'Sperm Wars' is the type of book to give sociobiology a bad name. Of course, to many social scientists, rightly or wrongly, sociobiology already has a bad name. This is why the term is now rarely used and euphemisms such as behavioural ecology and evolutionary psychology were invented.
(I use the term sociobiology rather than the more fashionable terms evolutionary psychology and human behavioural ecology advisedly, because many of Baker's claims actually deal with physiology rather than psychology or behaviour and therefore, strictly speaking, fall outside the remit of behavioural ecology and psychology.)
However, most of the charges levelled against sociobiology are fallacious (see The Triumph of Sociobiology
). However, in respect of `Sperm Wars', at least two of these charges, for once, have some merits.
Among the most familiar of the many charges levelled against evolutionary psychology are the claims, firstly, that evolutionary psychologists spin speculative untested (or even inherently untestable) `just-so' stories, and, secondly, that evolutionary psychologists are so-called `ultra-Darwinians' or `Darwinian fundamentalists' who claim that every human trait is necessarily an adaptation. In general, these charges have little merit.
Sperm Wars, however, is the exception that proves the rule. For once, both these familiar charges have some merit. In respect of virtually all of Baker's claims, an alternative non-adaptive explanation in which the characteristic in question is viewed as a by-product of more general purpose adaptations rather than itself adaptive is available and in some cases at least as plausible as Baker's own account. In accordance with the principle of parsimony (Occam's razor), the satisfaction of stringent criteria are usually required before the existence of an adaptation is recognised (see George C Williams's Adaptation and Natural Selection
). Yet Baker presents his material, in a format clearly oriented towards a popular mass lay readership, without giving a hint of how speculative and untested most of his ideas are.
Levels of Sperm Competition Among Humans
Baker claims that "worldwide... about 10% of children are in fact not sired by the man who thinks he is their father" and that "this is also the level found in Western industrial societies" (p60). In support, Baker cites the level of misattributed paternity reported by child support agencies. However, such agencies only generally test paternity where this is in dispute and therefore constitute an unrepresentative sample.
The level of misattributed paternity cited by Baker has now been discredited, at least as regards First World Western industrial societies of the sort which appear to be of primary interest to Baker (and presumably to his readership). Actual rate of misattributed paternity in First World Western industrial societies appear to be less sensational (Gilding 2005). (In fact, rampant misattributed paternity of the sort alleged by Baker would be problematic from the Darwinian perspective Baker purports to apply since it would raise the question of why `fathers' are so willing to invest in their putative offspring.)
This does not mean that sperm competition has played no part in human evolution. On the contrary, both male physiology (testicle size) and male psychology (male sexual jealousy) show evidence of adaptation to counter female infidelity. It does, however, suggest that sperm competition has played a lesser role in human evolution than some of Baker's more outlandish theories seem to presuppose.
Undoubtedly, Baker's best-known and most controversial claim is what he has termed `the Kamikaze sperm hypothesis'. Whereas sperm competition is usually conceived as entailing solely scramble competition, Baker suggests that contest competition is also involved. He argues that some sperm are designed, not for fertilising an eggs, but rather for preventing the sperm of rival males from so doing. The research on which this claim is based was, like most other research supporting Baker's contentions, conducted by Baker himself in collaboration with Mark Bellis.
In first positing this theory, Baker and Bellis (1988) pointed to sperm polymorphism. Large number of sperm - apparently "up to 40% in normal humans" - are apparently malformed. Baker claims that these sperm, rather than representing defective mutants, are in fact adaptive.
Given that there is likely to be strong selective pressure for fertile and competitive ejaculates, Baker and Bellis (1989) persuasively contend, "such consistent and abundant structures demand an adaptive explanation". Additionally, their claim that "sperm such as those with two heads or tails etc." are "morphologically adapted for intertwining and barrier formation" seems intuitively plausible and seems to be supported by their observation that such sperm are found to be disproportionately concentrated in the copulatory plugs (i.e. barriers of seminal fluid which are designed to perform a function analogous to that of a chastity belt, namely to block the access of subsequent ejaculations to ova) of bats.
A few months after its publication, Alexander Harcourt (1989) critiqued Baker and Bellis's theory on theoretical grounds. Whereas some of Harcourt's arguments are unconvincing, others are more compelling. In particular, he suggested that the disproportionate concentration of malformed sperm in the copulatory plugs in bats may reflect, not specialisation for this role, but rather simply the impaired swimming ability of such malformed sperm meaning they are unable to swim free.
More importantly, Harcourt presents data demonstrating that the proportions of polymorphic sperm contained in the ejaculates of males of different species does not correlate with the levels of polyandry (female promiscuity) characteristic of the mating system of the species in question. In other words, species with higher levels of sperm competition do not have higher levels of malformed sperm. In contrast, other traits thought to have evolved in the context of sperm competition (e.g. testicle size) are known to correlate with levels of polyandry. It has even been argued that it is the relative absence of high levels of sperm competition in species such as humans that has prevented abnormal sperm being selected against and hence reduced in number (Dixson 2009: p48).
In response, Baker and Bellis (1989) make the radical suggestion that "egg-getters may be the least, not the most, common sperm in an ejaculate and thus cannot simply be equated with the `normal' morph". On the contrary, "the possibility that egg-getters belong to a less common structural type than the normal morph should at least be born in mind".
However, the problem with this argument is that it undercuts many of Baker and Bellis's previous arguments. For example, if the sperm with extra heads and tails are actually the ones specialised for fertilisation, then the disproportionate concentration of such sperm in copulatory plugs is converted from evidence for Baker and Bellis's theory into strong evidence against it.
In addition to describing the supposed role of `blocker' sperm, Baker introduces the even less plausible concept of the `killer sperm', which purportedly detects the presence of sperm from different males inside the reproductive tract of females by "comparing the surface chemicals" on their own heads with those of their rivals and then jab the rival sperm with "corrosive poison". Among the many problems with this theory is the issue of how sperm detect whether a sperm which they encounter is from the ejaculate of a rival male.
Given that each sperm contains only half of the genes of the male from whom it originates (the other half of the genes necessary to create a person are, of course, provided by the mother's egg), even sperm from the same male may differ genetically from one another. Indeed, sperm from the same male could theoretically share no genes with one another whatsoever. Detection of whether sperm are from rival ejaculates or not on the basis of genetic markers is therefore problematic (Pound et al 2006: 24).
[In fact, assuming that a sperm's behaviour is programmed by the genes that it carries rather all of the genes of the parent organism, sperm may theoretically be programmed to poison, not only sperm from rival males, but also genetically dissimilar genes from the same male. This is because genes programme organisms (and sperm) to promote their own replication, not that of other genes that happen to come from the same parent. It would therefore be in the interests of a gene to programme a sperm to disable any sperm which does not contain the specific gene that programmes for that disabling behaviour. However, this seems to envisage something akin to what Dawkins refers to as a `Green Beard Effect', namely that a gene could code both for a detectable marker and the tendency to behave altruistically to others bearing that marker, which is regarded as implausible even in respect of organisms let alone single-cell sperm. It also raises the bizarre prospect that genetically dissimilar sperm from the same male may begin to wage war on one another while they are still inside that male's own body.]
In his defence, Baker seems to envision the more plausible prospect that sperm from the same male are detected by a shared chemical marker common to them all rather than direct detection of genetic similarity. Read more ›