sort of limp-walled balloon that can supply oxygen at rates only
one tenth as high as a dog's or chicken's of comparable body size.
Iguana hearts too are of low capacity, much smaller than a heart
from a mammal with the same size of body. Some other lizards—
the monitors, especially—have more thickly walled hearts and more
complex lungs than do iguanas, but no present-day reptile can match
362 I THE WARM-BLOODED METRONOME OF EVOLUTION
the heart-lung system typical of active, modern, advanced mam-
mals and birds.
How, then, was the heart and lung system of the dinosaurs
arranged? According to orthodoxy, the dinosaurs' physical exer-
tions alternated between episodes of stolid shuffling and long pe-
riods of somnolent inertia. But the dinosaurs' limbs, as we have
already seen, were not constructed for that kind of movement.
Tyrannosaurus's legs were built for speed, vigorous prolonged ex-
ercise. To keep such huge muscles functioning, tyrannosaurs would
have needed powerful hearts. There may even be some direct evi-
dence for the size of those hearts. Today, whenever a species has
a very small heart, the front end of the ribcage can be extremely
narrow, because the only internal organ filling it is the heart mus-
cle. Iguanas are a good example: their hearts are tiny and their rib-
cage seems very constricted in the front end. The ribcages of
dinosaurs, by comparison, tended to be very deep, and the ante-
rior ribs were often thick and long. What filled those deep noble
chests? Quite probably thickly walled hearts of heroic dimensions.
To maintain bodily activity at high levels, dinosaurs would have
needed lungs with a capacity to match the output of their hearts.
And those lungs have left some clear traces in their skeletal anat-
omy. Many dinosaurs had hollow cavities in their vertebrae. A sin-
gle bone of a Brontosaurus's spine is so full of holes and indentations
that the actual bony tissue is reduced to thin partitions, often a
few millimeters thick, folded and convoluted many times to pro-
duce the major structural contours. Allosaurus and other meat-eat-
ers also had such hollowed-out vertebrae, though to a lesser extent
than in the brontosaurs. What filled the vertebral holes and hol-
lows is not difficult to see because very similar hollow backbones
can be found in today's birds. In them, the hollows are filled by
air sacs connected by tubes to the lungs. Avian lungs are excep-
tionally efficient, better at extracting oxygen than mammal lungs,
and this is due to the air sacs and the resultant pattern of air flow.
The lungs of mammals and lizards have one fundamental flaw—
they're dead-end organs. Air must be sucked into the alveolar sacs
and then squeezed directly out again. Such a method is inefficient.
The lung would work better if air flowed in one direction only,
across the lung's surfaces and then out the throat. Birds solve this
problem by means of their complex system of air sacs. They draw
STRONG HEARTS, STOUT LUNGS, AND BIG BRAINS I 363
the air first into the system of air sacs. Then they pass it from the
sacs through the lung tissue proper, so that the exchange of gases
between air and blood can happen as the air flows in one direc-
tion, on its way out. Physiologists call this a "countercurrent ex-
change" (the blood in the lung's surfaces flows in the direction
opposite to the flow of the air). Countercurrent exchange allows
oxygen extraction and carbon dioxide venting at much greater ef-
ficiencies than are possible with our mammalian dead-end lungs.
The dinosaurs' vertebral hollows are so similar to birds' that
there can be little doubt an avian-style system of air sacs was at
work in these Mesozoic animals. Moreover, the holes in the bones
represent only the periphery of the total system. Birds locate their
largest air sacs between their flight muscles and in their body cav-
Deep dinosaurian lungs. Brontosaurus had a very deep chest that must have
enclosed large lungs and a large heart. A crocodile of the same weight would
have a much shallower chest and far weaker cardiac and pulmonary
machinery.
364 | THE WARM-BLOODED METRONOME OF EVOLUTION
ity. Allosaurus and Brontosaurus must have had huge air sacs dis-
tributed throughout their viscera. And it's a good supposition that
they had evolved lungs fully capable of intense, prolonged exer-
cise. No typical cold-blooded reptile has ever evolved a dinosaur-
bird type system of air sacs.
Some dinosaurs (duckbills, horned dinosaurs) exhibited no
vertebral hollowings, but I suspect they had located air sacs fully
within their body cavity. Most primitive dinosaurs had some hol-
lowed-out vertebrae, so the ancestral dinosaurs were probably
equipped with air sacs. Many modern species of bird that have air
sacs have solid vertebrae without any hollows (like the duckbills
and horned dinosaurs). In these avian species, the big air sacs re-
main nonetheless well developed within their body cavity. So
duckbill and horned dinosaurs, though they lacked peripheral sacs
inside their vertebrae, may well have had the main system of sacs
operating between the other internal body organs.
Myths about dinosaurs die hard. As a child, I first heard the
tale of the dim-witted, double-brained Stegosaurus, a fable created
in the 1880s and still popular a century later. This story had two
sources: the pea-sized brain of Stegosaurus, and its legendary after-
brain.
Let us consider the problem of brain size. Large animals need
large brains, because the mass of their active cells requires many
nerve channels to carry signals to and from the brain, and because
the brain needs adequate capacity for processing information about
all this physiological activity. An elephant is roughly as "smart" as
a dog—they have equivalent powers of learning. But the elephant
requires a three-pound brain to achieve this intellectual level while
the dog needs only four ounces. To gauge a stegosaur's intellect,
therefore, it is only necessary to compare its brain size to that of
a mammal of the same body size. It's not hard to measure brain
size in dinosaurs. Their braincase was nearly completely enclosed
by thick bone. When the first stegosaur skulls were discovered by
Professor Marsh in 1878, scientists at Yale simply sawed open the
braincase, and measured its volume. Calculating the brain's live
weight was straightforward. In big animals, the brain fills about half
the braincase; the other half is filled out by connective tissue, a
sort of cerebral padding, packed between the outer surface of the
brain and the inner walls of the braincase.
STRONG HEARTS, STOUT LUNGS, AND BIG BRAINS I 365
Brain and sacral "brain" cavities
in an elephant and a stegosaur.
Stegosaurus's live brain size occupied about half its braincase.
Calculations made on the basis of that suggested Stegosaurus had
been monumentally underbrained compared to modern mammals
&n
bsp; of the same size. This dinosaur had been endowed with two ounces
of brain cells at most. An elephant has a brain at least thirty times
larger. So dim-wittedness is a judgment hard to avoid when think-
ing about Stegosaurus. But the cranial end was only half the story
366 I THE WARM-BLOODED METRONOME OF EVOLUTION
of its neurological system. Between its hip sockets, deep within
the backbone where the pelvis attaches to the vertebral bodies,
was a huge enlargement of the spinal cord, a swelling of nerve tis-
sue thirty times larger than the volume of the brain itself—the
stegosaur's "second brain." Professor Marsh found sacral enlarge-
ments in other dinosaurs, too, but none as spectacular as in Stego-
saur us.
What did Stegosaurus accomplish with this afterbrain? En-
largements of the spinal cord came as no surprise to experienced
anatomists—nerves enter and leave all along its length from head
to tail. Each vertebral segment is endowed with its own set of out-
going and incoming signal lines, and wherever organs are espe-
cially big and complex, the cord is swollen by additional nervous
tissue to help organize preprogrammed reflexes. For example, ex-
tra nerve centers are needed to regulate the sequence that makes
all the muscles operate in the proper order to carry out the smoothly
coordinated movements of a leg or a tail. Stegosaurus had huge hind
legs and especially huge tail muscles, complexly subdivided and
capable of immensely powerful movements. Enlargements of the
spinal cord in the hip area therefore made perfect sense. Ostriches
are similarly heavy in the area of their locomotor muscles, because
their wings are atrophied and most of their contractile tissue is
concentrated in the hind legs. And ostriches have an enlargement
of the spinal cord inside the hip vertebra. Stegosaurus weighed
twenty times more than the largest ostrich, so its hind-end de-
mand for coordinated nervous activity was far greater. The other
large-tailed, big-rumped dinosaurs also tended to develop very
sizable sacral enlargements— Brontosaurus was especially well
equipped in that regard.
Big-rumped dinosaurs certainly did not "think" with their af-
terbrain. "Thinking" is usually defined as the highest level of neu-
rological exercise, encompassing analysis of incoming stimuli in the
context of experience stored in the memory, and decision making
that draws upon the inborn instincts and the immediate percep-
tion of circumstance. In all the Vertebrata, only two types of brain
tissue carry out these functions, and both types are found only
within the braincase, inside the skull. Mammals think with their
cerebrum, the part of the midbrain that first evolved to handle in-
formation coming in from the sense of hearing. Most present-day
mammals have enlarged cerebral compartments, so large that the
STRONG HEARTS, STOUT LUNGS, AND BIG BRAINS | 367
cerebral lobes, which grow upward from the midbrain, expand
forward to cover the underlying forebrain completely. Birds think
with expanded midbrain tissue too, but the avian intellectual ap-
paratus develops from a different layer, the corpus striatum. Both
bird and mammal thinking organs, however, look alike from the
top. And if a bird's brain is dissected, its overall shape strongly
resembles a mammal's: a pair of expanded "thinking" lobes cov-
ering most of the brain stem.
Never, absolutely never, does "thinking" tissue develop in the
sacral enlargements of modern birds or mammals. If the midbrain
lobes were removed from an ostrich, it could still run in circles for
a while, because the system of muscle-coordinating relays would
still be intact. But it certainly couldn't think, learn, remember, and
most certainly couldn't make decisions. Therefore the sacral ner-
vous tissue in the stegosaur's rump would have helped it move
gracefully and swing its spiked tail with dangerous precision. But
this "afterbrain" wouldn't have added even one small storage area
to its capacity for remembering and deciding.
As Professor Marsh's laboratory staff examined brain after
brain, from dinosaurs, fossil mammals, alligators, he espied a gen-
eral trend in cerebral history, a common thread running through
400 million years. His observation became known as Marsh's Law.
It stated that on average, any evolutionary line of birds' or mam-
mals' brains grew steadily in size over millions of years. And, on
average, for any given body size, present-day species were brainier
than their ancient ancestors. The modern jaguar has twice the brain
size of the jaguar-sized saber-toothed cat that stalked the Ne-
braska woodlands thirty million years ago. And the modern loon
possesses twice the brain of the loonlike birds Marsh excavated
from the Cretaceous sediments of Kansas. Most of this evolution-
ary upgrading was focused in the centers of higher learning—in
mammals, the cerebral lobes had enlarged under the guidance of
natural selection; in birds, it was the corpus striatum. Primates—
the monkey-ape clan—scored especially high in the cerebral
sweepstakes through the ages. A chimpanzee has a brain four times
larger than a jaguar of the same weight. But the example ne plus
ultra of Marsh's Law is ourselves. The average human brain is seven
or eight times larger than that of the average modern mammal of
the same size. And our bulging cerebral lobes are forty times the
368 | THE WARM-BLOODED METRONOME OF EVOLUTION
size of those found in the average 120-pound mammal of the Pa-
leocene Epoch, sixty million years ago. Since all the dinosaurs'
higher intellectual functions had to be carried out inside their tiny
braincase, Stegosaurus and all the other large dinosaurs must have
been single-brained and dim-witted—or so it must be supposed.
But Marsh's law doesn't apply to true Reptilia. The brains of
living alligators are no larger than those of their most ancient
crocodilian ancestor of Mid Triassic times, over 200 million years
ago. Turtles, too, took the low road in the cerebral race. Frogs,
snakes, salamanders, and most fish have been content with the low-
capacity mental equipment that was in vogue back in the Coal Age,
300 million years ago. Since turtle, lizard, frog, and snake species
together outnumber mammal species three to one, it must be con-
cluded that dimwits can be part of an ideal adaptive mode, at least
for small creatures.
Dinosaurs—with few exceptions—showed few evolutionary
tendencies toward developing greater intellectual prowess. Most
dinosaurs had brains no greater in size than a turtle or croc of the
same bulk. And the pin-headed dinosaurs with their tiny skulls
relative to their body mass had outstandingly small brains.
Big-headed dinosaurs, such as Triceratops, had larger brains than
stegosaurs but were still far short of the cerebral capacity of any
large modern mammal. Were dinosaurs then incredibly stupid? Most
popular works today still say so. And it is difficu
lt to deny that
most dinosaurs would seem dullards compared to a good Labrador
retriever, circus elephant, or jaguar. There is a rough correlation
between brain size and intelligence today, and there must have been
in the Mesozoic. Humans, chimps, large dogs, and adult alligators
are roughly the same size. Humans, if they're careful, can outwit
chimps. Chimps routinely outwit large dogs. And large dogs can
learn more and make cleverer decisions than can alligators. Most
dinosaurs probably would have fallen in the category of the alli-
gators, and no living species with brains as small as the dinosaur's
would be called clever.
As soon as the dinosaurs' average brain size became widely
known, some paleontologists jumped to the conclusion that tiny
brains proved cold-bloodedness. At first sight, this notion seems
reasonable. No living cold-blooded reptile has a big brain. Birds
and mammals are the only warm-blooded vertebrates in the pres-
STRONG HEARTS, STOUT LUNGS, AND BIG BRAINS I 369
ent ecosystem, and both have high average brain-to-body weight.
Some physiologists even went so far as to claim that large brains
caused warm-bloodedness—presumably the physiological coor-
dination required to balance heat production, sweating, panting,
and blood flow implied large areas of cerebral circuitry. But this
hypothesis is demonstrably wrong. Mammals and birds don't use
their expanded midbrain lobes for thermoregulation; that chore is
carried out in the brain stem, the oldest part of the brain in evo-
lutionary terms. There would be no difficulty in guiding a warm-
blooded system with an alligator-sized brain stem.
The link between being warm-blooded and having big brains
must be, at best, an indirect one. Brain tissue is vulnerable to
changes in temperature. Human brains addle when heated to 108°F
even for a few minutes, and higher cerebral functions become er-
ratic when the brain is chilled below 90°F. So warm-bloodedness
and a constant body temperature are prerequisites for a large brain.
It may well be that warm-bloodedness evolved first, and the evo-
lution of large brains followed. Warm-bloodedness would have been
an advantage even to an animal with an alligator's brain power,
because a constant high body temperature speeds growth, accel-
Robert T Bakker Page 37