In aging research, two types of age are studied: chronological age (the amount of time an organism has lived) and physiological age. It is the physiological age that determines lifespan which is species specific and is used as the timer for biological and genetic clock tick that is in agreement with the programmed theory aging.
According to rate of living theory of aging also known as this theory of maximum metabolic scope, the greater is the organisms rate of metabolism, shorter is its life span and vice versa. A large amount of data on energy metabolism in man, mammals and especially in birds leads to a fixed exponential mathematical relationship between lifespan (A), the rate of energy turnover (S) with overall body mass(M) for various organisms which is in principle the same for all species and for all phases of development including aging.
A=aM0.25 (a is a specie specific coefficient) — the higher the mass of species, the longer its programmed lifespan
S=bM-0.25 (b is a specie specific coefficient) — the higher the mass of species, the slower its metabolism
TM=SA=ab (TM: total mass-specific metabolism TM or energy during lifespan )
A=ab/S (the higher the rate of energy turn over or metabolism, the shorter the specie’s lifespan)
In other words, physiological lifetime (A) is expressed in units of energy metabolism, and is almost identical within an animal taxon. We reach a fixed physiological age as the organism works its way through a roughly constant quantity of energy until the internal clock initiates death. Of course, it is possible to find great differences in lifespan between species of different sizes and at different evolutionary levels. The absolute amount of energy that mitochondria can generate thus may ultimately define the lifespan of the host organism
