Comparative Biochemistry and Physiology Part B 132 (2002) 515–527 1096-4959/02/$ - see front matter  2002 Elsevier Science Inc. All rights reserved. PII: S1096-4959 Ž 02 . 00066-0 Review The acyl composition of mammalian phospholipids: an allometric analysis A.J. Hulbert *, Tahira Rana , Patrice Couture a, a b Metabolic Research Centre, Department of Biological Science, University of Wollongong, Wollongong, NSW 2522, Australia a Departement de Biologie, Universite Laurentienne, Sudbury, Ontario, Canada b Received 9 January 2002; received in revised form 5 March 2002; accepted 5 March 2002 Abstract Data concerning the acyl composition of tissue phospholipids from mammal species, ranging in size from the shrew (7g) to cattle (370 kg), has been collated from the literature and analysed allometrically. Phospholipids from heart, skeletal muscle, liver and kidney exhibited similar allometric trends whereby phospholipids had a significant decrease in unsaturation index (number of double bonds per 100 acyl chains) as species body size increased whilst there was no change in the percent of unsaturated acyl chains. Whilst total polyunsaturate content did not change with body mass, both heart and skeletal muscle phospholipids showed a significant allometric decrease in the omega-3 polyunsaturate content. The content of the highly polyunsaturated docosahexaenoic acid (22:6 n-3) in phospholipids showed significant and substantial allometric decline with increasing body mass in all four tissues (exponents ranged from y0.19 in liver to y0.40 in skeletal muscle). Brain phospholipids showed no allometric trends in acyl composition and were highly polyunsaturated in all species. These trends are discussed in light of the hypothesis that the relative content of polyunsaturated acyl chains in membranes, and especially docosahexaenoate (22:6 n-3), can act as a membrane pacemaker for metabolic activity.  2002 Elsevier Science Inc. All rights reserved. Keywords: Docosahexaenoic acid; Omega-3 polyunsaturates; Membrane lipids; Omega-6 polyunsaturates; Metabolism; Mammals; Phospholipids; Monounsaturates; Lifespan; Body size 1. Introduction More than 20 years ago, Gudbjarnason and colleagues published a fascinating correlation between the heart rate of mammals, ranging from mice to whales, and the docosahexaenoate content of their cardiac phospholipids (Gudbjarnason et al., 1978). This was a particularly intriguing cor- relation in light of the fact that the resting heart rate of a mammal is indicative of the metabolic intensity of the species. In a similar correlation, *Corresponding author. Tel.: q61-2-4221-3437; fax: q61- 2-4221-4195. E-mail address: hulbert@uow.edu.au (A.J. Hulbert). the high level of metabolism in a mammal com- pared to a reptile (of the same body size and body temperature) has been shown to be associated with a high docosahexaenoate content of liver and kidney phospholipids in the mammal compared to the reptile (Hulbert and Else, 1989). As part of a study of the allometric variation in metabolic intensity of tissues from five species of mammal, ranging in size from mice to cattle (Couture and Hulbert, 1995a), the authors also determined the acyl composition of tissue phospholipids from these species (Couture and Hulbert, 1995b). They found all tissues examined (but the brain) exhib- ited body mass-related trends in the level of