Friday, August 10, 2007

Chernobyl and Sexual Selection

This was fun to write about. A new study around Chernobyl has analysed the effect of radiation on bird health. Birds use antioxidants for a variety of activities. The research studied the health of birds which 1) migrate long distances 2) lay large eggs 3) Develop bright plumage using carotenoid, a type of antioxidant. All these three activities take up large quantities of antioxidants. Radiation creates free radicals which damages DNA. This damage can be limited if there are enough antioxidants in the body to soak up the free radicals. The researchers A.P. Møller and T.A Mousseau hypothesized that because birds in the above 3 categories had fewer antioxidants left to mop up dangerous free radicals, these birds would be most adversely affected by exposure to radiation around Chernobyl. Their hypothesis was supported by their test results.

The story was reported in a number of press releases (July 12 2007). The Economist has a different take on it. According to them the results cast light on the theory of sexual selection as well. This theory among other things tries to explain adornment and ornamentation in animals such as the peacock' s tail, bright plumage, the fantastic feathers of the birds of paradise. Such features are clearly detrimental to the bearers health. Why does the utilitarian process of natural selection allow such flagrant excess? Colorful plumage in birds is usually a sexual signal influencing mate choice. The signal may be arbitrary or it may be a sign of underlying good health or good genes. The Economist argued that since the carotenoid-based bright red and yellow plumage birds declined in health, the plumage really does come at a price indicating underlying good health.

But I think there is a problem with this argument. The report in the Economist was correct is saying that there is a debate in biology between those who think signals such as flashy feathers are essentially arbitrary and those who think they are signs of underlying health and good genes. Let's look at this using just female choice. In the "arbitrary" or "good taste" model first enunciated by R.A. Fisher, an initial bias towards a particular feature say bright color is set up by chance, essentially some females may take a fancy for a particular color. Because of this initial bias, natural selection will favor females who go along with the fashion because her son will inherit his father's bright color and her daughter will inherit her preference for bright color. This then sets an ever increasing evolutionary spiral of brighter colors in males and stronger discrimination for brighter color in females. The bright plumage develops because it acts as a label for sexual attractiveness. In this case even though being bright takes up large amounts of antioxidants, being fashionable by in itself is adaptive, since it leads to greater reproductive success.

In another model known as "good sense", the females actively are discriminating for underlying good health using bright color as an indicator. Loss of color may be an early sign of parasite infestation, so females can use brightness as a diagnostic tool. This will set up selection in males to deceive the female by false advertisement of bright color. In effect even an unhealthy male
who manages to produce a bright color will prosper. This in turn will set up a counter-selection in females to see through this deceit. Again male color and female discrimination will evolve, in evolutionary biologist Helena Cronin's memorable words, over generations "the females utilitarian stethoscope gradually burgeoning into a brilliant kaleidoscope". This model was modified into the "handicap" hypothesis by A. Zahavi who argued that advertisement has to be costly to reveal the true health of the male. Think of conspicuous consumption. If you buy an expensive car, it will cost you financially, but at the same time you are signalling to society that you are quite well off. What you lose in cash, you gain in enhanced social status and its benefits. In this sense, according to Zahavi females prefer the most handicapped males. In the "good sense" or handicap model only the healthiest males would be able to allocate large amounts of carotenoid antioxidants to color their feathers and still manage to stay alive at least long enough to reproduce.

After this long discussion, the conclusion is brief. The effect of both types of female choice ultimately results in bright plumage which means large usage of carotenoid which in turn imposes a health cost on the male. Carotenoid based plumage comes at a price regardless of which type of female choice produced it. So the harm to bright colored birds cannot be use to discriminate between the two hypothesis.

An interesting point to note here is that the original research although indicated that the bright plumage may be sexual signals, did not address the issue whether the decline in bright colored bird's health could be considered a test for the two models of sexual selection. This inference was drawn by the Economist reporter. I emailed my above objection to the science editor of the Economist. He was kind enough to write back saying "I do take your second point, though, that Fisher's original model can accidentally impose health costs. I shall investigate the matter further".

The first point was my objection that the bright color was a sexual signal in the first place. Sexual selection holds a special place in modern evolutionary theory. W.D. Hamilton one of the most influential evolutionary biologists of the 20th century once remarked, that if he understood why there are certain bird species in which both sexes are brightly colored, he would die happy. Unfortunately he died of a mosquito bite in March 2000 (complication due to malaria) without solving this mystery. The debate is sure to rage on in the coming years.

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