The comb of a cockerel can be used to judge health by both a potential mate and by farmers. The comb itself is an adornment, evolved through sexual selection, and so in many ways is a burden to survival:
- [The] comb is red because of the carotenoidpigments in it...The peculiar thing about caretinoids is that birds and fish cannot synthesise them within their own tissues; they extract them from their food - from fruit or shellfish, or other plants and inverterbrates. But their ability to extract caretinoids from their food and deliver it to their tissues is much affected by certain parasites. A cockerel affected by the disease coccidiosis, for example, accumulates less carotenoid in his comb than a healthy cockerel - even when both animals have been fed equal quantities of carotenoid. Nobody knows exactly why the parasites have this specific biochemical effect, but it seems to be unavoidable and it is therefore extremely useful to the female: the brightness of carotenoid-filled tissues is a visible sign of the levels of parasite infection....The size and brightness of such combs may be affected by parasites, but they are effected by hormones. The higher the level testosterone in the blood of a cockerel, the bigger and brighter will be his comb and wattles. The problem for the cockerel is that the higher his level of testosterone, the greater will be his parasite infestation. The hormone itself seems to lower his resistance to parasites. Once again nobody knows why, but cortisol, the 'stress' hormone that is released into the bloodstream during times of emotional crisis, also has a marked effect on the immune system. A long study of cortisol levels in the children in the West Indies revealed that they are much more likely to catch an infection shortly after their cortisol levels have been high. Cortisol and testosterone are both steroid hormones and they have a remarkably similar molecular structure: of the five biochemical steps needed to make cholesterol into either cortisol or testosterone, only the last two steps are different. There seems to be something about steroid hormones that unavoidably depresses immune defence. This immune effect of testosterone is the reason that men are more susceptible to infectious disease than women, a trend that occurs throughout the animal kingdom...It is as if male animals have a finite sum of energy, which they can spend on testosterone or immunity to disease, but not both at the same time.