Dinosaurs' metabolism

Up: A few things about dinosaurs

With time, it has become more and more probable that dinosaurs were warm-blooded. We give here some of the facts that lead to this conclusion, together with answers to arguments for cold-bloodedness.

We give here a succinct explanation of the terms (endothermy, homoiothermy, mass homoiothermy).

The recently found feathered dinosaurs were certainly warm-blooded. The other arguments' purpose is to show why the other dinosaurs would have been endothermic too.

Table: (this are the arguments and counter-arguments for warm-bloodedness, from most to least convincing in my view)

Feathered dinosaurs

As is now known, some dinosaurs had feathers, and some others had another type of integument. For a warm-blooded animal, feathers (or hair) mean of course a better keeping of heat, but for a cold-blooded animal, it would mean a good keeping of cold, and the animal would have been absolutely unable to extract the heat from its environment.

Thus, the small, feathered theropods were almost certainly warm-blooded. And as they were small, they were certainly not mass homoiotherms.

Dinosaur hearts and pterosaur endothermy

It is almost certain that the most birdlike dinosaurs were warm-blooded. But for the other dinosaurs, we can only say that the apparition of active homoiothermy (i.e. endothermy opposed to mass homoiothermy) occurred somewhere between thecodonts (the dinosaurs' ancestors) and birds. The problem is then to know where.

Recently, a skeleton of Thescelosaurus (a dinosaur not particularly close to birds) was found with remains of the animal's heart (see http://www.dinoheart.org/index.html). The anatomy of the heart is comparable with that of a mammal or bird heart, as opposed to a reptilian heart. That is, the heart contains four chambers, with separate ventricules. In casual reptiles, the two ventricles are partially fused, which results in oxygen-rich blood be mixed with already-used, poorer blood.

The fact that this dinosaur had a need for an efficient heart and oxygen-rich blood means that it had a high activity level. Although this does not necessarily imply a high metabolism, this is strongly correlated, since high activity level means a need for high body temperature.

Another point is that pterosaurs have been found with what looks like hair, and then was probably endothermic, for the same reason as feathered dinosaurs (brief discussion of pterosaur endothermy).

Thescelosaurus is one of the dinosaurs most distantly related to birds, and pterosaurs probably split off the dinosaur lineage a little bit before the separation of the Thescelosaurus and the bird branches. Endothermy could then have emerged three times (in pterosaurs, in some non-birdlike dinosaurs and in birds), or much more probably only once, before the differentiation of pterosaurs and dinosaurs, in which case all dinosaurs (not only the birdlike ones) would be endothermic, as well as pterosaurs.

Arctic and Antarctic dinosaurs

Dinosaurs have recently been found in South Australia and North Alaska. At the time these places were nearer to the poles than today. Assuming that the obliquity of the Earth did not change, these dinosaurs must have lived throughout six-month-long nights. The local average annual temperature at the time has been very roughly evaluated from 0°C to 13°C (using hints from the plants and the oxygen isotopes), which is higher than today, but during winter the minimal temperature could have been around -11°C.

However, some primitive reptiles (champsosaurs) have been found at quite high latitudes from the middle Cretaceous of Canada.

The Australian polar dinosaurs were smaller than their tropical relatives, which indicates that there were important ecological constraints, one more hint in favour of a low temperature. Some of them have been shown to have had unusually big optic lobes in their brains, probably an adaptation to the polar nights.

It has been supposed that these dinosaurs did migrate to reach warmer places. In the case of Australia, however, an arm of the sea prevented them from going directly North, and they would had to run thousands of kilometres, which is the less probable as they were (for dinosaurs) relatively small. Moreover, some young remains have been found (in Alaska), making migration even less probable. They could also have hibernated, but they were probably too big to bury themselves, and (?) had no protecting fur. In any case, the developed sense of sight would have been useless if they had been away or sleeping during the polar night.

Thus these dinosaurs probably remained active in the cold. Mass homoiothermy could be the solution, but (besides general considerations against it, see below) the polar dinosaurs would then have had a tendency to grow, not to become smaller than their ancestors.

See Australia's Polar Dinosaurs for more information on this topic.

Mass homoiothermy

Non-endothermic dinosaurs would have been mass homoiotherms for the big species, or simply cold-blooded for the (numerous) smallest ones. Yet most dinosaur families, sauropods excepted, contain both big and small forms (and the young of big forms were small), and it is unlikely that closely related species had had different metabolisms (which implies lots of differences in daily behaviour, if not in chemical metabolism). Note that big extant mammals, descended from small ones, do not lose their warm blood to become mass homoiotherms. Moreover, polar dinosaurs, thus probably true endotherms, are known from varied families.

Furthermore, if mass homoiothermy means a slower cooling of a warm body, it also implies a slower warming of a cold body. Thus, if mass homoiothermic dinosaurs were to be submitted to a long period of cold (a few days?) they would probably not be able to recover. Anyway mass homoiothermy seems quite inefficient as a way of warming : extrapolating data on alligators, it has been computed that it would take some 86 hours under the sun to a medium-sized (10 tons) dinosaur to gain only one degree (they probably ran fast enough to go round the globe in order to get more than 24-hour-long days).

Of course, a big endotherm loses less heat and thus can take advantage of its size, but this does not mean that it is ectothermic. Big dinosaurs were helped by their size to keep warm, whether the heat came from the sun or from their food.

Thus, mass homoiothermy as the only explanation for dinosaur metabolism is not satisfactory. The widespread opinion that dinosaurs would have had a particular kind of metabolism that is not known among the extant fauna seems reasonable, but we must keep in mind that the today last living dinosaurs, i.e. birds, are perfect endotherms, and so we have no reason to suppose more strange metabolism changes during evolution.

Predator / prey ratios

A very elegant way to estimate the degree of endothermy of extinct faunas has been suggested by Bakker, who noticed that an endotherm consumes much more food than an ectotherm in order to produce heat. So the flesh-eating animals are less numerous in an endothermic fauna (roughly 3%) than in an ectothermic one (roughly one third) because they must eat more plant-eating preys.

This method is very interesting since it allows to distinguish between true endothermy and mass homoiothermy. However, it only gives information about the carnivorous animals; but we can imagine that the dinosaur preys, to escape their predators, had roughly the same activity level.

In the case of dinosaurs, the ratio of carnivorous / herbivorous animals has been evaluated from 3% to 5%, indicating endothermy.

The main problem with this approach is the way the counting is done. For one thing, we don't exactly know which animals served as preys (for instance, some carnivorous animals are eaten by others). For another thing, the fossil evidence is not easy to understand (do several bones belong to a single individual ?), and some fossilisation bias could occur.

Bone histology

Dinosaur bones show so-called Haversian systems (abundant blood vessels in the bone). These structures are commonly found in birds and mammals, and were supposedly associated to endothermy. Yet the distribution of the Haversian system in the skeleton suggests that it is more linked to the mechanical constraints on the bones and to the size of the animal, than to its metabolism.

Another interesting bone (and tooth) structure has been studied in dinosaurs. In an poikilothermic bone some growth rings appear due to the slower growth during winter. A homoiotherm has a more regular growth, and the bones have no growth rings. Both structures of bone appear in dinosaurs (sometimes in the same animal). The absence of growth rings suggests homoiothermy, but it only means that the growth rate was constant, due either to mass homoiothermy or to true endothermy. The presence of growth rings indicates periods of slower growth, but a slower growth can result from a decrease in food resources rather than a lower internal temperature (for instance, deers show some growth rings) and so is compatible with endothermy. This topic is the more puzzling as the estimation of the age of an animal got through numbering the 'growth rings' can vary from bone to bone in the same specimen...

In short, the absence of growth rings excludes poikilothermy in some species, but cannot distinguish between mass homoiothermy and endothermy. However, we must remember that mass homoiothermy is excluded for the (numerous) small dinosaurs.

Besides, the variations of temperature in a few dinosaur skeletons have been measured, using oxygen isotopes. In an ectothermic animal, the extremities of the limbs are for instance much colder, relatively to the trunk, than in an endotherm. A mass homoiotherm would probably lie in between, but we have no living mass homoiotherm to ensure this. Dinosaurs seem to have had roughly the same level of variations than mammals, thus more probably endotherms than mass homoiotherms, although the latter cannot be excluded.

Posture, behaviour, growth

Since dinosaurs have an 'erect', not sprawling, stance (i.e. their limbs were held vertically under the body), and since the only living animals with erect stance are birds and mammals, which are precisely the only endotherms, this has been seen as a hint that dinosaurs are endotherms. It might be that the improved stance go together with endothermy due to a high level of activity, and that a running animal have a need for endothermy, but the causal relationship is not clear.

It has also been suggested that the social behaviour of dinosaurs (herds, care of the young), for which fossil evidence has been found, is a hint for endothermy, because it requires high activity levels and a developed brain. Once more, the causal relationship has to be made clearer.

Dinosaur eggs appear to be relatively small in comparison to their adult size. The young had to pass through an intense growth stage, which required a great amount of food and energy. Perhaps this was possible only with the help of endothermy (but endothermy would have required still greater amounts of food), but this is not certain. It may also be that dinosaurs grew very slowly, but in this case the young would have been too vulnerable. Moreover, dinosaur species changed quite often (as fast as mammals, and much faster than crocodiles), which could hint for rapid growing rates.

Dinosaurs occupied precisely the ecological niches that are today those of mammals, and never those who are since the Mesozoic those of cold-blooded reptiles. This could be a hint in favour of a similar metabolic rate. The more that small dinosaurs were in direct competition with mammals, thus having probably the same activity level (one specimen of Sinosauropteryx has been found with a mammal skeleton in its belly).

The Sauropod feeding problem

It has been claimed that due to the need for energy caused by endothermy, the giant sauropods would have had to eat more food than their small mouth allowed. Yet it has been suggested that sauropods used gastroliths to help them digest, and smooth stones have actually been found in the neighbourhood of sauropod fossils (see The case for terrestrial sauropods). Moreover, the possible presence of a trunk (see Sauropod trunks) would have increased their feeding efficiency, as well as, perhaps, multiple stomachs and a long intestinal tract, which their huge belly could easily contain.

Of course, even if true endotherms, sauropods were helped by their large size to preserve heat, like elephants.

The Sailbacks problem

Some dinosaurs (Spinosaurus, Ouranosaurus, Rebacchisaurus for instance) show elongated vertebral spines, which probably supported a thin 'sail' of skin on their back. Their function is quite mysterious, but they may have bee used as thermal regulators: perpendicular to the sun, the sail would have collected heat, and dissipated it in parallel position. The same is sometimes mentioned for the stegosaurs' plates.

Would the need for such a thermoregulation prove that these animals were ectotherms ? No, since for example the African elephant uses its ears to the same purpose. This is the more probable as the three animals mentioned above appear roughly at the same time in the same area (Africa), and that they belong to very different families whose other members do not show such sails - hence the sails probably helped for some peculiar local constraint. If the sails were used for general heat regulation (not for second-order help), once appeared in a species they would be kept during evolution, whereas they appear sporadically from the Permian to the Cretaceous in various species. And the animal would have had to walk North-South when it was warm, and East-West when it was cold...

Note that among the sail-backed animals are some sauropods (Rebacchisaurus, Amargasaurus), supposedly the best mass homoiotherms.

The most satisfactory explanation for these sails, to me, is the ne which states that the neural spines supported fat reserves for lean periods, like the extant bison or camel. This could explain why this characteristic occurs so many times at various places of the fossil record.

The success of dinosaurs

An argument in favour of endothermic dinosaurs is their evolutionary success. They supplanted mammals, which appeared at the same time, and they occupied lots of ecological niches which required a high activity level, during more than 150 millions of years. Nevertheless, this argument alone is not sufficient to prove endothermy.

Up: A few things about dinosaurs