How is Temperature Regulated in Mammals?

The emperor penguin (Aptenodytes forster) is over a metre tall and weighs about 35 kg, roughly half the weight of an average adult man. It lives in the Antarctic, one of the coldest places on Earth. Surprisingly, it is a warm-blooded animal and, even more surprising, it breeds in the middle of winter, when temperatures are at their lowest. At the beginning of winter, the penguins leave their feeding grounds in the sea and walk to their breeding grounds, up to 100 kilometres away. There, the female lays one egg, gives it to the male and then promptly returns to the open sea to feed.

The temperature plummets to -40°C but the males have no choice but to stand on the ice for the next two months, incubating the egg between their feet and a fold of fat. If the egg accidentally rolls on to the ice, the male must gather it up again immediately, or the chick will die. Despite these dreadful conditions, the core body temperature of the male penguin remains at 38 °C, about a degree higher than that of most mammals. The males fast for two months while they incubate their egg and lose as much as 15 kg of body weight. Like their fathers, the chicks instinctively huddle together to reduce their heat loss.

^{The life-cycle of the emperor penguin. Zina Deretsky, Public Domain}

The penguins maintain their body heat by respiring stored fat and by huddling together. Penguins are roughly cylindrical and snuggling up to their neighbours greatly reduces the surface area that is directly exposed to the freezing air. In an impressive example of social co-operation, the birds take it in turns to stand on the outside of the crowd. Scientists estimate that without this huddling behaviour, the males would not have enough fat to power the process of homeostasis that keeps them warm, let alone make the long return journey back to the open sea.

Temperature Control and Homeostasis

Life can exist at almost all the temperatures encountered on the Earth’s surface, from the extremes of cold that occur at the polar regions to the extremes of heat that occur in hydrothermal vents. This is possible because it is the internal, not the external temperature that is important to an organism. Animals that can withstand extremes of external cold, such as the emperor penguins, do so only because they maintain their body temperature inside this ‘operating range’, despite the outside temperature.

So, why do animals die when the external temperature exceeds a certain limit?

A common theory says that some vital enzymes become denatured, but probably not the whole story. It is more likely that death from excess heat is due to a metabolic imbalance, caused by enzymes working at different rates. As the temperature increases some enzymes work faster than others: some intermediate chemicals build up while some vital end-products become scarce.

Life can survive in a metabolically inactive dormant state at sub-zero temperatures. Sperm, eggs and even embryos kept in liquid nitrogen at -196 °C can be thawed and used successfully in infertility treatment. However, only a few organisms remain active when their body temperature drops below freezing. In most organisms, very low temperatures cause damaging ice crystals to form inside and also between cells.

If ice crystals damage the cellular structure, how do some cells survive being frozen in liquid nitrogen? It is because it takes time for ice crystals to develop. If you immerse a sample in liquid nitrogen, it freezes so quickly that ice crystals do not form. When the cells and organelles are thawed they are able to function normally as they have not been damaged.

What are the mechanisms of temperature control in animals?

Different animals control their body temperature in different ways. Many animals (fish, insects, amphibians, etc.) have a body temperature that is more or less the same as their surroundings. Mammals and birds usually maintain a constant body temperature, despite the external temperature. These two categories of animal are commonly described as cold-blooded and warm-blooded. But these terms are not really satisfactory: a cold-blooded animal, for example, is often not cold. In bright sunshine on a hot day, the core temperature of a reptile or amphibian may be higher than that of a mammal. Nor are the terms poikilothermic (cold-blooded) and homoiothermic (warm-blooded) strictly accurate. The word poikilos means changeable, while homeo means constant. However, many cold-blooded animals, especially aquatic ones, have a remarkably constant body temperature owing to the stability of their surroundings. Hibernating mammals, on the other hand, show a dramatic drop in their core temperature during their dormancy. To overcome these problems, two other terms can be used: ectotherm (heat from outside’) and endotherm (heat from within’).

During cold weather many animals increase their thermal inertia by huddling.

^{Birds huddling together. Blalonde, CC BY-SA 4.0}

Ectothermic organisms can control their body temperature only by changing their behaviour. All animals except mammals and birds are ectotherms. Endothermic organisms maintain a stable core body temperature using physiological and behavioural means. Mammals and birds are the only endotherms.

Thermoregulation in Ectotherms

Ectothermic animals thermoregulate (i.e control their body temperature) with difficulty. Aquatic ectotherms, such as fish, usually have the same temperature as their surroundings. Their gills, which have a large surface area to collect oxygen, are a liability in terms of heat loss because they carry large amounts of blood to within a few micrometres of the water. Airbreathing ectotherms, such as reptiles, thermoregulate more successfully. These organisms do not have the fine physiological control over their temperature practised by birds and mammals, but they manage to control their body temperature to a large extent by their behaviour.

The Peruvian mountain lizard is a classic example. This reptile lives in an environment where the days are hot but the nights are very cool. After a cold night, the lizard’s body temperature is well below the optimum required for activity. Its movement is sluggish and it cannot hunt or avoid predators. So, early in the morning, the lizard emerges from its burrow and basks in the sun until its body temperature reaches about 35°C. It is then fully active ad goes off in search of food. As the day wears on and gets hotter, the lizard seeks shade to avoid overheating.

^{The Peruvian mountain lizard. Image by abduljaleel1153 from Pixabay}

Thermoregulation in Endotherms

Endotherms regulate their body temperature so that it stays at a constant level. In the paragraph that follows, I will explain the mechanisms of thermoregulation in the mammal, using the example of a human.

Humans and heat

Some animals, such as polar bears, are obviously adapted to withstand the cold. In contrast, humans almost certainly evolved in Africa, and, as a species, we are adapted to a warm climate. We have a thin covering of insulating fat and our body hair is sparse. Most hairs are tiny and no use for insulation. We are one of the few animals to be covered in sweat glands, and our skin can make melanin, a dark pigment that blocks out harmful ultraviolet light.

It seems that we were able to spread to the colder areas of the planet only because of our ability to control our environment. We could build shelters, make clothes and control fire. These skills more than made up for the fact that we had few physical features to allow us to cope with the cold. The core temperature of the human body remains reasonably constant at around 37 °C. It can fluctuate by a degree or so (more during fever) but it is generally very stable. In a thermal image of an adult male, showing the definition of the body core; which includes the trunk, the head and the upper part of the arms and legs. The heat is produced inside the body as a by-product of metabolic reactions. Heat production occurs throughout the body, and is especially high in working muscles. It is often stated that the liver is a particular source of heat, but tests have shown that the temperature of blood in the hepatic vein is no higher than in other vessels, suggesting that heat production occurs more generally in the organs of the body. Since body heat is a by-product of metabolism, the amount of heat produced depends on the metabolic rate. This can be increased by doing more exercise and by the secretion of hormones such as thyroxine or adrenaline.

^{Thermographic image of a man. NASA/IPAC, Public Domain}

However, the principal way we control our body temperature is by increasing or decreasing heat loss to the environment, according to our needs. In humans, the basic mechanism that underlies temperature control, or thermoregulation, involves part of the brain called the hypothalamus. This acts as a thermostat. It can detect the temperature of the blood that passes through it and, if the temperature of the blood increases or decreases even slightly, the hypothalamus initiates corrective responses such as sweating or shivering.

When we encounter a particularly warm or cold environment, temperature receptors in the skin inform the hypothalamus. They also stimulate the higher, voluntary, centres of the brain. This means that we ‘feel’ hot or cold and decide to do something about it such as changing position, changing our clothing or turning the heating up or down. Often, this behavioural response corrects the situation without the need for any physiological response.

Before I look at temperature control in more detail, I need to quickly explain a little about the processes of heat transfer, and to study the structure of the human skin (this will be done in my next post)

ABOUT HEAT: Where does it come from? Where does it go?

For any organism to maintain a stable body temperature, its heat loss must equal its heat gain. Endotherms such as ourselves, which live in a temperate climate, produce their own internal body heat to keep warm but also control the amount of heat they lose to the environment. Heat can be gained or lost in four different ways: conduction, convection, evaporation, and radiation.

Conduction involves the transfer of heat between two objects that are in contact with each other. Heat is always conducted from a region of higher temperature to a region of lower temperature. When you sit on a cold seat, for example, heat is conducted from your body into the seat, until both are approximately the same temperature.

The efficiency with which a material conducts heat is called its thermal conductivity. Different materials have different conductivities. Air has a low thermal conductivity, water has a much higher one. A clothed human walking in an environment that is 15°C can maintain body temperature comfortably. If immersed in water at that temperature, it would not be long before his or her core temperature dropped and hypothermia began to set in. The heat loss into the water would be much greater – water can ‘draw out’ heat approximately 25 to 30 times faster than air at the same temperature.

Materials with a low thermal conductivity are very good insulators. Animals with fur keep warm largely by trapping air between the hairs. Humans rely more on fatty (adipose) tissue for insulation. This has a lower thermal conductivity than other body tissues and so conducts heat more slowly to the surroundings.

Convection is heat transfer due to currents of air or water. A person immersed in cold, absolutely still water would be able to heat up the water immediately next to the skin, and could reduce the heat loss to some extent by not moving. However, this situation does not happen in real life. For a person who has fallen into the sea or a river, there are usually strong currents that continually move the water over the skin, causing heat to be lost quickly by convection. Similarly, fast-moving air causes greater heat loss by convection. You have probably heard of the wind chill factor. A cold windy day feels a lot colder than a cold still day, even when the air temperature is the same. In air, our clothes reduce heat lost by convection by trapping a layer of air next to the skin. In water, divers reduce the risk of heat loss by convection by wearing wet suits or dry suits.

Most divers wear wet suits, even in tropical waters, because the body loses heat rapidly to any water that is below 29 °C. The wet suit traps a layer of water between the skin and the rubber. This warms up quickly as a result of conduction from the skin. The warm water layer cannot escape and so further heat loss is slow. For very cold water, dry suits are available. These trap a layer of air next to the skin. This provides even better thermal insulation.

Evaporation is the change in state from a liquid to a gas. The evaporation of water uses up a large amount of energy, and this is known as the latent heat of evaporation. What this means for everyday life is that evaporation of water from a surface has a great cooling effect. Anyone who has ever stepped out of the bath and stood in a draught will have felt the power of this cooling. It is also why sweating is usually an effective way of losing heat when we get too hot. Even hot air – such as from a hair dryer – can have a cooling effect provided that the skin is wet so that evaporation can take place.

Radiation is the loss of infrared heat into the surroundings. A human sitting in a room at 20°C radiates heat into the surrounding air, particularly from the exposed skin on the head, neck and hands. Under normal situations, at rest at a comfortable room temperature, most of our heat loss is due to radiation. The heat that we radiate is in the infrared range. This is why infrared cameras can be used to search out humans and other warm-blooded animals from their colder surroundings.

In conduction, convection and evaporation, heat transfer occurs as a result of the movement of molecules. Radiation is fundamentally different: it does not depend on molecular movement. This explains why radiated heat can pass through a vacuum.

Conclusively, with everything I have so far discussed, I hope I have been able to do justice to how temperature is being regulated in mammals. In my next post, I plan to look into studying the structure of the human skin in relationship with temperature regulation. I look forward to your comments and reviews. Remember to always stay safe. Thanks.