Patterns of fatty acid usage in two small insectivores, a reptile and a mammal.

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 by a nocturnal insectivorous gecko, Hemidactylus turcicus, and a nocturnal insectivorous shrew of similar body mass, Suncus etruscus. We compared the relative presence of 13C isotopes in their 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.