Birds of paradise

Birds of paradise are used to show how female choice can reshape the traits that evolve in a population. Their extravagant male tails illustrate both a runaway account of sexual selection and a rival explanation in which a costly handicap advertises genuine quality; the example makes visible how preference, inheritance, and reproductive advantage can reinforce one another without implying that evolution acts with foresight.

Mating success and female choice

In the book’s account of the “he-man” strategy, females receive little help from the father in raising offspring and instead seek good genes. They are described as choosing carefully among males rather than mating indiscriminately. If females share much the same information about male quality, their choices may converge on a small number of favored males. Those males can then obtain many matings at relatively little reproductive cost, while females’ selectivity limits which males gain that advantage. Birds of paradise are cited, alongside elephant seals, as a case in which a few males are presumed to achieve much of the mating success.

The proposed benefit to a female is indirect but potentially large: if her son becomes one of the males favored by females in the next generation, he may sire many offspring, giving her many grandchildren. Sexual attractiveness can therefore become valuable in itself. A trait need not remain useful only as evidence of strength, health, or survival; once it reliably draws female preference, producing attractive sons can benefit the genes associated with that preference and appearance.

Runaway evolution of the tail

The account begins with a plausible signal. A somewhat longer-than-average tail might initially have indicated a healthy constitution or some other advantage. A short tail, by contrast, could have signaled poor condition or a male less able to survive. The suggested explanations are hypotheses rather than established facts about the origin of the birds’ tails; the argument requires only that variation in tail length have some genetic contribution and that females prefer longer-tailed males.

Under those conditions, preference and trait can amplify each other. Females that favor longer tails have sons with longer tails, and those sons are more likely to be chosen by other females. A female whose sons do not meet the prevailing preference may leave fewer descendants through them. The preference can thus persist even after a long tail ceases to be a useful measure of some other quality. What began as a possible indicator becomes desirable simply because it is desired.

The tail may then lengthen beyond the point of practical advantage. The process is compared to a fashion trend or the increasing size of American cars: each round of preference pushes the trait farther, and the momentum does not stop merely because the original benefit has weakened. In this proposed runaway, selection ends the escalation only when the tail’s manifest disadvantages outweigh its mating advantage. Birds of paradise therefore make sexual selection’s counterintuitive possibility concrete: a trait can be favored because it attracts mates, even as it burdens its possessor.

The handicap principle

The book sets this runaway explanation against the handicap principle associated with A. Zahavi. The starting problem is deception: if females choose males by visible indicators of quality, males might evolve inexpensive imitations. A male could appear to have strong muscles, for instance, without possessing the underlying strength. If false advertisement were cheaper than the real quality it mimics, selection could favor the false signal; female discrimination would then have to evolve in response.

Zahavi’s proposed answer is that a successful signal must be costly enough to be difficult to fake. A male that can display strength despite a genuine burden offers more convincing evidence than one who merely looks strong. On this account, the long tail of a bird of paradise is not an accidental drawback attached to an attractive feature: its handicap is the very reason it can serve as evidence that its bearer is sufficiently robust to endure the cost. The book also cites peacock tails and deer antlers in describing the theory, but the bird-of-paradise tail remains a particularly clear example of how the display’s cost is made part of the proposed signal.

What the example establishes—and leaves open

The two explanations assign different roles to the same conspicuous feature. In the runaway model, an initially useful indicator may become exaggerated because female preference and male appearance reinforce one another. In Zahavi’s model, the burden itself helps make the display credible: a male’s capacity to bear it is evidence of quality. Both accounts connect female choice to inherited variation and unequal mating success, but they differ over why a seemingly disadvantageous trait persists.

The book’s narrator reports skepticism about the handicap account, while acknowledging less confidence in that skepticism than before. The concern is that a genuine handicap could disadvantage offspring as well as attract females, and the proposal requires the handicap’s effects to be confined to sons rather than daughters. Thus the birds of paradise do not simply illustrate a settled answer. Their tails stage a theoretical dispute about whether extravagant sexual traits become favored through self-reinforcing preference or because their costs make them honest signals of quality.