An ectotherm is an organism in which internal physiological sources of heat play a minimal or insignificant role in regulating body temperature. Such species (for example, frogs) rely on external heat sources, allowing them to function at very low metabolic rates.
Some of these animals live in habitats with nearly constant temperatures, such as those found in the abyssal ocean, and can thus be classified as homeothermic ectotherms. In comparison, many species habitually seek out external sources of heat or shelter from heat in areas where temperature varies so widely as to restrict the physiological behaviours of other types of ectotherms; for example, many reptiles control their body temperature by basking in the sun or finding shade when required, in addition to plenty of other behavioural thermoregulation mechanisms. Owners of pet reptiles in home captivity can use a UVB/UVA light device to aid the animals' basking behaviour. Endotherms, unlike ectotherms, largely depend, if not entirely, on heat produced by internal metabolic processes, while mesotherms adopt an intermediate strategy.
Fluctuating ambient temperatures may have an effect on body temperature in ectotherms. The word for such variations in body temperature is poikilothermy, though the definition is not generally accepted and its usage is diminishing. Poikilothermy is practically absolute in small aquatic creatures like Rotifera, but other creatures (like crabs) have more physiological choices, and they can switch to desired temperatures, prevent ambient temperature changes, or moderate their effects. Ectotherms, especially aquatic species, may exhibit homeothermic characteristics. Normally, their ambient environmental temperature range is relatively constant, and few seek to sustain a higher internal temperature due to the high associated costs.
Ectotherm, any cold-blooded animal—that is, any animal whose body temperature control is dependent on external sources such as sunlight or a heated rock surface. Fishes, amphibians, reptiles, and invertebrates are all ectotherms.
Ectotherms' body temperature is primarily determined by external heat sources. That is, the temperature of an ectotherm's body rises and falls in tandem with the temperature of the ambient world. Although ectotherms, like all living things, produce metabolic heat, ectotherms cannot increase this heat output to maintain a particular internal temperature. However, most ectotherms do control their body temperature to some extent. They simply do not do so by generating heat. Instead, they employ other techniques, such as actions (seeking light, shade, and so on) to identify conditions that fulfil their temperature requirements.
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Why Regulate Temperature?
Most animals have certain basic body temperature limits that they must adhere to in order to survive. Water freezes at 0 degrees Celsius, end text to form ice, at the other end of the spectrum. As ice crystals grow within a cell, the membranes are usually ruptured. At temperatures above 40 degrees Celsius, enzymes and other proteins in cells often lose shape and function, a process known as denature.
Why do many organisms, like you and me, keep our body temperatures within such a small range? Temperature affects the rate of chemical reactions, both because it affects the rate of collisions between molecules and because the enzymes that regulate the reactions can be temperature sensitive. Reactions tend to be quicker at higher temperatures, up to a point where their rate dramatically decreases as their enzymes denature.
Each species has its own metabolic network and collection of enzymes that are adapted for a specific temperature range. Organisms ensure the proper activity of their metabolic reactions by maintaining their body temperature within the target range.
Ectothermic Heating and Cooling
Many ectotherms exist in environments with little control, such as the ocean, where the ambient temperature is relatively constant. Crabs and other ocean-dwelling ectotherms can move to desired temperatures when required. Ectotherms who live mostly on land can control their temperature by basking in the sun or cooling off in the shade. Some insects warm themselves by vibrating the muscles that power their wings rather than flapping their wings. Ectotherms are slow at night and early in the morning due to their reliance on environmental factors. Many ectotherms must warm up before becoming involved.
Ectotherms in the Winter
Many ectotherms undergo torpor, a condition in which their metabolism slows or stops, during the winter months or when food is scarce. Torpor is essentially a short-term hibernation that can last anywhere from a few hours to an entire night. Torpid animals' metabolic rates will drop by up to 95% of their resting rate.
Ectotherms can also hibernate, which can last a season or, in the case of certain species like the burrowing frog, years. Hibernating ectotherms have a metabolic rate that is one to two percent of the animal's resting rate. Tropical lizards do not hibernate because they have not adapted to cold weather.
Certain ectotherms can control their body temperature to a useful degree thanks to a variety of behavioural patterns. Reptiles and many insects seek out sunny areas and locations that maximise their exposure to warm up; at dangerously high temperatures, they seek shade or cooler water. Honey bees huddle together to keep warm in cold weather. Butterflies and moths can orient their wings to maximise exposure to solar radiation before taking flight in order to build up heat. For thermoregulation, gregarious caterpillars such as the forest tent caterpillar and fall webworm benefit from basking in large groups. Many flying insects, including honey bees and bumblebees, increase their internal temperatures endothermically before flight by vibrating their flight muscles without violently moving their wings. This form of endothermal operation exemplifies the difficulty in applying words like poikilothermy and homeothermy consistently.
Physiological adaptations, in addition to behavioural adaptations, aid ectotherms in temperature regulation. Diving reptiles retain heat by heat exchange processes, in which cold blood from the skin takes up heat from blood flowing away from the body heart, reusing and thereby conserving some of the heat that would otherwise be lost. When it is humid, the skin of bullfrogs secretes more mucus, allowing for more evaporation cooling.
During cold periods, some ectotherms reach a state of torpor, in which their metabolism slows or, in some cases, stops completely, as in the case of the wood frog. Torpor can last for an hour, a season, or even years, depending on the species and circumstances.
Some species may alter their body chemistry in colder environments, where ectotherms may be exposed to freezing temperatures, to limit the growth of ice crystals in their cells and tissues or to prevent ice crystal formation entirely. Many ectotherms can produce and flood their bloodstream and tissues with cryoprotectants, which are ice-inhibiting compounds such as proteins, sugars, and sugar alcohols (e.g., sorbitol and glycerol), or they can use dissolved substances already present in the bloodstream, such as salts. These adaptations keep the cells of the animals from freezing by lowering the freezing point of water. The wood frog (Lithobates sylvatica), for example, survives the winter by producing excess sugars (specifically, by converting glycogen into glucose) that protect the animal's cells and tissues, despite the fact that much of the water in the frog's body may freeze. Similarly, ray-finned fishes that live in polar marine environments have high internal salt concentrations that prevent freezing, as well as glycoproteins that function as cryoprotectants.
Pros and Cons
Ectotherms depend heavily on external heat sources, such as sunlight, to maintain their optimum body temperature for a variety of bodily activities. As a result, they depend on ambient conditions to achieve operational body temperatures. Endothermic animals, on the other hand, sustain nearly constant high operating body temperatures primarily through the use of internal heat provided by metabolically active organs (liver, kidney, heart, brain, muscle) or even specialized heat generating organs such as brown adipose tissue (BAT). For the same body mass, ectotherms usually have lower metabolic rates than endotherms. As a result, endotherms typically consume more food, and typically food with a higher energy content. Such specifications can restrict a given environment's carrying capacity for endotherms in comparison to its carrying capacity for ectotherms.
Ectotherms are more sluggish at night and in the early mornings because they depend on environmental factors to regulate their body temperature. Many diurnal ectotherms need to warm up in the early sunlight after emerging from their shelter before they can begin their daily activities. Most vertebrate ectotherms' foraging behaviour is therefore limited to the daytime in cool weather, and in cold climates, most cannot live at all. Most nocturnal lizard species, for example, are geckos that specialise in "sit and wait" foraging strategies (see ambush predator). Such techniques do not necessitate as much energy as active foraging and do not necessitate as much hunting activity. From another perspective, sit-and-wait predation can necessitate extremely long periods of ineffective waiting. Endotherms, in general, cannot tolerate such long periods without food, however appropriately adapted ectotherms can. Endothermic vertebrate organisms are therefore less dependent on environmental factors and have evolved greater variability (both within and between species) in their daily activity patterns.
Ectotherm, any cold-blooded animal, that is, any animal whose body temperature control is dependent on external sources, such as sunlight or a heated rock surface. Fishes, amphibians, reptiles, and invertebrates are all ectotherms. An aquatic ectotherm's body temperature is normally very similar to the temperature of the surrounding water. Ectotherms do not consume as much food as warm-blooded animals (endotherms) of the same age, but they are more sensitive to temperature changes. Ectotherms that live in areas where temperatures fluctuate seasonally escape extremes by seeking refuge in burrows or similar locations, or by going dormant to some extent. Furthermore, ectotherms use biochemical techniques to counteract the effects of high temperatures. Ectotherms release heat-shock proteins during times of heat stress, which help stabilise other proteins and thus avoid their denaturation, since excessive heat can kill proteins in an animal's body.