Manfred's Column, February 2005: Engagement rings, parasites, and brains
Engagement rings, parasites, and brains
According to an unwritten law of American culture, the engagement ring should cost about two months salary. Once we assume that this custom is not the result of highly successful manipulative advertising strategies of the diamond monopolist DeBeers, we may ask, why does this custom exist? In particular, why does it exist in a society that prides itself in claiming that it economically optimizes practically all aspects of life? A diamond costs the giver a fortune but leads to very little benefit for the recipient. It is therefore quite the opposite of a reasonable investment. Why, we may ask again, are there such unreasonable investments, especially in a country where wise investment is regarded as as the ultimate goal of so many acivities?
This problem is a very general one and causes a stir among biologists discussing it, because within the animal kindom, a large number of similar behaviors can be observed that appear not to make any sense from the point of view of rational investment. The most widely known example in biology is probably the male peacock’s large and colorful tail feathers. The male uses them to form a wheel thereby impressing female peacocks. But apparently to his great disadvantage. It costs energy to to grow feathers and to carry them around and to maintain them. Moreover, they impede the rapid escape response to the presence of a predator. As all living beings, and their behavior, are the product of evolution, we may ask how peacocks with such large colorful tail feathers could come about and why they have not long gone extinct. In general, how can such useless features and behavior persist in nature?
Biologists have found that the peacock’s display actually works. It attracts the females which are more likely to produce offspring from those peacocks wearing the most glittering ornament. However, this observation does not lead us very far, as we may ask why peahens have such a weird taste. She, who reproduces with a male carrying genes for such an obvious obstacle to fitness for life, puts her own genes at risk in combining with this disadvantage and hence, decreases the probability of survival of her offspring. In other words. The special taste for large colorful tail feathers shares the same fate as large colorful tail feathers themselves: Both, the ornament and the taste for it, should quickly die out, as both incur costs for survival.
You need not assume that all the most beautiful peacocks get eaten. It is enough to make the assumption that growing and carrying large feathers costs resources that could otherwise be spent on more rapid growth and reproduction. In this view, evolution atomatically leads to what is currently called "lean capitalism", i.e., to the anihilation of useless features and behaviors.
If this is so, however, why do male nightingals sing and why are female nightingales attracted to the males with the most diverse songs? Why are some male beetles so bright and colorful (and hence easy prey for birds with sharp and trichromatic eyesight)? Why are the antlers of male elks so large and why have some male apes such colorful facial hair? Why are there so many useless features and behaviors in nature?
The answer which modern biology gives to this question briefly runs as follows. For specific reasons (whose exposition is beyond the scope of this paper; cf. Dawkins 1976, Buss 1994) sexual selection involves in many cases the competition among males for females. Females therefore have the possibility and power of choice, and they will choose the male for reproduction that appears to be the fittest, i.e., indicates the highest probability that the female genes will be carried to future generations. All other choice strategies are necessarily bound to die out, that is, if a female does not practice choice of the fittest male to mate with, she will necessarily have fewer and less fit offspring. So it is that genes that make females choose wisely will in the long run spread through the population and belong to the genetic makeup of females. Males are not only subject to the evolutionary pressures of the survival of the fittest but also to sexual selection, i.e., their selection by the female reproductive partner, since both of these are going to pass on their genes to future generations in common packages (in our species, these packages are usually called children).
How does the female detect the fitness of a male? Keep in mind that we are not talking about what some people do to their bodies in studios stuffed with machines to torture themselves. Genetic fitness is mathematically defined as the probability of an organism’s genes occurring in future generations. As deceptive behavior frequently occurs in nature (cf. Trivers 1985), females are having a hard time distinguishing between non-fit braggarts and truly fit males. Therefore, females use so-called fitness indicators to guide their decisions, and these fitness indicators have recently been studied by biologists in some detail.
A fitness indicator is a feature which males can afford to have, because the male is healthy and genetically fit. The feature therefore indicates the high genetic quality of the male. To take the example of the male peacock. Its large colorful tail serves as a fitness indicator because only peacocks who are strong, have sharp senses, powerful immunologic defences and quick response times can afford a large and colorful tail. Hence, as long as there are differences between male peacocks in these features or any combination of them, leading to the variability of genetic fitness, it makes good sense for female peacocks to chose those males for reproduction who behave most wastefull with resources. Peacock males with a large colorful feather wheel and peahens with a taste for such a wheel are therefore likely to produce fitter, i.e., more and higher quality, offspring.
This argument presupposes that there are variations of fitness within a species. Why, one might ask, are not all individuals of a given species optimaly adjusted to their environment? After all, there was ample time during evolution to come up with the perfect adaptation for everyone. As regards peacocks, why are not all peacocks strong, immune to disease, and have sharp vision, as more than enough time has passed for females to chose only the fittest males (and thereby breed the very fittest possible males). This deceptively simple question — why are there individual differences between members of a species in respect to fitness — vexed biologists for more than two decades. People have come up with basically two answers, (a) mutations and (b) parasites.
(a) Although organisms came into existence by the processes of spontaneous mutation and selection, most mutations have a detrimental effect on the fitness of an individual organism. For a species to survive in the long term, random mutations have to somehow be wiped out. As it happens, most random mutations decrease the fitness of the carrier, i.e., reduce the probability of its genes being carried by future generations. Mutations therefore always produce a variability of fitness needed for sexual selection to work. As regards female choice, we may add, it would be nice if a female somehow were able to gauge the amount of mutations that a male carries. In order to do this, she should evaluate an organ that is sufficiently complex to mirror the effects of as many genes as possible, because in this case the function of this organ would be a highly sensitive measure of the mutation frequency in a given male. Miller (2000) argues that the human brain, whose development is controlled by about half of all human genes, and whose functioning is controlled by about one third of all human genes, is an ideal candidate to serve the function at issue. The brain tells you more about the genetic fitness of its carrier than any other organ, as its functioning is the product of so many genes. In this statistical perspective, the brain is a reliable indicator of the genetic fitness of the individual. It represents something like an Achilles heel, indicating precisely the fittness of a male. It is exactly the high degree of proneness to genetically caused diseases that renders the human brain as a perfect fitness indicator for female choice.
(b) About half of all organisms on earth are parasites (Price 1980), and to all somewhat larger organisms — insects, reptiles, birds, mammals, including humans — there is not only the threat of random mutation, but also of parasites. This idea at first may appear strange, but at closer inspection, it can hardly be rejected (cf. Ridley 1993). No matter whether they come in the form of viruses, bacteria, or worms, parasites develop in parallel to their host organisms. As they have a faster generation cycle than their hosts, they are always ahead regarding their evolutionary adaptive development by means of mutation and selection. Whatever defense strategy a host organism may develop (by means of mutation and selection), the parasites will be quick in responding with counter measures, because they reproduce, and hence may evolve, faster. In short, parasites have the adaptive edge compared to their hosts. This is why they represent a continuous threat to their hosts, just as random mutations do. From this it follows, that no species of somewhat larger organisms is in an evolutionary stable equilibrium, as individual organisms permanently have to fight — with varying success — against the parasites that plague them. The large colorful ornamenal feathers of the male peacock signals to the potential female mate, that this male is relatively free of parasites, because if gut-worms, blood sucking schistosomae, bacteria, and viruses had been present in this male, it would not have been able to grow and carry such an ornament. Female peacocks with an inborn taste for males with large colorful feathered wheel-like tails therefore pass on their genes to offspring with good immunological defenses, and therefore, their offspring are less likely to suffer from parasites. In other words: female peacocks with different tastes have been evolutionary dead ends.
We have just seen that one important feature of fitness indicators is their uselessness. Just as the future husband, in giving an expensive engagement ring to his beloved fiancee, thereby signals that he can afford such wasteful luxury, and can therefore also afford to support children for decades, the elk uses his large antlers, the nightingale his complex singing, and the peacock his large colorful beautiful feathered wheel to signal one thing: I can afford this, hence I am fit.
To finish our argument, let us be reminded of the fact that many capabilities of the human brain could hardly be more useless. Why, we may ask within the framework of the ideas just discussed, is there music, painting, dancing, elaborate language and humor? These typical human capabilities obviously do not enhance survival. On the contrary, he, who sings, talks, or even tells jokes, at best wastes energy (which he could use to find food or a mate), or, worse, becomes food for an energy-hungry predator. Once looked at this way, many actions and capabilities of the human brain appear superfluous and useless. So why do they exist?
From what has been said so far, this question may easily be answered as follows: The brain with all its wonderful features and its wasteful use of energy (25% of what we eat is used by the brain, which makes up just 2% of our body) is the product of sexual selection. It is a fitness indicator.
This general view by no means rules out that our brains are useful. It merely shows that one needs to be cautious when thinking about the evolutionary usefulness of features or behaviors. Sexual selection may — according to the argument presented above — lead to the development of features and behaviors that are actually useless. Such processes were most likely of high importance in the course of evolution, as evolution by mutation and selection alone runs into problems when accounting for the development of entirely new complex features. It is hard to imagine, how organisms carrying the necessary intermediary steps in these developments were able to survive. Let us look at the following thought as an example (cf. Miller 2000, pp. 170-172).
It is hard to imagine how something as complex as the wings of birds could ever have evolved from the small undeveloped arms of the bird’s ancestors, the dinosaurs. A novel mutation may lead to the development of an extra skin-flap between the arms and the chest. It is unlikely however, that this extra skin enables its carrier to fly. On the contrary, it cost energy to grow and may have been an impediment to its carrier. Further mutational steps towards real wings suffer from the same drawback, i.e., they come with costs and yet without the benefit of flying. Hence, mutation and selection alone can hardly explain the survival of the many intermediary steps needed for the development of any complex feature or organs such as wings. It is this problem that sexual selection can easily solve. One only needs to assume that the extra skin-flap served to advertise the large size of the carrier’s body (by providing a much larger body outline) for a potential female mate. The extra surface area thus may not have evolved for flying but rather as a colorful ornament for displaying an apparently larger body than it actually has. Being entirely useless, thereby serving as a good fitness indicator for females who may have developed a taste for large skin-flaps.
From this example, it can be seen that sexual selection serves a function similar to venture capital in economics. It supports innovations which at first appear to be useless, but may later turn out to be the backbone of entirely new economic branches. If one were to invest only with immediate gains in mind (analogous to mutation and selection, acting on the short period of the survival of a single organism), truly new developments could hardly ever happen or at least took a very long time.
With respect to brain evolution, the mechnism described in this example may explain why the human brain evolved before the accompanying advantages became the main determinant features of our daily lives. We, i.e. individuals of the species Homo sapiens with our large brains, lived in Africa one hundred thousand years before supposedly essential cultural achievements, such as agriculture, the domestication of animals, the making, and use of tools, complex social structures, writing, law, and medicine, which are usually attributed to our brain’s capacity. Our complex mind was, as it were, at first nothing but the software equivalent of useless hardware such as the peacock’s colorful tail or the elk’s large antlers (cf. Dawkins 2000). It was much later that the brain turned out to be used for more than impressing females through paintings, creativity, music, poetry, and humor.
Large brains and complex useless cognitive functions as indicators of fitness (for the defense against mutations and parasites), are analogous to engagement rings — this idea may appear strange at first glance, but it nicely demonstrates the variety and creativity of current evolutionary thinking. Of course, the human brain may just as well be viewed as the result of the development of either increasingly complex communication for hunting (Pinker 1994, Hurford et al. 1998), or techniques of deception when fighting over the captured prey (Cosmides & Tooby 1992), or group warfare (Alexander 1989). Recent history, however, illustrates the dangers of such obsessions with violence and power. The idea that our brains evolved — despite all of its disadvantages for immediate survival — for art, poetry, music, and dance, and that survival and advanced communication, cheating and warfare came later as by poducts, is an unusual one. I like it!
Original title: Spitzer M (2000) Verlobungsringe, Parasiten und Gehirne. (Geist & Gehirn). Nervenheilkunde 19(7):415-417