Zahavi's handicap principle and the idea of condition-dependence are different perspectives on the same thing. The handicap idea emphasizes that sexual ornaments and courtship behaviors must be costly in order to be reliable fitness indicators. Their cost can take almost any form. They can increase risk from predators by making an animal more conspicuous with bright colors. They can increase risk from germs by impairing an animal's immune system (which many sex hormones do). They can burn up vast amounts of time and energy, like bird song. They can demand a huge effort to obtain a small gift of meat, as in human tribal hunting.
As with Veblen's conspicuous consumption principle, the form of the cost does not matter much. What matters is the prodigious waste. The waste is what keeps the fitness indicators honest. The wastefulness of courtship is what makes it romantic. The wasteful dancing, the wasteful gift-giving, the wasteful conversation, the wasteful laughter, the wasteful foreplay, the wasteful adventures. From the viewpoint of "survival of the fittest," the waste looks mad and pointless and maladaptive. Human courtship even looks wasteful from the viewpoint of sexual selection for non-genetic benefits, because, as we shall see, the acts of love considered most romantic are often those that cost the giver the most, but that bring the smallest material benefits to the receiver. However, from the viewpoint of fitness indicator theory, this waste is the most efficient and reliable way to discover someone's fitness. Where you see conspicuous waste in nature, sexual choice has often been at work.
Every sexual ornament in every sexually reproducing species could be viewed as a different style of waste. Male humpback whales waste their energies with half-hour-long, hundred-decibel songs that they repeat all day long during the breeding season. Male weaverbirds waste their time constructing ornamental nests. Male stag beetles waste the matter and energy from their food growing huge mandibles. Male elephant seals waste a thousand pounds of their fat per breeding season fighting other elephant seals. Male lions waste countless calories copulating thirty times a day with female lions before the females will conceive. Male humans waste their time and energy getting graduate degrees, writing books, playing sports, fighting other men, painting pictures, playing jazz, and founding religious cults. These may not be conscious sexual strategies, but the underlying motivations for "achievement" and "status"—even in preference to material sources—were probably shaped by sexual selection. (Of course, the wasteful displays that seemed attractive during courtship may no longer be valued if they persist after offspring arrive—there is a trade-off between parental responsibilities and conspicuous display.)
The handicap principle suggests that in each case, sexual selection cares much more about the prodigious magnitude of the waste than about its precise form. Once the decision-making mechanisms of sexual choice get the necessary information about fitness from a sexual display, everything else about the display is just a matter of taste. This interplay between waste and taste gives evolution a lot of elbow room. In fact, every species with sexual ornaments can be viewed as a different variety of sexually selected waste. Without so many varieties of sexual waste, our planet would not be host to so many species.
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