The bats that mix nature’s grossest perfume
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Humans are not the only species that makes perfumes, but you might think twice before sniffing the stinky cocktail mixed by this bat.

Women would do well to look, act and smell like flowers – at least if the etiquette guides of the 19th Century are to be believed. One such book of rules, called Manners, Culture, and Dress of the Best American Society, suggested women should emulate the moss-rose to become "beautiful in spirit and in intellect, when they are but half-unfolded." Another gave instruction in the use of floral perfumes, proposing "a faint suggestion of the violet or the wild olive is most delicate and lady-like." Men, on the other hand, have traditionally used fragrances based on musks and spices.

Of course humans have been masking their true odours for thousands, not just hundreds, of years. Archaeological discoveries suggest perfumes were being created from extracts of myrtle, cypress, and other plants in Mesopotamian palaces millenia ago.

In human life, smells play a variety of fundamental roles. From birth, infants can distinguish the scent of their mother's breasts. Since failure to feed can ultimately result in dehydration or death, some researchers think that the ability to identify those odours may itself be critical in promoting feeding-related behaviours. And as adults, certain body odours can determine a person’s attractiveness. Smells also have a direct influence on our emotions. Stephen Warrenburg, a researcher at International Flavors and Fragrances, a New York-based company, has found test subjects associate odours with particular feelings. For example, both clementine and vanilla are reported as pleasing, but while clementine is thought of as stimulating, vanilla apparently makes people feel more relaxed.

Smell is important to various other animals too. What is less well known is that the intentional creation of aromas is not a uniquely human trait. There is at least one other animal that practices the ancient art of perfume mixing, and perhaps the most disgusting is the male greater sac-winged bat, Saccopteryx bilineata.

Colonies of up to 60 of these bats are found from northern Argentina to southern Mexico. These are divided into a number of harems, each consisting of one male and multiple females, and controlling a territory of between one and two square metres. Males without partners roost on the outskirts of the harems, waiting to surreptitiously mate with females.

'Perfume containers'

Males with harems mark the boundaries of their territories with excretions from small, furless, nipple-like projections found under their chins called gular glands. But to court a female, they require a mixture of secretions from these glands, their genitals and urine. That's where their wing sacs come in.

The sacs, found on the inside of the wing, are 8-10mm in length, which is large given the animal’s size. Despite the presence of what researchers call "odiferous content" – smelly stuff, basically – within the wing sac, there isn't a scent gland inside. Instead of excreting directly into the sac, males transfer fluids from other parts of their body into them. "Thus wing sacs could also be called holding sacs or perfume containers," says Christian Voigt, of the Leibniz Institute for Zoo and Wildlife Research in Berlin.

Each afternoon, male greater sac-winged bats in a colony make a new batch of perfume. They begin by bending forward repeatedly, using their mouths to suck up a bit of urine, and then transferring it by licking the interior of their wing-sacs. They then bend forward as before, but this time they press their gular glands against their penises to combine excretions from both, before transferring the mixture to their wing sacs. This continues until both sacs have a sufficient amount of bat perfume. The process lasts 30 to 60 minutes. All 51 species in the family Emballonuridae have wing sacs, but S. bilineata is the only species known to engage in this complex form of perfumery.

At dawn and dusk, either at the start or the end of the nightly feeding, a male approaches a roosting female. While hovering and singing a courtship song, he opens his sacs and flaps his wings horizontally to blow the air scented with his perfume over her.

The males perform their excretory alchemy in both the mating and non-mating seasons. While critical for courtship, Voigt and other bat scientists think that the perfume must therefore play other social roles, perhaps underlying individual recognition or reinforcing a male's position within the dominance hierarchy.

Indeed, the fact that each male spends up to an hour each day cleaning and refilling his wing sacs strongly suggests that perfume – and smells, more generally – plays a fundamental role in the social communication of this species. It also means that bat noses, as with human noses, are critical to survival.

Perfume companies, with total global revenues of around $31.6bn last year, are of course always on the look-out for novel fragrance combinations. For readers from the industry wondering whether there might be potential in “eau de bat”, Voigt describes its smell as "sweetish, with a touch of bitter almond."

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