Respiration rates and inferred mass-specific field metabolic rates decline with body size among five sympatric cetaceans

Metabolic rates in animals scale allometrically with body mass, a relationship well-established in terrestrial mammals. Whether these scaling laws apply to fully aquatic mammals remains uncertain, due to key physiological and ecological differences. We estimated field metabolic rates (FMRs) for five sympatric cetaceans of varying sizes, inhabiting sub-Arctic Icelandic waters: harbour porpoises (Phocoena phocoena; mean body length ± s.d = 1.35 ± 0.19 m), white-beaked dolphins (Lagenorhynchus albirostris; 2.42 ± 0.17 m), minke whales (Balaenoptera acutorostrata; 7.53 ± 0.82 m), humpback whales (Megaptera novaeangliae; 9.44 ± 1.13 m) and blue whales (Balaenoptera musculus; 21.97 ± 0.96 m). Unoccupied Aerial Vehicle (UAV) photogrammetry and published data were used to estimate body size, while respiration rates (breathes min^-1) were obtained from UAV focal follows, biologging tags, and literature sources. From these data we predicted daily FMRs (MJ day^-1) using existing bioenergetic models. As expected, mass-specific FMR declined with increasing body size among species, consistent with scaling laws. However, FMRs across all species were elevated relative to terrestrial predictions, likely reflecting the greater energetic demands of aquatic life. FMR also scaled positively with the surface-area-to-volume ratio (SVR) of each species, supporting the hypothesis that thermoregulatory costs are driven by body shape and size, and influence energy expenditure. This was further supported by the positive relationship between FMR and heat loss rates. Overall, our findings suggest that large mysticetes benefit from reduced mass-specific FMRs, enabling long migrations and extended fasting that broaden their habitat use. Smaller cetaceans face higher metabolic demands and may be more dependent on smaller, prey-rich habitats. These size-dependent energetic constraints may influence species plasticity and vulnerability to environmental changes.