The strange animals that bend the rules of body heat

Background

Most of us grow up with the simple story: mammals and birds are warm-blooded; reptiles, amphibians, and fish are cold-blooded. Reality is far messier—and far more interesting. Across the tree of life, animals have evolved clever ways to steer their temperature up or down, locally or globally, minute-by-minute or season-by-season.

Why does this matter? Heat is biology’s hidden currency. The speed of chemical reactions, the flow of oxygen, and even the shape of proteins depend on temperature. Raise a body just a few degrees and metabolism sprints; drop it and everything slows. In a world where storms, droughts, fires, and cold snaps are intensifying and becoming less predictable, the ability to bend the rules of body heat can mean the difference between living and dying.

At the broadest level, animals fall along a spectrum:

• Endotherms generate most of their heat with metabolism (think birds and mammals).
• Ectotherms rely mainly on the environment and behavior (most reptiles, amphibians, and fish).
• Heterotherms can switch strategies or allow different body parts to be at different temperatures.

Layered atop that are the physics of heat transfer—conduction, convection, radiation, and evaporation—and a toolbox of structural tricks: blubber, feathers, countercurrent blood vessels, and heat exchangers in noses and fins. The result is a kaleidoscope of solutions to innumerable thermal problems.

What happened

Ars Technica profiled the remarkable organisms that don’t just passively cope with the weather but actively manipulate their internal temperatures—sometimes dramatically. The feature highlights field observations and lab work showing how creatures as different as hummingbirds, camels, sharks, frogs, bees, and even pythons bend the usual rules to survive everything from polar winters to desert heat waves to predator attacks. Below, we unpack the biology behind several of those strategies, add context from decades of thermal physiology research, and connect it to conservation challenges now unfolding.

Flip the switch: Torpor, hibernation, and “daily shutdowns”

Endothermy is expensive. Small birds and mammals burn fuel furiously to keep warm, and on a cold, stormy night there may not be enough food to meet the bill. Enter torpor: a reversible, controlled reduction of body temperature and metabolism.

• Hummingbirds are the reigning champions of daily torpor. Some species lower their body temperature by more than 20°C after sunset, reducing energy use by up to 95%. By morning, they rewarm—sometimes faster than a degree per minute—using brown adipose-like mechanisms and shivering muscles. This nightly shutdown lets them survive long, cold nights or days of poor nectar.
• The common poorwill, a North American nightjar, extends torpor to the scale of weeks. In sheltered rock crevices during winter, it can hover at near-ambient temperatures, the closest thing birds have to mammalian-style hibernation.
• Many small mammals—bats, mice, tenrecs, and dwarf lemurs—cycle between torpor and arousal across the season. Some Malagasy lemurs can store fat in their tails and enter months-long hibernation in tree hollows, a rarity among primates.
• Australian echidnas, an egg-laying mammal, take torpor a step further as a disaster response: studies have shown they retreat to burrows or log hollows after wildfires, lower their body temperature and activity, and wait out the barren, risky post-fire landscape until food returns.

A key distinction: torpor is shorter-term and can occur any day; hibernation is a prolonged, seasonal torpor with extensive physiological remodeling (immune changes, clotting resistance, and altered muscle maintenance). Bears are often debated, but the consensus is that their multi-month winter state—with deep metabolic depression and modest body temperature drops—is hibernation by any biological standard.

Cold cheats: Antifreeze, deep chill, and freeze tolerance

Not all animals fight the cold by making more heat. Many let themselves cool—and evolve safeguards.

• Wood frogs and a handful of other amphibians are freeze-tolerant. As temperatures drop, they flood tissues with glucose and urea that act as cryoprotectants. Ice forms outside cells, the heart stops, and brain activity flatlines, yet as spring warmth returns the frogs thaw and resume calling for mates.
• Painted turtles are legends of low-oxygen endurance. Adults overwinter under ice in ponds, their metabolism slowed by the cold. They can sit for weeks without breathing, buffering lactic acid with their shells. Hatchlings of some populations even tolerate partial freezing in the nest.
• Antarctic notothenioid fishes produce antifreeze glycoproteins to prevent ice crystals from seeding in their blood. Some species have lost hemoglobin entirely because cold water holds so much oxygen, letting them conserve energy otherwise spent moving thick, red blood.

These strategies showcase a different flavor of thermal mastery: instead of maintaining a human-like steady 37°C, they exploit physics to slow—and survive.

Heat as a weapon: Social thermogenesis and thermal signals

Temperature can be a tool, not just a target.

• Japanese honeybees defend their hives from giant hornets by swarming an intruder and vibrating their flight muscles. The “hot ball” rises to around 46°C—lethal to the hornet but survivable for bees—essentially cooking the predator.
• California ground squirrels confronting rattlesnakes heat their tails via blood flow and vigorous wagging. Rattlesnakes perceive infrared; a hot, flagging tail reads as a bold, high-risk opponent, discouraging strikes.
• Pythons, though reptiles, produce heat via muscular shivering when brooding eggs. Females can sustain their clutch well above ambient temperature, a rare case of parental endothermy in a reptile.

Here, the goal isn’t homeostasis—it’s winning a fight or protecting offspring with well-timed thermal bursts.

Sea athletes: Partial warm-bloodedness in the ocean

Large expanses of the ocean are cold, yet some predators sprint and dive with mammal-like vigor thanks to regional endothermy—keeping select tissues warm.

• Tunas and lamnid sharks (like mako, salmon, and white sharks) conserve muscular heat using dense networks of countercurrent blood vessels called retia mirabilia. Red swimming muscles, eyes, and even the brain can be kept far warmer than the water, boosting power and visual acuity.
• Billfishes possess “heater organs” derived from eye muscles that specifically warm the brain and retina, improving vision in deep, dim waters.
• The opah (moonfish) goes further: by insulating its gills and using heat exchangers, it maintains elevated temperatures across much of its body. It’s the closest thing we have to a fully endothermic fish.
• Leatherback sea turtles, though ectotherms, maintain warm cores through gigantothermy (a favorable surface-area-to-volume ratio), insulation, and countercurrent vessels in flippers. This lets them forage in frigid seas where jellyfish are abundant.

These solutions expand their niche: sustained speed, deeper dives, and dawn-dusk foraging windows off-limits to strictly cold-blooded competitors.

Heat in the heat: Riding out deserts and heat waves

In hot, dry places, the bottleneck is water, not calories. Evaporative cooling (sweating or panting) saves lives but wastes precious moisture. Several animals accept a hotter body to spare water—so-called adaptive hyperthermia.

• Camels can let their body temperature swing by 6–7°C daily, storing heat during the day and dumping it at night when air is cooler. This reduces the need for sweating under a brutal sun.
• Large desert antelopes and sheep use a carotid rete—a blood-cooling mesh in the head—that selectively chills the brain during heat stress. They can allow the core to run warm while protecting neural tissue.
• Many birds lack sweat glands and instead rely on panting and gular flutter (rapid throat vibrations). Some desert species allow their temperature to drift upward at midday, cutting their evaporative water loss by a third or more.
• Vultures and storks practice urohidrosis—wetting their legs with droppings—to promote evaporative cooling across the richly vascular skin of their lower limbs.

These are uncomfortable choices by human standards, but they are water-saving masterpieces for the animals that evolved them.

Behavioral hacks: Architecture, posture, and teamwork

Not every thermal trick is internal.

• Emperor penguins huddle by the thousands on Antarctic ice, creating a living windbreak. Minute shifts ripple through the crowd like waves, keeping the cluster optimally packed and warm while oxygen still diffuses.
• Bees keep brood remarkably close to 35°C year-round. Workers act as mobile radiators and air conditioners, fanning to ventilate or pressing warm thoraxes to combs to heat.
• Many shorebirds and ungulates exploit countercurrent heat exchangers in legs to let extremities run cool while cores stay warm. Arctic foxes and reindeer can hold foot temperatures near freezing without frostbite.
• Crocodilians and alligators thermoregulate behaviorally with exquisite precision—basking, gaping, and shifting between sun and shade. During sudden ice-ups, alligators will rest with nostrils protruding through the surface, letting the pond freeze around their snouts while they wait out the cold.

Behavior often closes the loop: anatomy sets the possibilities, and daily choices deliver the outcome.

Key takeaways

• Thermoregulation is not binary. Many animals are heterotherms that switch strategies or target temperatures by time of day, season, or body region.
• Torpor and hibernation are powerful energy-saving modes. Birds (like hummingbirds and poorwills) and mammals (from bats to bears to echidnas) deploy them to survive food shortages, cold snaps, and even the ecological aftermath of disasters such as wildfires.
• Cold survival spans antifreeze proteins, metabolic depression, and outright freeze tolerance. Wood frogs can literally freeze and revive; turtles overwinter under ice by running on near-zero oxygen.
• Heat is sometimes a weapon. Bees cook hornets, pythons warm eggs, and mammals and birds selectively heat or cool body regions for performance or signaling.
• The ocean hosts partial warm-bloodedness. Tunas, lamnid sharks, billfishes, leatherbacks, and the opah all warm key tissues to unlock speed, deep foraging, and sensory advantages.
• Desert survivors often let body temperature climb on purpose to save water, backed by specialized heat exchangers and nighttime heat dumping.
• Behavior and sociality amplify physiology. Huddles, fanning, posture, and microhabitat choices can mean as much as internal chemistry.

What to watch next

• Medicine from hibernation: Understanding how hibernators resist clots, muscle wasting, and organ damage during low flow could refine human therapeutic hypothermia, stroke care, and even long-duration spaceflight.
• Genes and switches: Researchers are mapping the hormonal and neural circuits that flip animals into torpor. Pinpointing master regulators could reveal why some species can shut down so deeply while close relatives cannot.
• Climate resilience and risk: Flexible thermoregulation may buffer some species against heat waves and cold snaps, but it isn’t a universal shield. Desert birds can exhaust their water budget in unprecedented heat; bats in torpor are vulnerable to disturbances that force costly arousals; marine warm-bodied predators may face shifting thermal habitats.
• New sensors, clearer pictures: Miniature biologgers that record temperature, oxygen, and heart rate are opening windows into free-living animals. Expect surprises as we instrument more species through storms, fires, and floods.
• Conservation by microclimate: Protecting shade patches, burrows, hedgerows, and cool refuges can be as important as preserving food. Thermal landscapes are habitat.

FAQ

• What’s the difference between torpor and hibernation?
  Torpor is a short-term, reversible reduction in body temperature and metabolism that can occur daily. Hibernation is a prolonged seasonal state with deeper, longer bouts of torpor and extensive physiological remodeling.

• Do birds really “hibernate”?
  Classic hibernation is rare in birds, but the common poorwill can remain torpid for weeks. Many small birds use daily torpor to bridge cold nights or storms.

• Are bears true hibernators if their temperature doesn’t drop much?
  Yes. Despite modest temperature decreases compared to small hibernators, bears show profound metabolic depression, bone preservation, and long fasting—all hallmarks of hibernation.

• Can fish be warm-blooded?
  A few can. Tunas and certain sharks keep muscles, eyes, or brains warm using countercurrent heat exchangers. The opah maintains elevated temperatures across much of its body, making it functionally warm-blooded.

• How do frogs survive freezing solid?
  Freeze-tolerant frogs load tissues with cryoprotectants like glucose and urea, which limit ice formation inside cells. They stop breathing and heartbeats cease, then thaw and recover when temperatures rise.

• Do reptiles shiver to make heat?
  Most do not, but some pythons produce heat through muscular shivering to warm their eggs during brooding—a rare case of reptile endothermy.

• How do desert animals avoid overheating without water to spare?
  Many allow body temperature to rise during the day (adaptive hyperthermia), rely on specialized heat exchangers to protect the brain, and dump stored heat at night when air is cooler.

• Do birds sweat?
  Birds lack sweat glands. They cool by panting, gular flutter, seeking shade, and bathing; some species use evaporative cooling through richly vascular skin on the legs.

Source & original reading

Read the full feature that inspired this explainer at Ars Technica: https://arstechnica.com/science/2026/03/the-strange-animals-that-control-their-body-heat/