The amount of energy burned by a creature in order to move. If you take the speed the creature is moving, the radius, and the distance they are moving along with this constant you can judge how much energy will be expended.

Note that RadiusSpeedDistance is equal to Radius * Speed * Distance. This means that the faster a creature moves the more energy will be consumed for equal distances. A creature should make sure only to move as fast as necessary to conserve energy.