A male bower bird in full glory. Credit:Leo Berzins This won't help with understanding the appeal of fluid dynamics or the night sky, but Prum is attempting to revive and expand on a view that Charles Darwin held, one that sounds revolutionary even now. The idea is that when they are choosing mates - and in birds it's mostly the females who choose - animals make choices that can only be called aesthetic. They perceive a kind of beauty. Prum defines it as "co-evolved attraction". They desire that beauty, often in the form of fancy feathers, and their desires change the course of evolution. All biologists recognise that birds choose mates, but the mainstream view now is that the mate chosen is the fittest in terms of health and good genes. Any ornaments or patterns simply reflect signs of fitness. Such utility is objective. Prum's - and Darwin's - notion of beauty is something more subjective, with no other meaning than its aesthetic appeal. Prum wants to push evolutionary biologists to re-examine their assumptions about utility and beauty, objectivity and subjectivity. But he also wants to reach the public with a message that is clear whether or not you dip into the technical aspects of evolution. The yearning to pick your own mate is not something that began with humans, he says. It can be found in ducks, pheasants and other creatures.

Yale evolutionary biologist Richard Prum: "We’ve been explaining away desire rather than actually trying to understand or explain it." Credit:Christopher Capozziello/New York Times To grasp his view, a little bit of history is in order. Darwin famously proposed the idea of evolution by natural selection, what is often called survival of the fittest. To put it simply, living things vary in their inherited traits, from speed to colour to sense of smell. The traits of the individuals who survive longer and have the most offspring become more common. So, over time, the faster antelope have more young, the fastest of them have more offspring, and antelope end up very speedy. But reproduction isn't just about surviving and staying healthy long enough to mate. You have to find a mate. And in many species, your mate must choose you. This process is sexual selection. Female birds are often the ones choosing. And their choices can produce male birds that are incredibly colourful, and some that are elaborate dancers or designers of striking boudoirs - like the bower birds. If, for example, females like males with long tails, then long-tailed males have more offspring, and the longest-tailed of those offspring reproduce more. In the end, that species becomes known for its long tails. Maydianee Andrade, an evolutionary biologist and vice dean at the University of Toronto, Scarborough, who studies sexual selection and teaches evolution, said that "the question is basically this. You can think of females when they are choosing a mate as foraging. So what are they looking for?" "If you're dragging a giant tail behind you, that might tell the female something," she said. "A male that survives carrying a large heavy tail is more impressive than a male that survives with a short tail."

But survival might not have anything to do with it. Some female finches use white feathers to line their nest, perhaps to camouflage white eggs. In one experiment, they also liked males with white feathers stuck on their heads better than other males. This seemed to be an aesthetic choice, and also proved that there is no accounting for taste. Darwin contended that selection-based mate choice was different from natural selection because the females were often making decisions based on what looked good - on beauty, as they perceived it - and not on survival or some objective quality like speed or strength. Scientists of that era reacted negatively, partly because of the emphasis on females. "Such is the instability of vicious feminine caprice that no constancy of coloration could be produced by its selective action," wrote St. George Jackson Mivart, an English biologist who was at first a great supporter and later a critic of natural selection. Alfred Russell Wallace, who came up with the theory of evolution at the same time as Darwin, preferred the idea that the colours and patterns meant something - either they were signs that this was a male of the right species, or they indicated underlying fitness. Perhaps only a strong, healthy male could support such a big, beautiful tail. At the very birth of evolutionary theory, scientists were arguing about how sexual selection worked. And they kept at it, through the discovery of genes and many other advances. Fast forward to the 1980s, when Prum was in graduate school at the University of Michigan, sharing an office with Geoffrey Hill, now a professor at Auburn University.

At that time, mainstream evolutionary thought took a big swing toward the idea that ornaments and fancy feathers were indications of underlying fitness. "Animals with the best ornamentation were the best males," Hill said. This was called "honest signalling" of underlying genetic fitness. The idea, he said, "almost completely ran over what was the old idea of beauty". Hill, for one, was completely convinced. "I was pretty sure I could explain all ornaments in all animals as honest signalling." But, he added, he has since reconsidered. There are some extreme forms of ornamentation that he thinks don't signal anything, but rather are a result of the kind of process Prum favours. "You can't explain a peacock's tale with honest signalling," Hill said. But, he said, he thought Prum had taken an important idea and gotten "a little bit carried away with it". The book, he said, "was a great read, and I could tell he put his heart and soul into it." But, he said, he found it "scientifically disappointing". Darwin himself, Hill said, "was completely unsatisfied with his work on sexual selection". And the mainstream of evolutionary biology is not hostile to a partial role for arbitrary female choice. Hill has recently argued for combining several different processes to explain sexual selection.

Prum is indeed given to enthusiasm, and to intellectual contention. He has been on the winning side of initially unpopular ideas before. As a graduate student, he sided with researchers who wanted to change the way animals are classified, to emphasise their evolutionary descent. The new idea was called cladistics and it is now the established idea. He has done groundbreaking research on both the physical structure and the evolution of feathers, and he was an early supporter of the notion that birds descended from dinosaurs, another new idea that is now the mainstream view. He has disagreed with the dominant view of sexual selection since graduate school and sees his new book, which he hopes will reach beyond scientists, as a kind of manifesto. It has too many parts to summarise. He takes a chapter, for instance, to speculate that same-sex attraction in humans evolved in our ancestors through female choices that undermine male sexual coercion. For a full account, you need to read the book. But one particular aspect of his argument is his distress at the idea that almost all evolutionary change is assumed to be adaptive, contributing to fitness. In other words, if a fish is blue, it must be blue for a reason. The colour must help it escape predators or sneak up on prey, or be otherwise useful in some way. Beauty, therefore, must be adaptive, or a sign of underlying qualities that are adaptive. Pick a behaviour or an ornament or a physical trait, and it is useful until proven otherwise. That's backward, says Prum. Take beauty. Since animals have aesthetic preferences and make choices, beauty will inevitably appear. "Beauty happens," as he puts it, and it should be taken as nonadaptive until proven otherwise.

In proposing this "null hypothesis," he draws on the work of Mark Kirkpatrick at the University of Texas, Austin, who studies population genetics, genomics and evolutionary theory and had read parts of The Evolution of Beauty. "I'm very impressed that Rick is taking on this crusade," Kirkpatrick said. He is not convinced that all aspects of sexual selection are based on arbitrary choices for perceived beauty, but, he said, if Prum can convince some other scientists to question their assumptions, "he will do a great service". For Prum, at least, there is a partial answer to the question posed by Prakash. Why are birds beautiful? "Birds are beautiful because they're beautiful to themselves." The New York Times