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Today, we're going to talk about one of the earliest animal senses, one that every life form we've ever found seems to have -- chemosensation -- or our sense of taste and smell. We'll discuss how animals use these senses to explore their environment and communicate, and how that pair of nostrils of yours is an example of convergent evolution. Also, before we wrap up our discussion of animal senses we're going to talk about a couple so specialized that seem straight from the pages of comic books -- the ability to sense electric and magnetic fields!

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Before sight, hearing, or even touch, before there were even animals, living things developed chemosensation or the ability to sense chemicals in the environment.

And we know this because every life form we’ve ever found has some form of chemosensation. But other animals have come up with even more specialized ways to interpret information in their environments with senses straight from the pages of comic books.

So in this episode, we’ll talk about the many ways animals have made their ancestral chemosensation their own, and how they’ve solved some of the challenges of following your nose. But we’ll also see electric colors with an unusual African fish, and follow pigeons trying to make their way home with magnets glued to their backs. I’m Rae Wynn-Grant, and this is Crash Course Zoology.

Chemicals are pretty much...everything. Water, sugar, air, -- all those liquids, solids, and gases around and inside of us are chemicals. And recognizing different chemicals is chemosensation -- which today we know better as our senses of smell and taste.

Specifically, chemosensation is the ability to recognize a chemical based on the molecule’s shape and electric charge. Only some chemicals can fit into the proteins in the body called receptors in the right way and not get repelled, which triggers signals getting sent to the brain. Which means chemosensation tells animals about the world around them a little differently than sight and hearing because it requires a physical interaction between the animal and the molecules it’s sensing.

Like taste happens either when a chemical binds to receptors on our tongue, or when parts of those chemical molecules enter taste cells. In both cases, the cells end up sending a bunch of signals to the brain, like “sour” for lemon juice or “sweet” for candy. Smell, also known as olfaction, works in the same basic way as taste.

Smelly particles bind to receptors, which also sets off signals from our nose that our brain interprets as “flowers” or “peppermint” or whatever. While animals usually have a few types of taste buds, they have hundreds or thousands of olfactory sensors. The big difference between taste and smell is that taste happens when the thing we are tasting directly contacts the receptors, whereas smell happens when molecules travel through a fluid, like air, into our smell organ, like a nose.

We’ll use a lot of smell examples in this episode, but remember both taste and smell are chemosensation. They’re related and in some cases use the same organ, like insect antennae that both taste and smell. Under the right conditions, smells travel way further than light or sound, and are harder to block with things like dense forest or rocks.

Chemosensation is dependable, so animals deploy it for a variety of important tasks. Like animals use smell to find things, especially food, that they can’t see or hear because it's hidden behind something or too far away. Like if you ask me, bears are amazing and their sense of smell is one important factor in their amazing-ness.

They’re such good smellers that we’ve never been able to truly test their limits. But they can sniff out food, like bee larvae and seals, from miles and miles away! So with the right nose, smell can be a super long range sense.

Which presents a new challenge because odors can slip through the tiniest of openings, spread out, and be detected at very low levels, all of which can make it really hard to tell where the smell is actually coming from. Not to mention that the one who dealt it might be long gone before the next animal smelt it. So animals have evolved adaptations to track where a smell is coming from.

The evolutionary solution is to have chemosensation organs that provide information about direction, especially in pairs to better pinpoint the smell. It’s so useful to have paired smelling structures, they’ve evolved over and over again in another example of convergent evolution. There are paired nostrils, paired antennae that can move to pinpoint smells and sounds, fan-like pectines in scorpions, forked tongues in reptiles like snakes, and much more.

Many of these tracking adaptations also help animals more generally to explore their environment. Animals also incorporate smell into special behaviors to communicate like to attract potential mates or scare away rivals. Like moths and orchid bees will climb up high in trees to make sure their attractive scent carries far in the wind.

And some animals leave pee, scat, and other smelly marks in well-traveled places that are like a short biography: how healthy they are, if they’re stressed, and if they’re looking for a mate. But it’s not just pee or scat that animals are sniffing. A lot of animals release pheromones, chemicals secreted by one animal to influence the behavior or physiology of another animal.

Some pheromones act as alarm cues, warning other animals away from dangers. Others attract or guide friendly animals down a safe path. Some mark territories, and others influence animal physiology and behavior, like pheromones that cause egg laying or mating behaviors.

Pheromones can even be used by predators like the bolas spider who uses pheromones to lay a trap for their moth prey! Chemosensation is the OG sense and it’s become incredibly useful for animals tracking, exploring, and communicating. But some animals have additional senses that basically make them real-life superheroes.

Like electroreception which is being able to sense the electric fields or currents that are pretty much everywhere in nature. In fact, every time we use our nervous system, we’re sending electrical signals through our body. So if an animal can pick up on those tiny signals, they can sense other animals when it’s hard to see, smell, or hear them, like in murky water.

Plus, the Earth’s atmosphere has electricity in it too, so electroreception can also tell animals about their environment! Besides seeing the occasional spark or lightning strike, we humans can’t see or sense electricity. Almost all electric-sensing animals like duck billed platypuses, star nosed moles, sharks, and a lot of fish who have this ability live in water because water conducts electricity much better than air or earth.

Though, we’d only really looked for electroreceptive animals within vertebrates. But starting in the 1960s, scientists have been learning that several arthropods react to electric fields. So electroreception is a lot more common than we once thought!

Some electrically-inclined animals go even further, evolving special organs to make their own electrical signals and actively sending them out into the world around them, either to find prey or to chit chat with other electric fish. Let’s experience a Day in the Life of one of them -- the elephant-nose fish! Allow me to introduce the Peters' elephant-nose fish of the dark and murky waters of the rivers of West and Central Africa.

Her beautiful, long snoz isn’t a nose at all, but an elongated chin covered in electricity-sensing cells. Each day, she swims around looking for snacks on the riverbed and waving her chin around like a metal detector on the beach. And her electric organ, which is made up of specially adapted cells in the tail, sends out weak electric signals every few seconds, creating an electric field.

Objects like worms and rocks affect the electric field, which she senses with her chin and tells her where they are so she can root them out from the gravel. Like most electric fish, her zaps aren’t powerful enough to stun prey or scare off attackers, but rather they help her understand the world. What she’s doing is similar to what bats do with sound and echolocation, which is why it’s called electrolocation, and it basically lets her see with electricity -- even a version of color!

As she’s looking for worm-snacks, the electricity-sensing cells in her nose-chin respond to two major characteristics as they sense electricity: how strong the signal is, and what shape it takes. Which is kind of how the cones in our eyes sense wavelengths of light. The fish’s brain then combines these signals in very specific ways -- just like lots of brains combine signals from different photoreceptors to perceive a color -- which gives each object an “electric color”.

Certain categories of objects like other electric fish or prey always have the same “electric color” -- imagine if your favorite food always glowed bright yellow! Have a nice day! Magnetism, or the force exerted by magnets as they repulse or attract each other, is another fundamental property that some animals can sense with magnetoreception, or the ability to detect a magnetic field.

Which is usually the Earth’s. While a surprising number of animals can shock you -- or at least use electricity -- no known animals make their own magnetic fields, so magnetoreception is entirely a passive sense. But useful!

The Earth is a big place, and lots of animals like birds and flying insects travel long distances to find mates, food, or to escape inclement weather, and they need some kind of compass. Other animals like mole rats and cave salamanders live underground, where they can’t see the stars or the Sun to tell them which way is east or west. And that’s where magnetoreception comes in!

It’s like a global compass that works in complete darkness. Magnetoreception is most well-studied in migrating birds like homing pigeons. In the early 1970s, a zoologist at Cornell named William Keeton glued magnets to the backs of pigeons, and observed that pigeons released on sunny days could find their way home, whereas those released on cloudy days got lost.

Later on, scientists would find that pigeons had two major systems for sensing magnetic fields. During the day they could use proteins called cryptochromes in their eyes that responded to blue light in such a way that they reacted to magnetic fields. And at night, clusters of iron in their beaks would be drawn vaguely north, kind of like how a compass works.

If that sounds mysterious, well, it is! Exactly how magnetoreception actually works is still something scientists are trying to work out. For example, a lot more animals have cryptochrome in their eyes than we thought -- even us!

So there are some senses that are older than animals and some that we’re just discovering because animals are always exploring their environments with light, sound, chemicals, and even electricity or magnetic fields to pick up crucial information. And even though there are so many different animals out there, we all sense and learn about the world in very similar ways. Metazoa really is one big family.

So far in this course, we’ve focused mostly on single animals on their own. But next episode, we’ll get into what happens when animals get together to pass down their genes! I’ll see you then.

Thanks for watching this episode of Crash Course Zoology which was produced by Complexly in partnership with PBS and NATURE. It’s shot on the Team Sandoval Pierce stage and made with the help of all these nice people. If you’d like to help keep Crash Course free for everyone, forever, you can join our community on Patreon.