Effect of Selection for High Activity-Related Metabolism on Membrane Phospholipid Fatty Acid Composition in Bank Voles Clare Stawski 1, * Teresa G. Valencak 2 Thomas Ruf 2 Edyta T. Sadowska 1 Geoffrey Dheyongera 1 Agata Rudolf 1 Uttaran Maiti 1 Pawel Koteja 1 1 Institute of Environmental Sciences, Jagiellonian University, Gronostajowa 7, 30-387 Kraków, Poland; 2 Department of Integrative Biology and Evolution, Research Institute of Wildlife Ecology, University of Veterinary Medicine, Savoyenstrasse 1, 1160 Vienna, Austria Accepted 6/29/2015; Electronically Published 7/21/2015 ABSTRACT Endothermy, high basal metabolic rates (BMRs), and high locomotor-related metabolism were important steps in the evo- lution of mammals. It has been proposed that the composition of membrane phospholipid fatty acids plays an important role in energy metabolism and exercise muscle physiology. In particular, the membrane pacemaker theory of metabolism suggests that an increase in cell membrane fatty acid unsaturation would result in an increase in BMR. We aimed to determine whether membrane phospholipid fatty acid composition of heart, liver, and gastroc- nemius muscles differed between lines of bank voles selected for high swim-induced aerobic metabolism—which also evolved an increased BMR—and unselected control lines. Proportions of fatty acids significantly differed among the organs: liver was the least unsaturated, whereas the gastrocnemius muscles were most unsaturated. However, fatty acid proportions of the heart and liver did not differ significantly between selected and control lines. In gastrocnemius muscles, significant differences between selection directions were found: compared to control lines, membranes of selected voles were richer in saturated C18:0 and unsaturated C18:2n-6 and C18:3n-3, whereas the pattern was reversed for saturated C16:0 and unsaturated C20:4n-6. Neither unsaturation index nor other combined indexes of fatty acid proportions differed between lines. Thus, our results do not support the membrane pacemaker hypothesis. However, the differences be- tween selected and control lines in gastrocnemius muscles reflect chain lengths rather than number of double bonds and are prob- ably related to differences in locomotor activity per se rather than to differences in the basal or routine metabolic rate. Keywords: bank voles, basal metabolic rate, endotherm, evo- lutionary physiology, fatty acids, membrane pacemaker theory of metabolism. Introduction Endotherms (birds and mammals) have high basal metabolic rates (BMRs) in comparison to ectotherms (reptiles and am- phibians), and explaining this striking difference has become one of the main themes in evolutionary and ecological physi- ology of vertebrates. One direction of the research has been focused on hypotheses concerning selection forces that could be responsible for the evolution of such an energetically wasteful strategy (e.g., Bennett and Ruben 1979; Hayes and Garland 1995; Ruben 1995; Farmer 2000, 2003; Koteja 2000, 2004; Kemp 2006; Bergman and Casadevall 2010; Nespolo et al. 2011; Lovegrove 2012; Sadowska et al. 2015). The other framework concerns proximate physiological and biochemical factors underlining the high BMR observed in endotherms at the organismal level. One of the plausible proximate explanations is an increased size of metabolically active organs (especially the brain, heart, liver, kidneys, and skeletal muscle), which generate a large proportion of BMR (Konarzewski and Diamond 1995; Brzęk et al. 2007). Endotherms also differ from ectotherms at a lower level of organization, for example, a fourfold increase in total mi- tochondrial membrane surface area (Ruben 1995) and an in- crease in the proton leakiness of their cell membranes (Hulbert and Else 1999, 2000). The latter observation has led to devel- opment of the membrane pacemaker theory of metabolism, which suggests that an increase in the unsaturation index (UI; the aver- age number of double bonds per 100 fatty acid molecules) in cell membrane fatty acids would result in an increase in BMR, as molecular activity is thought to increase with membrane poly- unsaturation (Hulbert and Else 1999, 2000; Hulbert 2003, 2007; Brzęk et al. 2007). This hypothesis has recently received consid- erable attention in relation to theories of aging as the life span of an animal may be predicted by the polyunsaturated fatty acid (PUFA) content of their membranes, termed the membrane *Corresponding author. Present address: Centre for Behavioural and Physi- ological Ecology, Zoology, University of New England, Armidale, New South Wales 2351, Australia; e-mail: cstawsk2@une.edu.au. Physiological and Biochemical Zoology 88(6):668–679. 2015. q 2015 by The University of Chicago. All rights reserved. 1522-2152/2015/8806-5047$15.00. DOI: 10.1086/683039 668