How do poikilotherms thrive in a wide range of core temperatures? [closed]

Question

What is it about the metabolism of a poikilotherm that enables them to adapt to the surrounding temperature?

Answer 1

I'm not an expert in this topic but I have read a reasonable amount on animal thermal regulation, especially in insects, so maybe I can give some insight into your question. First of all, an important thing I learned is that one must differentiate poikilotherms from homeotherms and endotherms from ectotherms. According to Randall et al (1997):

• Endotherms are animals which generate their own body heat through metabolism, while ectotherms have a hard time generating enough body heat to maintain their physiological functions, as they have low metabolic rates.

• Homeotherms are animals which are able to maintan their body temperature at a more or less constant rate, while in poikilotherms body temperature fluctuates more or less with the environmental temperature.

This a somewhat troubling classification. Insects, for example, are considered ectotherms/poikilotherms, but they may show a condition called heterothermy: they are (most of the time) ectotherms, but they may be able to generate heat through some metabolic activity, and in this situation, they are showing endothermy. This happens quite often in winged insects, which may raise their body temperatures by flapping their thoracic muscles which move their wings. They may also ventilate themselves using their wings, lowering their body temperature, which (I'm not sure about this) may be considered an endothermic behaviour, as they are lowering their temperature in spite of the ambient temperature. Bees and wasps are even able to control their nest temperatures when the whole colony uses their wings to heat or cool their nest. So classifying animals in one of these categories is not so straightforward, but I'm just pointing this out because this is important. I don't think this invalidates your question, so I will talk about poikilotherms/ectotherms for now on.

One important thing to know about these animals is (Randall et al 1997):

Often, ectotherms have low rates of metabolic heat production and high thermal conductances-that is, they are poorly insulated. As a result, heat derived from metabolic processes is quickly lost to cooler surroundings. Accordingly, heat exchange with the environment is much more important than metabolic heat production in determining an ectotherm's body temperature. On the other hand, the high thermal conductance allows ectotherms to absorb heat readily from their surroundings.

As for how they control their temperatures...well, there are lots of mechanisms. I'll state some of them.

• As Remi.b stated, behaviour is an important mechanism of temperature regulation in poikilotherms/ectotherms. This happens with basking/avoiding sunlight (as previously said), but also happens in the example of wing flapping I mentioned. Another interesting behaviour mechanism for temperature control is microhabitat selection: desert ants, for example, may face soil temperatures as high as 70° C when searching for food. They lower the harmful effects of such a high temperature by standing high on their legs and/or climbing onto small grasses or rocks which they find through their way. This may seem useless to us, but it plays a big difference between life or death for them (Marsh 1985).

• Some animals have antifreeze substances in their blood which prevents them from freezing to death (see Randall et al 1997). This is common in arthropods.

• Colour may play an important part in temperature regulation, as lighter body colours make the animal absorb heat less quickly than darker colours.

• Some bumblebees may heat up by producing heat at the cost of ATP (a clearly endothermic characteristic) (May 1979).

• Body size plays a huge part in body temperature regulation, not only in endotherms, but in ectotherms too. Lighton et al (1994) demonstrate that bigger workers from a species of ant called Veromessor pergandei can withstand dessication from higher temperatures for longer than smaller workers (and they show only a few milimeters difference in their sizes).

Well, as you may see, I don't think there is a straightforward answer. The way I look at it is that, as almost anything in biology, different organisms may show different strategies of coping with thermoregulation. I suggest you take a look at the following book if you want to know more or if I haven't quite answered your question (and the articles I have cited are the ones below):

Book: Randall et al. 1994. Eckert Animal Physiology: Mechanisms and Adaptations. W. H. Freeman and Company.

Journal articles: May, M. 1979. Insect thermoregulation. Annual Review of Entomology.

Lighton et al. 1994. Is bigger better? Water balance in the polymorphic desert harvester ant Messor pergandei. (The name of the ant above changed, that's why I called it Veromessor pergandei)

Marsh, A. C. Microclimatic factors influencing foraging patterns and sucess of the thermophilic desert ant, Ocymyrmex barbiger. Insectex sociaux.