The acyl composition of tissue phospholipids varies in a systematic manner among species. Phospholipids, and thus membrane bilayers, from the tissues of small mammal and bird species have a high content of docosahexaenoic acid (DHA) compared to large species. A similar difference exists between the tissues of endothermic mammals and ectothermic reptiles. High DHA content in phospholipids is associated with high metabolic activity and this observation has led to the development of the "membrane pacemaker" theory of metabolism. This proposes that highly polyunsaturated acyl chains impart physical properties to membrane bilayers that enhance and speed up the molecular activity of membrane proteins and consequently the metabolic activity of cells, tissues and the whole animal. The brain has highly polyunsaturated membranes irrespective of body size and possible reasons for this are discussed. Highly polyunsaturated acyl chains are very susceptible to peroxidative damage. It is suggested that these chemical properties of highly polyunsaturated membrane acyl chains have important implications for understanding aging and the determination of longevity.