Dinosaur warm-bloodedness: The whole dinosauria (non-avian), Part 3

Evidence for warm-bloodedness in the whole of the dinosaur group was discussed in Part 1 and Part 2. Now, we will look at some evidence against that idea. These have been used to argue that all non-avian dinosaurs (i.e. all dinosaurs except birds) were cold-blooded. We know that birds definitely are warm-blooded, but this evidence suggests that that feature is not rooted deeper in the dinosaur group, i.e. that warm-bloodedness started with the birds, or at least within their closest ancestors.

1. The first arguments relates to water loss associated with warm-bloodedness. Recall from the Part 2 that warm-blooded animals need a high rate of blood flow through their bodies in order to supply all the cells with oxygen required for aerobic food conversion into energy. From Part 1, I hope you could understand that oxygen is supplied to the blood by gas exchange, and that this cannot occur without ventilation, or breathing. Therefore, efficient oxygen supply to the body’s cells requires both high blood pressure, as discussed in Part 2, and high ventilation rates.

Gas exchange is actually not made directly between the air in the lungs and the blood vessels of the lungs. The oxygen and carbon dioxide gas molecules are first dissolved in the thin lining of water that covers the inner surface of the lungs. The dissolved gases then travel across the blood vessel walls and into the blood, where they remain dissolved, of course.

The lining water evaporates as it is heated up by the warm blood that is constantly pumped near it. This is why warm-bloods have a moist breath, which explains why glass windows get hazy when you breathe on them: that is the water vapour that evaporated from within your lungs that came out with your breath and then condensed into tiny liquid droplets on the cool window glass.

Therefore, rapid ventilation, which is needed to sustain the metabolism of a warm-blood, results in considerable water loss. And we know well that water is essential for any organism!

Both mammals and birds have solved this problem independently but in the same way: by respiratory turbinates, which are spiralled sheets of thin bone covered in moisture-trapping tissue, forming a large surface area for absorbing the water from exhaled air. These are located in the nasal cavity (which is why the air you breathe out from your nose is not as moist as that which comes out of your mouth). No dinosaurs have been found with such structures, however.

The four pictures above illustrate this point showing the independently evolved adaptations of the only known warm-blooded animals to avoid water loss during ventilation, and comparing it to crocodiles, a know cold-blood and the closest living relatives to the ancestors of the dinosaurs. Finally, it is shown that the dinosaur skulls that have been studied in this light have shown features similar to that of a crocodile, rather than of definite warm-bloods. Images from http://faculty.plattsburgh.edu/thomas.wolosz/turbinates.htm

Although I would not agree that this “strongly” suggests cold-bloodedness among the dinosaurs, it presents a difficult obstacle to advocates of the warm-blood hypothesis. Of course, there are more than one way of preventing water loss, and some can be expected not to show up in the fossils.

For example, I can easily imagine that dinosaurs may have possessed a similar structure, but made up of cartilage, or any other material that is rarely fossilised. Cartilage decomposes much more easily than bone, which is why for example shark fossils are almost always teeth. Sharks belong to the cartilaginous fish, whose entire skeleton is made up of that material; only the teeth are mineralised and thus tough as bone, which is why they are preserved far more frequently. Constructing a respiratory turbinate out of cartilage within the nasal cavity, rather than by bone, is cheaper in terms of energy investment in tissue production and maintenance. Since I doubt those structures need to be hard, it would have been more economical to build them from a softer but cheaper material. It is also possible that these cartilaginous respiratory turbinates were later mineralised into bone later in birds! However, I am not sure of why birds might have needed this: maybe there would have been more pressure or strain on them during flight? On the other hand, since birds want to be as light as physically feasible, to fly more efficiently, mineralising a cartilaginous structure into heavy bone seems counter-productive! The idea of a cartilage-based structure in dinosaurs resembling respiratory turbinates needs some more thinking about, but it seems worth looking up more information to see if there might be something to it!

Another possibility is that the trachea, the air tube that goes from the lungs to the nose, may have been lined with similar soft, moisture-absorbing tissue that the respiratory turbinates have. Dinosaurs had long necks, so this may have produced a more than sufficient surface area for efficient reabsorption of water that evaporated in the lungs. One must understand that the important features are the soft tissues that absorb the moisture, not where they are or whether they are attached to a specialised structure or not. And these tissues would not have been fossilised unless under exceptional conditions, making it highly unlikely to ever find solid evidence for this question.

While it is questionable to argue that dinosaurs might have had such tissues elsewhere – but they just have not been preserved – it is also dubious to argue that dinosaurs didn’t have them because there is no evidence. Both of these are, in essence, cases of argument ad ignorantiam, or arguing based on negative evidence – i.e. saying that something is true because there is no evidence against it – which is a logical fallacy (see The scientific method, part one).

Therefore, since the reasoning is not water-proof (hehe), the absence of respiratory turbinates in dinosaurs (or, really, the absence of any evidence for adaptations to reduce water loss during ventilation) is no solid argument against warm-bloodedness in dinosaurs. However, the issue of water loss is still an important obstacle for those who favour the idea of dinosaurs as warm-bloods. How did the dinosaurs handle this problem? An explanation is needed here!

2. The second argument is similar in nature to many of those in favour of warm-bloodedness that are based on correlation, albeit with decent theoretical connections. This time, it is about lines of arrested growth, or LAGs, which are much like growth rings on trees, but found instead in the bones of dinosaurs, as well as most cold-blooded animals, but not in warm-bloods.

Bones grow by adding new bone material on the outside of the old. LAGs are produced if the growth rate of the bones is reduced for some time. If this reduction occurs in cycles, the bone will appear banded if sliced up, with periods of normal bone growth repeatedly separated by LAGs. This is the same principle in many trees.

LAGs, just like tree rings, usually represent seasonal reduction or cessation of growth. It is usually during the winter, when it is colder, that their growth slows or stops. Since the body temperature of cold-bloods changes with that of its surroundings (see the post on the meaning of warm-bloodedness), their metabolism is more strongly affected by cold seasons. When the ambient (surrounding) temperature, and thus also the body temperature, is lower, the metabolic reactions occur at a slower rate; bone production is not an exception. As a result, cold-blooded animals, which are unable to regulate their body temperature by producing heat internally, grow less during the cold season. This is reflected in the LAGs of their bones.

LAGs of Xenopus, a common frog. Image from http://www.botany.uwc.ac.za/presents/focuson/frogs/freeze.htm

Most mammals and birds lack LAGs, because their internal heat regulation allows their metabolism to operate at normal rates largely unaffected by the outside temperature, so their bone growth is continuous. I am not sure about exceptions among birds, but there are at least a few mammals that hibernate during winter. When hibernating, the metabolism is slowed considerably, and so is bone growth, I presume. I can imagine that polar birds such as penguins may have LAGs as well. So, there are a few exceptions to the correlation (as usual…).

LAGs have been found in many dinosaurs, and have been well studied, because they can be used to, for example, determine the age of an individual, if the LAGs can be assumed to be annual. Their presence suggests that dinosaur metabolism was not stable throughout the year, and makes the warm-bloodedness hypothesis less plausible.

But, a worthy counter-argument to that interpretation points to the fact that the global temperature showed only little seasonality during the Mesozoic, the era during which the dinosaurs lived. The seasons were not as extreme back then as they are today, which invites the thought that maybe the LAGs in dinosaurs were caused by other factors than ambient temperature. I think the best would be to compare the growth rings of dinosaurs to those of contemporary cold-bloods, which would allow us to make fairer judgements on the cause of the LAGs, and, as a result, on the interpretation of them.

Moreover, as mentioned earlier, there are warm-bloods known to have LAGs, so their connection with cold-bloodedness is not definite. However, whereas we can explain the LAGs of these warm-bloods, I have not heard of suggestions for their presence in dinosaurs other than that they would have been cold-bloods.

3. The final piece of evidence against warm-bloodedness in all dinosaurs is the most persuasive, in my opinion. It basically argues that the large dinosaurs simply cannot have been warm-blooded, but would have been better off as cold-blooded because their size would have given them similar advantages, without the cost of higher food consumption.

Size plays an important role here because large bodies loose heat more slowly, or, in other words, are better at retaining heat that tries to escape. Conversely, they also gain heat at a slower rate, i.e. they are not good at absorbing heat from the surroundings. This is because heat is created during chemical reactions within every living cell in the body, while heat is transferred to the surrounding medium (air or water) only across the surface of the body. Therefore, the rate of heat production is proportional to the volume of the animal, while the rate of heat exchange with the surroundings is proportional to its surface area.

We know from simple maths that volume of a sphere is proportional to the cube of its radius (r³), while its surface area is proportional to the square of the radius (r²). Therefore, as the radius increases, i.e. as the sphere gets bigger, the volume will increase relatively more than the surface area.

Scientist usually speak of the surface-area-to-volume-ratio, which is the same as the surface area divided by the body volume:   

Since we use r to represent body size (i.e. simplifying body size by thinking about it as a sphere), the surface-area-to-volume-ratio becomes inversely proportional to body size, i.e. proportional to 

A keen mathematician would recognise that

and that the value of this fraction therefore will decrease if r increases. (Apologies for the weird use of these fraction expressions, but this blogger format does not work with fractions, so I have to include them as figures...) This means that the surface-area-to-volume-ratio decreases if body size increases, and vice versa. The bigger you are, the smaller your surface-area-to-volume-ratio. I will explain why this is important shortly.

Heat always travels from warm to cold. Always. So, a warm body surrounded by cooler air will lose heat, while a cool body will gain heat from the hotter air. The greater the difference in temperature is, the faster the heat is transferred. Thus, the rate of heat exchange depends on two main things: the thermal gradient (the temperature difference between the body and the surrounding medium) and the surface-area-to-volume-ratio. For example, an animal with a large surface-area-to-volume-ratio in an environment where the thermal gradient is small will lose heat very slowly.

Losing heat, i.e. cooling down, can be good or bad, depending on the situation. If you need to be warm, losing heat is bad, but if you have built up too much heat – usually when performing physical activities – there is a risk of overheating, which is very bad too; then, getting rid of excess heat is essential.

Overheating is naturally a severe problem for warm-bloods! Since they produce enough heat to maintain their bodies at about 37°C (mammals) and 40°C (birds) at rest, they will become considerably hotter if they start running, fighting, or any kind of strenuous physical activity. When the muscles work, their cells release large amounts of heat. Unfortunately, it is not possible to slow down the basal metabolism when active, so a net gain of excessive heat is inevitable, especially since the advantage of being warm-blooded comes when they are active (see Part 1) – i.e. it is a waste for a warm-blood to spend a lot of time at rest only to avoid overheating.

So, if they will inevitably gain excess heat, it follows that warm-bloods need ways of cooling down. Since mammals and birds usually have fur and feathers, respectively, covering most of their bodies, losing heat becomes rather difficult. Furs and heathers are highly efficient insulators – materials (in this case, structures) that do not let heat pass through easily. Both work by trapping air, creating a thin ‘layer’ of still air just over the body surface. Heat travels very slowly across still air. So, really, it is the air layer that acts as an insulator; the fur and feathers only really create that layer. If you think about it, that explains why you get cold when your body hair gets wet – water weighs the fur hairs and feather barbules down so they lose their ability to trap air, and the insulating air sheet is lost, and you start to lose the heat your body produces from within. That also shows why warm-bloods need these insulatory structures in the first place: without them, the heat we produce internally would escape too! That would mean that we would need even higher basal metabolic rates to maintain our ordinary body temperatures, by using up more food, which is not good. Consequently, losing heat presents a difficulty for warm-blooded animals. For that reason, modern forms have evolved various ways of cooling down.

Now, if we also consider surface-area-to-volume-ratios, it becomes clear that larger warm-bloods have much greater difficulties cooling down than small ones. Just think about the big, bulky elephants. They only weigh about 5 tonnes at most, but have lost nearly all hair, have enormous, thin flat ears (i.e. with a relatively large surface area with a small volume) and special spots on their bodies where their skin is extra thin, where they can lose heat more easily from, and a habit of bathing in cool mud, all to cool down their bodies that otherwise would overheat massively, especially considering that they live in the lower latitudes, i.e. with high air temperatures all year round. The average dinosaur probably weighed about half of that of an elephant, but a large number of them were much, much larger, and the temperatures worldwide in the Mesozoic were about the same as near the equator today. In other words, large dinosaurs would have had it much worse than the elephants if they were warm-blooded!

There is little evidence that the largest dinosaurs would have been capable of cooling down rapidly enough if they had been warm-blooded. It has been suggested that their long limbs, tails and necks may have worked as areas of relatively low volume, where some heat could be dissipated, but, on a hunch, it doesn’t seem quite sufficient. Some large dinosaurs have evolved thin skin sails on their backs (e.g. Spinosaurus, Ouranosaurus), while others show evidence of considerable vascularisation (i.e. concentration of blood vessels) in flat bone structures that would have been held high up (e.g. the back plates of stegosaurs, and the neck frill of ceratopsians), all which have been interpreted to have been designed for efficient temperature regulation; however, these interpretations are not solid, and there are alternative explanations for all of them.

Since it seems highly unlikely that the large dinosaurs would have been able to live if they had been warm-blooded, it appears unfeasible that all dinosaurs would have been warm-blooded.

That idea is cast in further doubt when you realise that the large dinosaurs may actually have received the benefits of being warm-blooded without high internal heat production, but by just being big. Since their large bodies conserve heat very efficiently, they may actually have warmed their bodies up sufficiently just by the heat released during muscle activity. This thermophysiological strategy is called gigantothermy (also known as mass homeothermy).

Normal cold-bloods gain heat from their surroundings, usually by basking in the sun during the days. The larger they are, the slower the sun will warm them up completely; it takes longer to get warm on the inside. They prefer it this way, because their small bodies and non-insulating skin cannot conserve any heat produced from the inside very well; consequently, there is no gain in generating internal heat.

Gigantotherms, on the other hand, can retain this internal heat better because they have large bodies. The wonderful thing about them is that their internal temperature is raised by ordinary muscle activity, rather than deliberately high and inefficient chemical food consumption. The heat generated during muscle work may not be as high as that produced metabolically by ordinary warm-bloods, but coupled with heating from the outside, it is enough to achieve fairly high and, more importantly, stable body temperatures needed to sustain an aerobic metabolism.

If you remember the different types of warm-bloodedness in the strict sense from an earlier post, you can work out that gigantotherms then are bradymetabolic homeotherms, and actually endotherms, since they utilise internal heat to warm up (although they may also use external heat sources as well… but the main heat comes from within). Whether gigantotherms should be considered to be warm- or cold-blooded, I don’t know… for that reason, I prefer think of them as separate from both; to me, they are their own kind, so to speak.

Gigantotherms can achieve high, stable body temperatures, enabling them to rely on an aerobic metabolism, but without the considerable food demands suffered by typical warm-bloods, and without the risk of overheating any large-bodied warm-blood would run. In a way, it may be considered the optimal solution, but there is one disadvantage: gigantothermy only works above a certain size, and all animals start off small. Until they reach that size threshold, they will have to live like regular cold-bloods, which may put them at a disadvantage against warm-bloods, particularly in terms of growth rates (remember that cold-bloods cannot grow continuously the whole year).

However, there is some evidence pointing towards some dinosaurs having feathers as hatchlings and juveniles, but losing them as they grow larger. I have not exact information of which dinosaurs this refers to, but it seems like a plausible solution to the problem. So, if the large dinosaurs were gigantotherms (which I have implicitly suggested), it is possible that they were regular warm-bloods in their early years, and gradually lowered their metabolic rates as they grew, becoming gigantotherms later. In that case, I would say that dinosaurs were at least much closer to warm-blooded than cold-blooded, since they must have had the necessary adaptations initially, and then simply strangle their metabolism as they grow. It becomes something of a grey area, but that is the closest I can come to arguing for some sort warm-bloodedness in the largest dinosaurs. Otherwise, it seems highly unlikely that at least those dinosaurs were full warm-bloods.

In the final part, I want to discuss two lines of evidence that I find show beyond reasonable doubt that at least the dinosaurs closest to the birds were warm-blooded. As you might suspect, it has to do with flight.