The dietary and thermoregulatory role of blubber as revealed by fatty acids

Abstract

Blubber was a crucial adaptation for mammals living in water. Blubber serves as an energy reservoir, where surplus energy is deposited in the form of fatty acids (FAs). Most FAs are obtained from an animal’s diet (dietary FAs), thus a predator’s FA signature has the potential to provide dietary information. However, some FAs in the predator may be synthesised or modified intrinsically to fulfil physiological demands (non-dietary FAs), which complicates dietary studies. The aim of this thesis was to understand how FAs signatures relate to the dietary and thermoregulatory roles of blubber. I analysed FAs of leopard and crabeater seals to determine how FAs vary across the blubber depth and how this variability influenced dietary interpretations. I found that blubber is not uniform; where some FAs are abundant in the outer (superficial) layer others are dominant in the inner (deepest) layer. This suggests that the inner layer has a dietary role whereas the outer layer has a more structural role. The FA signatures from the predator’s inner layer resembled more closely those of their prey than the FAs in the outer layer. Trophic predictions were clearer when using only the dietary FAs rather than all FAs; this indicates that there are other factors influencing the metabolism of the non-dietary FAs. To examine if a mammal’s thermoregulatory requirements impact the shifts in FAs, I conducted a meta-analysis including 48 mammals from terrestrial, semi-aquatic, and fully-aquatic environments. I found that the FAs of aquatic mammals are more highly desaturated than those of terrestrial mammals. Higher desaturation helps reduce heat loss and ensures that blubber remains flexible in cold environments. FA desaturation is correlated with latitude and fur density in semi-aquatic mammals. Thus, they increase FA desaturation when living in colder habitats and when they have sparser fur. I compared the FAs of three sympatric Antarctic seals and found that FA desaturation changes as seals grow, which suggests that the thermal efficiency of blubber develops with age. In order to obtain better results in dietary studies, the effect of thermal habitat, fur density and age, on the metabolism of FAs must be considered.