Dinosaur warm-bloodedness: The meaning of 'warm-blooded'

 Being ‘warm-blooded’ has nothing to do with the temperature of the blood. It is a possibly misleading colloquial name for a certain physiological condition. What it means cannot be explained in one sentence, and one could write several books about the vast implications for the biology of an animal.

To really appreciate the debate about whether some dinosaurs were warm-blooded or not, it is of course important to have an understanding for what warm-blooded actually means.

The formal meaning of ‘warm-’ and ‘cold-blooded’

There are three pairs of contrasting terms that can be equivalent of warm- vs. cold-blooded. Each pair relates to a certain aspect, and usually people really refer to different aspects when they just say ‘warm-blooded’. All three are however very intimately linked, so the only time you need to distinguish between them is when you want to be really precise about one specific aspect.

I will first give you a simple table that summarises these six terms, but do not get stuck on it if you do not understand: it will be explained in the text that follows.

 

Warm-bloods are typically animals with stable body temperatures, or homeotherms. The opposite – i.e. cold-bloods – is then a poikilothermic animal. These terms are only descriptive: they say nothing about how the animal achieves this, or whether these temperatures are high or low. A homeotherm is simply an animal that maintains a steady body temperature throughout the day, be it by means of behaviour, chemical reactions, or whatever, and be it warm or freezing cold. (Many homeotherms, including us humans, can further maintain different temperatures in different parts of their bodies – referred to as regional homeothermy – but they are still homeotherms because those different temperatures are each kept constant.)

When discussing warm- and cold-bloodedness, most scientists talk about endothermy and ectothermy, and, thus, these are the words most commonly thought to be equal with warm- and cold-bloodedness. Endotherms are animals with an internal heat source, while ectotherms use an external source of heat, usually the sun.

Typically, endotherms trigger chemical reactions in their metabolism that are deliberately inefficient and thus convert large amounts of chemical energy into thermal energy, which is released as heat. The chemicals that are used up are the end-products of food processing, chiefly the compound called ATP (short for adenosine triphosphate). ATP acts as storage for chemical energy extracted from food. Endotherms use up some of their ATP in wasteful reactions for the purpose of releasing heat from inside; this warms them up.

Ectotherms, on the other hand, use heat from somewhere in their environment to warm up. This heat almost exclusively comes from the sun, either directly or indirectly. (Other sources include heat from the Earth’s interior, which sips out in vents on the very deep ocean floors and supports quite large communities of marine animals; however, all dinosaurs were land-living, so let us forget about that now.)

Before we move on to the final terms, let us think about how the first four are related. Ectotherms rely on the sun to heat up. The sun sets for a large part of the day in most areas. So, ectotherms have no direct source of heat during the night. Since heat spreads from warm to cold areas, the warmed-up ectotherms will lose heat to their colder environment during the night. Thus, ectotherms are usually also poikilothermic! On the other hand, most endotherms keep their heat-inefficient chemical reactions going all day, so their body temperatures remain the same even at night (rather, they use other methods too cool down during the hottest hours of the day), so endotherms tend to be homeothermic. There are notable exceptions, but I want to discuss those later.

The final pair of scientific terms described as warm- or cold-blooded is tachymetabolic and bradymetabolic. These relate to the level of the animal’s BMR (basal metabolic rate): the amount of metabolic energy released during rest. This is essentially how much of those energy-inefficient metabolic reactions goes on while the animal is at complete rest. (During activity, the reactions switch to involving more of the efficient processes, for the purpose of converting the ATP energy into muscle movement, so to get a good measurement of the standard metabolic activity, we want the animal to inactive.) Tachymetabolic animals have a relatively high BRM, while bradymetabolic animals have low BMRs.

Again, these are purely descriptive concepts. Even though endotherms in essence are tachymetabolic, the word ‘tachymetabolic’ means ‘having a high basal metabolic rate’ and nothing more: whether this metabolic activity is used to produce heat energy, chemical energy or mechanical energy does not matter in the strict sense. However, BMR is measured as only the amount of heat energy produced, simply because we lack the technology to measure the rest (without hurting the animal).

For that reason, tachymetabolic and endothermic are synonymous in practice. Similarly, being bradymetabolic usually means the BMR is too low for an animal to be endothermic, so it must in effect rely on external heat and therefore be classified as ectothermic. Moreover, tachymetabolic endotherms are usually homeothermic, and bradymetabolic ectotherms are typically poikilothermic. The former is what is colloquially referred to as warm-blooded, and the latter is what we mean by cold-blooded.

I hope that makes sense. If it is too complicated, do not struggle too much with the details, but revise the table and make sure you at least grasp the essence of the different meanings. The next section will explain some of the main advantages and disadvantages of being warm-blooded or cold-blooded. As a little challenge for those who think they have grasped this, consider this before reading on: for warm-bloods, the advantage lies in being homeothermic and the disadvantage has to do with being tachymetabolic.

Implications of warm-bloodedness

I have been researching this since the end of my second year in high school. I needed to explain the concept in an important essay, and struggled immensely to making it easily understandable without being too wordy. Since then, whenever I am going to explain it again, I get too lazy and just quote my old essay. That is what I am going to do now as well.

The essay is titled “Would Tyrannosaurus rex, as an obligate scavenger, have had an optimal foraging efficiency if it was endothermic? – Speculations about whether an adult, scavenging

Tyrannosaurus rex would have favoured endothermy” and basically consists of a mathematical model and a theoretical argument about whether T. rex would have been better off as an endotherm if it was assumed to be an obligate scavenger (i.e. incapable of obtaining food by other means than scavenging dead carcasses; I made that assumption not because I believed it to be true – I did and do not – but because it made the model far easier!). That is why I keep referring to Tyrannosaurus in the text.

Since this is not a published piece of writing, I can get away with not using the appropriate formatting for long quotes (smaller font size and a slight marginal indent), but for the sake of clarity I will just use a different font. 

Note that I had not come across the different terms described in the last section when I wrote this, but I erroneously equated all with endothermy and ectothermy. Please do not be confused, simply understand that I did not know better back then.

Aerobic and anaerobic metabolism

Aerobic metabolism is a series of elaborate chemical reactions that convert food molecules into a form in which its energy is more rapidly mobilisable: ATP (ade­no­sine triphosphate) (Eckert & Randall 1983, Kent 2000). Aerobic metabolism requires oxygen.

     When oxygen is deficient (e.g. during in­tense activity), ATP must be produced through anaerobic metabolism, which produces far less ATP per food molecule (Bennett 1982, Eckert & Randall 1983, Kent 2000). Yet, anaerobiosis generates ATP at a considerably faster rate. However, given this and its low ATP-yield per food molecule, anaerobiosis clearly consumes much more food than aerobic metabolism (Bennett 1982).

     Anaerobiosis is an excellent means for mobilising vast quantities of energy quickly, since its rate of ATP production is high. However, its relatively low effici­ency in producing ATP per food mole­cule results in a rapid depletion of food stores, and prolonged use of anaerobic metabolism soon ensues in utter exhaus­tion. Therefore, the use of anaerobic metabolism is not sustainable (Bennett 1982, see also Bennett 1994). Anaerobic metabolism is thus ideal for energising short, explosive bouts, which are useful for short-term pursuit of prey or escape from predators (Bennett 1994, 1982). However, such bouts are unlikely to have been effective for a ponderous scavenger like Tyrannosaurus, with presumably few natural predators; for Tyrannosaurus, anaero­bio­sis would not have been useful to acquire food nor to avoid danger.

     Aerobic metabolism, in contrast, grants stamina rather than speed. Its greater ef­fici­ency in the generation of ATP rela­tive to food consumption makes aerobiosis a superior means for fuel­ling and sustaining more moderate levels of activity for a considerably longer period of time (Bennett 1982, Bennett & Ruben 1979). Such stamina should be more helpful for a scavenger, which does not need burst speed to obtain food (carcasses), but rather needs to be able to search for carrion for a longer period of time.

     Table 1 shows a summary of the relevant differences between aerobic and an­aerobic metabolism.

Table 1: A summary of the significant differences between aerobiosis and anaerobiosis.

     The efficiencies of an animal’s respiratory and cardiovascular systems set an upper limit to the amount of oxygen it can utilise, and consequently limits the amount of ATP that can be synthesised through aerobiosis (which requires oxygen). When more energy than what can be mobilised by anaerobiosis is needed, anaerobic metabolism must be used to create additional energy to supplement that produced through aerobiosis. Anaerobiosis is only used when aerobic metabolism has reached its limit and cannot supply enough energy to sustain the activity; anaerobic metabolism is then used in addition to aerobic metabolism. Since anaerobic metabolism consumes considerably more food, it is undesirable to be forced to utilise it (see Bennett 1982). A greater capacity for maximal oxygen consumption increases the amount of energy that can be supplied by aerobic means, and thus enhances the intensity of activity that can be sustained aerobically (i.e. that do not require anaerobic input). In other words, the greater the maximal oxygen consumption is, the faster can the animal move without using anaerobiosis.

     The minimum oxygen consumption of an organism is equi­valent to its BMR (basal metabolic rate) – the metabolic rate at rest (Bennett 1982, Eckert & Randall 1983, Kent 2000). Since energy consumption is proportional to oxygen con­sumption (see Bennett 1982, Eckert & Randall 1983, Schmidt-Nielsen 1972), the BMR and maximum oxygen consumption (C_max) respectively represent the mini­mum and maximum rates of aerobic con­version of food – i.e. maximum and mini­­mum food consumption. Note, however, that this ‘maximum food consumption’ actually refers to food consumed by means of aerobic metabolism; additional food may be consumed by the use of anaerobiosis to supply additional energy. There­fore, it is more useful to regard C_max as representing the maximum energetically economical food consumption. In addition, C_max also indicates the maximal level of activity (or speed) that can be sustained by aerobic means.

Endothermy and ectothermy

Endothermic animals are such that utilise elevated BMRs to maintain their body temperatures at optimal levels (Eckert & Randall 1983). That is, they increase the rate of metabolic conversion of food and use the resulting heat production to warm their bodies. This implies a substantial increment in energy demands, but grants excellent control over body temperatures, which then can be kept at levels optimal for chemical processes, including aerobiosis. They thus attain a greater C_max (Bennett & Ruben 1979). Thus, endotherms can sustain a wider range of speeds aerobically (Bennett & Ruben 1979) – at the price of greater food requirements. The only extant animals that possess endothermy are birds and mammals.

     Ectotherms, in contrast, are animals with low BMRs. They instead absorb or lose heat to their surroundings (Eckert & Randal 1983). Consequently, their body temperatures fluctuate with the temperature of their surroundings, and are difficult to maintain at optimal levels (Bennett 1982, Eckert & Randall 1983). Endotherms also exchange heat with their surroundings, but to a substantially lesser degree, as they possess insulatory structures (e.g. feathers or fur). The low BMR of ectotherms entails lower food requirements, but also a much lesser C_max compared to endotherms (Bennett 1982, Bennett & Ruben 1979, see also Bennett 1994). Reptiles are typical ectotherms.

     Since endotherms possess an adequate C_max to sustain prolonged activity at modest levels, they mainly use aerobic metabolism to mobilise energy. Con­versely, ectotherms, having a low C_max, must rely heavily on anaerobiosis during activity (Bennett 1982). Thus, endotherms have more stamina than endotherms, while ectotherms possess greater burst speed.

     Table 2 summarises the essential differences between endotherms and ecto­therms.

Table 2: A summary of some main differences between endotherms and ectotherms.

     Dinosaurs are closely related to both birds and reptiles: dinosaurs descend from (and are classified as) reptiles, and birds descend from dinosaurs (Fastovsky & Weishampel 2009). This is a major cause behind the discrepancy concerning dinosaurs’ thermal physio­logy: were they ectotherms like reptiles or endotherms like birds?

If you are curious, the references are given below:

Bennet, Albert F. 1994. “Exercise performance of reptiles”. Advances in Veterinary Science and Comparative Medicine. Vol 38B. Pp. 113-138.

—. 1982. “The energetics of reptilian activity”. In Biology of the Reptilia, edited by C. Gans and W. R. Dawson. New York. Pp. 155-199.

Bennet, Albert F. and Ruben, John A.  9 November 1979. “Endothermy and activity in vertebrates”. Science. Vol 206. Pp 649-654.

Eckert, Roger and Randall, David. 1983. Animal Physiology – Mechanisms and Adaptations. USA. Kingsport Press.

Kent, Michael. 2000. Advanced Biology. Oxford, UK. Oxford University Press. 

Schmidt-Nielsen, Knut. 21 July 1972. “Locomotion: energy cost of swimming, flying, and running”. Science. Vol 177. Pp 222-228.

The arguments about what would have been good for a scavenger is not really relevant here, but I thought it could be a nice way of showing how these concepts can be applied. (The argument does not end there, but the rest gets a bit too technical to be of any use in this post.)

Normally I would want to say something more about what I just quoted, and weave it in with what has been discussed before. But, since I will come back to this multiple times in the later posts, I think I might as well leave you here, free to think and wonder for yourselves a little!

The next post will be about evidence for and against warm-bloodedness in the whole dinosaur group.