Topics in Depth
The Theme of Temperature Regulation in Animal Movement
Temperature regulation is, perhaps, the most variable and the most noticeable form of homeostasis. There are enzymes within all cells that require optimal temperatures for them to remain functional. If the body temperature goes outside of the acceptable range, the cells will not be able to perform their chemical reactions. In addition, if the contents of a cell freeze, ice crystals can form inside the cell, which will damage the cellular structures.
Sensors in the blood vessels are constantly reporting the internal temperature to a part of the brain called the hypothalamus. The hypothalamus analyzes these readings, and if necessary, it signals the animal to make appropriate changes. Since many of these changes are behavioral adjustments, they can be observed easily in animals. For this reason, there has been a lot of data collected to help us understand temperature regulation.
In the past, animals were called either warm-blooded or cold-blooded. This led to confusion since the actual temperature of the animal's blood did not determine which category they belonged to. Now, the naming system is much more specific. It's also a little more complicated to understand. Don't worry; we've got you covered.
Animals can be divided into three categories depending on how they regulate their temperature:
Homeotherms: animals who maintain a constant internal body temperature across a wide range of environmental conditions. Humans are homeotherms.
Poikilotherms: animals whose temperature changes according to the environmental temperature. Snakes are poikilotherms.
Heterotherms: animals which usually keep a constant body temperature, but have specific periods where their temperature is different, such as during hibernation. Ground squirrels are heterotherms.
A wild boar in the snow. Boars are mammals and produce their own heat. To retain their heat and keep out the cold they also have special fur to insulate their bodies.
They can also be divided by another classification system based on whether their heat primarily comes from an external source or an internal source:
Ectotherms: Animals which receive heat primarily from external sources (reptiles, amphibians, and fish).
Endotherms: Animals which create their own heat (mammals and birds).
These two systems overlap. Homeotherms and heterotherms can also usually be classified as endotherms, and poikilotherms can also usually be classified as ectotherms. However, these rules are not always true. For example, there are deep sea fishes who are ectotherms and do not produce their own heat, but since they live in constant temperature then they are considered homeotherms. There are also insects that are poikilotherms with body temperatures that change with the environment. In order to fly, these bugs produce their own heat. Therefore, they are not true ectotherms.
The chief benefit for ectotherms is that they do not have to expend too much energy generating their own heat. The downside is that they are at the mercy of their environmental temperature, and their cells must be able to function over a wide range of internal temperatures.
Endotherms, on the other hand, benefit from staying at a constant optimal temperature. This allows all of their enzymes can work at full efficiency all of the time. They are also capable of withstanding extreme temperatures for a short time without a serious detrimental effect. It's why people can do the polar bear plunge. Unfortunately, though, they need to spend a great deal of energy on temperature regulation.
Keep these basic concepts about heat transfer in mind. Heat can be gained or lost by conduction, convection, and radiation. Conduction means that heat can be transferred between two objects that are touching. Convection means that heat can be transferred between objects and the liquid or gas that surrounds them. Radiation means that absorption of electromagnetic waves (AKA sunlight) can cause heat.
Since ectotherms do not produce their own heat, they must regulate their temperature via other means. The circulatory system plays a big role in temperature regulation. The temperature of the blood is key to the temperature of the entire animal. For this reason, ectotherms need to maintain a close connection between their blood vessels and their skin for quick heat exchange. Therefore, they usually lack hair or feathers.
Most heat exchange between the blood and the environment occurs in an animal's extremities such as its feet or tail. To prevent heat loss, an animal can constrict the blood vessels in the extremities in order to limit blood flow to these vulnerable areas. When it wants to get rid of excess heat, it can increase blood flow to the extremities so that the heat can be released.
Some types of fish have a way of conserving heat in their blood called a counter-current heat exchanger. As discussed in the circulation section, the blood of a fish needs to circulate past the gills for gas exchange. Unfortunately, this also means that the warm blood comes close to the cold water, which is an opportunity for heat loss.
Diagram of blood vessels in countercurrent heat exchange setup in fish.
The fish retains its heat by setting up the blood vessel going to the gills running parallel to a blood vessel returning from the gills. As warm blood goes towards the gills, it passes cool blood that is returning from the gills. The heat from the warm blood transfers to the cool blood. This causes the blood going towards the gills to get cooler and the blood returning to the body to get warmer.
In this way, the heat remains in the body and is never lost to the outside water. The blood returning from the gills is warmed before it pumps through the body. Countercurrent heat exchange can also be found in the legs of the arctic fox, so that heat is not lost when the warm blood passes through the cold paws.
Ectotherms regulate their temperature with behavioral changes as well. Usually, this involves placing themselves in the best position to receive heat from an external source (or to transfer heat to an external source). On cool days, many reptiles will sun themselves on sunny rocks and receive the heat of the sun and the heat of the heated rock. On hot days, however, they will be found on wet, shady, cool rocks instead, transferring their excess body heat away.
An iguana sunning itself on a warm rock to get warm.
Iguanas are known to feed in the cold seawater. Then they find a hot rock to lie on for digestion. The enzymes required for digestion cannot function at cold temperatures, so iguanas have learned that they must seek out warmth in order to digest their food.
There are many other examples of ectotherms adapting their behavior to conserve, receive, or lose heat. Fish can change their depth in the water column to adjust their temperature. Snakes find dark crevices to avoid absorbing heat on hot days. Butterflies and other insects will spread their wings wide right before flight to absorb as much heat as possible. This warms their flight muscles.
Honeybees handle temperature regulation in terms of the entire hive. They cluster themselves together, which benefits the entire hive rather than individual bees. This clustering also helps to regulate the temperature of the developing larvae, much like birds and reptiles will warm nests of eggs.
Ectotherms are capable of surviving at a range of temperatures since they never know what their environment will be like. Still, quick temperature changes can be deadly. For example, when moving fish, it is important to slowly acclimate them to their new water, especially if there is a temperature difference.
Although fish have a variety of enzymes that can work at various temperatures, each enzyme still has an optimal temperature at which it works. Careful acclimation is important so that they have time to switch the enzymes.
As ectotherms rely on external production of heat, they often have periods of inactivity that correlate with cooler temperatures. When their internal temperature drops, their enzymes become less effective and their metabolism decreases. Lower metabolism means less energy, and less energy means less movement.
For this reason, they are often inactive at night and in the winter. During periods of inactivity, ectotherms seek out a buffered site that protects them from extreme temperatures and from predators. Often this wintering site, or hibernaculum, is deep within the ground below the frost line.
Both cars and ectotherms need to prepare for winter by filling up with antifreeze, or in science language, cryoprotectants. A cryoprotectant is something that prevents things from freezing. Although it is not directly involved in temperature regulation, it can prevent some of the effects that extremely low temperatures can cause. Ectotherms that live in extreme climates fill up on special carbohydrates and amino acids before winter which helps prevent their cells from freezing. One of those carbohydrates, propylene glycol is actually the same chemical used in automotive antifreeze.
Endotherms can exhibit many of the behavioral changes for temperature regulation as ectotherms. Pigs, for example, are notorious for soaking in a pile of cool mud on a hot day. While we all like to sit by the fire on a cold day, we endotherms can also create our own heat through metabolism. For this reason, endotherms need to consume more food than ectotherms, and the basal rate of metabolism for an endotherm is approximately six times greater than the rate of a comparably sized ectoderm with the same temperature.
The cycle of temperature regulation in endotherms.
Endotherms use blood vessel constriction or dilation to regulate their temperature. In addition, the ability to create heat from within has allowed endotherms to develop body coverings such as hair or feathers, which prevent the environmental temperatures from affecting them. Marine mammals have a layer of blubber that helps them twofold; it provides neutral buoyancy so they can expend more energy on temperature regulation and it also acts as a layer of insulation from the frigid deep ocean water.
Polar bears and other arctic species have several layers of fur to keep the harsh winds off of their skin. Tiny muscles at the base of the hairs can contract and make the hair stand on end in a process called piloerection. This is often also seen during fear or arousal. During temperature regulation, warm air is trapped close to the skin and cold air is prevented from getting in. Birds have a similar form of piloerection whereby muscles connected to feather tracts enable them to fluff up and trap air beneath them.
When necessary, mammals and birds can produce heat by shivering. Shivering is caused by tiny muscle contractions. These contractions cause muscles to create heat; it is also what causes teeth to chatter together. Despite his snowsuit, this pug is still shivering to stay warm. Mammals can also produce heat with a special tissue called brown fat, which is specialized for producing heat. Infants often rely on brown fat exclusively for warmth.
When they need to release heat, many mammals use evaporation. Humans and horses have sweat glands that release liquid that can then be evaporated from the skin, taking heat with it. Other animals, such as dogs pant to release saliva for evaporation. Kangaroos, are known to lick their front paws to stay cool.
Some endotherms are heterotherms who sustain periods of lowered body temperature. If this occurs during the winter months it is called hibernation. If this occurs in the summer months it is called estivation. These endotherms are able to survive during this period because their metabolism decreases to a very low level. In fact, they can survive on only one-fiftieth of their normal metabolic rate.
In preparation for dormancy, animals eat large amounts of food and store it as fat in order to sustain their low metabolism through the winter. Hibernating ground squirrels allow their body temperature to drop to environmental levels. They then remain in a deep slumber, unable to be roused until the spring. Bears, on the other hand, keep their body temperatures at a moderate level, and are able to be roused without much difficulty. Although waking a sleeping bear is NOT recommended.
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