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The most popular language on earth isn’t spoken, it’s smelled. Those smells are made up of terpenes, a multipurpose class of chemical compounds.

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You've probably never heard of it by name, but there's a language on Earth more popular than Mandarin, more widespread than English, and more universal than interpretive dance. It isn't spoken. And it isn't viewed. It's smelled. 

I'm talking about terpenes-- a multipurpose class of chemical compounds that can be used to communicate both within and across species. They're used in all kinds of ways: From hungry, eavesdropping microorganisms to brief, back-and-forth conversations between creatures you would never think would be up for a chat. 

All a molecule has to do to be considered a terpene is to have the right structure. They're made up of multiple subunits of an organic molecule called isoprene, which has five carbon atoms. These subunits can be arranged in a line or form rings, or both, leading to a diverse collection of over 30,000 known compounds. 

Because it's so broad a label, terpenes can have a lot of different biological functions, like as vitamins, hormones, or part of the immune system. They're also a really common source of scents in plants.

You might've heard some people talk about terpenes as the chemicals that give different strands of cannabis different aromas. And they're responsible for the scent of things like peppermint and fir trees. One reason these smells are strong is that terpenes tend to be volatile, meaning that they evaporate easily, which also makes them super useful for communication-- they can travel long distances and can be detected by a second party.

Some terpenes are used to communicate within the same species, in which case we'd call them a pheromone. One example is androstenol, which is emitted by male pigs to signal a nearby female to assume a mating stance.

By definition, pheromones are only used to communicate within the same species, but because terpenes are so diverse, some can be used to send information between entirely different kingdoms. Kingdoms of life, I mean, not like Rohan and Gondor. 

Terpenes aren't the only kinds of molecules species use to communicate with each other; but, they are some of the most common. Lots of sweet-smelling flowers, for instance, use terpenes to attract pollinators; but, plants can also use them to keep critters away. Like the compound in catnip that cats love so much? That's actually an insect-repelling terpene. 

Plants can even use terpenes as a chemical cry for help. If a caterpillar picks the wrong leaf to munch on, it might just find itself a meal, because the plant releases terpenes that attract one of that caterpillar's natural predators or parasites. Like when the tobacco plant responds to the saliva of tobacco hornworms by releasing a signal that attracts the big-eyed bug. Or the terpene signal can be less of an SOS and more of a "Save yourselves!", telling other plants in the area to bolster any defenses they have in case the herbivore comes their way.

Lima bean leaves can do this when they're being attacked by spider mites even when they're not attached to a plant. But like with any form of communication, there's a chance the message will be intercepted.

For example, some kinds of protists can hunt for bacteria in a pitch black maze of soil by eavesdropping on the terpenes sent between different bacteria. Based on the specific materials detected, the protists can identify the terpene-sender's species. If it's a species the protist eats, it can make a beeline straight for the bacteria. 

All the examples I've mentioned so far have been more of a monologue. Another creature might act on the information they get, but they aren't really returning the call. But researchers have seen what appears to be more of an actual conversation using terpenes. For example, in a paper published in the journal Scientific Reports in April 2017, researchers found that when a particular fungus emits some terpenes in the vicinity of a certain species of bacteria, the bacteria not only reacts to the fragrance by moving around, but responds to the fungus by emitting terpenes of their own. 

One of those terpenes goes by the name sodorifen, which has a super unusual structure compared to all the other terpenes we've studied. Basically, it has two cyclic structures smushed together, and every carbon atom on the ring has another carbon atom hanging off of it. We're still not entirely sure how or why bacteria make sodorifen, so we'll need a lot more research before we know exactly what these bacteria are trying to say to the fungi. But deciphering this language of terpenes, as well as the other, less common compounds used to communicate between species, is a major area of research.

Most organisms produce complex mixtures of terpenes, not just one or two, which allows for a wider vocabulary and a more complex conversation. So, if scientists can work out how the different molecules signal different species, we can use terpenes to issue our own orders. And that has a lot of potential for the agricultural industry, especially as an alternative to the pesticides and fertilizers that are used now. With more research, terpenes could eventually be used to protect crops from hungry pests, or fungal or bacterial infections, or to improve crop growth and health. We might even be able to genetically engineer plants or microorganisms to produce these terpenes. 

So, you'll probably never be able to sit down and have a conversation with your house plant; but, maybe at least you'll be able to say "Smell you later." 

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