We’ve all seen ants carrying seeds, crumbs, other insects — things whose mass is a good fraction of their own. Most of us couldn’t lift things that are so heavy relative to ourselves, and so ants and many other tiny creatures have reputations for being strong. Is this justified?
Answering this question brings up the notion of scaling: different physical forces and properties have different dependencies on size. Mass and the associated pull of gravity are in general proportional to volume, while strength depends on area. It’s therefore tough to be big: the forces pulling you down scale more steeply with size than the forces keeping you up. This is been known for centuries — Galileo in fact wrote about it. One way around this conundrum is for larger animals to have limbs that are disproportionately thick, trying, perhaps successfully, to have their mechanical strength keep up with gravity. Conversely a small animal, like an ant, can be strong as a simple consequence of size: as we shrink organisms, the ratio of strength to mass gets larger. The ant, as wonderful as it is, can’t take much credit for this.
I give a more leisurely description of scaling in my recent book, and I also explore other manifestations: the challenge of walking on water, why a bacterium that tries to swim with the same strokes as whales wouldn’t get anywhere, and even the contentious question of why small animals live fast and die young.
My goal here though it’s not so much to describe the answer but the question. Is this biophysics?
The field of biophysics tends to focus mostly on small scales: membranes, proteins, DNA, and perhaps cells and small collections of cells. The larger scales have tended to fall under the umbrellas of physiology or biomechanics. Conceptually, though, if we care about the intersection of biology and physics, it shouldn’t matter the scale at which phenomena take place. There’s plenty of fascinating stuff at all sizes, from those of molecules to whole ecosystems. At first glance the physics governing larger scales may seem obvious, just variants on standard mechanics, but a deeper look reveals intersections with how the properties of “active” fluids or complex materials influence the living world, motivating current work on systems like swarms of insects or snakes sliding on sand. Even issues of simple mechanics are often not as simple as they may seem, though, and can influence living things in profound ways.
Our ant, then, is a perfectly fine example of biophysics.
Today’s illustration I drew a “turtle ant,” based on this photo (Andreas Kay).
— Raghuveer Parthasarathy; March 13, 2022