Energy metabolism is fundamental for animal fitness because it fuels locomotion, growth, and reproduction. Mitochondrial capacities often acclimate to compensate for negative thermodynamic effects. Our aim was to determine the importance of transcriptional regulation and membrane fatty acid composition in modulating oxidative capacities at body temperatures selected in a cold and a warm environment by a reptile (Crocodylus porosus). In the cool environment (mean selected Tb p 21C), mRNA concentrations of the transcription factor peroxisome proliferator–activated receptor gamma (PPARg) and its coactivator PPARg coactivator 1 alpha (PGC-1a), as well as of the cytochrome c oxidase (COX) subunits COX1 and COX5, were significantly higher in the liver but not in skeletal muscle compared with animals in the warm environment (mean selected Tb p 29C). FOF1-ATPase subunit a mRNA concentrations were significantly higher in both muscle and the liver in the cool animals. A positive relationship between PGC- 1a and PPARg mRNA concentrations, with an indicator of mitochondrial density (16S rRNA) in muscle and COX and FOF1-ATPase subunit a mRNA concentrations in liver, suggest that these proteins regulate quantity increases of mitochondria during acclimation. The percent saturated fatty acids in liver membranes of cool animals was significantly lower, and the n3
fatty acid content was significantly higher, compared with in warm animals. The n3 fatty acid content was positively related to COX enzyme activity in the liver, and there was a negative relationship between n7 fatty acid content and COX activity in muscle. Hence, metabolic acclimation is mediated by both transcriptional regulation and membrane fatty acid composition. The importance of PGC-1a and PPARg in a reptile indicate that the mechanisms that regulate metabolism are conserved among vertebrates.