skeletons are rare, but there is some suggestion that one gender's
(presumed to be female) spines were shorter than the other's. Even
at its shortest, however, Dimetrodon's spiny back-sail made a splen-
did spectacle as it strutted slowly, puffed itself up, and displayed
itself broadside to potential mates and sexual rivals. Dimetrodon
couldn't waggle its crest, because each supporting spine was rig-
idly anchored to a vertebra. But the skin stretching between those
spines might well have contained some message indicated by the
pattern of scales. Maybe, like many amphibians and reptiles today,
Dimetrodon's scales changed color during the mating season.
Dimetrodon s> spines generated a hundred years of debate within
the paleontological community—a great deal of it unnecessary, in
my opinion. The learned Edward D. Cope of Philadelphia made a
tongue-in-cheek suggestion: the fin on Dimetrodon's back was a sail
to allow it to scud across Permian ponds like a scale-covered rac-
ing yacht. Al Romer, who spent a lifetime studying Dimetrodon and
related clans, believed the sail worked well as a radiator. In the
SEX AND INTIMIDATION: THE BODY LANGUAGE OF DINOSAURS I 333
morning, when the creature was cold from the night air, its sail
would be turned toward the sun to soak up the warming rays. When
the noonday Permian sun became too hot, and Dimetrodon was in
danger of overheating, the sail could be turned into the breeze for
a cooling effect. Two quantitative paleontologists developed ele-
gant mathematical models to show how blood could flow to and
from the skin of Dimetrodon s sail to provide both solar heating
and wind cooling.
This heating-cooling hypothesis is widely accepted, but it has
weaknesses. The chief problem is that Dimetrodon had a close rel-
ative, Spbenacodon, that didn't have a dorsal sail. Sphenacodon was
identical to Dimetrodon in all the details of its anatomy. Only the
spines of its back differed. Sphenacodon's spines were only very
slightly elongated. If we accept the heating-cooling theory, it would
have to be concluded that Sphenacodon was very different from Di-
metrodon in its thermoregulatory adaptation. That implies a most
unusual evolutionary development. In today's ecosystem, closely
related species usually exhibit far greater differences in their
courtship behavior than in the way they use heat. In other words,
evolution usually works faster in changing display behavior than
in changing thermoregulation. Several genera of lizards alive to-
day include some species with backbone crests and others with no
elongation of the spines. Except for the spines, these clusters of
closely related species are adaptively very similar. On balance,
therefore, it's far more reasonable to interpret Dimetrodon's sail as
a display for sex and intimidation. It might indeed have been a
radiator—anything sticking out from the body might be. But the
overall pattern of evolution implies that display organs evolve more
rapidly into grotesque shapes than do such utilitarian devices as
radiators.
At least four other early Permian creatures carried equally
extraordinary display sails on their backs. A distant relative of
Dimetrodon was Edaphosaurus ("earth lizard"), a small-headed,
barrel-bellied reptile, up to eight feet long, that preferred swampy
habitats. Edaphosaurus evolved its fin totally independently of
Dimetrodon and even featured extra ornamental devices—knobby
crosspieces sticking out sideways from the long spines. Edapho-
saurus itself had a close relative with simple spines.
Permian amphibians were hardly upstaged by the skeletal
334 | THE WARM-BLOODED METRONOME OF EVOLUTION
Body billboards in the armadillo toad. Platyhystrix, a close relative of Cacops, strutted around the Early Permian landscape with a sexual billboard
constructed from armor-plated vertebral spines. The three-feet-long
Platyhystrix and its smaller kin Cacops both belonged to a very successful
family of strong-legged amphibians that sported armor plate over their spinal
columns, a trend that gave them the nickname "armadillo toads." (Platyhystrix
went even further by evolving armor over some of the ribs.)
theatrics evolved among the finback reptiles. The amphibians
evolved an outstanding finback of their own. A strong-legged, three-
foot-long amphibian, Platyhystrix ("flat-spine"), evolved a dorsal
display piece every bit as baroque as the edaphosaur's. Just as with
Dimetrodon, Platyhystrix had close relatives that hadn't evolved such
a crest. A single quarry at Rattlesnake Canyon, Texas, has pro-
SEX AND INTIMIDATION: THE BODY LANGUAGE OF DINOSAURS | 335
duced specimens of Platyhystrix, Edaphosaurus, and Dimetrodon.
What was so special about Early Permian times that they should
have produced so many giant dorsal displays in such profusion?
No one knows.
The Golden Age of the finbacks is, however, a sobering dis-
covery for scientists who believe in the principle of extreme uni-
formitarianism. This theory insists that evolutionary processes work
the same way at all times. But there has never been another age
of finbacks to compete with the Early Permian. And why this ep-
isode in the evolution of body organization should have remained
unique is one of the great unsolved mysteries in the history of life.
The Age of Finbacks ended suddenly about halfway through
the Permian Period, and the Age of Head-Butting began. Proto-
mammals (generally called mammal-like reptiles) took over the
leading roles in the land ecosystem, and they generally did not in-
dulge in extravagant visual displays—a curious state of affairs be-
cause protomammals descended from some close relative of
Dimetrodon. With them, courtship and intimidation evolved along
very different lines. From their very beginning, mammals evolved
hornlike growths, thickened skull roofs, or knobs and bumps to
cover their faces. Earlier paleontologists were at a loss to explain
these cranial excesses (of course the theory of racial decadence was
invoked). But in recent years Herb Barghusen, a Chicago anato-
mist, has developed a strong case for head-to-head butting matches
during the mating season as the most likely explanation.
Some of the head-butters had wide, flat snouts, enabling two
males to indulge in a pre-mating shoving match. The most ex-
traordinary protomammals were the dinocephalians, the "terrible
heads." These animals evolved skulls that, at a distance, looked a
lot like a bowling ball with a snout attached. All the skull bones
around the forehead, cheeks, and eyes were enormously thick-
ened, endowing the beast with a bony puffiness all over its face.
There can be no doubt that such heads were designed for butting.
Their necks entered their skulls from a right angle, so a charging
male could lower its head and bash its opponent with the mass of
its forehead facing forward. When two charging males collided, at
full speed, the Late Permian air must have resounded with lo
ud,
clunking crashes.
Dome-headed protomammals raise an interesting question
336 I THE WARM-BLOODED METRONOME OF EVOLUTION
concerning the evolutionary connection between sex and warm-
bloodedness. Recently, popular health and diet magazines have
discovered what physiologists have known for a century—sex can
be a strenuous exercise and often an important way to burn up
calories. Sexual practices embrace not only the physical act of cop-
ulation, but all the pre-mating ritual, strutting, dancing, brawling,
and the rest of it. They can consume enormous amounts of en-
ergy. Successful bull elephant seals are tattered, scarred, and ex-
hausted after the long mating season of wrestling matches against
rivals. A male moose may lose weight during the rut, because of
the exertions expended when running into other males. Mammals
can afford to squander vast amounts of energy during courtship
and mating since their warm-blooded system produces a huge
amount of energy. In comparison, a totally "cold-blooded" animal
produces a tiny amount of energy and therefore cannot expend as
much effort in sexual athletics. When a five-hundred-pound moose
spends 50 percent of its total energy in mating contests, the total
calories burned are ten times greater than those burned when a
five-hundred-pound tortoise uses 50 percent of its energy for sex,
because the tortoise starts out with much less.
Protomammal head-butters of the Late Permian: the one-thousand-pound
Tapinocephalus
Knobby-snouted protomammal of the Late Permian. Aulacephalodon was a
plant-eating two-tusker of the Tartarian Epoch and may have had a warm-
blooded pre-mating style, complete with vigorous competitive butting and
pushing. It was about four feet long and two hundred pounds adult size.
Judging from a consideration of the evolution of available en-
ergy, the contrast between the Age of Finbacks and the Age of
Head-Butting is intriguing. The amphibians and primitive reptiles
of the Early Permian adopted a low-energy approach to sex and
display; the tall sails on their backs were visual signals that really
didn't require violent body language to be effective. Head-butting
was something else again. The robust construction of its head and
neck testifies to the vigor of the protomammal's physical effort.
There may be an important clue here to a major increase in avail-
able energy that had occurred during the evolution of the first
protomammals. Quite possibly they expended much larger totals
338 I THE WARM-BLOODED METRONOME OF EVOLUTION
of calories than do typical reptiles today. As we'll see in a subse-
quent chapter, there is excellent evidence from other sources that
suggests the Late Permian head-butters were indeed the first warm-
blooded animals ever to evolve. Whatever their metabolic rate, in
any case, it must be recognized that the Late Permian head-butters
were certainly the first land vertebrates to escalate sexual gymnas-
tics into a high energy level.
Protomammals continued to butt their heads until the end of
the Triassic Period, when dinosaurs took over the roles of large
herbivore and carnivore on land. The dinosaurs' approaches to sex
and intimidation ran the entire gamut from elaborate dorsal dis-
plays to head-butting and perisexual symphonies. Largest of the
dinosaurs resorting to display was the appropriately named Spino-
saurus, the "spine lizard," a forty-foot predator probably related
distantly to Allosaurus. All specimens of Spinosaurus are frustrat-
ingly fragmentary, but it's clear a tall sail decorated its back, rising
six to eight feet above the backbone. A strutting Spinosaurus must
have been a singular sight—striding on its long hind legs, its head
twenty feet above the ground, turning broadside to dare its rival
to test its potency. Sex also probably explains the tails of duckbill
dinosaurs. Those tails were very deep from top to bottom and well
suited for conveying messages. Some duckbills even evolved true
sails constructed from vertebral spines over the base of their tail.
Torso and tail were not the only sites of sexual adornment.
The Early Jurassic carnivore Dilophosaurus evolved a striking cra-
nial profile: two tall crests, very thin from side to side, rose from
the edges of its skull from snout to forehead. Lower, thicker crests
in the same location decorated the heads of the Late Jurassic Al-
losaurus and Ceratosaurus and the tyrannosaurs of the Cretaceous.
Dilophosaur crests were so thin that they could have been only
for visual effect. But the bony crests of the later meat-eaters were
heavy and covered by stout layers of horny skin. Allosaurus and its
relatives probably butted heads during confrontations on the field
of sexual valor. To be sure, a pair of male allosaurs, driven by their
hormones, could have bitten each other to death. But I suspect
such terminal contests were relatively rare. Evolution tends to fa-
vor the sexual soldier who can win multiple contests and who
therefore, by implication, does not run the risk of being dismem-
bered in his first bout. Less than lethal horns would confirm this
•
SEX AND INTIMIDATION: THE BODY LANGUAGE OF DINOSAURS I 339
Wagtail fintail Montanoceratops. Like
its close relative Protoceratops,
Montanoceratops had incredibly tall
tail spines that made the tail a
billboard for social messages. The
tail was highly flexible from side to
side and so the entire tail could be
wiggled. Total length about four
feet long.
general theory. The allosaurs' ancestors possessed little in the way
of crests or horns but they did have dangerously sharp teeth and
quick-biting jaws. These earlier carnivores could have resorted to
biting to settle mating contests, but they were probably restrained
by genes that programmed for less dangerous encounters. And the
success of genetic changes that increased the disposition toward
head-butting among the later, larger carnivores indicates that but-
ting, not biting, was the best strategy for maximizing success in
mating. Big mammals show the same pattern—clans with danger-
ous teeth often evolve nonlethal horns.
Dinosaurs as a class must have owned large quantities of en-
ergy to pour into the rigors of courtship and mating, because head-
butting evolved several times in different families. Tyrannosaurus
rex's massively thick skull edges, covered with horn, represent the
acme of the evolutionary trend toward head-ramming among car-
nivores. Pachycephalosaurs, thick-headed dinosaurs described in a
previous chapter, evolved huge, bowling ball—shaped skulls very
much like the ones carried by the dome-headed protomammals of
the Late Permian. Some Polish scientists have suggested that head-
ramming was too powerful to be used against sexual rivals and that
the head-down charge at full tilt must have been employed against
predators. However, the domes both on dome-headed p
rotomam-
mals and on dome-headed dinosaurs really resembled bony box-
ing gloves built atop the skull. The boxing glove, even one of bone,
delivers a blunt, stunning blow. If evolution had really been work-
ing to produce a deadly antipredator weapon, a spearlike point on
the head would have been much more effective than a blunt dome.
It seems far more feasible therefore to envisage the domes as ideal
weapons for sexual contests without incurring the danger of esca-
lating the match to the point where even the winner goes away
mortally wounded. A mortally wounded winner doesn't win in the
game of evolution because dead heroes can't mate.
Quite the reverse, however, must hold true for the headgear
of Triceratops and the other long-horned dinosaurs. Recently, a pair
of American paleontologists, apparently caught up in the rush to
reinterpret dinosaur features as organs for sexual display, sug-
gested the horns and frill on Triceratops were not for defending
against Tyrannosaurus but for display to other Triceratops. But the
head ornaments of Triceratops were simply much too deadly to serve
a sexual purpose. Those long, sharply pointed horns were for kill-
SEX AND INTIMIDATION: THE BODY LANGUAGE OF DINOSAURS | 341
Nonlethal butting crests of the
meat-eating dinosaurs.
Tyrannosaurus had thick, low
butting ridges, Allosaurus had
shorter, sharper crests, and
Dilophosaurus had tall, thin snout
crests.
ing. The wide frill, sometimes edged with horn-covered spikes,
served for protection against dangerous bites. The little horned
dinosaurs, Protoceratops and its relatives, probably had started out
designed for nonlethal jousting. Their snouts were strong, but only
the slight suggestion of a blunt horn grew above their nostrils. Large
suites of Protoceratops skeletons from the red Mongolian sand dunes
indicate that males had larger heads and stronger nose horns than
females. So springtime probably did bring thoughts of sex and
snout-butting into the minds of the little horned dinosaurs. But
that could no longer have been true of their larger, far more le-
thally endowed descendants.
By far the most spectacular devices for sex intimidation were
Robert T Bakker Page 34