Physiological adaptations to semi-aquatic habits: the thermogenic macronutrient intake of the Eurasian otter

Maintaining thermal homeostasis in water is a major challenge for semi-aquatic mammals, which entails morphological, behavioural and physiological adaptations. Foraging strategies and dietary-induced thermogenesis (DIT) contribute to regulating body temperature in cold environments. As DIT differs among macronutrients, semi-aquatic mammals may be expected to regulate their macronutrient ratios to enhance their capability of thermoregulating. We estimated the nutritional intake target of a widespread semi-aquatic mustelid, Eurasian otter Lutra lutra by reviewing the available literature on otters' diet throughout Europe and assessing the nutritional composition of non-commercial fish species forming the bulk of otter diet through laboratory analyses. The macronutrient intake target for the otter was assessed as 61% protein and 39% lipid energy ratios. The mean percent protein energy was higher than that reported for typical hypercarnivores and tended to increase with decreasing water temperatures. The additional source of heat provided by the thermogenic effect induced by protein digestion may contribute to balance heat loss, compensating for the costs associated with protein overconsumption. If, as suggested by fossil records, semi-aquatic habits appeared after the development of otter's 'fish specialist' dentition, the thermogenic effect of its fish-based diet may have promoted the tight relationship occurring between otters and water habitats. Our results show that percent protein energy of Eurasian otter's diet is higher than that of typical hyper-carnivores and tends to increase with decreasing water temperatures. Protein foods exhibit higher thermogenic effects respect to lipids and carbohydrates and may contribute to mantain thermal homeostasis in water. image