Sperm Wars: Dispatch From a Conscientious Objector

Revisiting the notion that humans are biologically adapted for sperm competition

Posted Aug 07, 2013

Direct competition between ejaculates is possible whenever two or more males mate with the same female sufficiently close in time for viable sperm populations to overlap. Among evolutionary biologists, sperm competition has long been a hot topic and several authors have proceeded to claim that humans are biologically adapted for conflict between ejaculates. Putting it bluntly, this implies that women are adapted to seek sex with two or more men within a single cycle and that men adjust their ejaculates correspondingly. In his 2006 book Sperm Wars, Robin Baker strongly advocates adaptation for sperm competition as a core feature of human reproduction. Numerous spin-off studies have taken this as a starting-point. But we first need convincing biological evidence that humans are actually adapted for sperm competition.

Sperm competition in chimpanzees

Field studies have clearly shown that chimpanzees—our closest biological relatives—mate promiscuously. And numerous features indicate biological adaptation for sperm competition. Males have notably large testes and short, muscular ejaculatory ducts, whereas females have relatively long oviducts, increasing sperm travel distance. Moreover, after ejaculation—in a process that Steve Jones graphically describes as “competitive corking”—chimpanzee semen forms a copulatory plug that blocks the vagina.

This is where an inexcusable short-cut is often taken. Humans, it is argued, descended from chimpanzee-like ancestors, so sperm competition was necessarily present at the outset. This is a prime example of “frozen ancestor” thinking: one living species is taken as a model for the origin of another. Contrary to widespread misrepresentation, Darwin’s message was not that humans are descended from apes but that we shared a common ancestor. Instead of simply assuming that the ancestor was chimpanzee-like, we must carefully reconstruct its features.

Mating systems and sperm competition

Mating systems vary widely across primates but fall into three main categories: monogamous, polygynous and promiscuous. Regarding sperm competition, the key distinction is between promiscuous groups, in which several resident males regularly mate with in-group females, and monogamous or polygynous social systems, with mating largely limited to a single resident male. Among apes, gibbons are typically monogamous, while orangutans and gorillas essentially live in one-male systems as well, although orangs are dispersed whereas gorillas live in cohesive harem groups. Chimpanzees are the only apes that live in promiscuous, multi-male groups, so it is not parsimonious to infer that this was the ancestral condition. Moreover, broad surveys have shown that human societies are typically monogamous or polygamous, whereas polyandry is rare and none is promiscuous like chimpanzees.

Humans actually show no convincing evidence of biological adaptation for sperm competition. Unlike chimpanzees and other promiscuously mating primates, such as baboons and macaques, men have relatively small testes and their ejaculatory ducts are long and not very muscular. Moreover, women have short oviducts. Although human semen does coagulate briefly after ejaculation, it soon liquefies without forming a plug.

But arguments based on testis size and other dimensions of male and female reproductive tracts are potentially flawed. It is inherently assumed that those features are genetically fixed. Yet in seasonally breeding species testis size usually varies markedly across the year, and reproductive dimensions may vary flexibly to fit local conditions. Matt Anderson and Alan Dixson elegantly circumvented this problem by examining sperms themselves. Mitochondria packed in the midpiece provide energy to power the tail for progression. Anderson and Dixson found a convincing association between sperm midpiece size and mating system. In promiscuously mating baboons, macaques and chimpanzees, the midpiece is significantly larger than in pair-living gibbons or harem-living gorillas. In human sperms, the midpiece is notably small, clearly falling into the range of primates living in one-male groups and well below values for promiscuously mating species. Indeed, the human midpiece is among the smallest recorded for any primate. This alone directly indicates that humans are not biologically adapted for sperm competition.

Molecular evidence from competitive corking

The clincher comes from evidence concerning copulatory plugs. Chimpanzees are the only apes that produce a firm plug. Evidence from gene sequencing indicates that promiscuously mating primates with relatively large testes are specially adapted to produce copulatory plugs. In primate ejaculates, the main proteins are two kinds of semenogelin, which are directly involved in coagulation. Michael Jensen-Seaman and Wen-Hsiung Li reconstructed the evolution of both semenogelin genes. They found that our two genes have changed relatively little from the likely condition in the common ancestor of gorillas, chimpanzees and humans. Contrastingly, in common chimpanzees the first gene has almost doubled in length. Promiscuous mating of chimpanzees is clearly a secondary condition. In gorillas, both genes actually show signs of degeneration.

In sum, substantial evidence—ranging from testis size through sperm dimensions to genes controlling the physiology of semen—indicates that humans have no special biological adaptations to cope with sperm competition.

References

Anderson, M.J. & Dixson, A.F. (2002) Motility and the midpiece in primates. Nature 416:496.

Baker, R.R. (2006) Sperm Wars: Infidelity, Sexual Conflict and Other Bedroom Battles. New York: Basic Books.

Baker, R.R., & Bellis, M.A. (1995) Human Sperm Competition: Copulation, Masturbation and Infidelity. London: Chapman & Hall.

Birkhead, T.R., & Møller, A.P. (eds.) (1998) Sperm Competition and Sexual Selection. London: Academic Press.

Dixson, A.F. (2012) Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes and Human Beings (Second Edition). Oxford: Oxford University Press.

Dixson, A.F. & Anderson, M.J. (2002) Sexual selection, seminal coagulation and copulatory plug formation in primates. Folia Primatologica 73:63-69.

Jensen-Seaman, M.I. & Li, W.-H. (2003). Evolution of the hominoid semenogelin genes, the major proteins of ejaculated semen. Journal of Molecular Evolution 57:261-270.

Jones, J.S. (2003) Y: The Descent of Men. Boston: Houghton Mifflin Harcourt.

Kingan, S.B., Tatar, M. & Rand, D.M. (2003) Reduced polymorphism in the chimpanzee semen coagulating protein, semenogelin I. Journal of Molecular Evolution 57:159-169.