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that his mate does not share: long, curved tail feathers, a bright ruff around the neck, a loud dawn call and a red comb on the crown of his head, to name the most obvious. Marlene Zuk wanted to find out which of these mattered to female junglefowl, so she presented sexually receptive hens with two tethered males and examined which they chose. In some of the trials one of the cocks was reared with a roundworm infection in his gut, which affected his plumage, beak and leg length very little, but showed clearly in his comb and eye colour, both of which were less colourful than in healthy males. She found that hens preferred cocks with good combs and eyes but paid less attention to plumage. She failed, however, to make hens go for males with fake red elastic combs on their heads; they found them too bizarre. None the less, it was clear that hens paid most attention to the most health-informative feature of a cock.53

Zuk knew that poultry farmers, too, observe the comb and wattles of a cockerel to judge his health. What intrigued her was the idea that the wattles were more ‘honest’ about the state of a cockerel than his feathers. Many birds, especially in the pheasant family, do grow fleshy structures about their faces to emphasize during display: turkeys grow long wattles over their beaks, pheasants have fleshy red ‘roses’ on their faces, sage grouse bare their air sacs, tragopans have expandable electric-blue bibs beneath their chins.

A cockerel’s comb is red because of the carotenoid pigments in it. A male guppy fish is rendered orange by carotenoids also, and a house finch’s or a flamingo’s red plumage depends on carotenoids, too. The peculiar thing about carotenoids is that birds and fish cannot synthesize them within their own tissues; they extract them from their food – from fruit or shellfish, or other plants and invertebrates. But their ability to extract carotenoids 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. Little wonder that red or orange is a common colour in fleshy ornaments used in display, like the combs, wattles and lappets of pheasants and grouse.54

The size and brightness of such combs may be affected by parasites, but they are effected by hormones. The higher the level of 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.55 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 children in the West Indies revealed that they are much more likely to catch an infection shortly after their cortisol levels have been high.56 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.57 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. Eunuchs live longer than other men, and male creatures generally suffer from higher mortality and strain. In a small Australian creature called the marsupial mouse, all the males contract fatal diseases during the frantic