Why sex is strictly for the birds

Some pigeons are so sexy that they are literally irresistible to female birds. But why is male attractiveness still so variable?

Our brains are designed for sex. The relatively enormous size of the human brain, compared with that of most other animals, has evolved to help us acquire mates. So says one theory of human evolution which argues that big brains allow men to demonstrate how clever they are, and allow women to ensure that men are not bluffing.

Our brains are designed for sex. The relatively enormous size of the human brain, compared with that of most other animals, has evolved to help us acquire mates. So says one theory of human evolution which argues that big brains allow men to demonstrate how clever they are, and allow women to ensure that men are not bluffing.

Much has been written about the possible role of art in providing a sexually alluring persona. Geoffrey Miller's book, The Mating Mind, suggests that whether we are conscious of it or not, art is motivated by sex and that human creative achievements are the equivalent of a peacock's tail, or more precisely, the bower birds' bower.

In the animal kingdom, too, of course, colour and adornment play an important part in mating rituals.

In the small town of Gaudix, east of Granada, an unlikely contest takes place each weekend during the spring and summer. The contest involves half a dozen men each bearing a male pigeon. At an isolated spot on the outskirts of town the male pigeons are presented to a single female. The males are painted with bright colours to facilitate recognition by the judges and the female is specially marked for the competition by the addition of three long extra feathers to her tail.

As soon as their owners let them go, the males court the female with ferocious intensity, using all the strutting, tail-sweeping, bowing and coo-ing we see so commonly in street pigeons. Eventually the female accepts one of the males, signalling her choice by flying back to his loft with him, at which point the male's owner collects his winnings from the other men, possibly to spend on his female friends.

Male pigeons that are successful in the competition are also very capable of attracting females when they are flying free. They are considered a nuisance by other pigeon owners, and referred to as palomo ladron - "thief pigeons". Especially attractive males specialise in accumulating female pigeons, luring them back to their loft, copulating with them and then leaving them to rear the offspring alone.

An exceptional male can even induce an incubating female to abandon both her partner and her eggs - something virtually unheard of in the study of wild birds - and return with him to his loft. Such monumentally sexy pigeons are not confined to the Iberian peninsula. Charles Darwin was aware of them among British pigeons, but he didn't pursue the implications of their seditious success. He should have, for thief pigeons hold the key to an evolutionary puzzle.

The paradox is this. If certain males are irresistibly attractive to females and thus enjoy greater than average reproductive success, why, over time, don't the genes for irresistibility spread through the entire population? If male attractiveness is heritable, then the male offspring of super-sexy males will also tend to be attractive, and the genes for irrestibility will spread, so that eventually all males will be irresistible. But they are not, and that raises the key question in sexual selection: what is it that keeps male attractiveness so variable?

In fact, pigeon attractiveness is heritable: breeders of thief pigeons can selectively breed for sexier pigeons, creating males that are more attractive to females than any wild bird. In the wild, natural selection must put a limit on extreme attractiveness, because in evolutionary terms attractiveness is "costly". The cost is reduced lifespan - pigeons that are too sexy for their own good live less long - either because their preoccupation with sex leaves them with insufficient time to feed, or because it renders them vulnerable to predators.

The Scots have also exploited super-sexy male pigeons, not for status itself, but for money, which may amount to the same thing in the end. Scottish thief pigeons may even have originated in Spain, but north of the border they are known as "horsemen", an abbreviation of "Horseman Thief Pouters".

Horseman in old Scots means "highwayman", or robber, and the term was coined because back in the 1600s these birds were used to catch other birds as a source of food for their owner's starving families. The term "pouter" refers to the bird's ability to inflate its crop. Among street pigeons males pump up their crop as they launch into courtship, but breeders, recognising this as the male pigeon's greatest assest, have selectively bred birds whose fully inflated crop is the size of a football.

The unnaturally inflated crop of a domestically bred male is irrestibly sexy to female pigeons. But sexy and stupid is not enough: pigeon fanciers have also deliberately bred males, not just with intense sexual motivation and tremendous sexual athleticism, but also with the brains to lure their prizes home with them.

In recent years, unemployed men in inner city areas of Glasgow have turned pigeon stealing into a good source of income, with the pigeons doing the main part of the dirty work. On areas of waste ground, men set up "do-cots" (dove-cotes) - the avian equivalent of tenement blocks from which the horsemen scour the local area for talent.

Sometimes owners pit their birds against against each, releasing two or more horsemen together and betting on who will bring home the females. The winner wins a double whammy for having secured some females, they can either be held to ransom, swapped or sold in special back-street "do" shops. It all seems fairly innocent - which is exactly the point - but pigeon-thieving is a lucrative, illegal and dangerous business.

In exactly the same way that the horseman's display is costly, stealing other people's pigeons can also be costly. The owners of the thief pigeons visit pigeon racing events to establish what is worth stealing and target high quality racers, which they sell on for substantial sums. Since no one likes having their prize birds stolen, retribution can be swift and painful. One of my informants knew of someone who suffered a severe kicking resulting in a broken leg after a horseman incident.

This is sexual selection: males which produced bigger or better extended phenotypes obtained more matings and father more offspring. There are no free lunches, however. For sexual selection to work, the displays that males use to enhance their attractiveness or status, whether they are gaudy plumes, antlers or violin concertos, must genuinely reflect their owner's quality. If males could fake displays, all males would look great and be equally attractive. In evolutionary terminology, sexual displays must be "costly" - they must be energetically demanding or they must render their owners more vulnerable to a risk or danger, whether it be charging bulls for young men in parts of Spain or a goshawk for pigeons.

Tim Birkhead is professor of zoology at the University of Sheffield