Oxidation of linoleic and palmitic acid in pre-hibernating and hibernating common noctule bats revealed by 13 C breath testing.

Mammals fuel hibernation by oxidizing saturated and unsaturated fatty acids from triacylglycerols in adipocytes, yet the relative importance of these two categories as an oxidative fuel may change during hibernation. We studied the selective use of fatty acids as an oxidative fuel in noctule bats ( Nyctalus noctula ). Pre-hibernating noctule bats that were fed 13 C-enriched linoleic acid (LA) showed 12 times higher tracer oxidation rates compared with conspecifics fed 13 C-enriched palmitic acid (PA). After this experiment, we supplemented the diet of bats with the same fatty acids on five subsequent days to enrich their fat depots with the respective tracer. We then compared the excess 13 C enrichment (excess atom percentage, APE) in breath of bats for torpor and arousal events during early and late hibernation. We observed higher APE values in breath of bats fed 13 C-enriched LA than in bats fed 13 C-enriched PA for both states (torpor and arousal), and also for both periods. Thus, hibernating bats selectively oxidized endogenous LA instead of PA, probably because of faster transportation rates of polyunsaturated fatty acids compared with saturated fatty acids. We did not observe changes in APE values in the breath of torpid animals between early and late hibernation. Skin temperature of torpid animals increased by 0.7°C between early and late hibernation in bats fed PA, whereas it decreased by -0.8°C in bats fed LA, highlighting that endogenous LA may fulfil two functions when available in excess: serving as an oxidative fuel and supporting cell membrane functionality.