Isaac Newton would never have discovered the laws of motion if he had only studied cats.
Suppose you carry a cat, belly up, and drop it from a second-story window. If a cat is simply a mechanical system obeying Newton’s laws of matter in motion, it should sit on its back. (Okay, there are some techniques – like doing this in a vacuum, but ignore that for now.) Instead, most cats usually avoid injury by twisting their way to landing on their feet.
Most people are not mystified by this trick – everyone has seen videos proving the acrobatic prowess of cats. But for more than a century, scientists have wondered about the physics of how cats do it. Clearly, the mathematical theorem that analyzes the falling cat as a mechanical system fails for living cats, as Nobel laureate Frank Wilczek points out in a recent paper.
“This theorem 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 … powering mechanical motion”.
However, the laws of physics apply to cats as well as any other animal, from insects to elephants. Biology does not shy away from physics; embraces him. From friction on microscopic scales 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 imposed and the possibilities allowed by physics.
“Living organisms are … systems whose actions are constrained by physics at multiple scales and time scales,” write Jennifer Rieser and coauthors in the current issue of the Annual Review of Condensed Matter Physics.
While the field of the physics of animal behavior is still in its infancy, substantial progress has been made in explaining individual behaviors, along with how those behaviors are shaped through interactions with other individuals and the environment. In addition to discovering more about how animals perform their diverse repertoire of skills, such research may also lead to new physical insights gained by examining animal skills that scientists do not yet understand.
Creatures in motion
Physics applies to animals in action on a wide range of spatial scales. At the smaller end of the range, attractive forces between nearby atoms facilitate the ability of geckos and some insects to climb walls or even walk on ceilings. On a slightly larger scale, textures and structures provide adhesion for other biological gymnastics. In bird feathers, for example, small hooks and barbs act like Velcro, holding the feathers in position to increase lift when they fly, Rieser and colleagues report.
Biological textures also aid locomotion by facilitating friction between animal parts and surfaces. The scales of California king snakes have textures that allow for rapid forward gliding, but increase friction to slow backward or sideways movement. Some side-swirling snakes have apparently evolved different textures that reduce friction in the direction of movement, recent research suggests.
Small-scale structures are also important for animal interaction with water. For many animals, the microstructures make the body “superhydrophobic” – able to block the penetration of water. “In humid climates, the shedding of water droplets can be essential in animals, such as flying birds and insects, where weight and stability are essential,” note Rieser, of Emory University, 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 helping to protect the animal from dangers such as skin parasites and other infections,” the authors of the Annual Review explain. And in some cases, removal of foreign material from an animal’s surface may be necessary to maintain surface properties that improve camouflage.