The laws of physics are rigid and fixed. Animals, therefore, are restricted in their use of available energy in accordance to the constraints of mechanics and thermodynamics. Evolutionary success dictates that a large proportion of the available energy reserves be allocated to reproductive effort despite the demand by other energy consuming functions. Therefore, mechanisms that reduce energy costs for non-reproductive functions relative to total energy reserves have an adaptive benefit for individuals (Fausch, 1984). As the laws of physics are inflexible and the available energy limited, animals have found ways to exploit these laws for their own benefit.