It turns out that elephants have a sixth toe. They’ve adapted one of their wrist bones into a strut that supports their giant squishy feet. I wrote about this for Nature (excerpt below), but here’s my full interview with John Hutchinson, the man behind the discovery, and the owner of (probably) the world’s largest collection of frozen elephant feet.
Elephants walk on the world’s biggest platform shoes. Now, John Hutchinson at the Royal Veterinary College in London and his team have found that their footwear also contains hidden stiletto heels. Even though an elephant’s leg looks like a solid column, it actually stands on tip-toe like a horse or a dog. Its heel rests on a large pad of fat that gives it a flat-footed appearance. The pad hides a sixth toe — a backward-pointing strut that evolved from one of their sesamoids, a set of small tendon-anchoring bones in the animal's ankle. This extra digit, between 5 and 10 centimetres long, had been dismissed as an irrelevant piece of cartilage. Almost 300 years after it was first described, Hutchinson finally confirmed that it is a true bone that supports the squishy back of the elephant’s foot. The ones on the hindfeet even seem to have joints.
Elephants are very familiar animals. How did this part of their body go ignored for 300 years? It’s weird. People noticed this weird cartilaginous rod, but they forgot that this thing was even there. It only came up in specialist papers on elephant anatomy, and even then just a sentence or two: “Oh yeah, there’s this weird thing, it’s probably cartilage”. It’s really an outstanding structure. I think it was just the right matter of the right specialist not looking at it with the right tools. It was just serendipity that I happened to be studying it at the same time as teaching my students about the panda’s thumb and it hit me – the elephant’s toe is basically a panda’s thumb, with the same connections and everything. It really struck me. We held back for three years collecting more and more data after writing an anatomy paper that was published in an obscure book chapter. We really threw the kitchen sink at the structure to figure out as much about it as possible. It was about being in the right place and the right time - understanding the evolutionary literature as well as the anatomical stuff. There’s a nice comparison to mole thumbs – it’s a repeated theme in evolution. Instead of evolving a new finger, turn one of your sesamoids into a functional finger. You must have looked at a lot of elephants? Is that difficult? Elephant feet are difficult because the structures are so embedded in the flat pad. You can’t see them from the outside. You’d never tell they were there from the outside. Most imaging techniques wouldn’t work through the foot, especially in a live animal. So the only way we could approach it was with cadaveric material. Fortunately, I have the dubious distinction of having previously assembled perhaps the world’s largest collection of frozen elephant feet and just started CT-scanning all of them. We were one of the few people to bother CT-scanning these structures. Obviously, you can’t do that with a live elephant. It’s extremely impractical and unsafe. They tend to die a lot under anaesthesia. That’s when it really hit me that these weird structures weren’t just cartilage – they’d show up on the CT scans as brightly coloured as the surrounding bones. These are mineralised; they’re not just cartilage. They’re showing a weird pattern that looks at least like calcified cartilage, if not true bone. That’s when I called some histologists and imaging experts on board. They got really excited – they said this is absolutely bizarre bone, unlike anything we’ve seen before. The pre-digit is really integrated into the fat pad. There are all these weird fibres and compartments, muscles and tendons weaving their way through, and this structure is in the middle of it all. Unless you really cut through carefully and think about the anatomy, it might seem like an odd piece of cartilage. It’s why you don’t see it in museum specimens – it doesn’t fit with the search images of what should be there. And museum preparators usually throw away pieces of cartilage because they’re hard to preserve in a skeleton. So, er, why exactly have you amassed the world’s largest collection of frozen elephant feet? I was working on elephant locomotion in the US. We got a grant to look into more detail at their anatomy. I’m in contact with a bunch of zoos and other people – I got to be trusted as someone to send stuff to. The most common thing to send was the feet, since those are portable enough. The whole limbs are very hard to transport or even cut off. It can take a couple of hours to get a whole leg off and they’re a couple of hundred kilos. The zoos were largely interested in foot problems in elephants. They have a lot of arthritis and lumbar diseases. In a lot of cases, the elephants have to be put down because of their foot problems. About 50% of mortality in captive elephants is due to foot problems, and they often want people in the Royal Vet College to check these feet, to see what’s going on and to CT-scan them for diagnosis. So I’ve got all of these feet and they’re very interesting. I thought let’s take a look at them and find out how they work. How many feet do you have? Around 70. We had to get a walk-in freezer and 4-5 chest ones So it looks like the toe is acting as a stabiliser, right? That’s accurate. Because of its position, it’s uniquely placed to support the fat pad. There’s nothing else in the back of the foot behind the toes that could support this huge high-heel of fat and give it control. Otherwise, you just have this giant squishy pad and stiff toes in front and there’d be no way for the nervous system to control what’s going on in the fat pad. This structure, which has some muscles and tendons associated with it, provides the capacity to control what happens in the fat pad. Certainly, it stiffens the pad by being a stiffer tissue in the middle of compliant tissue. We noticed that there are interesting differences between the fore and hind-feet, which we think relates to differences in posture. The front leg is very vertical, and the pre-digits seem to be specialised or that. The hindfoot pre-digit is probably acting in a different way. We noticed that the prehallux [the sixth toe on the hind leg – Ed] has what looks like a joint. That seems to move. That’s a mystery we still need to solve. The stabiliser idea seems to explain why the bones harden so much later than the rest of the skeleton. Only the very largest adults need the extra support. I’d agree with that. It’s probably more important to mineralise it in a larger adult. There’s still the issue that we get these feet pretty much entirely from captive elephants that are kept in different conditions to wild elephants. So how much of the ossification [hardening of bone – Ed] is normal and how much is pathological? We don’t know. There’s so much variation there and different degrees in pathology that it’s hard to say. I think it probably does relate to becoming mature or bigger but it would be nice to look at wild elephants. Why did the elephants evolve to stand on tip-toes over time? Mechanically, it makes good sense for an animal to make its whole limb more vertical if it becomes bigger. It turns the skeletal structure into a column. Bone is really strong in compression. If you align all the bones in one line, the force acts straight through them, so the muscles have to be less active to support the joints. That’s a general pattern in terrestrial animals – they tend to get more straight-legged as they get bigger. You didn’t find similar sixth toes in other large animals. Is that because they don’t exist or have you just not found them? We’ve looked at rhinos – we’ve got a pretty good collection of rhino feet too. But they’ve lost their first toe which is where the structure attaches – you lose that and you lose this sesamoid as well. We speculate there’s a correlation there. I suspect that if you have lost your first digit quite early in evolution, then you can’t easily evolve one of these pre-digits. That may be why hippos and rhinos and other large mammals don’t have the same fat-pad.