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Locusts don't have to be locusts. When they grow up by themselves, they lead pretty regular, grasshopper lives. But when conditions are right, well, it’s swarming time.

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There's something transformative about being part of the crowd.

You may have felt it at a concert or a football game. That moment when hundreds or thousands of people all seemed to share a single will.

It’s a powerful feeling. But what if it wasn't just a feeling? What if being part of a crowd actually had the power to change you into something else?

Well, then you might be a locust [ ♪ Intro ] There are more than 12,000 species of grasshoppers in the world, but fewer than 20 of them can become locusts. Locust is what we call these grasshoppers when they are in their swarming phase. The rest of the time they are just regular old grasshoppers hanging out by themselves, eating plants.

But when conditions are right, well, it's swarming time. These periodic outbreaks, when they're big enough and include individuals from different breeding areas across a region, are sometimes literally called plagues. The individuals can eat their body weight in vegetation every day, and there can be up to 70 billion of them in a swarm.

Locusts are found on every continent except Antarctica. Though the species that wreaked the most havoc in North America, the Rocky Mountain Locust, became extinct in the late 1800s. Sometimes they even move between continents, like the swarm in 1988 that traveled across the Atlantic Ocean from Africa to South America in just ten days.

Now how a solitary grasshopper becomes a swarming locust starts with a change in its environment. The desert locust, one of the most notorious species, is found in arid desert and semi-desert regions from Africa to India. Usually these regions can't support huge numbers of grasshoppers, but when there's more rainfall than normal, the soil stays wetter, more vegetation grows, and conditions become more favorable for the grasshoppers to breed.

And that means more baby grasshoppers, called nymphs, hatching together at around the same time. If these nymphs are spread out across the landscape and stay away from each other, they will grow up into grasshoppers. But if they are crowded, they will transform into locusts.

There's something about the sight and smell of other grasshoppers that changes their behavior. Drawing them together making them more gregarious and starting the feedback loop that leads to locusts. Once enough of them have started to congregate, they will almost inevitably come into physical contact.

These touches seem to increase their serotonin levels, the same neurotransmitter that in us boosts our mood, among other functions. And the touches also produce physical changes. For example, in one study, researchers raised South American locusts in either isolated or crowded conditions isolated nymphs where pale, greenish or brownish eventually becoming brighter green as adults.

Nymphs that were raised in crowded conditions had orange heads and dark spots on their bodies. And the grasshopper and locust phases of another species called the migratory locust look so different than Linnaeus thought they were two separate species. And it wasn't until 1921 that anyone realized they're actually the same one.

How does that even work? How can the DNA of a single species produce two totally different outcomes just by being part of a crowd? Well, it has to do with a thing that biologists call phenotypic plasticity.

Every locust has its own genotype or set of genetic material. The DNA instruction booklet for making a locust, And every locust or grasshopper also has its own phenotype or set of observable traits. How you get from genotype to phenotype depends on how the DNA instructions interact with the environment.

DNA by itself isn't everything. And potentially having some wiggle room there is generally a good thing. It makes organisms more adaptable to changing conditions.

For some traits, the environment can only change the possible outcome a little bit. But in the case of the locust, the environment, crowded or not, matters a lot. We would say that this insect has high phenotypic plasticity and that it is density dependent.

It's so extreme that it is even considered a special kind of phenotypic plasticity called phase polyphenism. This means that the two forms are phases that these insects can take a grasshopper or locust are separate and distinct. As for what drove this adaptation in the first place, it might have to do with ancient changes in climate, at least for one of the genera in the family that includes all the locusts.

See in this genus, the desert locust is the branch of the family tree that split off earliest, probably around 8 million years ago in Africa. According to one study at that time, the Sahara Desert was becoming more arid as a result of the closing of ancient seaways that had linked the Mediterranean Sea to the Indian Ocean. These also potentially made the African monsoon belt more sensitive to the tilt and wobble of the Earth's orbit, which might have contributed to the edges of the Sahara shifting back and forth.

All these things could have made for a less stable environment for the ancestors of the desert locust, which might have selected for individuals who were more adaptable, giving rise to the extreme plasticity we see in locusts today. They could be grasshoppers in the lean times and locusts when there was more vegetation for them to eat. And this is both pretty bizarre and also maybe not that bizarre, after all.

We've known for decades that DNA isn't destiny. There are all kinds of things that can change the outcome of this so-called “blueprint.” But it feels just a little more extreme for the locusts, maybe because the trigger seems so simple. It is literally just them bumping into each other that turns them into an entirely different form, which makes me really glad that it is not the same for us.

You can sign up for the Bizarre Beasts Pin Club from now through the end of March 6th. When you sign up, you will get the locust pin in the middle of the month and the pins after that around the time each new video goes live. And you I mean, let's be honest, want these locust pins.

Sometimes we do like cute little ones like this elephant seal, and sometimes we make beautiful works of art. We're really excited about how this turned out, so much so that we also turn them into stickers. So now you're going to have a plague all your own.

It is also time for the mystery pin sale. So we always have like a few pins left over. So for the whole month of March, we were going to be selling the pins of Bizarre Beasts passed at

Which ones are you hoping to get? And as always, profits from the Pin Club and all of our merch go to support our community's efforts to decrease maternal mortality in Sierra Leone. [ ♪ Outro ]