No living mammal is cold-blooded in the strict textbook sense, but several species come so close that the boundary between “warm-blooded” and “cold-blooded” starts to feel like a polite fiction. The naked mole-rat has been formally described in the scientific literature as an “operative poikilotherm,” meaning its body temperature drifts with its surroundings much the way a lizard’s does. Sloths, tenrecs, dwarf lemurs, and even some tiny marsupials also bend or temporarily abandon the warm-blooded rule that supposedly defines their class. The real story is that mammalian thermoregulation sits on a spectrum, and the cold end of that spectrum is stranger than most people realize.
The Naked Mole-Rat Is the Textbook Exception
If you had to name one mammal that comes closest to being cold-blooded, it would be the naked mole-rat (Heterocephalus glaber). These wrinkly, nearly hairless rodents live in underground colonies in East Africa, and researchers have found that they are unable to effectively maintain a stable body temperature through their own metabolism. Instead, their body temperature rises and falls with the temperature of their tunnels.1PubMed. Cold-induced changes in thyroid function in a poikilothermic mammal, the naked mole-rat That makes them functionally poikilothermic, the technical term for what most people mean by “cold-blooded.”
Naked mole-rats manage this because their underground habitat stays relatively stable in temperature year-round. They also huddle together in groups, essentially borrowing body heat from each other the way reptiles bask on warm rocks. Their metabolic rate is far lower than what you would expect for a mammal of their size. In a sense, they have traded the caloric expense of maintaining a constant body temperature for the energy savings that come with letting the environment do the work. For an animal that eats tough underground tubers in a resource-scarce environment, that trade-off appears to have paid off handsomely: naked mole-rats live for over 30 years, an astonishing lifespan for a rodent their size.
Sloths Run Their Metabolism in Reverse
Three-toed sloths (genus Bradypus) are not quite as extreme as naked mole-rats, but their thermal biology is bizarre by mammalian standards. Their body temperature swings with ambient conditions, sometimes fluctuating by several degrees over the course of a day. They eat extraordinarily little, averaging about 17 grams of food per kilogram of body weight per day, and even that meager intake is affected by temperature: they eat more when it is warmer.2PubMed Central. Sloths like it hot: ambient temperature modulates food intake in the brown-throated sloth (Bradypus variegatus) The likely reason is that the microbes in their gut ferment food faster at higher temperatures, so a warmer sloth can actually process more of what it eats.
What makes sloths truly odd is how their metabolism responds to temperature. Most mammals ramp up their metabolic rate when it gets cold, burning more energy to stay warm. Sloths appear to do something closer to the opposite: their oxygen consumption increases as the temperature rises, peaking in a comfortable zone before declining again at high temperatures. Researchers have described this as a reversal of the standard pattern seen in warm-blooded animals, and they believe it allows sloths to minimize energy expenditure across a wide range of conditions.3PubMed Central. The metabolic response of the Bradypus sloth to temperature This is not torpor or hibernation. It is a sloth’s everyday operating mode, and it looks more like what you would expect from a cold-blooded animal than from a fellow mammal.
Tenrecs Push the Boundary Even Further
Madagascar’s tenrecs are small, often spiny mammals that look like a cross between a hedgehog and a shrew. They are placental mammals, firmly in the mammalian family tree, yet their thermoregulatory flexibility exceeds that of most other mammals with variable body temperatures and approaches that of ectothermic reptiles.4PubMed Central. Physiological responses to hypoxia are constrained by environmental temperature in heterothermic tenrecs Tenrecs can allow their body temperature to drop dramatically, entering torpor readily and sometimes maintaining body temperatures that would be dangerous for most mammals.
One particularly revealing finding involves reproduction. In tenrecs, pregnant females become substantially more warm-blooded than males or non-reproducing females. Gestating females maintain body temperatures roughly 13 degrees Celsius higher than males under the same cool conditions and increase their resting metabolic rate enormously to do so.5The Company of Biologists (Journal of Experimental Biology). Increased homeothermy during reproduction in a basal placental mammal This suggests that the tenrec’s default state is something close to cold-blooded, and it cranks up the thermostat only when the developing embryos demand it. Once reproduction is over, the metabolic furnace dials back down. The implication is striking: for tenrecs, being warm-blooded is not a permanent condition but something they switch on when they need it.
Research on tenrec brown fat has added an evolutionary dimension to this story. Brown adipose tissue, the specialized heat-generating fat found in mammals, appears to have been functional in tenrecs before the broader expansion of consistently warm-blooded placental mammals. This has led some researchers to propose that the ability to generate heat through brown fat came first, and sustained warm-bloodedness evolved on top of it later, rather than the other way around.6Nature Communications. Brown fat in a protoendothermic mammal fuels eutherian evolution
Hibernation and Torpor Are Temporary Cold-Bloodedness
Even mammals that are reliably warm-blooded most of the year can temporarily abandon that status. Hibernation and torpor involve dramatic drops in body temperature, metabolic rate, and heart rate. During deep hibernation, some mammals let their core temperature fall to levels that would kill a human.
The arctic ground squirrel holds perhaps the most extreme record. During winter hibernation in Alaska and northern Canada, these squirrels allow their abdominal body temperature to drop below the freezing point of water, reaching average lows around minus 1.9 degrees Celsius, without actually freezing. Soil temperatures at the depth of their burrows can fall to minus 6 degrees Celsius. Remarkably, the squirrels’ blood shows no evidence of antifreeze molecules; they appear to supercool, staying liquid below zero through mechanisms that are still not fully understood.7PubMed. Freeze avoidance in a mammal: body temperatures below 0 degree C in an Arctic hibernator Periodically, they rewarm themselves with bursts of metabolic activity before sinking back into torpor, a cycle that repeats throughout the long arctic winter.
If ground squirrels represent hibernation in extreme cold, Madagascar’s fat-tailed dwarf lemur represents something even more conceptually jarring: hibernation in the tropics. This primate hibernates for about seven months of the year, even though winter temperatures in its habitat rise above 30 degrees Celsius. During hibernation, its body temperature passively tracks the ambient temperature, fluctuating between roughly 10 and 30 degrees Celsius with the daily cycle.8PubMed. Hibernation in the tropics: lessons from a primate The quality of the tree hole matters: animals in poorly insulated holes experience wider temperature swings, while those in well-insulated holes maintain steadier temperatures and periodically arouse.9PubMed. Physiology: hibernation in a tropical primate In captivity, hibernating dwarf lemurs have been shown to thermoconform across a range of about 10 to 30 degrees Celsius without arousing from torpor at all, passively riding the temperature wave for weeks or months.10Scientific Reports. On the modulation and maintenance of hibernation in captive dwarf lemurs
A hibernating dwarf lemur, with its body temperature drifting in lockstep with the air around it, is functionally indistinguishable from a cold-blooded animal for the better part of a year. That this is a primate, a close relative of humans on the mammalian family tree, makes the discovery all the more unsettling to the tidy warm-blooded/cold-blooded divide.
Some Mammals Bask Like Reptiles
Behavioral thermoregulation, using sunlight, warm surfaces, or social huddling to manage body temperature, is a hallmark of reptiles. But it turns out that some small mammals do the same thing, and they rely on it heavily.
The long-tailed planigale, one of the smallest marsupials in Australia, regularly basks during cold conditions. When researchers measured energy use during basking at an ambient temperature of 15 degrees Celsius (typical of winter nesting sites), the planigales reduced their resting energy expenditure by about 58 percent compared to what would be expected without solar heating.11PubMed. Basking behaviour in relation to energy use and food availability in one of the smallest marsupials That is an enormous energy saving for a tiny animal that cannot afford to waste calories.
Another small Australian marsupial, the stripe-faced dunnart, takes it a step further. Researchers found that torpid dunnarts emerged from cracks in the soil with body temperatures as low as 14.6 degrees Celsius and positioned themselves in the sun to rewarm, much the way a lizard sits on a rock at dawn.12PubMed. Torpor and basking in a small arid zone marsupial These findings suggest that basking is more widespread among small mammals than previously thought, particularly in species that use torpor. For these animals, solar energy partially replaces metabolic energy, blurring the line between the warm-blooded and cold-blooded strategies once again.
An Extinct Goat That Grew Like a Crocodile
The most dramatic case of a mammal crossing into cold-blooded territory comes from the fossil record. Myotragus balearicus was a goat-like bovid that lived on the Balearic Islands in the Mediterranean from the Pliocene into the Pleistocene, going extinct only after humans arrived. When researchers examined its long-bone histology, they found something unprecedented for a mammal: the bone was made of lamellar-zonal tissue throughout the cortex, a growth pattern previously seen only in ectothermic reptiles.13PubMed Central. Physiological and life history strategies of a fossil large mammal in a resource-limited environment
The bone microstructure tells a story. Myotragus grew slowly and flexibly, similar to modern crocodiles. It ceased growth periodically, presumably during lean seasons, and did not reach full adult size until around age 12, extraordinarily late for a mammal of its size. This developmental pattern is consistent with an animal that synchronized its metabolic needs with fluctuating food availability on a resource-poor island. In other words, Myotragus appears to have evolved a reptile-like growth and metabolic strategy to survive in an environment where there simply was not enough food to support typical mammalian metabolism year-round. When humans arrived and began hunting, the same slow growth and low reproductive rate that had kept Myotragus alive on a sparse island made it fatally vulnerable to predation.
Monotremes and the Roots of Warm-Bloodedness
The platypus and echidnas, the only surviving egg-laying mammals, offer a window into what early mammalian thermoregulation may have looked like. Early physiological studies found that monotremes have notably low body temperatures and metabolic rates compared to other mammals. The short-beaked echidna in particular can show large daily swings in body temperature and undergoes seasonal hibernation, leading some researchers to view it as a useful model for what a “proto-endotherm” might have been: an animal partway between cold-blooded and warm-blooded.14PubMed Central. Energy Homeostasis in Monotremes
The platypus has about twice the basal metabolic rate of an echidna, though their maximum metabolic output is similar. Neither animal runs as hot or as consistently as a typical placental mammal. Monotremes are unambiguously mammals, with fur, milk production, and all the other hallmarks, yet their thermoregulation sits closer to the cold-blooded end of the spectrum than almost any other mammalian group. They are a living reminder that warm-bloodedness was not an all-or-nothing switch in evolution but something that developed gradually, with plenty of intermediate stages along the way.
Broader evidence from bone histology and isotopic analysis suggests that endothermy in the mammalian lineage emerged in a stepwise fashion beginning in the Late Permian period and accelerating after the catastrophic Permian-Triassic mass extinction. Survivors of that extinction event show evidence of insulating body coverings and bone growth patterns consistent with higher metabolic rates, suggesting that the devastation may have favored lineages that could generate their own body heat.15Gondwana Research. The origin of endothermy in synapsids and archosaurs and arms races in the Triassic
Cold Limbs, Warm Core
Even mammals that maintain a warm core temperature do not keep their entire body at that temperature. Marine mammals are especially adept at regional heterothermy, allowing their extremities to cool dramatically while protecting their vital organs. Seals, for instance, have an elaborate vascular system in their flippers that can route blood through different pathways depending on need. Arterial blood heading to the flippers can pass through counter-current heat exchangers that recapture warmth before it reaches the skin, keeping heat in the core. Alternatively, blood can be shunted through skin capillaries and return through large superficial veins, carrying cold blood back toward the heart and effectively turning the flipper into a radiator.16PubMed. Selective brain cooling and its vascular basis in diving seals
The result is that a seal swimming in near-freezing water may have flippers that are only a few degrees above the water temperature while its brain and internal organs stay at a normal mammalian temperature. This is not cold-bloodedness, but it is a deliberate, controlled departure from uniform warmth. Whales, dolphins, and Arctic-dwelling land mammals like wolves and caribou use similar counter-current heat exchange systems in their legs, tails, and noses. The principle is the same one used by cold-blooded animals that selectively warm certain body parts, just running in reverse: mammals selectively cool certain parts to avoid losing too much heat overall.
Shrews That Shrink Their Own Brains
Small mammals face a unique thermoregulatory problem: they lose heat fast because of their high surface-area-to-volume ratio. Some shrews have arrived at a startling solution. In a process called Dehnel’s phenomenon, certain small mammals undergo reversible seasonal shrinkage of their skulls, brains, and overall body mass to reduce their metabolic demands during winter when food is scarce.17PubMed. Programmed seasonal brain shrinkage in the common shrew via water loss without cell death
The numbers are remarkable. In red-toothed shrews, individual braincases shrink by an average of about 15 percent heading into winter, then partially regrow by about 9 percent in spring. Overall body mass drops by roughly 18 percent before winter and then surges back up by over 80 percent the following spring.18Current Biology. Profound Reversible Seasonal Changes of Individual Skull Size in a Mammal This is not a cold-blooded strategy per se, but it is an adaptation driven by the same fundamental problem that pushes other mammals toward cold-blooded-like states: the staggering caloric cost of staying warm when you are small. By literally reducing the amount of tissue they need to fuel, shrews manage to survive winters that would otherwise be energetically impossible.
Why Climate Change Threatens the In-Between Strategies
Mammals that rely on torpor and hibernation have finely tuned their energy budgets to local conditions. Rising temperatures are now disrupting those budgets. Warmer winters can elevate the temperature inside hibernation dens, which shortens torpor bouts, increases the frequency of costly arousals, and depletes the fat reserves that hibernators depend on to survive until spring.19PubMed Central. Are Hibernators Toast? Global Climate Change and Prolonged Seasonal Hibernation For a mammal that has spent millions of years calibrating its survival strategy around a predictable winter, even a modest temperature increase can be the difference between making it to spring and starving in the den.
Modeling work on hibernating marsupials has shown that there is a temperature threshold above which hibernators arouse from torpor involuntarily, dramatically increasing their energy needs at a time when no food is available.20PubMed Central. Climate change and population persistence in a hibernating marsupial The irony is sharp: these mammals evolved flexible thermoregulation as a survival advantage in unpredictable environments, but the flexibility has limits. If the environment shifts too far too fast, the very strategy that kept them alive becomes a liability. Animals like the mountain pygmy possum in Australia, which depends on stable cold conditions for hibernation, are already showing signs of population stress as winters warm.
When Cold-Blooded Fish Become Warm-Blooded
If the question “are any mammals cold-blooded?” destabilizes the boundary from one side, a deep-sea fish called the opah destabilizes it from the other. The opah (Lampris guttatus) is the first fish known to maintain whole-body endothermy, generating heat through constant flapping of its large pectoral fins and retaining that heat via counter-current heat exchangers in its gills.21Science. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus Other fish, like tuna and some sharks, warm specific muscle groups or their eyes and brains, but the opah keeps its entire body warmer than the surrounding water. It is, for practical purposes, a warm-blooded fish living among cold-blooded relatives.
The opah matters here because it reveals that endothermy and ectothermy are not locked to particular branches of the tree of life. They are strategies, shaped by ecology and natural selection, that have been gained and lost and modified repeatedly across evolutionary history. Mammals inherited endothermy from their synapsid ancestors, and most have kept it running at full blast. But as naked mole-rats, sloths, tenrecs, dwarf lemurs, and even an extinct island goat demonstrate, being a mammal does not make warm-bloodedness mandatory. It makes it the default, and defaults, as any of these animals would tell you, are made to be broken.