POIKILOTHERMS: EVOLUTION, REGULATION, BENEFITS - BIOLOGY - 2025

Poikilothermo (poikilos, multiple or varied) refers to those animals that cannot regulate their internal temperature, so their body temperature fluctuates with room temperature.

Historically, zoologists used other more widely used terms such as "cold blooded" to refer to a diverse group of animals. However, it is a term that in a strict sense is inefficient to separate two groups of animals.

Source: Bjørn Christian Tørrissen

Another term widely used to refer exclusively to the source of body heat is "ectotherm," like those animal groups that depend almost entirely on environmental heat sources. Thus, the combination of these terms provides valuable information on the way in which animals regulate their body temperature.

Temperature regulation in poikilothermic organisms

Animals throughout their evolution have used strategies to preserve their internal environment in optimal conditions and maintain normal cellular functioning, in addition to optimizing the expenditure or saving of metabolic energy.

Poikilothermic animals produce comparatively less metabolic heat than endothermic animals. Therefore, the exchange of caloric energy with the environment is of great importance to determine your body temperature.

In this sense, a poikilothermic animal absorbs heat from the environment if it needs to raise its body temperature, behaving as thermal conformists, since they depend on the temperature of the environment. In energy terms, they constitute poorly isolated animals.

First of all, they have low rates of metabolic heat production that dissipates quickly into the surrounding environment and does not contribute significantly to the increase in body temperature. On the other hand, they have a high thermal conductivity, which allows ectotherms to absorb heat easily.

In most cases, ectothermic organisms have a behavioral regulation of body temperature. For example, snakes and lizards bask until they reach a suitable temperature for efficient muscle function, mitigating the effects of the environment through behavior.

Metabolism in poikilothermic animals

It is well known that biochemical reactions are sensitive to temperature, since the activity of many enzymes has an optimum temperature. Any change in temperature alters the efficiency of the enzymatic machinery, constituting an impediment for the animals.

If the temperature drops to a critical level, the speed of metabolic processes is compromised, reducing energy production and the amount that can be used by animals for their activities and reproduction.

In contrast, if the temperature rises too much, metabolic activity is unstable and even destroyed. This allowed to establish optimal ranges for the development of life between 0 ° C to 40 ° C.

Body temperature in poikilothermic organisms is not constant as in the case of homeothermic (endothermic) organisms.

In this case, although heat is generated as a product of metabolic activity, the latter is lost as quickly as it is generated. The internal temperature does not depend on the mechanism of combustion of food as in the case of homeotherms.

Generally, poikilothermic animals are associated with bradymetabolic-type metabolism. However, it is a condition that only strict ectothermic organisms meet, bradymetabolism being the metabolism in a state of rest.

Poikilothermia in nature

Poikilothermia is the most common type of thermoregulation in the animal kingdom. Within this group are the groups of lower vertebrates such as fish, amphibians and reptiles and the vast majority of terrestrial and aquatic invertebrates (with some exceptional cases).

In aquatic poikilotherms, the body temperature is basically the same as that of water due to its caloric characteristics. On the other hand, terrestrial organisms could have a temperature higher than the air temperature due to the effect of radiation.

Ectothermic animals, through their behavior, exploit places with more favorable temperatures, however, as previously indicated, the source of energy used to increase their body temperature comes from the environment and not from inside the body.

In tropical areas, ectotherms such as reptiles compete effectively with mammals, in many cases surpassing them in abundance of species and individuals. This is because the constant temperature of the tropics allows activity throughout the day and also dedicates the energy saved in reproduction activities and survival.

This advantage tends to diminish in temperate environments where, due to adverse conditions for ectothermy, endothermic organisms are favored.

Benefits and costs of poikilothermia

Since the body temperature of many ectotherms depends to a considerable degree on the environment, ectotherm species that live in places with temperatures below freezing can have problems.

However, they have developed responses as substances to prevent nucleation of ice crystals in extracellular fluids and thus protect cytoplasmic fluid, supercooling and antifreeze substances in body fluids.

In hot environments, the tissue functions of most ectotherms are hampered. Due to a lower affinity of hemoglobin for oxygen within the higher temperatures of the body temperature, it prevents the animals from performing demanding activities, due to its low rates in aerobic metabolism.

The latter brings with it the development of an oxygen deficit during anaerobic respiration and the limitations of achieving large sizes.

Ectothermy is a slow life form with small energy flows, that is, with modest energy requirements. The latter allows them to occupy unused terrestrial niches by homeothermic vertebrates, investing less energy to produce heat and more in growth and reproduction activities.

Evolution of ectothermy in dinosaurs

Since the raising of the first fossils there has been a debate as to whether dinosaurs were homeothermic or poikilothermic. As we already know, ectothermy involves low metabolic investment to generate heat and instead energy available from the environment is used to regulate body temperature.

This obviously brings with it a series of problems such as the lack of radiation or solar energy at night or the fact that the habitat is warm and cold. Traditionally, given the relationships between dinosaurs and current reptiles, dinosaurs were classified as ectotherms.

However, due to the lifestyle that was deduced about dinosaurs, several arguments support that they were endothermic animals.

The first is that they had superficial insulation (feathers in Archeopteryx), which would constitute a barrier for the absorption of energy from radiation and for an endotherm, assuming a way to maintain metabolic heat.

Many of the fossil finds occurred in temperate zones, which is why it is considered to be endothermic to survive the climate with metabolic heat. Other evidence suggests that the relationship between predator and prey is characteristic of endothermic and non-ectothermic animals.

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