What makes a mammal?

Is there more to mammals than fur and milk?
08 October 2017

Interview with 

Liam Drew




From tiny shrews to enormous blue whales, polecats to polar bears, mammals have expanded across the planet to fill a wide range of environmental niches from the frozen Arctic to the fiery desert. But what actually makes a mammal a mammal? That’s a question that bothered neuroscientist-turned science writer Liam Drew when he accidentally took a very painful football to the groin while playing goalkeeper - as only mammals keep their vital reproductive organs dangling about on the outside.

Luckily this didn’t stop him becoming a father, and during that process he became fascinated by the underlying biology - a live baby growing in the womb, fed through a placenta, and then nourished on milk produced by the mammary glands from which mammals get their name. You might think of other traits, such as having fur - but if all it takes is hair and milk, then is a coconut a mammal? To get to the bottom of this mammalian mystery, Kat Arney caught up with Liam at the Wellcome Collection in central London for a chat about his new book on this very subject - I, Mammal.

Liam - You look it up in the dictionary and it says a mammal is a warm-blooded vertebrate with hair and mammary glands, maybe a few other traits if your dictionary is a good one. And so, I thought you know, to define what a mammal is, I’ll write a list of the traits we have hair, a unique type of cerebral cortex, three bones in our middle ears, a unique jaw joint... And I figured that each chapter of the book would describe fairly independently how each of these traits had evolved. And so, it was great fun, looking at how milk had evolved or…

Kat - Boobs and willies, you know.

Liam - Yeah, well I guess I was inspired by reproductive biology, so it did start in the bathing suit area and I’d like to try and move beyond that...

Kat - This is a scientific book obviously!

Liam - Yes, exactly. Well, I did end up trying to arrange the book, having started with the scrotum and saying, “Look! Life begins when sperm are made there and obviously, when eggs are made in the ovaries.” And then I tried to trace the arc of a mammalian life. But to go back to your question, I have this list of traits and this was very 18th century really. You know, the actual classification of mammals can be traced to Linnaeus in the mid-18th century, 1758.

Kat - He did love a bit of classification.

Liam - He did love a bit of classification! And so, it’s in the same book that he coined the term Homo sapiens, that he coined the term mammals. But anyway, next to these 200 primarily European mammals, he had a list of the traits they had and that’s how they did it then. And of course, Darwinian Theory a hundred years later really turned everything on its head. But I think what was important in writing the book, in trying to come up with what a mammal was, was that the chapters weren’t independent at all.

When I was writing about the scrotum, it wasn’t like this made sperm production better. It was like, this is a necessary adaptation to either mammals becoming more warm-blooded or essentially because they started galloping in a certain way which made the abdomen susceptible to these waves of pressure. So we have these two traits of the animal which changed its characteristic.

And then once you went to mammary glands, the sort of main theories on why milk evolved was that before it was a food stuff, it was a useful secretion to either keep eggs moist or to stop from dehydrating, or that the secretion contained components of the innate immune system which killed microorganisms which would have proliferated more on eggs as mammals became warm blooded. So you have this warm-blooded animal laying warmer eggs that needed to be protected either from dehydration or from microorganisms. And so again milk was tied to the warm bloodedness.

There was this fantastic essay written in 1977 on why lactation was important - it was actually published in a month when I was actually being breastfed, which I kind of liked that connection - by Caroline Pond at Oxford University and she listed how as soon as milk could have evolved it changed the landscape again for the possibilities of the animals that possessed it. So suddenly, by the newborns being fed milk, they didn’t have to hunt their own food. And so, they didn’t need such good teeth and so they could grow to a sort of fairly adult size and have an adult jaw and only grow their teeth in an adult jaw.

Mammals have one set of milk teeth and an adult dentition. One of the really defining things of mammals is how sophisticated their teeth are. And one of the things about them is that they interlock perfectly between upper and lower jaws which allows us to chew food very efficiently which helps power this warm blooded metabolism of ours. So again, it was just sort of link upon link. So as soon as I got to the warm-blooded chapter, it was like, “Hey, remember. All these other stuff was spoken about.”

Kat - You can't forget any of it. It seems like being a mammal is absolutely not this like 18th century tick list of, if you’ve got all seven of these, you're definitely a mammal. It’s more like kind of, “do you fit into this weird Venn diagram?” because there must be exceptions. There must be mammals that are weirdly exceptional.

Liam - So, as soon as you say a mammal is an animal with hair, you have a problem with dolphins – they have hair in utero and a few hairs around their mouths when they're first born. But then they have…

Kat - Yeah, like a tiny mustache.

Liam - Yes, apparently useful for finding the mother’s teat. But the dolphin has lost that hair. It’s lost this definition or trait, but it’s still defined by its high energy level lifestyle and all these other traits which allow it to function.

Kat - When we’re talking about Linnaeus, we’re talking about the idea of what is a mammal, Linnaeus and the taxonomists, they love to classify things and say, “Well, it’s got this structure and that form, and so, it must be this kind of animal.” But now, we have genetics. What is genetics telling us about what it is to be a mammal and where we came from?

Liam - When people starting doing taxonomy after Darwin, which was based upon shared history and inter-relatedness, the groups look quite similar with what Linnaeus had come up with because one sort of assumed that a shared physical characteristic meant you are very closely related.

Kat - If you look like a mouse, you're probably some kind of like rodenty mousey thing.

Liam - Yeah, precisely. For a century after Darwin, people were doing taxonomy on morphological basis. And then in the late 1950’s and early ‘60s, this idea of using – it was actually Francis Crick who, in a 1957 lecture said we can start using protein sequences or DNA sequences to trace relatedness. Instead of looking at morphological features, look at protein or DNA sequences of different species and and then further interrelatedness from that.

And it was a wonderful idea. Linus Pauling did this with primate hemoglobin and of course, DNA sequencing was incredibly arduous and hardwork for a long time. Some of the morphologists didn’t really trust this and you can kind of imagine that, right? People who know animals incredibly well and infer everything from all their features, suddenly, guys with test tubes of DNA are like, “Hold on buddy. I've got this covered.” People who’ve never even held an animal could use DNA to infer inter-relatedness.

Kat - It must be a bit like having a paternity test going. “No. That’s definitely my kid.” And it’s like, “DNA says no. I'm afraid that is not a rodent.”

Liam - Exactly. And so, this back and forth went on and on, and in the early ‘90s, some molecular biologists said, “Actually, we don’t think a guinea pig is a rodent” and everyone just sort of dismissed it as very silly. And then suddenly, in 1997, this group in California led by Mark Springer suddenly published this paper entitled, “African Mammals Shake the Mammalian Family Tree.” They basically showed on really robust genetic evidence – they’d really assembled more genetic evidence than anyone had ever previously done – and they suggested that actually, there was a group of African mammals which were more closely related to one another than any other types of mammals.

Kat - So, when people started to discover that DNA was telling us different stories about mammalian history than the fossils, and then the actual physical shapes of mammals, should this change the way that we think about what it means to be a mammal? Is there a genetic definition of what it is to be a mammal?

Liam - The genetics data is now overwhelming in this classification of four groups. One of the really interesting things that it did is that it really tied mammalian evolution to geography. And so, there was the original African group and then the data showed very convincingly that the South American mammals, the sloths, the ant eaters, and armadillos were very closely related forming another group. And there was a third group which could be subdivided in two which evolved on Eurasia. So it actually tied really nicely together the genetic story and the geography of the world.

Kat - I love how in the book you describe mammals as being almost like luxury items in evolutionary history.

Liam - Yeah and I had a moment - that was actually in Darwin’s garden, I think - I was just wandering around there and it’s a beautiful house and it was a weird moment of thinking, ‘there's a lot of plants there’. I was all alone in the woodland behind his house where he used to walk. I'm just thinking, “Look at all these wood and all these plants.”

I mean, it’s absurd – the amount of energy we use. We’re not energy efficient. 95 per cent of our energy consumption goes on just maintaining our temperatures, maintaining our physiology. Together, we are a luxury. You couldn’t have a world made of mammals. We’re reliant on just eating and consuming all the time.

Kat - We are biology’s luxury.

Liam - Yeah, maybe!

Kat - Liam’s book, I, Mammal, is published by Bloomsbury Sigma on the 2nd November, and is available from all good retailers/


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