Abstract
We propose two different approaches to defining variable units of intrinsic time (physiological time units in a strict sense, or PTU). For continuously growing animals, we suggest the use of specific mass growth rates; and for animals that cease to grow at some point, we recommend specific metabolic rates. Longevity of animals in terms of PTU is equal to the total specific rate (per lifetime) of the respective processes. A method is proposed to describe age-related changes in respect of specific metabolic rates of non-growing constant-mass adult birds. Maximum intrinsic longevity values have been estimated for certain fish species (continuously growing animals), and birds (that cease growth). Estimates of the maximum PTU longevity across both passerine and non-passerine groups differ slightly and are actually estimates of the Rubner constant for birds.
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