title: Physiological time and time-invariance.
abstract: An operational definition of physiological time is suggested that involves a non-linear transformation of a physical time-scale such that a certain temperature-dependent process becomes time-invariant. The definition expresses that physiological time has the dimension of time, and can be measured in units such as days or weeks. It is shown that the quantification of a physiological time-scale requires the specification of an arbitrary constant, which equals the rate of change of the process in physiological time. When the process-rate also depends on some internal variable, the interaction of this variable with temperature must be multiplicative, if a single physiological time-scale is to be used. The relationship with conventional uses of physiological time is discussed. The time-scale, as defined here, reduces to degree-day summation or development accumulation if certain multiplicative constants are deleted.
mesh_terms: D000818:Animals; D008954:Models, Biological; D010827:Physiology; D013696:Temperature; D013995:Time