Alien Sex

©1982 by Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., "Alien Sex," Analog 102(June 1982):46-52



This paper contains material originally drawn from the book Xenology (1979) by Robert A. Freitas Jr.



Reproduction takes many forms even on Earth – and profoundly influences other aspects of life. In a universe teeming with life, the variety must be far greater and the influence no less.


Of all the important things lifeforms do, self-reproduction seems quite unique. Deprive an animal of its food or drink, draw off its blood, or cut away its skeleton, and it dies. But prevent an animal from reproducing and, usually, nothing happens. The species may eventually become extinct, but the individual organism lives out its lifespan. Reproduction of self is an important asset but is not absolutely essential for life – even on Earth.

This is true despite protests that self-replication is somehow the entire point of biological activity. The vast majority of social insects never engage in personal self-reproduction, yet these species are extremely successful. The anatomy of domesticated turkeys has been altered by breeding for plumpness so that these animals can no longer mate in the natural way and must be artificially inseminated with human help. A number of higher Earth species such as the mule are quite sterile, yet do not become extinct.

Indeed, an intelligent extraterrestrial race might lack the capability of individual direct self-replication. We might imagine two closely allied nonsentient alien species among whom, when a successful interspecies mating occurs (or in a special way or in a special environment), sterile but intelligent “mule” offspring are the result of the union. Clearly there is no bar to the rise of intelligence in such a situation – the hybrid's brain mass. neural complexity, or level of organization may be qualitatively greater than those of its non-sentient parents. Our intelligent but sterile race would maintain their numbers by corraling and manipulating the “dumb” mixed parental population much as stockmen raise choice cattle and stablemen breed champion thoroughbreds.

It is entirely possible that some very complex extraterrestrial living creatures may have no need to reproduce themselves at all, either personally or at the species level. One class of such beings might be self-creating but non-replicating organisms, analogous to very advanced robots capable of making continual repairs and of upgrading their own mechanisms periodically. Other nonreproductive lifeforms might increase their numbers simply by physically expanding and then dividing into pieces of various sizes – biomass increases as easily by growing to larger volumes as by replicating a large number of small originals.

There could even exist a race which evolves by means of acquired characteristics. Such lifeforms would neither die nor reproduce, but would instead modify their parts to survive in a changing environment. Selection would act internally on their constitutions, rather than on a succession of descendent organisms. The closest analogies, according to Dr. P.H.A. Sneath, are terrestrial soils, which don't reproduce in the usual sense but are complexly organized systems nevertheless.[1] Soils respond to environmental changes, arise where there is rock and wind to erode it, and are virtually immortal. If ever they tried to “compete” with their neighbors, such soil-like organisms would blend together with a total loss of individuality.

Finally, reproduction is not a prerequisite for sex. Two dissimilar growth systems could trade genetic information about their expansion patterns, then each continue growing in a slightly different way. This would be an example of “sexual growth” without replication. Of course, self-reproduction does have many advantages. Whole-body duplication allows rapid dispersion into new niches and produces abundant biological alternatives upon which natural selection may operate. It is a telling observation that most complex terrestrial creatures are capable of self-replication. Assuming Earth is a typically exotic planet, we should expect that many, though certainly not all. extraterrestrials will be reproducers.


Is Sex Necessary?

If reproduction is a useful convenience for a species, sex seems almost pure luxury. Certainly there is no fundamental reason why evolution and diversity cannot thrive in its absence. There is no universal law prohibiting asexuality.

In fact, asexuals can be vastly more prolific in the short run. Microorganisms chum out literally billions of copies in the space of a few hours, relying almost exclusively on such simple techniques as binary fission and budding. No “opposite sex” is customarily required. While it is true that many sexual species are also quite fecund, as a general rule fewer offspring are produced than among the asexuals.

Furthermore, asexual reproduction is good economics from the personal point of view. An organism which copies itself without sex passes undiluted its entire genetic heritage to its young. Offspring are exact duplicates of the originals. A bisexual parent, on the other hand, normally contributes only half of its own genes towards the construction of an offspring. The other half must be donated by the second parent. From the standpoint of the selfish gene, sex entails a rather poor profit margin in comparison to no-sex.

Except ...

A completely asexual species produces a population of virtual duplicates, save an occasional mutation. Since variation is the raw material of evolution, and the lack of sex decreases the breadth of this variation, such creatures are a distinct disadvantage when competing with their sexual brethren. New genetic combinations in asexual species can accumulate only by a sequence of fortuitous mutations in the same family lineage. Asexuals must “stand in line” to wait for a series of rare mutations.” Change spreads only slowly through the gene pool.

Sex allows the accumulation of variation in parallel, rather than in series. In a sexual species many new genes can spread rapidly throughout the population because gene-jumbling produces a novel combination (possibly of several new genes at once) with each act of reproduction. Rare mutations become more widely distributed. So great are the advantages of sex that even many normally asexual organisms have occasional sexual encounters to beef up the waning gene pool. This is especially true in particularly harsh or rapidly changing environments.

For example, both the freshwater hydra and the aphid reproduce asexually for most of the year. As winter approaches. with hard times ahead, these animals switch over to sexual reproduction. This ensures genetic diversity when the colonies disband and disperse with the arrival of cold weather.

In the billion years or so since its invention, sex has proven remarkably successful – if we are to judge from the fossil record of life on this planet. Sexual species dominate the animal world, and the most widespread and important groups are all but exclusively sexual in their mode of reproduction. What of the creatures of other worlds? We don't know whether all alien species must have chromosomes, genes, or some other information-carrying molecules – perhaps some extraterrestrials reproduce by a process akin to xerography. But two things are clear: Variability is the key to biological complexity and survival, and sex reshuffles the biological data deck nonpareil.


How Many Sexes?

Not all Earth creatures are bisexual. Terrestrial biology offers several examples of multisexual reproduction. One interesting case is the lowly paramecium, which has between five and ten sexes depending on how you count. These are distinct mating forms which arise at different times under definite conditions, and which can only mate in certain specific combinations. Another example is certain quadrisexual fungi, notably Basidiomycetes, in which there are four distinct sexual groupings. Among the higher animals, greylag geese display an evolved sociobiological “behavioral trisexuality.” One goose “marries” and mates with two male ganders. Multisexuality is clearly a viable alternative.

Why, then, are the vast majority of terrestrial sexual lifeforms bisexual?

The answer seems to be that one sexual partner is just enough to properly shuffle the genetic deck. Each healthy individual has a reasonable chance of mating with a member of the opposite sex. Apparently, two are both necessary and sufficient. ^More than this may seriously impair the chances for species continuity. The more sexes required for successful reproduction, the more difficult it is to bring them all together properly at just the right time. The greater the number of links in the mating chain, the greater is the chance that the species may become vulnerable to certain predators or other environmental severities, thus jeopardizing the future of the entire race. And it is not clear how, say, three sexes could generate variability very much more effectively than two.[2]

So while extraterrestrial multisexuality cannot be ruled out, requiring more than two sexes for reproductive activity seems an unnecessarily complicated solution to a problem elegantly resolved using only two. It's a safe bet that bisexuality is the overwhelmingly dominant mode of sexual reproduction among the alien lifeforms in our Galaxy.


The Bisexual Universe

Assuming that most sexually-reproducing ETs will have just two sexes, bisexuality does not necessarily demand the existence of distinct male and female forms. A case in point is the black mold Rhizopus nigricans, which displays an unusual form of reproduction known as “heterethallism.” This species of fungus is bisexual, inasmuch as two organisms are required for fertilization and replication to take place. However, the two sexes are physically indistinguishable. There are no constant differences between members of opposite mating groups other than their reciprocal behavior when crossed. Thus, it is impossible to designate one form of the black mold as male and the other as female. Customarily the complementary groups are labeled merely “+” and “-” for convenience during experiments.

One can imagine a race of intelligent extraterrestrials apparently unisexual to our undisceming eyes but which actually practice heterothallic sex. Such beings would most certainly lack secondary sexual characteristics, those hormone-induced physical landmarks such as beards and breasts to which we humans are so pleasantly accustomed. They might even lack distinctive primary sexual characteristics such as internal or external gonads. Norms of marriage, inheritance, language, religion and social behavior would be profoundly affected by this state of affairs. The usual social tensions caused by sexual competition in human cultures would be more diffuse in a society in which every member was a potential mate and in which all could become pregnant. though sexual undercurrents might arise in all interpersonal relationships. The disparate male/female roles in human social roles and courtship rituals would defy their understanding, and to heterothallic ETs, human males – who participate in reproductive acts for pleasure but cannot become pregnant as a consequence – might be judged especially pitiful, handicapped, even perverted creatures.

Assuming maleness and femaleness exist among most bisexual alien species, there are again major variations in Earthly biology. It is quite possible to have an organism which is neither strictly female nor strictly male, but rather exhibits some alternating or intermediate condition. For example, simultaneous hermaphrodites possess at once both female and male sex organs. Ovaries and testes are present together in the same individual. Matings occur in pairs, with each partner serving both sexual roles at the same time. Planarians, earthworms, sponges and snails fall into this category, and a few simultaneous hermaphrodites among the more highly evolved vertebrates are known, such as the banded flamefish Serranus subltgarius.

Such intersexual animals can be sex-mosaics in time as well. Many creatures start life as one sex and finish it as another. These sequential hermaphrodites come in many varieties. For instance, in protoandry an animal is first male and later female; proterogyny is the converse, with young females metamorphosing into functional males as they age. Or the process can be cyclical. Oysters are bom as males, then spend the rest of their lives switching back and forth between male and female in irregular cycles a few months long.

What would a society of sequential hermaphroditic aliens be like? We can take a few clues from the life history of the freshwater shrimp Gammarus pulex. Each of these individual crustaceans is both male and female, but not at the same time. Newborn animals spend early life in a neuter stage, after which they pass through puberty and enter the first sexually active phase as functioning males. After a while, the maleness is exhausted. Latent ovaries ripen into maturity, and the organism spends the remainder of its life as a full-fledged female. Eggs are shed by middle-aged mothers and are fertilized by energetic youthful males still in the middle of their first cycle.

It is a magnificent bisexual system, one which works quite well on Earth. No individual is excluded from any phase of the reproductive process. Still more significant, each member of the colony plays both male and female roles during his/her life. Drawing an analogy to the human life cycle, zoologist Norman J. Berrill of McGill University in Montreal imagines that all halfgrown individuals, about ten years old and weighing about 34 kilograms, would be males – the only males – ready to act as such both sexually and “probably in other wayward ways.”[3] Like their truly human counterparts, as troublemakers they would be kept in line by a closed society of matriarchs, roughly equal in number to the males but each twice the size and much older and wiser. This wisdom would be not merely of a general character, as among human parents, but also in the special sense of each having been a male herself, as understanding as a mother with a child and as little likely to put up with any nonsense, perhaps wistfully looking back to her youthful manhood. Womanhood would bud as usual when masculinity had faded, with growth continuing and full female maturity yet to come.

The institution of monogamous marriage as we know it would be quite impossible in such a society. Husbands would be forever changing into wives and males would be too immature psychologically to be treated as other than “child-lovers.” Such pedophilia is viewed as a sexual perversion in many human societies, but for our intelligent shrimps it would seem quite normal. Incest prohibitions might be inordinately complex, since all fertile middle-aged females in the family in theory could mate with any or all male children. To offset the negative effects of inbreeding, exchanges of matriarchs could occur between families, doubtless accompanied by the same pomp and ceremony as upon “giving the bride away” in our society. Love in the traditional human sense probably would not exist – females could have strong affective and familial non-sexual ties with other females, whereas relations between females and males would be characterized more as controlling playfulness than by affectionate cooperation. Our usual concepts of male/female love might seem quite alien to them.



Given these tremendous potential cultural and biological differences, one wonders if meaningful interspecies social-sexual relations would be possible at all between humans and extraterrestrials. Many science fiction authors have tried to deal sensibly with this touchy question, such as Philip Jose Farmer in The Lovers, in Flesh, and in Strange Relations, Walter Tevis in his The Man Who Fell to Earth, and a host of others. There have been “reports” of sexual molestations of humans by the occupants of UFOs. And Star Trek’s own Mr. Spock is a prime example of xenogamy, the product of a marriage between a human female alid a male alien from the fictional planet Vulcan.

It is not at all implausible that interspecies copulation can occur. Given the prevalence of the complementary male and female organs throughout the animal kingdom on this planet, such activity may indeed be possible even between creatures of “gross morphologic disparity.”[4] Alfred Kinsey’s researchers turned up accounts of attempted couplings between a female eland and an ostrich, a male dog and a chicken, a female chimpanzee and a tomcat, and a stallion and a human female. Obviously, relations between humans and other beings even roughly humanoid in shape can happen.

If such activity is possible, is it likely? Could humankind and an alien race derive sexual pleasure from mutual physical encounters? These are very difficult questions, mainly because the ET is such an unknown quantity. Extraterrestrials may have organs, appearances, sensitivities, and responses wholly incompatible with any conceivable human style of lovemaking.

And yet – in 1948 Kinsey reported that some 17 of all rural farmboys had experienced sexual congress with various barnyard animals, and had achieved orgasmic satisfaction in this way. (Less than a tenth of a percent of all females interviewed admitted such coition, although 1.5 of the sample reported some form of sexual contact with animals.) If bestiality occurs so regularly among human populations, can we state with any assurance that “xeniality” will not also occur when humans mingle socially with alien races? The evidence, scanty though it may be,suggests that interspecies sexual contacts are not only possible but probable.

One last question remains. When humans and aliens sexually join. will anything result from the union? Again, this is a difficult question because an unknown alien physiology is involved. Different species on Earth have been mated successfully from time to time – for instance, the hybrid offspring of a mallard and a pintail duck is fertile.

In 1975 a chance mating of two very different species of ape in the Grant Park Zoo produced the first reported ape hybrid. The offspring, dubbed a “siabon,” was the result of a mating between a male gibbon and a female siamang confined in a single cage. “Obviously,” remarked one researcher, “they had been sexually involved for some time.” Gibbon cells have 44 chromosomes, whereas siamang cells have 50, and thus are farther apart genetically than human beings and the great apes. The “siabon” offspring, believed sterile, has a mixed bag of 47 chromosomes – 22 from the father and 25 from the mother. Still, in the first analysis, xenobiologists recognize that interspecies fertilization, and especially hybrid fertility, is a rather rare phenomenon.

In the context of extraterrestrial matings, natural interspecies fertility should be even rarer. (Of course, with advanced technology almost anything may be possible – the first interkingdom clones combining plants and animals were achieved during the late 1970s.) We know that slight changes in the environment can cause enormous variations in planetary biochemistry. Nucleic acids, genes and codons may not be needed by ETs, or these may be essential but in different forms than are found on Earth. Many complicated and highly unlikely coincidences must occur for an alien/human mating to produce viable results. The two species must have identical amino acid sequences for proteins (assuming they even have proteins), the same optical rotation in their biomolecules, closely matched chromosomes with similar size and shape, the same kinds of genes located on the same chromosomes at the same locations, and so forth – all of which is highly improbable.[5] It has not even been shown that humans can produce interspecies offspring with their own closest biological relatives – apes and other primates who share most of man's biological heritage.

So interspecies matings involving humans aren't likely to result in pregnancy. If pregnancy somehow does occur, the hybrid offspring probably won't be viable. (It has been estimated that up to 50% of all normal human pregnancies may end in spontaneous abortion.) Finally, if somehow viable and carried to term, the interspecies hybrid will most likely be sterile or maladapted for natural survival, much like the mule or the liger. Hybrid vigor is unlikely in the offspring of parents of such widely varying genetic constitution.

This does not augur well for Mr. Spock.



1. P.H.A. Sneath, Planets and Life, Thames and Hudson, London, 1970.

2. J. Maynard Smith, The Evolution of Sex. Cambridge University Press, Cambridge, 1978.

3. Norman J. Berrill, Worlds Without End: A Reflection on Planets, Life, and Time. The Macmillan Company, New York, 1964.

4. Alfred C. Kinsey et al. Sexual Behavior in the Human Female, W.B. Saunders Company, Philadelphia, 1953.

5. Robert A. Freitas Jr., “Xenobiology,” Analog 101(30 March 1981):30-41.


Last updated 30 December 2008