The central
hypothesis in this series of posts on the debate about whether the dinosaurs
were warm-blooded or not is that actually some types of dinosaurs were more warm-blooded
and others were less; i.e. that there were different degrees of
warm-bloodedness among different parts of the dinosaur group.
This idea calls
for an in-depth analysis of all the different subgroups in the Dinosauria, but
first I want to give an account of some of the evidence that has been used to
argue about all dinosaurs as one.
(Note that I naturally exclude the birds
here: we know they are all fully warm-blooded.)
I hope this both serves as an introduction to the main lines of reasoning, and
also gives you an idea of why it makes more sense to split the diverse
dinosaurs into smaller groups rather than treating them all equal.
I will start off
with evidence in favour of warm-bloodedness in the whole Dinosauria, then move
to evidence against, and finish off with an argument that warm-bloodedness must
have at least evolved in one dinosaur group: the one close to true birds.
Something you
need to understand about how paleontologists gather evidence for such things as
warm-bloodedness is that there is no way
of measuring it directly from fossils. Recall from the post explainingthe different types of ‘wam-bloodedness’ that the term refers to animals
with stable body temperature (homeotherms), internal heat source (endothermic),
and/or a high basal metabolic rate, or BMR (tachymetabolic); usually
‘warm-blooded’ comprises all three in combination. It is not possible to measure
the live body temperature or metabolic activity of a dead animal that was
turned into solid rock millions of years ago. So, there is no way we can truly show conclusively that the dinosaurs fit
into any of these categories. What we can do is look at other features –
features we can tell from the fossils, and that we have reasons to suspect are related to warm- or cold-bloodedness. We
do this by examining differences between the modern animals we know are
warm-blooded (mammals and birds) and those we know are cold-blooded (the rest),
thinking about which differences could be linked to warm-bloodedness and why,
and finally seeing if we can find the same features in the fossils. In other
words, all evidence here is indirect evidence. It is important
to be aware of this, because it poses a strong, inherent limitation to the
quality of the evidence; but then again, it is the best we can do!
Evidence for
warm-bloodedness
1. The reason the
idea of warm-blooded dinosaurs was proposed in the first place is actually not
that palaeontologists realised that they were closely related to birds, as you
might think. Warm-bloodedness was suggested already in the 1980s when the
comparative anatomist Sir Richard Owen noticed that the dinosaurs had an upright (or parasagittal) limb posture,
a feature today only seen in birds and mammals, the only living warm-bloods. The
other land vertebrates typically have a sprawling
posture, their limbs splaying out to the sides, the upper part being closer
to horizontal.
The main types of limb posture of land-living
vertebrates. Sprawling is seen in most amphibians and ‘reptiles’, erect in
dinosaurs (including birds) and mammals, and the pillar-erect in rauisuchians,
ancient relatives of crocodiles, but extinct today. Image from http://en.wikipedia.org/wiki/File:Sprawling_and_erect_hip_joints_-_horiz.png
Of course, it is
not enough to base such a claim on pure correlation: there needs to be a link
between a parasagittal limb posture and warm-bloodedness. (In scientific terms,
there must be a causal relationship
between the two features.) And there is! It might be a bit complicated though,
but bear with me and I will try to guide you through.
Recall from the postexplaining aerobic and anaerobic metabolism that the aerobic variant is
about twenty times more energy-efficient, but demands and consumes oxygen,
while anaerobic metabolism works in oxygen-free conditions, but produces much
less energy. Warm-blooded animals, which use a large proportion of the energy
they gain from metabolic processes to convert into heat to warm their bodies
from the inside, therefore require a metabolism that is predominantly aerobic.
Otherwise, they would not gain enough energy from their metabolic conversion of
food to fuel their internal heat machine.
Thus, warm-bloods
must rely on a predominantly aerobic metabolism, and therefore need a large and
constant oxygen supply, in order for their aerobic metabolism to work. If they
do not get enough oxygen, they will need to resort to anaerobic metabolism to
get the energy, but that will consume twenty times as much food, which will run
out quickly.
Warm-blooded
animals then need an efficient respiratory system – i.e. a
system to get oxygen from the air into the cells of their body.
Vertebrates have lungs, sacs of highly vascularised
tissue attached to muscles around the ribs. The millions of tiny, thin blood
vessels in the lungs make up a huge surface area for exchange of gases between the blood and the air inside the lungs.
The blood that enters the lung vessels has passed through the body, delivering
oxygen for the cells to maintain themselves and taking up carbon dioxide they
leave as a waste product. Therefore, that blood has plenty of dissolved carbon
dioxide but little oxygen. When it comes in contact with the air, which has relatively less carbon dioxide but more
oxygen, both gases are exchanged. They move from where there is plenty to where
there is less, in order to balance out between the two sites. In effect, the
blood gets rid of some carbon dioxide and picks up essential oxygen.
However, after gas
exchange, the air in the lungs has as much carbon dioxide and oxygen as the
blood, so when new blood enters the lungs, they will already be balanced and no
more exchange will take place! For that reason, we need to replace the air in
the lungs with fresh air from the outside. We call this ventilation, or breathing;
for that, we use the ribs.
The rib muscles
perform cycles of contractions to alternately expand and compress the torso,
and, in effect, the lungs as well. When the lungs expand, the air pressure
inside becomes lower than the pressure of the air outside. This pressure
difference causes air to flow from the outside into the lungs. (Air always
flows from a region of high air pressure to where the pressure is relatively
lower; this is what causes wind, for example.) When the lungs contract, they
force out some air from the lungs. This is how we get fresh air in and used air
out. Without ventilation, gas exchange
will not occur, and we will quickly starve our oxygen supplies, and, as a
result, not be able to carry out aerobic metabolism.
Now you must be
wondering what this has to do with having straight or bent legs. I will come to
that just now! The thing is, except those with a parasagittal limb arrangement,
all vertebrates move forward primarily by waving their bodies sideways. We all
know that fish swim that way, and that set a precedent for their close
land-living descendants. Maybe you have not thought about it, but most
amphibians and reptiles in general move this way as well. I will not go into
detail on their biomechanics (mostly because my understanding is not great),
but, basically, their limbs are not flexible enough to reach far forward and
backward, so their strides would be very short if they only used the limbs. By also turning their limb girdles, they
achieve a considerably longer stride length, allowing fairly efficient
locomotion.
This mode of
moving is not only a residue from their fish ancestors, but is kept also in
advanced amphibians and reptiles that have lost their limbs, e.g. caecilians (a
very worm-like type of amphibians) and snakes. Exceptions to this include frogs
(which jump rather than stride) and chameleons (which actually have more of a
parasagittal limb posture than most reptiles, but this is because they are fairly
large lizards moving around on rather narrow tree branches, so they need to
have their limbs straight in order to achieve sufficient… narrowness to get a
decent grip). However, these are special
cases; they do not have a wave-like body motion for special reasons,
unlikely to be related to their metabolism.
The importance of
this locomotion style has to do with how
they turn their limb girdles: the muscles on the sides of the body, between the
front and hind legs, contract and relax alternatingly. When the trunk muscles
on the left side contract, the left hind foot is moved forward, closer to the
left front foot, and the right front foot is moved forward, away from the right
hind foot. The next step is to let the trunk muscles on the right side contract,
while those on the left relax. If the animal keeps the left hind foot and right
front foot still in the ground, the remaining two feet will be moved forward,
this time taking the left front foot further away from the left hind foot, and
bringing the right hind foot closer to the right front foot. The animal moves
forward by repeating these steps again and again, in a cycle.
I hope that made
sense, but if not, do not be alarmed, it is not essential to grasp this.
Actually, if you just look up some videos of walking reptiles on YouTube, I’m
sure it will become much clearer. Regardless, the key thing to note is that half of the body contracts with every step.
This contraction compresses the lung on that side of the body, forcing air
out.
Wow! So they breathe
by moving!... You might think so, but that is a hasty conclusion. While one
lung is compressed, the other is not expanded (I think… might need to check
this up, actually…), so air is not sucked in there. And, even if it did, it
would still mean that they only use half their lung capacity while moving. This puts a severe
restriction on their maximum oxygen intake. Since oxygen is needed for aerobic
food conversion, their ability to perform this is equally limited. As a result,
most amphibians and reptiles become completely
exhausted after a few minutes of strenuous activity, because anaerobic food
conversion is so inefficient.
Now, this means
that these sprawling-gaited animals are not
very active animals. Because their energy production is so inefficient
while they are moving, they spend most of their days resting. (In general, they
are active for only a few hours every day, during which they forage for food, carry
out domestic chores, etc.)
Now, here is where
the argument might get quite confusing: because they are not particularly
active, they would not gain much from
having an active metabolism (i.e. having a high BMR; tachymetabolic). An
active metabolism here means a metabolism that is predominantly aerobic – i.e.
the bulk of the food is converted into energy aerobically. As explained in the previouspost on this topic, such a metabolism provides stamina in the form of a fairly high and continuous energy
production, while the alternative – a predominantly anaerobic metabolism –
normally has low energy output, but is capable of mobilising vast amounts of
energy very rapidly, thus granting impressive burst speed. As a result, predominantly
aerobic animals can be active for longer periods of time. That is the main advantage of such a
metabolic strategy, but the sprawlers
would still be restricted on this very aspect due to their less efficient
walking style. They would gain little from having an aerobic metabolism, as
the advantage it would grant would still have been limited due to their poor
oxygen intake.
The advantage of
aerobic metabolism is manifested when the animal is active. But this is also when the oxygen intake
of amphibians and reptiles is as lowest, meaning that they will not be able to rely on an aerobic metabolism during activity. In other words,
their particular gait thwarts the aerobic processes at the precise point where
they would have been effective.
The disadvantage of a predominantly aerobic
metabolism is that the metabolic rates
are also elevated at rest. This means they consume more food during
physical inactivity compared to animals relying on an anaerobic metabolism,
which have very low rates of energy production (and thus also energy usage)
during rest. Having a high energy production while not using any energy is simply a waste, as there is no gain, only
higher costs of food.
To summarise, the
way most amphibians and reptiles move severely limits their oxygen intake by
compressing one of their lungs with every step. If they had been warm-blooded –
i.e. had an active metabolism – this problem would have thwarted the advantage
of stamina during activity. With the main advantage removed, the only effect of
having such a metabolism would be suffering the main disadvantage of higher
food consumption while at rest.
This is the reason why a sprawling limb posture is
associated with cold-bloodedness. Conversely, it is
also the reason why a parasagittal limb posture is thought to have a
connection with warm-bloodedness. Having the limbs straight under the body
allows them to swing forward and backward comfortably using only muscles from
the limbs and girdles. No other parts of their body need to contract or expand
during such movement, so no lungs are compressed; they can breathe to their
full capacity even while running, and, as a result, an aerobic metabolism would
prove advantageous for them!
Now, a keen
critical thinker would realise that the argument that dinosaurs were
warm-blooded because they had a parasagittal limb arrangement is based on the assumption that dinosaurs would have
evolved warm-bloodedness simply because it would have granted them an advantage.
This is just as valid as arguing that ancient apes would have evolved brains of
size and complexity rivalling that of humans because it would have granted them
an advantage. Clearly, that did not happen.
Evolution just
does not work like that. A human-sized brain is arguably an advantage to any animal (except sessile or planktonic
filter-feeders… but let us not go there now…), but it is only us, among all the
billions of species of animals that have ever evolved, that successfully
achieved such cognitive complexity. Why? Because we had the genes for it!
Without the necessary genetic material, the trait cannot develop in the first
place.
This is why one
cannot assume that dinosaurs would have developed full warm-bloodedness just
because it would have been advantageous, not without having any reason to
suspect they had the necessary genetic background. The fact that birds are warm-blooded gives us a clue
that this genetic background at least evolved somewhere within the dinosaur
group, but perhaps not necessarily at the base of it, i.e. not necessarily at
the same time as they evolved a parasagittal limb posture.
The only
difference these limb posture variants make is that if any sprawling-gaited
amphibian or reptile had acquired the necessary genetic material to achieve
full warm-bloodedness, their limb posture means they would not have gained any
advantage from it, but rather a big disadvantage, and so it would not have
become prominent in these groups. Dinosaurs, on the other hand, had the potential to gain an advantage from
being warm-blooded. That is all.
Dear God, this
discussion got far longer than I would have expected. I wanted to keep it brief
and simple. But, my desire to explain it from the most basic level made it
necessary to describe all parts of the argument thoroughly. Therefore, I
decided to actually cut here, or the post would be dauntingly long. The
remainder of the arguments for dinosaur warm-bloodedness will be the topic of
Part 2.
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