Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

For a female, an ideal sexual partner should simultaneously be maximally fit, sufficiently sexy and simultaneously willing to invest the greatest possible amount of time and energy into care for his offspring.It follows from the existence of conditional strategies that it is very improbable that such a partner will be found.Thus, the female must decide on some sort of compromise, reducing her demands in one or the other area or must find a suitable counter-strategy.  The most effective strategy according to which the female can resolve problems connected with selection of a sexual partner consists in separation of the roles of care-giver and biological father.She can achieve this shrewd solution in a simple way, which is called extra-pair copulation (EPC) in animals, while we use a commoner term amongst humans – partner infidelity.For females, the optimum state exists when the partner in biparental care for offspring (or even exclusive care for offspring) is the winner in the game for “who is the dumbest”, a male willing to invest the greatest amount into care for his offspring, while the biological father of the greatest number of progeny will be the male with the best combination of viability, fertility and sex-appeal.Direct observations in nature, in combination with modern molecular-biological methods of determining paternity, have shown that, in a large number of species of mammals and birds, females achieve this result with surprisingly high frequency (Sundberg & Dixon 1996), {12659} (Fig. XIV.9).

Fig. XIV.9 Extra-pair copulation (EPC) amongst buntings. The female of the yellowhammer (Emberiza citrinella) very frequently achieves that the biological father of at least some of her offspring is a more attractive male than her social partner. Of 32 analyzed nests, 69% contained at least one offspring whose father was not the social partner of the female. It was possible to determine the biological father of 23 “cuckoo” young using genetic methods. It is apparent from the graph that, in most cases, the biological father of the young was a male with a greater color index than that of the “deceived husband”. In one case, the males “placed the horns” on one another (see the arrows). According to Sundberg a Dixon (1996).

            It is obvious that a situation that is optimum from the standpoint of the female is disadvantageous from the viewpoint of the male caring for foreign progeny, and thus that the emergence of an appropriate male counter-strategy can be expected.The males mostly try intensely to prevent partner infidelity.They guard the female in the critical period and chase other males out of the territory.In a great many species, the male delays the first copulation to a time when it is obvious that the female has not been previously fertilized by a different male.In other cases, he mechanically prevents entrance into the sexual organs of the female following copulation.For example, some spiders create such a “chastity belt” in that, following copulation, they break off and leave part of their own sex organs in the sex organs of the female.In a great many species of mammals, including those species of primates in which their life style and reproduction strategy make competition of sperm very probable, the sperm coagulate in the female reproductive organs and form a copulation plug (Polak et al. 2001; Baumgardner et al. 1982; Dixson & Anderson 2002).

            A very common counter-strategy, through which the male at least minimizes losses following from partner infidelity, consists in reducing care for those offspring for which there is a greater risk that that they come from foreign fathers.In starlings and warblers, it has been found that the amount of paternal care is directly proportional to the time during which the male had the female “under control” in the critical period (Wright & Cotton 1994; Dixon et al. 1994) (Fig. XIV.10).

Fig. XIV.10 Reduced care for offspring amongst starling males exposed to the risk of infidelity. Long before the fertile period, i.e. 17-19 days prior to laying eggs (control) or in the fertile period, i.e. 0-5 days prior to the beginning of egg-laying (“unfaithful female”), starling females were caught and closed for 20 minutes in a cage located next to the cage of a foreign male within view of her partner. Males who witnessed this potential unfaithfulness of their females subsequently cared far less for their offspring than the males of unaffected pairs or pairs where the female was placed in the cage long before egg-laying. According to Wright and Cotton (1994).

Fig. XIV.11 Relationship between the size of testicles and the probability of sperm competition in birds. Birds whose reproductive system is characterized by the greatest probability of competition of sperm in the female reproductive organs (i.e. polyandric species) have the greatest relative testicle size. The relative testicle size was calculated as the residuals from regression between the testicle size and the average weight of a male of the particular species. In addition to average values, the graph also shows the standard error of the mean and the number of species in the particular category. According to Birkhead (1995).

            In humans, it has been found that children, according to newer studies only sons, are similar to their fathers, especially in the first years of life, when it could be useful to ensure a suspicious father that the offspring is really his (Christenfeld & Hill 1995){12874}.It has also been found that fathers, in contrast to mothers, were willing to invest in their children in proportion to how similar these children (on computer-modified photographs) are to them {11457}.Critics of these studies objected that, in prehistoric time, fathers did not have any mirrors, so that they had limited information about their own appearance (Bains 1996).However, it can be assumed with probability bordering on certainty that malicious uncles, aunts and friends were very willing to provide them with the relevant information even in prehistoric times.

            The male can also attempt to increase the probability of his biological paternity by copulating with the female repeatedly and, in some species, with high frequency.Simultaneously, the amount of sperm in the ejaculate is frequently far greater than the theoretical biological requirement.If the female is guilty of only isolated partner infidelity, there is high probability that the less numerous sperm of the foreign male will lose out in competition with the more numerous sperm of the social partner.It is known that, in many taxa, the size of the testicles and thus the number of sperm produced is positively correlated with the probability of repeated copulation of a single female with particular males (Hosken 1997)(Fig.. XIV.11 and Fig. XIV.12).Studies performed on humans (Baker & Bellis 1993a)have also shown that the amount of ejaculate in sexual intercourse is increased with the length of time during which the partners were not together in the previous period (and thus with the probability of partner infidelity and possible sperm competition).

Fig. XIV.12 Testicle size in bats is related to the probability of sperm competition. Comparative studies performed on 31 species of bats in twenty genera showed that the male testicle size is positively correlated with the probability of sperm competition, to be more exact with the size of the population of which the bats of the particular species are members at the time of reproduction in nature. The residual testicle size and group size were calculated so as to eliminate the effect of body size and phylogenetic relatedness. According to Hosken (1997).

            Thus partner infidelity is extremely advantageous for females for two reasons.On the one hand, it can lead to the optimum result (a male willing to invest the maximum amount of energy in care for progeny will take care of the offspring of a genetically ideal male) and, on the other hand, forces the male to invest substantial efforts in ensuring his biological paternity in the initial stages of reproduction (production of a large number of sperm, guarding the female, etc.), so that this reduces the advantageousness of the male strategy “if not this one, then another one”.