For a long time, scientists have known that body size and temperature shape metabolic rates. “Smaller animals tend to burn energy faster per unit mass than larger ones.” “Warmer environments increase metabolic speed.” These ideas form the backbone of metabolic theory in ecology. But most of what we know comes from ectotherms , animals whose body temperature tracks the environment. The ocean, however, is not only home to ectotherms. It also contains mesotherms , species like certain sharks and tunas that can elevate parts of their body above ambient water temperatures. They sit in a metabolic middle ground that has been difficult to quantify.

There’s a bit at stake here: if we misunderstand how these animals process energy, we also misunderstand how they shape food webs, how they move through oceans and how they respond to environmental change. Using a new modelling approach to estimate metabolic heat production across fishes, from tiny larvae to massive ocean predators, researchers have begun to map how routine energy expenditure scales with size and temperature. And what has emerged is not a smooth curve of increasing demand but a more complicated pattern. Mesotherms, it turns out, use roughly four times more energy than ectotherms of similar size and conditions! So what does it mean for an animal to live life at four times the energetic cost?

As fish grow larger, their bodies do something unexpected. Heat production increases faster than heat loss. At small sizes, this imbalance is negligible, but as mass increases, it becomes more pronounced. Bigger fish are not just larger versions of smaller fish, though; they are warmer, sometimes significantly so, because their bodies retain metabolic heat more effectively than they can shed it. This creates a scaling mismatch, also known as a persistent overheating problem. It is not dramatic in the way we might imagine physiological stress, as there is no sudden threshold where things fail. Instead, it is a gradual tightening of constraints. The larger the mesotherm, the more its own metabolism warms it beyond the surrounding water. This actually helps explain a long-standing ecological pattern in which many large mesotherms are found in cooler waters or migrate through regions where temperatures help offset internal heat production. It is not just preference or prey distribution that shapes these biogeographies but physics. After all, heat must go somewhere!

We are quickly accelerating to a reality where the boundary between internal heat production and environmental temperature becomes even narrower. So if warm-bodied fish are already operating under this energetic pressure, what happens when oceans warm further? One of the most important implications of this work is the realization that these animals are operating near the limits of thermal balance. Their high fuel demands already require substantial ecological support and if you add rising ocean temperatures into the mix, the margin for error shrinks. And history offers a warning here, as mesothermic species have shown disproportionate declines during past climate shifts.

From a broader perspective, this story goes beyond sharks or even fish physiology. Work like this turns our focus on how energy moves through living systems and how scaling laws shape ecological reality. If a one milligram larva and a three tonne predator are governed by the same fundamental rules but express them so differently, then what does that say about predictability in biology? It also makes us question on how we think about ocean health! Often, we focus on population numbers… but are glossing over energy budgets when we consider ocean conservation? Metabolism sets the pace for all life, so perhaps changes in energetic balance might be amongst the earliest signals of ecological stress and we should be sounding off alarm bells around this trait. No one really knows how flexible these metabolic systems are under rapid warming or whether an mesotherms can adjust their heat production. Their physiology might already close to fixed limits imposed by size and physics — we just don’t know! There is still much to uncover. But is there enough time to figure it all out?