What the Mysterious Mating Habits of an Enigmatic Species Reveal About the Secrets of Evolution

In all animals mating is a deal: one sex donates a few million sperm, the other a handful of eggs, the merger between which—unless a predator intervenes—will result in a brood of young. Win-win for the parents, genetically speaking. But there are few creatures that behave as if sex is a dull, simple or even mutually beneficial transaction and many that behave as if it is an event of transcendent emotional and aesthetic salience to be treated with reverence, suspicion, angst and quite a bit of violence.

In the case of Black Grouse the males dance and sing for hours every day for several exhausting months, selling their little packages of sperm as passionately and persuasively (and frequently) as they can. To prepare for the ordeal they grow, preen and display fancy, twisted, bold-colored feathers. They gather together in one spot, putting themselves at conspicuous risk of attack by hawks and forgoing opportunities to feed. They fight with deadly intent again and again, suffering significant injuries. As excitement builds they expand the bright red, swollen, fleshy combs over their eyes, covered with hundreds of tiny tentacles like vermillion sea anemones. The act of sexual congress itself, the consummation of the deal, takes seconds. The rest has taken months of practice and preparation and is elaborate, extravagant, exhausting and elegant. Why?

The full answer to this question is, as I say, still mysterious. Evolutionary biologists can explain why males are generally (not always) the eager sellers, females the discriminating buyers of gametes. They can begin to explain why some species have gaudy males, others beefy ones, still others dull ones. But they struggle to explain—or at least they disagree passionately—why it is that in some species this extravagance goes beyond the mere gaudy and into baroque and bizarre shapes and postures. And as I say they don’t seem to be able to satisfactorily explain why bird displays generally seem beautiful to us.

Among the enduring enigmas, Black Grouse represent the purest example of one. For here is a bird that is obsessively choosy—on the females’ part—when it comes to sex, and in which a single male gets almost all the matings at any one lek. Given that this also happened a generation before, and that males live near where they were born, it follows that some of the rivals on this lek will probably be brothers and half-brothers, fathered by the same male. For generation after generation the local population goes through a narrow genetic filter, with just one male doing most of the fathering each year, so the genetic diversity in the population is inevitably low. There must be relatively little to choose between the males genetically compared with other species. So in this species, far more than others, there is surely little point in being choosy.

Yet female Black Grouse are—as I say—obsessively choosy, breeding only from the best, as if the males were racehorses. Indeed, racehorses suffer from the same paradox, because breeders pay gigantic sums for access to the sperm from the best stallions despite the entire breed being massively inbred (‘thoroughbred’) from a handful of original stallions with very little genetic variation compared with other breeds of horse.

This puzzle has been known to biologists since 1979 as the “lek paradox” and it remains unresolved. The scientist who coined the term, Gerald Borgia, was drawing attention to the fact that if females are consistently choosing the same males, they will soon cause any variety to diminish, maybe even disappear. So the choosiest species seem to have the least reason to be choosy. Resolving this puzzle to my own satisfaction is one of the tasks I have set myself by watching this lek. Incidentally, unlike in racehorses, lekking does not make the species problematically inbred, because female Black Grouse emigrate from their native area so they bring fresh blood to each lek. The females attending this lek were probably fathered in a different dale, twenty miles away or so. But a glance across the lek now confirms that there is very little to choose between the males and unless you carefully count missing tail feathers or notice scars about the mouth, you cannot even recognize individual males.

As I watch, I ponder a human analogy for the lek paradox, though very inexact because human beings are not a lekking species. Imagine an unmarried young woman, Miss Bennet, turning up in a village in which there are fourteen single men, many of whom are half-brothers or cousins, all of whom look roughly alike, wear identical frock coats and live in similar small houses on the main street: Tom Darcy, Dick Darcy, Harry Darcy, Fitzwilliam Darcy, etc….There is nothing to choose between them in wealth or breeding, and very little in looks or dress. They are all qualified chartered accountants.

Though they spar continuously among themselves, the Darcys are scrupulously polite and attentive to the new woman, each one bursting into the same rendition of “O Sole Mio” to serenade her whenever she passes down the village street. Yet somehow every woman who has arrived in the village before has agreed that Tom Darcy is the man for her, and the rest are nothing. The village is full of young ladies bringing up Tom Darcy’s babies. Have they decided this independently or are they copying each other? We don’t know. Our protagonist Miss Bennet, on learning this, obediently follows suit and submits to the same gentleman’s caresses. Why?

I am here watching the lek for the umpteenth time mostly because I love being here—watching rare birds at close quarters, while being serenaded by an avian dawn chorus may be as close as I get to feeling spiritually uplifted, strange fellow that I am. But also I am convinced that one day, just by watching very closely and paying attention to the details, I will finally resolve not only the lek paradox but the full mystery of sexual display in birds, and therefore of the origin of beauty itself, to my own satisfaction at least. To do this I really should have captured each male bird, then weighed, measured, marked and genetic-fingerprinted him, and staked out the lek with evenly spaced markers. I did a good bit of exactly this with other species of bird many years ago when I was a professional scientist but today I am being more of a naturalist.

Fortunately, a former Oxford colleague of mine by the name of Rauno Alatalo began to do exactly these scientific things at Black Grouse leks in his native Finland, and at a separate site in Sweden, more than twenty years ago. Although Rauno suffered a stroke in 2008 and sadly died in 2012, his legacy continues as colleagues are still trying to understand this enigmatic species. The Nordic scientists catch Black Grouse in traps baited with oats during the late winter and spring in an area of bogs, lakes, forests and open areas used for peat digging.

For each male, they weigh the bird, estimate its age from its plumage and measure the length of its wing and the length of its lyre-shaped tail.

Samples of feathers are cut from the breast and in 2006, Heli Siitari took a few sample feathers from each bird back to a laboratory, placed them on a black velvet background in a dark room and illuminated them with a xenon fiber-optic light source at a consistent angle so that a spectroradiometer could read out the blueness of the feather: the peak reflectance is in the violet-blue part of the spectrum. (Some bird species can see in the ultraviolet, but in grouse the peak of their short-wave vision is only a little shorter than that of human beings.)

Later work has also estimated the size of the fleshy red combs above the eyes and measured just how red the combs are, using a spectrophotometer. For some birds the scientists also extract a sample of blood for genetic and hormonal analysis and to measure parasites. Females are also weighed and blood-sampled. The birds are then released with a metal ring and a couple of colored plastic rings on their legs for recognition. Over the years the Finnish study has trapped and measured more than 1,200 females and 1,700 males, many of them more than once.

Meanwhile the diligent, and presumably sleep-deprived, scientists literally stake out five or more leks, with white markers placed to create a grid on each one. Every day for two weeks during late April and early May from 3 a.m. to 8 a.m. they watch and video the leks, recording each male’s position and behavior as completely as possible: its distance from the center of the lek, its total time at the lek each day, how often it fights and displays, and of course whether and when it mates. The color rings allow individuals to be identified at distance and any matings and behaviors assigned to a specific individual.

This exhaustive, invaluable study, to which I will refer repeatedly throughout this book, has enabled somebody like me to interpret in far more detail what I see on the lek I watch, where I can only identify some of the individual birds from a few distinguishing marks. The information it has revealed is remarkable and far from taking the mystery out of nature, it only makes it more magical, in my eyes at least.

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From Birds, Sex and Beauty: The Extraordinary Implications of Charles Darwin’s Strangest Idea by Matt Ridley. Copyright © 2025 by Matt Ridley. Reprinted courtesy of Harper, an imprint of HarperCollins Publishers. Available wherever books are sold.