Animals walking on walls: challenging gravity

How do insects, spiders or lizards for walking on smooth vertical surfaces or upside down? Why would not be possible for Spiderman to stick on walls the way some animals do?

Scientist from several areas are still in search of the exact mechanisms that allow some animals to walk on smooth surfaces without falling or sliding. Here we bring you the latest discoveries about this topic.

Animals walking on walls: challenging gravity

Competition for space and resources (ecological niche) has led to a lot of amazing adaptations throughout the evolution of life on Earth, like miniaturization.

When nails, claws or friction forces are insufficient to climb up vertical smooth surfaces, dynamic adhesion mechanisms come into play. Dynamic adhesion mechanisms are defined as those that allow some animals to climb steep or overhanging smooth surfaces, or even to walk upside down (e.g. on ceilings), by attaching and detaching easily from them. The rising of adhesive structures like adhesive pads as an evolutionary novelty has allowed some animals to take advantage of unexplored habitats and resources, foraging and hiding from predators where others could not.

Gecko stuck on a glass surface. Picture by Shutterstock/Papa Bravo.

Adhesive pads are found in insects and spiders, some reptiles like geckos and lizards, and some amphibians like tree frogs. More rarely they can be also found in small mammals, like bats and possums, arboreal marsupials native to Australia and some regions from the Southeast Asia.

The appearance of adhesive pads among these very different groups of animals is the result of a convergent evolution process: evolution gives room to equal or very similar solutions (adhesive pads) to face the same problem (competence for space and resources, high predation pressure, etc.).

Adaptation limits (or why Spiderman could not climb up walls)

Studying the underlying processes of the climbing ability of these animals is a key point in the development of stronger adhesive substances. So, a lot of research regarding this topic has been carried out to date.

Will humans be able to climb up walls like Spiderman some day? Labonte et al. (2016) explain us why Spiderman could not be real. Or, at least, how he should be to be able to stick on walls and do whatever a spider can.

Will humans be able to climb up walls like Spiderman some day? For now, we will have to settle for this sculpture. Public domain image.

Apart from the specific mechanisms of each organism (of which we will talk in depth later), the main principle that leads the ability for walking on vertical smooth surfaces is the surface/volume ratio: the smaller the animal, the larger is the total surface of the body with respect its volume and smaller is the amount of adhesive surface needed to avoid falling due to the body weight. According to this, geckos are the bigger known animals (i.e. those with the smallest surface/volume ratio) able to walk on vertical smooth surfaces or upside down without undergoing deep anatomical modifications.

And what does ‘without undergoing deep anatomical modifications’ mean? The same authors say that the larger the animal, the bigger is the adhesive pad surface needed for walking without falling to the ground. The growth of the adhesive pad surface with respect the size of the animal shows an extreme positive allometry pattern: by a small increase of the animal size, a bigger increase of the adhesive pad surface takes place. According to this study, a 200-fold increase of relative pad area from mites to geckos has been observed.

Picture by David Labonte

However, allometry is led by anatomical constraints. Therefore, if there was an animal larger than a gecko able to climb up smooth surfaces, it should have, for example, extremely large paws covered by an extremely large sticky surface. While this would be possible from a physical point of view, anatomical constraints would prevent the existence of animals with such traits.

Now we are in condition to answer the question ‘Why Spiderman could not stick to walls?’. According to Labonte et al., to support a human’s body weight, an unrealistic 40% of the body surface would have to be covered with adhesive pads (80% if we only consider the front of the body) or ridiculously large arms and legs should be developed. Both solutions are unfeasible from an anatomical point of view.

Great diversity of strategies

Dynamic adhesion must be strong enough to avoid falling as well as weak enough to enable the animal to move.

A great diversity of dynamic adhesion strategies has been studied. Let’s see some of the most well-known:

Diversity of adhesive pads. Picture by David Labonte.

1) Wet adhesion

A liquid substance comes into play.

Insects

Insects develop two main mechanisms of wet adhesion:

Smooth adhesive pads: this mechanism is found in ants, bees, cockroaches and grasshoppers, for example. The last segment of their legs (pretarsus), their claws or their tibiae present one or several soft and extremely deformable pads (like the arolia located in the pretarsus). No surface is completely smooth at microscale, so these pads conform to the shape of surface irregularities thanks to their softness.

Cockroach tarsus. Adapted picture from the original by Clemente & Federle, 2008.

Hairy adhesive pads: these structures are found in beetles and flies, among others. These pads are covered by a dense layer of hair-like structures, the setae, which increase the surface of the leg in contact with the surface.

Chrysomelidae beetle paw. Picture by Stanislav Gorb et al.

A thin layer of fluid consisting of a hydrophilic and a hydrophobic phase located between the pad and substrate comes into play in both strategies. Studies carried out with ants show that the ends of their legs secrete a thin layer of liquid that increases the contact between the pretarsus and the surface, filling the remaining gaps and acting as an adhesive under both capillarity (surface tension) and viscosity principles.

Want to learn more about this mechanism in insects? Then do not miss the following video about ants!

Tree frogs

Arboreal or tree frog smooth toe pads are made of columnar epithelial cells separated from each other at their apices. Mucous glands open between them and secrete a mucous substance that fill the intercellular spaces. Having the cells separated enable the pad to conform to the shape of the surface and channels that surround each epithelial cell allow to spread mucus over the pad surface to guarantee the adhesion. These channels also allow to remove surplus water under wet conditions that could make frogs to slide (most tree frogs live in rainforests).

Red-eyed tree frog (Agalychnis callidryas), distributed from Southern Mexico to Northeastern Colombia. Public domain image.

In the next video, you can see in detail the legs of one of the most popular tree frogs:

Smooth toe pads of tree frogs are similar to those found in insects. In fact, crickets have a hexagonal microstructure reminiscent of the toe pads of tree frogs. This led Barnes (2007) to consider the wet adhesion mechanism as one of the most successful adhesion strategies.

Different species of tree frogs (a, b, c) and their respective epithelia (d, e, f). Figure g corresponds to the surface of a cricket’s smooth toe pad. Picture by Barnes (2007).

Possums

The most detailed studies on possums have been carried out about the feathertail glider (Acrobates pygmaeus), a mouse-sized marsupial capable to climb up sheets of glass using their large toe pads. These pads are conformed by multiple layers of squamous epithelium with alternated ridges and grooves that allow them to conform to the shape of the surface and that are filled with sweat, the liquid this small mammal use to adhere to surfaces.

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Acrobates pygmaeus. Picture by Roland Seitre.
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Frontal toe pads of Acrobates pygmaeus. Picture by Simon Hinkley and Ken Walker.

2) Dry adhesion

Liquid substances do not come into play.

Spiders and geckos

The adhesion of both spiders and geckos depends on the same principle: the Van der Waals forces. Unlike insects, tree frogs and possums, these organisms do not secrete sticky substances.

Van der Waals forces are distance-dependent interactions between atoms or molecules that are not a result of any chemical electronic bond. These interactions show up between setae from footpads of geckos (which are covered by folds, the lamellae) and setae from spider paws (which are covered with dense tufts of hair, the scopulae), and the surface they walk on.

Spider paw covered with setae. Picture by Michael Pankratz.
Diversity of footpads of geckos. Picture by Kellar Autumn.

However, recent studies suggest dry adhesion in geckos is not mainly lead by Van der Waals forces, but by electrostatic interactions (different polarity between setae and surface), after confirming that their sticking capacity decreased when trying to climb a sheet of low energetic material, like teflon.

Anyway, the ability of geckos to climb is impressive. Check this video of the great David Attenborough:

3) Suction

Bats

Disk-winged bats (family Thyropteridae), native to Central America and northern South America, have disk-shaped suction pads located at the base of their thumbs and on the sole of their feet that allow them to climb smooth surfaces. Inside these disks, the internal pressure is reduced, and the bat stick to the surface by suction. In fact, a single disk can support the weight of the bat’s body.

Thyropteridae bat. Picture by Christian Ziegler/ Minden Pictures.

Now that you know about all these animals’ ability for climbing smooth walls, do you still think Spiderman is up to the task?

Main picture by unknown author. Source: link.

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