Scents appeal

The witches in Macbeth were convinced that the more diabolical their concoction, the more effective the spell. " Fillet of a fenny snake,/ In the cauldron boil and bake,/ Eye of newt and toe of frog,/ Wool of bat and tongue of dog."

The witches in Macbeth were convinced that the more diabolical their concoction, the more effective the spell. " Fillet of a fenny snake,/ In the cauldron boil and bake,/ Eye of newt and toe of frog,/ Wool of bat and tongue of dog."

A little bat from Costa Rica operates under exactly the same principles. In the 1950s, a zoologist captured a very small bat (weighing just 7g - the same as a 2p piece) that appeared to have its ears on its wings.

On closer inspection these large forearm cavities seemed to be glands rather than ears. When researchers watched these bats in their daytime roosts, they couldn't quite believe what they were seeing, for in a protracted afternoon ritual the bats concocted a mixture more diabolical than anything Shakespeare could ever have imagined.

Massaging their genitals, the bats collected a variety of substances and placed them deliberately inside their wing cavities. Research has recently revealed that what the sac-winged bats, as they are called, are doing is manufacturing their own perfume, albeit from an unlikely set of substances.

We all smell. We are aware of other people's body odour, especially when it is bad, but we are generally unaware of the way body odours influence our lives. There's a story, probably apocryphal, but with more than a whiff of truth about it, that at country dances Italian men would place a handkerchief under their armpit as they danced, and at an appropriate moment would discreetly remove it and offer it so their partner could mop their brow. The effect of the now subtly scented kerchief was said to be irresistible, with inevitable carnal consequences.

As far as I am aware this idea has never been systematically tested, but other aspects of human odour have. Our personal body odours are determined to some extent by our genes, and particularly by a set of genes known as the major histocompatibility complex (MHC). This region of the genome is principally concerned with fighting disease, and its enormous variability - which can now be "typed" using molecular tools - is thought to help protect us from an enormous diversity of disease-causing organisms.

The more variable the MHC the better, and what better way to produce a healthy baby than to make their MHC as variable as possible? The best way to do this, of course, is to make a baby with someone whose MHC complements our own. Here's the ingenious trick that evolution has used to ensure that we don't make a mistake. By labelling MHCs with a distinct body odour, we may be able, subconsciously, to choose an appropriate partner.

In tests in which women were asked to sniff T-shirts worn by men, the women ranked them as smelling pleasant or unpleasant. Those considered sweet-odoured had been worn by men whose MHC complemented the sniffer's. Conversely, sour-smelling T-shirts were those that had been worn by men whose MHC was similar to the sniffer. Remarkably, all this seems to be occurring without our being aware of it. It is only under rather specific circumstances that we become aware of the immense power of body odour to affect our lives. When we miss a loved one, we have only to catch a fleeting hint of their scent to trigger powerful emotions and memories.

For most other mammals it is a different story. Their lives are dominated by body odours created by special scent glands dotted around the body. The main ones in foxes, badgers and dogs are located near the tail and their secretions are often extruded along with the faeces. Rabbits and cats possess scent glands on their face or lower jaw, which is why they often rub their chin on the ground or on their owner. In all cases, the odours from these glands are generated by special cells - usually modified sweat glands - and used for a variety of purposes.

These chemical signals, known as pheromones, are detected by special scent-detecting vomeronasal organs in the nose. In their urine, cows release pheromones that are break-down products of the hormones that regulate their sexual cycle. A bull will stretch out his neck and curl back his top lip to maximise the contact these smells make with his vomeronasal organs, savouring a cow's scent and deciding on her sexual receptivity. It was once thought that humans did not possess vomeronasal organs, but in fact they are on the septum just inside our nostrils.

Dogs and foxes use their odorous anal secretions to delineate the boundaries of their territory; badgers use these same glands to mark each other and create a common social identity. And male sac-winged bats, like Italian gigolos, use odour to seduce a mate. Here's how they do it.

When the average male sac-winged bat rouses himself in the afternoon, still upside down, he starts to inspect, lick and clean his wing cavities. After a few moments he leans forward and laps up a few drops of urine from his penis. Keeping the urine in his mouth, and with an occasional exaggerated chewing motion, he continues to lick the cavity, placing the mixture of saliva and urine inside it. Next, he bobs down to his groin again where exuded from his penis is a blob of yellowish matter. The bat collects the blob on to his chin and swiftly transfers it to the cavity.

As far as is known, the blob originates from his preputial glands - one of a series of paired accessory glands plumbed into the reproductive system that help, in most other species, to make up the seminal fluid. And it's not over yet. Opening his mouth wide in an exaggerated yawn, the bat forcibly squeezes a drop of liquid from a tiny gland on his chin that also goes into the wing sac. You might think this enough, but this witch's brew requires one final unseen ingredient. Millions of bacteria feast on the nutritious mess inside the wing cavity. As they multiply they modify and enrich the mixture's odour.

This whole elaborate process takes up to an hour every day, but once it's done the male is ready for action. Spotting one of his harem females he launches himself into the air and hovers alluringly in front of her. With wing sacs gaping wide, he wafts his genital-salivary perfume towards her. Hanging there, open-mouthed, she laps up the odours through her wrinkled nose. This is it. On the basis of how good he smells the female will decide whether or not to copulate with him.

The researcher Christian Voigt at the Institute for Zoo and Wildlife Research in Berlin is convinced that the odour of each male is different and in some way reflects the male's quality - though whether it signals his MHC is so far unknown. "The wing sacs and their contents," he says, "have all the hallmarks of a sexually selected trait." That is, females prefer the smells of certain males, and the preferred males are those that pass on not just their smells, but their genes as well.

The fragrance coming out of those wing sacs is the bat's equivalent of a peacock's tail or a nightingale's song. A male's chances of reproducing are determined entirely by how well he has organised his odours. If he's got it right, he will succeed in copulation. To make his point, Voigt passes me a small vial. "Go on," he says, "open it and sniff." Cautiously, I do so, and am amazed: the smell is like spring flowers. I'm impressed. Perhaps I could have some of this to put on my handkerchief.

Tim Birkhead is a professor of evolution and behaviour at the University of Sheffield