How old were you when you found out whales were mammals? It feels like an obvious fact now, but did you ever wonder who first decided to put these fish-like titans of the ocean in the same group as mice and foxes? In this episode, Brandon Gertz follows the story of how whales became mammals, from the birth of natural history over 2,000 years ago to modern genetics.
Brandon is the Communications Lead for the Deep Ocean Stewardship Initiative (DOSI) and a graduate of the Duke Marine Lab. His work focuses on connecting the work that scientists do in the deep sea to both policymakers and the public. In his spare time, Brandon enjoys putting together pieces like this podcast that highlight some of the surprising stories in ocean science.
Seas the Day Podcast
How Do We Know Whales are Mammals?
Diving with Whales by KEVOY plays
Brandon: Few animals have captured the human imagination like the whale. We are captivated by their gargantuan size, their grace as they breach the surface, and their surprising intelligence. Throughout history, whales have been revered as gods, feared as monsters, and relied on as a resource by all sorts of cultures.
And of course: at some point in your life, you learned a fact that, at the time probably surprised you: Whales, those finned, ocean dwelling creatures that look an awful lot like fish and whose name comes from a word literally meaning “large sea fish”, are actually mammals. If you never knew that before this episode, then congratulations! Now you do. If you did learn that before today, though, you probably followed up with a question: “Why?”
The answer you got could have been different depending on who you were talking to. Maybe whoever it was said that whales are mammals because they breathe air and make milk to feed their babies. Or maybe they said it was because whales are descended from extinct mammal ancestors that used to live on land. Both would be pretty good answers that make the fact that whales are mammals seem obvious. And we could end the conversation there. But those answers leave one key question: How do we know? How did we find out so much about whales, and who decided that having milk and breathing air or having some land animal for a great-grandparent makes a whale a mammal instead of a fish in the first place?
Oyster Waltz by Joe Morton plays
The answers to those questions lie in a fascinating story of discovery, tragedy, and triumph that burned for two thousand years on its way to what we now think of as a cold, hard fact. So, yes. We know whales are mammals. But getting there took more time and effort by smart, creative people and more surprising twists than you might think. This show is about appreciating and understanding those people who developed today’s obvious ideas: their successes, their setbacks, and the world of science that they helped build. My name is Brandon Gertz, and this is Seas the Day, a podcast from the Duke University Marine Lab. Today, I invite you to join me in asking the question: How do we know whales are mammals?
Tropical Cruise by code_box plays
Our journey begins with one man’s vacation on the Island of Lesbos just south of ancient Greece. The year was 340 BC, and the philosopher Aristotle had just arrived. And during his time on the island, Aristotle started to really think hard about whales.
Now, this is a big deal for science. Aristotle’s work, while often incomplete and sometimes just wrong, is responsible for inspiring a huge part of how we understand the world today. This guy has been called the father of almost every science there is, so having him on the case to think about what a whale is and how we should classify it was a great thing. But why Aristotle ended up on an island where he had the chance to study whales is the result of some pretty shocking coincidences.
You see, Aristotle had spent 20 years in the Greek cultural center of Athens as a student and eventually as a teacher at the academy of Plato, another Greek philosopher. But then, one day, he quit, packed up his things, and left.
The reason Aristotle quit his job isn’t totally clear. Traditional stories say that Aristotle was upset at the direction that Plato’s nephew was taking the academy in after taking over when Plato died in 347 BC. He might also have been worried about discrimination: Aristotle was originally from Macedonia, a country that Athens had just gone to war with.
Whether he quit his job because of disgust with his school’s administration or fear for his life, we do know that Aristotle made his next stop in the town of Assos in modern-day Turkey. There, he opened a new Academy with the support of the local ruler, who, after Aristotle married his daughter, also became his father-in-law.
But the good times didn’t last long: just a few years later, that same ruler was kidnapped on the orders of the king of Persia. Word eventually came back to Assos that Aristotle’s father-in-law, refusing to give up Greek military plans, had been tortured and finally assassinated.
With his patron dead, Aristotle, who now definitely had a reason to fear for his life, decided it was probably time to take a sabbatical from his Academy in Assos. A 3-year sabbatical. Packing up shop, Aristotle fled to the island of Lesbos. And there, purely by coincidence, and as something partway between a teacher, a tourist, and a refugee, is where Aristotle made his contribution to our understanding of whales.
Aristotle stayed in a house that overlooked a beautiful lagoon. And he was fascinated by the marine life he saw there. He was especially impressed by the giant beasts that spouted water high into the sky that he could see in the distance. And then, he studied. Making his own observations from the beach, riding along with fishermen on their boats, and taking notes on the stories of travelers, Aristotle learned about the bodies and behavior of the dolphins, porpoises, and fin whales that lived in the area. That collected knowledge was then written down in one of Aristotle’s few surviving works: Historia Animalium, or “History of Animals.”
This writing is pretty much the first surviving natural history document we have. While creating it, Aristotle had to decide how to categorize and separate the creatures in his world. What groups should he put them in, and for what reasons? Our modern definitions for things like mammals, fish, reptiles, and so on wouldn’t exist for over a thousand years. If you were faced with every living thing and had to divide them up yourself, how would you do it?
Well, working with the ideas of the time, Aristotle decided that the best way to divide up nature was through two main differences: Blood, and whether something laid eggs. Humans, for example, who clearly had blood and had live children instead of laying eggs, were not in the same category as insects, which laid eggs and, to Aristotle, did not really seem to have blood. He also grouped animals together by what he saw as their shared qualities, like birds having feathers. Aristotle called each one of those smaller groups a genus, which is a word we still use today.
Thanks to his time observing and researching whales on Lesbos, Aristotle had a good idea of where they should fit in his new system. Whales, along with dolphins and porpoises, had blood, did not lay eggs, and shared certain characteristics with each other. That led Aristotle to classify those animals under a single name that we also still use today in its Latin form: cetaceans, meaning, well… “like whales.” To clarify what a cetacean was, he wrote that they have a blowhole to breathe and, although they lived in the ocean, they had hair and milk like humans or horses. Aristotle also argued that cetaceans were not fish because they had no gills and their bones were more similar to those of land animals than they were to the ones fish had. For the first time in known academic history, whales had been separated from fish.
That makes it sound like Aristotle was calling whales mammals, right? Well, no. Aristotle didn’t go that far, though he did point out similarities between his cetaceans and some of the four-legged animals full of blood and milk that wandered the land. For Aristotle, though, taking that last leap in logic to connect whales and land animals with hair and milk had a problem, or more accurately, four problems: legs. The whales didn’t seem to have any. In the end, that seemed to be enough of a reason for Aristotle to divide them from what we would call other mammals now. After all, animals like lizards have legs, which in a way makes them seem more similar to a fox or a mouse than a whale does.
That difference was especially important to Aristotle because his system for classifying animals also included a feature that we don’t really use anymore: Ranking. Basically, of living things, humans were at the top. The best of the best. with lots of hot blood, live babies, and a good number of legs. The less human-like Aristotle thought a group of animals was, the further down the list it was ranked. Cetaceans managed to beat fish by one spot thanks to their hot blood and live babies, but lost to both two-legged birds and egg-laying, four-legged creatures like lizards and frogs. Still, progress had been made toward our modern view. Whales might not have been mammals yet, but thanks to the coincidences that had brought Aristotle to Lesbos, they had managed to escape being lumped in with their fishier friends.
Sadly, though, the change didn’t last. After a while, as the scholarly language of the Mediterranean and Europe switched to Latin and Greek texts became harder to find, Aristotle’s writings fell out of common use in European academia. His research wouldn’t be brought back into the spotlight for quite a while. Instead, much of the way that whales were classified across the middle ages and through the renaissance depended on the work of another famous figure, one born in ancient Rome nearly 350 years after Aristotle’s death: Pliny the Elder.
Pliny the Elder
Fantasy Classical Themes by TheoTer plays
Pliny the Elder was a Roman writer who, in the final years of his life, started working obsessively on a truly massive project: Naturalis Historia, or Natural History. His goal was nothing less than writing down all knowledge of everything ever about the natural world, which even in Roman times, was a lot: By the time it was done, Naturalis Historia filled 37 books. And because it covered… well, everything, it naturally included facts about whales. The books were also meant to be a resource for everyone, not just academics: Pliny dedicated Naturalis Historia first to, quote “the common people, the mass of peasants and artisans, and only then for those who devote themselves to their studies at leisure.” Unquote. And his plan worked: for centuries, Naturalis Historia was a bestseller. While Pliny’s books were deeply flawed from a modern point of view (He tended to write down information he heard without much fact checking, including stories of magical creatures) they were so popular that they became the model that future writers and scientists followed. Even after Aristotle’s work was rediscovered by medieval scholars, Pliny’s writings held onto their lead: Aristotle’s Historia Animalium had been released in about 12 editions by the year 1500, while Pliny’s Naturalis Historia had been published 39 times.
And with that fame came influence: Influence over how people saw whales. You see, Pliny had a very different idea about how animals should be grouped than Aristotle did. While Aristotle focused on shared features, like hair and breasts and blood, Pliny was much more interested in dividing animals based on where they live. Volume 9 of Naturalis Historia contained every animal that lived in the water, including oceans, rivers, and lakes.
Because of the way Pliny’s creatures were lumped together, it was hard to separate the whales from the fish. Pliny knew from Aristotle that whales and dolphins had live babies, made milk, and had no gills, but that difference from fish seemed less important to him than their shared environment: the water. And so, for the next 1600 years, Pliny’s decisions held their grip on tradition. Books almost always kept fish and whales together: Water creatures in one section, and land creatures in the other. Many scientists noted how similar whales were to land creatures, and some even followed Aristotle in dividing the so-called “fishes” into those with blood and without blood, but no one took that last, critical step to break from Pliny’s tradition and somehow unite whales with land animals. Until, in the 1700s, a friendship between two Swedish scientists changed everything.
Peter Artedi and Carl Linnaeus: Tragedy and the birth of Taxonomy
Vivaldi: Four Seasons: Spring performed by John Harrison with the Wichita State University Chamber Players plays
The part of our story where whales finally became mammals starts with a college student named Peter Artedi. Artedi was supposed to be at Uppsala University in Sweden to study religion, but found that he was a lot more interested in fish. After switching tracks to science, Artedi made friends with a new transfer student: Carl Linnaeus. Linnaeus had transferred to Uppsala so that he could study botany, and the two became very close. So much so that, when they went their separate ways in 1732, Artedi and Linnaeus made a promise: If either of them died an untimely death, each agreed to publish the other’s research in their memory.
Artedi continued his work after college, which was so valuable to our understanding of fish that he is often called the father or ichthyology. But what’s most important about his work to our story was the categories he had decided to put fish in. If you drew it on paper, Artedi’s system would look like an upside-down, branching tree. All fish were included at the top, but as you moved down the tree, the different groups would split into branches, and then into twigs based on how many features they had in common. But what about the whales? Well, like past scientists, Artedi knew that they were different: He separated cetaceans, which he called plagiuri, into their very own branch of the tree. He also followed Aristotle by separating cetaceans from fish based on their bodies: This time, the difference Artedi chose to focus on was the flat tail fin as opposed to the vertical one fish have. That choice moved away from Pliny’s focus on grouping animals based on whether they lived on land or in the water. Still, Artedi’s work was mainly about fish, not whales or land animals. and tradition led him to include the whales, dolphins porpoises, and even manatees in the book on fish he was writing, titled “Ichthyologia.”
In 1735, though, tragedy struck. While his focus was on his research, Artedi needed a job to pay the bills. During a visit, his friend Linnaeus had introduced Artedi to a wealthy Dutchman who paid him to write descriptions of fish for his book about animals. One night, while returning home from work, Artedi slipped, fell into a canal, and drowned. Olof Celsius, a less famous relative of the Celsius who worked on temperatures, wrote Artedi’s epitaph, which read:
Here lies poor Artedi, in foreign land pyx’d
Not a man nor a fish, but something betwixt,
Not a man, for his life among fishes he past,
Not a fish, for he perished by water at last.
Linnaeus, hearing about the accident two days after it happened, rushed to Amsterdam. With Artedi dead, there was nothing he could do but follow through on their promise from when they were both students. He collected Artedi’s incomplete book, Ichthyologia, and published it in his stead.
But that wasn’t the only impact of Artedi’s research. Linnaeus had been working on his own book that would change how we classify life forever: Systema Naturae. Rather than working like a hierarchy of how good a creature is by virtue of its blood, or separating them based on their environment, Linnaeus’s new system followed Artedi by looking like an upside-down, branching tree. This time, rather than just fish, the tree started with all of nature in the trunk, and then split based on the characteristics that different things had in common. Linnaeus developed the basis for the modern way that we tell one kind of life apart from another. It wasn’t identical to the modern system. For example, without getting too deep into the organizational weeds, scientists now usually divide life into broad kingdoms that include groups like animals, plants, fungae, and so on. Linnaeus’s 3 kingdoms were animals, plants, and minerals, the last of which was gotten rid of by later scientists. But the main ideas stuck. From that point on, people would define species based on where they were found in that upside-down tree of life. Modern taxonomy had truly arrived.
Compared to Pliny’s 37 books, the first edition of Systema Naturae published in 1735 wasn’t much: just 12 pages. Following both tradition and his friend’s lead, Linnaeus also placed cetaceans, including whales, with the fish. But in the much larger 10th edition of Systema Naturae in 1758, Linnaeus made a judgement call that became the turning point of our story. He invented a new group of animals. Animals that had warm, red blood, birthed live young, and fed those young with milk. Linnaeus named this new group Mammalia, after the Latin word for breast. And he placed whales inside it. From a scientific standpoint, in 1758, whales officially became mammals.
I feel like now is a good time to point out just how amazing the creation of this upside-down tree and the idea of a mammal was because of one key thing: None of the scholars that we’ve talked about: Aristotle, Pliny the Elder, Artedi, or Linnaeus, had the idea of evolution to guide their thinking. Linnaeus organized the tree of life that we still use today in modern science, but he didn’t believe that he was actually describing where these animals had come from. To him, species were real groups that were “god-given.” But he never considered the idea of a common ancestor that would make these different species within a group like mammals related to one another. Most levels of the upside-down tree, like kingdom and class, Linnaeus just saw as tools to help other scientists think and talk about living things. It was sort of a filing system. Like books in a big library, there’s a lot of life out there. and finding what you’re looking for can be hard. If you were looking for a book about an animal and knew that the animal had hair and gave milk, Linnaeus would tell you to check in the mammals section. Very convenient! But not a literal interpretation of the history of life. As Linnaeus himself stated his belief: “God created, Linnaeus organized.” The fact that we can use Linnaeus’s tree today as a reference for how life evolved is just a lucky coincidence based on hundreds of years of gradually improving guesses about which features of an animal are important for dividing them into different groups.
So what changed between then and now? How did we learn that whales literally came from other mammals, rather than just sharing traits with them? Well, Charles Darwin, the scientist who first really established the idea of a common ancestor for living things a hundred years later, had his suspicions about whales’ relatives on land. Since it was accepted by that point that whales were mammals, the only real question left for scientists like Darwin in the 1800s was how exactly the whales were related to the rest. For his part, Darwin said that he could imagine how a bear-like ancestor that switched to feeding only in the water might be able to give rise to big marine mammals over a lot of generations. But there really weren’t many good fossils showing a transition from whale’s ancestors on land for scientists to work with.
Because of that lack of information, debates on which mammals whales evolved from cropped up and kept going. Some scientists favored the idea that whales came from land-dwelling carnivores, while others were more drawn to the idea that whales’ closest living relatives were the hippopotamus. How long do you think that uncertainty about where the whales came from went on for? 20 years? 50? Try 140. It was only in 2001 that information from fossils and molecular biology, a science that can identify relationships between species based on their DNA, really sealed the deal: Hippos were in fact the whale’s closest cousins.
And so, we come to the end of our story. We’ve reached the present, where whales are definitely, securely mammals. We know what they’re like, what they share in common with other mammals, and, thanks to modern science, we even know where they came from. So does that mean that scientists are finally done arguing about where whales belong?
Well, no. Not even close. Because as much as we love to label and rank animals, it’s actually really hard to draw clear boundaries between different kinds of living things. Remember the people who first told you that a whale was a mammal? We mentioned the two arguments that they probably used to convince you. But we can challenge both of those pretty easily. First, let’s think about the person who told you whales are mammals because they have mammal features: They give milk, have hair, and have live babies. But that argument doesn’t really work for something like the platypus: We call it a mammal. It evolved from other mammals. But it lays eggs. Does that make it something new? Or just a sort of weird mammal?
The second argument someone might have used to convince you is that whales are mammals because they’re descended from other mammals. But relying on ancestry like that has sort of a chicken and the egg problem. When was the first mammal born? Was its parent also a mammal? Or was it something else that was… almost a mammal? And since mammals share a common ancestor with fish, wouldn’t using the ancestry logic mean that whales (and people) are fish after all? Hard lines and life don’t really mix, and it’s important to remember that taxonomy is still very much a human tool that we use to simplify the vastly complex scope and history of life.
Thoughtful Piano by tictac9 plays
So, how do we know whales are mammals? It can seem like an obvious fact of our modern world, but it’s really the result of a lot of science and even philosophy that shifted how we look at nature and the “correct” places of creatures within it. We’ve come a long way from Aristotle’s ranking of animals by whether or not they have blood or Pliny the Elder’s groupings based on where animals lived, but that story of change and mistakes and growth is important. So the next time that you hear or read something that points out the obvious -that whales are mammals- be thankful to the people who were willing to ask silly questions. Who challenged the status quo, made mistakes, and did their best with the information and tools that they had. Because if the story of how we know whales are mammals tells us anything, it’s that there’s always more to learn.
Diving With Whales by KEVOY plays
Oyster Waltz by Joe Morton plays
Thanks for listening to this episode of How Do We Know on Seas the Day Podcast! This episode was written and produced by me, Brandon Gertz. Our theme music, Oyster Waltz, was written and recorded by Joe Morton. If you have an ocean-themed question that you think might make for an interesting episode of How Do We Know, send us a message on Twitter @seasthedaypod and we may use your question in a future episode. A good part of the information in this episode came from a paper called “When Whales Became Mammals: The Scientific Journey of Cetaceans from Fish to Mammals in the History of Science” by Aldemaro Romero, so be sure to give it a read if you want to learn more. Thanks for listening, and we’ll see you next time.
Black, Riley. (2010) “How Did Whales Evolve?” Smithsonian Magazine. https://www.smithsonianmag.com/science-nature/how-did-whales-evolve-73276956/
Encyclopedia.com. “Artedi, Peter.” https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/artedi-peter
Gebhart, Tim. (2021) “Meet Six of the Beasts in Pliny’s “Natural History.” Medium.com. https://medium.com/exploring-history/meet-six-of-the-beasts-in-plinys-natural-history-55753fbf9e72
The Politics. (2010) Müller, Sabine “Philip II”. University of Chicago Press. ISBN 978-0-226-92184-6. In Roisman, Joseph; Worthington, Ian (eds.). A Companion to Ancient Macedonia. Oxford: Wiley-Blackwell. pp. 166–185. ISBN 978-1-4051-7936-2.
Romero, Aldemaro. (2012) “When Whales Became Mammals: The Scientific Journey of Cetaceans From Fish to Mammals in the History of Science.” In New Approaches to the Study of Marine Mammals, edited by Aldemaro Romero. InTech. https://doi.org/10.5772/50811.
Society for the Diffusion of Useful Knowledge (1844) “Artedi.” Biographical Dictionary, Volume III Part II. Longman, Brown, Green, and Longmans.
University of Hawaii CriticaLink. | Aristotle (384 – 322 BCE) https://english.hawaii.edu/criticalink/aristotle/life.html
World History Encyclopedia. “Aristotle.” https://www.worldhistory.org/aristotle/#:~:text=When%20he%20was%2018%2C%20Aristotle,for%20the%20next%2020%20years.&text=He%20was%20an%20exceptional%20student,faculty%20teaching%20rhetoric%20and%20dialogue
Wright, John. (2014) The Naming of the Shrew: A Curious History of Latin Names. Bloomsbury Publishing.
Music and Sound Effect Sources
AaronGNP, “Dramatic Tension” https://freesound.org/people/AaronGNP/sounds/54615/
code_box, “Tropical Cruise” https://freesound.org/people/code_box/sounds/573973/
Eelke, “Calm Seawaves” Freesound – “calm seawaves” by Eelke
John Harrison with the Wichita State University Chamber Players, “Vivaldi: Four Seasons: Spring” https://freemusicarchive.org/music/John_Harrison_with_the_Wichita_State_University_Chamber_Players/The_Four_Seasons_Vivaldi
Kevoy, “Diving with Whales” https://freesound.org/people/KEVOY/sounds/82325/
Saviraz, “Boxing Bell Signals.”
Sonsdebarcelona, “Group Fighting.” https://freesound.org/people/sonsdebarcelona/sounds/221960/
TheoTer, “Fantasy Classical Themes” https://freesound.org/people/TheoTer/sounds/511311/