Battle of the titans: terramegathermy (hooray) versus gigantothermy (boo hiss)

[M Murphy <4mjmu@home.com>]

I appreciate the posting of parts of my work on terramegathermy and
related 
matters. Less appreciated are some of the dicussions which, aside from
being 
too extensive to read in detail, often seem to consist of superficial 
commentary not backed by an in depth understanding of the extensive 
literature on energetics or my papers. Some of the comments are
profoundly 
erroneous or at least dubious. I do not have time to answer all of them,
but 
to clarify some of my work I offer the following. Unless otherwise
noted, 
relevant sources are referenced in my papers.  

It is interesting that after 300 million years of evolution classic, 
bradymetabolic, bradyaerobic land reptiles have never become truly
gigantic 
either in height or mass. But dinosaurs became very tall and reached
many 
tonne status soon after they appeared, and mammals also evolved real
giants 
fairly quickly after giant dinosaurs went extinct. Ever wonder why? This 
disparity has not really received that much attention, probably because
the 
view that terrestrial bradyaerobes can become gigantic remains popular
in 
some circles. But can they?  


Gigantothermy - This was proposed By Paladino et al in a paper in
Nature. 
Part of the paper was an alledged measurement of the resting MR of the 
leatherback sea turtle. They claimed it was elevated above the level
expected 
in a reptile its size. The very same summer Lutcavage et al published an 
accurate measurement of the leatherback RMR which showed a typically 
reptilian value. The erroneous Paladino et al data has since been
withdrawn. 
    Based on the mismeasured data, Paladino concluded that the MRs of
giant 
reptiles and mammals converge with one another. From this they derived
the 
hypothesis of gigantothermy, which argues that great bulk causes the 
energetics and thermodynamics of giants to converge, unlike the 10 fold 
difference in MR seen in small mammals versus reptiles. In this view
reptiles 
should be as suitable for becoming huge on land as mammals. 
    Since gigantothermy was based on faulty data it was ill founded.
Besides, 
even if leatherbacks had elevated MRs this would not necessarily have
meant 
that all big reptiles converge with big mammals, it have instead might
have 
meant that leatherback MRs were elevated above those of other reptiles.
Giant 
tortoise and croc MRs show no evidence of being above the predicted
reptile 
line. Deep core body temperature measurements of multi-tonne basking
sharks 
show that they are poikilothermic and therefore almost certainly 
bradymetabolic, with MRs probably one tenth the whale level (probably
true of 
whale sharks also). There is no evidence that the 10 fold difference in
MRs 
does not apply to giants as well as teeny weenie vertebrates. Of course 
giant, tachymetabolic, tachyaerobic rorquals are far more active and 
energetic than filter feeding sharks (scuba divers can easily keep up
with 
the latter, not the former), confirming that large size does not force a 
major convergence. At this time, it is not certain what the hypothesis
of 
gigantothermy is based upon, or even what it actually means. Nor does it 
explain why bradymetabolic/aerobic reptiles have failed to evolve into
land 
giants for so long. 
(Despite being bradymetabolic leatherbacks are homeothermic endotherms, 
because they use the constant activity of swimming and countercurrent
heat 
exchangers to maintain an elevated level of body heat and temperature.
The 
same is true of some tuna and sharks. These options are not available to
land 
animals which are not constantly active, and cannot thermally isolate
their 
thermally exposed leg muscles with countercurrent heat exchangers).  


Terramegathermy - Bradymetabolic fish grow almost as large as
tachyaerobic 
whales. Water is buoyant, and streamlined swimmers can move from 6 
(leatherback) to 12 (tuna) times more energy efficiently than an animal
of 
the same mass can walk the same distance under the influence of 1 G. It
is 
therefore easy for any swimmer of any metabolic status to cruise at
fairly 
high speed, swim very long distances (little fishes swim across entire 
oceans), and gather enough food at little expense in order to grow fast 
enough to become gigantic. Since only mammals are as massive as blue
whales 
it is possible that only tachyaerobes can grow fast enough to become so 
gigantic. It is therefore possible that there is a weak form of 
pelagomegathermy. 
    On land mammals have reached 20 tonnes, classic reptiles only a
tonne or 
two (there is no way that the biggest tortoises weighed 4 tonnes which
is as 
big as African elephants, it is not yet certain whether the super
moniter 
Megalania was fully terrestrial). This is opposite the common but
illogical 
assertion that sauropods grew so big because they had low MRs. 
Terramegathermy is based on the logical and rather obvious premise that
since 
land animals live in 1 G, tall and/or massive giants need to have high
energy 
power systems in order to cope with living under the pull of an entire 
planet. The extremely low aerobic exercise capacity of all bradyaerobic 
animals of all sizes, and regardless of limb design and posture, limits
them 
to sustained walking speeds under 2 km/h (claims in the literature that
oras 
regularly move at 5 km/h are not substantiated by data sets [when I 
asked 
one researcher for his data he said it was lost] and are spurious, oras
plod 
along at 1-2km/h like other big lizards), so they cannot migrate long 
distances regardless of size and limb form and none do so (crabs, snakes
etc 
migrating relatively short distances do not count). (Nor are migrating
flying 
insects pertinent since all over 1 g are tachyaerobic endotherms with 
exercise energetics as high as those of birds. Besides, flying is about
three 
times more energy efficient than walking in terms of distance traveled).
It 
cannot be overemphasized that body temperature stability does absolutely 
nothing at all in any way or regard to solve this problem for reptiles, 
because a reptile with a body temperature of 98F 24 hrs a day every day
of 
the year will still have the same pathetic aerobic exercise capacity it
would 
if its temperature drops 20 degrees at night. Because according the refs 
cited in my papers long migration (1000s km) is extremely energy taxing
and 
physically arduous, only tachyaerobic mammals move far on land. Claims
in the 
literature that bradyaerobes can actually migrate farther than
tachyaerobes 
while walking at speeds of 3-4km/h are false in that bradyaerobes simply 
cannot sustain such high speeds, and are improbably speculative in face
of 
the inability of land bradyaerobes to migrate long distances.
Bradyaerobic 
land giants would therefore at best be slow, and unable to move very
far. 
    That's if they would work at all. Because bradymetabolic animals 
inherently have low capacity central organ dependent respiro-circulatory 
systems, they cannot oxygenate and feed large, high aerobic capacity
sets of 
muscles, so they are inevitably bradyaerobic (ergo, all reptiles have 
relatively small leg muscles, anchored upon very short ilia in the
thigh). It 
is questionable whether such small, low capacity muscles can adequately 
support a giant body in 1 G, at least without frequent belly resting
which 
would be a tad awkward for something weighing 50 tonnes or so. We KNOW
that 
the large, high aerobic capacity of bradyaerobic mammals can carry huge 
bodies around all day long and more (and their large limb muscles are 
anchored upon long pelves at the thigh). 
    (The Bennett et al. study on anaerobiosis induced fatique in big
crocs 
does not offer definitive evidence of the ability of big reptiles to use 
anaerobiosis for sustained activity. For one thing they did not actually 
measure the amount of work being done by the struggling crocs. A big
croc 
doing anything in close quarters might seem periodically "explosive",
yet may 
be only achieving modest levels of exercise. In any case we have no idea
how 
much walking speed the effort would translate into because there were no 
power measurements. More informative is the case of spindly legged gnu,
which 
after being snatched at a waterhole by a croc twice its size was able to 
brace itself and resist being pulled into the water for 20 minutes
[until 
other crocs came along] because the massive reptile so quickly exhausted 
itself that it was not able to overcome the superior sustainable aerobic 
capacity of its smaller mammalian prey. Besides, big crocs often die
after 
doing heavy anaerobic work. Finally, animals just do not regularly use 
anaerobiosis to regularly do sustained work such as walking for half an
hour 
or more each day, its too toxic and will damage the creature in the long 
term. Modeling dinosaurs as using anaerobiosis for regular movements is
not 
going to get very far, as it were.) 
    It is questionable for inherent circulatory reasons that others have 
discussed whether a high pressure, truly four chamber heart is
compatible 
with bradymetabolism. Conversely, bradymetabolic organs probably cannot 
support a heart powerful enough to produce high pressures. Because
reptiles 
have only low pressure hearts, they are all low slung even when large. 
Tachymetabolic bird and mammal hearts can produce the very high
pressures 
needed to carry the head well above heart level, resulting in
giraffes.   
    Because tachymetabolic animals have stable body temperatures even
when 
small they than can gather food for a larger portion of the day than can 
reptiles. Because tachyaerobic metabolic systems allow them to sustain
high 
walking speeds and extended bouts of high activity without profound
fatique, 
they can gather much more food per unit time. This allows 
tachymetabolic/aerobic mammals and birds to grow far faster than
continental 
reptiles (although some marsupials and primates grow no faster than the 
fastest growing reptiles) under natural conditions (spare me the notion
that 
since farm raised crocs kept constantly warm all day long and/or feed
vast 
quantities of food at no expense to themselves suggests that reptilian 
dinosaurs could magically have grown as fast, since no one was raising 
dinosaurs on Mesozoic farms where they could be kept constantly warm all
day 
long and/or feed vast quantities of food at no expense to themselves). 
Because it is not possible for continental animals exposed to numerous 
disease vectors and predators as well as accidents to live more than 60
years 
or so, all giant animals must grow rapidly. It is probable that on land
only 
tachymetabolic, tachyaerobic beasts can grow fast enough to become
gigantic 
before they go belly up.  
    If reptiles became truly gigantic without elevating their aerobic 
exercise capacity to mammalian levels, they would be much slower,
shorter 
ranged, shorter, and slower growing than equally large tachyaerobes. At
best 
gigantothermy fails to show that land giants will be similar in
thermodynamic 
attributes and performance regardless of MR, at worse it is impossible
that 
land bradyaerobes could become really massive at all. 
    Giant slothes were almost mesometabolic and mesoaerobic, so this
shows 
that animals with such moderate energetics were able to grow that large, 
albeit with rather low locomotary performance and a limited ability to 
migrate. The much larger indricotheres were almost certainly
suprametabolic, 
it is possible that high MRs are necessary to exceed 5-10 tonnes.  


Dinosaurs - Some people, those who prefer to not carefully read my
papers, 
actually think that I'm one of these hardcore "hot-blooded" dinosaur
people 
that think all dinosaurs had the energetics of hummingbirds on speed. Of 
course the more knowledgeable among you, those who have truly read my
papers, 
are rolling on the floor with laughter at the thought. 

Mesoschian proto/dinosaurs - Lagosuchians, eoraptors, staurikosaurs, 
prosauropods have ilia whose length is higher than measured in reptiles
even 
the semi-bipedal ones, but shorter than those of birds and mammals. This 
means that they were intermediate in terms of leg muscle mass and
exercise 
energetics. The legs were fully erect however, a combination not seen in
any 
living animal. Therefore basal dinosaurian energetics could not have
been 
like that of birds and mammals, and must of been of a marginal
endothermic 
kind no longer extant. Because long erect legs work as pendulems that
force 
walking speeds to be over ~3 km/h, it is probable that the combination
of 
high sustained walking speed legs and lower sustained walking speed leg 
muscle mass was somewhat mismatched, and unstable in the longer term. It
is 
therefore interesting that the mesoschian dinosaurs were fairly soon 
replaced. The limited leg musculature also appears ill suited for
gigantism. 
It is correspondingly interesting that none of these archaic dinosaurs 
exceeded a tonne or two in mass (some prosauropods). 

Longoschian dinosaurs: All other dinosaurs had long, deep ilia able to
anchor 
large leg muscles. Larger cnemial crests in the knee suggest the same
thing. 
This upgrade in aerobic muscle capacity would have solved the leg speed 
problem, and indicates that aerobic exercise capacity was approaching or 
equalling the lower avian range and was well into the mammalian range. 
Because the centralized vertebrate respiro-circulatory system probably 
requires a tachymetabolic condition when the exercise system was tachy
aerobic, it is probable that longoschian RMRs were elevated above the 
reptilian maximum, and were within the mammalian range (not that that's 
saying much since manatee RMRs are essentially reptilian, and some other 
mammals are not far above reptiles), and in many cases approached or
equalled 
the ratite condition (lower than that of most other birds). With such 
elevated levels of power production these would have been endotherms in
terms 
of acquiring most body heat internally. Currently unknown is how
aggressively 
longoschian dinosaurs defended their body temperatures. Bone isotope
data 
suggests that the temprature stability and energy budgets of dinosaurs
may 
have not been as high as in most placentals, but this leaves them within
the 
range of marsupials well as some canids, raptors, ratites and the like.
The 
relatively sluggish armored dinosaurs and therizinosuars may have been
less 
energetic than other dinosaurs. 
    With such large, highly aerobic leg muscles dinosaurs could easily
have 
sustained the 3-10km/h walking speeds that something like 97% of their 
trackways (sample ~400) show. They could therefore migrate as most
dinosaur 
researchers argue that some of them probably did (however, I have
repeatedly 
explained that ultralong migrations away from the poles are probably 
impossible for any land animal. But then, strongly endothermic dinosaurs 
would not have had to move away from the polar winters [bone isotope
data 
suggests that a fairly high latitude armored against attack even when 
immobile ankylosaur may have hibernated through the dark winter]).  
    Because they had such large leg muscles and the other energetic 
attributes associated with tachymetabolism and tachyaerobism,
longoschian 
dinosaurs were excellent candidates for evolving true gigantism, which
some 
of them did. They could grow fast enough to become as big as whales in a
few 
decades, as the latest bone microstructure and growth ring counts show
that 
giant dinosaurs did (unlike slow growing supercrocs). The corresponding
high 
power, high pressure hearts were able to pump blood far up against the 
gravity well. Viola, sauropods. (As I have explained multiple times, the 
common idea that giant endotherms are in exceptional danger of
overheating 
are mythical.) Terramegathermy explains why longoschian dinosaurs and
mammals 
have had little trouble becoming gigantic and tall on land, they have
had the 
high power tachyaerobic system needed to do so. 
    There is currently nothing in the nature of gigantic longoschian 
dinosaurs that indicates that they were not tachymetabolic tachyaerobic 
endotherms that had completely diverged from the reptilian pattern. Does
not 
mean that they were all exactly like birds and mammals in their 
thermodynamics, but they were much closer to them than to reptiles. This 
further contradicts speculat