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Understanding Temperature: Cold-Blooded versus Warm-Blooded Animals

Howard Glicksman


Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News is delighted to offer this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

the-designed-body4.jpgAmong the other dynamics of nature, the body must contend with heat (the transfer of energy from one object to another) and temperature (the random motion within an object or its internal energy). The body’s core temperature is directly related to how much heat it produces through metabolism, the sum total of all of its chemical reactions. The human cell is able to harness only about one-quarter of the energy released from the breakdown of complex molecules like carbohydrates, fats, and proteins. The remaining three-quarters is released as heat into the body. As with any working machine, the more active the body, the more heat it releases.

In addition to the heat released by its metabolism, the body’s core temperature is also directly related to how much heat it loses to, or gains from, its environment. Sit directly in the sun on a tropical island and your body will quickly gain a lot of heat. Go out at night on the frozen tundra wearing just a T-shirt and jeans and your body will quickly lose a lot of heat. The body must take control of its core temperature because if it isn’t just right, it can adversely affect enzyme function and the integrity of the plasma membrane and other cellular structures.

In my last few articles, I’ve shown that the body’s normal core temperature is set by the hypothalamus at 97o– 99oF (36o-37oC). Studies indicate that this temperature range is the one in which the enzyme systems of the body work best. Thyroid function contributes to the core temperature by setting the basal metabolic rate (BMR), which is how much heat the body generates at complete rest. But life is a dynamic process where to survive the body must stay active, releasing more heat while living within an environment where temperatures fluctuate. The hypothalamus receives data from the central thermoreceptors and keeps the core temperature at its set-point by using both voluntary means (shedding or donning clothing) and involuntary means (shivering or sweating).

These irreducibly complex systems use their natural survival capacity to keep the core temperature right where it should be so the enzyme systems within the cells can work at peak efficiency. Clinical experience teaches that if our earliest ancestors could not have kept their core temperature within the normal range they never could have survived long enough to reproduce. Since humans, like other mammals and birds, can control and keep their core temperature relatively high through internal processes, scientists consider them warm-blooded. In contrast, the core temperature of most insects, amphibians, reptiles and fish is dependent on their surroundings and so they are considered cold-blooded. This article will look at what it means to be cold-blooded and warm-blooded and what might be required for one to develop into the other as evolutionary biologists claim.

Humans, like birds and most mammals, are able to regulate their core temperature at a level that is usually above their surroundings, and sometimes lower than it as well. They accomplish this through increasing their cellular respiration and releasing more heat from their metabolism, altering blood flow in the skin, sweating, panting, shivering, and releasing heat by breaking down fat. In this way they are able to control their core temperature from within. They are therefore called endotherms (endo = within + therm = heat). Since they can keep their core temperature relatively stable, they are also known as homeotherms (homeo = same). The increased need for energy to accomplish this type of thermoregulation requires a high resting metabolic rate, so these organisms have a tachymetabolism (tachy = fast + metabol = to change). In general, birds and mammals are endotherms and homeotherms with a tachymetabolism and are called warm-blooded.

Most insects, reptiles, fish, and amphibians, are not able to maintain a regular core temperature from within, and are therefore more dependent on the temperature of their surroundings. They are therefore called ectotherms (ecto = outside + therm = heat). Since their core temperature is quite variable, they are also known as poikilotherms (poikilo = varied). In order to live within these temperature guidelines, these creatures do not need to provide themselves with as much heat energy as those that are warm-blooded. These creatures tend to have a lower resting metabolic rate or bradymetabolism (brady = slow). In general, insects, reptiles, fish, and amphibians are ectotherms and poikilotherms with a bradymetabolism and are called cold-blooded.

There are advantages and disadvantages to being either cold-blooded or warm-blooded. In particular, since the efficiency of chemical reactions in the cell is dependent on the core temperature, being warm-blooded allows for more activity in colder environments. Warm-blooded animals are, in general, able to forage for food faster and defend themselves better in a wider temperature range than cold-blooded animals. Additionally, warm-blooded animals can support highly-complex energy-dependent organs like the mammalian brain.

However, to maintain a core temperature that is often far higher than its environment, warm-blooded animals must use more of the energy they obtain from food as heat. This means that warm-blooded animals require much more food (often about five to ten times more) than cold-blooded animals to survive. Compared to cold-blooded animals, warm-blooded ones are nature’s equivalent to the gas-guzzling and energy-inefficient automobile, since they use so much energy to maintain their core temperature to keep their organ systems working properly. Cold-blooded ones are eco-friendly, energy efficient, and more in tune with their environment because they don’t need to use up as much fuel to keep their organ systems working properly.

Conventional scientific wisdom says that warm-blooded animals evolved from cold-blooded ones. Little else is said about how this evolutionary development could have taken place or what viable transitions between these two steps would look like. Converting a cold-blooded animal into a warm-blooded animal would be like converting a Model-T Ford into a Lexus. Instead of cranking the engine to start, sitting in a drafty vehicle, and moving in a herky-jerky motion from shifting gears, the modern driver electronically starts the engine from a distance, sits comfortably in a climate-controlled airtight vehicle, and enjoys smooth acceleration from the automatic transmission.

An Exercise in Critical Thinking

The more you understand what it takes for life to survive within the laws of nature, the more you realize how inadequate and simplistic the theories of evolutionary biologists are. Imagine an exercise in critical thinking: Given the facts of current biology, determine the challenges that face evolutionary biologists in explaining how cold-blooded animals evolved into warm-blooded ones. Consider these three questions and responses for the exercise.

(1) Whether cold or warm-blooded, all life forms, even bacteria and amoebae, have some sort of thermoregulatory mechanism. Since temperature is one of many physiological parameters that must be controlled to maintain life, shouldn’t evolutionary biologists have to describe each of these thermoregulatory mechanisms and how they became more sophisticated?

Each of these thermoregulatory mechanisms requires that the organism sense the change in temperature, decide what needs to be done, and then effect an adequate change in function to correct the situation. For example, when the core temperature of warm-blooded animals drops below the set-point, they can automatically increase their production of heat while at the same time limiting heat loss. Most cold-blooded animals, on the other hand, can only get warmer by lying out in the sun. How could such an irreducibly complex system have evolved while remaining functional and allowing for survival?

(2) One of the main differences between warm-blooded and cold-blooded organisms is that the former can generate more heat from their metabolism than the latter. It is important to note that when cold-blooded animals increase their level of activity, they give off more heat just like warm-blooded ones do. The key difference between them is that, in general, whether at complete rest or with activity, warm-blooded animals tend to give off more heat than cold-blooded ones. Wouldn’t you think that in trying to show how cold-blooded animals evolved into warm-blooded ones, evolutionary biologists would first need to explain the mechanism behind this phenomenon and the changes that must have taken place along the way?

In fact, it appears that not only do the cells of cold-blooded organisms have fewer mitochondria and so release less heat through cellular respiration, but the process of cellular respiration seems to be different as well. In the last few decades, scientists have shown that there are uncoupling proteins (UCPs)within the cells of most organisms, which, particularly in warm-blooded ones, seem to reduce the amount of energy their cells store as ATP and cause the release of more heat. Although thyroid activity is present in most invertebrates and vertebrates, it would appear that one of its unique functions in warm-blooded animals is to activate these UCPs and increase the production of heat. The production and control of thyroid hormone is irreducibly complex and requires natural survival capacity because having too little or too much of it is incredibly harmful. This is a second very important point that should be addressed by evolutionary biologists before claiming to understand how cold-blooded animals evolved into warm-blooded ones.

(3) If, to keep the enzyme systems that make up the metabolism in their cells working at peak efficiency, warm-blooded animals must maintain their core temperature within a certain range to survive, how do cold-blooded animals stay alive at these lower temperatures? In other words, before claiming to know how cold-blooded animals evolved into warm-blooded ones, don’t you think evolutionary biologists should address this other obvious difference in basic cellular function?

It appears that, when it comes to very important metabolic reactions, most cold-blooded animals have several different enzyme systems in place that are able to work at different temperatures to allow for survival. This means that, in general, when it comes to the genes that code for important metabolic processes, the cells of cold-blooded organisms usually have more than warm-blooded ones. This would mean that while cold-blooded animals were evolving into warm-blooded ones they would have been removing the genes for these various important metabolic processes at each step along the way. How the intermediate organisms could have survived during this transition — involving a loss of metabolic flexibility and the development of increased heat production along with thermoregulatory control — is another conundrum that evolutionary biologists need to address.

As biologist Ann Gauger has pointedly noted here at Evolution News, “Evolutionary biology’s explanatory power is inversely proportional to its rigor.” I maintain that if thoughtful adults were educated not just about how life looks, but how it works to survive within the laws of nature, views about evolution would look very different from how they do today.

Image credit: Jim Skea [CC BY-SA 2.0], via Wikimedia Commons.

Howard Glicksman

Dr. Howard Glicksman practices palliative medicine for a hospice organization. He is the author of “The Designed Body” series for Evolution News, and is currently working on a book on the same topic with Steve Laufmann. The book is slated to be released by Discovery Institute Press in the fall of 2022.



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