Why are humans hairless? / Energy considerations…

My interpretation is at end of post.

A review of basic animal physics and chemistry – Why are humans hairless?

Thanks to: The American Chemical Society Website.

Warm-Blooded or Cold-Blooded?

The most important adaptation is how animals regulate their body temperature. Animals can be either warm-blooded or cold-blooded.

Warm-blooded animals, which are mostly birds and mammals, need to maintain a relatively constant body temperature or they would suffer dire consequences. It doesn’t matter what the outside temperature is—they must maintain the same internal temperature. For us, the commonly accepted average body temperature is 98.6 °F (even though it may vary among individuals). Most other mammals range from 97 °F to 103 °F; birds have an average body temperature of 105 °F.

Cold-blooded animals do not maintain a constant body temperature. They get their heat from the outside environment, so their body temperature fluctuates, based on external temperatures. If it is 50 °F outside, their body temperature will eventually drop to 50 °F, as well. If it rises to 100 °F, their body temperature will reach 100 °F. Most of the rest of the animal kingdom—except birds and mammals—are cold-blooded.

In most instances, the size and shape of an organism dictate whether it will be warm-blooded or cold-blooded. Think about some large animals—elephants, whales, and walruses. Their volume is so large that relying on the outside environment to heat them up would be inefficient and would slow their response times, putting their survival at risk. For that reason, nearly all large animals are warm-blooded.

What about all the birds and mammals that are not large, such as mice and sparrows?  The other factor—body shape—comes into play here. Small warm-blooded animals tend to have a rounded shape, which ensures that the interior of an organism stays warm the longest time possible. Most cold-blooded organisms have either an elongated or a flat shape. If you look at a typical fish, their bodies tend to be flat when viewed head-on from the front. Snakes, lizards, and worms tend to be long and slender. These shapes ensure they can heat up and cool down rapidly.

Within a given species, animals tend to be larger in colder climates and smaller in warmer climates, an observation known as Bergmann’s rule. For example, whitetail deer in the southern part of the United States tend to have a smaller body size and less overall mass than whitetail deer in the far northern states.

There are exceptions but, overall, this rule holds true, for the following reason: As the volume of an object decreases, the ratio of its surface area to its volume increases. In other words, the smaller an animal is, the higher the surface area-to-volume ratio. These animals lose heat relatively quickly and cool down faster, so they are more likely to be found in warmer climates. Larger animals, on the other hand, have lower surface area-to-volume ratios and lose heat more slowly, so and they are more likely to be found in colder climates.

Generating Energy

Warm-blooded animals require a lot of energy to maintain a constant body temperature. Mammals and birds require much more food and energy than do cold-blooded animals of the same weight. This is because in warm-blooded animals, the heat they lose is proportional to the surface area of their bodies, while the heat they produce is proportional to their mass. This means that larger warm-blooded animals can generate more heat than they lose and they can keep their body temperatures stable more easily. Smaller warm-blooded animals lose heat more quickly. So, it is easier to stay warm by being larger. Warm-blooded animals cannot be too small; otherwise, they will lose heat faster than they can produce it.

This energy produced by warm-blooded animals mostly comes from food. Food represents stored chemical energy (potential energy), which is converted into other forms of energy within the body when the food is metabolized. Metabolism refers to the all of a body’s chemical reactions.

The metabolism of food within the body is often referred to as internal combustion, since the same byproducts are generated as during a typical combustion reaction—carbon dioxide and water. And like combustion reactions, metabolic reactions tend to be exothermic, producing heat.

For a warm-blooded animal, food is not just a luxury—it is a matter of life and death. If food is not available for energy, the body’s fat is burned. Once fat reserves are used up, death is imminent if a food source is not found. The smaller the warm-blooded animal, the more it must eat—relative to its body size—to keep its internal furnace stoked. That’s why most songbirds fly south for the winter.

On the other hand, cold-blooded animals require less energy to survive than warm-blooded animals do, because much of the energy that drives their metabolism comes from their surroundings. It is common to see turtles basking in the sun on rocks and logs. They are not trying to get a suntan, but rather are revving up their metabolism. The sun gives them an energy boost. Muscle activity in cold-blooded animals depends on chemical reactions, which run quickly when it is hot and slowly when it is cold (because the reacting molecules move faster when temperature increases). Some reptiles, such as the python, can go a year without eating, because they do not use food to produce body heat. And if they lie still, they use little energy, so they can afford to eat little.

Cold-blooded animals have a disadvantage compared to warm-blooded animals: There is a certain temperature below which their metabolism just won’t work. The reason is that all chemical reactions slow down as the temperature is lowered, so at low temperatures, all the chemical reactions in an organism slow down. You may notice that few cold-blooded animals are active in the winter, and the farther north you go, the rarer they become. By contrast, warm-blooded animals are present in a wider variety of environments and for a longer part of the year than cold-blooded animals.


For warm-blooded animals that don’t migrate, one way to survive the winter is to sleep through it. Hibernation is a great strategy that enables animals to conserve energy when food is scarce. During hibernation, body temperature drops, breathing and heart rate slows, and most of the body’s metabolic functions are put on hold in a state of quasi-suspended animation.

It is almost as if the warm-blooded animal becomes cold-blooded, as its body temperature drops considerably. But they are still alive, and they live off their fat reserves. Hibernation for extended periods of time is only accomplished by those animals that can store a great deal of body fat, such as bears, groundhogs, and chipmunks. A black bear loses 15%–30% of its weight while hibernating.

Cold-blooded animals hibernate, too. But they need to store less fat than warm-blooded animals because they require less energy. Turtles and frogs bury themselves in mud under lakes and ponds for up to six months at a time, and for all practical purposes, they appear dead. There are no external signs of life.

Keeping Warm

When it is cold outside, you put on more clothes. Your winter coat does not keep out the cold, but rather keeps in the heat. (Cold itself doesn’t exist—it is simply the absence of heat; see the article titled “Why Cold Doesn’t Exist,” on p. 10.) Birds and mammals also rely on insulation to prevent heat loss. The most effective insulation traps air, since air is one of the best insulators. Wool tends to be warm because its fibers are curled, effectively trapping air and keeping you (and sheep) warm. Birds fluff up their feathers when they want to stay warm, since fluffing introduces air.

For mammals without hair, insulation is accomplished by blubber, a thick layer of fat tissue which helps to insulate an animal’s body because fat does not transfer heat as well as muscle and skin. This blubber may be two feet thick in some whales! Whales, tuna, dolphins, and other warm-blooded marine animals also rely on another ingenious method to conserve heat… aquatic animals rely on a “countercurrent heat-exchange method,” in which the arteries that carry warm blood away from the heart are positioned directly against the veins that carry cool blood to the heart. So, the warmer blood leaving the heart through the arteries warms the cooler blood entering the heart through the veins.

Keeping Cool

When you get hot, what’s the first thing that happens? You start to sweat. The average adult has 3 million sweat glands. It’s not the sweating that cools you, but rather the evaporation of this sweat. Evaporation is an endothermic phase change, meaning it must absorb energy to occur. This energy is drawn from your body, making you cooler.

Anytime you lose energy, your body will feel cool. Evaporation requires energy because forces of attraction between water molecules—called intermolecular forces—need to be broken when water goes from a liquid to a gas. In liquid water, the molecules are close together and are attracted to one another. Evaporation requires energy because the intermolecular forces of attraction between water molecules in the liquid phase must be overcome when water goes from a liquid to a gas. The energy that goes into overcoming these attractive forces comes from your body.

Do animals sweat?  Most don’t, but some do. Dogs sweat mainly between the pads on the bottom of their paws. One notable exception is the American hairless terrier, which has sweat glands all over its body, illustrating the fact that fur tends to inhibit sweating because if the sweat can’t evaporate it doesn’t help in the cooling process.

The key to surviving in hot climates is not only to keep your body from overheating but also to prevent water loss. Animals that are adapted to desert life are not heavy sweaters—because water is scarce, they cannot afford to lose it by sweating. Also, a great deal of water is lost through breathing out, so desert animals expel dry air, reabsorbing the water in their breath before it has a chance to be expelled. This info impacts our ideas about hairlessness and sweating in Hominids and in Homo sapiens. It is obvious that warm-blooded animals in hot and cold desert environments RETAIN fur – they are not hairless – although properties and distribution vary. And, in HUMID tropical environments, the effectiveness of sweating – evaporation drops considerably.

Why and when did bipedal apes become hairless? In other words, did hairlessness, or reduced hairiness, come first, with “sweating” as a subsequent mechanism of heat regulation – compensation for the loss of hair or fur as the insulator / heat regulator? Evolution is full of trade-offs. Hair or fur is the “preferred” adaptation in warm-blooded land animals in ALL types of environments: In birds – feathers.

Proto-bird dinosaurs produced feathers before evolving full flight;  lightweight feathers are an “appropriate” insulator for a flying animal. The other “hairless” mammals (elephant, rhino, hippo) are massive (efficient in cold environments) – but each utilizes specific heat regulation strategies in adapting to hot environments. Very large “hairless” marine mammals store blubber – an effective and BOUYANT insulator.

So, what advantage would a lack of hair, fur, or feathers provide a bipedal ape?

CLUE: Modern humans easily store fat, which takes advantage of “excess calories” when available. This back up energy supply is essential for pregnant females in any environment. Growing fetuses and nursing infants require moms to generate lots of energy over extended periods of time.  A covering of fur or hair as insulation, was not nearly as critical as fat storage for “extended” development of big-brain babies – both pre and post birth.

Fat provides both insulation and fuel; more bang for the energy buck and a “positive” adaptation. It’s all about reproduction.

Of course, this line of reasoning demonstrates that female Homo sapiens cannot be ignored in  “logical” explanations that may aid in the untangling of human evolution… a bitter pill for males, who have utterly dismissed this “crazy” notion that females are equal players in evolution.







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