Animals running on land are supported by a solid substratum. Animals that swim and fly move in fluid media and have no solid support; they are supported by the medium through which they move. Fish have nearly the same density as water, and the energy they use for locomotion goes into overcoming the resistance of the medium. A flying bird must also overcome the resistance of the medium, but in addition it must keep from falling to the ground; that is, it must provide lift equal to its body weight.

Fish

Because fish are nearly neutrally buoyant, they expend little or no energy to support themselves, but energy is needed to overcome the resistance of the medium. The resistance that a swimming fish encounters is called the drag. To overcome the drag, the fish must provide thrust that equals the drag. There are two components to the drag on a fish moving through the water: pressure drag and friction drag.

Friction drag can be thought of as the drag on a thin, flat plate being pulled through a fluid parallel to its plane. Pressure drag can be thought of as the drag on the plate if it is moved through the fluid in a direction vertical to its plane.

Pressure drag is difficult to calculate accurately. It comes from the necessity to displace water during forward movement, and it is determined by the frontal area (the projected body area onto a plane normal to the direction of swimming) and by the shape of the body.