Dwarf Elephant Evolution
At one stage there was a situation where you had a one metre tall elephant and a swan that was probably bigger than it
When you want to understand dwarf elephant evolution, you have to kind of think about the environment they were living in and what else was going on. And the first thing you notice is that all of these dwarf species occur on islands. That’s the common feature, so it’s clearly something to do with the island environment. And it’s got to be something that’s common to islands across the board; I mean, these islands are in different places, with different sort of plants there, you know, different kinds of topographic features, different latitudes in some respects, so Siberia versus the Mediterranean.
And yet, for some reason, in that situation all these elephants have evolved to become smaller. And elsewhere in the fossil record on those islands you find other things, so you often find evidence of dwarf hippopotamus and you find evidence of dwarf deer, but then you also find giant dormice or giant rats, sometimes giant swans. So, on Malta at one stage there was a situation where you had a one metre tall elephant and a swan that was probably bigger than it, because it was a swan that was probably about 30% bigger again than the largest swan living today, this swan, and that’s a big swan. I’m quite scared of swans, I don’t fancy meeting that!
But, yes, you’ve got that, then you’ve also got these dormice which were about that big, about the size of a cat as opposed to, you know, the kind of…in your hand, in the teapot sort of dormouse, the sort of thing you might imagine. And so you’ve got this situation where there’s many things changing their size and the big things, particularly the mammals, seem to be getting smaller and the small things, particularly the mammals, seem to be getting bigger. And so it’s a question of, you know, what is it that could be driving these things at the same time in the same situation and it all comes down to this idea of there potentially being, we think, an optimum body size.
Now start off there, cause then you’ve got to roll back a bit, you’ve got to start thinking to yourself, OK, well, if we want to understand why they’re getting small, let’s first think about why elephants are big in the first place on the mainland. Because size is really important, it’s one of those key characteristics, but we know that big animals tend to live slower but longer lives. Now, that’s fine, but ultimately if you sort of think natural selection’s working to increase the number of genes passed on to the next generation – so anything that can pass on more genes will be selected for – then any sort of life history strategy which is a slow version is going to be susceptible to disruption if given the opportunity.
Now across the whole of the evolution of life, elephants have evolved to fill this big animal size niche and that’s part of a much bigger thought that there’s always if you like, you know, room at the end of the size spectrum. So it might not be great to be big cause it takes you nine or ten years to becomes sexually mature if you’re a female elephant, a bit longer, about 30, before you become properly sexually mature if you’re an elephant male, you can…well, you’re sexually mature a bit younger but you very rarely have any offspring until you hit the musth stage, you’re about 30 then. That’s a long period to wait before having your first offspring. And then if you’re a female elephant it takes you 22 months to gestate a baby. And so, even if you have a lifespan of about 60 years, that’s not a huge offspring output when you’re only having one baby at any one time.
So, um, but if you are big, then you can actually live in a situation where you can live off really poor quality food. So elephants can exploit the kind of poor quality food end of the kind of habitat spectrum. They eat pretty much everything: they eat nice, high quality young grass but they also can eat really, really old stuff, kind of gnarly thorns, see the African elephants kind of wind their trunks around sort of baobabs and sort of spiky things, acacia plants, and they do that by basically eating all day, they eat about 150 kilograms of food a day and it takes them almost that entire time to chug it through their digestive system. And if you look at what comes out the other end, which is elephant poo, you will find that elephant poo is actually quite pleasant on the sort of poo spectrum, it’s a bit…in the same way that horse poo is nicer than cow poo, cow poo is nicer than dog poo if you like, but horse poo is quite fragrant, it’s not too unpleasant, elephants is even nicer than that, it’s actually very, very undigested, it’s got lots of kind of good stuff in there and, um, if you look at, say, a Chris Ofili sort of artwork, you kind of see this kind of big ball of elephant poo and, actually, it’s not too bad. I mean, we can make paper from it, things like that. And that’s because actually very little energy is being taken out of each kilogram of food that’s passed through, it’s actually still got quite a lot of energy in it, it’s not that different in calorific value.
So, these are inefficient, but they get over that inefficiency by being big and having a big gut, it’s kind of slowly extracting it. Now if you were suddenly in a situation on an island where you aren’t kept away or aren’t kind of competed out of the high quality food niche, then perhaps that removes one of the reasons to be big. And, similarly, if you’re a big elephant on the mainland, yes you take a long time to grow up and have a baby, but you probably have quite a low mortality rate cause you’re big and very few things hunt you.
Now if you remove the predator from the situation, then perhaps that removes another reason to be big. And when you look at islands, you always see on an island…a feature of them because they are small pieces of land effectively, is you always see a reduced number of species overall and that tends to be particularly seen in the predators. And so on islands you tend to get an elephant either with no competitors or maybe just one or two, maybe a deer or a hippo and absolutely no predators…maybe a hyena, a couple of them have hyenas on them. And it may be they’ve been released from the reasons to be big on the mainland.
Now they’re the three hypotheses people have come up with: lack of predation, lack of competition and the other one is a lack of general resources overall which makes it difficult to maintain a large population of large size organisms. It gets a bit squiffy there because you start to get into group selection ideas, people haven’t quite worked that one out. And that’s all great and those three things work, but it’s really hard to get a handle on them, and particularly a lot of energy has been expelled and has been used up debating which one of those three things is the most important, you know, which is the key driver of this body size change.
And that’s great but, because it’s happening on islands, it brings in this idea of island biography, we call it, which is that idea that you can predict…or rather it’s predictable or it’s easy to see how many species you’re going to get, the diversity on an island, based on the island size and how near it is to the mainland. So it’s immigration rates versus extinction rates and how many species a piece of habitat can support. And so because these seem to be tied in intrinsically with those things, those two things, how easy it is to get there in the first place and how easy it is to sort of, to not go extinct, or rather how hard it is to sort of maintain a viable population, you start to see that those three hypotheses must massively co-vary. So a small island is going to have few resources on average, it probably won’t support many species so you’re going to have few competitors and it certainly is unlikely to support predators, whereas a large island is going to have more resources, may support more species and may actually be able to maintain a predator population. So it’s teasing those things apart, you know, it’s become one of those big muddles. And so people have made lots of meta analyses, they’ve looked at the literature and they’ve gone back and they’ve measured size versus species living together at the same time and, you know, different analyses turn back different results.
And when you actually start to unpick these things you start to realise that one of the key problems we have is that we can’t really necessarily be 100% sure which species may be completely contemporaneous with each other on these islands because we haven’t done the proper groundwork at the beginning in the excavations. Because all these excavations were back in the Victorian period where there wasn’t so much interest in things like stratigraphy or the context of the environment that things were sitting in because the questions were different.
And so if you fast forward you end up with a lot of lists of animals from different places people have kind of made about who might be living together, so they’re kind of like, oh, you know, there’s that site there and that site there, I think they’re about the same age so, that’s got an elephant in and that’s got a hippo in, probably they were living together. But we can’t be 100% sure so that’s what kept me going really, is that I want to come back and say OK, let’s try and work out who exactly was living with each other when and then can we put that into the analyses to get finally a better answer.