common frog is a poikilotherm and is able to function over a wide range of body core temperatures.
A poikilotherm (/ˈpɔɪkələˌθɜːrm,pɔɪˈkɪləˌθɜːrm/) 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 lipid order and can cause protein unfolding and denaturation at elevated temperatures. It is the opposite of a
homeotherm, an animal which maintains
thermalhomeostasis. While the term in principle can apply to all
organisms, it is generally only applied to animals, and mostly to
vertebrates. Usually the fluctuations are consequence of variation in the
ambient environmental temperature. Many terrestrial
ectotherms are poikilothermic. However some ectotherms remain in temperature-constant environments to the point that they are actually able to maintain a constant internal temperature and are considered
homeothermic. It is this distinction that often makes the term "poikilotherm" more useful than the vernacular "cold-blooded", which is sometimes used to refer to
ectotherms more generally.
Poikilothermic animals include types of vertebrate animals, specifically some fish, amphibians, and reptiles, as well as many
invertebrate animals. The
naked mole-rat and
sloth are some of the rare mammals which are poikilothermic.
The term derives from
Greekpoikilos (ποικίλος), meaning "varied," ultimately from a root meaning "dappled" or “painted,” and thermos (θερμός), meaning "heat".
Sustained energy output of a poikilotherm (a
lizard) and a
mouse) as a function of core body temperature. The homeotherm has a much higher output, but can only function over a very narrow range of body temperatures.
Poikilotherm animals must be able to function over a wider range of temperatures than homeotherms. The speed of most chemical reactions vary with temperature, and in order to function poikilotherms may have four to ten
enzyme systems that operate at different temperatures for an important chemical reaction. As a result, poikilotherms often have larger, more complex
genomes than homeotherms in the same
Frogs are a notable example of this effect, though their complex development is also an important factor in their large genome.
Because their metabolism is variable and generally below that of homeothermic
animals, sustained high-energy activities like powered
flight in large animals or maintaining a large
brain is generally beyond poikilotherm animals. The metabolism of poikilotherms favors strategies such as sit-and-wait hunting over chasing prey for larger animals with high movement cost. As they do not use their
metabolisms to heat or cool themselves, total energy requirement over time is low. For the same body weight, poikilotherms need only 5 to 10% of the energy of
Adaptations in poikilotherms
Some adaptations are behavioral. Lizards and
snakes bask in the sun in the early morning and late evening, and seek shelter around noon.
The eggs of the
yellow-faced bumblebee are unable to regulate heat. A behavioral adaptation to combat this is incubation, where to maintain the internal temperatures of eggs, the queen and her workers will incubate the brood almost constantly, by warming their abdomens and touching them to the eggs. The bumblebee generates heat by shivering flight muscles even though they are not flying.
Termite mounds are usually oriented in a north–south direction so that they absorb as much heat as possible around dawn and dusk and minimise heat absorption around noon.
Tuna are able to warm their entire bodies through a heat exchange mechanism called the
rete mirabile, which helps keep heat inside the body, and minimises the loss of heat through the
gills. They also have their swimming muscles near the center of their bodies instead of near the surface, which minimises heat loss.
Gigantothermy means growing to large size in order to reduce heat loss, such as in
sea turtles and
ice-agemegafauna. Body volume increases proportionally faster than does body surface, with increasing size; and less body surface area per unit body volume tends to minimise heat loss.
Camels, although they are homeotherms, thermoregulate using a method termed "temperature cycling" to conserve energy. In hot deserts, they allow their body temperature to rise during the day and fall during the night, adjusting their body temperature to cycle over approximately 6 °C.
It is comparatively easy for a poikilotherm to accumulate enough energy to reproduce. Poikilotherms at the same
trophic level often have much shorter generations than homeotherms: weeks rather than years. Such applies even to animals with similar ecological roles such as
This difference in energy requirement also means that a given food source can support a greater density of poikilothermic animals than homeothermic animals. This is reflected in the predator-prey ratio which is usually higher in poikilothermic fauna compared to homeothermic ones. However, when homeotherms and poikilotherms have similar niches, and compete, the homeotherm can often drive poikilothermic competitors to extinction, because homeotherms can gather food for a greater fraction of each day.
^Milton Hildebrand; G. E. Goslow Jr. (2001). Analysis of vertebrate structure. Principal ill. Viola Hildebrand. New York: Wiley. p. 429.
^"Body size shifts influence effects of increasing temperatures on ectotherm metabolism". Global Ecology and Biogeography.
^Daly, T.J.M., Williams, L.A. and Buffenstein, R., (1997). Catecholaminergic innervation of interscapular brown adipose tissue in the naked mole-rat (Heterocephalus glaber). Journal of Anatomy, 190: 321-326.
^Sherwin, C.M. (2010). The Husbandry and Welfare of Non-traditional Laboratory Rodents. In "UFAW Handbook on the Care and Management of Laboratory Animals", R. Hubrecht and J. Kirkwood (Eds). Wiley-Blackwell. Chapter 25, pp. 359-369