Abstract

The metabolic rates Г_o = a_oM ^0.73 [W], a ₀ = 3.6, of animals of different body masses M [kg] determines their heat production Q′ and their mechanical power output P. The metabolic heat production of small warm blooded animals living in a cold environment sets lower size limits for the body mass of birds and terrestrial mammals of M_t,b ≥ 0.01 kg and M_aq ≥ 10 kg for fast aquatic mammals. Birds must travel at the speed υ_fl = 15M ^1/6 [m/s] in order to stay aloft, but their muscles only generate the metabolic velocity υ _Г ≤≈ 50M ^−0.25 [m/s]. These conditions yield an upper body size limit for flying birds: M_{b ,max} ≤ 15 kg. Walking speeds scale with body mass as υ _walk ∝ M ^1/6, while propagation speeds for endurance running are nearly independent of body mass. This makes walking faster than endurance running for animals with body masses M ≥ 600 kg.