forty years of expeditions to the richest finback-bearing strata of
Texas and New Mexico. Romer was always gracious and gener-
ously shared even his unpublished information. He supplied me
with precise details of what and how much he had found in the
quarries.
Even before I started counting, it was obvious that the fin-
backs' predator-to-prey ratio was more like the spiders' than the
380 I THE WARM-BLOODED METRONOME OF EVOLUTION
Some predator-prey ratios. Present-day spiders have very high scores—25
percent or more—and so did the finback predators of the Early Permian.
Dinosaurs and mammals scored much lower, mostly ranging between 1 and 5
percent. Protomammals and crimson crocs had scores intermediate between
the fully warm-blooded and the fully cold-blooded.
lions'. Mammalian top predators are always rare, but finbacks were
overwhelmingly abundant. Romer noted that in one of his first pa-
pers published about Permian faunas in the 1920s, Dimetrodon was
the single most common genus in most locales. That went against
all the laws of bioenergetics, unless the predator had a very low
metabolism and its interaction with the ecosystem permitted it to
reach its maximum theoretical abundance. I invested half a year in
measuring every specimen Romer possessed, and extended the
census to all the other samples of finbacks housed here in the
United States and abroad. In nearly every quarry, in every for-
mation, in every habitat, the result was the same: The finback
EATERS AND EATEN AS THE TEST OF WARM-BLOODEDNESS | 381
predators were the first or second most common genera among all
the fauna. The theory of predator-to-prey ratios had proved stun-
ningly reliable with relation to the cold-blooded finbacks. If met-
abolic level was the primary agent determining the abundance of
predators, then cold-blooded Dimetrodon should have been highly
abundant in all kinds of predator-prey systems, and over species
of prey in many different habitats. The hundreds of specimens I
catalogued showed that was precisely the case. Dimetrodon was ex-
tremely catholic in its choices of prey. In sediments laid down in
quiet Permian lakes, the commonest large prey was a fish-eating
reptile, Ophiacodon. At other sites deposited in swampy flood-
plains, Ophiacodon was rare but another semi-aquatic species, the
big-headed amphibian Eryops, assumed the role of supplying food
to the finback.
Dimetrodon had no choice but semi-aquatic, fish-eating prey,
because at this stage of evolutionary history the large plant-eaters
hadn't developed very far and were only rarely of good size. But
could Dimetrodon have maintained its extraordinary abundance in
normal ecosystems, where it fed on land-living vegetarians? This
was a key question, because ultimately Dimetrodon s predator-to-
prey ratio had to be compared to that for meat-eating dinosaurs
which fed almost exclusively on plant-eaters. Fortunately, Dime-
trodon's fossil record did include two dry, floodplain habitats where
a big plant-eater—a buck-toothed reptile called Diadectes—was its
most common prey. In these two habitats Dimetrodon proved to
be as successful and abundant as it was in the systems where fish-
eaters were its main fare.
Were there any ecological situations where Dimetrodon could
not keep its populations at a very high level? I found only one. Al
Romer had excavated a highly unusual quarry in Texas called the
Geraldine Bone Bed. There the fossil skeletons were mingled with
segments of fossil logs and dark, carbon-rich stains which per-
meated the entire mass of sediment. The Geraldine was the
strangest of all Dimetrodon's habitats. Animals usually common
elsewhere were rare or absent entirely— Eryops, for example. And
animals usually very rare in the rest of Early Permian beds were
superabundant at Geraldine—the big finback herbivore Edapbo-
saurus and the eel-like amphibian Archeria. Romer concluded that
the Geraldine Bone Bed was the remains of a stagnant backwater,
382 I THE WARM-BLOODED METRONOME OF EVOLUTION
Finback meat-eaters of the Permian—efficient coldbloods in every habitat.
Finbacks provide the perfect test case for predator-prey theory. Finback
anatomy is so primitive that all scientists agree cold-bloodedness was the only
possible adaptive level. Therefore, finback predators should have been
superabundant in most habitats—and they were. Quarries dug in floodplain
sediment show high abundance, and so do quarries dug in lake limestone and
pond mudstone.
a fetid swamp where rotting vegetation had choked the river chan-
nel. Those conditions kept Dimetrodon out. Only a few tiny juve-
nile specimens were found. But this was the sole exception to the
rule that Dimetrodon maintained extremely high abundances.
At this point it might be valuable to explain a bit more about
EATERS AND EATEN AS THE TEST OF WARM-BLOODEDNESS
383
precisely how predator-to-prey ratios are calculated. A bioener-
getic census isn't made on raw numbers of specimens alone but on
estimates of total body weight as well. Body size determines how
much food energy a predator requires. And the size of the prey's
carcass, of course, determines the amount of meat available to the
predator. My technique for calculating predator-to-prey ratios
involved two steps. First, I sculpted scale models from careful re-
constructions of the animal's appearance in life. Top, side, and cross-
sectional views of the skeleton served as the basis for restoring
major muscle masses in clay. When a model was completed, I im-
mersed it in water to measure its volume. Once this volume was
measured, it was a simple step to calculate the volume of the full-
sized animal. And since nearly all vertebrate bodies are almost as
dense as water (the average carcass is 95 percent as heavy as the
equivalent water volume), the live weight could be figured with
precision.
After the live weights were determined for all the common
species, I made a census of the total meat available, the total weight
represented by all the fossils from the habitat under study. Some-
times these calculations profoundly changed the traditional pic-
ture of predator-and-prey relations. For example, in the floodplains
haunted by Dimetrodon, the commonest prey were smallish, flat-
bodied amphibians, including the boomerang-headed Diplocaulus.
Paleoecologists had traditionally reconstructed these ecosystems
with Diplocaulus in the role of chief meat-supplier to the fin-backed
predator. However, most Dimetrodons were medium-sized and big
animals, between twenty and a hundred pounds in weight. Diplo-
caulus weighed only a pound or two on average, at best a Permian
hors d'oeuvre for the bigger animal. All the Diplocaulus carcasses
together didn't amount to enough meat to keep even one mated
pair of Dimetrodons alive and healthy. The two large species of prey,
Eryops and the Diadectes, were only one tenth as
common as Di-
plocaulus, but those animals were a hundred times heavier, on av-
erage. An adult Eryops or Diadectes weighed about two hundred
pounds, heavier than most adult Dimetrodons. So most of the meat
supply for Dimetrodon came from the rarer but bigger species. This
illustrates an important general rule: Large predators obtain most
of their food from large prey.
Armed with new confidence in this method of predator-prey
384 I THE WARM-BLOODED METRONOME OF EVOLUTION
analysis, I returned to my primary goal—evaluating the dinosaurs'
metabolism. Thanks to a census by a Canadian paleontologist, a
complete count of the very rich dinosaur beds in the Judith Delta
and later sediment was available. These formations yield Late Cre-
taceous fauna, duckbill and horned dinosaurs, and other plant-eat-
ers, all of which were hunted by the tyrannosaurids. If orthodoxy
were correct, tyrannosaurs should have been as common as Di-
metrodon had been hundreds of millions of years before. But, if my
hypothesis were right, dinosaurs would show the same low pred-
ator-to-prey ratio as fossil mammals. To compare these dinosaurs
to warm-blooded mammals, I calculated predator-to-prey ratios from
some recent publications that supplied counts for saber-toothed cats
and hyenalike hunters found in South Dakota. When I calculated
the body weight of each fossil saber-tooth and of its prey and added
all the columns of data, the final ratio between the mammal pred-
ators and their total available prey proved nearly identical to the
tally for dinosaurs—both the tyrannosaurs and the mammals added
up to between 3 and 5 percent of the weight of their prey. The
case for warm-blooded tyrannosaurs was beginning to look good.
If only one dinosaur habitat had the same predator-to-prey ratio
as one fossil mammal habitat, the case for warm-bloodedness would
obviously have been weak. But, in fact, dozens of fossil dinosaurs
and dozens of fossil mammals from the full variety of sediments
exhibited the identical range. This consistent pattern had only one
logical interpretation: Dinosaurs and mammals were fundamen-
tally similar in their metabolic needs and both had a much higher
metabolism than cold-bloods like the finbacks.
I published summaries of my findings in Nature and in Scien-
tific American. Dale Russell from the Canadian National Museum
was the first to notice a curious twist in the evidence for my case.
Dinosaurs did indeed have a much lower ratio to their prey than
did finback reptiles or spiders. But their ratios were still higher
than those obtaining today in the Serengeti, in Indian game parks,
or in most ecosystems today where large mammals are the top
predators. Predatory dinosaurs average about 3.5 percent of their
prey. In the best-studied modern game park, the Serengeti, the
predators average only one tenth of 1 percent or less—in other
words, their prey is nearly a thousand times greater in number than
the predators. The average ratio to prey of all modern predatory
EATERS AND EATEN AS THE TEST OF WARM-BLOODEDNESS I 385
mammals is 1 percent or less—three or four times less than the
ratio of the predatory dinosaurs.
If dinosaurs had been as warm-blooded as modern lions, why
were their predator-to-prey ratios so much higher? That was a
question requiring an answer. Dale Russell concluded that the di-
nosaurs' metabolic rates must have been three or four times lower
than the mammals'. But I believe his conclusion was incorrect. The
predator-to-prey ratios for fossil mammals average about 3 or 4
percent, much higher than those calculated for today's mammals,
and identical to those of the dinosaurs. Do these averages imply
that the extinct mammals were cold-blooded? That is hardly likely.
The extinct predators that established the percentages were per-
fectly normal mammals—saber-toothed cats, hyenalike carnivores,
giant wolflike bears. Nothing in their anatomy has ever suggested
they were cold-blooded. Paleontologists who had studied them have
universally assumed—correctly, I think—that they were as warm-
blooded as any modern mammals.
This evidence, however, did present a lovely paleontological
paradox: Dinosaurs—supposedly cold-blooded—and fossil mam-
mals—supposedly warm blooded—both exhibited the same pred-
ator-to-prey ratios, which were higher than those of any modern
mammal habitat. Did such an apparent paradox have a solution? I
suspect it will be found in a proper understanding of a basic geo-
logical axiom called "uniformitarianism." Usually defined as mean-
ing that the present is the key to the past, the central assumption
of uniformitarianism is the idea that the natural processes seen in
operation today are the only forces that were at work in the past.
In general, that is a reliable assumption. But taken to an extreme,
the concept is used to argue that all ancient ecosystems were or-
ganized exactly like present-day habitats. Such an argument would
insist that no extinct warm-blooded predator could reach a ratio
of 4 percent of its prey because warm-blooded carnivores today
rarely attain that level. Extreme uniformitarianism would also be
forced to maintain that 3 or 4 percent ratios for fossil mammals
were wrong, the result of unknown distortions in the processes of
fossilization.
Such criticism of the argument from predator-to-prey ratios
assumes that today's world is normal and typical of all of the earth's
history. That is simply not the case. In many ways modern ecosys-
386 | THE WARM-BLOODED METRONOME OF EVOLUTION
Why the Serengeti lion is so inefficient. Five sets of forces team up to make a
lion's life hard on the Serengeti Plains: 1) The most common plant-eater is
the wildebeest, an antelope that prefers the wide-open, treeless plains where
a lion has a hard time stalking prey unawares; 2) Severe summer droughts kill
off thousands of wildebeest each year, and most of this meat is wasted as far
as the lion is concerned; 3) Some plant-eaters are so big and aggressive that
lions can't make kills; 4) Human herders and hunters harass the meat-eaters;
and 5) Human tourists in minibuses add more aggravation.
But back in the Eocene Epoch, warm-blooded mammal predators had a much
easier time. Dense forest gave lots of opportunities for ambush and most of
the plant-eaters were small enough to be caught and killed easily.
EATERS AND EATEN AS THE TEST OF WARM-BLOODEDNESS
387
Dinosaurian predators played by hot-blooded rules. Some dinohabitats were
like the Serengeti—summer droughts killed masses of plant-eaters,
woodlands were open and made ambush difficult, and the plant-eaters were
huge. The Jurassic at Como was like this, and here the predators were rare
and inefficient. But in the Late Cretaceous of Alberta, the plant-eaters were
much smaller, the forest was denser, and the summers were far less dry. So
the Alberta predators were more common and more efficient.
terns are abnormal, di
storted by unusually dry climates and by the
intrusions of human activity. The following figures are instructive.
Under ideal conditions, like those found in a game park or a well-
run zoo, lions require a minimum of ten times their own weight
in meat per year to live healthily and reproduce. So 10,000 pounds
388 | THE WARM-BLOODED METRONOME OF EVOLUTION
of meat suffice as a full year's supply for a family of lions weighing
a total of 1,000 pounds (one 350-pound male, two 250-pound fe-
males, and three 50-pound cubs). To supply this, a herd of deer
or antelope weighing 20,000 pounds would be required. In this
ideal situation, therefore, the predator-to-prey ratio can be found
by dividing 1,000 pounds of lion by 20,000 pounds of prey—5
percent. Why, then, are the ratios in the Serengeti only one tenth
of 1 percent?
The answer is that the grasslands and woodlands of the Ser-
engeti are very far from ideal. The savannah covered by short grass
is poor hunting ground because there isn't sufficient cover to al-
low the lions or hyenas to approach their prey. As a consequence,
the predators are inefficient and do not catch enough prey to make
an ecological difference, so the vegetarian herds grow bigger and
bigger. And humans compound the situation. Herdsmen and
ranchers kill off predators to protect their livestock. Poachers and
pelt hunters kill for the skins. Hordes of tourists insist on harass-
ing the predators during their hours of rest. In consequence, the
Serengeti predators never build their populations to full potential.
Is it any wonder why the predator-to-prey ratios are so far below
the maximum possible with prey multiplying so abundantly and
predators multiplying at such a minimum? Clearly, the predator-
to-prey ratios of this modern game park are most unreliable guides
for any understanding of the past.
Most of the habitats frequented by fossil mammals or dino-
saurs were not nearly as hard on predators as the Serengeti. The
ancient ecosystems were not generally as treeless and, of course,
were free of any interference from humans. It would not be sur-
prising therefore to find higher predator-to-prey ratios obtaining
in the fossil samples. In addition, many ancient predators enjoyed
Robert T Bakker Page 39