The red jungle fowl is the ancestor of the domestic chicken.
Like a farmyard rooster, the cock is equipped with a good many THE PEACOCK ' S TALE
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ornaments that his mate does not share: long, curved tail feathers, a bright ruff around the neck, a red comb on the crown of his head, and a loud dawn call, to name the most obvious. Marlene Zuk wanted to find out which of these mattered to female jungle fowl, 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 color, both of which were less colorful than in healthy males. Zuk found that hens preferred cocks with good combs and eyes but paid less attention to plumage. She failed to make hens go for males with fake red elastic combs on their heads, however; they found them too bizarre. Nonetheless, it was clear that hens paid most attention to the most health informative feature of a cock."
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, 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, and 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 housefinch ' s and a flamingo ' s red plumage also depends on carotenoids. 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, shellfish, or other plants and invertebrates. But their ability to extract carotenoids from their food and deliver it to their tissues is greatly affected by certain parasites. A cockerel affected by the bacterial 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
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specific biochemical effect, but it seems to be unavoidable and is therefore extremely useful to the female: The brightness of carotenoid-filled tissues is a visible sign of the levels of parasite infection. It is not surprising that red and orange are common colors in fleshy ornaments used in display, such as the combs, wattles, and lappets of pheasants and grouse."
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 his comb and wattles will be. The problem for the cockerel is that the higher his level of testosterone, the greater 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 children in the West Indies revealed that the children are much more likely to catch an infection shortly after their cortisol levels have been high because of family tension or other stress:" 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: S6 There seems to be something about steroid hormones that unavoidably depresses immune defense. This immune effect of testosterone is the reason that men are more susceptible to infectious diseases 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 breeding season and die. It is as if male animals have a finite sum of energy that they can spend on testosterone or immunity to disease, but not both at the same time."
The implication for sexual selection is that it does not pay to lie: Having sex-hormone levels that are too high increases the size of your ornaments but makes you more vulnerable to parasites, which are revealed in the state of those ornaments. It is possible
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that it works in the other direction: The immune system suppresses the production of testosterone. In Zuk 's words, "Males are thus necessarily more vulnerable to disease as they acquire the accoutrements of maleness. ""
The best proof of these conjectures comes from a study of roach, which are small fish with reddish fins, in the Lake of Biel in Switzerland. Male roach grow little tubercules all over their bodies during the breeding season, which seem to stimulate females during courtship as the fish rub against each other. The more parasites a male has, the fewer tubercules he grows. It is possible for a zoologist to judge, just from a male 's tubercules, whether he is infested with a roundworm or a flatworm. The implication follows: If a zoologist can deduce which parasite is present, a female roach probably can as welclass="underline" This pattern results from different kinds of sex hormones; one can be raised in concentration only at the expense of leaving the roach vulnerable to one kind of parasite; the other can be raised only at the expense of lowering defenses against 59
another kind of parasite:
If cockerels ' wattles and roach tubercules are honest signals, so presumably are songs: A nightingale that can sing loud and long must be in vigorous health, and one that has a large repertoire of different melodies must be experienced or ingenious, or both. An energetic display such as the pas de deux of a pair of male manakins may also be an honest signaclass="underline" A bird that merely shows its feathers, such as a peacock or a bird of paradise, might be a cheat whose strength has been sapped by bad habits since he grew the plumes: After all, peacock feathers still shine brightly when their owner is dead and stuffed. Perhaps it is no surprise, then, that most male birds do not molt just before the breeding season but adopt their spring plumage the autumn before. They have to keep it tidy all winter: The very fact that a male has looked after his plumes for six months tells a female something about his enduring vigor: Bill Hamilton points out that white fluffy feathers around a bird's rear end, which are common in grouse of various kinds, must be especially hard to keep clean if the bird has diarrhea. b°
Zahavi certainly believed that honesty was a prerequisite of
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handicaps, and vice versa. To be honest, he thought, an ornament must be costly; otherwise it could be used to cheat. A deer cannot grow large antlers without consuming five times its normal daily intake of calcium; a pupfish cannot be iridescent blue unless it is genuinely in good condition, a fact that will be tested by other male fish in fights. On the assumption that anybody who refuses to play the game and use an honest signal must have something to hide, males are likely to find themselves dragged into honest displays. Therefore, display ornaments are examples of "truth in advertising. "61