Isaac Newton would never have discovered the laws of motion if he had only studied cats.
Imagine holding a cat, belly up, and dropping it out of a second-story window. If a cat were simply a mechanical system following Newton’s rules of matter in motion, it would have to land on its back. (Okay, there are some technicalities, like the fact that this has to happen in a vacuum, but ignore that for now.) Instead, most cats avoid injury by spinning on the way down to land on their feet.
Most people aren’t surprised by this trick: Everyone has seen videos of cats performing acrobatic stunts. But for more than a century, scientists have puzzled over how the physics of cats do this. Clearly, the mathematical theorem that analyzes the falling cat as a mechanical system doesn’t work for living cats, as Nobel Prize winner Frank Wilczek points out in a recent paper.
“This proposition is not relevant to real biological cats,” writes Wilczek, a theoretical physicist at MIT. They are not closed mechanical systems and can “consume stored energy … and thus produce mechanical motion.”
Nevertheless, the laws of physics do apply to cats, and to every other kind of animal, from insects to elephants. Biology doesn’t avoid physics; it embraces it. From friction on a microscopic scale to fluid dynamics in water and air, animals use the laws of physics to run, swim, or fly. Every other aspect of animal behavior, from breathing to building shelters, depends in some way on the constraints of physics and the opportunities it provides.
“Living organisms are … systems whose actions are constrained by physics on multiple length scales and time scales,” Jennifer Rieser and coauthors write in the current issue of the Annual Review of Condensed Matter Physics.
Although the field of animal behavioral physics is still in its infancy, significant progress has been made in explaining individual behavior, along with how that behavior is shaped by interactions with other individuals and the environment. In addition to discovering more about how animals perform their diverse repertoire of skills, such research can also lead to new physics knowledge gained by investigating animal abilities that scientists do not yet understand.
Moving creatures
Physics applies to animals in action at a wide range of spatial scales. At the smallest end of the scale, attractive forces between nearby atoms facilitate the ability of geckos and some insects to climb walls or even walk across ceilings. At a slightly larger scale, textures and structures provide adhesion for other biological gymnastics. In bird feathers, for example, tiny hooks and barbs act like Velcro, holding feathers in place to increase lift during flight, Rieser and colleagues report.
Biological textures also aid in locomotion by facilitating friction between animal parts and surfaces. The scales of the California kingsnake have textures that allow for rapid forward gliding movements, but increase friction to slow backward or sideways movement. Some sidewinder snakes have apparently evolved other textures that reduce friction in the direction of movement, recent research suggests.
Small-scale structures are also important for how animals interact with water. For many animals, microstructures make the body “superhydrophobic,” allowing it to block water penetration. “In wet climates, shedding water droplets can be essential for animals, such as flying birds and insects, where weight and stability are critical,” note Emory University’s Rieser and co-authors Chantal Nguyen, Orit Peleg, and Calvin Riiska.
Water-blocking surfaces also help animals keep their skin clean. “This self-cleaning mechanism … may be important in protecting the animal from hazards such as skin parasites and other infections,” the Annual Review authors explain. And in some cases, removing foreign material from an animal’s surface may be necessary to maintain surface properties that enhance camouflage.
