ABSTRACT
This chapter examines physiological maintenance, the base to which costs of activity and thermoregulation are added. The most direct way to determine maintenance requirements is energy balance trials using several feeding levels, preferably including those which cause energy gain and energy loss. If the efficiency with which energy is used for maintenance is known, energy requirements can be calculated from energy expenditures. Ion movement along electrochemical gradients followed by increased operation of active transport in the opposite direction is true maintenance expenditure since no net work is performed in maintaining the gradient. A variety of factors influence heat production of animals at rest in a thermoneutral environment. Particularly important is the increment associated with digestion and metabolism of nutrients. A variety of hormones influence metabolic rate. Thyroid hormones have well-documented effects on energy metabolism, exerting control on the mass and properties of mitochondria and the activity of the sodium pump.
