"The courtship chamber was placed on top of an identical chamber, with the chambers separated by muslin gauze," reports geneticist Yael Grosjean in a Methods section fit for a paperback romance. Then the "perfumed" portion of the experiment began. An aphrodisiac scent was presented on the other side of the muslin gauze. Scientists watched to see whether the subject, a male fruit fly, would be compelled to start courting his partner. To ensure that the female wouldn't influence the results with her responses, she had been recently frozen to death.
(This may seem unromantic, but keep in mind that in another part of the study, the females were headless. It didn't deter the males.)
A male fruit fly displays a scripted set of actions when courting a female, beginning with a buzzy sort of love song and ending with the deposition of his extraordinarily long sperm. Many animals use pheromones, chemical messages wafting through the air, to attract partners. But, Grosjean says, scientists haven't yet found the hardware in a male fruit fly's brain that would respond to pheromones. So what other chemical signals might the fruit fly be sensing when it decides to court a female?
Grosjean's research team identified neurons in male Drosophila melanogaster that detect scent and extend into a part of the brain involved in sexual behaviors. They found that when these neurons weren't working properly, males failed to court (headless) females. Once they knew the neurons were important for courtship behaviors at the brain end, the researchers investigated what was happening at the "nose" end. They exposed the neurons to the smell of fly bodies, both up close and at a distance (as they would be if pheromones were involved). But the neurons didn't respond.
The researchers proceeded to test another 163 odors on the frigid cells until they found a couple of smells that turned them on: phenylacetic acid and phenylacetaldehyde. These aromatic compounds come not from flies, but from plants. (They also lend their honey-like scent to some of the perfumes manufactured by humans.)
These molecules are common in fruit and vegetable matter, including overripe bananas and prickly-pear cactus, two ofDrosophila's preferred foods. To understand why mealtime puts fruit flies in the mood for mating, it helps to know that they lay their eggs in their food. (I guarantee this will occur to you the next time you see buzzing visitors inside the pastry display case at your favorite coffee shop.) To a fruit fly, vegetables and fruits are good places to eat, mate, and start a new generation.
It's a surprising evolutionary solution to the problem of helping tiny flying animals find each other and mate. Pheromones work for some other species, but for fruit flies, the smell of a good egg-laying environment might be enough.
The question of whether humans release or detect pheromones, tantalizing though it is, remains unresolved. Maybe scientists would have more luck if they looked for environmental cues humans respond to, rather than molecules released by other humans. I wouldn't expect prickly-pear cactus to be the next hot perfume, but you never know.
Grosjean, Y., Rytz, R., Farine, J., Abuin, L., Cortot, J., Jefferis, G., & Benton, R. (2011). An olfactory receptor for food-derived odours promotes male courtship in Drosophila Nature, 478 (7368), 236-240 DOI: 10.1038/nature10428