Why Spider-Man would fall off buildings

Have you ever dreamt of climbing up the outside of a building like Spiderman? Making that dream a reality would require specially adapted adhesive feet that can take advantage of weak molecular forces of attraction between your body surface and the wall, much like a gecko. Felicity Bedford spoke to David Labonte from Cambridge University who has been investigating whether there's a good reason that this wouldn't work for humans...

David - There's only a certain range in sizes that actually use a sticky pad.  So you find  you find tiny mites, you find big geckos, you find frogs and spiders and many different insects in between, and the question that we were interested in is whether there's actually a limit, or what is the limit to the size you can grow to where you can still use adhesive pads.  And there's an interesting problem hidden in trying to stick to surfaces because as they grow in size, the amount of surface area that they have per body volume, goes down, and  it's probably easiest to illustrate that problem if you think of a tiny ant.  If you look at the ant you mostly see surface area and there's a tiny bit of volume, and if you look at an elephant you see a lot of volume but there's really not much surface area.

Felicity - It's difficult to imagine and elephant climbing with sticky feet.  You mentioned quite a diversity of different animals there, from insects all the way through to reptiles.  Are the sticky pads and the mechanisms that they use to climb the same across those groups?

David - From all the evidence we have, they're surprisingly similar and, in fact, adhesive pads in climbing animals have been a prime example of what evolutionary biologists call 'convergence,' where you find the same solution appearing in very different animal groups.  So, if you think, for example, of spiders and geckos - so they are really, really different animals in terms of their evolutionary history but, if you look at their feet, they just look almost identical.  And this, of course, hints at the possibility that these structures are really good in what they do and, therefore, they have evolved multiple since then.

Felicity - So what do these structures look like?

David - So on the feet of geckos, and spiders, beetles, and flies, you would find what we call 'hairy pads' and they are essentially dense arrays of tiny little hairs and on the feet of many ants, and cockroaches, and tree frogs, you would find what we call 'smooth pads' that are essentially unstructured, at least on a microscopic level so you can best think of them as a soft cushion.

Felicity - In front of me we have an array of different machines...

David - Yes, so what you're seeing here is one of the devices that we can actually use to get an idea of how well animals can actually stick, and what you see is a centrifuge here.  So we can essentially spin it around at different speeds and we can put tiny little insects on the centrifuge so that they're spun around and at some point they start to slide outwards and, using very simple physics, we can calculate how much force is actually acting on these animals and is pulling them outward.

Felicity - So it's a bit like a merry-go-round in a play park, except insects not children.

David - Exactly, and that for insects it's not very dangerous because they're small enough to not be harmed at all by these techniques.

Felicity - Can we switch it on?

David - Yes, of course you can.  If we look at the ant here, right now, and we switch it on and it slowly increases in speed... You can see, at some point, the ant slowly starts sliding outward.  So that's the point at which it starts to slide and, if we go faster, and faster, and faster, suddenly it's of the surface.

Felicity - That's quite impressive really.  That's certainly not something I would be able to do if this was scaled up.

David - No.  Some of the ants can, actually, withstand forces up to 1,000 times it's own body weight so it's really, really impressive what they can do with their adhesive pads.

Felicity - We're all familiar with the Hollywood version of sticky pads in the form Spiderman.  How accurate is their representation of him clinging by his fingertips?

David - So, I mean of course, there's a large number of issues if you talk about the scientific accuracy of Spiderman, but if you phrased the question differently and say, well why are there no larger animals with adhesive pads?  Then we can start saying something about this because if we scale up our calculations with our data that we have then we find if you go to the size of a human, you probably end up with an unrealistically large fraction of your surface area that needs to be covered in sticky pads and what we calculated it somewhere in the area of something like 40% of your total body area or 80% of your frontal area, which I think is UK size 89.

Felicity - That's going to make it difficult to climb.

David - Exactly, and that's just to illustrate the problem that you face if you grow bigger and bigger.  It comes from very simple physical laws that effectively put constraints on what evolution can do.

Felicity - Larger animals - many of them do climb...

David - So, if you go larger than a gecko say, for example, or if you go to a certain size, then what starts to happen is that suddenly the world doesn't look flat to you any more.  If you think again of an ant and it climbs on a twig, the twig is effectively flat for the ant, but if you think of a monkey, the monkey can actually grasp around the twig and that's why you start to see a departure from these clinging pads, or adhesive pads, going to something like hands or massive claws, or so on and so forth.