Practice question

Mammals maintain a nearly constant, elevated body temperature through continuous metabolic heat production. This endothermic maintenance raises respiration, so a smaller fraction of assimilated energy remains for somatic growth, offspring, or stored biomass. Production efficiency is therefore generally lower than in ectotherms, whose metabolic costs fall when environmental temperature falls. Body size modifies the absolute and mass-specific rates, but homeothermy provides the mechanistic explanation; rapid growth would increase, not reduce, production efficiency when other costs are comparable. The distinction between gross transformation and net nutrient release is important: simultaneous microbial uptake can conceal substantial biochemical turnover. Environmental effects are often nonlinear; drought suppresses microbial access to substrates, waterlogging restricts oxygen, and extreme heat can reduce activity despite faster kinetics. Substrate stoichiometry links carbon processing to nutrient demand, so identical mass loss can accompany mineralization in one litter type and immobilization in another. Microbial enzymes act outside cells, making surface area and molecular accessibility as important as the nominal energy content of the dead material.