Homo Mysterious: Evolutionary Puzzles of Human Nature (21 page)

Several other proximate causes have been proposed and deserve mention. For example, there appears to be a birth-order effect: One factor that evidently predisposes toward a male being homosexual is if he has older brothers. This association appears somewhat robust, which in itself is notable, given that even correlations—never mind causation—have been so elusive.
⁷¹
The “fraternal birth-order effect” might be due to the impact of the uterine environment on a developing fetus, since it seems likely that when a woman repeatedly bears a male fetus, she accumulates antimale antibodies, which could modify the developing brain of subsequent children.
⁷²
But no one knows if this really happens.

It has long been suspected that hormonal factors are also implicated, but their actual role, if any, remains unproven. It does seem clear, however, that among male homosexuals, sexually relevant brain regions respond to a derivative of testosterone in a manner not found among male heterosexuals.
⁷³
Significantly, perhaps, a comparable estrogen derivative activates parallel brain regions of lesbian but not heterosexual women.
⁷⁴

There have also been numerous reports—many of them conflicting—regarding brain differences between straight and gay people,
⁷⁵
notably the claim of differences in structure of the hypothalamus.
⁷⁶
In addition, it is well established that hormones—especially steroids such as estrogen and testosterone—are typically important in potentiating sexual behavior, including same-sex mounting. This is especially true for females, while the impact of testosterone and its various chemical relatives seems to be more complex.

One problem with these and other such findings is that even if there are consistent neuroanatomic and/or hormonal distinctions between gays and straights, and even controlling for likely questions as to whether a subject was
^xiv
“really” homosexual (as opposed to bisexual, for example), it isn’t possible to conclude with confidence
that such differences are the cause of homosexuality, as opposed to being a result of many years of homosexual practice. In addition, let’s imagine that gay men are that way because of a difference in their hypothalami, or some other brain region such as their super-chiasmatic nuclei. This would still beg the evolutionary question: Why has natural selection favored the existence of such mechanisms in the first place?

Or, stated slightly differently: Why do such differences persist in the human population, given the fitness decrements we have already discussed?

We also need to consider a possibility we shall shortly examine with regard to the evolution of religious belief and of the arts, namely, that homosexual behavior might be not so much
mal
adaptive as simply
non
adaptive. That is, it might not have been selected for, but persists instead as an incidental by-product of other traits that presumably have been directly favored.

One of these traits could have been the simple (actually, not so simple) yearning to form a pair bond, seeking emotional or physical gratification. As to why such an inclination would exist at all—that is, why human connections are perceived as pleasurable—the answer may well be that historically (and prehistorically) it has often been in the context of an ongoing pair bond that individuals were most likely to reproduce successfully. And this, in turn, would be due to the fact that in a species such as
Homo sapiens
, which gives birth to helpless young that require substantial parental investment over many years, evolution would have equipped people with a tendency to form such bonds. If a suitable heterosexual partner isn’t available—as we have already considered in the case of prison populations—it is imaginable that a same-sex relationship will be consummated instead.

Carrying this a step farther, it is now well established that same-sex pairings are relatively common among animals, especially many bird species. The typical situation in such cases is that female–female couples form when there is a shortage of males. These avian “lesbian” couples often reproduce, so clearly their “homosexuality” is not exclusive; given the well-known proclivity
for males to take advantage of opportunities to copulate outside their pair bond as well as within it, no one should be surprised that females paired with females also get fertilized.
⁷⁷
It is noteworthy that such females don’t reproduce as successfully as do male–female pairs, but they always do better than solitary individuals, since—especially among birds, with their high metabolic rates, and hence, their need for constant provisioning of nestlings—two adults have a definite advantage over “single parents” when it comes to rearing a brood.
⁷⁸

An interesting example comes from a colony of Laysan albatrosses, on Hawaii, where 31% of all couples consisted of two females. In this case, such pairings were remarkably long term: several female–female couples staying together for at least 8 years, with one pair remaining “a unit” for 19 years! These animals face an interesting dilemma, since they normally lay just one egg per year, and that is all that an albatross nest can accommodate. When two females cohabit, they continue to make a one-egg-sized nest even though each female commonly lays her own egg. The extra one typically rolls out, isn’t incubated, and doesn’t hatch. It may be that the long duration of Laysan albatross female–female mate-ships is itself a prerequisite for such a social system to exist at all, since it provides an opportunity for a female whose egg was excluded one year to recoup and be successful during the next.
⁷⁹

It could be argued that female–female pairing removes “excess” females from the group, thereby reducing the group’s overall reproductive rate to the advantage of all, just as male–male pairings removes “excess” males. The problem, once again, is how (aside from, say, kin selection) natural selection could favor any voluntary reproductive self-restraint on the part of same-sex paired individuals. It seems more parsimonious to attribute such pairing to the individuals in question having been unable to establish a heterosexual mateship.

This situation, although intriguing, seems to offer little or no insight into the human situation, which often involves individuals who seem to be no less attractive than their heterosexual counterparts. In the case of human beings, moreover, sexual orientation is almost entirely a function of personal proclivity (whether generated by genes, environment, or an ineffable combination of the two) rather than because opposite-sex options aren’t available.

In some cases, same-sex behavior can even result from simple mistaken identity, something strongly implicated in animals, from invertebrates to vertebrates.
⁸⁰
Walk through a marsh in the American South during early spring: You might well find the toe of your boot (maybe even both boots!) clasped by a male bullfrog, eager to mate. In fact, these fellows are so undiscriminating when it comes to copulation that they have even evolved a particular identifiable croak that essentially says, “Get off, you dummy, I’m a male!” Such occurrences speak to a very low threshold for sexual stimulation on the part of certain individuals (most commonly males) of certain species. They do not, however, seem to qualify as homosexual behavior, but are simply mistakes—which have not been strongly selected against, because the potential payoff is large and the cost of an error is relatively low. There is little reason to believe, however, that gays or lesbians are “that way” because they cannot tell men from women. Rather, they prefer individuals of one sex or the other as sexual partners and it is that preference—an important part of the proximate causation of homosexual behavior—that we are seeking to explain in terms of its ultimate or evolutionary causation.

Maybe it is neither preference nor mistaken identity, but rather simply a consequence of heterosexual deprivation. The so-called “prison effect” is well known: When deprived of suitable heterosexual partners, some individuals act sexually toward whoever is available. Frequently, in such cases, the more active partner in particular denies being homosexual, and it is at least conceivable that the behavior is more an example of dominance signaling than sexuality (recall the American bison described earlier). Nonetheless, sexual arousal is typically involved.

Closely related to deprivation resulting from strict sexual segregation and isolation is a kind of “overflow” hypothesis, by which homosexual behavior might result from the combination of a high sex drive and an inability—for a variety of possible reasons—to obtain heterosexual satisfaction.
⁸¹

There is yet another perspective on high sex drive, namely, the possibility that homosexual acts are driven primarily (and proximally) by the simple prospect of obtaining sexual pleasure, using any willing individual. Think back to those female Japanese macaque monkeys who commonly mount males in an effort to
stimulate them to return the favor. In pursuit of the likely adaptive significance of this behavior (getting themselves fertilized by desirable heterosexual partners), female macaques may well have been proximally induced to mount certain males by obtaining immediate pleasurable sensations—possibly including orgasm—via physical stimulation, presumably of their clitoris. Once female macaques had biologically discovered the potential for sexual satisfaction via genital stimulation, they might also have found that it was available from other females as readily as from males.
⁸²
Males, too, could have made a comparable discovery. And there is no reason, of course, for all these revelations to have been limited to nonhuman animals.

By contrast, at least one peculiar mechanism, found in some animals, has not (yet?) been found to operate in human beings: the strange case of host manipulation by a parasite or pathogen. Numerous examples are known, for example, of pathogens essentially hijacking the behavior of their host victims, causing them to behave in ways that benefit the pathogens rather than the hosts. For example, the life cycle of a parasitic flatworm known as
Dicrocoelium dentriticum
involves doing time inside an ant, followed by a sheep. Getting from its insect host to its mammalian one isn’t easy, but the resourceful worm has found a way: Ensconced within an ant, some of the worms migrate to its brain, whereupon they manage to rewire their host’s neurons and hijack its behavior. The manipulated ant, acting with zombielike fidelity to
Dicrocoelium
’s demands, climbs to the top of a blade of grass and clamps down with its jaws, whereupon it waits patiently and conspicuously until it is consumed by a grazing sheep. Thus transported to its happy breeding ground deep inside sheep bowels, the worm turns, or rather, releases its eggs, which depart with a healthy helping of sheep poop, only to be consumed once more by ants.

A remarkable story, and one that has many other parallels in the annals of host–parasite interactions. Take cholera. The terrible, watery diarrhea for which this disease is known probably isn’t just a symptom, but a manipulation of the human gastrointestinal tract whereby the cholera bacillus,
Vibrio cholerae
, gets to infect new hosts. It is currently unclear what, if anything, such cases have to do with sexual behavior, among human beings or other animals, although one might wonder whether STDs are able, in any way,
to manipulate the behavior of their victims so as to facilitate their own transmission.

Certainly, concern about AIDS has greatly enhanced awareness of male–male transmission of this particular disease, but such traditionally heterosexual STDs as syphilis and gonorrhea should not be overlooked. When the role of pathogens is considered, nearly always the question is how sexual behavior influences pathogen transmission. But there is at least the possibility that the connection occasionally goes in the other direction, with pathogens influencing sexual behavior. There is a fungus,
Entomophthora muscae
, that kills domestic flies that it infects. Dead victims develop a notably swollen abdomen, which strongly resembles the body of a healthy female, loaded with eggs and, hence, sexually desirable. Uninfected males are then drawn to copulate with these corpses, a behavior that transmits the fungus yet further.
⁸³
This raises the striking prospect that, to a degree not often realized, pathogens may actually manipulate the sexual behavior of their hosts, although it must be emphasized that no such connections to human homosexuality have ever been identified, or even seriously proposed.

By contrast, as we have seen, there are lots of hypotheses for the evolution of homosexuality. Although we can state with complete confidence that same-sex preference is definitely not a simple “lifestyle choice”—instead, it is clearly founded on biology of some sort—we must also conclude that it remains a tantalizing evolutionary mystery. Unlike the US military’s ill-conceived and now defunct “don’t ask, don’t tell” policy, however, many reputable investigators are asking not
who
is homosexual, but
why
are there homosexuals, and we can be quite confident that eventually, nature is going to tell.

1
. Haldane, J. B. S. (1927). A mathematical theory of natural and artificial selection. Part v: Selection and mutation.
Proceedings of the Cambridge Philosophical Society, 23,
838–844.

2
. Bell, A. P., & Weinberg, M. (1978).
Homosexualities: A study of diversity among men and women.
New York: Simon & Schuster; Saghir, M. T., & Robins, E. (1973).
Male and female homosexuality: A comprehensive investigation.
Baltimore, MD: Williams & Wilkins.

3
. Kinsey, A. C., Pomeroy, W. B., & Martin, C. E. (1948).
Sexual behavior in the human male
. Philadelphia: W. B. Saunders Co.