Robert T Bakker

by The Dinosaur Heresies (pdf)

  erates reproduction, optimizes muscular output, and increases

  digestive efficiency. And once acquired, it might have launched an

  evolutionary tendency toward larger brains.

  More importantly, a survey of brain sizes in today's creatures

  leads me to conclude that large brains aren't essential for having a

  high metabolism. Humans have the biggest brains ever evolved for

  our weight class. But we don't possess a higher metabolism than a

  German shepherd, which has a brain one seventh the size of ours.

  Ostriches have tiny heads for their bodies, and ostrich brains are

  only one fortieth the size of that of a human of the same weight.

  Does an ostrich therefore have a metabolism one fortieth that of

  a human? On the contrary, its metabolism is higher, pound for

  pound, than ours. It is thus very difficult for me to believe that

  metabolism and brain size evolved in a kind of evolutionary lock

  step.

  I would contend that the only effective way to analyze the

  connections between the evolutions of brains and of metabolism

  is to reconstruct the fossil history of intelligence separately from

  the fossil history of warm-bloodedness. Then the two stories may

  be compared side by side. The microtexture of bones can be em-

  370 | THE WARM-BLOODED METRONOME OF EVOLUTION

  ployed to compute metabolic levels. Judging by such criteria, even

  the primitive dinosaurs of the Triassic Period had a metabolism as

  high as a modern mammal's. The debut of warm-bloodedness in

  the Dinosauria certainly occurred when their brains were small.

  The same pattern emerges in the history of our own Class Mam-

  malia. Bone texture demonstrates that the ancestors of mam-

  mals—the protomammals of the Permian Period—had already

  evolved the essentials of warm-bloodedness quite long before the

  first large-brained mammal made its appearance. This similar his-

  tory in both the dinosaur and mammal lines makes a good case for

  warm-bloodedness coming first, followed, much later, by larger

  brains.

  If dinosaurs were warm-blooded as far back as the Triassic, it

  could be expected that at least one of their later lines might have

  evolved some sort of higher intelligence. And indeed some did.

  Dale Russell from the Canadian National Museum discovered the

  top of the braincase of a turkey-sized predator named Stenonycho-

  saurus, in the Judith River sediments of Alberta. Clearly im-

  pressed into the inner roof of this braincase were indications left

  by a pair of bulging midbrain lobes. Russell concluded that his Al-

  berta dinosaur had possessed a brain at least as large as that of many

  present-day birds of the same size. The dune sand laid down in

  Mongolia during the Cretaceous has preserved several skulls of

  small dinosaurs closely related to Stenonychosaurus. The Mongo-

  lian species also seem to have carried brains far larger than those

  of alligators and lizards of comparable weight. These large-brained

  dinosaurs were evolving quickly in many of their adaptive com-

  partments. And they probably were every bit as endowed as the

  Late Cretaceous mammals that scampered over those very same

  sand dunes.

  Why didn't these dinosaurs of Alberta-Mongolia continue to

  evolve ever larger cerebral systems? Why didn't they eventually

  produce super-intelligent species capable of making stone tools,

  smelting iron ore, programming computers, or writing master's

  theses about Dino-Proust? Dale Russell believes they could have

  if the dinosaurs had been given longer to live. Unfortunately, the

  larger-brained dinosaurs were among the last, and the merciless

  hand of extinction fell upon them just as it fell on all the Late Cre-

  taceous groups.

  But Russell has indulged in a bit of "what-if" paleontology.

  STRONG HEARTS, STOUT LUNGS, AND BIG BRAINS I 371

  The turkey-sized Stenonychosaurus, a

  predator from the Late Cretaceous

  of Alberta, had a brain as large as

  many modern birds of the same

  size.

  What if the Cretaceous extinctions hadn't wiped out the dino-

  saurs? What if the Alberta bigbrain had continued to evolve? Rus-

  sell has reconstructed the final evolutionary product which he

  believes the large-brained dinosaurs would have produced had they

  survived until the present: a hundred-pound biped with bulging

  forehead, scaly skin, and clawed hands capable of cleverly manip-

  ulating objects. One could quibble about details, but Russell is

  probably correct in general. Moreover, those large-brained dino-

  saurs were certainly clever for their time and probably hunted the

  rat-sized mammals of the period. Russell believes, in fact, that they

  were the chief predators on Cretaceous mammals, and I tend to

  agree. As long as they existed, the mammals could not and did not

  evolve to any size larger than a cat. And if these dinosaurs had

  continued to evolve past the end of the Cretaceous, it's a fair bet

  they would have continued to suppress mammal evolution. The

  i

  372 | THE WARM-BLOODED METRONOME OF EVOLUTION

  Big-brained ostrich dinosaurs. Close relatives

  of Deinonychus and stenonychosaurs and

  tyrannosaurs, ostrich dinosaurs like

  Struthiomimus had brains as large as modern

  ostriches of the same weight.

  dinosaurs would then have continued their own history of adap-

  tive proliferation, right down to the present era.

  Then how would our ecosystem be organized if Russell's sce-

  nario had been real rather than hypothetical? You and I, dear

  readers, would probably be members of some tiny species, eking

  out a terrified living under the ever-present shadow of a dinosau-

  rian overlord. And this book would have been written by a super-

  intelligent dinosaurian—a member of the elite species that had

  evolved four-pound brains, invented language, and built printing

  presses—on the subject of his own history. If dinosaurs had evolved

  to write their own history, they certainly wouldn't make the mis-

  take of believing their Mesozoic forebears were cold-blooded.

  374 | THE WARM-BLOODED METRONOME OF EVOLUTION

  18

  EATERS AND EATEN

  AS THE TEST OF

  WARM-BLOODEDNESS

  Afinal test for the theory of warm-blooded dinosaurs is what

  they ate. A warm-blooded animal consumes ten times as many

  calories per year as a cold-blooded creature of the same size. If a

  seven-hundred-pound Allosaurus were producing metabolic heat

  every minute of its life at a rate as high as a modern seven-hundred-

  pound bear, its meat consumption would have to be enormous.

  But if that allosaur operated like the traditional cold-blooded di-

  nosaur, then it could bask in the warm Jurassic sunshine, soaking

  up the solar calories until it reached its preferred body tempera-

  ture without squandering energy derived from food. Which hy-

  pothesis comes closer to the truth?

  By 1970, my studies of dinosaur limbs had already persuaded

  me that dinosaurs were designed for high levels of loco
motor ac-

  tivity. I had also suspected that the dinosaurs' metabolism more

  closely resembled a giant bird's than a giant tortoise's. How else

  could they have suppressed the evolution of mammals for more

  than a hundred million years? But how could anyone measure me-

  tabolism in a fossil? It seemed a completely forlorn prospect.

  Sometime in 1970, Elwyn Simons, professor of primate pa-

  leontology at Yale (now a member of the National Academy of

  Science), provided me with an invaluable insight. He was discuss-

  ing the fossil mammals he had been excavating for a decade in

  Wyoming, in Egypt, and in India's Siwalik Hills. He observed that

  EATERS AND EATEN AS THE TEST OF WARM-BLOODEDNESS | 375

  How much do you feed a sentry-guard lizard? Warm-bloodedness is

  wasteful—so much body energy is spent on keeping warm. A one-hundred-

  pound guard dog (plus puppies) demands one thousand pounds of wet dog

  food per year for an active outdoor existence. But cold-bloodedness is far

  cheaper. A one-hundred-pound guard lizard (plus hatchlings) is happy with

  only one hundred pounds of wet lizard chow per year.

  numerous large predators were never found in the fossil record;

  they were always rare. This was because the big meat-eater sub-

  sisted at the very top of the ecological pyramid. Its food had to

  come from the plant-eaters below. And it took roughly a hundred

  zebra to maintain the supply of meat for one lioness and her cubs.

  I realized his remarks about the scarcity of predators would apply

  perfectly to dinosaurs. If predatory dinosaurs required as much meat

  per week as warm-blooded mammals, then they would have to be

  rare. The predator-prey relationship might well serve therefore for

  the calorimeter I was looking for.

  The theoretical concept is straightforward: The higher the

  metabolic needs of a predator, the scarcer in number it will be. To

  determine the allosaurs' metabolism, all that was required was a

  count of the number of specimens and a comparison with the

  number of prey specimens found in the same strata. If allosaurs

  were always rare compared to all their prey, as rare as lions are

  relative to zebra and antelope, it would provide direct evidence

  that the predatory dinosaurs needed a very large weekly ration of

  meat. But if allosaurs were very common, say ten times more

  abundant relative to their prey than are lions, tigers, or hyenas, it

  would provide strong support for the orthodox view that dino-

  saurs shared the leisurely metabolic style typical of snakes and other

  cold-blooded animals.

  I determined on making a predator-to-prey census through the

  entirety of geological history, from bottom to top, beginning with

  the very primitive reptiles of the Coal Age, through successive levels

  of dinosaurs, and ending with the game parks in Africa and India

  today. So far my studies have taken ten years, but I believe they

  have been amply justified by the results. They have revealed a

  spectacular story of metabolic evolution, a saga of hunters and

  hunted stretching throughout the 300-million-year record left by

  evolving ecosystems—one that at last places dinosaurs in their

  proper place in the grand progression of evolution.

  Before starting the count of fossil fauna, I sought some con-

  firmation of the idea that the metabolism of predators does indeed

  regulate their scarcity and abundance. Interactions in nature are

  often so complex and unpredictable that perhaps counting preda-

  tors and prey would yield no reliable information about metabo-

  lism. For example, even if a species of allosaur was cold-blooded,

  EATERS AND EATEN AS THE TEST OF WARM-BLOODEDNESS I 377

  Dinosaur energy budgets—the food-chain restaurant metaphor. Imagine that

  a family of ceratosaurs lived their whole lives, generation after generation, in

  a restaurant where all the garbage was fossilized in a nearby river. As they

  died, the ceratosaurs would be dumped into the river along with the chewed

  remains of all the prey they had eaten.

  and therefore could have existed in relative abundance, diseases

  might keep its number low, much lower than the maximum hy-

  pothetically permitted by its metabolism. And many ecological

  agents could depress the numbers of top predators: parasites, bad

  weather, fighting between predators, competition from the scav-

  engers. What was needed was at least one test case from living

  ecosystems to show that predator-to-prey ratios might work as cal-

  orimeters for cold-blooded predators.

  Spiders came to the rescue. They can be thought of as eight-

  legged, hairy lizards, for they are perfectly cold-blooded, operat-

  378 | THE WARM-BLOODED METRONOME OF EVOLUTION

  ing at a very low metabolic level. Because spiders do not hunt over

  large territories—a few square yards are the entire dominion of a

  big wolf spider—spider predation is fairly easy to study in detail.

  Wolf spiders catch most of their prey on the move as they prowl

  their turf. Hence the analogy between a wolf spider and a cold-

  blooded vertebrate predator such as a carnivorous lizard or a pos-

  sibly cold-blooded Allosaurus is a good one.

  If metabolism determined the abundance of predators, then

  spiders should produce huge populations compared to their prey.

  In Africa's game parks, the ratio of predator to prey among mam-

  mals is 1 percent or less—that is, there is roughly one lion or hyena

  for every one hundred large prey animals (zebra, wildebeest,

  warthog, bushbuck, etc.). Spiders have such a low metabolism that

  they could, in theory, reach a ratio ten or twenty times higher.

  And so they do. Study after study showed spider populations

  achieving levels of 10, 15, and 20 percent of their prey popula-

  tions, impossibly high by mammalian standards. There's no doubt,

  of course, that spider ecology is complicated, and that they suffer

  from the usual share of parasites, diseases, and disastrous die-offs

  from bad weather. Nonetheless, their cold-blooded metabolism

  does, on average, show through this overlay of ecological noise.

  Predator-to-prey ratios work for spiders; they correctly indicate

  cold-bloodedness.

  Would such ratios test equally well for big, cold-blooded ver-

  tebrate carnivores? In today's world there exists no predator-prey

  system in which both predator and prey are large, cold-blooded

  vertebrates. Pythons (cold-blooded) feed on deer (warm-blooded),

  and Komodo dragon lizards (cold-blooded) kill pigs and tourists

  (warm-blooded), but nowhere does a giant lizard or snake feed on

  giant lizards or snakes as its principal prey. To test the predator-

  to-prey method of analysis for this case, I had to go into the fossil

  record, back to the earliest land vertebrates that evolved into the

  role of large top predator. A top predator by definition is a car-

  nivore that eats the flesh of the largest available prey. It must

  therefore develop adaptations for dismembering the carcasses that

  are too large to swallow whole. The earliest vertebrates that evolved

&nbs
p; the requisite meat-slicing teeth were the fin-backed reptiles, which

  first appeared very late in the Coal Age, about 300 million years ago.

  Finbacks provided the ideal fossil test case for the predator-

  EATERS AND EATEN AS THE TEST OF WARM-BLOODEDNESS I 379

  to-prey concept. In the first place, they were unquestionably cold-

  blooded. Their anatomy was still on a very primitive level, more

  primitive in most aspects of their limb and backbone than today's

  lizards. Even the structure of their bone appeared emphatically cold-

  blooded under the microscope. The canals left by blood vessels

  were few and far between, proving that metabolic activity pro-

  ceeded at a very modest pace.

  In the second place, the finbacks were large—early species grew

  to the size of wolves and leopards, forty to eighty pounds, and later

  species were larger still, up to two hundred pounds or more, the

  size of the average lioness. These big finbacks seemed large enough

  and well enough armed to deal with any animal in their ecosys-

  tem. Their heads were proportionately large, and armed with strong

  killing teeth in front and razor-sharp rear teeth for cutting up even

  the largest carcass. Moreover, these creatures are found in nearly

  every fossil habitat: swamps, lakes, streams, swampy floodplains,

  dried-out floodplains. With such ecological diversity, it was possi-

  ble to determine whether the predator-to-prey ratio changed from

  habitat to habitat. And finally, all the species of prey available to

  them were incontestably cold-blooded as well.

  By the time I had fixed upon the finbacks as a key test for my

  method, I left Yale for Harvard. There I met Al Romer, the world's

  leading expert on finbacks. He was even fond of them—especially

  of one genus, Dimetrodon. On his office door he kept a cartoon

  that featured Dimetrodon digging up a human skull. Although he

  didn't assume the role of quantitative paleontologist in his pub-

  lished articles, he was always careful to pick up the skulls and limbs

  of every creature he found, and so he built up a great store of

  unexploited data other scientists could use for quantitative re-

  search. (Some excavators will "high-grade" a deposit, collecting only

  the rarer species, thus ruining the sample for reconstruction of the

  entire ecosystem.) In his office were the results of his life's work—