Sperm competition and the Kamikaze Sperm Hypothesis

This is the very controversial and highly sensational scientific theory that the book Sperm Wars was based upon. It was originally put forward in the early 1990′s by Robin Baker and Mark Bellis, and later adopted by various self-styled Seducers, as a justification for their even more bizarre notions about alpha biology amongst humans. I shall endeavour to recount the theory here, with comments and analysis:

Apparently, reasonably young, healthy, heterosexual couples have sex approximately every three days. Perhaps this is coincidental, but this mating frequency maintains an almost continuous supply of sperm inside the female reproductive tract. And so any extra-pair copulations may then lead to Sperm Competition (unless the female had not been having sexual intercourse with her husband, and she only has one additional partner). However, it is very difficult to say exactly how often humans engage in adulterous relationships, and even more difficult to say how frequently a woman might end up mixing sperm from two or more men. For this to happen, a woman would have to have sex with two different men, within approximately a five day period.

Baker and Bellis report that while intra-pair copulations are divided more or less evenly throughout a woman’s reproductive cycle; by contrast, extra-pair copulations are more frequent when women are most fertile (even if fertilization is thwarted by the use of birth control). Baker and Bellis say “at some time in their lives the majority of males in western society place their sperm in competition with sperm from another male and the majority of females contain live sperm from two or more different males.” further, they estimate that in Great Britain1 4 to 12% of children are conceived by “sperm that has prevailed in competition with sperm from another male.” This would be consistent with standard estimates2 of ‘paternal discrepancy‘ (calculated to be about 10%) amongst human beings generally. And, on the surface, this might suggest an opportunity for sperm competition. Additionally, when almost 4,000 sexually experienced women (having had sex at least 500 times) were surveyed1, 1 in 200 claimed to have had sexual intercourse with two different men within half an hour of each other on at least one occasion; Within 24 hours, the figure jumped to almost 30%. Again, suggesting more opportunities for sperm competition.

Many insects and birds have a ‘last male advantage‘. This means that the last male to copulate with a particular female is more likely to fertilise most of her eggs. In the case of mammals, things are rather less straight forward; and so, it is currently completely unknown (particularly amongst humans) whether the first male, the last male or any other male in-between that mates with a female has any advantage at all.

It is quite possible that amongst human beings the probability of fatherhood may simply be a case of depositing the most sperm into the vagina; and then frequent regular sex with a woman could be seen as ‘topping up‘ her reproductive tract, replacing sperm that has become old or disabled since the last copulation. This would keep a pretty much constant supply of sperm inside the female, and would be adaptive under any circumstances, because it would maximize the chances of fertilization (this would be especially true, if the woman might have sex with other men too). Furthermore, from a sperm competition perspective, masturbation could then be a way of making sure that the sperm available to be ejaculated in sexual intercourse have a long ‘shelf life’: If the body removes older sperm, then whatever is left should be fresh, and the younger sperm would be better able to compete, and especially more able to penetrate cervical mucus.

Baker and Bellis go on to claim that women can (unconsciously) control the outcome of a sperm competition, and we accept that since women evaluate men by many criteria (emotional, intellectual, physical, financial etc.) while searching for the appropriate qualities in a parent, colleague, friend, lover, protector etc. that there is no reason why a woman should not find herself socially paired with one man, but disposed to have extra-pair sex with another man.

Women are quite likely to be concerned about the overall quality of the male who fertilizes her eggs; and the internal reproductive tract3 of a women produces many barriers, including anti-sperm antibodies that can interfere with fertilization by immobilizing, or even destroying sperm and by impairing their ability to penetrate the egg, while other antibodies act against the egg’s membrane to prevent early egg cleavage and development. The key point here is that these antibodies do not necessarily reduce fertility; instead they diminish the fertility of certain male-female pairings. Therefore, a woman may enhance her reproductive success by seeking a different sexual partner, while retaining her social marital partner (all quite possibly unconsciously).

Another behavioural strategy that may be likely is that of ‘flowback‘. Up to one-third of the seminal fluid ejaculated within the vagina leaks out within a few minutes, and semen can also be expelled with substantial force when urinating, as opposed to dribbling out after sex when a woman stands up (or even if she remains laying down). About 12% of the time, this flowback results in the expulsion of virtually all the sperm ejaculated inside the reproductive tract; so, women are certainly capable of exercising some control over sperm.

Baker and Bellis somehow managed to convince a number of women to capture flowback after both extra-pair copulations and intra-pair copulations; the results they found showed a lower level of sperm retention after intercourse with their main partner. According to Baker and Bellis “women achieve higher sperm retention during extra-pair copulations by reducing their frequency of non-copulatory orgasms via masturbation” So, contractions during female orgasm may actually push out semen and thus by masturbating less (and therefore having fewer orgasms) women end up retaining more sperm from extra-pair copulations.

Insects and birds have sperm storage organs, whereas mammals lack any similar organs. However, it has been claimed that sperm are stored in ‘cervical crypts‘ which are tiny cavities lining a woman’s cervix. It is further claimed that from here they could be released over a period of time, after sex. This would be important for human sperm competition, because the ability to store sperm would encourage sperm competition among successive males. The key question (which scientists are still debating) is how long after ejaculation can human sperm remain viable? and thus capable of competing to fertilize a woman’s egg. Estimates vary from two to ten days; so if we were to assume a median value of five to six days to be true, that would mean that if a woman had sex with someone within five or six days of having sex with another man, the sperm of the two men could be in competition.

Ultimately, a ‘Sperm competition‘ begs the question: How does one win? For a woman, this would be by making the best choice (i.e. having the opportunity to choose more than one man), as well as being able to select ‘good’ sperm, possibly by setting up a competitive situation. For a man, winning would be simply by having ones sperm succeed in fertilizing a woman’s egg.

A male strategy may therefore be as simple as making lots of sperm, we could assume that fertilizing the egg was a type of lottery (i.e. own more tickets and you’re more likely to win). Biologists have therefore asked “What is the best strategy for males, in terms of dividing their sperm between intra-pair copulations and extra-pair copulations?”. And Geoffrey Parker concluded from a detailed mathematical model that males with a partner should generally ejaculate more sperm during extra-pair copulations than intra-pair copulations (assuming that the male was able to maintain an adequate sperm level inside his main sexual partner). The only exception being4 when a male has determined that his partner has been unfaithful, in which case he should increase his sperm numbers; although it is still to be proven whether or not this actually occurs. However, fertilization may be less a lottery than a race; in which case it would be important to make sperm that move quickly.

Baker and Bellis in their ‘Kamikaze Sperm Hypothesis‘ have proposed that it is a war, in which men’s sperm literally do battle with each other, and that only a small percentile of human sperm are actually intended to function as ‘egg-getters‘, which fertilize eggs. Whilst the balance are Kamikazes on a suicide mission, whose goal is simply to stop the sperm of other males. In addition to these ‘Blockers‘ (sperm with coiled and kinky tails) there are supposedly others that go on ‘search-and-destroy’ missions. These, it is claimed carry out chemical warfare, via specialized structures known as acrosomes, which are located on the tip of the sperm.

A normal human male produces a wide variety of different types of sperm (amorphous, coil-tailed, crook necked, double headed (bicephalous), pin-sized, short-tailed, weirdly shaped). As an example, the smallest sperm in a single human ejaculate can have 14% of the volume of the largest, and there is more variation within a single human ejaculate than in the mean sizes of sperm from all the different primates put together.

This wide variety of different sperm types has always been thought to be because sperm are so difficult to produce (about 30% of human sperm are acknowledged to be defective in some way). And may be a reason why males produce so many of them; but if natural selection had acted on males to produce ‘egg-getters,’ why should so many sperm be lame, defective, slow or deformed?

Baker and Bellis argue that semen should be seen as another human organ, comparable to the immune system. As such, it would be made up of many different types of highly specialized cells, all of which work together to get an important job done. Amongst rats, sperm from copulation will form a copulatory plug which will get in the way of sperm from the next male; and the sperm which form that plug are those which have smaller heads, and which are more likely to be decapitated. Human sperm does not form copulatory plugs, but there could be an evolutionary advantage for men whose sperm was especially nasty to anyone else’s. Baker and Bellis would suggest that older sperm serve as blockers, guarders, seek-and-destroyers, kamikazes etc.) Further, Baker and Bellis claim that when sperm from two different men are mixed, they could see sperm stuck together in the process of killing each other. They also saw an increase in the proportion of acrosome-reacted sperm (including that they had used their acrosome in the killing process), and they also saw many more dead sperm in mixtures than in single-sperm samples that had been separated and then re-mixed.

This sounds pretty convincing, but other studies from different mammals and birds reported nothing resembling warfare. And, subsequently Harry Moore, working in collaboration with colleagues at a local fertility clinic, reran Baker’s and Bellis’s experiment, but at a much higher level of sophistication. They differentially labelled sperm from the different males to see whether the agglutination Baker and Bellis had reported was between sperm from different males, as it would have to be. But, whatever agglutination there was turned out to be random with respect to the sperm sample; also, mixing sperm from different men caused no induction of the acrosome reaction, and no increase in sperm mortality.5

Some evidence of male sperm competition comes from analysing the detailed makeup of the sperm ejaculate. Human ejaculation takes place in a series of three to nine spurts, and through mammoth efforts, scientists have examined split ejaculates, obtained by capturing a few squirts of semen from various stages of ejaculation. In the experiment, early and late spurts were different. The final squirts actually containing a spermicidal substance which may ambush those sperm of the next male to ejaculate into the same female. At the same time, chemicals6 present in the first half of the ejaculate contribute some protection to sperm in the second half, and possibly against any chemicals deposited by the final spurts of a preceding male.

It is well known that men produce larger ejaculations when their sex lives are interrupted, and then resumed. This is most likely because seminal fluid has had a chance to accumulate over time; and with any emptying, the volume of ejaculation becomes less, so it’s not surprising that Baker and Bellis discovered that when a man spends time away from his female partner, he produces more sperm per ejaculate, once sexual relations have been restored. More interesting is their discovery (from analysing condom contents) that sperm concentration is higher when males engage in sexual intercourse than when they masturbate. Additionally, during sex, they claim that the amount of sperm transferred is adjusted according to the risk of sperm competition, especially how long it has been since the last copulation with the same woman, and even how much time the two have spent together recently. To quote Baker and Bellis1 “Males may not look very sophisticated in the moments leading up to and during ejaculation but… some very sophisticated adjustments are taking place.”

Evidence from primates suggests that human males are less adapted to competing with the sperm of other men, than male chimpanzees, which have to deal with polyandrous, promiscuous females (human beings are less prone to extra-pair copulations than chimps) As evidence, human sperm concentration diminishes more rapidly, with repeated ejaculations. In one intriguing experiment, men engaged in a ‘10-day depletion experience,‘ (averaging 2.4 ejaculations per day). Afterwards, their sperm output7 remained below their earlier pre-depletion levels for more than five months! By comparison, male chimps8 can ejaculate every hour for five hours, after which their sperm count is only halved, and they recover very rapidly. From this we can conclude that whatever the importance of sperm competition in humans, it is likely not as pronounced as it could be.

Biologists, especially Alexander Harcourt, have criticized specific parts of Baker and Bellis’s work more directly, pointing out that mammals are unlikely to produce designated, non-fertilizing sperm for a number of reasons:

  • Given a high natural loss, males may be unable to afford production of sperm that are certain not to be potential fertilizers, and whose supposed aggressive / defensive properties may never actually be called for.
  • Secretions from the accessory gland of males appear sufficient by themselves to coagulate semen and generate copulatory plugs (in other mammals); a male who used these secretions to form a copulatory plug, and continued to produce fertilizing sperm, instead of diluting his ejaculate with Kamikazes would then be at an evolutionary advantage.
  • Males of polyandrous species are under much more intense sperm competition than in monandrous species, in which females mate with only one male. However, a review of research findings shows that polyandrous species do not produce a greater number, or even a higher percentage of deformed (non-fertilizing) sperm. Neither do they produce more slow-swimmers, which you would expect to see if such Kamikazes were designed to stay behind, and fight with a competitor’s sperm.

Harcourt concludes9 that sperm competition (at least in mammals) occurs via what ecologists call ‘scramble competition,’ in which contestants struggle individually towards a goal, irrespective of the competition, as opposed to ‘contest competition,’ in which individual contestants would seek to physically better their opponents.

Other parts of the theories for specialist sperm were severely criticised10, when in a review of Baker and Bellis’s book, Roger Short pointed out that the sperm which had been designated ‘egg-getters’ were about as likely to be egg-getters as they were to contain little men. Twenty years previously, Short and three colleagues had published a paper in the scientific journal Nature, pointing out that these large-headed sperm were in fact production errors carrying twice the normal chromosome complement, and as a consequence they were incapable of producing a normal embryo. Somehow, Baker and Bellis had overlooked this publication.11

The fact that males produce12 such vast quantities of sperm may therefore be because fertilization is a simple ‘raffle,’ rather than a direct competitive struggle. A raffle would still involve sperm competition, but a competition in which the contestants compete by buying as many tickets as possible, rather than by tearing up each other’s entries. Another answer may be that males make lots of sperm simply because, considering their very high mortality rate (even without sperm competition) it benefits men to make as many sperm as possible, if fertilization is to occur at all. The warmth and moisture of the vaginal environment is ideal for bacteria to enter the female body, so the low pH is as harsh on human sperm as it is on other intruders. Phagocytes roam through every woman’s reproductive tract, and many sperm will end up being absorbed into the uterine wall. Sperm have a long way to swim, as well as the fact that about half will swim up the wrong fallopian tube, whilst a fertile egg awaits at the other.

The reproductive adaptations proposed by Baker and Bellis are rather more extreme than seems likely to have occurred by evolution. It is therefore not surprising that, so far, many of Baker’s and Bellis’s results have failed to stand up to scrutiny, and many of the rest of their conjectures are still far from proven.

In summary, the importance of sperm competition may be overblown and the incredible view of sperm competition that Baker and Bellis have perpetuated, whilst catching the public’s imagination, is little more than a sexual fantasy.


  1. Baker, R. R., and M. A. Bellis, Human Sperm competition (London: Chapman & Hall, 1995)
  2. Macintyre, S., and A. Sooman, ‘Nonpaternity and prenatal genetic screening’, Lancet (1992), 338: 839
  3. Kanada, M., T. Daitoh, K. Mori, N. Maeda, K. Hirano, M. Irahara, T. Aono, and T. Mori., ‘Etiological implication of autoantobodies to zona pellucida in human female infertility’, American Journal of Reproductive Immunology (1992), 28: 104-109; Ahmed K., and R. K. Naz., ‘Effects of human antisperm antibodies on development of preimplantation embryos’, Archives of Andrology (1992), 29: 9-20
  4. Parker, G. A., ‘Sperm competition: Sneaks and extra-pair copulations’, Proceedings of the Royal Society of London (1990), B, 242: 127-133
  5. Moore, H. D. M., M. Martin and T. R. Birkhead, ‘No evidence for killer sperm or other selective interactions between human spermatozoa in ejaculates of different males in vitro’, Proceedings of the Royal Society of London (1999), B, 266: 2343-2350
  6. Lindholmer, C., ‘Survival of human sperm in different fractions of split ejaculates’, Fertility and Sterility (1973), 24: 521-526
  7. Freund, M., ‘Effects of frequency of emission on semen output and an estimate of daily sperm production in man’, Journal of Reproduction and Fertility (1963), 6: 269-286
  8. Marson, J., D. Gervais, S. Meuris, R. W. Cooper, and P. Jouannet, ‘Influence of ejaculation frequency of semen characteristics in chimpanzees’, Journal of Reproduction and Fertility (1989), 85: 43-50
  9. Harcourt, A. H., ‘Sperm competition and the evolution of nonfertilizing sperm in mammals’, Evolution (1991), 45: 314-328
  10. Birkhead, T. R., H. D. Moore, and J. M. Bedford, ‘Sex, science and sensationalism’, Trends in Ecology and Evolution (1997), 12: 121-2
  11. Short, R. V., ‘Review of R. R. Baker and M. A. Bellis, Human Sperm competition: Copulation, Masturbation and Infidelity’, European Sociobiological Society (1997), 47: 20-23; Seuanez, H. N., A. D. Carothers, D. E. Martin, and R. V. Short, ‘Morphological abnormalities in spermatozoa of man and great apes’, Nature (1977), 270: 345-7
  12. Parker, G. A., ‘Why are there so many tiny sperm? Sperm competition and the maintenance of two sexes’, Journal of Theoretical Biology (1982), 96: 281-294

Sperm Wars: The Science of Sex – Reviewed and appraised

We have seen the book ‘Sperm Wars‘ cited as some type of super seduction guide several times, by several different people. This is quite puzzling, because the highly controversial ‘Kamikaze Sperm Hypothesis, which is the main theory behind the motivation for this book was only ever a biological theory that caught the media’s attention for a short span of time.

The Kamikaze Sperm Hypothesis has never been proven, and the author offers very little evidence for his theories in this book, or even his previous ones. In fact, the book has no index, no bibliography and mentions no other researchers; it therefore denies the reader the opportunity to examine more rigorous scientific studies that may give some evidence for the proposals put forward in the book.

Sperm Wars could by no means be classed as serious science; I suppose this is why the cover notes feature quotations from Cosmopolitan and Elle magazine, rather than any vaguely scientific publication.

The author, Robin Baker, proposes that something called ‘Sperm warfare‘ is the basis of all human life, and that everything we do, either consciously or unconsciously is to ensure the survival of our genes, and to pass our DNA to future generations. So, apparently every aspect of sex i.e. doing it, wanting it, talking about it etc. is because of this theory.

The author then takes his insights into the biology of sexual behaviour further, and mixes in a lot of speculation with it. The resulting book is a collection of thirty seven sexually explicit narrative sexual scenarios: such as sex in the woods, bad dates, drunken confrontations and husband-swapping; each of which Baker attempts to explain in terms of evolutionary biology, and Sperm competition.

Whilst the book claims to be grounded in evolutionary theory and natural selection, the reader is not given any sort of detailed explanation of either.

For those who may not be aware, Sperm competition is a scientific theory that has been around for about thirty or forty years. It has most often been applied to non-human animals, and uses evolutionary biology as an attempt to provide an explanation for how ejaculations from different males compete to fertilize the eggs of a single female. Robin Baker and a former collaborator (Mark Bellis) took sperm competition several steps further, and used extrapolation of animal models and behaviours, to claim that this theory works in humans.

So, the Wars referred to in the title of the book concerns competition between two or more men’s sperm within the same women, and also a sort of ‘battle of the sexes‘ that may occur between a male and female sexual pairing.

In the first type of sperm war, Baker claims that different types of sperm are each programmed to carry out a specific function. Some are ‘Egg getters’, programmed to attempt to fertilize the female’s ovum. The remainder, often the vast majority, are programmed for a Kamikaze role. Instead of attempting to find and fertilize ova themselves; their role is to reduce the chances that the egg will be fertilized by sperm from any other male.

In the second type of sperm war, Baker claims that a woman’s body might be ‘trying’ to avoid conceiving with a particular man, whereas a man’s body will be ‘trying’ to fertilize her egg.

Both types of ‘Wars’ are highlighted through the book, to illustrate various sexual behaviours and experiences that fulfil the objectives of each type of battle.

Unfortunately, the author fails to mention that other researchers have failed to reproduce most of the experiments that underpin his research, and that his Kamikaze Sperm Hypothesis has been disproved repeatedly by scientists, using more modern data and analysis.

The book therefore only seems to be a cursory nod to science, followed by an immoderate attempt to appeal to Frat boys, using fierce imagery of chemical warfare and destruction occurring inside women’s bodies, as different sperm do battle to the death.

A brief history of sperm

I’m always surprised at how little the people interested in having as much sex as possible actually know about reproduction and procreation. Much of the history is fascinating stuff, filled with interesting characters who had amazing ideas.

Things that we take for granted today weren’t always so, and by way of a quick primer, I should like to offer this extract from the work of Tim Birkhead (Professor of Behavioural Ecology at the University of Sheffield) who explains this subject far better than I could, and really brings the history to life.

Eggs and sperm – the stuff of occasional miracles and frequent accidents. Although we now take it for granted that sperm and egg must fuse to produce a new being, the road to this discovery was as long and winding as the oviduct itself. This is hardly unexpected given the microscopic size of sex cells and the temporal separation of insemination, fertilization and birth. It is also not surprising that our understanding of male and female roles in reproduction should have fluctuated through the course of time, In Homer’s day females ruled supreme in reproduction and pregnancy was thought to result from microscopic ‘animalculae’ carried in the air which somehow found themselves inside the female. The man’s role was unimportant and the concept of paternity unknown since ‘man lacked all sense of responsibility for survival of the species’1 The very term ‘Mother Nature’ stems from this period in which goddesses were all important and females dominated reproduction. Easter is the pagan legacy of this; named after the goddess Oestrus and celebrated with the ultimate symbols of female fertility – Easter eggs.
Only when the parallel between planting seeds into the womb of mother earth and the impregnation of a female with semen became apparent did the male’s role in reproduction assume a special significance.1 That insemination was known to be an integral part of reproduction is clear from the Greek story of Pasiphae and her husband Minos. Fed up with his persistent infidelity, Pasiphae put a spell on Minos so that in subsequent affairs he ‘poured forth in his semen a swarm of poisonous snakes, scorpions and centipedes, which devoured the woman’s entrails’.

In his book Ornithologia, published in 1599, the Italian scholar Ulisse Aldrovandi commented on the great lustfulness of the rooster.2 In contrast to other birds like the eagle and sparrow, who ‘copulate less frequently and are content with a single partner, the rooster treads his numerous wives fifty times a day.’ Aldrovandi also noted that aggression among cockerels was not associated with the acquisition of food or protection of their offspring, but was motivated entirely by the desire to maintain sole control over their females: ‘The rooster fights because he does not wish any of his hens to be touched by anyone and he thus performs the functions of a wise father protecting his honour.’ The cause of the cockerels’ salacity, Aldrovandi suggested, was their ‘especially abundant genital semen: since they cannot endure the irritation it produces they hurry towards sexual satisfaction.’ The Italian anatomist Fabricius ab Aquapendente (1537-1619) was the first to identify the ovary of the hen as the source of ova, but was unable to transpose the concept to humans because the ovaries of birds and women are so different in appearance. Fabricius taught and greatly influenced William Harvey (1578-1657), whose main claim to fame was discovering the circulation of the blood. The two men differed on a number of points relating to reproduction. Fabricius thought that hens could store viable sperm for an entire breeding season (several months) following a single insemination, but Harvey accurately showed that thirty days was the maximum duration.3 Fabricius thought that semen stimulated the generative process without entering the egg; Harvey was convinced that embryonic development was initiated by semen penetrating the egg, but without a microscope he was unable to demonstrate this.

The story of the discovery of spermatozoa – literally ‘sperm animals’ – by Anton Leeuwenhoek (1632-1723), or more likely by his student Johan Ham, is well known.4,3 Leeuwenhoek reported to the Royal Society how, by means of his home-made microscope – comprising a single exquisitely ground lens, which magnified 300 times – he had observed in his own semen millions of vigorously swimming spermatozoa. His letter to the Royal Society is somewhat coy: ‘What I investigate is only what, without sinfully defiling myself, remains as a residue after conjugal coitus. And if your Lordship should consider that these observations may disgust or scandalise the learned, I earnestly beg your Lordship to regard them as private and to publish or destroy them as your Lordship thinks fit.’ Luckily, the Royal Society thought it appropriate to publish Leeuwenhoek’s findings, in which he suggested that is was the minute microscopic creatures swimming in the semen that entered the egg and resulted in fertilization. This was controversial stuff, and some of his colleagues at the Royal Society thought that all that Leeuwenhoek had seen were parasites. After all, Leeuwenhoek had shown the existence of numerous microscopic animals when he examined the scrapings from his teeth!

It wasn’t until a further century had passed that another Italian, Lazzaro Spallanzani, a priest cum scientist, provided unequivocal evidence for Leeuwenhoek’s spermatozoa hypothesis for fertility. Given its current unease with matters sexual, it seems rather surprising that the Church should have provided Spallanzani with both moral protection and financial assistance in his efforts to establish the role of semen in reproduction. Spallanzani worked mainly with frogs, whose reproductive behaviour had been lovingly described by the Dutch biologist Jan Swammerdam (1367-80) in his Book of Nature.3 During the breeding frogs:

“become so eagerly intent on the business of propagation, that they take no care in a manner of their own safety … The male frog leaps upon the female, and when seated on her back, he fastens himself to her … and throws his forelegs round her breast … He most beautifully joins his toes between one another, in the same manner as people do their fingers at prayer … and closes them so firmly that I found it impossible to loosen them with my naked hands … At last the eggs are discharged in the female’s fundament in a long stream, and the male … immediately fecundifies, fertilizes or impregnates them by an effusion of his semen. As soon as these eggs have escaped from the female body, between hers and the male’s hinder legs, and have been impregnated by the male’s semen, the two frogs abandon each other.”

Inspired by some novel but unsuccessful experiments of two colleagues, Spallanzani made pairs of prophylactic oilskin trousers for male frogs to prevent their semen from reaching the females eggs.3 The experiment worked: despite the encumbrance of the trousers the males grasped the females, whose eggs were not fertilized, ‘for want of being bewedded with semen’. Spallanzani then conducted the other essential part of the experiment. Recovering the drops of semen from inside the trousers, he applied these to a female’s eggs which subsequently developed. Moving swiftly from external to internal fertilization and from frogs to dogs, Spallanzani performed the ultimate experiment. He took a female spaniel and before she came on to heat placed her under lock and key inside his apartment, away from male dogs. When she was obviously in oestrus Spallanzani found a male spaniel ‘which furnished me, by spontaneous emission, with nineteen grains of seed, which were immediately inseminated’ into the female. Sixty-two days later ‘the bitch brought forth three very lively puppies’ which resembled both the male and the female. This was the first ever successful artificial insemination involving internal fertilization. With some justification Spallanzani was delighted with his efforts: ‘the success of this experiment gave me a pleasure which I have never experienced in any of my philosophical researches.’

Spallanzani’s studies demonstrated unequivocally for the first time that semen was essential for fertilization, and in doing so dispelled the centuries-old concept of spontaneous generation. Notwithstanding these clever experiments Spallanzani still thought that ‘spermatic worms’ played no role in fertilization. The reason for this was the outcome of another ingenious investigation in which he filtered semen in order to establish which component of semen – sperm or seminal fluid – was responsible for fertilization. A mixture of filtered semen and eggs generated fertile eggs and Spallanzani deduced, entirely logically, that it was the seminal fluid rather than the spermatozoa that triggered development. What he had not realized was quite how difficult it was successfully to separate sperm from seminal fluid and it is now obvious to us that some sperm must have remained. On the basis of these experiments Spallanzani believed that the seminal fluid stimulated the foetal heart, which lay pre-formed inside the egg, and triggered development.5 It was nearly another century before George Newport in 1853 showed, again using frogs, that sperm actually penetrated the egg and were essential if fertilization was to occur.4

Spallanzani was an ‘ovist’, believing each egg to contain a pre-formed embryo. In rather vigorous contrast, the ‘spermists’ thought that the sperm contained the entire embryo and that copulation and insemination were little more than embryo transfer. In the spermist’s scheme the female was regarded merely as a recipient vessel to provide the optimum environment for the embryo’s growth. Nicholas Hartsoeker (1665-1725) encapsulated the spermists view of the male’s central role in reproduction in his drawing of a sperm containing an extremely cramped homunculus with a gigantic head, In fact, Hartsoeker never claimed to have seen the little man inside a sperm, merely that if he could this is what it would look like. Nevertheless, the idea of a pre-formed body inside each sperm was an appealing, and not unreasonable, one during the seventeenth and eighteenth centuries. It did, however, worry James Cooke, an English doctor, who wondered in 1762 what happened to all the sperm that did not give rise to a new person.4 He thought they might not die, but ‘live a latent life, in an insensible or dormant state, like Swallows in Winter, lying quite still like a stopped watch when let down, till [they] are received afresh into some other male Body of the proper kind’.

But it was these wasted sperm, together with their minute size, that finally brought the demise of the spermist viewpoint. The French physician Pierre-Louis Moreau de Maupertius summed up the spermist’s problem in 1744: ‘This little worm, swimming in the seminal fluid, contains an infinity of generations, from father to father. And each [pre-formed creature inside the sperm] has his seminal fluid, full of swimming animals so much smaller than himself.’ Sperm within sperm within sperm … on and on into infinity. Hartsoeker tried to calculate how small the sperm in the original rabbit would have to have been to account for all the rabbits that had ever lived. But it didn’t add up. Or, rather, it did, but the answer was so incredible, a figure involving 100,000 zeros, that it seemed ludicrous.

Exactly the same problem faced the Ovists.3 The mother of us all was Eve and her ovaries must, like a Russian doll, have contained an endless series of smaller and smaller homunculi to sustain the human race. Hardly a likely scenario. There were other objections: the ovist view could not, for example, account for the occurrence of hybrids: if the ovum of a horse contained a pre-formed horse, where do mules come from?

The alternative to the pre-formationist view of both the spermists and the ovists was epigenesis – the idea that embryos resulted from the fusion of male and female sex cells, an idea favoured by William Harvey, among others. But even this had its problems. While the observations of early embryologists were consistent with the generation of new structures arising during development, there had to be some sort of pre-formation to account for the resemblance between offspring and their parents. The transfer of this information, it was deduced, must occur at conception.4 The turning point came in 1875 when Oscar Hertwig showed, using sea urchins, that the sperm head fused with the female genetic material inside the egg to form the nucleus of a new being.


  1. Jöchle, W., ‘Biology and pathology of reproduction in Greek mythology’, Conception (1971), 4, 1-13
  2. Lind, L. R., Aldrovandi on Chickens (Norman: University of Oklahoma Press, 1963)
  3. Pinto-Correia, C., The Ovary of Eve: Egg and Sperm Preformation (Chicago: University of Chicago Press, 1997)
  4. Moore, J. A., Science as a way of Knowing (Cambridge, Massachusetts: Harvard University Press, 1993)
  5. Laurila, A., and P. Seppa, ‘Multiple paternity in the common frog (Rana temporaria): genetic evidence from tadpole kin groups’, Biological Journal of the Linnean Society (1998), 63, 221-32