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Why People Have Sex

Neither procreation nor recreation tells the whole story.

Key points

  • Humans produce offspring through sexual reproduction, but not all species do the same.
  • Some species of vertebrates (which include humans) reproduce through parthenogenesis, where non-fertilized eggs develop into viable animals.
  • Certain species can actually choose whether to reproduce through parthenogenesis or sexual reproduction.
  • Closely examining the conditions under which such animals choose sex over parthenogenesis provides fresh clues about why humans have sex.

Ella Fitzgerald’s famous song about falling in love starts with:

Birds do it

Bees do it,

Even educated fleas do it…

It turns out that Fitzgerald's ode to love was a more comprehensive description of activities leading to reproduction than she may have realized.

Yes, many species of birds, bees, and fleas—whether in love or not—do engage in sexual reproduction, but certain species of birds, bees, and fleas reproduce, not solely through sex, but also through a process called parthenogenesis.

A conjugation of the Greek parthenos (meaning virgin) and genesis (meaning birth) parthenogenic animals develop eggs that hatch into a viable animal without ever being fertilized by a male’s sperm. Some parthenogenic offspring are simple clones of their mothers, but others, through gene shuffling in a cell division process called meiosis, have genes different from their mother’s, and can even be males. Examples of parthenogenic animals include condors (birds), honeybees (bees), and daphnia (water fleas).

Advantages of parthenogenesis (over sexual reproduction) are that it consumes less energy (no hunting for a mate, no lengthy mating rituals, no exertion) than sexual reproduction and may be less risky (injuries or infections from sex, avoiding distractions in the presence of dangerous predators, and so forth).

Of particular importance to the question of “Why we really have sex” are species that not only exhibit parthenogenesis, but have a special ability called facultative parthenogenesis in which the animal can, by choice, reproduce either through sex or through parthenogenesis.

In these creatures, having sex is not strictly required for the survival of the species, but under some circumstances, the animals choose to do it anyway.

Digging deeper into just why animals who don’t need to have sex but sometimes do have sex could provide important clues about why humans have evolved exclusively sexual reproduction, instead of simply popping out unfertilized eggs that develop into viable babies.

Zoologists who have studied facultative parthenogenesis in a range of species from monitor lizards to aquatic arthropods have concluded that females with a choice of reproductive method opt for parthenogenesis when there are no males around, or in the absence of environmental stressors, such as food shortages or unfavorable climate.

But when things go bad, such as if a toxic spill occurs in a habitat (and there are abundant males), females, more often than not, choose sex over parthenogenesis.

Zoologists believe the reason some species choose “sex under stress” is that sexual reproduction leads to more novel gene combinations in offspring of sexual reproduction than are present with parthenogenic offspring, and that, when conditions turn unfavorable, fresh genes may be better adapted to harsh environments and are needed for the survival of the species.

In other words, evolution has decided that ‘new’ genes are more likely to succeed in new, challenging worlds than ‘old’ genes.

One reason that humans engage in sex is to produce offspring who are likely to be better suited than their parents to uncertain futures. Given the ever-changing environments that confronted our ancestors (climate change, disease, famine, predation), evolution has determined that having a steady stream of novel genes through sex is worth the added metabolic costs and risks.

Another reason that humans reproduce exclusively through sex is that our bodies may be too complicated to develop without merging male and female genes. The overwhelming majority of parthenogenic species are relatively simple invertebrates, and only 0.1 percent of more complex vertebrates (including turkeys and some fish and lizards) are parthenogenic. Biologists theorize that once organisms pass a certain threshold of complexity (e.g., the size of the central nervous system) parthenogenesis lacks the sophistication of male/female gene combinations necessary for embryos to develop to maturity.

Tantalizing hints that the complexity of human biology may at least partially explain our sole reliance on sex can be found in cases where unfertilized human eggs “self-activate” (for unknown reasons) and start to develop into rudimentary embryos, only to stop developing at a stage where complex tissues such as skeletal muscle would normally start to form. (Such malformed, unfertilized embryos constitute a type of benign ovarian tumor called a teratoma).

The bottom line of why we really have sex: both the ever-changing world we live in and our own bodies are far too complicated to do it any other way.

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Savage, Thomas F. (September 12, 2005). "A Guide to the Recognition of Parthenogenesis in Incubated Turkey Eggs". Oregon State University. Retrieved 2006-10-11.

Schuett, G.W.; Fernandez, P.J.; Gergits, W.F.; Casna, N.J..; Chiszar, D.; Smith, H.M.; Mitton, J.B.; Mackessy, S.P.; Odum, R.A.; Demlong, M.J. (1997). "Production of offspring in the absence of males: Evidence for facultative parthenogenesis in bisexual snakes". Herpetological Natural History. 5 (1): 1–10

Smith, John Maynard (August 24, 1978). The Evolution of Sex. CUP Archive. p. 42. ISBN 978-0-521-21887-0.

Watts, P. C.; Buley, K. R.; Sanderson, S.; Boardman, W.; Ciofi, C.; Gibson, R. (2006). "Parthenogenesis in Komodo dragons". Nature. 444 (7122): 1021–1022.