Large, warm-bodied fish, like sharks and tuna, may owe their dominance to being able to retain their own body heat, but that advantage comes at a cost. According to a new study, these mesothermic species require nearly four times more energy than other fish, and as oceans warm, their tendency to generate heat faster than they can lose it may push these already vulnerable species closer to the brink of extinction. A small fraction of fish species, like tuna and some sharks, have evolved the ability to retain metabolic heat within the body – a strategy known as mesothermy – which can enhance their physiological capabilities. However, while the advantages allow such species to dominate as top ocean predators, they also come with elevated energetic costs, as maintaining elevated body temperatures and high activity levels demands substantial energy. However, the energetics of warm-bodied mesotherms, which can heavily influence marine food webs, are poorly understood, particularly in rapidly warming ocean environments.
To address this gap, Nicholas Payne and colleagues developed a method to estimate routine metabolic rate (RMR) in fish by analyzing heat exchange in tagged individuals and combining the results with published respiratory data for the species. This allowed Payne et al. to assemble a comprehensive dataset spanning nearly the full spectrum of fish sizes – from microscopic larvae to massive 3-ton sharks – from a wide range of ocean temperatures for both ectotherms and mesotherms. Then, using this framework, the authors evaluated how body size, environmental temperature, and heat-retaining physiological adaptations shape energy demands. The findings show that mesothermic fish require nearly four times more energy than their cold-bodied counterparts, high energy costs that likely constrained body size and contributed to extinction risk in both living and extinct species. Moreover, the analysis revealed a scaling mismatch between heat production and heat loss, in which rates of heat production increase faster than heat loss as fish species grow larger, meaning larger mesothermic fish become increasingly warm-bodied. According to the authors, this creates an “overheating predicament,” which may explain why such species are more commonly found in cooler, deeper, or higher-latitude waters. However, as these cooler waters warm under climate change, large mesothermic fishes – many already vulnerable and under severe pressure from overfishing – face increasing energy demands and substantial overheating risk, elevating their threat of extinction.
