Dr Victoria Herridge
Victoria Herridge obtained her PhD in paleontology at University College London. Her current research at the Natural History Museum focuses on the evolution of island mammals, particularly dwarf elephants, during the last Ice Age. During this research she co-discovered the smallest mammoth ever known to have lived. Her work has led to a greater understanding in evolutionary responses to extreme climate change.
Elephants are really, really, really weird.
On becoming aware of dwarf elephants
I first became aware of dwarf elephants and the research behind them when I saw a PhD advert for them. So it’s not something I brought up myself, which is the way of much science. I was looking for a PhD and I’ve always, always loved the Ice Age. So I wanted a PhD that ticked these boxes, I wanted to do evolutionary biology, and I really wanted to work on something that linked to the Ice Age for personal kind of romantic reasons, I just kind of like the period and I wanted something that sounded cool. And I kind of wanted to be in London, I wanted to stay in London. So I was looking for various PhD adverts and this one jumped out at me: the evolution of dwarf elephants on Mediterranean islands, a natural experiment in paleo evolution it said. I thought, ah, that sounds good and the rest is history.
On writing a book about dwarf elpehants
In a nutshell, it’s a dwarf elephant guide to evolution which sounds kind of obscure; I mean, dwarf elephants aren’t the first thing that springs to mind when you think of evolutionary biology, but the very first dwarf elephant fossil was discovered and described almost coincidentally with the publication of On the Origin of Species and from that point onwards dwarf elephant research has tracked beautifully every single shift in evolutionary thought and so by following the discovery and the interpretation right the way through to modern day science you really get a…a kind of precis of every single aspect of evolutionary biology and the thinking behind it.
On why dwarf elephants matter
Well it is fascinating, it gets under your skin. I mean, I think all research questions get under your skin eventually, I mean, unless you’re a very unimaginative person, when you’ve got a question to answer and you can’t quite get at it, you just want to keep on going and each question you do answer opens up more. I think that would be the same for any subject, but for your PhD, if it doesn’t kill you, then you really do fall in love with what you’re doing and so, yep, dwarf elephants are cool.
I actually wasn’t that particularly interested in elephants more than the average person – I mean, elephants are wonderful – until I started my PhD. And then the more reading I did about both living elephants and extinct elephants, the more fascinating they became and one of the best things about elephant research is that you really are working at the extremes of our understanding of evolutionary biology and form, animal form, ‘cause of course, elephants are the largest living land mammal and body size is one of the most important aspects of an animal’s biology.
So if you are small then you have a different kind of life than if you’re a large animal, your body size determines where you can live, what you can eat, how long you’ll live, how many babies you will have, how quickly you will have those babies and, generally speaking, small animals live fast lives, fast short lives, and big animals live slow, long lives.
Now the elephant is at the far end aspect of that and so everything we really think we understand about the extremes of size in animals is ultimately down to what we understand about elephants, ‘cause that’s our only living thing we’ve got to compare things to. So if you’re looking at dinosaurs and you want to understand the constraints on dinosaur size, you kind of have to look at an elephant to kind of get an idea of what’s going on there.
But elephants are really, really, really weird. So if you were to go and look across the evolutionary tree of mammals and you were to trace back the point at which the elephant lineage – which doesn’t just contain elephants, it contains hyraxes and dugongs, things like that, golden moles – that branch split off about 65 million years ago, which is about the same time that dinosaurs went extinct, so really it’s at the root of this big explosion of mammalian diversity.
So because of that they’re these really odd animals that are on this massive what we call long branch in the mammal family so it brings up a question, it begs the question, what is it about an elephant that is unique because it’s big and what is it about an elephant that is unique because it’s an elephant, right? And so dwarf elephants which are secondarily small, they’ve evolved and become smaller from big ancestors, kind of give you an in to that question. And then, on top of that, they evolved many, many times so whenever you see an elephant on an island in the fossil record, they’re generally a small size version of their mainland ancestors. And so I work in the Mediterranean where there are many islands, holiday destinations – Sicily, Malta, Cyprus, Crete, the Greek islands – all those islands have got at least one species of dwarf elephant fossil there and most of them, most of them evolved from one mainland ancestral species called the straight tusked elephant and so, if you like, you’ve got a kind of repeated example of this thing happening, so you’ve got a real life experiment which is awesome because experimental biology is the way to get out answers, you can start to tease apart questions and hypotheses. But if you’re only limited to lab animals then you really, really can’t get that easily at kind of big creatures like humans, but like elephants too that you maybe want to get answers about, at the other end you’re kind of limited to flies, zebra fish, frogs, mice, you know, your typical lab organism. So when you see an opportunity for an actual experiment, particularly something so unusual like an elephant, you’re gonna jump at it, and it is just fascinating.
But the real reason that I am still working on it is that, well, we’re finding there are more questions than answers because even though there’s been 150 years of research on these things, people often think they understand the reasons behind why elephants evolved become smaller on islands; when you actually start to dig a bit deeper you find that the whole set of hypotheses behind it are kind of based on a slightly wobbly foundation, our basic data is a bit poor. And so I guess my quest is to kind of put the solid data in there so we can actually get some decent answers.
On elephant evolution
So they’re all in…so, I suppose we should say here now, when I say elephants, I mean elephants and mammoths, and the reason why I mean that is that if you were to look at the elephants living today, you’ve got the African elephant, you’ve got the Asian elephant, so, if you like, two points on the family tree of life. And these two species actually separated from each other about six million years ago. So that’s actually quite a big depth, time depth in that split, that’s about the same evolutionary distance between you, me and a chimpanzee.
Now the mammoths are more closely related to the Asian elephant: they sit within the African and Asian elephant family tree. So if we call an African elephant an elephant, and an Asian elephant an elephant, then a mammoth has to be a type of elephant; if you like the “elephant” term brackets both of them.
So, dwarf elephants sit within their two and there are several types. I have never as far as I know come across…as far as I know, no one’s ever found an island dwarf African elephant; I don’t know where we need to look, maybe we should go look in São Tomé or somewhere’d be cool, but you do get straight tusked elephants which are part of the Asian elephant mammoth family. We actually don’t know which it’s more closely related to, the mammoth or the Asian elephant, because we haven’t got any DNA and the morphology can’t resolve the family relationships at that point so they kind of sit in that cluster and we get dwarf mammoths as well so they kind of sit within those lineages, so you get dwarf Columbian mammoths off the coast of California, and you, as I mentioned earlier, you get dwarf woolly mammoths off the coast of Siberia and we actually found a dwarf what we call the, um, well it’s called the step mammoth in Europe – mammuthus trogontherii – there’s some, well, a small number of fossils which they think are small mammoths from Sardinia for that species and there’s a dwarf Southern mammoth which is a very primitive kind of mammoth which was in Europe about three and a half million years ago, there’s a dwarf version of that in Crete, but then pretty much everywhere else is dwarf straight tusked elephants.
And dwarf straight tusked elephants are now completely extinct, they’re part of that mammoth/Asian elephant family but they used to be all over Europe from about 800,000 years ago and went extinct on the mainland about 34,000 years ago. And they were called straight tusked elephants as their common name because they had quite straight tusks: that’s quite interesting. So mammoth tusks, if you were to look at them, they kind of twist in a corkscrew. You can imagine it really, really clearly on the woolly mammoth, if you imagine a woolly mammoth in your head and you picture those big tusks coming out that usually kind of sweep round and cross over at the front, it’s kind of describing a screw shape. Whereas the straight tusked elephant only curves in one plane, so, like that [demonstrates the shape]. I mean, that’s why they’re called the straight tusked elephant, but they were big, on the mainland they were about four metres tall, probably weighed about 10 tonnes – that’s bigger than the living African elephant – and when these straight tusked elephants got to islands like Sicily or Cyprus, they evolved to become really, really small, some of the smallest species were just one metre tall as adults which is the size of a newborn baby elephant.
On questions left to answer
So the biggest, if you like the biggest questions that are facing not just dwarf elephant researchers, I guess, but island biology researchers both in the present day and in the past is a question of rates; how quickly can things evolve and how quickly can things respond to perturbations in the environment. So, from a paleontological point of view, the things I work on all evolved in the last two and a half million years, probably even a bit more recently than that, and that’s a period of massive climatic change, so it’s a period where you start to see the onset of what we call the Ice Age and that Ice Age has existed until the present day. An Ice Age is just a period where you’ve got permanent ice at the poles. But within that long period of Ice Age, um, you get periods of really, really cold and periods of quite warm like the present day, so we’re currently in what’s called a warm stage.
But every hundred thousand years or so you get an Ice Age, a kind of an expansion of the polar ice sheets. The ice in the UK came down as far south as Finchley Road in London, Hornchurch 500,000 years ago, I mean that’s a lot of ice. And when you’ve got that much ice then that water has to come from somewhere. So all that ice is freezing up the water in the water cycle and that generally causes a drop in sea level. At the peak of the last Ice Age we think the sea level was about 125 metres lower than it is today and so of course that’s gonna have a massive and dramatic effect on the land shape, particularly islands. Some islands become connected to the mainland, there’s a land bridge between them, and other islands become closer and bigger. We think that it’s quite likely that these climatic changes were important to both the evolution and extinction of these dwarf species.
Of course, we want to understand how quickly that may have happened, how quickly things were able to respond to these very dramatic climatic changes because it starts to give us an idea of response rate which might be relevant to modern day species, not living on islands but kind of living in island habitats which are now within a sea of land, so we’re kind of impinging on big jungle areas, you start to create kind of habitat islands within a kind of sea of humanity around them and, yeah…is there ever gonna be a possibility that, you know, that these rates may be fast enough to respond at a kind of an ecological human scale which seems really unfeasible, except that we know that deer that live on Jersey, they evolved to become about 50% smaller in probably about 6,000 years which is quite fast in an evolutionary scale and that’s a kind of a maximum level because that’s kind of our bounding roots. And so it’s, you know, with things like size as a kind of typical habitat response we wanna understand how labile it is. That’s what I’d guess are the big questions, the rate thing.
And then, secondarily to that, I think also a really key question is challenging or trying to sort of really pick apart some of our key assumptions about size in general. So I’ve talked a lot about long slow lives and short fast lives but these are patterns that can observe over really, really big scales, across the whole of the tree mammalian life.
Now the question is, can that big scale pattern really apply at a smaller scale? So the evolution of dwarf elephants on islands is almost, kind of like a within species evolutionary response. I mean, you can have a really long debate about whether these things constitute a good species or not, these separate populations on islands. But the question is, can we assume that a dwarf elephant, cause it’s a metre tall, lived a faster life than it’s four metre tall ancestor? And I don’t think we actually know that: we think we know that and that’s part of our assumption underlying our hypotheses about why this dwarfism might have evolved, but I think we do need to unpick that. A couple of reasons for that: I mean, you get some really fascinating things if you look at, say, artificial selection in dogs. Now large dogs do not live longer lives than small dogs – quite often it’s the reverse – and so there clearly is a possibility for the decoupling of these things, it’s not a mechanistic follow on. And then on top of that, recent work on this really weird island species that’s completely extinct called the myotragus – it translates to “mouse goat”, which was found on the Balearic islands – recent research with people who’ve looked at their teeth, by sectioning their teeth and counting daily growth bands in the enamel – you can count the age of your teeth like a tree ring if you like ‘cause your enamel cells have a daily activity pattern and they lay the enamel down in bands – by looking at that, they’ve actually looked at the lifespan of these animals and found out they’re actually living quite long lives, which is contrary to what they were expecting in this island situation.
And then a colleague of mine – which is very exciting – has just published a preliminary piece of work looking at some dwarf elephant teeth from Cyprus and comparing them to teeth in big elephants and doing a similar sort of thing, trying to count these daily growth bands in elephants, which is really hard, ‘cause elephant teeth are weird. It took a long, long, long time, she did it in her spare time, she works on primates mostly. It’s a woman called Wendy Dirks at the University of Newcastle and she looked at these dwarf elephant teeth from Cyprus and her results suggest that, yes, there’s a smaller tooth so it takes less time to grow than these big elephant teeth which are about this big [demonstrates with hands], that one’s about this big [demonstrates dwarf elephant tooth size with hands] and it took less time to grow but not as little time as you might predict based on its size. So it took longer to grow than you would predict based on an animal of that size, and so these may not be living the fast lives that we think they are and I think that’s a really key thing because that starts to kind of get at one of the really big fundamental questions in biology, which is how do you reconcile the micro evolutionary patterns with the macro evolutionary patterns? Cause if natural selection is acting on variation in populations and that variation in populations is what will ultimately give rise, based on the kind of selection and selective sweeps and things like that through the evolution, the whole of the evolution of humanity and mammalian form etc. etc., if those things are disjointed and disjunct, you know, what are you picking out in the macro record that can really tell you about the micro record? And I’m really fascinated about how you get from that variation in the patterns that don’t seem to do the same thing to the big broad scale pattern across the whole of the tree of life and it’s one of those little bits of data that starts to kind of throw up new questions.