Whispers in the Wind

Anyone with a dead-tree or digital subscription to the Times of London will have received their copy of the monthly science magazine Eureka over the weekend. The May issue is about plants and I have a piece in it about plant scents titled “Whispers in the Wind.” Here's a taste:

Orchids, it turns out, have raised trans-species sexual deception to an art form. About one-third of all orchid species dupe their pollinators by mean of chemical mimicry. Insects often depend on smell to find mates and are hard-wired to be exquisitely aware of a few key odour molecules produced by females. Orchids take advantage of the insect’s narrow sensory focus by providing the lure but not the payoff. We have known since 1793 that orchids use false odour cues, but have been slow to fully credit the evolutionary cleverness of plants. Even Charles Darwin doubted the existence of such “an organized system of deception.” He wrote that anyone who believes in “so gigantic an imposture” must necessarily “rank the sense or instinctive knowledge of many kinds of insects, even bees, very low in the scale.” Today, we understand how powerfully insects are led by their nose (or antennae); still, some might consider an orchid’s deceit to be a harmless evolutionary prank. But from the point of view of a short-lived insect, the time wasted is costly indeed.

Sexual odours are not the only ones exploited by plants. In an equally perverse ruse, some plants attract blowflies by mimicking the stench of rotting meat. A female blowfly wants to lay her eggs on a ripe piece of carrion but is misdirected by the olfactory posings of Helicodiceros muscivorus, a Mediterranean lily also known as the dead-horse arum. The lily gets pollinated but the wasp’s offspring are destined for a brief and hungry life. Recently, directors of botanical gardens have taken to using rotting flesh scent as a revenue-enhancing tactic. Morbidly inclined members of the public will pay for the privilege of smelling the six foot tall flowering stalk of Amorphophallus titanum, an exotic tropical tuber with the stage name of Corpse Flower. A given tuber produces a stalk every six years or so, and the stinking protuberance lasts only a few days, or what carnival pitchmen used to call a short stand. Step right up, folks, and sniff while the sniffing’s good!

Plants are also quite adept at mimicking the smell of dung, a resource of great interest to many insect species which comes in a variety of olfactory shadings. Horse dung, for example, is characterized by monoterpenes, such as limonene, while the typical aroma of cow and pig manure is produced by p-cresol. In carnivores, such as dogs, the defining scat note is phenol. The quality of a plant’s false fecal odor is precisely tuned to the poop preference of its insect dupe.