Robert T Bakker Read online

  conserving properties. The elephant's ratio of 22 times less skin

  per pound compensates for its ten times less heat production per

  pound, granting it a net advantage of 22^-10, or about 2.2. The

  elephant produces body heat 2.2 times faster per pound per square

  inch of skin than the bunny.

  The biothermal bottom line here is this: It is easier to chill a

  mouse than an elephant. Zoo keepers know this from experience.

  Big mammals, even species from tropical homelands, adapt to winter

  outside in northern zoos better than do small species. Open the

  cage window, let the cold draft in, and the elephant doesn't feel

  much. But the poor mouse starts shivering immediately and liter-

  ally will shiver itself to death in an hour or so. (Shivering is the

  mammalian body's way of increasing heat production to meet the

  thermal crisis. Shivering burns up the calories at enormous rates,

  up to five times the standard metabolism, and a shivering mouse

  literally can burn itself out quite quickly because the body is so

  small.)

  Today's Reptilia share their own version of this mouse—to—

  elephant formula. Big reptiles, like giant tortoises, produce less body

  heat per pound than little ones, and big reptiles also have much

  less skin area per pound. So it happens naturally that a giant lizard

  or tortoise doesn't chill as quickly as a little one. If you take heat

  lamps and warm your three-hundred-pound tortoise and your three-

  pound box turtle to 90°F body temperature, and put both of them

  outdoors on a cool cloudy day, the big tortoise will lose its body

  92 I THE CONQUERING COLD-BLOODS: A CONUNDRUM

  Why big animals stay warmer easier. An elephant is ten thousand times

  heavier than a rabbit and produces body heat one tenth as rapidly. But still

  the elephant keeps warmer because it has much less skin area per pound and

  much more body heat per square inch of skin.

  heat much less quickly. However, no matter how big the reptile

  is, its metabolism will always be lower than that of a "warm-blooded"

  mammal or bird of the same weight—about four times lower. So

  a three-hundred-pound pig keeps warm more easily than a three-

  hundred-pound tortoise (in terms of the standard metabolic for-

  mula, M = kfW~' ;A that means the reptile k is one fourth the

  mammal k, and the reptile metabolic rating is A

  l

  that of the mam-

  mals).

  The theory of mass homeothermy starts by assuming a point

  of view. Assuming that dinosaurs were cold-blooded and had low

  metabolism, can we explain their success? The theory begins by as-

  suming "cold-bloodedness." Stuck with the model of hypothetical

  dinosaurs who produce low body heat, the only way to make their

  body temperature stay reasonably high and constant is to make their

  bodies as huge as possible. So the theory says that dinosaurs were

  successful because they evolved gigantic body size and conse-

  quently their body temperature was maintained without the need

  for excessive metabolism. This theory would work best in a warm,

  tropical climate. And fossil plant evidence shows the Mesozoic

  world was, on average, much warmer farther up toward the poles

  than it is today. So the environmental context would seem perfect

  for giant homeotherms with low metabolism.

  Orthodox paleontologists have rallied round the standard of

  homeothermy, triumphantly proclaiming it obviates any need even

  to consider the hypothesis of high metabolism in dinosaurs. But

  their enthusiasm is ill-founded. The theory doesn't work and it's

  fairly easy to demonstrate its flaws. A few years ago a young bio-

  physicist, Jim Spotila, worked up a computer program to show how

  a two-ton reptile with low metabolism might regulate its body

  temperature. Jim placed the hypothetical beast into a nearly ideal

  climate—present-day southern Florida—and never allowed rainfall

  to influence its body heat. Jim's theoretical two-tonner did pretty

  well. It could maintain its body temperature between 30°C and 38°C

  for most of the year. And its body temperature never rose to where

  it might addle its brain, nor fell to where it might get frostbite.

  The advocates of mass homeothermy seized upon Jim's com-

  puterized two-ton lizard as proof positive that dinosaurs didn't need

  high metabolism. But they missed an important point: The two-

  ton lizard might certainly regulate its temperature better than a

  little two-ounce lizard, but a two-ton dinosaur with high metabo-

  94 | THE CONQUERING COLD-BLOODS: A CONUNDRUM

  Why the mass homeothermy theory doesn't work. Compare the performance

  of a hypothetical two-ton cold-blooded lizard with a two-ton warm-blooded

  dinosaur. In a warm climate with clear skies the two-ton cold-blood can keep

  its body temperature high and constant during the daylight hours. But on

  cool nights the lizard's temperature will slip ten degrees below the warm-

  blooded dinosaur's, so the dinosaur would have a distinct advantage. And

  during the rainy season, when the sun is blotted out for weeks on end, the

  two-ton cold-blood simply won't have the body heat to prevent a disastrous

  fall in body temperature.

  Mesozoic nightmare—being a cold-blooded dinosaur during the rainy season.

  If big dinosaurs really were mass homeotherms, then the rainy season would

  have sapped their body heat and left them torpid and vulnerable to the

  warm-blooded mammals.

  lism would do much better than that, much better. If Jim's com-

  puterized beast had possessed the high metabolism typical of

  mammals, it could have kept its body temperature between 38°C

  and 38.5°C all year, 6 or 8 degrees less variation than the low-

  metabolism model. And if Jim's computer had allowed rain to fall

  on the beast, the high-metabolism version would greatly surpass

  the low-metabolism model, because the high-metabolism model

  could shiver to raise heat production so high that rain wouldn't

  lower body temperature at all (no reptile can shiver like a mam-

  mal or bird).

  What sort of advantages would a dinosaur with high metabo-

  lism garner from temperature regulation that keeps variation up

  to eight degrees less than one with low metabolism ? As we have

  seen, a drop of eight degrees in body temperature implies a drop

  of 20 to 50 percent in physiological prowess (the exact drop, re-

  member, depends upon Qio)- If two-ton, high-metabolism dino-

  saurs met two-ton low metabolism dinosaurs in southern Florida,

  sooner or later the animals with low metabolism would find them-

  selves outclassed in all the ecological contests necessary for sur-

  vival—running, fighting, digesting, mating, growing. In direct

  confrontation, high metabolism always conquers low metabolism,

  even when bodies are huge and climate warm. Even if the advan-

  tage of high metabolism was only 10 percent, the laws of evolu-

  tion would force the low-metabolism model to extinction.

  Geneticists in the 1930s proved that even a tiny net advantage,

  say 5 percent, would imply that one adaptive s
ystem would re-

  place another over hundreds of generations.

  Applying these ideas to the questions of the Mesozoic, how

  could dinosaurs have suppressed mammals for over a hundred

  million years? Merely by being big? No, it wouldn't work. Dino-

  saurs could not have maximized their physiological output simply

  by being big. Those Jurassic and Cretaceous mammals must have

  had some sort of high metabolism—since all the most primitive

  living mammals do. If the dinosaurs were equipped only with low-

  metabolism biothermal weaponry, they couldn't have prevented the

  Mammalia from evolving to fill all the large-bodied niches. Low-

  metabolism dinosaurs would have survived only by staying small,

  hiding in their holes, nipping out to forage when the big mammals

  weren't looking—just as modern reptiles do.

  DINOSAURS SCORE WHERE KOMODO DRAGONS FAIL I 97

  Mass homeothermy falls into another error: it entirely ig-

  nores the small- and medium-size dinosaurs. There weren't any tiny

  (less than two-ounce) dinosaurs—or at least none have been found.

  But there were ten-, fifty-, and hundred-pounders, and this size

  range contained the dinosaur species that would have interacted

  with the Mesozoic mammals. A twenty-ton Brontosaurus probably

  didn't interact with the two-ounce mammals of Como Bluff. The

  brontosaur ate tree leaves and would have swallowed a mammal

  only by accident, as we might swallow a caterpillar hiding in a chef's

  salad. Ornitholestes was another story. It is a twenty-pound Como

  dinosaur with big eyes, sharp teeth, and quick legs for darting

  through the underbrush and hunting Jurassic small prey. Ornitho-

  lestes must have hunted mammals—the Como furballs were just the

  right size to fit the predator's jaws. Before Ornitholestes, the earli-

  est dinosaurs of all, Lagosuchus of the Triassic Period, were also

  small, lively hunters. Hence all through the Mesozoic, the dino-

  saurs supplied mid-sized predators that must have continuously

  confronted the Mesozoic Mammalia.

  These small, mammal-hunting dinosaurs were far too little to

  reap the theoretical benefits of big body size in keeping tempera-

  ture constant. The only way Ornitholestes could have kept its body

  temperature high and constant was by having a high constant me-

  tabolism. The fact that Ornitholestes and its brethren succeeded in

  keeping the Mammalia small for over a hundred million years is a

  powerful argument that these dinosaurs possessed basic physio-

  logical equipment equal to or better than a mammal's.

  Ornitholestes was an impressive little dinosaur, and even the

  diehard defenders of orthodoxy yield a little to admit that perhaps

  Ornitholestes and its kin might have had high metabolism. Such a

  concession, however, would lead to yet another inconsistency in

  the theory of mass homeothermy. Big dinosaurs, all of them,

  evolved from small-dinosaur ancestors. The idea that little ances-

  tors had high metabolism and their bigger descendants didn't, would

  be tantamount to arguing that evolution reversed itself. (In math-

  ematical terms, that means the constant k would get smaller as size

  got bigger.) Modern elephants and rhinos evolved from small

  ancestors with high metabolism, without reversing their metabolic

  rating (and without changing their k). If big mammals didn't lose

  high metabolism, why should big dinosaurs have? In fact, there's

  98 | THE CONQUERING COLD-BLOODS: A CONUNDRUM

  The Late Jurassic forty-pound predator

  Ornitholestes terrorizes a mammal.

  not a single documented case of a large descendant completely

  abandoning a high-energy heritage handed down from a small

  ancestor. It could happen—in theory—but given the evidence, it

  is more logical to assume that a small high-metabolism dinosaur

  would produce big, high-metabolism descendants.

  The assumption that big dinosaurs didn't require high metab-

  olism also ignores the fact that each dinosaur community co-evolved

  with others—the big plant-eaters interacted with medium-size meat-

  eaters, which interacted with small meat-eaters, and so on

  throughout the ecological web of relationships. Ornitholestes was a

  close relative of big Allosaurus, a predator that reached a ton or

  more in size. It's difficult to believe that Allosaurus had a physio-

  logical structure very different from its little cousin—the bony ar-

  chitecture is startlingly similar. So if Allosaurus was 100 percent

  warm-blooded, with a high metabolism, then the plant-eaters that

  had to cope with it would have required matching physiological

  adaptations.

  Another weakness in the theory of mass homeothermy is its

  assumption that dinosaurs succeeded only where the climate was

  warm and tropical. It's true that most of the best-known Creta-

  ceous graveyards—the Judith River Delta in Montana and Al-

  berta, for example—yield strong evidence of warm habitats with

  mild winters. Big fossil crocodiles and soft-shelled turtles can be

  found there, and these clearly reptilian types required year-round

  warmth. Fossil leaves from these sediments represent plants of

  tropical aspect (the leaves of dicots in the tropics tend toward

  "whole margin" shapes, with the leaf edge smooth and not sculp-

  tured into complicated edges; leaves from habitats with cold win-

  ters, on the other hand, tend toward complex shapes like those of

  our New England oaks). The chemistry of fossil seashells from the

  nearby marine beds also show that winters were warm (the ratio

  of the oxygen isotopes O 1 6 and O 1 8 in the lime shells indicates the

  temperatures of the seawater when the animals were alive). It's also

  true that the best-known Jurassic dinosaur beds yield fossil plants

  that indicate warm habitats (Jurassic flora at Como feature many

  tropical-type ferns). And finally, all the evidence from plants, fos-

  sils, and geochemistry demonstrates that tropical conditions pre-

  vailed farther up toward the poles all during the Mesozoic than

  they do today, so that tropical conditions were present even as high

  as latitude 45°.

  100 I THE CONQUERING COLD-BLOODS: A CONUNDRUM

  But—and this is a big but—dinosaurs were also the dominant

  big-bodied land life form in less well publicized sites where the

  climate was much cooler in Mesozoic days. A good example is in

  South Australia, where Early Cretaceous dinosaur bones are found

  in lake beds deposited at 70° south latitude. Fossil plants and geo-

  chemistry show that winters here were cold while those dinosaurs

  were alive—cold enough so that frost formed and lakes froze over.

  How could a cold-blooded giant dinosaur have survived those chilly

  Australian winters? Giant tortoises and crocodiles can't cope with

  such winters today.

  The final major shortcoming of the orthodox theory of mass

  homeothermy is that it ignores what really happens to genuinely

  giant reptiles with low metabolism. The theory holds that all the

  benefits of constant body temperature can be enjoyed in a tropical
r />   climate, without high metabolism, if body size is large enough. But

  if being a low-metabolism giant reptile were so efficient, why aren't

  today's tropics overrun with two-ton lizards and frogs? How many

  species of multi-ton reptile lurk today in warm terrestrial habitats?

  None. On the other hand, how many species of tropical high-me-

  tabolism mammal presently grow to one ton or larger? Quite a

  few—three rhinos, a hippo, two elephants, giraffes, some races of

  water buffalo.

  So mass homeothermy doesn't work in today's ecosystems. The

  message from the tropics is unambiguous: To be a successful big

  land animal, you must cope with mammals, and to cope with

  mammals you must be a mammal yourself, or at least have metab-

  olism as high as a mammal's. And big mammals have suppressed

  big reptiles in our tropics for the last sixty-five million years. So

  how can the dinosaurs' success over mammals' be explained? By

  assuming that dinosaurs had low-energy metabolic styles? Not very

  likely.

  To understand the dinosaurs, we need a new theory, a heresy.

  Or rather we need the renaissance of an old nineteenth-

  century view, which believed that the dinosaurian system pos-

  sessed key elements not found in the adaptive tool kit of modern

  Reptilia.

  DINOSAURS SCORE WHERE KOMODO DRAGONS FAIL

  101

  PART 2

  THE HABITAT

  OF THE DINOSAURS

  5

  THE CASE OF THE

  BRONTOSAURUS:

  FINDING THE BODY

  At Como there is a limestone ledge called Cam Bench, named

  for a Camarasaurus skeleton that lies there, eroding out bit

  by bit. The dinosaur's pale gray, weatherbeaten fragments are slowly

  disintegrating beneath the endless blows of sun and rain. Camara-

  saurus was a smallish brontosaur, with rather long neck and tail,

  probably no more than eight tons alive. It has been left to decay

  because the skeleton isn't complete enough to justify the two weeks

  of quarrying necessary to chip it free. It is nonetheless quite im-

  portant because preserved around it is a trail of fossil clues that

  stare out at us from the day, 140 million years ago, when this an-

  imal died. This body can tell us something.

  How did this Camarasaurus die? If this question can be an-