Have you heard the expression “dogs are people’s best friend”? Where did it come from? And why are dogs and humans such great pals? Molly and co-host Kha-ai jump back in time to learn how dogs became humans’ best friends, and answer the question why do dogs wag their tails? Plus, we’ll also explore how dogs help humans and even other animals like cheetahs. And keep your ears perked for a brand new Mystery Sound!

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KAI: You're listening to Brains On, where we're serious about being curious.

SUBJECT: Brains On is supported in part by a grant from the National Science Foundation.

MOLLY BLOOM: Kai, we're so excited you're co-hosting with us today. Before tapings, I usually warm up by performing a choreographed routine I made up to the Beyoncé 2003 hit song "Crazy In Love." Would you like to join?

KAI: Sure, these jazz hands are always ready to rock. (WHISPERS) Jazz hands.

MOLLY BLOOM: That's what I like to hear. Come on, we can dance in the gym. Huh, looks like someone's already there.

SANDEN: Go, Penelope, go! And time. That's a fetch record. Ha, nailed it.

MOLLY BLOOM: Sanden and his dog friend Penelope Poodle, are you two here for the pre-show dance too?

SANDEN: Oh, no, we're just training for Amazing Chase. That's the world's biggest game of fetch.

KAI: Oh, I've heard of that. You team up with your best friend and compete together to fetch all sorts of stuff.

SANDEN: Yeah, sticks, tennis balls, frisbees, floppy disks, a 1986 Topps Mark McGwire rookie card, half-caff oat milk latte-- don't spill it, it's hot-- one lonely Argyle sock, buckets of goo.

MOLLY BLOOM: Buckets of goo?

KAI: Mystery goo, to be exact. It's an amazing chase staple.

SANDEN: Yeah, they keep you on your toes. But between Penelope's speed and my great hair, we're going to ace this. Right, Penelope?


You know what they say, dogs are Sanden's best friend.

MOLLY BLOOM: I think it's dogs are people's best friend.

SANDEN: Tomato, tomato, potato, potato, people, some other way of saying people. All I know is Penelope here is my bestie. Give me paw, peeps.



MOLLY BLOOM: You're listening to Brains On from APM Studios. I'm your host, Molly Bloom. And today, I'm here with Kai from Princeton, New Jersey. Hi, Kai. How's it going?

KAI: I'm good. I'm really excited to be here.

MOLLY BLOOM: We are very excited you're here too. Today's episode is all about dogs, their friends, and how they communicate.

KAI: If you want to learn more about human friendships, check out last week's episode.

MOLLY BLOOM: Sometimes a dog's friends are fellow pups, humans, or even cheetahs.

KAI: More on the cheetah later.

MOLLY BLOOM: We've gotten a lot of questions about dogs and friendship, like this one from Vera and Sonya.

VERA AND SONYA: Why are dogs man's best friend?

MOLLY BLOOM: Kai, I'm curious, have you heard this expression, why are dogs man's best friend, before?

KAI: I've heard of this expression before, yeah.

MOLLY BLOOM: So do you think it's true? Are dogs really people's best friends?

KAI: I mean, yeah. I personally think cats are but--

MOLLY BLOOM: You don't have a dog, you have a cat, right?

KAI: Yeah.

MOLLY BLOOM: How do you think a cat expresses their friendship for humans differently than dogs express their friendship for humans?

KAI: So from what I know, dogs wag their tails, and then cats, when they try to smile, they like, slowly blink their eyes.

MOLLY BLOOM: Yes, I live with a cat too, but I grew up with dogs. And cats are definitely a little more standoffish, not as eager to please, I would say. At least that's my cat. Is your cat like that?

KAI: My cat loves people.

MOLLY BLOOM: Oh, that's amazing. Yeah, my cat really doesn't. So I have a different kind of cat in my life. So since you have a cat at home, what inspired you to write a question about dogs?

KAI: Because since I have a cat, like, I know a lot of things about cats and not as many things about dogs. So I'd like to know both.

MOLLY BLOOM: Excellent. Well, we are going to tell you all about it today. Dogs and humans can be best buds.

KAI: And that's because dogs evolved to become our best buds.

MOLLY BLOOM: Right, when we say evolved, we mean an animal changed over many, many generations to become something different. In this case, it all started thousands of years ago with wolves. This was back when people were hunter-gatherers, way before farms, towns or cities even existed. It was so long ago, that we don't know exactly how it happened. And in fact, it may have happened separately in many different places at different times. But the story might have started something like this.

KAI: Picture a group of humans resting by a warm fire after a long day of hunting.

HUNTER 1: Hey, buddy. Really great job hunting today.

HUNTER 2: Oh, gosh, Thank you. You did a great job, too. If you hadn't seen those deer tracks, we'd be going to sleep hungry.

HUNTER 1: Sometimes I wish I could track by smell or run faster than deer. It'd be so much easier.

MOLLY BLOOM: Meanwhile, a couple of hungry wolves nearby have smelled what's cooking.


WOLF 1: Those humans have food. I want their food. Let's go take their food.

WOLF 2: But there are eight humans and only two of us. And they have sharp pointy sticks and fire.

WOLF 1: We have sharp pointy teeth. And the food smells good. And maybe the humans also taste good.

WOLF 2: No way, brother. I am not charging in there and getting a pointy stick thrown at me. It would ruin my beautiful fur coat. Humans aren't delicious enough to take that risk. Why don't we just wait till they leave in the morning, and then chow down on the scraps and leftovers?

KAI: Over time, wolves probably realized that humans threw away delicious bones and other tasty treats, So they might have started hanging out around the places where people left their trash.

MOLLY BLOOM: At the same time, the humans probably noticed that the wolves chased away other predators from their campsites so they could keep those yummy garbage piles to themselves.

KAI: And people who study this think that over time, these wolves became less frightened of humans.

MOLLY BLOOM: And when those wolves had puppies, the pups were also less frightened of people. And within a few generations of wolves and people, something like this might have happened.

WOLF PUP 1: I bet the humans have even more meat that they're not putting in the bone pile. I'm going to follow them and see. Don't tell Mom.

WOLF PUP 2: Bring me back a leg.

HUNTER 1: Look, I went to the garbage pile to throw out some leftover hooves, and this little wolf pup followed me back. I mean, look at him.

HUNTER 2: So fuzzy. These wolves aren't as scary when they're small, huh?

HUNTER 1: I bet if we give him some meat, he'd stay and maybe decide he likes us. Maybe he would even share his meat with us when he grows up.

WOLF PUP 1: Bark bark, food please.


KAI: In some places, wolves and humans probably started living closer together.

MOLLY BLOOM: The wolves that lived with or near humans had puppies. And eventually, some humans probably started raising the wolf pups that were extra friendly. Those pups grew into adult wolves, but still stayed with the humans who raised them.

HUNTER 1: Oh, my goodness. Wolf, look at your fluffy belly. Who's a good boy? Who's a good wolf? Oh, yes, you are. Wait, what is that voice? Why is my voice doing that?

WOLF PUP 1: What is that voice? Why do I really like it? Hey, what is my tail doing?

HUNTER 2: Oh, your wolf is waving his tail around. He looks happy.

HUNTER 1: Good wolfie. Nice wolfie.

WOLF PUP 1: I am a nice wolfie. More meat, please. Oh, and scratch my ears.

MOLLY BLOOM: Researchers think over thousands of years, as wolves became more and more used to living around people, some of them started to look different from their wild cousins.

KAI: Their ears got floppier, they learnt how to bark to communicate with their human friends.

MOLLY BLOOM: These were not quite wolves and not quite dogs. After even more generations of living around people, some of these early dogs even evolved a special muscle above their eyes. This muscle lets them make the puppy dog expression that says, please give me a bite of your sandwich.

KAI: That muscle is something that wild wolves don't have.

MOLLY BLOOM: Humans shaped this canine evolution process so much that thousands of years later, a totally new species was born, dogs. So wolves and dogs are two different species.

KAI: And now, dogs can compete with us to fetch mysterious buckets of goo.

MOLLY BLOOM: And speaking of tail wags, Kai, you had a question about this.

KAI: Yeah, I wanted to know, when dogs wag their tails, does that mean they're trying to be your friend?

MOLLY BLOOM: That's a great question. So you've encountered dogs, and you've seen them wag their tails. Do you think that means they like you?

KAI: Yeah, I think it means that, like, they like you and they're happy.

MOLLY BLOOM: So we don't have tails. So how do you show somebody you're trying to be their friend?

KAI: I usually ask if I can walk with them, talk with them, or play with them, and yeah, and then we just do that a lot, and then we become friends.

MOLLY BLOOM: So you're able to use your words instead of your tail. Is there a body language that you can use to show someone, like, hey, I'm happy to see you?

KAI: You can wave.

MOLLY BLOOM: True. Tail wags are one way the dogs communicate with each other. It's like body language for dogs. When humans are happy, we might do a little wave or a smile or a thumbs up.

KAI: But sometimes, dogs communicate by shaking their butts.

MOLLY BLOOM: It's true. To learn more about the ways of the wag, we talk to Alexandra Horowitz. She studies dog minds at Barnard College in New York City.

ALEXANDRA HOROWITZ: One of the great pleasures of living with or knowing or meeting dogs is their tail wag. So yeah, I think the question of why they wag their tails, and additionally, what they mean when they wag their tails is a little bit more complicated than one might think. But it starts with what you probably already know, which is one of the tail wags is an expression of pure happiness.

KAI: I know that one. It's a classic. Their tails go back and forth like a windshield wiper.

ALEXANDRA HOROWITZ: So that loose broad tail wag that you get from a dog when you come home again, that's them happy to see you. That's them just thrilled at the sight of your face.

MOLLY BLOOM: But not all tail wags were created equal. Alexandra says there are different kinds of wags.

ALEXANDRA HOROWITZ: The other types include a really low tail wag. So the tail is dropped down and kind of twitching between the legs. That might indicate that the dog feels nervous or worried about something. And there's also a tail wag where the tail is straight up, in dogs who have tails that stick straight up, and kind of rigidly wagging quickly. And that's a tail wag that indicates the dog is feeling really excited and maybe even a little tense, like when they're seeing another dog who they've never met.

KAI: Something like, new dog alert, new dog alert!

MOLLY BLOOM: Dogs might wag their tails at other dogs or even other animals.

KAI: And they do it to communicate with humans, too.

MOLLY BLOOM: So tail wags are for people, fellow pups, and any animal in between. And these tail wags can get even more specific. Researchers found that when a dog wags its tail to one side or another, it can mean totally different things.

ALEXANDRA HOROWITZ: So for instance when the dog is confronted with their person, they wag their tail a lot. And the researchers found that they wagged it more strongly to the right side than to the left side. On the other hand, when the dog saw, like, a strange dog, a dog they didn't know, or an unknown person, they also wag their tail, but they wagged it more to the left.

MOLLY BLOOM: Wow, nothing like some good rump shaking research.

KAI: All in the name of science.

MOLLY BLOOM: These different wags are like the differences between a smile, a frown, or raising your eyebrows. It's an expression.

KAI: Right, it's a way for dogs to tell people or other dogs how they're feeling.

MOLLY BLOOM: But it's not just tails. Dogs use their whole bodies to communicate.

KAI: Their ears might perk up when they're alert.

MOLLY BLOOM: They might growl or bark to get your attention.


KAI: Or use their awesome sense of smell to check out just about anything, from your neighbor's tulips, to the newest dog at the park.

MOLLY BLOOM: By the way, we have a whole episode on dogs' sense of smell. You can listen to that one at brianson.org.

KAI: So dogs use lots of different ways to communicate, whether it's a bark, a really excited tail wag, or even perked up ears.

MOLLY BLOOM: You know, if my ears could perk up, they'd be standing straight up right now because it's time for the--

(WHISPERING) Mystery sound.

All right, are you ready? Are your ears ready, Kai?

KAI: Yeah.

MOLLY BLOOM: All right, here it is. What do you think?

KAI: Ooh, that's a hard one.

MOLLY BLOOM: Do you want to hear it again?

KAI: Yeah.


KAI: I feel like it could be, like, shaking a towel, or like some sort of clothing out, outside or like-- yeah.

MOLLY BLOOM: So you heard some shaking. You heard some outdoor noises. Well, we will hear it again and get another chance to guess after the credits. So stick around. Hey, friends. We're working on an episode all about bugs.

KAI: Bugs are incredible.

MOLLY BLOOM: Some bugs pollinate our plants, others help keep soil healthy, and lots end up as delicious meals for other animals.

KAI: Some bugs even eat other bugs, like how dragonflies eat mosquitoes.

MOLLY BLOOM: So cool. Kai, do you have a favorite bug?

KAI: Actually, I have not such a fan of bugs having lived through the brood x cicada epidemic in 2021. But my favorite would probably be a butterfly.

MOLLY BLOOM: OK, so you've got to fill us in. Tell us, what was it like to live through that cicada? So that was when tons of cicadas were hatched all at the same time, right?

KAI: Yeah.

MOLLY BLOOM: So can you describe cicadas for people who might not have gotten to live through that?

KAI: So they're these pretty big bugs that have wings and red eyes. But the worst thing is when they land on you. So like, one time, I guess it landed on me, and I didn't notice. And it crawled up my pants and into my shirt, like, my bare skin.


KAI: And I screamed so loud at camp.

MOLLY BLOOM: Oh, my goodness. Yeah, I understand why they're not your favorite.

KAI: Yeah.

MOLLY BLOOM: That is a lot to deal with. Yeah, so cicadas, they also make that really loud noise, right?

KAI: Yeah, when there's a lot of them, especially, like, that's the only thing you'll hear.

MOLLY BLOOM: Yeah, so I get why bugs are not your favorite. But you do like butterflies.

KAI: Yeah, like, I like the look of butterflies, but I don't really like when they land on me.

MOLLY BLOOM: Yeah, yeah. I'm sort of the same. I think insects are super cool. I really like to think about them and look at them in pictures, but I would prefer if they don't land on me if possible.

KAI: Yeah.

MOLLY BLOOM: My daughter, however, loves boxelders and will let them crawl all over her arms. She likes them. She likes the-- she always tries to keep them as a pet. Listeners, we want to know what's your favorite bug and why. Maybe it's a praying mantis.

KAI: Or a bumblebee.

MOLLY BLOOM: Whatever it is, we want to hear about your best bugs. You can send in your recordings to us at brainson.org/contact.

KAI: And while you're there, you can send us mystery sounds, drawings, and questions, like this one.

LILO: Hi, my name is Lilo. And I was wondering why the eggs were turning from clear to white when they were cooked?

MOLLY BLOOM: Again that's brainson.org/contact.

KAI: And keep listening. You're listening to Brains On. I'm Kai.

MOLLY BLOOM: And I'm Molly. We're talking all about dogs and their friends.

KAI: Dogs evolved from wolves over thousands and thousands of years. Over that time, they found ways to communicate with humans.

MOLLY BLOOM: Now they communicate so well with us, that they can learn all sorts of things.

KAI: Like sit, stay, roll over, and fetch that bucket of mystery goo.

MOLLY BLOOM: And we trust dogs to help us in lots of very important ways. Dogs have jobs now.

KAI: What if dogs had a career day at school? I bet it would go something like this.


INSTRUCTOR: OK, quiet down, pups. Shh! OK, welcome to career day. We have an important job as humans' best buddies, but that could mean a lot of different things. You could do lots of different jobs, depending on what kind of help your humans need.

PUP 1: I want to be a sled dog.

PUP 2: I want to be sheep dog.

PUP 3: I want to be a baloney sandwich dog. Is that a job to eat sandwiches?

INSTRUCTOR: You could be anything you want if you trained hard enough. We could be companions and help keep our humans safe. We could help them with their work. And they give us kibbles and ear scratches and throw sticks.

PUP 2: Stick, stick, stick.

INSTRUCTOR: Oops, shouldn't have said stick. OK, hush, puppies. Now to show you how many different jobs there are for us dogs, I've made a presentation of some of the great working dogs in my family. Here's my cousin Rudy. He lives on a ranch down in Texas and helps his humans move their cows from place to place.

PUP 1: What's a cow?

PUP 3: What's a Texas?

INSTRUCTOR: Texas is a place, and a cow is like a big dog that goes moo and makes milk, I think. That's what Rudy says. OK, next. This is my granddaughter Asha. She's a search and rescue dog. If something bad happens, like, if a building falls down, and humans get stuck inside, dogs like Asha use their noses to sniff out anybody who needs help. I'm so proud of her.

BONNIE: One time I was using my nose to sniff and-- and I found a poop.

PUP 3: Whoa, awesome.

INSTRUCTOR: That's great, Bonnie. Good sniffing. You can also use your nose to work with human police and soldiers to sniff out anything dangerous people might be carrying with them. Our noses are way more powerful than human noses. So if there's something dangerous, we can usually smell it long before humans see it. It's a cool trick.

PUP 1: I know a trick. Want to see? Look, I can bite my tail. Hang on, I almost got it. (MUMBLING) I caught it.

INSTRUCTOR: You got it! If you're good at making humans laugh and feel happy warm feelings, you could be a professional snuggler. Sometimes, humans who are sad or sick need a big furry hug and some slobbery kisses.

PUP 2: I like licking humans. They taste good.

PUP 3: And sometimes little baby ones are sticky.

INSTRUCTOR: OK, last one, pups. This is my sister Daisy. While we were puppies, she got picked for a special program to train her to guide humans who need help seeing. She keeps her human safe when they cross the street, helps her get around town, and lets her know if there's anything dangerous around. OK, that's it for today, pups. Does anyone have any questions?

PUP 1: Can I go pee? I really got to go pee.


Never mind.

INSTRUCTOR: All right, everybody. Let's head outside and sniff some grass.


ALL: Brains on!

MOLLY BLOOM: Wow, dogs are so helpful to us humans.

KAI: And dogs help other animals too, even cheetahs.

MOLLY BLOOM: Usually, we think of cheetahs as solitary creatures. But they can be pretty social, especially when they're younger. Cheetah cubs like to run and wrestle with their siblings. But sometimes a cub is abandoned by its mother or gets sick or injured, and needs help from a zoo. But even those cheetahs can do better with a pal. That happened to one cheetah at the San Diego Zoo.

NICKI BOYD: It was about 40 years ago, where we had a young cheetah cub that needed a companion. And so we had brought a dog in to be that companion. It was a golden retriever. And it kind of started from there.

MOLLY BLOOM: That's Nicki Boyd. She works with animals at the San Diego Zoo.

NICKI BOYD: Anna was the dog, and Arusha was the cheetah.

MOLLY BLOOM: She says the two of them quickly became friends. And visitors loved seeing them run and play together. Soon, the zoo started pairing other cheetahs with dogs.

NICKI BOYD: Cheetahs have some dog-like qualities they don't have retractable claws. So they have almost dog-like feet. And dogs love to run, and so do cheetahs. So we can take them out into our big campground area and let them run around together. And so it just worked. It was something that it seemed to work for both the animals most of the time. And it was fun for people to see.

MOLLY BLOOM: They didn't always bond right away. That's where trainers like Nicki helped. It's their job to get the animals comfortable with each other. After all, cats and dogs are usually not the best pals.

NICKI BOYD: Yeah, it's like the odd couple, right? So the dog's usually slowly introduced, maybe through a fence, just let them sniff each other. Reinforce for calm behavior. So give them their favorite treats for being relaxed. And then maybe we go on some walks, or walk in the habitat together a little bit. The dogs were usually the boss, so to speak. They were usually the more confident and outgoing, being domesticated, where cheetahs are not. And so the cheetahs kind of took the dogs' lead, which really kind of helped with a lot of the training.

MOLLY BLOOM: Soon enough, Nicki says they'd be playing and running around. And from then on, the cheetah and dog would be a pair, spending all of their time together. Well, almost.

NICKI BOYD: The only time that they were separated was for feeding because the dog would wolf down all its food and then try to go eat the cheetahs food. So we-- it wasn't more for aggression, it was more to make sure they got the right amount of food for each of them. Some of the pairings were really tight. You'd see them both grooming each other and sleeping together. And others wanted their space. And so they would sleep separately. So it just depended on the pairing.

MOLLY BLOOM: There's still one cheetah and dog who live together at the San Diego Zoo today, but Nicki says they're moving away from the program. Instead, they're trying to pair single cheetahs with other single cheetahs, sometimes from other zoos. But dogs are still helping out in other ways. Nicki says the zoo works with dogs trained to help sniff out endangered tadpoles in the wild. They're called frog dogs.


And they help scientists find these at-risk critters So they can help preserve them. That's a good doggie.



ANNOUNCER: Let's all give a big old round of applause for this year's winners of the Amazing Chase, Sanden and Penelope.


SANDEN: Hot dog! I cannot believe we did it. I'm going to be honest, I did not know what was going to happen after we spilled the mystery goo everywhere in round two. I was a little nervous when the goo accidentally dribbled into my mouth. But turns out it was delicious.

Spoiler alert, it's totes pistachio pudding, I'm sure of it. OK, honestly though, it gave me the sugar rush I needed to take the crown in the final round. And I have never seen Penelope fetch a beanbag chair with such ease. Who needs opposable thumbs when you've got a bite like that, right, Penelope?


I couldn't have said it better myself. Thanks, everyone. See you next year.


KAI: Humans and dogs evolved closely together over thousands of years, sharing food and eventually friendship.

MOLLY BLOOM: Dogs use different tail wags to communicate with dogs, humans, and even other animals.

KAI: They can learn how to do all sorts of amazing things to help people.

MOLLY BLOOM: And they help other animals too, from cheetahs to tadpoles.

KAI: That's it for this episode of Brains On. This episode was produced by Molly Bloom, Rosie Dupont, Anna Goldfield, Aron Woldeslassie, Anna Weggel, Nico Gonzalez Wisler, Ruby Guthrie, and Marc Sanchez.

MOLLY BLOOM: Our editors are Sanden Totten and Shahla Farzan. This episode was sound designed by Rachel Brees. We had engineering help from Peter Haggans and Josh Savago. Beth Perlman is our executive producer. The executives in charge of APM Studios are Chandra Kavati, Alex Shaffer, and Joanne Griffith. Special thanks to Irene Small, Koko, Cece, and Lulu.

KAI: Brains On is a nonprofit public radio program.

MOLLY BLOOM: There are lots of ways to support the show. Head to brainson.org.

KAI: While you're there, you can send in your questions and fan art.

MOLLY BLOOM: And you can subscribe to our smarty pass, ad-free episodes and rad bonus stuff just for you. OK, Kai, are you ready to hear that mystery sound again?

KAI: Yes.

MOLLY BLOOM: Wait, before we hear it this time, I'm going to give you a little hint, that it is related to the topic of this episode today, OK? So keep that in mind as you hear it. Do you want to hear it one more time?

KAI: No, I think I have it.

MOLLY BLOOM: OK, what do you think?

KAI: I think it is a dog wagging its tail.

MOLLY BLOOM: Hmm, very good guess. Do you want to hear the answer?

KAI: Yes.

MOLLY BLOOM: OK, here's the answer.

MARC SANCHEZ: That was the sound of my dog Ella flapping her ears back and forth. Every morning when I wake up, she is very, very excited to see me. And I go and give her a bunch of scratches. And as soon as I'm done, she makes this exact sound by shaking her head back and forth.

KAI: I was so close.

MOLLY BLOOM: You were so close. It was a dog shaking something, just not the tail, the ears. Have you seen dogs do that?

KAI: Um, well, like, when they're wet.

MOLLY BLOOM: Yes, totally. Yeah, they do it a lot. Like, when they get up for some reason, they're probably like, I don't know if it's a way to wake themselves up or get comfortable. Not really sure. We should find out.

KAI: Yeah.

MOLLY BLOOM: And that was producer Marc Sanchez talking about his dog Ella. So if you were going to send in a mystery sound of your cat, what would you send in a mystery sound of?

KAI: It would probably be of her purring.

MOLLY BLOOM: Mm, that's a very satisfying sound. When does your cat purr?

KAI: Whenever you put her in the right spots, or sometimes, like, she'll sit in your lap or go across your lap.

MOLLY BLOOM: Now it's time for the Brains Honor Roll. These are the incredible kids who keep this show going with their questions, ideas, mystery sounds, drawings, and high fives.



We'll be back next week with more answers to your questions.

KAI: Thanks for listening.

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