or they have dismissed it as an aberration when they encountered it. The relatively shallow pool of knowledge pertaining to animal homosexuality makes it difficult to arrive at solid conclusions about how it evolved and exactly what adaptive functions it plays in animal populations and reproductive success. As complex an issue as all sexuality is, however, the factors causing and governing homosexuality are doubtlessly complex and many.
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