Warm-blooded And Cold-blooded Animals: What Is The Difference?

Lizards are ectothermic animals that often bask in the sun to keep its body warm.
Lizards are ectothermic animals that often bask in the sun to keep its body warm.

Organisms are classified into two broad categories of warm-blooded animals and cold-blooded animals based on their ability to regulate their body temperature in relation to the surrounding temperature. Warm-blooded animals are mostly mammals and birds as they maintain relatively constant body temperature. Most birds have an average body temperature of 105 degrees F while the body temperature in most mammals ranges between 97 degrees F and 103 degrees F. Humans have an average body temperature of 98.6 degrees F, although the temperature can vary among individuals. On the other hand, cold-blooded animals are unable to maintain constant body temperature. They rely mainly on the external environment for heat, which means that their body temperature fluctuates based on external temperatures. For example, if the surrounding is 45 degrees F, their body temperature will gradually drop to 45 degrees F, and when the external temperature rises to 90 degrees F, their body temperature will also increase progressively to 90 degrees F.

Dealing With Heat And Temperature

Animals have to deal with various dynamics of nature. Their bodies have to contend with temperature (random motion within an object) and heat (transfer of energy to another object). Apart from external influence, the body’s temperature is also tied to the amount of heat it can produce through metabolism, which is the sum of all chemical reactions in the body. For example, the human cell harnesses about 25% of the energy released after the breakdown of carbohydrates, proteins, and fats. The remaining 75% is released as heat within the body. Apart from metabolism, the body’s temperature is also influenced by the amount of heat it loses or gains from the environment. For example, basking in the sun on a tropical island leads to an increase in heat while sitting in little to no clothing in the tundra will lead to a considerable loss of heat.

Warm-blooded Animals

Most mammals and birds regulate core temperature to a level that is above that of the surroundings or, in some cases, below external temperature. That is accomplished through cellular respiration and the release of heat through their metabolism, sweating, altering the flow of blood in the skin, shivering, panting, and breaking down fat to release heat. Through such mechanisms, animals can control their core body temperature from within. Such organisms are, therefore, also known as endotherms (endo meaning within + therm meaning heat). Their ability to keep their core body temperature relatively stable also means that they can also be referred to as homeotherms (homeo meaning the same + therm meaning heat). An increased need for energy is required to achieve this type of thermoregulation. Such organisms, therefore, have a high resting metabolic rate, which is called tachymetabolism (tachy meaning fast + metabol meaning to change). In general, mammals and birds that are endotherms and homeotherms and have tachymetabolism are referred to as warm-blooded animals.

Regulation Of Heat In Humans

 Humans are warm-blooded, and our body’s core temperature is set at between 97 degrees F and 99 degrees F by the hypothalamus. Like other warmblooded animals, human bodies have to control core body temperature so that it does not adversely affect the functioning of enzymes and the integrity of cell structures. Thyroid function also contributes to the body’s core temperature by setting the BMR (basal metabolic rate), which is the amount of heat a body generates at rest. The hypothalamus receives information from central thermoreceptors and keeps the temperature at a set point. That is done by invoking various involuntary actions such as sweating and shivering. The body temperature can also be controlled through voluntary activities such as shedding or putting on more clothing. 

Cold-blooded Animals

Most reptiles, amphibians, fish, and insects are unable to maintain their core body temperatures from within. They are, therefore, dependent on the temperature from their surroundings. Such organisms are therefore known as ectotherms (ecto meaning outside + therm meaning heat). Their core temperature also varies significantly, which means that they are poikilotherms (poikilo, meaning varied + therm meaning heat). As a result of their nature, such animals do not require to generate as much heat as warmblooded animals. These animals tend to have a lower resting metabolic rate, which is also known as bradymetabolism (Brady meaning slow + metabol meaning to change). In general, reptiles, fish, amphibians, and insects that are ectotherms and poikilotherms and have a bradymetabolism are known as cool-blooded animals.

Advantages And Disadvantages

There are various disadvantages and advantages associated with either warm-blooded or cold-blooded organisms. The efficiency of cellular chemical reactions, for example, is dependent on the organism’s core temperature. For that reason, warmblooded animals are generally able to defend themselves better and forage for food faster across areas with wider temperature ranges compared to cold-blooded animals. Warm-blooded animals can also support complex energy-dependent organs such as the mammalian brain. The downside is that warm-blooded organisms have to use a large amount of energy obtained from food for heat. Warm-blooded animals, therefore, consume a relatively large amount of food (usually up to five to ten times larger) than cold-blooded animals. In that respect, warm-blooded animals are comparable to gas-guzzling and energy-inefficient automobiles due to the high amounts of energy that they require to maintain core temperatures to ensure that organs are working optimally. Coldblooded animals can be compared to the energy-efficient and eco-friendly automobiles that are more in tune with the environment as they require less energy to keep their organs working optimally. 

Use Of The Terms Warm-blooded And Cold-blooded

It has been argued that the terms “warm-blooded” and “coldblooded’ are misleading labels. Biologists use other contrasting terms such as homeotherm and poikilotherm or endotherm and ectotherm while describing the thermal physiology of organisms. The characteristics of organisms categorized under the various traits tend to be correlated. For example, most ectotherms are poikilotherms, and most endotherms are homeotherms. There are, however, exceptions. Desert lizards are ectotherm, which means that they receive their heat from sunlight but are also considered behavioral homeotherms. They maintain near-constant activity temperature by basking in the morning to bring their core temperature up to activity level. At mid-day, they look for shade to avoid overheating. They can, therefore, be referred to as homeothermic ectotherms during the day. While at night, they are poikilothermic ectotherms. 

Hummingbirds are classified as endotherms. To conserve energy at night, hummingbirds go into dormancy and allow their core temperatures to drop. They can thus be classified at poikilothermic endotherms at night, while during the day, they are homeothermic. The term cold-blooded is particularly misleading since while insects such as butterflies and bees are poikilothermic, they can generate heat in their flight muscles. For example, a flying drone honey bee has a body temperature of over 1300F, which is considerably higher than the average human body temperature of 98.60F. Desert lizards have daytime core temperatures that are higher than those of mammals living in the same environment. As such, it would be inappropriate to call such organisms cold-blooded if we are to consider the strict interpretation of the term. 

Evolution Of Warm-blooded And Cold-blooded Animals

Scientists have, over the years, proposed various theories that try to explain why endotherms evolved relatively high and stable body temperatures. They include the need to aid physiological processes, the need for animals to maintain activity over more extended periods, and because it allows some animals to take care of precocial offspring. Such theories have, however, not received strong support in the scientific community. While various theories have some truth to them, some experts argue that the likely cause was something that significantly impacted their chances of survival and reproduction. Otherwise, traits of endothermy would have been too costly as a strategy and would therefore not be favored during natural selection. The assertion is debatable. A more popular theory suggests that pathogens may have facilitated the evolution of warm-blooded animals. Scientists in favor of this hypothesis believe that the ability to maintain a high core temperature would have allowed animals to mount a rapid fever response to invading pathogens compared to cold-blooded animals. Cold-blooded organisms would have to rely on external sources of heat to obtain fever-like temperatures. That means that cold-blooded animals would have to search for the ideal microclimate to initiate a fever; they would also struggle to mate and forage and would be exposed to predators. The theory emerged as a result of recent discoveries in fields such as animal physiology and immunology. Scientists, however, agree that rigorous tests, experiments, and data collection has to be carried out to strengthen the hypothesis. For example, the theory also suggests that species that maintain the most stable and warmest temperature should also experience a higher frequency of virulent pathogens or disease outbreaks. That is yet to be confirmed.

Critique Of Theories On The Evolution Of Warm-blooded Animals

While scientific theories have suggested that warm-blooded animals evolved from cold-blooded animals, little has been said on the evolutionary development and what the viable transitions between the two would have looked like. Some experts believe that considering what it takes to survive within the laws of nature, the theories suggested by evolutionary biologists are too simplistic.


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