Having just started on my science post, and realising it will be longer than expected, I figure I should feed you some of the last useful posts from my old website. Also, this Monday we are having a one-day fieldtrip (we just found out...), so I might bring something worth telling from there as well. Meanwhile, I apologise for the time it is taking to write the big thing up, but stuff keeps coming up that need to be done – such is university!
Tetrapods
(land-living vertebrates) breathe because our internal chemical reactions
consume oxygen. Without oxygen, we would simply not be able to live. (This
should not surprise you.)
The chemical
reactions in question are described in detail in the article about aerobic
and anaerobic metabolism. Here, I will briefly summarise the key points.
Aerobic metabolism refers to a series of chemical reactions involved in aerobic cell respiration, which
basically uses oxygen to release energy from food for the cells to carry out
their functions. There is an alternative way of extracting energy from food
without the use of oxygen – anaerobic
cell respiration – but it consumes about twenty times as much food to
provide a given amount of energy.
There is a limit to
how much oxygen an animal can absorb; this depends on its lung capacity. A large
influx of oxygen is essential, because when the oxygen supply cannot meet the
energy demand of the cells (e.g. during intense activity), anaerobic cell
respiration must be used to supply the additional energy required – which is
undesirable since it consumes much more food.
Lung capacity can
partly be related to posture. Dinosaurs, just like mammals and birds (as well
as some archosaurs, such as pterosaurs and rauisuchians) had an erect or parasagittal limb posture – they held
their limbs vertically under their bodies (seen from the front). This is
contrasted by the sprawling limb arrangement of most other reptiles, and
amphibians, where the upper bones of the arms and legs are held almost
horizontally and pointing away from the trunk.
Animals with a
sprawling posture move very differently from those with a parasagittal limb
arrangement. Sprawlers move by alternately contracting their left and right
flanks. This helps moving the limbs forward, making each step considerably
longer, but at a fatal cost: when contracting each side, the lungs are also
compressed, preventing air from flowing in. Consequently, sprawlers cannot breathe while they run. Thus, their lung capacity,
and therefore also oxygen influx, is severely limited during strenuous activity
– precisely when they need oxygen the most. When animals with parasagittal
limbs move, the lung capacity is not affected. There is, of course, still a
limit to how much oxygen they can extract, but the lungs are not impeded by
movement, allowing them to maintain a continuous inflow of oxygen, reducing the
need for anaerobic supplement.
How does this relate
to warm-bloodedness, then? The above example illustrates that animals with a
parasagittal limb posture can rely on a predominantly aerobic metabolism – i.e.
they extract their energy mostly by aerobic cell respiration. This allows them
to extract more energy from a given amount of food.
Modern animals with
parasagittal limbs (mammals and birds) exploit this advantage by spending a
substantial amount of energy on producing heat internally. They can afford this
because their energy economy is so efficient. The heat is produced by speeding
up chemical reactions in their cells – they are said to have a high basal
metabolic rate (BMR), or being tachymetabolic.
In other words, they are actively consuming considerable amounts of food even
when at rest, in order to keep their bodies warm. Their efficient respiratory
system enables them to maintain high metabolic rates even when engaged in
active movement. Sprawlers, however,
cannot have high BMRs because their respiratory system is too limited (either,
they are not able to maintain elevated metabolic rates even at rest, or the
additional demand during strenuous action simply becomes too much).
Another way the
parasagittal limb posture is linked to warm-bloodedness is by activity levels.
Warm-blooded animals have a superb advantage over cold-blooded ones because
they can be active for a considerably longer proportion of the day. Cold-bloods
tend to spend a lot of time lying still, either in ambush or for behavioural
thermoregulation (sun basking to warm up, or cooling down in the shade), while
mammals and birds are active for a substantial proportion of the day (or
night). And a parasagittal stance is designed for being capable of long-term,
energy-efficient activity.
There is nothing
saying that dinosaurs must have been tachymetabolic and/or highly active
because they had a parasagittal limb posture. But the fact that the only
tachymetabolic animals today are those with parasagittal limb arrangement
suggests that there is a tight relationship. We may infer that dinosaur at
least were capable of being
warm-blooded thanks to their erect stance.
Birds have the by far
most efficient respiratory (breathing) system known. For a detailed account of
this system, please see the article on birds. In brief, they rely on
so-called air sacs – cavities in
their bones – where they pass the air after gas exchange (i.e. oxygen has been
replaced with carbon dioxide), before it is exhaled; this allows the birds to
maintain a continuous flow of air through the lungs – in contrast to other
tetrapods, where the lungs are alternately filled and emptied – and so enables
optimal oxygen extraction from the air. It is thanks to this ingenious design
that birds can fly across oceans in their long seasonal migrations: not only
does it allow for extensive aerobic respiration; it also makes their skeletons
lighter.
Ample fossils of
dinosaurs with pneumatised bone have
been found. This bone contains air cavities like those of birds, inviting the
idea that dinosaurs possessed a similar, if not identical respiratory system to
that of modern birds. Although few dinosaurs flew, impressive stamina and a
light body can provide many advantages, such as allowing predators to engage in
fast, lengthy pursuit of prey.
However, since
mammals do not possess such adaptations, bone pneumatisation does not have a
strong connection with warm-bloodedness. It is not a requirement, but more of
an extension to enhance the efficiency of the fundamental feature – aerobic
metabolism. Pneumatic bones supports the idea of dinosaurs as using aerobic
metabolism to a considerable extent – perhaps more, perhaps less, than modern
warm-bloods. On the other hand, this is not compulsory: it is possible that the
sole purpose of the pneumatisation was weight reduction, which may have been
important for the larger dinosaur forms. However, this line of reasoning does
not explain why the bone pneumatisation was most well-developed in the smaller
forms!
A final piece of
evidence related to the aspect of breathing is the possession of a secondary palate – the structure
dividing the nasal and oral cavities and so enables breathing while chewing – by hadrosaurids and ankylosaurids, as
well as mammals. This is highly useful for herbivores employing extensive
chewing of their food. Without the secondary palate, there would have been a
greater risk of food ending up in the lungs, if the animal would try to breathe
during mastication. The ability to chew and breathe at the same time removes
the need to compromise between food and oxygen intake, both being crucial to
sustaining a high metabolic rate.
Now, a secondary
palate is by no means necessary for warm-blooded herbivores: birds do not
possess them. Moreover, cold-blooded animals such as crocodiles and turtles
also have secondary palates, making arguments for dinosaurian warm-bloodedness
based on this correlation very weak. Rather, it seems that the secondary palate,
like bone pneumatisation, would improve
warm-bloodedness, if already present. (Note that it can improve the conditions
of cold-blooded animals too – recall the crocodiles and turtles!)
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