Size-specific strategies of sympatric cetaceans to reduce heat loss.

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Maintaining a stable core body temperature is essential for endotherms. Cetaceans live in a highly thermally conductive medium, requiring special adaptations to reduce heat loss and maintain homeothermy. We employed a combination of aerial photogrammetry and existing data sources to estimate heat loss rates in five sympatric cetaceans of varying sizes, inhabiting the sub-arctic waters (∼3.7 °C) of NE-Iceland: harbour porpoises (Phocoena phocoena, 1.0 - 1.6 m, n=50), white-beaked dolphins (Lagenorhynchus albirostris, 1.1 - 2.9 m, n=294), minke whales (Balaenoptera acutorostrata, 4.4 - 8.6 m, n=30), humpback whales (Megaptera novaeangliae, 6.0 - 14.2 m, n=282) and blue whales (Balaenoptera musculus, 13.2 - 24.2 m, n=29). Further, we investigated the effect of body size (length), body shape (surface-area-to-volume ratio, SVR), body temperature, and blubber thermal conductivity and thickness on heat loss for all species. Smaller species had higher volume-specific heat loss compared to larger species due to their higher SVRs, a fundamental consequence of scaling. Apart from body size, blubber thickness had the largest effect on heat loss, followed by thermal conductivity. Smaller cetaceans seem to rely primarily on physiological and morphological adaptations to reduce heat loss, such as increased blubber thickness and lower thermal conductivity, whereas larger species offset heat loss by having larger bodies and lower SVRs. Our findings provide valuable insights into the thermal biology of these species and its implications for habitat use and prey requirements.