Pungent body odour from sweaty adult human skin is unique in the animal kingdom.
Humans turn out to be particularly smelly because odours are released from nearly every part of the body while other species living on us are simultaneously emitting odours too.
Our powerful scent attracts at least two species of mosquitoes, according to a paper accepted for publication in the journal Trends in Parasitology.
Understanding what it is about human odour that attracts these bugs could help prevent the spread of diseases such as malaria and yellow fever.
Lead author Renate Smallegange explains that "the microorganisms on our skin use the materials present on our skin and in our sweat for their own metabolism. The microorganisms convert non-volatile compounds into volatile compounds."
Mozzies descend for a sweaty treat
Smallegange, an entomologist at Wageningen University in the Netherlands, and colleagues Niels Verhulst and Willem Takken, analysed data on the chemical structure of human sweat.
They conclude that: "Sweat-associated human volatiles are probably the primary determinant factor in the host preference of anthropophilic mosquitoes." These insects can carry life-threatening diseases, such as dengue, yellow fever and malaria.
So far, the 'recipe' for synthetic human sweat appears to contain a complex blend of carbon dioxide, ammonia, lactic acid, and seven other carboxylic acids. The latter "have a sweaty smell," Smallegange says. Mosquitoes are very attracted to this odorous concoction when scientists whip it up in the lab.
While we tend to think of other animals as smelling more unpleasantly than we do, prior studies on both birds and other mammals revealed that significantly fewer volatile organic compounds emanate from the skin of these non-human species. Only a few carbolic acids have been detected on chicken feathers and giraffe hair, for example.
For this latest study, the researchers compared the distribution, function and secretion of different types of human skin glands with those of other primates. Based on the information, humans would be predicted to smell more like chimpanzees and gorillas, due to similarities in the distribution of these glands, but chimps and gorillas release more oils, probably to safeguard their body fur.
Adult humans instead frequently emit water, proteins, amino acids, urea, ammonia, lactic acids and certain salts - much of which can stink. During puberty, the glands that release these components mature and are colonised by bacteria.
"So even though parents can recognise their pre-adolescent children by olfaction, children have a less 'pungent' body odour compared with adults," the researchers explain, adding that children also produce sweat at a lower rate than adults do.
The mosquitoes studied by the scientists bite sweeter smelling infants and children less frequently. Having a strong body odour can be useful at times, however. Smallegange mentions that the odours that emerge during and after puberty are likely tied to "sexual maturity and mate choice." Prior research determined that we can even distinguish ourselves based on hand smell alone.
The scientists further report that men sweat more than women do during exercise. Nevertheless, the concentrations of smelly, volatile carboxylic acids are basically the same for men and women.
Searching for the molecules that attract mozzies
Many other studies have focused on mosquitoes and other insect vectors that are attracted by carbon dioxide when we exhale. Smallegange points out, however, that some mosquito species are likely repelled by human breath. Adult human sweat smells are therefore more important to certain bloodsuckers.
James Logan, a member of the Faculty of Infectious and Tropical Diseases at the London School of Hygiene & Tropical Medicine, says that while he and other scientists are aware that "human odour is distinct from other mammals and that this plays an important role in the attraction of mosquitoes to human beings," it's less clear exactly what it is in human sweat that attracts mosquitoes.
It's hoped that future research will better reveal these elements. Then synthetic human sweat could be used in mosquito studies, sparing human subjects from the misery of being bitten.