Not too hot and not too cold: Jargon in heat regulation; Its Greek to me! (Part 3)

Warm blooded and cool blooded are easily understood terms. Unfortunately in describing heat regulation of different animal species they are not always correct. There are lots of variations in how mammals, birds, reptiles, & fishes regulate their temperatures and thus many terms are used. These terms were coined long ago by learned scientists, who often used Greek for the terminology. We will briefly discuss the origin of the various terms, & their physiological meaning. In future blogs, with these basics under our belts, we will discuss the fascinating mechanisms that different animals use in different environments to regulate their temperature.

The term “warm- blooded” is commonly used to describe mammals & birds and “cold -blooded” commonly used to describe reptiles and fishes. The problem is that some mammals do allow their blood to go cold i.e. hibernation & reptilian blood can rise to temperatures much hotter than so-called warm-blooded mammals e.g. hot desert.

Thus a more accurate though more complex terminology has gradually been developed. Let’s see if we can gently wade through the terms.

Poikilotherms: “therms” is derived from the Greek word “thermos” for heat I so that part is  easy to remember. But ”poikilos” meaning “varied” is only used in science talk. “Poikilotherm” refers to animals with fluctuating body temperatures that track the changes in the ambient temperature. So this is a more appropriate term than cold-blooded for snakes etc. although harder to remember. (see figure;  adapted from Eckert Animal Physiology 5th ed; Randal D et al; Freeman & Co, 2002, p.708).

Classification of animals as homeotherms or poikilotherms based on their response to ambient temperature (Click to enlarge)

Homeotherms: this term refers to animals which maintain a stable body temperature e.g. mammals, birds, despite changes in the ambient temperature. “Homoios” means “similar” in Greek. We will later on again see the use of the word, when we discuss “homeostasis” which is the ability to maintain a stable internal environment e.g. blood glucose levels, acid base balance, as well as temperature. But the term homeothermy also has inadequacies. Fishes can also a very stable body temperature if they inhabit water with a stable temperature i.e. depths of the ocean, & some birds & mammals do allow their body temperature to vary!

Thus to overcome these fuzzy definitions a more rigorous and widely applicable classification has been developed. This is based on the source of the heat stored in the body. Animals that mainly generate their own heat through  the body’s metabolism (endogenous production) to maintain their temperature are referred to as “endotherms” and animals that mainly rely on external heat sources (exogenous) from the environment e.g. sun are referred to as “ectotherms”. “Endon” & ektos” are the Greek words for ‘within” & “outside” respectively

Animals are classified as endotherm or ectotherm based on the main source of heat, endogenous or exogenous) used to maintain their body temperature (click to enlarge)

Now lets finish off our Greek lessons with just three more words: stenothermal, eurythermal, & heterothermal. The range of body temperature that species can tolerate varies. In the Antarctic there are fishes which live at sea water temperature  i.e. between +2° & -1.86°C (lower than the latter is the freezing point of the salty sea water).  After millions of years of adaptation they are now only  able to live in this narrow range and they die if water rises above 4°C. “Steno” is Greek for “narrow” & these fish are referred to as stenothermal. Other species e.g. those living in tidal zones, must survive wide variations in temperature as the tide swells & ebbs on the sea-shore. “Eury” is Greek for “wide” and these intertidal inhabitants are referred to as eurythermal.

Hetero is Greek for other & herothermal  is the term referring to endotherms that do allow their temperature to vary when conditions demand it. Maintaining a stable body temperature in homeotherms requires a large energy intake or large fat reserves. Thus some animals allow their temperature to fall in winter time i.e. they are heterotherms. Another example of  heterothermy is found in hummingbirds who allow their body temperature to fall at night when they are unable to feed.

We needed to learn these terms to understand the following blogs on thermal stress & adaptation but enough lessons on Greek grammar for now. In the next blogs we move  to the ins & outs of thermal stress & disuss how animals adapt to it

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