Most animals on Earth either cannot sweat or sweat so little that it barely matters for cooling. Humans are genuinely unusual: our skin carries roughly ten times the density of eccrine sweat glands found in our closest primate relatives, making us one of the few species that relies on sweating as a primary defense against overheating. The rest of the animal kingdom has evolved an impressive and sometimes bizarre toolkit of alternatives, from rapid throat vibrations in owls to deliberate defecation on one’s own legs in storks. How each species stays cool depends on its anatomy, habitat, and evolutionary history, and many animals combine several strategies at once.
Why Humans Are the Exception
Mammals generally have two types of sweat glands: apocrine and eccrine. Both can contribute to thermal cooling, and research has shown that apocrine glands, once dismissed as “primitive,” function comparably to eccrine glands in furred mammals.1PubMed. The evolution of sweat glands But what sets humans apart is the sheer number of eccrine glands spread across nearly hairless skin. Compared to chimpanzees and macaques, whose eccrine gland densities are strikingly similar to each other, human eccrine gland density is about ten times higher.2PubMed Central. Comparative evidence for the independent evolution of hair and sweat gland traits in primates This expansion appears to have evolved exclusively in the human lineage, likely hand-in-hand with the loss of thick body fur and the shift to walking upright. Natural selection has shaped the ratio of eccrine to apocrine glands and their distribution across the body differently in different primate lineages, reinforcing the idea that sweating capacity is not a default mammalian trait but a specialized adaptation.3PubMed. The evolution of eccrine sweat glands in human and nonhuman primates
Which Animals Don’t Sweat
The list is long. Dogs and cats have sweat glands only on their paw pads, which do almost nothing for whole-body cooling. Birds, reptiles, and amphibians lack sweat glands entirely. Pigs are a commonly cited example: despite having structures in their skin that look like apocrine sweat glands, pigs do not appear to sweat in any meaningful way. Even when overheated, the moisture lost through pig skin is small enough to be explained entirely by passive evaporation driven by warmer skin, not active glandular secretion.4Nature. Evaporative Cooling in the Pig Elephants similarly lack functional sweat glands. Horses and some livestock do sweat, but they are exceptions among mammals, not the rule. For the vast majority of animals, staying cool means doing something other than sweating.
Panting and Gular Flutter
Panting is the most familiar alternative to sweating, used by dogs, many ungulates, and birds. It works by rapidly moving air across wet surfaces in the mouth, throat, and upper airways, evaporating water and carrying heat away. A panting animal breathes faster but takes shallower breaths, which keeps the airflow concentrated in the upper respiratory tract rather than deep in the lungs. This matters because over-ventilating the lungs would wash out too much carbon dioxide from the blood, disrupting the body’s acid-base balance.5PubMed. Mechanisms for the control of respiratory evaporative heat loss in panting animals Birds face the same challenge and many species have evolved ways to keep panting shallow, reducing the effective dead space in their respiratory tract to roughly 15% of its total volume, so oxygen and carbon dioxide exchange stays balanced even as airflow ramps up.6PubMed. Panting and acid-base regulation in heat stressed birds
Some birds go further with a trick called gular fluttering: rapidly vibrating the throat and the thin, moist skin of the gular pouch (the fleshy area under the beak). This drives evaporative cooling with very little extra metabolic effort. In small owls, activating gular flutter boosted evaporative heat loss by 44 to 100%, while metabolic rate increased by less than 5%.7PubMed. Avian thermoregulation in the heat: metabolism, evaporative cooling and gular flutter in two small owls Nightjars, which are active in hot environments at dusk and dawn, have pushed this even further. At extreme air temperatures above 50°C, nightjars dissipated heat equivalent to more than 500% of their own metabolic heat production through gular fluttering, all while keeping metabolic rate within about 20% of resting levels.8PubMed. Avian thermoregulation in the heat: efficient evaporative cooling in two southern African nightjars That is an extraordinarily efficient cooling system, arguably more effective per unit of energy spent than human sweating.
Built-In Radiators
Many animals shed heat not through evaporation but through radiation and convection, using body parts that are thin, well-supplied with blood vessels, and exposed to moving air. The toco toucan’s bill is one of the most dramatic examples. By adjusting blood flow into its oversized beak, a toucan can use the bill as a controllable radiator, dumping body heat into the surrounding air. Relative to the bird’s size, the bill rivals an elephant’s ears in its ability to radiate heat.9PubMed. Heat exchange from the toucan bill reveals a controllable vascular thermal radiator A broader look across bird species shows that bill heat exchange can range from about 2% to as much as 400% of basal heat production, depending on the species and conditions. Bills can partly substitute for evaporative water loss at high temperatures, which is a significant advantage in dry habitats.10PubMed. The evolution of the avian bill as a thermoregulatory organ
Mammals use the same principle with different body parts. The fennec fox, the tiny desert carnivore with famously oversized ears, dilates blood vessels in its ears and legs once the surrounding air approaches body temperature. This increases its thermal conductance, letting heat flow outward without requiring any evaporative cooling.11Journal of Zoology. Thermoregulation and metabolism in a small desert carnivore: the Fennec fox (Fennecus zerda) (Mammalia) Jackrabbits do the same with their long ears, and elephants rely heavily on their enormous, thin-skinned ears to radiate heat. In water, dolphins manage heat exchange seasonally through their flippers and flukes: in summer, these appendages maintain uniformly high heat-flow values, acting as continuous radiators, while in winter they restrict blood flow to conserve warmth, with thicker blubber lowering overall heat loss.12SpringerLink (J Comp Physiol B). Seasonal patterns of heat loss in wild bottlenose dolphins (Tursiops truncatus)
Selective Brain Cooling
For animals like gazelles, sheep, and cats, overheating the brain is the most dangerous consequence of high body temperature. These animals have evolved a specialized piece of vascular plumbing called the carotid rete: a dense network of small arteries carrying blood toward the brain, surrounded by a sinus filled with cooler venous blood returning from the nasal passages. Water evaporating from the moist surfaces inside the nose cools that venous blood well below arterial temperature, whether or not the animal is panting. As arterial blood passes through the rete, heat transfers into the cooler venous blood, so the blood reaching the brain can be more than 1°C cooler than blood elsewhere in the body.13Conservation Physiology. Body water conservation through selective brain cooling by the carotid rete: a physiological feature for surviving climate change? This has been described as one of the most effective selective brain cooling mechanisms among land vertebrates.14PubMed. Physical and computational fluid dynamics models for the hemodynamics of the artiodactyl carotid rete
The practical upshot is that these animals can let their overall body temperature climb during heat exposure without frying their brains. The brain stays protected even as the rest of the body runs a mild fever. This also conserves water: rather than sweating or panting heavily to keep every tissue cool, the animal tolerates a warmer body and spends its cooling budget only on the organ that cannot afford to overheat.
Self-Wetting Strategies
Some animals create their own evaporative surface when they lack the glands to do it automatically. Red kangaroos lick their forearms when heat-stressed, spreading saliva over skin that sits above a dense network of superficial blood vessels. The evaporation of that saliva cools the blood flowing through the forearms, which then circulates back into the body core.15Comparative Biochemistry and Physiology Part A: Physiology. Forelimb blood flow and saliva spreading in the thermoregulation of the red kangaroo, Megaleia rufa Rats do something similar, spreading saliva across their fur and skin in proportion to rising temperatures.16PubMed. Severe impairment of heat-induced saliva-spreading in rats recovered from lateral hypothalamic lesions
Storks take self-wetting to a place that seems deeply unappealing: they defecate on their own legs. This behavior, called urohidrosis, has long been assumed to cool the blood supply in the legs, but only recently has the effect been quantified in wild birds. In breeding white storks, urohidrosis reduced leg surface temperature by an average of about 4.4°C, with drops as large as 6.7°C.17Ecosphere. Keeping cool with poop: Urohidrosis lowers leg surface temperature by up to 6°C in breeding White storks Vultures do the same thing. It is not elegant, but it works.
Wallowing, Mud, and Elephant Skin Engineering
Pigs wallow in mud and water not because they enjoy being dirty but because they have essentially no other option. With no functional sweating and limited panting ability, external wetting is a pig’s primary thermoregulatory tool.4Nature. Evaporative Cooling in the Pig Water buffalo, rhinos, and wild boar use the same approach. Mud has the added advantage of drying more slowly than plain water, extending the cooling window and providing sun protection.
Elephants, which also lack sweat glands, take wallowing a step further thanks to the unusual architecture of their skin. African elephant skin is deeply sculpted with wrinkles and crevices arranged in geometric patterns. These features are not just cosmetic: the textured surface holds five to ten times more water than a flat surface of equal area, acting like a sponge after the animal bathes or sprays itself.18Journal of Zoology. Surface sculpturing and water retention of elephant skin African elephants have more deeply sculpted skin than Asian elephants, retaining significantly more moisture, which aligns with the African species’ adaptation to hotter, drier habitats. The cracks that characterize African elephant skin turn out to form through a mechanical process: as the outermost layer of dead skin cells grows outward over curved underlying tissue, it stretches and eventually fractures in a brittle failure, creating the network of channels that traps water so effectively.19Nature Communications. Locally-curved geometry generates bending cracks in the African elephant skin
The Hippo’s Red Secretion
Hippos are often said to “sweat blood,” but what they secrete is neither sweat nor blood. Hippo skin produces a viscous, initially colorless fluid that turns red and then brown as it is exposed to air. The pigments responsible, called hipposudoric acid and norhipposudoric acid, serve at least two functions beyond any thermoregulatory role. They absorb strongly in the UVA and UVB range, acting as a built-in sunscreen, and they have potent antibiotic properties that may help protect skin wounds in the murky, bacteria-laden water hippos live in.20PubMed. Probing the molecular and electronic structure of norhipposudoric and hipposudoric acids from the red sweat of Hippopotamus amphibius: a DFT investigation Whether this secretion provides meaningful evaporative cooling is less clear; what is clear is that it is produced by specialized glands unrelated to either eccrine or apocrine sweat glands. Hippos rely primarily on their semi-aquatic lifestyle, spending most daylight hours submerged, to manage body heat.
Letting the Thermostat Float
One of the more counterintuitive strategies is simply allowing body temperature to rise. Camels are famous for this. Rather than spending water to keep body temperature locked at a set point, camels let their core temperature swing widely over the course of a day. Research on free-ranging dromedaries shows that bulls in rut start the day about 0.6°C cooler than at other times and let their daily maximum climb about 0.45°C higher, expanding the total daily cycle.21PubMed Central. Strategic (adaptive) hypothermia in bull dromedary camels during rut; could it increase reproductive success? This pattern was first described decades ago in water-deprived camels: by absorbing heat during the day and radiating it at night when the desert cools, they can avoid sweating away water they cannot afford to lose. The same broad strategy, called adaptive heterothermy, shows up in various desert-adapted mammals and even some birds.
Camels and other large ungulates also conserve water through nasal heat exchange. As exhaled air passes through the nasal passages, the cooler tissue absorbs heat and causes water vapor to condense before it leaves the body. In giraffes, exhaled air can be 7 to 13°C below core body temperature depending on ambient conditions, recovering an average of about 56% of the water that would otherwise be lost if air were exhaled at body temperature. Across several species of wild and domestic ungulates, nasal water recovery ranged from about 24 to 58%.22Elsevier / ScienceDirect (Respiration Physiology). Nasal heat exchange in the giraffe and other large mammals This is not a cooling mechanism per se, but it makes panting and other respiratory cooling strategies more sustainable by reclaiming a significant fraction of the water they cost.
Going Underground and Shutting Down
Behavioral strategies are often the first line of defense, especially for small ectotherms that cannot generate enough airflow or blood-flow manipulation to shed heat physiologically. Desert lizards are a good case study. The great desert skink of central Australia spends only about 4% of its time active on the surface, primarily at dusk. Its burrow system provides an extraordinary thermal buffer, reducing temperatures by roughly 40°C compared to the baking surface above, while maintaining humidity near 100%.23PubMed. Under the weather?-The direct effects of climate warming on a threatened desert lizard are mediated by their activity phase and burrow system For these animals, the burrow is the thermoregulatory organ. Many desert rodents, snakes, and invertebrates follow the same playbook: avoid the heat entirely by being underground during the day and active at night.
Some animals take avoidance to an extreme by entering estivation, a state of metabolic suppression triggered by heat, drought, or food scarcity. Estivating ectotherms can dramatically reduce their metabolic rate without necessarily lowering body temperature, which conserves energy reserves and reduces water loss through breathing.24Comprehensive Physiology. Metabolic Flexibility: Hibernation, Torpor, and Estivation Lungfish, certain snails, some frogs, and a handful of mammals (like the Malagasy fat-tailed dwarf lemur) all use estivation to ride out periods when staying active would cost more water and energy than the environment can support.25PubMed Central. Aestivation in Nature: Physiological Strategies and Evolutionary Adaptations in Hypometabolic States
Amphibians, Insects, and Other Edge Cases
Amphibians occupy a strange position in this story. Their skin is highly permeable to water in both directions, which means they lose moisture rapidly in dry conditions but can also absorb it when it is available. Some tree frogs in the genera Phyllomedusa and Litoria have found a partial solution: they secrete lipids from cutaneous glands and wipe these over their entire body using their legs, creating a waxy coating that dramatically reduces water loss.26Comparative Biochemistry and Physiology Part A: Physiology. Review Cutaneous adaptations to water balance in amphibians For most frogs, though, the permeable skin that provides free evaporative cooling also means they are tethered to moist habitats. Their cooling strategy and their vulnerability are the same trait.
Insects face a different set of constraints because of their small size, which gives them a very high surface-area-to-volume ratio. Most insects rely on behavioral thermoregulation: seeking shade, adjusting posture, or timing activity to cooler hours. But honeybees are a notable exception. During flight at high temperatures, honeybees greatly increase evaporative heat loss, and many bee species use “tongue-lashing,” repeatedly extending the tongue to spread a thin film of liquid that evaporates quickly. Flying honeybees also reduce their metabolic rate as the air warms, generating less internal heat as an additional thermoregulatory strategy.27Oxford Academic. Mechanisms of Thermoregulation in Flying Bees
When These Strategies Hit Their Limits
Every cooling strategy has a ceiling, and rising global temperatures and humidity are pushing some species toward theirs. Evaporative cooling, whether through panting, gular flutter, or cutaneous water loss, becomes less effective as humidity rises because the air can hold less additional moisture. Recent experimental work on hornbills exposed to high-humidity conditions found that these birds lost coordination at a wet-bulb temperature of about 32°C, at which point they could no longer keep their body temperature below the environmental temperature.28PubMed Central. Increases in humidity will intensify lethal hyperthermia risk for birds occupying humid lowlands This is the same fundamental limit that threatens humans during extreme humid heat waves: when the air is already saturated with water vapor, evaporation stalls and body heat has nowhere to go.
Species that depend on behavioral thermoregulation face a different kind of pressure. A burrow that currently buffers temperatures by 40°C still provides relief, but if surface temperatures rise enough, the window of time available for foraging on the surface may shrink to the point where an animal cannot meet its energy needs. The carotid rete that cools an antelope’s brain by a degree or two may not keep pace if the baseline body temperature that system starts from keeps climbing. Animals that wallow depend on water availability, which is declining in many regions. The diversity of cooling strategies across the animal kingdom is a testament to evolutionary ingenuity, but each strategy was calibrated to a climate that, in many places, no longer exists in its historical form. How quickly species can adjust their physiology or behavior, or shift their ranges, will shape which lineages thrive and which face lethal heat exposure in the decades ahead.