Sexual promiscuity

In species in which uniparental care for progeny is possible, r-strategy, mostly of males, the greatest possible promiscuity, copulation with the greatest possible number of females, is evolutionarily advantageous. In contrast, careful choice of the biological fathers of their offspring, i.e. copulation with the best male in the population, is more advantageous for K-strategists, mostly for females. However, this theoretical conclusion has been repeatedly thrown into doubt by the results of the observations of the behavior of organisms in captivity and in nature. It has been found in a number of species that, not only males, but also females copulate with a great many partners.

            The simplest explanation of this, at first glance illogical, behavior of females is that organisms do not subject their reproductive behavior to attempts to achieve the greatest possible fitness, but to attempts to gain a reward in the form of satisfying feelings of physiological pleasure. However, this explanation is apparently not sufficient. An individual can subject and probably does subject his behavior to this aspect, to what his/her nervous system feels to be pleasant; however, evolution makes the final decision. With a certain degree of exaggeration, we can say that we do not eat an apple because it is sweet but that we experience an apple as being sweet because it is evolutionarily advantageous to eat it. Like almost everything in biology, this principle is not one hundred percent valid. A smoker doesn’t enjoy a cigarette because it is evolutionarily advantageous to smoke, not to mention other types of addictive drugs.

            The above-mentioned arena hypothesis is a frequently mentioned hypothesis explaining the function of female promiscuity. This hypothesis assumes that a female copulates with a greater number of males to obtain sperm from various individuals in the population to create conditions for intergamete competition. Through copulation with a greater number of males, the female can insure herself against infertile males or against males with reduced fertility.

            There are a number of other hypotheses that attempt to explain the phenomenon of female promiscuity. In some species, it can be assumed that the sperm or other components of the ejaculate constitute not only a source of genetic material for the female, but also a source of energy for nourishment of the developing embryos or even for herself (Wedell 1994). In other species, it is assumed that the females ensure greater genetic diversity of their offspring in this way, reducing their mutual competition for resources (see the elbow room hypothesis, XIII. and also increasing the chance that an individual ideally adapted to the conditions of any particular micro-habitat will be present amongst the offspring (see the lottery model, XIII.

            Other models assume that, in species living in groups, it is advantageous for females if they prevent the males from finding out with certainty who is the biological father of the offspring. If the female copulated at least once with any male in the group, then no male can exclude that he is actually the biological father of any offspring and will thus not behave in an unfriendly manner towards the offspring. The phenomenon of concealed ovulation, which is encountered in many species of animals, and the ability of the female to manipulate the ejaculate inside her reproductive organs simultaneously allows the female to affect which of the copulating males finally becomes the biological father of the offspring (Pizzari & Birkhead 2000; Pizzari, Froman, & Birkhead 2002). In human beings, similar to other animals, these processes frequently occur to a major degree outside of the consciousness of the woman. However, experimental studies have shown that, in the fertile phase of the menstrual cycle, women prefer different types of short-term sexual partners (more dominant and masculine types) than in phases where conception is less probable (Penton-Voak & Perrett 2000; Penton-Voak et al. 1999). The results of studies performed on the students of the Prague Faculty of Science of Charles University also demonstrated that the smell of dominant males was preferred in the fertile phase only by women who had a long-term partner at the time of the experiment and who were thus probably unconsciously more interested in “good genes” than in a “good care-giver” for their offspring {12514}.

The greater probability of conception in cases of rape than during normal sexual intercourse could also be connected with concealed female choice at subconscious level (Starks & Blackie 2000). A study in the U.S.A. showed that, in a single year, 32,000 pregnancies resulted from rape, of which 38% of the women finally gave birth to their babies. The data indicated that the probability of conception following rape was 5%, compared to approx. 1.2% in normal sexual intercourse. Only 11.8% of the pregnancies ended in spontaneous abortion, compared to 13.8% of normal pregnancies. Because of the size of the data set, all the observed differences were statistically highly significant. 

            Female (and male) promiscuity is, however, sometimes explained in an entirely different way. This need not in any way be behavior advantageous for its bearer, the promiscuous female, but may consist in behavior controlled by a parasite and advantageous only for this parasite.

            Control of the behavior of a host by a parasite is apparently a very common phenomenon (see XIX.6.5). There are a great many parasites whose transmission from one host to another occurs at the moment of copulation or during behavior directly or indirectly connected with reproduction of the host species. Promiscuity could provide certain advantages for the host species; however, for the sexually transmitted parasite, it is the most important parameter influencing the effectiveness of its spreading in the population, thus a question of life and death. If a mutation occurs in a parasite that allows it to affect the nervous system of the host organism in a way so that it will behave in a more promiscuous manner, this will greatly increase the chance for the parasite that it will spread in the population horizontally from one individual to another. It is understandable that such a mutant very rapidly predominates in a population of parasites. A parasite can achieve the required result in various ways, either by direct intervention in the nervous system of the host or through its endocrine system, through production of suitable hormones or their analogues, or very indirectly, for example in that it causes itchiness of the sexual organs of its host (Dawkins 1976).

            The parasite need not be transmitted during the actual copulation. It is, at the very least, suspicious how frequently the mouths come into contact as part of the precopulation behavior of various species (mutual feeding, licking, kissing). In general, it can be stated that we kiss one another because it is pleasant. This undoubted fact is, however, uninteresting from an evolutionary standpoint. What requires an explanation is who and why programmed our nervous system so that kissing is pleasant. One of the surprising answers could be – a parasite.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more