Editing Fatty acid (section)

====Variation between animal species====
Studies on the [[cell membrane]]s of [[mammal]]s and [[reptile]]s discovered that mammalian cell membranes are composed of a higher proportion of polyunsaturated fatty acids ([[docosahexaenoic acid|DHA]], [[omega-3 fatty acid|omega−3 fatty acid]]) than [[reptile]]s.<ref name=hulb1999/> Studies on bird fatty acid composition have noted similar proportions to mammals but with 1/3rd less omega−3 fatty acids as compared to [[omega-6 fatty acid|omega−6]] for a given body size.<ref name=hulb2002/> This fatty acid composition results in a more fluid cell membrane but also one that is permeable to various ions ({{chem2|H+}} & {{chem2|Na+}}), resulting in cell membranes that are more costly to maintain. This maintenance cost has been argued to be one of the key causes for the high metabolic rates and concomitant [[warm-blooded]]ness of mammals and birds.<ref name=hulb1999/> However polyunsaturation of cell membranes may also occur in response to chronic cold temperatures as well. In [[fish]] increasingly cold environments lead to increasingly high cell membrane content of both monounsaturated and polyunsaturated fatty acids, to maintain greater membrane fluidity (and functionality) at the lower [[temperature]]s.<ref name=hulb2003xa/><ref name=rayn1991/>