In 1988 Baker and Bellis first proposed the "Kamikaze sperm"
hypothesis (Baker and Bellis
1988; Baker and Bellis 1989;
Baker and Bellis 1993; Baker
and Bellis 1993) . This was an extention of Parker's ideas on the
role of plugs and blocking agents in sperm competition (Parker
1970; Parker 1984; Parker
and Begon 1993) . In essence, they envisaged two types of human
sperm, "egg-getters" and "kamikaze" forms specialized
either to block competitors or to roam around the female tract
seeking to destroy competitors (using acrosomal enzymes?). This was
fleshed out in a highly entertaining and best-selling book, "Human
Sperm Competition. Copulation, Masturbation and Infidelity"
(Baker and Bellis 1995) , and has been
widely publicised including a TV documentary, "Sperm Wars". Roger
Short has a devastating review from the 47th
newsletter of the European
The book actually provides a wonderful read, even though many of its conclusions are pretty wild and have not been published in peer-reviewed journals (sperm heads changing shape in mixed ejaculates, for example). My major problem is that they produce a most complicated explanation for what may be a straightforward problem and there is no convincing direct evidence for adaptive roles for the various morphs (Gomendio and Roldan 1991; Gomendio and Roldan 1993) . If you accept that humans produce abnormal sperm simply because we are not very fertile and the quality control mechanisms in the testis are deficient, then there is no need for an obsessive need to find an "adaptive" explanation for every aspect of sperm biology. In fact Occam's razor demands that we should always prefer the simplest explanation even if it doesn't grab the popular imagination.
Harcourt (1991) has shown that the numbers of abnormal sperm in mammals -- and especially in Primates -- do not correlate with mating pattern. If the "kamikaze sperm" hypothesis were correct one would expect to see more abnormal forms in species with high intensity of sperm competition, whereas if anything the reverse is true. Baker and Bellis responded by suggesting that variation is likely to be greater in species that have cryptic estrus (like humans) but of course this doesn't explain high levels of pleiomorphism in sperm of gorillas where the chances of sperm competition are pretty minimal, nor in a diverse range of species ranging from thoroughbred stallions (cryptic estrus in mares?) to koalas, cheetahs and other big cats (Cummins 1990) .
I suspect that Baker and Bellis have fallen into the Panglossian trap of assuming that every aspect of sperm biology is adaptive. Indeed abnormal sperm can't even be considered to be exaptations (features that may enhance fitness but that were not designed by natural selection for their current role) (Gould and Vrba 1982) because they do not normally participate in fertilization. While abnormally shaped sperm (in mice) can generate normal embryos when microinjected into oocytes (Burruel, Yanagimachi et al. 1996) , in normal circumstances barriers in the reproductive tract such as cervical mucus and the zona pellucida actively select against abnormal forms (Katz, Morales et al. 1990; Liu and Baker 1994) and are probably the major selective mechanisms for defining the heredity of sperm form (Bedford 1991) .
Kamikaze sperm? I'm not convinced. I'm happy to post any comments or rebuttals, though.
Baker, R. R. and M. A. Bellis (1988). Kamikaze' sperm in mammals?Animal Behaviour 36: 937-980.
Baker, R. R. and M. A. Bellis (1989). Number of sperm in human ejaculates varies in accordance with sperm competition theory Animal Behaviour 37: 867-869.
Baker, R. R. and M. A. Bellis</A> (1993). Human sperm competition: ejaculate management by females and a function for the female orgasm Animal Behaviour 46: 861-885.
Baker, R. R. and M. A. Bellis (1993).Human sperm competition: ejaculate management by males and the function of masturbationAnimal Behaviour 46: 861-885.
Baker, R. R. and M. A. Bellis (1995). Human Sperm Competition. Copulation, Masturbation and Infidelity. London, Chapman and Hall.
Bedford, J. M. (1991). The coevolution of mammalian gametes.. A Comparative Overview of Mammalian Fertilization.. B. S. Dunbar and M. G. O'Rand. NY, Plenum Press: 3-35.
Burruel, V. R., R. Yanagimachi, et al. (1996). Normal mice develop from oocytes injected with spermatozoa with grossly misshapen heads;Biology of Reproduction 55 (3): 709-714.
Cummins, J. M.</A> (1990). Evolution of sperm form: levels of control and competition. In Fertilization in Mammals B. D. Bavister, J. M. Cummins and E. R. S. Roldan. Norwell, Massachusetts, Serono Symposia, USA: 51-64
Gomendio, M. and E. R. Roldan (1993). "Coevolution between male ejaculates and female reproductive biology in eutherian mammals." Proc R Soc Lond Biol 252(1333): 7-12.
Gomendio, M. and E. R. S. Roldan (1991). Sperm competition influences sperm size in mammalsProc R Soc Lond B 243 181-185.
Gould, S. J. and E. S. Vrba (1982).Exaptation - a missing term in the science of form.Paleobiology 8: 4-15.
Harcourt, A. H. (1991). Sperm competition and the evolution of nonfertilizing sperm in mammals.Evolution 45(2): 314-328.
Katz, D. F., P. Morales, et al. (1990). Mechanisms of filtration of morphologically abnormal human sperm by cervical mucus. Fertility and Sterility 54(3): 509-512.
Liu, D. Y. and H. W. G. Baker (1994). Acrosome status and morphology of human spermatozoa bound to the zona pellucida and oolemma determined using oocytes that failed to fertilize in vitro.Human Reproduction 9(4): 673-679.
Parker, G. A. (1970).Sperm competition and its evolutionary consequences in the insects.Biological Reviews 45: 525-567.
Parker, G. A. (1984). Sperm competition and the evolution of animal mating systems In "Sperm competition and the evolution of animal mating systems". R. L. Smith. (Ed) London, Academic Press: 1-60.
Parker, G. A. and M. E. Begon (1993). Sperm competition games: sperm size and sperm number under gametic controlProc R Soc Lond B 253: 255-262.