The Infinite Monkey Cage - The Scale of Life (or were dinosaurs just too big?)

Episode Date: November 29, 2023

Brian and Robin are joined by palaeontologist Dr Susie Maidment, evolutionary biologist Dr Tori Herridge and comedian Dave Gorman to pitch giant creatures against tiny creatures in their bid to avoid extinction. They explore the scale of life and ask why some organisms are large and some small and what the optimum size for successful survival is. From the prokaryotic cell to the grandest dinosaur, how does the modern synthesis explain the huge variation in scale, form and function? What are the advantages and disadvantages to being huge like the dinosaurs, or was it their size that really did them in, in the end?New episodes released Wednesdays. If you're in the UK, listen to the newest episodes of The Infinite Monkey Cage first on BBC Sounds: bbc.in/3K3JzyFExecutive Producer: Alexandra Feachem

In our new podcast, Nature Answers, rural stories from a changing planet, we are traveling with you to Uganda and Ghana to meet the people on the front lines of climate change. We will share stories of how they are thriving using lessons learned from nature. And good news, it is working. Learn more by listening to Nature Answers wherever you get your podcasts. This is the first radio ad you can smell. The new Cinnabon pull apart only at Wendy's. It's ooey gooey and just five bucks with a small coffee all day long. Taxes extra at participating Wendy's until May 5th. Terms and conditions apply. BBC Sounds. Music, radio, podcasts.

Hello, I'm Brian Cox. And I'm Robert Ince. Welcome to the Infinite Monkey Cage. Now today we are exploring the scale of life. Why are some organisms large and some small? From the prokaryotic cell to the grandest dinosaur. How does the modern synthesis explain the observed variation in scale, form and function? Eloquently put and I presume that everyone who ever listens to this knows that we spend most of our time

exploring deep questions and carefully formulating our prose to transfer these ideas to listeners with economy and precision, not dissimilar to the ideas of the Darwin paradigm. It didn't go out there that's what we've actually spent the whole afternoon after about five minutes of of you know doing some research it's talking about why do big things always talk like this if you have

a big elephant and all the little things always talk like this like if they're in bagpipes we will fix it we will mend it we did that for three hours and that's basically the show that we've got is the equivalent of watching Johnny Morris' Animal Magic, with him doing the voice, Hello, I'm a very big bear. I'm a very small thing. I'm Brian Cox, look at that. Today, we are exploring the scale of living things

and the advantages and disadvantages that come with being big or small. To discuss these matters without silly voices, well, probably with some silly voices, but anyway mainly without silly voices we're joined by a paleontologist, an evolutionary biologist and a comedian who came to fame by asking the question are you Dave Gorman? Now I don't know if I've given away any of the panel there with that introduction, we'll find out in a moment. They are? I'm Susie Maidment, I'm a paleontologist at the Natural History Museum and I think the most astounding fossil ever found was the stem tetrapod, Acanthostega. Acanthostega is a fish, but it has limbs.

It's from the Devonian period. It's very, very old, about 450 million years ago, something like that. And it's a fish. It lived in rivers. It's got quite a fish-like body, a fish--like tail but it has limbs and it has digits but biomechanical modeling suggests that it couldn't stand on these limbs and this has been used to indicate that actually limbs first evolved for kind of moving through shallow water and pushing off the bottom so uh you know this is basically our oldest relative and just to set the scene there so 450 million years ago so the land was there was no plants on the land at the time? Yeah, some early plants, yeah, very early plants, not much else though, some insects and things like that.

But of course, yeah, it was the first sort of vertebrates to come out onto land, yeah. Hello! I love that idea, so people at home can go, oh, so Tauri's very, very small. Sushi very big. Hello, I'm Tori Herridge. at home can go oh so so tori's very very small hello i'm tori herridge i'm an evolutionary biologist from the university of sheffield and i think the most astounding fossil ever found i'm cheating it's not quite a fossil yet it's the permafrost remains of woolly mammoths because they still have all of their innards sometimes i have literally had the chance to hold a woolly mammoth liver and floppy gooey corpsey and when you get to hold

the floppy flesh of your extinct animal it doesn't get better than that your your choice there was excellent in the speed in which an audience have gone from ooh to eww. I'm Dave Gorman. I'm a comedian. I live in Bournemouth, and I think the greatest fossil discovery is actually quite a simple fossil. It's an ammonite, but they found so many of them, they were able to package them and give one to everyone on a recent budget airline flight I took.

They called them Danish pastries. Very good. And this is our panel. Thank you. Susie, could you just define the parameters here? What are the largest animals that have ever lived? Or the smallest animals? OK, well, the largest terrestrial vertebrates that have ever lived,

so things that are walking on land, that we know of are the Titanosaurian dinosaurs. So these were things that lived in the Cretaceous period. And the largest one is about 60 tonnes. Now, we can get to how you estimate mass in an extinct animal at a later point in the show but that is fraught with difficulty as you might be able to guess it was about 30 meters long absolutely enormous if you want to see one there's one on display at the natural history museum right now and they are

absolutely huge and so they're the largest terrestrial animals. Of course, we also have things like blue whales, which are also very, very, very enormous. Yeah. And then in terms of the smallest? For mammals, it's the bumblebee bat, I think it is. And for birds... Under two grams, I think. Yeah, for birds, the same.

Bee hummingbirds. But, I mean, if we're going to insects, then we're getting, you know, microscopic. Yeah, and then you go down to single-celled organisms and everything. I mean, that's the thing about size it it covers everything yeah well brian originally wanted to spend the whole show just going so if we start with the subatomic particles get to the size of the universe there might be a time problem you know whether it exists

it brings me to the obvious question from an evolutionary perspective why that huge range clearly started with single-celled organisms why do things get so big oh that's a really really really big question and it depends on your perspective i guess like you know why are animals plants the size they are now you know you could have many different answers you could say well first and foremost size is fundamental to your biology it determines where you can live what you can eat how long you live how quickly you grow up how many babies you might have in your lifetime all those things we know tie in with size right so you can sort of say you know a big animal is one kind of thing and a small animal is one kind of thing people have even suggested there might be an

optimal body size which for mammals might be around a kilogram or 100 grams depending who you think but of course not every single animal can be one kilogram if it determines where you live right because we can't all live in the same space and so if you want to fill up all the space that's available then you change your size and you change the kind of animal that you are so that's the kind of bigger picture but if you take the other perspective the one that Susie and I you know naturally take as evolutionary biologists and paleontologists, and look back in time and say, you know, when did things start to get big? What's the baggage we all bring with us? You know, where does it all come from?

Then you get different answers. And actually, if you look across the evolutionary time period for animals, I can't do plants, I'm sorry, things don't get big, like really big, big like elephant big for quite a long time in the history of the earth you're talking i guess about 300 million years ago before you see something kind of big it's really only when you get to the dinosaurs that you see stuff that is kind of bigger than two and a half meters in fact two meters seems to be the limit of like you get ediaca and stuff really old weird who knows what they are type things

you get discs two metres wide. You go through to later periods. When were the giant millipedes? Oh, Carboniferous. Carboniferous, yeah. 350 million years ago. Again, like two metre millipedes in Northumberland. In Northumberland?

In Northumberland. And other places, but also Northumberland. Two metre millipedes. Like two metres seems to be this kind of thing. And then you hit like the Permian, bit bigger, you get something maybe two and a half bit bigger. And then you hit the Triassic. Dinosaurs, we don't like dinosaurs.

I love that reaction to the two metre millipedes. There was a real sense that some people here are not keen on that idea. I was so tempted to just go, release the millipedes. Why could they exist then? Because now that two metre millipede would not susie why would it be harder now for for that to exist well the theory is that the atmosphere has a different composition probably a little bit more oxygen than it does today and of course

these insects rely on diffusion through their cuticles so the idea is that they could get a lot bigger than today because there's a higher partial pressure of oxygen in the atmosphere so that diffusion could occur. It's one of the justifications for expanding you, Les. I just got slightly lost. Very early on, you said size is all to do with perspective, and I thought, well, yeah, obviously. If you're closer, it looks big.

I get perspective, but that's not what you meant at all. That would be great, wouldn't it, if we eventually discovered the only reason dinosaurs seem to be big was because everyone was standing too close to them. How did we not work that out before? A fossil record is a lie. Another win for the creationists. Tori, you mentioned there, just in passing,

that we carry evolutionary baggage with us. So if you ask the question, why are we the size that we are, you're implying that that's because of our history, partly, where we came from. If you could basically reset the clock to the world right now, you wouldn't necessarily design animals, plants, to be the size and shape that they were. They're not perfect.

They're pretty good, because evolution does a good job but evolution can only work on what came before and what came before came from a different time. You've got to work with the mess you're given. A really obvious question is why is there nothing as big as a dinosaur

walking around on land today? Part of the reason is that everything big got totally smashed to smithereens by an asteroid that hit 66 million years ago. And that reset the clock to some respect, but it basically made big things go extinct. The things that survived were generally smaller, and then the clock sussed again, and then everything that's come on from that is working from that small baseline. I've got a good mass extinction fact so 66 million years ago when the dinosaurs went extinct everything that was over 150 kilos went extinct

everything so the only things that survived were under 150 kilos so it was very size selective yeah and it takes like you know 20 million years after that before you get to the big stuff again stuff that we would go wow big mammals you mammals. You have the Parasauetheriums which are kind of rhino relatives so they're kind of like rhinos with long necks and they were massive. Some of them reached something ridiculous like 24 tonnes 5 metres at the shoulders

they had long necks as well. They were found in Africa and the Southern Plateau, all over the world and at that same time period you start to get the first big elephant like things as well but they're still not as big as dinosaurs. No, tiny, tiny mate. It's not a competition, Susie. By the way, just to say, that's on land.

So in the ocean, the big stuff survives. Not at the end of Cretaceous mass extinction. Everything in the land and in the oceans goes extinct. So we do have big things in the ocean, not quite as big as today but we've got big nocosaurs, we've got other other marine reptiles are really really large and they all go extinct so it is literally globally everything that we know about at least so far it's a reset yeah it can't be possible to sort of tell over the time frame that we have actually witnessed as a species but are things getting bigger and do you think that is evolutionary or is there just not a big enough sample size?

Things are getting smaller. Are they? Again, it's like a perspective thing. So we are losing big animals far faster than we're losing small species. And we are living in a time of extinction and the big things are going first. And so you see that pattern, it changes. So we actually think of today's time slice as

being we call like size de pauper we actually are missing the big things and the other times in earth history we always have big things now that how big that big thing is varies they used to be big things but they've all gone extinct so we used to have mowers and we used to have loads of different sorts of elephants yeah loads of them and things like this and of course the megafauna in australia and they have been wiped out as humans evolved and there's a bit of debate around the impact of humans versus the impact of climate change in different places and this is one thing that actually we haven't touched on but i think is important to mention that that first question you asked brian about you know why do we have big things or why is there a big size variation is that not only are there you only evolution has to work on what it has

and all the things that Tori's already said, but also there's not one answer. Ecologists love coming up with rules and giving them names of people, usually white men. All those different rules are competing against each other the whole time. So today, this size, depauperate fauna that we have could be because of humans, but it could also be because of this sort of unique set of climatic conditions

that we find ourselves in after the last ice age. So why did these dinosaurs get, what did you say, 30 metres long in some cases? As I say, there's probably not a single answer to that question. But one of the kind of leading theories about why, I think it's safe to say that most of the herbivorous dinosaurs are not running anywhere so today if you think about the savannah and you think about the plane you've got antelopes running away from lions and stuff and that you know the lions are chasing the antelopes you've got pursuit predation and that hadn't evolved in the mesodermic that's not how predation worked then so a lot of dinosaurs that we know evolved quite elaborate mechanisms for defending themselves against predators so we

have armored dinosaurs we've got dinosaurs with horns and frills they may have been display evolved quite elaborate mechanisms for defending themselves against predators. So we have armoured dinosaurs, we've got dinosaurs with horns and frills. They may have been display structures to make themselves look more scary. And of course, they all got bigger. So one way you can stop getting eaten is to just be too big, too big to prey on. Yeah, but then the predators get bigger.

Right, exactly. So then you get Allosaurus in the Jurassic. So then everything, you know, then your prey has to get a bit bigger. So then your predators get even bigger. So, you know, you start to get a t-rex size thing in the cretaceous so the idea is that as the predators get bigger the prey then get bigger to defend themselves and of course there were other methods as well for the sauropods the long necked long tailed dinosaurs we know they also lived in herds now of course they hatched out of eggs about the

size of a melon so they had to grow incredibly rapidly to get to that protected adult body size. So probably a lot of the juveniles were preyed upon. But if they managed to get over some sort of critical threshold, then they would have been pretty much safe from being attacked. So this is really different in some ways to how we think about mammals. What Susie's describing there is so different to how an elephant would do it, which is the biggest sort of land mammal we have today. Now, elephants do not lay an egg the size of a melon.

An African elephant female, like four tonnes maybe, elephant baby, 100 kilograms, and it takes 22 months to gestate it, and then you invest a lot of energy in it, looking after it, feeding it. It's a high investment strategy. Everything happens slowly, but hopefully your baby makes it. Very few babies die. And so you have a high investment strategy everything happens slowly but hopefully your baby makes it very few babies die and so you have a very different strategy to say these mega

like huge size sauropods i personally don't understand how they got enough energy to grow that quickly without just eating plants with those silly small heads yeah i mean i think that is probably you know an excellent question possibly for another show. How long do they live, typically? We don't really know. It's difficult to tell that. One of the things we can do is slice up the bones and look at growth rings. So bones have growth rings in them. The idea is that these are probably seasonal,

they're probably deposited when growth slows at times of limited resources. But one of the things we find with the sauropods is that they grew incredibly rapidly, such that they didn't very often deposit these growth rings. So it's actually pretty difficult to date a sauropod. But I mean, I think they could have lived to probably 70 years or, you know, something like that. So they probably lived a long time.

I always thought with evolution, sort of everything would be happening in some species somewhere. So why wasn't there some herbivore going, be small and quick? Oh, there was. Everyone else is getting huge. Yeah, but there would be though, wouldn't there? They would be involved running. Yeah. I mean, there was, and I should be clear there,

there were small two-legged herbivores that were quick and there were small two-legged meat eaters that were quick. But when we get up to the bigger things, you know, four legs, and that four-leggedness is kind of a really key thing that we see in the dinosaurs, because all dinosaurs were two-legged. They And that four-leggedness is kind of a really key thing that we see in the dinosaurs, because all dinosaurs were two-legged. They evolved from two-legged ancestors. And so we see them evolving four-leggedness

multiple different times through their evolution as herbivores. And this is probably related to a herbivorous diet, actually. They needed this big kind of barrel-like gut to digest all this food that they needed to be able to grow really fast and all that sort of thing. And in order to suspend your massive barrel-like gut and not constantly topple forward, you need to go down onto four legs. So that's one idea about why they maybe evolved this four-leggedness.

And then, you know, I guess a kind of corollary of having to be four-legged and having this big gut is you can no longer run around very fast. And you set the scene for the race towards getting bigger and bigger and bigger. How big do you think it would have got if the asteroid hadn't hit? I mean, it's interesting, isn't it? Because every time they find a new one, every time somebody finds a new one, they go, well, this must be close to the limit of what it's possible to be and live on land as a terrestrial vertebra.

And then they find another one and go, oh, no, we were wrong. Actually, they could be a bit bigger. And I suspect that we'll probably find bigger ones in the future. But the amazing thing about the sauropods, about the long-necked, long-tailed dinosaurs, is that they were not just like scaled-up mammals in terms of any aspect of their biology, but particularly in terms of their respiration. So they have a bird-like respiration. This is totally different to what we have. They have a unidirectional airflow where their lungs have this extension of air sacs and those air sacs invade into the bones. So the bones effectively have these big air spaces in them which when the animal breathes in the oxygen gets pumped from the

lungs into the air sacs and so oxygen is extracted from the in-breath but also from the in-breath, but also from the out-breath. So it's an incredibly efficient form of respiration. It's what allow birds to fly today and be very, very active. And of course, it made the sauropod skeleton much lighter than the equivalent mammal skeleton today. It is raised the question, Toria, is there a limit to how big a land animal could get? Well, again, that will depend on the setting in which it lives, right?

There are these rules that go with size. Because if you think, say, a mammal can go from, say, two grams to 100,000 kilograms if you're a whale, but they're all made of bone. But basic, basic laws of physics, and I'm terrible at maths, so this is up to you, Brian, to make sure I get it right. If you basically scale something up, because the way the volume goes to the cube and surface area and cross-sectional areas go with the square as you get bigger you're putting all that weight

through your limb bones if you're you know walking around on the ground you're going to end up breaking your bones because you get heavier faster than your bones can keep up with it structurally and so we see this trend of bones getting more and more robust as things get larger or things change the way they stand. So you go from kind of like crouchy to very upright and columnar in an elephant, for example. If you get to that limit, then you basically only get so much bigger

before you can change again. And then you're looking at changes in structure. So I've got another good fact along that line. Well, I think it's a good fact. T-Rex couldn't actually run. T-Rex couldn't perform a running gait. So a running gait is where you have both feet off the ground for just a split second or so at one time. And biomechanical modelling has shown that if T-Rex performed that, the forces on its bones would have been such that its legs physically would have broken if it had run. So it could walk really

fast. Actually, it could still walk faster than we can run. So if you are ever being chased by a T-Rex, do make sure you're not the slowest person in the room. It raises that question, doesn't it, of a T-Rex hunting a big herbivorous dinosaur. And you think of it as being very dramatic, but it's just a load of things walking around. I always thought that was just the speed

that they could do claymation. I didn't realize that was the actual speed. In our new podcast, Nature Answers, rural stories from a changing planet, we are traveling with you to Uganda and Ghana to meet the people on the front lines of climate change. We will share stories of how they are thriving using lessons learned from nature. And good news, it is working. Learn more by listening to Nature Answers wherever you get your podcasts. So that's what they could do. I'm the least scientific person on this panel,

but I think they have found the largest dinosaur that they're ever going to find, and I'll tell you for why. I live in Bournemouth. As someone who has recently had to go through the beach looking for a lost earring, I tell you, it's really easy to find big things and really hard to find small things. How could they not have found the biggest thing so far

if they were looking? They're the easiest things to find. It's true. There's a massive size bias on the fossil record. It's also a kind of collector bias. There's a Far Side cartoon in which, you know, they say, well, there's no prestige in finding the most average-sized dinosaur,

the most medium-sized dinosaur. So there's not only a collector bias, but also a preservation bias. They just preserve better. If you have little tiny things, their bones just smash up. They get transported transported down a river they're going to smash up they just don't get fossilized just when you were talking about the the very very slow pursuit have you ever seen the film cockneys versus zombies there's a beautiful scene in which

richard bryers is being chased by a zombie richard bryers has a walking frame but because of a zombie it's a traditional zombie, both of them are moving, so it's the most beautiful slow chase you will ever see. I mean, this is such a sidetrack, but I was once doing a show in Colorado. It had a matinee, and they hadn't sold any tickets for the matinee, and so they bust in an old folks' home.

And there was some material in the show about creationism and it offended many of this audience and I had a man with a walker which had a squeaky wheel and it also had an oxygen tank that was going up with a pipe to his nose and it was just a walkout that took seven minutes of a man being so outraged by what I was saying. And it was, for me, the funniest part of the show. You've got a child, and I always found with when my son was little the fact that all my dinosaur facts were wrong. Because what we were brought up with, we're a similar generation,

it turns out a huge number of these ideas have changed. I mean, it's one of those beautiful things to see the speed of science and the speed of understanding. So did you find yourself perhaps when, you know, about the age of four or five where you're constantly being told off by that child? No, that's not wrong. Brontosaurus, don't be so silly, Dad. Absolutely, and they go through this phase.

And it is an amazing phase, because it's just as they're developing language, so many boys get obsessed with dinosaurs. Probably girls as well. I have a boy. But suddenly, these people who are struggling with language are able to pronounce these really complicated terms that we struggle with.

The whole thing becomes completely mesmerizing suddenly you've got a top trump's deck of cards that you've somehow memorized and then he watches something else on tv his obsession changes and i've forgotten it all but for two years i was hot to trot on dinosaurs and it all came from a starting with a four-year-old boy being so excited and suddenly having a facility for language which kind of meshed with his curiosity at exactly the right time there's actually nothing more terrifying than doing you know a talk or something in front of a bunch of six to eight year old boys I mean they know so much more about

dinosaurs than I do you know they'll say what about this dinosaur and I'm like never heard of that dinosaur before I just I've got nothing I'm sorry I don't know I don't know who would win in a fight I just I just don't know my my son and one of his friends had a big fight that almost I mean there were tears because one of them had read in a book that all birds are related to dinosaurs. And so the fight became, are dinosaurs still around? No, dinosaurs are extinct. You're wrong, actually, because that's a dinosaur

pointing at a seagull. And the one who thought that all birds were dinosaurs would not back down, and the one who thought that all dinosaurs were now extinct would not back down and they basically had to be separated and they're not allowed to see each other anymore i mean it sounds like every day of my life it's reasonably recent that isn't it yeah yeah early 90s when they i mean so that theory has actually been around for a really long time that you know that birds evolved from dinosaurs went right back to thomas henry huxley in the latter part of the 19th century,

but was basically forgotten about and ignored. And then suddenly in the 90s in China, they started to find these incredible fossils, which are fossilised with soft tissues and these beautiful feathers. And all these bird-like features, but were clearly not fully evolved into birds yet. And we now have many, many, many steps

on that evolutionary lineage. And it's as close to fact as we get in paleontology that birds are dinosaurs. That's a trick they're missing in Jurassic Park, aren't they? When you were saying seagull, immediately all I saw was Sam Neill in Jurassic Park having a lovely 99 there. And then a pterodactyl going, boom! Mine read just all over again sorry you mentioned just in passing that large things tend to live a long time and smaller things tend to live shorter lives so why is that well again what's your why do you want the evolutionary why or the kind of like the mechanistic why because well yeah i mean

so the evolutionary reason i guess is that to get big you've got to grow right and there's a finite amount of time you could spend looking for food and so effectively those things all come together to can put constraints on how quickly you can get big unless you're a sauropod clearly but as a mammal it will take longer to get bigger than it will to if you're a mouse and so that factors into the kind of basic strategy you take and we call it life history so it's kind of just there and you get exceptions to it so you might get say i don't know big dogs for example don't live longer than small dogs but they're domesticated so that's kind of interesting so there's that that aspect to it but then there's a kind of is there some kind of fundamental

constraint right on the way biology works now we know that say metabolism like your actual metabolism that also scales we say scale as it basically changes with size. So if you are a shrew or a mouse, you have a really fast metabolism. If you're an elephant, you have a slow one. If you just kept the same metabolism, if metabolism scaled up equally, as you got bigger and you doubled in size, you doubled your metabolic rate,

an elephant's blood would boil. So to stop that from happening mechanistically, that's a pretty strong selection pressure against having metabolism that goes up in proportion to size. So is it evolution or is it just mechanism? I say mechanism because there is some discussion about whether it's about fractal properties of the way things like mammals work.

So our blood goes around our body. So I've already mentioned volume and an area. Well, volume, your body size is basically volume. The amount of blood you have in you is also volume so they ought to go together right you double in size you double in blood volume but your blood goes through your body it goes through like big veins coming out of your lungs through the smaller veins it's capillaries spread out and all those come out in a branching fractal pattern with differing cross-sectional areas and of course those cross-sectional areas

are what determine how easily things can get across from the blood into your muscle tissue. And that's a physics limitation, a chemistry limitation. And that in itself may, it's been suggested, explain mechanistically why you have this two-thirds relationship. So your metabolism scales to body mass to the power of 0.75. Well, that might explain that, but there's also quite a good selective reason why it doesn't work either. I suppose it's also the surface area,

so the heat's got to get out. So then you come to these big rules named after boring white men. So there's a rule called Berber's rule, which is things that live in colder places, so away from the equator or at higher altitudes, for example, tend to be larger. And one of the reasons explaining,

in inverted commas, that is this surface area to volume rule. volume rules as you get bigger you have more volume in the surface area and so if you're in a cold area being bigger is better because you can basically keep the heat in but these amazing ecological rules are you know they're raw they're more just guidelines really so bergman's rule apparently um good for mammals but not so much reptiles so you do see in butterflies though interestingly it comes a kind of a game like you know do we see bergman's rule in this species or this group or this group and and the big one is cope's rule for body size which is that things get bigger through time but that endless debates as to whether or not it is actually things getting bigger or just more variation because of course like we mentioned

small things going extinct for example exactly yes so we have this thing if you reset the clock every mass extinction back to small size stuff again basically you've got a lot of small size things the only spaces left are the bigger ones right so you might expect an increase in variation that manifests as things looking bigger through time do we see any pattern of behavior that seems to fit with different sizes i mean in terms of say the amount of social behaviour or group behaviour? Well, brain size scales with body size, and so it's interesting. But yes, so that in itself is interesting to think about, sort of larger social mammals, you tend to have larger brains.

But it is interesting, isn't it? If you're a large-sized animal, like an elephant, and you have no predators, unless you're very old and infirm or a baby, then could you afford to be a bit more placid? Sorry, what about herbivory what about herbivory do we see any really large carnivores yeah herbivores are always bigger than carnivores right so which makes sense and that's partly because you can get really circular here so if you are a herbivore you need to be big because if you're eating lower quality food so

you know leaves do not have as much energy as meat in them and so you have to be bigger to have a larger gut to process and allow that food to chug through it unless you then have an evolutionary innovation which allows your stomachs to multiply and you start to ferment but if you're going to basic like hindgut fermented like an elephant you need that big gut because it takes a long time for that food that you're eating to get from out the other end. And when it comes out the other end, it's hardly been digested. People have done experiments, bomb calorometer experiments on elephant poo. And it's kind of got the same calorific value as it had when it went in.

But they just do it really slowly and enough comes out and they just keep on eating and so on and so forth. And that's one of the reasons why you can make paper out of elephant poo and it's quite nice. And elephant poo is not as noxious as say dog poo and all these various things when you go down that route of herbivory then you one on one hand need to be big and then of course you can get bigger because you can just keep expanding your gut size and your body size and just eat more slowly with your lower metabolism you can accommodate that lower resource food and so it's it's enabled and then it's interesting to think like you know it's the constraint on the body size of carnivores as a result yeah you know what's really interesting

to me as you've described these are seem like simple questions what's the biggest thing what's my thing why are they big why they're small but as you described it it depends on absolutely everything about a planet and its history basically it's the amount of oxygen in the atmosphere the gravitational pull of the planet the strength strength of bone, the history. Would it be gated by an asteroid or not? All the other animals you're living in the ecosystem with. So the thing that I mostly work on are elephants that lived on islands. Now, these elephants are dwarf elephants. During the Ice Age, we had all kinds of different elephant species all across the world, everywhere apart from Antarctica and Australia Australia and some of the ones that lived in Europe were massive. You have these straight tusk elephants that you could find in warmer periods even here in London for

example and they were 10 tons like four meters at the shoulders that's bigger than an African elephant. These were very common all across the Mediterranean and when they became isolated on islands like Malta or Sicily or Cyprus or Crete they always always always evolved to become smaller which is telling you something about the fact that being big is a bit rubbish on these islands you have no predators usually you have fewer competitors but you also have limited resources because you're in a tiny island and you're a big elephant and some of these elephants were really small and so the smallest ones were a meter tall as adults that is the same size as a newborn baby african elephant and we have skeletons from caves in sicily of baby dwarf elephants they're like

puppy sized and they're just like little tiny elephants so that was my like that was a good competitor for the most astounding fossil ever found really for me because these baby dwarf elephants are so tiny. And that's really interesting, again, because, of course, because it happens again and again and again, it's a great system for testing evolutionary hypotheses about size. And one of the things we see is that they get smaller, but they still stay chunky.

And because we know that small things live faster than large things, the expectation was these smaller elephants were evolving smaller because they were kind of racing towards a faster life history having more babies more quickly because that would make sense from evolutionary perspective when we looked at their bones the cross-sectional growth lines these tiny one meter tall elephants were taking as long to grow up they lived to 70 years exactly the same as a full-size elephant but then the opposite is also true isn't it because we hear of island gigantism. Yeah, in rodents.

Christmas Island, where the crabs, the coconut crabs or the rubber crabs, are enormous things, aren't they, relatively? It's another rule. Sometimes called Foster's Island Rule, but we just call it the Island Rule these days. On islands, typically, small things get big and big things get small.

And again, this is this idea, maybe they'd all kind to go into wars, this kind of optimal sweet spot given half a chance. Do we have any idea how many, for example, tyrannosaurs have been found? I mean, is it hundreds or is it thousands or is it tens? Is that including the secret ones in people's private collections? Well, yeah, so I mean, that's a complicated question. I think there's around about 30 more or less complete T-Rexes that are known.

There's probably lots of others in private collections and there's lots and lots of things that are like a foot bone or a single backbone. We very rarely find complete fossils, complete articulated specimens in the fossil record. Because when you say, oh, maybe the Spinosaurus was bigger or whatever, if in millions of years they're finding fossils of humans and we've died out and there's somebody else discovering them,

but all of the fossils they found are where a coach taking a basketball team ended up in a river. That plane that crashed. They're going to skew the perception of what humans were. Absolutely. And this is you know this is something that makes me a little bit crazy actually um most dinosaur species are known from a single partial bone or you know a few bones we don't have complete skeletons for most dinosaur

species now in the media we see quoted oh this one was 60 tons or this one was 20 tons or this one was 30 tons well how do we work out that body mass from a small selection of these bones and that itself is quite a difficult thing to thing to do as you can imagine and there's a couple of different methods but a very popular one is that if you measure the humerus so the upper arm bone and the femur the upper leg bone if you measure the circumference of those bones and you plot it against body mass in living animals you find a nice straight line relationship and so what people have done is go and do that with dinosaurs if you've got the forelimb and the hind limb preserved then you can measure those circumferences you can

plot it on this graph and you can read it off now of course you might have to extrapolate that graph a bit further because the largest animal in that data set is an elephant at six tons and the largest reptile is an alligator 150 kilos so you know but we'll just ignore that now the method has enormous error bars associated with it so we did it for the stegosaur that we have at the natural history museum which is the world's most complete stegosaur and we got a body mass range of anywhere between one and six tons so it's quite a big it's quite a big range i mean i go up and down around christmas it's more than just having a big poo. I mean, I go up and down around Christmas. It's more than just having a big poo, though, isn't it?

That's still one of my favourite theories about the dinosaurs dying out. You probably remember it from the 1970s. There was a theory that there was some kind of plant that died out which had helped them digest and they all died of constipation, which is just such the agony of the small-armed Tyrannosaurus. Now, Dave, I just want to get to something quite serious now, which is the William Shatner film Kingdom of Spiders.

Probably remember William Shatner's cows are eaten by a giant spider. In the 1970s, these books were everywhere. The books and films about giant, weird creatures. As you know, I'm a big fan of the Guy N. Smith giant killer crab books. Crabs on the Rampage, Night of the Crabs, Crabs Moon, Origin of the Crabs. You know, the rest of them. So were you ever into that kind of genre? I wasn't into that at all.

I was aware that they existed and there would be a shop where for 50p there'd be different paybacks. I just assumed we're all the same. Somebody had gone control CC, Control-V, and rabbits became slugs, and the same novel got produced. In the 1970s, that Control-C and Control-V didn't exist, because that comes through its windows, it's cut and paste, and that would be Windows 3.1, I think. It wasn't around in the 90s.

Finally, I see Brian come alive during the show. If you look at the history of operating systems, Control-C, Control-V. He's fallen into my trap. We can leave. I'm just saying. I'm sure you know in Crabs on the Rampage, there's a moment where one of the characters says,

Crabs, Crabs, Cra crabs the size of beach donkeys now my question is could crabs be the size of beach donkeys well there were giant sea scorpions that reached about two and a half meters long so let's just go with why not let's go with it good i just wanted to make sure that the giant killer crab novels by garland smith were scientifically accurate excellent now the final question sorry can i just check where a beach donkey is compared to make sure that the Giant Killer Crab novels by Guyon Smith were scientifically accurate. Excellent. Sorry, can I just check, where are beach donkeys compared to other donkeys? I think it was merely because they were the things being eaten and that gave them something to actually...

Right. Yeah. Because I'm no script editor, but I would have said just say as big as donkeys. You know when the word count just isn't quite what you need it to be? Now, this is my favourite question we've ever typed out, and then I've turned the page and gone, we really did type this out.

On the moon, how big would elephants be? Yeah, that was... We genuinely started... I think I'll take this one. It's the last question, but it's a question about scaling. It's a question about gravity. it's a question about scaling. So the question is about gravity. It's a sixth of the gravity. Well, yeah, this is actually a really critical question, Brian.

So what is size? What is size? Is it weight? Is it mass? Is it height? So an elephant would be the same size on the moon, but it would weigh a different weight, right? But it's evolved on Earth, so it would have brought different weight right but it's evolved on earth so it

would have brought all of its metabolism everything with it but it structurally would wouldn't need to be as chunky as it is elephants actually have got quite slender legs and quite delicate feet and they kind of stand on tippy toes and they have a kind of sort of a wedge-like pad with a kind of cartilaginous stiletto going through it so they'd be fine maybe they could run on the moon they could jump on the moon that'd be jump on the moon. That'd be great. Because elephants can't run either, like T-Rexes. They just kind of groucho run and kick their legs behind them like bunny hopping.

But if they evolved on the moon, that would be a different question. I'm so excited by telling this to a 1970s disaster movie. My God, what's wrong with the tides? They're not moving in the way they normally do. Oh my God, elephants jumping on the moon. I am just aware, and you sort of mentioned moving in the way they normally do. Oh my God, elephants jumping on the moon! I'm just aware, and you've sort of

mentioned all these rules named after boring white men, and I'm aware that I can't help the fact that I have inherited a culture of boring white men. And part of it means that at the back of my head, when you said elephants look like they've got chunky feet, but they're not. They've got tippy toes, not like on stilettos. A part

of me looked at the elephant text and went, oh, she's got that wrong. They've got big, chunky feet, I know elephants. I had to stop the boring white man coming to the fore. Gorman's rule says elephants have chunky feet. We asked the audience a question as well. What would you most like to dig up on an archaeological dig and why? Oh, I've got a leg bone of an ancient

man. It would be a real shin dig. Oh! I thought that was very well done. You don't listen to them, Sue. You keep going. What have you got, Brian? The lost city of Atlantis. Why?

It guarantees the funding for another season. That's a Mr G, Hancock. Keys to the car that I lost 20 years ago. It might stop my husband reminding him of it every time we have an argument. What would you most like to dig up on an archaeological expedition? My dignity. I lost it in my 20s and i've never quite recovered

there's a similar one here actually from from dave he said my geology degree certificate portsmouth 1981 lost it days later two two and proud pavlov's dog because i'm sick of the bloody bell ringing so they're the uh Something more valuable than bottle tops, because bling, oh Brian, you know bling can only get better. It's always on. You get royalties if I say it, if I sing it. Anything can only get better.

Thank you very much to our panel, Tori Herridge, Susie Maidman and Dave Gorman. Now, next week, we're going to be talking about science and the movies

and we're going to discover whether it really is true about the giant killer rabbit attacks. Now we've found out that in fact, the giant killer crabs really are a threat

and some of them might be larger than beach dogs. He's not listening to anything you said. The whole thing just washed over him. All I was thinking about was William Shatner's cows.

Anyway, you'd probably get, I reckon, a job on a science show. I think you'd be a science advisor on a movie about a giant killer trout because you're a Pisces and I think you'd have a kind of innate understanding. Oh, so now all of a sudden you're not into star science. This is getting ridiculous. It is true, I've said it before, but any time you find one of Brian's books in a second-hand shop, move it to astrology.

It is such a lot of fun. Thanks very much, everyone. Bye-bye. APPLAUSE In the infinite monkey cage. In the infinite monkey cage. Turned out nice again. Hello, I'm Dr Michael Moseley, and in my BBC Radio 4 podcast, Just One Thing, I'm investigating some quick, simple and surprising ways to improve your health and life.

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