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Re: dinosaur endothermy (long)



The claim in this venue by Terry Jones that there is some sort of religious
fervor in favor of dinosaur endothermy is ironic. Ironic, because his
assertions that there is "currently NO evidence" (his emphasis) that any
dinosaurs was either endothermic - or remained nestbound for an extended
period - is itself more an statement of unsubstantied belief than sound
science. There is of course adundant evidence suggesting that many if not all
dinosaurs were endotherms (produced most of their body heat internally) and
tachyaerobic (had high aerobic exercise capacity [AEC]), rather than being
bradyaerobic (low AEC) ectotherms (dependent upon the environment for the
majority of their body heat). The following is not an in depth or completely
balanced analysis, but a quick look at just some of the evidence pro and con.
While I am at it, I will make some comments on the sometimes interesting but
often misleading dinosaur article in the Dec. Discover on the work of Tomasz
Qwerkowicz. 

Growth rates - As noted recently by Leahy on this board, the fastest growing
wild land reptiles grow as rapidly as the slower growing birds (kiwis
especially) and mammals (primates, some kangaroos, monotremes, some
insectivores), so TO's observation that his captive lizards grow as fast as
his captive hedgehogs is not important - nor did the Discover article say
whether his more tachyaerobic squirrels were growing faster and by how much!
Besides, growth rates of captive animals living under ideal conditions with
food being brought to them by kind humans is not applicable to dinosaurs,
since to the best of my knowledge none (of the nonavian sort of course) did
not live in zoos, farms, labs or homes. As far as I know the fastest growing
wild nonmarine reptile yet observed is a male gator that reached only 55 kg
in 6 years! (McIlhenny 1934 Copeia:80). Consider that a bison is full grown
in a couple of years. Some dinosaurs do seem to have grown slowly like
reptiles and some mammals, others appear to have grown faster. Troodon may
have grown to 50-70 kg in 3-5 years according to Varricchio (1993 J Vert
Paleo 13:99), slower than big birds, but faster than any big reptile. There
is evidence from bone ring counts and bonebed size distribution patterns that
horned dinosaurs, duckbills and smaller sauropods grew as fast as rhinos and
elephants. But if you want really fast dinosaur growth consider giant
sauropods. For reasons that we shall not detail here animals must become
sexually mature at at least a substantial fraction of adult mass by age
20-30, and statistically speaking no animal can avoid being killed much later
than 150 yrs. So super-sauropods need to grow at least 15-50 tonnes in 20-30
years, and reach 50-100 tonnes in 50-100 yrs. The monsters must have had peak
growth rates of many kilograms EACH DAY, a rate only matched by whales! Such
hyper-fast growth is almost certainly far beyond what can be achieved by land
reptiles. Why reptiles grow so slowly is not well understood. My theory
(which is mine for you Monty Python fans) is that it takes large amounts of
food to grow rapidly, and on land (where moving is very energy expensive)
only tachyaerobic animals can gather enough food to grow fast (note that fast
growing captive reptiles are fed by hard working tachyaerobic caretakers such
as Owerkowicz!). Also, animals grow faster when warm, so homeotherms have the
advantage. Ergo, fast growing dinosaur juveniles and/or their parents should
have been more tachyaerobic and homeothermic than reptiles.

Wee baby dino nestlings - One of the advantages of cybertext is that you can
get away with titles like this without some Dilbertian editor getting freaked
out. Anyhow, TJ makes the interesting point that raising helpless altricial
nestlings in open nests in areas where habitats abound is unusual, but this
does not constitute direct evidence regarding nesting habits of baby
duckbills. Evidence that baby duckbills were staying in the nest includes the
trampling of the egg shells into little bits and pieces. If the young leave
the nest soon after popping out of the eggs than large fragments and even
almost complete shells should have been left behind. Also, juvenile duckbills
a few times larger than the eggs have been found in and near the nests,
indicating that they experienced a period of growth while living in the nest.
As for the degree of bone ossification, John Horner has been working on bone
microstructure for years (and another who works on bone microstructure and
saw his SVP talk thought it was sound), and one should be careful in
dismissing his recent conclusions that juvenile duckbill bones were too
poorly ossified for them to have been precocial and thereby able to leave on
their own.  It is possible that baby hadrosaurs were neither entirely
precocial or altricial, which may explain the mix of bone characterisitics
observed so far. Perhaps they were largely nest bound, but had just enough
locomotary ability to temporarily jump ship when a small predator escaped
their parents attention and got into the nest. Why be altricial in the face
of predators? Because being largely inactive while being fed by parents
greatly promoted very rapid growth. Largely altricial nestlings have
 implications for endothermy in two ways. First, because they had to be fed
by the parents, the latter would have had to have been tachyaerobic in order
to gather large amounts of food for their young. Second, living in open nests
left the juveniles exposed to blazing hot sun, and cold nights and rain. If
so they needed a combo of high metabolic rates and good thermoregulatory
abilities to survive and thrive (see moi in Dinosaur Eggs & Babies 1994 K.
Carpenter et al eds.).  

Brooding dinosaurs - Multiple specimens show that Oviraptor lay directly atop
exposed egg rings in a manner almost exactly the same as observed in many
birds, big ratites especially. The theropod even draped its arms over the
eggs! Contrary to implications by TJ and others, this is an entirely
nonreptilian way of brooding eggs. The only reason to leave eggs exposed is
in order to incubate them with body heat. It is improbable that theropods,
with their short stiff bodies, could have generated sufficient body heat
through muscle contractions (pythons use rythmic contractions of their
powerful body muscles to incubate their eggs). Instead a bird-like method of
generating body heat is most probable.  Illustrations showing naked
oviraptors brooding the egg rings are not logical since the eggs are left
largely exposed, therefore some form of insulating pelage is indicated. Which
brings us to........... 

Feathered dinosaurs - TJ is correct we need to verify the new feathered
Chinese theropod before we get all in a dither about it. Funny (odd, not ha
ha), though, how the Oregon people often derive dramatic conclusions from
their new data before it has  been verified. In any case, the presence of a
feathery body covering if confirmed will be powerful evidence for endothermy
over a reliance on the environment for body heat. Every single example of a
feathered or furry vertebrate is endothermic. Not surprising because
insulation interfers with acquiring external heat. That some birds raise
their body temps a few degrees with basking only proves that endotherms can
do so, it in no way shows that a feathered reptile is a workable proposition.


Bone isotopes - Poor Barrick & Showers, they get no respect for using bone
isotopes to restore the thermoregulation of dinosaurs. Funny (same as above)
too in that bone isotopes had been used for years to restore the feeding
preferences of fossil herbiovres with not much fuss being made about it. If,
as TJ asserts, bone isotopes reflect the temperature of ground
water/sediments rather than the fossils, did a Cretaceous lizard's bones show
a typical pattern of reptilian heterothermy while various dinosaur's bones
from the same and other formations consistently indicate degrees of
homeothermy? (See Barrick et al 1996 Palaios 11:295). If this pattern
continues to hold up then the B&S method will be tested and verified
according to the scientific method. (Among the tests, sample reptiles and
mammals in a series of Cenozoic formations, and see how well the results
correspond to reality.) Because dinosaur bones show homoethermy even in small
and juvenile (including baby duckbills!) examples, high metabolic rate
endothermy (rather than thermal stability due to bulk) are indicated.   

Hip size - Bradyaerobic reptiles of all sizes have very small hips because
they have very small thigh muscles. Tachyaerobic birds and mammals of all
sizes have much larger hips because they have large thigh muscles that burn
up large quantities of oxygen when they walk and run. Most dinosaurs of all
sizes had big hips, which strongly suggest they had large tachyaerobic leg
muscles. Reptilian energetics could not power such large oxygen hungry
muscles. On the other hand, the earliest dinosaurs have hips not much larger
than those of reptiles, strongly indicating that they were only marginally
tachyaerobic (they also show that the larger hips of most dinosaurs are not
the result of having an erect gait). 

Trackways - It is little appreciated that we have direct evidence of the
aerobic capacity of dinosaurs. They left countless trackways, each one a
record of how fast they moved, and how much oxygen they burned doing so. The
maximum aerobic exercise capacity of all reptiles is so pathetic (barely
above the RESTING metabolism of birds and mammals) that they cannot walk
faster than 1-2 km/h for more than a few minutes (moving faster requires
application of anaerobic power, which quickly becomes toxic). This precludes
reptiles form migrating on land, or moving in herds. High AEC birds and
mammals almost always walk faster than 2-3km/h, and some migrate in enormous
groups. (Limb design and posture do not account for this difference because
all limb types are equally energy inefficient. Nor can large animals
aerobically power speeds much higher than small ones, which is why squirrels,
dogs and elephants normally walk at about the same speed). Of my current
sample of 400 dinosaur trackways (itself a tiny fraction of the total
available data base) of all sizes and limb types only 8 are below 2 km/h.
Mass single species bonebeds and shoreline parallaling mass trackways of
horned and duckbilled dinosaurs mimic those of migrating ungulates. As far as
I know there is no way reptilian energetics could power movements that are so
consistently fast, and range so far, on land. 

Limb posture - Sprawling legs are well suited for slow walking reptiles
because they provide a stable gait even at slow speeds. Erect legs and
bipedalism are less stable and less suitable for slow speeds, the same
instability and the pendulem action of erect legs tend to force high walking
speeds. Speeds that only high AEC can power. Ergo, erect legs and bipedalism
probably force the the evolution of tachyaerobiosis. If so then there is a
causal link between limb posture and metabolism.  

Big tall dinosaurs - Classic land reptiles never grow over about one tonne
(recently extinct tortoises and giant monitors of S Asia and Australia
respectively). Land mammals and many dinosaurs get to 10-20 tonnes, and a few
sauropods exceeded 20 tonnes. Being big on land probably requires very rapid
growth, enormous and highly aerobic limbs muscles to carry the weight
indefinitely, and large high pressure hearts to oxygenate all those musles,
and to pump blood up to the brain if it is normally held well above heart
level. Bradyaerobic reptiles are probably not able to do manage any of these
things, so they remain modest sized,  low slung forms. 

Polar dinosaurs - It is true that the Mesozoic poles were not nearly as harsh
as they are today, but with long dark winters they were not reptile heaven
either. On the north slope of Alaska large numbers of dinosaur bones and
teeth have been found in locales that ENTIRELY lack any trace of reptile
remains. If dinosaurs were reptiles in terms of their energetics, then why
were they able to inhabit high latitude climes that ectotherms seem unable to
have lived in? It is interesting that polar dinosaurs both north and south
seem dwarfed compared to their more equatorial relatives, a reversal of
Bergmann's Rule. This may indicate that dinosaur thermoregulation was not as
good as in mammals and birds. 

        Clearly TJ's claim that there is no evidence for dinosaur endothermy
is false to the point of being absurd. This does not mean that dinosaurs have
been proven beyond any doubt to have been endothermic, although the data may
pass the *reasonable* doubt test. Perhaps TJ is not understanding and
therefore incorrectly applying the term evidence. In a criminal trial, for
instance, there can be valid evidence that indicates a particular person did
it, and valid evidence that he did not, even though only one set of evidence
is ultimately correct. (By the same token, G. Olshevky's assertion that there
is no evidence, just speculation, on dinosaur energetics is not correct.
Example. The bloody glove found on OJ's property is evidence, not
speculation. What is speculation is how the darn thing got there.) Also
peculiar is TJ's assertion that the evidence for dinosaur ectothermy is
"obviously ignored". The work of John Ruben and others has received
considerable attention in the press, and on this list. Now, I am not going to
say that there is NO evidence for dinosaur ectothermy. What is true is that
so far there has been very little in the way of arguments actually and
positively FOR ectothermy, most arguments have been AGAINST the evidence for
 endothermy (which is then often and incorrectly assumed to be evidence for
reptilian energetics). The positive evidence for dinosaur ectothermy is not
as strong as TJ thinks. 

Respiratory turbinates - This is one argument that actually tries to
demonstrate dinosaur ectothermy. I am not going to go into this more because
I am close to finishing a study that pretty much refutes the argument that
dinosaurs could not have had RT. Also, a few birds and mammals breath through
nasal passages as narrow as seen in some dinosaurs. 

Bone histology - It has turned out that deep cortical growth rings are fairly
common in certain but not all kinds of dinosaurs, as well as early birds.
This has been cited as positive evidence for ectothermy. However, deep bone
rings may be more common in mammals (especially marsupials) and recent birds
than realized according to Leahy and others. As for the early birds, the
individuals are fairly large adults, and the bone deposited during juvenile
growth has been lost because the bones are so thin walled. The growth rings
observed only show that the birds continued to grow slowly as adults, like
reptiles and some marsupials. How fast the baby birds grew is not yet known.
The presence of bone rings in various dinosaurs may be evidence that they
thermoregulated less well than most mammals and birds, or maybe not. 
         Because it has not been published in detail T. Owerkowicz's work on
bone growth cannot be fully considered, but some comments are pertinent.
First his work is not positive evidence for dinosaur ectothermy, but merely
an attempt to refute evidence for dinosaur endothermy, that subtle but
critical point many fail to appreciate. Besides, his work may actually
SUPPORT the presence of high AEC and energy budgets in dinosaurs. TO claims
that activity via exercise can cause bone to become bird and mammal like.
Maybe so. But what TO and many others seemingly fail to appreciate is that
all activity is in the end entirely the result of the burning of food and
oxygen (this is true even of initially anaerobic power). The more total
activity there is, the more oxygen and food that must be burned to power it.
(This point is a pet peeve if mine, because many workers seem to detach
activity from power production, and think that dinosaurs could have been more
active than reptiles yet had similarly low energy budgets. Hello! This
violates the known laws of physics!) I bet that TO's heavily exercised
animals are eating and breathing a more than his couch potatoes. If so, then
the total metabolic rates of his subjects with heavy haversion reconstruction
should be higher than those with more reptile-type bones (if TO is not
measuring and recording the food consumption of his subjects, he is missing a
major research opportunity). In this case, if haversion reconstruction is
more extensive in dinosaurs than in living land reptiles, then this indicates
that the former were more active, and therefore more energetic, than
reptiles. Think about it! 

Flight muscles in Archaeopteryx - Ruben argues that Archaeopteryx may have
been ectothermic, in which case their dinosaurian ancestors should have been
too. Basically, he believes that Archaeopteryx could not support enough
flight muscles (at least 15% of body mass) to power fly aerobically, so it
must have had a smaller set of anaerobically powerful flight muscles of the
type only found in bradyaerobic reptiles. This is positive evidence for
ectothermy. Problem. Some birds with poor to fair powered flight abilities
have flight muscles making up only ~10% of total body mass, and Archaeopteryx
has arm muscle attachment areas far larger than in reptiles and other
theropods. Therefore, a modest tachyaerobic flight musculature sufficient for
the minimal degree of powered flight expected in such a basal flying form is
by no means unreasonable. 

Things may or may not change, but to date the evidence for dinosaurs having
nonreptilian energetics is way out in front of the evidence that they were
essentially reptiles. Consider that the hypothesis that dinosaurs had
nonreptilian energetics would be falsified (in the Popperian sense) if they
all had small hips, consistently left low speed trackways, had truly
reptilian bone microstrucute indicative of very slow growth, etc. Instead,
such matters as trackways effectively falsify the presence of truly reptilian
locomotary energetics in dinosaurs (unless it can be shown that dinosaurs
were not really fast walkers, or that reptile can power such fast walks
without poisoning themselves in the process). (This ability to falsify
hypothesis means that we can scientifically restore the thermodynamics of
dinosaurs even though they are remote in time, just as well as we can restore
the thermodynamics of stars that are remote in distance and whose cores are
inherently inaccessible. Therefore paleometabolics are not mere speculation)
This does not mean that dinosaurs all had a classic avian-mammalian metabolic
and thermoregulatory system. Certainly the small hipped early dinosaurs did
not, and even larger hipped dinosaurs may have had somewhat lower resting
metabolic rates and less well developed thermoregulatory abilities than
condors or antelopes. On the other hand, metabolic rates and thermoregulation
are highly variable in mammals as well. What we need, and will get
(especially when those smart roboscientists take over) is a much better data
base both for living and extinct tetrapods. It is good that physiologists
like JR have joined the effort, what bothers some of us is how they seem to
run way ahead of the data in terms of their conclusions. 

While on this subject, I have noticed recently some posted questions as to
whether ideas such as "warmbloodedness" and other "radical" ideas concerning
dinosaurs are scientifically valid, or just something some people use to sell
books. Of course one could assert the opposite, some researchers seem to
loath the notion of bird/mammal-like dinosaurs in part because it IS so
popular. Of course, science is not popularity contest, and lay opinion has no
legitimate bearing on the problem. After all, in late 1903 most people
thought the notion of manned flight absurd. These days 32% believe dinosaurs
lived with people. BTW, has anyone seen a poll on public opinion concerning
dinosaur energetics? 

GSPaul