Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm


12.1  Is Sex Necessary?

If reproduction is merely a useful convenience, we must admit that sex is pure luxury. There is no fundamental reason why evolution and diversity cannot thrive in its absence. There is no law against asexuality.

In point of fact, asexual reproduction is vastly more prolific in the short run. Bacterial lifeforms churn out literally billions of offspring in the space of hours, relying solely on such simple techniques as binary fission and budding. No "opposite sex" is required. And while it is true that many sexual species are also quite fecund, as a general rule fewer offspring are produced and survive to adulthood than among the asexuals.

Furthermore, asexual reproduction is good economics. An organism which copies itself without sex passes its entire genetic heritage to its young undiluted. Offspring are exact duplicates of the originals.

A sexual parent, on the other hand, may contribute only half of its own genes towards the construction of a child. The other half, in the case of a bisexual species, must be donated by the other parent. From the standpoint of the selfish gene, sex has a lousy profit margin in comparison to no-sex.

Nevertheless, there is a more subtle difficulty with asexuality that turns virtue into vice.

A completely asexual species produces a population of virtual duplicates -- except for an occasional mutation. Since variation is the raw material of evolution, and the lack of sex decreases this variation, such lifeforms should be at a distinct disadvantage when competing with their sexual brethren. New genetic combinations in asexual species can only proceed through a sequence of fortuitous mutations in the same family lineage. Asexuals therefore must "stand in line" to wait for a series of rare mutations. Change spreads only slowly through the gene pool.1044

But sex allows the accumulation of variation in parallel, rather than in series.1045 A sexual species is able to spread many new genes rapidly throughout the population, because gene-jumbling allows a new combination, a new throw of the genetic dice, with each act of reproduction. Rare mutations become widely dispersed. 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 harsh or rapidly changing environments.

For example, 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 have come to dominate the animal world, and the most widespread and important groups are all but exclusively sexual in their mode of reproduction. These broad brush strokes of nature should paint a similar picture elsewhere in the universe.

Of course we don’t know if aliens have genes, or even if information-carrying molecules are necessary at all. For all we know, extraterrestrials may reproduce by xerography85 or in direct response to the environment by inheriting acquired characteristics.22 But one fact is clear: Variability is an advantage in the quest for biological complexity. And sex provides a unique opportunity for shuffling the data deck -- genetic or no -- which asexual techniques simply cannot match.

If sex is necessary, then how many sexes are best? Can there be more than two?

Terran lifeforms provide several examples of multisexuality, although they are few and far between. The lowly paramecium, for instance, 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 includes fungi, notably Basidiomycetes, in which there are four distinct sexual groupings. Fungi are quadrisexuals. Still another example is found among the greylag geese -- a rather clear case of behavioral trisexuality.455 (One goose "marries" and mates with two male ganders.) Multisexuality is clearly a viable alternative.*

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

The answer seems to be that two sexual partners are just enough to provide the requisite genetic recombination. Each healthy individual has a reasonable chance to mate with a member of the opposite sex. Apparently, two are both necessary and sufficient.

More than a single pair of sexes may seriously impair the chances for species continuity. The more sexes required for successful reproduction, the more difficult it becomes to bring them all together properly at just the right time. If there is a weak link in the mating chain -- as where one member of a reproductive triad is characteristically vulnerable to certain predators or other environmental severities -- the future of the entire species would be jeopardized. Finally, it is not clear how, say, three sexes could shuffle the genes very much better than two.

There are no compelling reasons to exclude the possibility of a thriving population of alien multisexuals on another planet. That is, extraterrestrial multisexuality cannot be ruled out. But requiring more than two sexes for reproductive activity seems to be an unnecessarily complicated solution to a problem elegantly solved by only two.

It’s a safe bet that bisexuality is the overwhelmingly dominant mode of sexual reproduction among the biological alien lifeforms in our Galaxy.


* Science fiction writers and many others have toyed with the implications of intelligent trisexual and multisexual aliens for years. See especially Asimov,2485 Farmer,2500 Niven,753 Ritner,1550 and Stapledon.1946 Norms of marriage, inheritance, language, religion and social behavior would be profoundly affected by this state of affairs. Indeed, they might prove virtually incomprehensible to us. The normal social tensions caused by sexual competition would be greatly aggravated in a society in which every member was a potential mate. In their eyes, humans might appear perverted.97


Last updated on 6 December 2008