Eran Levin

Dietary fatty acids (FAs) have been demonstrated to be differentially stored or used as a metabolic fuel, depending on carbon chain length or saturation level. However, intestinal absorption also differs among FAs, potentially biasing conclusions on functional differences and their subsequent implications. We tested dietary FA usage in a nocturnal insectivorous reptile and a nocturnal insectivorous mammal of similar size: the gecko Hemidactylus turcicus and the shrew Suncus etruscus. We compared the relative presence of 13C isotopes in breath and feces following ingestion of three isotopically enriched fatty acids: linoleic acid (a polyunsaturated FA), oleic acid (monounsaturated) and palmitic acid (saturated). Both species oxidized linoleic and oleic acids at much higher levels than palmitic acid. Egestion of palmitic acid in feces was much higher than that of linoleic and oleic acids. The major difference between geckos and shrews was that the latter digested fatty acids much faster, which was best explained by the difference in the metabolic rates of the species. Circadian differences were evident for gecko metabolic and FA oxidation rates, peaking at night; for shrews, peak oxidation was achieved faster at night but rates did not differ. Our study is among the first to integrate oxidation and absorption patterns, as well as metabolic rates and their rhythms, providing important insights into the utilization of different dietary FAs in different species.

The scaling of the energetic cost of locomotion with body mass is well documented at the interspecific level. However, methodological restrictions limit our understanding of the scaling of flight metabolic rate ( MR ) in free‐flying insects. This is particularly true at the intraspecific level, where variation in body mass and flight energetics may have direct consequences for the fitness of an individual. We applied a 13 C stable isotope method to investigate the scaling of MR with body mass during free‐flight in the beetle Batocera rufomaculata . This species exhibits large intraspecific variation in adult body mass as a consequence of the environmental conditions during larval growth. We show that the flight‐ MR scales with body mass to the power of 0.57, with smaller conspecifics possessing up to 2.3 fold higher mass‐specific flight MR than larger ones. Whereas the scaling exponent of free‐flight MR was found to be like that determined for tethered‐flight, the energy expenditure during free‐flight was more than 2.7 fold higher than for tethered‐flight. The metabolic cost of flight should therefore be studied under free‐flight conditions, a requirement now enabled by the 13 C technique described herein for insect flight.