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What's that awful smell? Cat urine? Semen? Rancid butter? Possibly one of these gorgeous city trees?

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It’s a gorgeous day outside.

The sun is shining, the city streets are lined with leafy, flowering trees. it’s a perfect day for a morning jog, you take a step out your front door, take a deep breath of that fresh morning air. Which is when you start gagging because you just inhaled a lungful of air that smells very strongly of vomit.

And it’s not left over from your neighbor stumbling home last night. The smell is coming from a beautiful nearby tree. There are a bunch of different kinds of trees that smell just really, truly awful.

But there is a good reason for it, at least, good for the trees. Even though we might not like the scent, it is meant to attract other animal species that can help pollinate and spread seeds so that the trees can thrive. And because a few of these stinky trees happen to also be pretty resilient, we tend to plant them in cities despite the smell.

Because, you know, cities really needed to smell worse. One popular tree is the Ginkgo biloba, also known as the Ginkgo or Maidenhair tree. It has beautiful fan-shaped leaves that turn golden in the fall, and it’s been stinking up the world for a really long time, with fossils dating back to over 200 million years ago.

It made its way to our cities after the Industrial Revolution, when the increase in smog and pollution started killing trees in cities like London, but not the Ginkgo. City developers started planting them all over the world, and today you’ll find Ginkgoes pretty much everywhere, from London to Seoul to New York. The only downside is that in the fall they smell like a delicious mix of vomit and rancid butter.

At least, it would’ve smelled delicious to dinosaurs, according to paleontologists. Ginkgo trees are dioecious, meaning there are separate male and female trees to carry out the reproductive cycle. The males produce pollinating cones, and the females produce ovules, which become seeds after pollination.

Around September or October, the pollinated seeds drop to the ground, where they mature in preparation for spring, then go dormant for the winter. To survive the cold, the seed coat is made of three layers: a soft outer layer called the sarcotesta, a hard middle layer called the sclerotesta, and a thin inside layer called the endotesta. It looks kind of like an oversized, yellow cherry.

And it’s the soft, fruit-like layer that’s responsible for the horrible smell. When it starts to rot, it releases two aromatic compounds: butyric acid and hexanoic acid. And when I say aromatic, I only mean that in the technical sense of the word, unless you are a fan of the smell of puke.

Researchers think that the tree developed this smell millions of years ago to attract dinosaurs and other prehistoric animals. If the animals ate the fallen seeds, then they’d poop them out somewhere else, spreading the seeds around to grow more trees. But if it’s just the female trees that produce the seeds, you’d think urban developers would just plant the male ginkgoes so the city could reap all the benefits of the tree without the stinky seeds.

But that doesn’t work. Because it turns out that the ginkgo tree can and will spontaneously change its sex to make sure there are roughly the same number of male and female trees around. So you can plant as many male trees as you want, but about half of them are going to turn into females and keep stinking up the place.

But that is not the only smelly tree you will find in cities. There’s also the beautiful but nauseating Pyrus calleryana, or the Callery pear tree. In spring, it produces delicate white flowers that look a lot like cherry blossoms.

But these flowers smell anything but sweet. They are often described as smelling like rotting fish, or like semen. The odor comes from chemicals released by the flowers, specifically, trimethylamine and dimethylamine, which are some of the same compounds that give fish and shellfish their “fishy” smell.

Those compounds are derivatives of ammonia, which happens to be present in semen. So it makes sense that they’d smell kind of similar, although as far as I can tell no one has ever done a study to check how similar they smell. If you feel like verifying that for yourself, by all means.

It’s thought that the strong smell of the flowers is to attract bees that will help spread the pollen to other trees, because the Callery pear can only reproduce through cross-pollination. That adaptation is a great evolutionary advantage for the species, but it becomes a problem in urban settings where the trees are planted too close together. The seeds spread out very quickly, and in a lot of places the trees have become an invasive species, replacing other native plant life.

So even though that cluster of white flowery trees might look beautiful in the city, the smell isn’t the only problem. Ready for another awful-but-stupidly-common tree? Good.

Because yes, there is another one. It’s called Ailanthus altissima, or the Tree of Heaven. And it branches outward like a fern, and it can grow to around 25 meters in height, which is pretty tall for a city tree.

If you happen to be wondering right now why you’re feeling like you’re back in your 9th grade classroom, that’s probably because the Tree of Heaven was used for a lot of the symbolism in the novel A Tree Grows in Brooklyn. And it’s taught in a lot of schools, in America. Like the others I’ve mentioned, it is a tough tree, but it comes with a pungent price.

It smells a lot like spoiled peanut butter or cat urine, which I’ve never actually considered similar, but then again I don’t normally walk around sniffing old peanut butter or my cat’s pee. Like the Ginkgo, the Tree of Heaven is dioecious, with male and female trees. But in this species, it’s the males that stink — probably to attract pollinators that will spread the pollen from male to female trees.

The tree also evolved another special adaptation to ensure its survival. It produces an herbicidal chemical that kills other nearby plants, meaning that the Tree of Heaven outgrows native plant life. So even though the lovely greenery was once welcome in cities, it’s now become an unstoppable invasive species.

When it comes to planting trees in cities, it makes sense that our choices are limited — a lot of native trees wouldn’t usually survive in tight, crowded city spaces. And the pollution doesn’t help either. Unfortunately, some of the trees city planners picked happened to smell terrible, and often end up knocking out whatever native plant life might have otherwise been able to survive.

There are efforts to promote native plant life and control the spread of these trees, though. Which, thankfully, will also help to control their stink. Thanks for watching this episode of SciShow!

For more tree weirdness, check out our video on the oldest tree in the world. Turns out there are a couple of different contenders.