The giant manta ray (Mobula birostris), often shortened to “birostris” or informally called the “fish biro,” is classified as a cold-blooded animal. Like the vast majority of fish, it relies on the surrounding water to regulate its body temperature rather than generating its own internal heat. But the full picture is more interesting than that simple label suggests, because manta rays have evolved at least one remarkable exception to their cold-blooded status: a system that keeps their brain warm.
What Cold-Blooded Actually Means
Cold-blooded animals, technically called ectotherms, don’t maintain a constant internal body temperature the way mammals and birds do. Instead, their body temperature rises and falls with their environment. For a giant manta ray, this means its muscles, organs, and blood are roughly the same temperature as the ocean water it swims through. When a manta ray dives from warm surface water into the cold deep ocean, its body cools down accordingly.
This isn’t a flaw. Ectothermy is enormously energy-efficient. Warm-blooded animals burn a huge portion of their calories just keeping their internal furnace running. Cold-blooded animals skip that cost entirely, which means they can survive on far less food. For a filter-feeding animal like the manta ray, which eats tiny plankton and other small organisms, conserving energy is a major advantage. Research on giant manta rays suggests they need to supplement their diet by diving into deep, cold waters to access additional food sources, and being ectothermic helps them stretch those calories further.
A Cold-Blooded Fish With a Warm Brain
Here’s where the giant manta ray breaks the mold. While its body is cold-blooded overall, it possesses a network of specialized blood vessels around its skull called a cranial rete. This structure works as a counter-current heat exchanger: warm blood flowing toward the brain passes close to cool blood flowing away, transferring heat inward and keeping the brain warmer than the surrounding water. Scientists call this cranial endothermy.
The brain is metabolically expensive tissue, and it functions better when kept at a stable, warm temperature. For a manta ray that moves between warm surface waters and deep, cold zones hundreds of meters below, protecting the brain from dramatic temperature swings likely improves nervous system performance. The giant manta ray’s large brain, one of the biggest relative to body size among all fish, generates enough metabolic heat to make this system work. Researchers have described the brain itself as potentially acting as a heat-producing organ.
This is not the same as being warm-blooded. The manta ray lacks the red muscle endothermy and visceral heat exchangers found in some of its relatives, like the Chilean devil ray. Those additional systems allow the Chilean devil ray to retain heat across more of its body during extreme deep dives. The giant manta ray warms only its brain, leaving the rest of its body subject to the ocean’s temperature.
How Manta Rays Cope With Cold Water
Giant manta rays are found across tropical and subtropical oceans, typically in water ranging from 19°C to 30°C depending on the region. Off the U.S. East Coast, they’re commonly spotted in waters between 19 and 22°C, while populations near Indonesia and the Yucatan peninsula prefer warmer water between 25 and 30°C. During feeding, they often stay in shallow water less than 10 meters deep, but tagging studies have recorded dives between 200 and 450 meters, with some exceeding 1,000 meters.
Those deep dives expose the manta ray to much colder temperatures, and without the ability to retain body heat the way its Chilean devil ray cousin can, the giant manta ray cools down faster. This likely forces shorter deep dives. One study tracking oceanic manta rays in New Zealand found they spent about 70% of their daytime hours in surface waters shallower than 5 meters, with occasional dives to 365 meters. Interestingly, the researchers found that temperature was a poor predictor of how long the rays spent at the surface afterward, suggesting they may not need extended “basking” periods to rewarm the way some other large cold-blooded fish do. Their cranial heat exchanger may be efficient enough to protect the brain without requiring a long recovery at the surface.
Why Most Fish Stayed Cold-Blooded
Only a handful of fish lineages have evolved any form of internal heat generation. Tunas, some sharks, the opah, and certain rays are the notable exceptions, and even among those, full-body warmth is rare. The reason most fish remain ectothermic comes down to physics and economics. Water conducts heat about 25 times more efficiently than air, which means any warmth a fish generates bleeds away rapidly through its gills. Maintaining a body temperature above the surrounding water would require burning enormous amounts of energy, a cost that only makes sense for high-performance predators like tuna that need fast muscle contractions to chase prey.
The giant manta ray sits in an interesting middle ground. It’s a filter feeder, not a high-speed hunter, so warming its entire body would waste energy it can’t afford. But it dives deep enough that its brain benefits from thermal protection. Evolution solved this by giving it a targeted warming system for just the organ that needs it most, while leaving everything else cold-blooded and energy-efficient. It’s a precise, economical adaptation rather than an all-or-nothing switch.