What the Nose Knows - Avery Gilbert [20]
NATURAL BOTANICAL scents have a soft-focus, flower-child ambience about them. They are perceived as innocuous and innocent, a gift from Earth Mother Gaia to aromatherapists everywhere. In reality, they are biological communication systems, a way for plants and animals to talk to each other. This also makes them instruments of deception and treachery. Once a smell is used as a signal, other organisms can turn it to their selfish advantage. (Ask a female Large White how she feels about the parasitic wasp on her back.) A Mediterranean plant called the dead-horse arum fakes the stench of rotting meat. It attracts blowflies looking to lay eggs on a nice ripe carcass. The blowfly gets fooled into pollinating the plant for free, traveling from one stinky plant to the next carrying pollen on its legs, in what has been called “a striking example of evolutionary cunning that exploits insects for pollination purposes.” Other examples are more sinister and almost perverse. An Australian orchid emits a smelly molecule called 2-ethyl-5-propylcyclohexan-1,3-dione, which happens to be the exact molecule produced as a sex attractant by females of the wasp species Neozeleboria cryptoides. When the orchid joins the action, the result is an aroma-based, cross-species sexual deception in which hapless male wasps attempt to copulate with the orchids. In the end, the orchid is pollinated and the male wasp is frustrated. Sex and exploitation are never far apart.
In nature, smells also serve defensive purposes. Essential oils, cherished as healing elixirs by aromatherapists, are really weapons in the ongoing battle between a plant and its predators. Take the orange tree as an example. It provides three different materials used in perfumery: neroli oil from its flower, orange peel oil from its fruit, and pettigrain from its leaves. Orange trees didn’t evolve for the perfumer’s convenience. Flowers smell good to attract pollinators; fruits smell and taste good to attract seed dispersers. A leaf releases volatile aromatic compounds as soon as an herbivore bites into it. This makes the leaf unpalatable or even toxic to the attacker (a caterpillar, say) and simultaneously alerts predators (such as wasps) that food is available. To an aromatherapist the orange tree is a repository of healing oils; to a caterpillar it looks like a weapons depot ringed with alarms and booby traps.
In college I lived for a time near the eucalyptus grove at the West Gate of the Berkeley campus. I loved to walk through its aromatic shade on the way home from class. The fresh astringency of the trees, like the fog that sometimes shrouded them, was to me a key element of Bay Area aesthetics. Back then I took a simple pleasure in that smellscape, and I still do. But today I also see it another way: as the lingering haze of biological warfare. Eucalyptol, chief among the fragrant molecules wafting about the West Gate, wards off leaf-eating bugs and suppresses the growth of seedlings of competing tree species.
The Web of Nature
Near Guaraqueçaba in southern Brazil is a remnant of the rain forest that until recently covered all 4,650 miles of the country’s Atlantic coast. While prospecting there one spring for unusual smells, Roman Kaiser found the forest suffused with a strong fruity-floral scent. He tracked it to a tree with white bottle-brush flowers. Nearer the tree the scent took on a blackcurrant quality; close to the flower itself the smell resembled cat pee. With chemical analysis, Kaiser was able to trace both odors back to a single molecule: 4-mercapto-4-methylpentan-2-one, or MMP. (It is one of many molecules whose odor character depends on airborne concentration.) For most people, that would be the end of it: Odd Molecule