Ectotherms Definition and Examples

What are Ectotherms?

Ectotherms are organisms that regulate their body temperature solely based on the environment. The word ectotherm originates from the Greek word ektos’ meaning outside and thermos’ meaning hot. These organisms’ internal physiological heat sources are of negligible importance in regulating their body temperature. Rather, they depend on the environmental sources of heat which they can operate at an economical rate of metabolism. Usually, they depend on sunlight or heated rock surfaces as an external source to regulate their body temperature.

Ectothermic organisms normally live in environments with constant temperatures like the ocean or tropics. Usually, the body temperature of an aquatic ectotherm is relatively close to the temperature of its environment. These organisms have adapted various behavioral thermoregulation mechanisms. A common example of such behavior is seeking shade to reduce their body temperature or basking in the sun to increase their body temperature.

Ectotherms are cold-blooded organisms and include fishes, insects, amphibians, reptiles, and some invertebrates. Naturally, ectotherms do not need as much food as their counterparts, endotherms (warm-blooded) require. Though ectothermic organisms are usually affected by fluctuations in temperatures. Ectotherms actually rely on their external environment to provide heat for digestion, metabolism, immune function, and reproduction. Ectothermic organisms are at the mercy of their environment compared to endothermic organisms. This is because to control their body temperature, they use ambient water temperature. Once ectothermic animals emerge from their shelter, they need to heat up in the early sunlight before they can begin their daily activities. Therefore, in cold weather, the foraging activity of ectothermic animals is restricted to the daytime.

Examples of ectotherms

  1. Many reptiles and insects search for sunny places and take a position that maximizes their level of exposure to the sun.
  2. Reptiles and insects at harmful high temperatures search for cooler water or shade.
  3. Honey bees huddle together when the weather is cold to retain heat.
  4. For moths and butterflies to build up heat, they move their wings in order to maximize their exposure to the sun before taking off.
  5. Caterpillars like the fall webworm and forest tent caterpillar bask in large groups to regulate their temperature.
  6. Before a flight, many flying insects vibrate their flight muscles without any violent wings movement to raise their internal temperature e.g honey bees and bumblebees.
  7. Bullfrogs, when it is hot secrets more mucus to allow more cooling by evaporation.
  8. Diving reptiles through a heat exchange mechanism conserve heat. The cold blood from the skin gets heat from blood moving outward from the body core.
  9. Some ectoderms like wood frogs during cold enter a state of torpor or hibernation where their metabolism slows down or stops.
  • Ectothermic animals

  • Amphibians
  • Ectothermic fish
  • Lizards
  • Poikilotherm
  • Snake
  • Orthoptera
  • Atlantic bluefin tuna
  • Migratory locust
  • Ectothermic vertebrates

  • Amphibians
  • Lizards
  • Desert iguana
  • True toads
  • Vipers
  • Green iguana

Body regulation of temperature

A fundamental problem faced by animals is keeping their internal temperature in a state that permits them to go about their normal cell function. This is because biochemical activities are sensitive to temperature and the chemical environment. All enzymes have an optimum temperature. Thus, the function of enzymes is impaired at temperatures above or below its optimum temperature.

Therefore, the temperature is a serious constraint for animals. Once the temperature drops too low, metabolic processes slow down. Due to this, the amount of energy the animal can use for reproduction or activities is reduced. Also, when body temperature rises too high, the metabolic reactions become unbalanced. Thus, enzymatic activities are hampered or even destroyed.

However, organisms can only succeed in a restricted temperature range. Normally, the temperature range is between 0 degrees to 40 degrees celsius. Hence, they must find a habitat that they do not have to contend with extreme temperatures. Moreso, at times they develop means of stabilizing their metabolism despite the extreme temperature.

However, thermoregulation is an organism being able to keep its body temperature within certain ranges despite the environmental temperature. Interestingly, by contrast, there are thermo-conforming organisms. These organisms simply just adopt the environmental temperature as their own body temperature. Hence, they avoid the need for internal thermoregulation. The process of internal thermoregulation is an aspect of homeostasis.

Homeostasis is the state of optimal functioning for an organism. It includes many variables like fluid balance and body temperature being kept within certain limits. An organism’s body should be able to maintain a normal temperature. Once it is unable to maintain a normal temperature and increases significantly above normal levels. Hyperthermia occurs, which is a condition of increased temperature above normal levels. Humans, however, may experience lethal hyperthermia. Especially when the temperature of the wet-bulb is sustained above 35 °C for 6 hours.

Furthermore, hypothermia, the opposite condition occurs when body temperature decreases below normal levels. This happens when there is a malfunction of the homeostatic control mechanisms of heat within the body. Hence, making the body lose heat faster than producing heat. Around 37 °C is the normal body temperature. If the body temperature goes lower than 35 °C, hypothermia occurs. This condition is usually caused by exposure to cold temperatures for a very long time. It is usually treated by methodical attempts to raise the temperature of the body back to a normal range.

Furthermore, internal thermoregulation is responsible for an animal being able to maintain homeostasis within a certain temperature range. However, physiological processes are affected as the internal body temperature of an organism rises. Such physiological processes like enzyme activity. Even though enzyme activity increases initially with temperature, it begins to denature and lose its function at higher temperatures. This happens around 40-50 degrees Celcius for mammals.

Hypothermia happens as internal body temperature decreases below normal levels and other physiological processes are affected. However, there are various mechanisms that animals use in order to regulate their internal body temperature. This is why thermoregulation in organisms could be either endothermy or ectothermy. Endothermy involves the organism creating most of its heat through metabolic processes and such organisms are called warm-blooded. Then, ectothermy involves organisms using external temperature sources to regulate their body temperatures. Such organisms are called cold-blooded even though the temperature of their body usually stays within the same range of temperature as warm-blooded organisms.

How do ectotherms regulate body temperature?

Ectotherms regulate their body temperature differently from endothermic animals. An ectothermic organism will use external temperature sources to regulate its body temperatures. Such organisms are called cold-blooded even though the temperature of their body usually stays within the same range of temperature as warm-blooded organisms. Ectotherms however regulate their body temperature and achieve temperature independence in two ways:

  1. Behavioral adjustments
  2. Metabolic adjustments
  • Behavioral adjustments

Since ectotherms cannot regulate their body temperature physically, they regulate it behaviourally. Usually, they have the option of searching for areas in their environment that has a favorable temperature. The area has to have an environmental temperature that suits the animal’s activities.

Some ectotherms like the desert lizards, to keep their body temperature constant have to exploit hour-to-hour changes in solar radiation. In the early hours of the morning, the lizard exposes its head to absorb the heat energy from the sun. Then by midday, it emerges from its burrow and basks in the sun with its body flattened to absorb heat. Later on, as the day warms up, it turns to face the sun in order to reduce its exposure. It raises its body from the hot substrates. Then, in the hottest part of the day, it may return to its burrow or move into a shade. Later it emerges to bask as the sun sinks lower and the temperature drops.

Picture illustrating how ectotherms regulates its body temperature. Using lizard as an example.
Photo Credit: Textbook (integrated Principles of Zoology, Fifteenth Edition) by Hickman, Roberts, Keen, Eisenhour, Larson, I’Anson Pg 682

However, this behavioral pattern helps to maintain a relatively steady body temperature of 36 – 39 degrees Celsius, whereas the air temperature ranges between 29-44 degrees celsius. Some lizards can stand extreme midday heat without shade. Moreso, the desert iguana of the Southwestern United States, for instance, prefers a body temperature of 42 degrees celsius when active. It can even tolerate a rise to 47 degrees celsius which is a temperature that is lethal to all birds and mammals, even some lizards.

Other behavioral adjustments examples are:

  • Honey bees huddle together when the weather is cold to retain heat.
  • Insects at harmful high temperatures search for cooler water or shade.
  • For moths and butterflies to build up heat, they move their wings in order to maximize their exposure to the sun before taking off.
  • Caterpillars like the fall webworm and forest tent caterpillar bask in large groups to regulate their temperature.
  • Metabolic adjustments

Most ectotherms can regulate their body temperature even without behavioral adjustment. They can adjust their metabolic rates to the prevailing temperature. It is done in a way that the intensity of the metabolism remains mostly unchanged. This process is referred to as temperature compensation. It involves complex cellular and biochemical adjustments. For example, these sorts of adjustments enable a salamander or fish to benefit from almost the same level of activity in both warm and cold environments. Endotherms achieve metabolic homeostasis by maintaining their body temperature independent of the temperature of the environment. Whereas, ectotherms achieve much the same by directly maintaining their metabolism independent of body temperature. However, such metabolic regulation is also a form of homeostasis.

Thermoregulation in ectotherms

  • Cold environment

Some species of ectotherms in colder environments may alter their body chemistry. In environments where they are exposed to freezing temperatures, they change their body chemistry. This is done to hinder ice crystal growth in their cells and tissues and to prevent the formation of ice crystals.

Many ectothermic animals can manufacture cryoprotectants and flood their bloodstream and tissues with them. These cryoprotectants are ice-inhibiting compounds such as sugars, proteins, and sugar alcohols like glycerol and sorbitol. Sometimes, ectothermic animals can use other dissolved substances, like salts that are already present in the bloodstream. These adaptative mechanisms protect the cells of the animals from freezing. This is achieved by the substance lowering the freezing point of water.

The wood frog, for example, survives winter by producing excess sugars. It converts glycogen into glucose which protects the tissues and cells of the animal cells even though most of the water in the body of the frog may freeze. Likewise, ray-finned fishes in polar marine environments possess high internal salt concentrations. This high internal concentration of salt inhibits freezing. Also, the fish produce glycoproteins that act as cryoprotectants.

  • Hot environment

Ectothermic animals, by taking shelter in burrows or shades avoid extreme temperatures in regions where the temperatures fluctuate seasonally. Some may become dormant to some degree. For instance, fishes may rest near the water bottom in winter. Also, ectothermic organisms adopt biochemical strategies to help them combat the effect of extreme temperatures. Ectotherms release heat-shock proteins during periods of heat stress. Since excessive heat can damage proteins in an animal’s body, the heat-shock proteins released help stabilize other proteins. Hence, preventing the denaturation of the protein’s molecular structure.

Ectotherms vs Endotherms

The difference between ectothermic animals and endothermic animals is basically how they regulate their body temperature and whether it is dependent or independent of the external environment. However, there are three distinctive features between ectotherms and endotherms. They are:

  1. Regulation of body temperature
  2. Number of mitochondria
  3. Amount of food needed
  • Regulation of body temperature

Ectotherms actually rely on their external environment to regulate their body temperature whereas endotherms regulate their own body temperature through internal metabolic processes. Endotherms usually maintain a narrow range of internal temperatures and generate most of their heat from metabolism.

Once the environmental temperature is cold, endothermic animals increase metabolic heat production. This is to keep their body temperature constant. They maintain nearly constant high operational body temperature by relying on internal heat produced by metabolically active organs like the kidney, heart, liver, brain, and muscle. Internal heat is produced too by heat-producing organs like brown adipose tissue (BAT). Hence their body temperature is more independent of the environmental temperature. Endothermic vertebrate species are therefore less dependent on the conditions of the environment. They have developed a higher variability in their daily patterns of activity.

Ectotherms, on the other hand, use environmental temperature to regulate their body temperatures. They are, however, the opposite of endotherms when it comes to regulating internal temperatures. For ectothermic animals to achieve optimal body temperature, they rely on external heat sources like sunlight for various bodily activities. Hence, in order to reach operational body temperature, they rely on ambient conditions. The internal physiological sources of warmth in ectotherms are of negligible importance as environmental influences help them to maintain adequate body temperature. Ectothermic animals live in areas like the tropics or the ocean that maintain a constant temperature. However, they have adopted a wide range of behavioral mechanisms that enable them to respond to external temperatures. Such behaviors like sunbathing or seeking the cover of shade.

  • Number of mitochondria

Many endotherms have a large number of mitochondria per cell. These mitochondria enable endothermic animals to generate heat. This is done by increasing the rate at which they metabolize sugars and fats. Hence endothermic animals possess a larger number of mitochondria per cell than ectothermic animals.

  • Amount of food needed

Normally, ectotherms have low metabolic rates. Their metabolic rate is low compared to endotherms at a given body mass. To sustain their high metabolism, endothermic animals must eat more food more often, compared to ectothermic animals. This is why endothermic animals generally rely on higher food consumption and food of higher energy content. For instance, an endothermic mouse must eat food every day to sustain its high metabolism. Whereas an ectothermic snake may only eat once a month because its metabolism is much lower.

Poikilotherm vs Ectotherm

Zoologists usually use poikilothermic and homeothermic as alternative terms to cold-blooded and warm-blooded respectively. A poikilotherm is an animal whose body temperature fluctuates with the environmental temperature. Homeotherms, on the other hand, is an animal that maintains thermal homeostasis. This means that their body temperature is constant and regulated independently of environmental temperature.

The term poikilothermic and homeothermic can apply to all organisms but it is generally applied to animals, especially vertebrates. These terms refer to the variability of body temperature and are more precise but still offer difficulties. Hence, physiologists prefer another way to contrast and compare thermoregulatory mechanisms. This is by using terms that would reflect the fact that an animal’s body temperature is a balance between heat gain and heat loss. Moreover, all animals produce heat from cellular metabolism but in most animals, the heat is conducted away as fast as it is produced. This, however, brought the classification of ectotherms and endotherms. Ectotherms are classified as animals that their body temperature is solely determined by environmental context.

However, many terrestrial ectotherms are poikilothermic. Also, some ectotherms remain in temperature-constant environments to the extent that they become homeothermic. This means that they eventually maintain a constant internal temperature. This distinct example, however, is the reason why the term poikilotherm is more useful than the vernacular cold-blooded. Poikilotherm is used more generally sometimes to refer to ectotherms. Poikilothermic animals include many invertebrate animals as well as several types of vertebrate animals, like some fish, reptiles, and amphibians. However, the sloth and naked mole-rat are some of the rare mammals that are poikilothermic.

Advantages of ectotherms

  1. Ectotherms need less amount of food for survival.
  2. As ectotherms, they eat less food and may be able to survive for long periods of time in habitats with food scarcity.
  3. They use a greater proportion of the energy (over 50%) obtained from food for growth and reproduction.

Disadvantages of being ectothermic

  1. Ectotherms are less active in cooler temperatures. Hence, they have to warm up at sunset in order to be active which can put them at risk of predation.
  2. During winter, they are inactive because they are not able to warm up. Though, they have enough stored energy to carry them through the winter.
  3. Ectotherms are not able to increase their rate of respiration in order to generate internal heat.
  4. Being ectothermic puts an organism at the mercy of the environmental temperature.

Are fish ectotherms?

Fishes together with amphibians and invertebrates are not capable of creating and storing internal metabolic heat. Hence, most fishes are ectothermic. This is because they have no control over their body temperature. The core body temperature of fishes fluctuates widely and conforms entirely to ambient temperature. However, how a fish functions at different temperatures is dependent on the species of the fish and individual fitness. The organs and enzymes of the fish need to be capable of functioning at a range of temperatures. This is why ectothermic fish has adopted and evolved several mechanisms to maintain optimal thermal habitats.

Nevertheless, there are some fish species that can be considered to be warm-blooded. The opah is a common example. Since, ectotherms actually rely on their external environment to provide heat for digestion, metabolism, immune function, and reproduction. For fishes, their external environment is their surrounding aqueous home. Many tropical fish species are stenothermal and have a narrow range of temperatures they can tolerate.

Koi and goldfish are eurythermal and can tolerate a wide range of temperatures. This is why they strive very well in outdoor ponds. Usually, outdoor ponds are fed depending on the temperature of the water. This is because when the water is too cold, these fishes can have difficulties metabolizing some diets. Different protein sources, sometimes, are utilized which are easier for the fish to break down and digest. Once the food comes out the back end looking exactly the same as when it went in, one can tell the fish wasn’t able to digest their diet.

Even the immune system of a particular fish species is seriously hindered when the water is too cool. This eventually prevents antibodies from working as it is designed to. Antibodies need assistance from the immune system. The immune system, being hindered also prevents healing after surgery. In cold water, fish that are recovering from surgery cannot protect themselves against infection. Also, they cannot break down enough energy to heal their skin surface or incisions properly.

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