Membrane acyl (fatty acid) composition of vertebrates varies dependent upon body size, body temperature, developmental stage, phylogeny (endothermic or ectothermic) and diet. These differences often involve increases or decreases in the amount and level of unsaturation of the polyunsaturated fatty acids (PUFA), changes that can readily alter the propensity of membranes to undergo peroxidation. These changes are examined in vertebrates using the peroxidation index (PI), a value that calculates the relative peroxidisability of membranes based on the bisallylic methylene concentration of PUFA. A brief discussion of the relevance of PI to tissue peroxidation is presented (with consideration of antioxidant mechanisms). It is proposed that contrary to the deleterious effect of high levels of peroxidation, that low levels of peroxidation may have a positive effect on growth and metabolism in vertebrates. Practical applications: Potential implications of this work include understanding how vertebrates modulate their metabolism and growth; with changes in body size, as a result of the evolution and development of endothermy, and how ectothermic vertebrates can sustain relatively increased rates of metabolism and growth in cold environments. Membrane fatty acid composition varies amongst vertebrates. This often includes changes in relative amount and unsaturation of polyunsaturated fatty acids (PUFA). Such changes can alter the peroxidisability of membranes (PI) and when combined with membrane density differences could have significant effects on the biology of vertebrates. This idea is investigated in terms of increased basal rates of peroxidation stimulating growth and metabolism.