But not if you can avoid it
Most scientific research starts with an observation. And when in Antarctica I watched penguins, how they walk, hop, swim and (very occasionally) slip on the ice. I, on the other hand, would have slipped and fallen all the time, hadn’t I worn special footwear with spikes, termed in-step crampons, fastened under the soles of my boots. Now penguins don’t wear boots with crampons but hardly ever slip when marching along on slippery, icy surfaces. How come? I examined the underside of the feet of a dead penguin by microscopy and found numerous tiny bumps and protuberances that could have been there to improve the feet’s steadfastness on the ice. But how about the African or Galapagos species of penguins? They never see any ice and their only risk of slipping comes from climbing onto wet and slippery rocks along the seashore. Their feet still await to be examined. And so do the feet of the Adelie penguins when in the ocean for many months and not breeding on land.
Sliding along on a slippery surface can, on rare occasions, be advantageous: for example, when penguins go into a mode of locomotion called ‘tobogganing’ or when pond skaters slide along the water surface. But more usually slipping is to be avoided as it entails the risk of getting injured. The problem is to find a compromise between reducing speed and increasing grip, i.e. stability – and that often depends not only on structures involved in permitting or avoiding sliding, but environmental conditions. L. Heepe et al. found in 2016 that the feet of ladybird beetles (Coccinelllidae) achieve the highest attachment forces (i.e. the least amount of slipping) at a humidity of 60%, but lower and higher humidities would lead to a decrease in attachment ability. Not getting stuck too tightly is, of course, another problem that animals that possess adaptations to prevent slipping, need to control.
A pioneer in connection with adhesiveness and slipping on wet surfaces was Prof Jon Barnes, whom I had invited to give a lecture on his research with frogs when I had been in charge of our department’s weekly seminar series. Jon gave an incredibly memorable and exciting lecture, in which he described his and his colleagues’ observations on frogs of different sizes and their adhesive abilities on wet and dry surfaces. Another pioneer in the field of adhesive mechanisms in animals is Prof S. Gorb, who explains that the wet adhesive system depends mainly on capillarity (think of a wet paper plastered onto the window glass) while dry adhesiveness involves molecular interactions, known as van der Waals forces. The latter have been identified in gekkos, who are known to scuttle along upside down on the ceilings of houses. However, in frogs sitting on slanted and wet surfaces of leaves, a mechanism akin to peeling sticky tape off a surface comes into play. Their attachment forces are significantly enhanced by close contacts and boundary friction between the frog’s toe pad epidermis and the substrate. These toe pads are wet with watery mucus, assisting attachment due to the fluid-filled close coupling between the pad and the substrate, but wet adhesion alone would not hold a frog on a slippery and vertical surface. Incidentally, frogs always choose to rest on a slippery surface head-up; if the surface is turned, the frog readjusts its position as the turn reaches approximately 55 degrees. The underside of the frog’s toes features “peg-studded hexagonal cells separated by deep channels into which mucus glands open” and some vague similarities to the structures I found on the underside of the penguin’s feet, seemingly to prevent slipping, are apparent.
Frogs, of course, unlike penguins, have four legs on the ground and spiders have even eight. I had always wanted to study if each leg contributes equally to the total adhesiveness of an animal, but this question was answered by S. Gorb’s group in an interesting publication of 2014, which showed that the whole was more than the sum of all its parts. What apparently still has not been repeated was a little study of mine jn 1993 in which I reported that flies adhere to surfaces significantly longer when it is dark than under illumination. That makes sense, of course. But it needs a scientific explanation and I have so far not been able to follow up that result (but I do hope someone will).
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