Some large, wide-ranging teleost and elasmobranch fishes are converged to have regional endothermy, retaining metabolic heat via vascular countercurrent heat exchangers. Yet, its adaptive significance remains debated. While previous studies proposed potential benefits of elevated body temperature, enhanced controllability of body temperature enabled by heat exchangers may also be important. Some endothermic teleosts (e.g., bigeye tuna) alter rates of body temperature change depending on dive phases to maximize foraging time in deep, cold waters while minimizing recovery time in shallow, warm waters. However, whether endothermic elasmobranchs possess similar abilities remains unclear, leaving a major gap in understanding the adaptive significance of fish endothermy.
　Using animal-borne tags, we recorded diving behaviors and muscle temperatures of shortfin mako sharks, a possible elasmobranch equivalent to bigeye tuna. Fieldwork was conducted off southeastern Taiwan in June-July 2023 and January 2024. Sharks were captured by longline, brought onboard, measured, instrumented, and then released into the ocean. Muscle temperature was measured by a stalk temperature sensor inserted into the swimming muscle. The tag automatically detached from the shark and popped up to the sea surface after 18−30 h of deployment, then retrieved by a boat. Warming and cooling rates were estimated for each individual with a simple heat exchange model.
　All four mako sharks repeatedly dived below the thermocline with ambient temperature changes of up to 7–14°C (range across individual). The average temperature difference between muscle and the ambient water was 1.5–3.9°C, with a maximum of 5.3–12.0°C. Two individuals dived deep (up to 286–327 m) and showed 14–47 times higher warming rate than cooling rate, only comparable to endothermic bigeye tuna and swordfish among fishes studied to date. One individual warmed its muscle above sea surface temperature before a deep dive, possibly preparing for the coming deep excursion using internal heat sources. Other two individuals dived shallowly and exhibited 1–2 times differences.
　Our results demonstrate that enhanced temperature controllability has convergently evolved among some endothermic teleosts and elasmobranchs that inhabit low-to-middle latitude waters with strong thermal gradients. By contrast, some other endothermic species (e.g., salmon sharks and Atlantic bluefin tuna) that migrate to subpolar waters are specialized for body temperature elevation. We propose that the controllability and elevation of body temperature have different adaptive significance, reflecting species’ habitats and foraging ecology. Our findings help explain the diversity and success of endothermic fishes as apex predators across the world’s pelagic oceans.