![Lizard Thermoregulation](https://upload.wikimedia.org/wikipedia/commons/1/15/Lizard_Thermoregulation.svg)
An ectotherm (from the
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
() "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling
body temperature
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
.
[Davenport, John. Animal Life at Low Temperature. Publisher: Springer 1991. ] Such organisms (for example
frog
A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-f ...
s) rely on environmental heat sources, which permit them to operate at very economical
metabolic
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
rates.
Some of these animals live in environments where temperatures are practically constant, as is typical of regions of the abyssal ocean and hence can be regarded as
homeothermic
Homeothermy, homothermy or homoiothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate environmen ...
ectotherms. In contrast, in places where temperature varies so widely as to limit the physiological activities of other kinds of ectotherms, many species habitually seek out external sources of heat or shelter from heat; for example, many
reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s regulate their body temperature by
basking in the sun, or seeking shade when necessary in addition to a whole host of other behavioral thermoregulation mechanisms. For home captivity of pet reptiles, owners can use a UVB/UVA light system to assist the animals' basking behaviour.
In contrast to ectotherms,
endotherms
An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inste ...
rely largely, even predominantly, on heat from internal metabolic processes, and
mesotherm
A mesotherm (from Greek μέσος ''mesos'' "intermediate" and ''thermē'' "heat") is a type of animal with a thermoregulatory strategy intermediate to cold-blooded ectotherms and warm-blooded endotherms.
Definition
Mesotherms have two basic c ...
s use an intermediate strategy.
In ectotherms, fluctuating ambient temperatures may affect the body temperature. Such variation in body temperature is called
poikilotherm
A poikilotherm () is an animal whose internal temperature varies considerably. Poikilotherms have to survive and adapt to environmental stress. One of the most important stressors is temperature change, which can lead to alterations in membrane ...
y, though the concept is not widely satisfactory and the use of the term is declining. In small aquatic creatures such as
Rotifer
The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals.
They were first described by Rev. John H ...
a, the poikilothermy is practically absolute, but other creatures (like
crabs
Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...
) have wider physiological options at their disposal, and they can move to preferred temperatures, avoid ambient temperature changes, or moderate their effects.
[ Ectotherms can also display the features of ]homeothermy
Homeothermy, homothermy or homoiothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate environmen ...
, especially within aquatic organisms. Normally their range of ambient environmental temperatures is relatively constant, and there are few in number that attempt to maintain a higher internal temperature due to the high associated costs.
Adaptations
Various patterns of behavior enable certain ectotherms to regulate body temperature to a useful extent. To warm up, reptiles and many insects find sunny places and adopt positions that maximise their exposure; at harmfully high temperatures they seek shade or cooler water. In cold weather, honey bees huddle together to retain heat. Butterflies and moths may orient their wings to maximize exposure to solar radiation in order to build up heat before take-off.[ Gregarious caterpillars, such as the ]forest tent caterpillar
The forest tent caterpillar moth (''Malacosoma disstria'') is a moth found throughout North America, especially in the eastern regions. Unlike related tent caterpillar species, the larvae of forest tent caterpillars do not make tents, but rathe ...
and fall webworm
The fall webworm (''Hyphantria cunea'') is a moth in the family Erebidae known principally for its larval stage, which creates the characteristic webbed nests on the tree limbs of a wide variety of hardwoods in the late summer and fall. It is con ...
, benefit from basking in large groups for thermoregulation. Many flying insects, such as honey bees and bumble bees, also raise their internal temperatures endothermally prior to flight, by vibrating their flight muscles without violent movement of the wings (see insect thermoregulation
Insect thermoregulation is the process whereby insects maintain body temperatures within certain boundaries. Insects have traditionally been considered as '' poikilotherms'' (animals in which body temperature is variable and dependent on ambient t ...
). Such endothermal activity is an example of the difficulty of consistent application of terms such as poikilothermy and homiothermy.[
In addition to behavioral adaptations, physiological adaptations help ectotherms regulate temperature. Diving reptiles conserve heat by heat exchange mechanisms, whereby cold blood from the skin picks up heat from blood moving outward from the body core, re-using and thereby conserving some of the heat that otherwise would have been wasted. The skin of bullfrogs secretes more mucus when it is hot, allowing more cooling by evaporation.
During periods of cold, some ectotherms enter a state of ]torpor
Torpor is a state of decreased physiological activity in an animal, usually marked by a reduced body temperature and metabolic rate. Torpor enables animals to survive periods of reduced food availability. The term "torpor" can refer to the time ...
, in which their metabolism slows or, in some cases, such as the wood frog
''Lithobates sylvaticus'' or ''Rana sylvatica'', commonly known as the wood frog, is a frog species that has a broad distribution over North America, extending from the boreal forest of the north to the southern Appalachians, with several nota ...
, effectively stops. The torpor might last overnight or last for a season, or even for years, depending on the species and circumstances.
Pros and cons
Ectotherms rely largely on external heat sources such as sunlight
Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
to achieve their optimal body temperature
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
for various bodily activities. Accordingly, they depend on ambient conditions to reach operational body temperatures. In contrast, endothermic animals maintain nearly constant high operational body temperatures largely by reliance on internal heat produced by metabolically active organs (liver, kidney, heart, brain, muscle) or even by specialized heat producing organs like brown adipose tissue
Brown adipose tissue (BAT) or brown fat makes up the adipose organ together with white adipose tissue (or white fat). Brown adipose tissue is found in almost all mammals.
Classification of brown fat refers to two distinct cell populations with si ...
(BAT). Ectotherms typically have lower metabolic rates than endotherms
An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inste ...
at a given body mass. As a consequence, endotherms generally rely on higher food consumption, and commonly on food of higher energy content. Such requirements may limit the carrying capacity
The carrying capacity of an environment is the maximum population size of a biological species that can be sustained by that specific environment, given the food, habitat, water, and other resources available. The carrying capacity is defined as t ...
of a given environment for endotherms as compared to its carrying capacity for ectotherms.
Because ectotherms depend on environmental conditions for body temperature regulation, as a rule, they are more sluggish at night and in early mornings. When they emerge from shelter, many diurnal ectotherms need to heat up in the early sunlight before they can begin their daily activities. In cool weather the foraging activity of such species is therefore restricted to the day time in most vertebrate ectotherms, and in cold climates most cannot survive at all. In lizards, for instance, most nocturnal
Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite.
Nocturnal creatures generally have highly developed sens ...
species are geckos specialising in "sit and wait" foraging strategies (see ambush predator
Ambush predators or sit-and-wait predators are carnivorous animals that capture or trap prey via stealth, luring or by (typically instinctive) strategies utilizing an element of surprise. Unlike pursuit predators, who chase to capture prey us ...
). Such strategies do not require as much energy as active foraging and do not require hunting activity of the same intensity. From another point of view, sit-and-wait predation may require very long periods of unproductive waiting. Endotherms cannot, in general, afford such long periods without food, but suitably adapted ectotherms can wait without expending much energy. Endothermic vertebrate species are therefore less dependent on the environmental conditions and have developed a higher variability (both within and between species) in their daily patterns of activity.
References
{{reflist
Animal physiology
Thermoregulation