Part 2: Terramegathermy (very long, too)

["David Marjanovic" <david.marjanovic@gmx.at>]

After I've shown :-( the disadvantage of an e-mail over a letter, I continue
and hope that this helps more than it stuffs mailboxes :-] ...

Very short:

...Migrators must be tachymetabolic. InMRs probably aren't enough to surpass
5 tonnes. Fluctuating MRs can't be exploited very far because hard heart
work must be sustained to prevent torpor. Dinosaurs grew as fast as birds &
mammals; the difference of reptilian and bird & mammal growth rates
increases with size, rather than disappearing, as advocates of
gigantothermic megadinosaurs assume. "Reptiles" grow too slowly to evolve
giant sizes, while populations of megamammals are small and unstable and
can't easily be refilled by reproduction. Desert elephants never overheat,
so HiMR sauropods shouldn't have either. Therizinosaurs, and maybe early
dinosaurs with small ilia, probably had InMRs like giant sloths. Sauropods
didn't always carry their necks horizontally and so had to pump blood up all
the way which requires more blood pressure than a bradymetabolic heart can
produce. Gigantothermy would require unreasonable amounts of fat. Sauropods
didn't have feeding problems.

Very long:

    "Long anterior airways pose a respiratory problem because they hinder
ventilation of the lungs. Even so, sperm whales (Fig. 4) inhale enough air
through long anterior airways to sustain HiMRs with modern oxygen levels.
This is true despite the small size of their lungs, the lack of respiratory
air-sacs, and the need to respire during brief periods at the surface
between long dives."

There once was an article called, IIRC, "An alternative to hot-blooded
dinosaurs: the happy wanderers" or something like that. Well, nothing of the
sort:

    "*Cruising and migration. -* In order to forage long distances on a
daily basis, or to migrate very long distances on a yearly basis, sustained
walking speeds should be above 2-3 km/h. Because moving on land is energy
expensive, high aerobic capacity is needed to power such high cruising
speeds for many hours (Bennett, 1991). This is true of large as well as
small animals. The 2-7 km/h walking speeds observed in elephants for example
(Fig. 5) are easily achieved aerobically. Although swimming leatherbacks
cruise at 3-5 km/h, the sustainable aerobic capacity of leatherbacks can
power walking speeds of only 0.5-0.8 km/h (Fig. 5). The long migratoins of
leatherbacks are possible only because they swim so cheaply, and exploit
favorable currents [...]. Anaerobiosis does not produce power long and
efficiently enough to power high walking speeds, so calculations that
bradyaerobes can migrate farther than tachyaerobes on land (Spotila et al.
1991) are incorrect, and no land reptile migrates."

Aren't there some "exceptions that prove the rule"? They've slipped my
mind...

Fig. 5 plots the speeds of elephants and rhinos (from videos), ornithopods +
theropods, ceratopsids, nodosaurids and sauropods (estimated from
trackways), which all begin at 2 km/h upwards, and "the low sustained
walking speeds predicted by the reptilian aerobic metabolism of a cruising
leatherback sea turtle" under 1 km/h.

    "Galloping rhinos do not have higher resting MRs than slower elephants,
although their exercise MRs may be higher. The most gigantic extinct mammals
were 10 to 20 tonne, HiMR proboscideans and indricotheres with long striding
legs [...]. Giant extinct edentates and marsupials with heavy awkward limbs
never exceeded about 5 tonnes [...], and these rather sluggish beasts
probably had InMRs like their living relatives (McNab, 1983). If so, land
animals much over 5 onnes may need HiMRs.

*Heterometaboly. -* Farlow (1990) suggested that nonmammalian giants may be
able to save energy by sharply dropping their MRs from high to low levels on
a seasonal basis, or when they complete growth. [John R. Horner's concept
called "mesothermy" is, IIRC, similar.] Birds and mammals can drop mass
specific MRs by about a third under similar circumstances. Greater metabolic
declines are probably not feasible in vertebrates because suppressing MRs
strongly decreases cardiac work and circulatory pressures, resulting in
impaired aerobic capacity and torpor.

    *Growth and reproduction. -* Fig. 6 shows that land reptiles grow more
slowly than birds and all but a few terrestrial HiMR mammals (Case, 1978).
Note that the divergence between terrestrial reptilian and mammalian growth
rates _increases_ _with_ _size_; this negates the premise of gigantothermy
that the growth rates of land giants should converge towards a common
level."

Fig. 6 is impressive. Growth rates in g/day are plotted against body mass
for reptiles, marsupials, placentals in general, whales, ungulates,
elephants, hippos, altricial and precocial birds, a hadrosaur nestling, a
hadrosaur (at what age?), a ceratopsid and the "[m]inimum growth rates
needed for giant sauropods to reach sexual maturity at 1/3 adult mass". The
area occupied by placentals touches that of reptiles at small sizes but
diverges clearly at large ones, small marsupials overlap with reptiles while
bigger ones overlap with placentals and middle ones are in between,
cetaceans diverge even more strongly, rhinos, hippos, hadrosaurs and
ceratopsids are on the lower border of the ungulate range, elephants are
below, the minimum for sauropods lies on the extrapolation for elephants and
the hadrosaur nestling on that of the upper border of the range of altricial
birds, which have the highest growth rates.

"The inability of bradyaerobic juveniles with low foraging speeds and ranges
to gather enough food is one reason they grow slowly. It has been suggested
that elevated growth rates of farm-raised alligators and captive
leatherbacks show that reptiles can grow rapidly. Raising alligators is an
energy expensive and labor intensive proposition that involves providing
idle reptiles with large quantities of food (Grenard, 1991). The relevance
of captive and[/]or aquatic reptilian juveniles to natural land conditions
is nil.

    In order to maintain stable populations over time, generational turnover
must be sufficiently rapid. Giants must therefore reath sexual maturity
within about twenty years and their lifespans should not be much greater
than a century (Dunham et al., 1989). Big ungulates, rhinos, elephants, and
whales fit these characteristics (Owen-Smith, 1988). [There is a recent New
Scientist article "The old man of the sea" that says that the oldest whale
caught was 211 years old, based on the aspartic acid racemization in its eye
lenses.] Note that the more gigantic an animal is, the higher the rate of
growth must be in order to keep the juvenile stage and lifespan within
reasonable limits. [Whales fit this, Fig. 6.] We conclude that HiMRs are
necessary to grow more than 5 tonnes. The large size of some extinct
marsupials and edentates suggests that InMRs are sufficient to grow to about
5 tonnes, and LoMRs can grow animals to only about 1 tonne.

    A problem with being a tachyaerobic giant is that each adult consumes
large amounts of food, so the total adult population size is rather small
(Farlow, 1993; Paul, 1994). [...] In general, small populations are less
stable than larger ones over geological time. [...] Under optimal natural
conditions megamammals can expand their populations about 6-12% per annum
(Owen-Smith, 1998). These modest rates of population expansion have allowed
megamammals to evolve moderately gigantic masses during the Cenozoic. Large
reptiles lay large numbers of eggs, but their slow growth and generational
turnover hinders their ability to exploit their rapid reproduction to evolve
giant dimensions. We predict that if giants combine high rates of growth
with high rates of reproduction, then the resulting high rates of population
expansion [...] should allow them to survive as species even if the adult
populations are so small that they are prone to periodic crashes. If so,
then fast breeding tachyaerobic giants heve the potential to have smaller
populations of larger adults living off of the same resource base than
observed among slow breeding big mammals (Paul, 1994)."

Now it goes into physics...

    "*The big overheating myth. -* It is a nearly universal truism that
giant HiMR endotherms are in danger of 'frying' or 'melting down' in hot
climates. Spotila et al. (1991) calculate that an inactive 3.6 tonne
tachyaerobe will hav a body temperature of 53°C when the environmental
temperature is 35°C."

If the poor beast is a BLACK BODY, that is.

"The reality is that desert elephants traverse shadeless land in the middle
of hot days, and even when chased by helicopters elephants do not heat up to
dangerous levels (Bartlett & Bartlett, 1992 and Osborn, 1992, pers. comms.).
In hot droughts the big bulls suffer the lowest mortality, and females and
calves die largely from starvation (Owen-Smith, 1988; Haynes, 1993), there
are no documented examples of elephants dying from heat stroke under natural
conditions. Extinct elephants, [interglacial North American] mammoths and
indricotheres of 10 to [hm] 20 tonnes thermoregulated in hot climates."

Sort of negative gigantothermy:

    "Giant tachyaerobes have relatively low MRs per unit body mass, enormous
heat storage capacity, and high body temperatures of 36-39°C. [...] An
active tachyaerobe of 4 tonnes with a normal body temperature of 39°C can
resist the inflow of an external heat load well over 40°C by raising its
body temperature to 43-46°C. [Isn't that _lethal_?] Internal heat is safely
stored for about six hours [...], and is later dumped into the night sky.
[...] The temperature stability of LoMR inertial homeothermy does not
provide giants with the power they need to be so big. It is water giants
that have no need for high blood pressures or large volumes of hard working
limb muscles."

*Hips, legs, and cruising. -* Early dinosaurs - eoraptors, staurikosaurs,
herrerasaurs and prosauropods - had erect legs like birds and mammals, yet
they retained short, reptile-like ilia [...]. These ilia could have only
supported narrow thigh muscles like those of reptiles. The combination of
erect limbs and reptile-like hips was an unusual and exotic combination that
is now extinct. It suggests that these early 'brevischian' dinosaurs had
aerobic metabolics that were neither reptilian or avian-mammalian in nature,
and that MRs, circulatory pressures, cruising speeds and growth rates were
insufficient to achieve great size - so it is not surprising that no
small-hipped dinosaur became very big. These archaic dinosaurs may fit the
definition of 'damned good reptiles'."

Human ilia, however, aren't so much longer IMHO, and *Aliwalia*
(herrerasaurid or -grade theropod) and *Euskelosaurus* did become big, 11
and 12 m in length, respectively. (Of course, the former is rather based on
scrap, but the latter...) I'd rather say that "brevischian" (why not
"brevilian"?) dinos just were less adept runners and migrators than later
ones.

And now into biomechanics and myology:

    "The early dinosaur condition was not a very satisfactory one because
the full potential of the long erect legs could not be realized until the
size of the ilium and the leg musculature expanded to avian-mammalian
proportions. This is the condition observed in 'longoschian' [why -o-
instead of -i-, and, again, why the ischium?] theropods, therizinosaurs,
ornithischians and sauropods of all sizes [...]. Among megadinosaurs, the
ilial plates of tyrannosaurs are so large that a high endurance limb
musculature suitable for chasing down large prey is indicated over ambush or
scavenging habits. The prey of tyrannosaurs - hadrosaurs and especially the
ceratopsids - also had long ilia that appear to have supported large sets of
aerobically capable muscles suitable for running. Slower moving armored
dinosaurs and sauropods are restored with large tachyaerobic limb muscles
suitable for bearing great bulk. There is nothing reptilian about the hips
and legs of longoschian [so the o is not a typo] megadinosaurs; instead,
their form is bird- or [to a much lesser extent] mammal-like. The suggestion
that the muscles of large dinosaurs were small and hyperanaerobic is
therefore contra-indicated.

    [...] Figure 5 [...] proves that megadinosaurs walked in the same fast
lane as HiMR megamammals, not in the reptilian slow lane. Only the bizarre
advanced therizinosaurs [...] had awkward feet suggestive of InMRs like
those of giant sloths. It is widely agreed that some megadinosaurs migrated
long distances (Currie & Dodson, 1984; Horner & Gorman, 1988); such journeys
demanded high aerobic capacity.

    *Circulatory pressures. -* It has been widely accepted that big
theropods had strongly S-curved necks that carried the brain well above
heart level, and the same was true of the therizinosaurs [...]. There has
been much more controversy over the neck posture of sauropod dinosaurs. It
has been argued that the long necks of sauropods evolved for high browsing
and must have been held erect, or that circulatory pressure problems
compelled them to carry their necks horizontally (Dodson, 1991), but no one
has examined the articulation of sauropod necks in order to restore their
true posture. Articulated specimens of _Camarasaurus_ and Chinese sauropods
consistently show an upward flexion at the base of the neck [...] [Fig. 9].
The tall shoulders present in many sauropods (a cetiosaur, brachiosaurs,
camarasaurs, [...] euhelopids [sic], many titanosaurs) favored an erect neck
posture. Low shouldered diplodocids had horizontal necks, but large sacral
complexes and heavy tails suggeset they reared up to feed. Retroverted hips
suggest that camarasaurs [...] and euhelopids [and titanosaurs] also reared
up often."

"*FIGURE 9 -* Upwardly flexed articulated neck bases and beveled
cerv[ic]o-dorsals indicating habitually erect neck carriage in the sauropods
[...] _Camarasaurus_ and [...] _Euhelopus_." Apparently drawn as in situ.
The authors don't mix the upward curvature of the neck bases up with
post-mortem distortion like in the C-shaped neck of the *Jobaria* holotype.

    "One way or another, sauropods had to pump blood all the way up their
necks. Consider the problem faced by a 30 tonne _Brachiosaurus_ (Fig. 4). We
conservatively presume that even with special vascular adaptations a low
power reptilian heart [even if four-chambered] could not pump blood up 10 m.
Even if the sauropod had a normal sized high pressure heart of 180 kg, the
metabolic rate of the heart alone would be greater than that of the entire
resting metabolism of a giant reptile (Table 1). If the heart had
giraffe-like proportions it would have weighed 400 kg, but even this would
not suffice to pump blood up over 30 ft. Seymour (1976) calculated that
_Brachiosaurus_ needed a supersized heart of over a tonne, and a one tonne
heart is the largest that could fit into the sauropod's ribcage. Such a
heart would be inefficient, and Choy & Altman (1992) made the interesting
and controversial suggestion that sauropods had extra hearts in the neck so
that the main heart would not need to be so huge. In either case,
conservative calculations of cardiac heat production are many times higher
than the resting metabolism of a reptile. When the heat production of the
other internal organs is added in, it is clear that the resting MR of
_Brachiosaurus_ was as high as those of giant mammals, and many times higher
than expected in a reptile of such size (Table 1)."

No problem with long necks:

    "It has been calculated that long necked sauropods could not draw enough
air down their long trachea to sustain HiMRs (Daniels & Pratt, 1992 [and
several SVP meeting abstracts from 1998]). Alternately it has been
calculated that elevated oxygen levels were necessary to sustain sauropods
(Hengst, this volume [a very weak article IIRC]). Although we take no
particular position on Mesozoic oxygen levels, we strongly question whether
the respiratory capacity of dinosaurs can be calculated accurately enough to
predict past oxygen levels. Nor do we predict that sauropods had any more
trouble breathing large volumes of air with modern levels of oxygen than do
sperm whales. The surface of sauropod trachea[e] may have been
aerodynamically configured to maximize airflow. Although sauropods probably
lacked a mammalian diaphragm, thin walled, pneumatic vertebrae strongly
suggest the presence of pulmonary air-sacs. Because sauropods were not close
bird relatives, we predict that their air-sac/lung system operated in a
different manner (note even some birds have sternal plates that are too
small to ventilate abdominal air-sacs [...]). Abdominal air-sacs operated by
long posterior ribs [and/or gastralia] probably improved pulmonary air
exchange enough to oxygenate HiMRs."

No problem with narrow ribcages:

    "Large theropods had pneumatic vertebrae that suggest a pre-avian
air-sac system was being developed. Progressive elongation of posterior over
anterior ribs suggest that ventilation of abdominal air-sacs became
important in large theropods [...]. Perry (1983) suggested that the prepubis
and retroverted pubis/ischium of large ornithischians supported abdominal
muscles that functioned like a diaphragm. Large ornithopods had a lumbar
space that lacked long ribs, and was preceeded [sic] by long mid-dorsal
ribs. This was a very mammal-like condition [...], and strongly suggests
that ornithopods paralleled mammals in developing a vertical transverse
diaphragm. [This does not preclude the presence of air-sacs.] Giant
dinosaurs appear to have had high capacity respiratory systems designed to
oxygenate their high capacity circulatory systems. [...]

    *Megadinosaurs did not meltdown. -* [...] A 30 tonne HiMR tachyaerobic
sauropod with a high body temperature would have been able to safely store
internally generated heat for 12 hours [...]. We restore tropical
megadinosaurs with 2-7% body fat (as in tropical ungulates and
proboscideans, Ledger, 1968; Haynes, 1991), rather than heavy domestic
animal-like fat deposits postulated for gigantothermic dinosaurs (Spotila et
al., 1991). Polar megadinosaurs probably built up fat deposits for winter
use; whether they used it for insulation is more problematic (see Haynes,
1991).

    *Could tachyaerobic sauropods feed themselves? -* Astute observers of
_Jurassic_ _Park_ noticed that the brachiosaur's head was big enough to
swallow the kids whole. A 30 tonne HiMR brachiosaur needed to eat about half
a tonne of fodder/day, only 1.5% of its own mass. If the beast took six
bites per minute for twelve hours per day [and it didn't chew, so more are
possible] (as per giraffes and elephants) each bite would be a mere four
oz., hardly a problem for a mouth that was 42 cm broad. A 10 tonne HiMR
diplodocid needed only 2 oz. bites.

    *Megadinosaurs were not weak. -* Over the years it has been asserted
that sauropods could not move on land, rear up, feed HiMRs, or pump blood up
their long necks, that large dinosaurs had limited breathing capacity and
moved slowly, and that big theropods were mere scavengers - it is amazing
that the 1-100 tonne weaklings survived at all! Examining the structure of
megadinosaurs reveals strong animals of high aerobic capacity and great
athletic ability. Figure 1 shows that at any given size, megadinosaur
skeletons (especially their vertebral columns) were more strongly built than
those of megamammals.

*SUMMARY AND CONCLUSIONS*

[...] The giant dinosaur's fast growth was possible only because the
juveniles had fast running metabolisms, and dramatic fall offs in MRs with
maturity are not only contra-indicated but may have been reversed in tall
sauropods" because cardiac work seems to increase faster than body mass with
size.

    "We do not assert that the physiology of megadinosaurs was identical to
that of megamammals. The evolution of megadinosaurs in a warm Mesozoic world
may have left low latitude examples with less well developed
thermoregulatory controls and auxiliary heat production than is present in
birds and mammals - but these features may have been present in polar
dinosaurs. Smaller dinosaurs may have been more prone to entering daily
torpor than modern birds and mammals. This may help explain why dinosaurs
were more prone to laying down bone growth rings as they matured than are
birds and mammals (Reid, 1990; Varricchio, 1992; but deep set postcranial
rings are also observed in mammal bones (Leahy, 1991; Varricchio, 1992)).
But, contrary to the argument that many dinosaurs had some form of
transitional metabolics, the anatomical evidence shows that this condition
was limited to early brevischian dinosaurs [...]. There was little or
nothing reptilian in the energetics of big bodied and/or big hipped
dinosaurs. [...] We find the recent tendency to cite marine and captive
reptiles as primary analogs for dinosaurs as unconvincing as it is
perplexing. Giant dinosaurs were not good reptiles, or damned good reptiles.
They were marvelous archosaurs whose anatomy and aerobics converged with
megamammals. It is only logical that the closest living models for extinct
land giants are living land giants with aerobic metabolisms, circulatory
systems, and growth patterns suitable for terrestrial gigantism under
natural conditions. [...]

In water, either low or high MRs work in animals up to 15 tonnes. It is
possible that only very fast growing tachyaerobes can become larger in the
sea. On land, insects with high active MRs are small because of their
decentralized respiratory systems (see Heinrich, 1993; tiny flying insects
have adaptations that minimize oxygen consumption). [...]

    Owen-Smith (1988) stressed the extinction resistance of slow breeding
megamammals. It has been little appreciated that even the biggest dinosaurs
were prolific 'weed species' with much higher recovery potentials than
mammals. It is very difficult to understand how a diverse array of thermally
sophisticated Late Cretaceous dinosaurs adapted to living in climates
ranging from tropical to polar could have been _totally_ extinguished when
environmentally sensitive birds and amphibians survived. This is true
regardless of the proposed extinction agent - massive impacts, vulcanism,
climatic shifts, marine regressions, oxygen declines, floral changes, etc.
The loss of nonavian dinosaurs remains one of the most extraordinary and
inexplicable [hm] events in Earth History, and may have as much to do with a
bad roll of evolutionary chaos [???] as with a specific cause or causes."

Phew...
Finally... done... in... gigantothermy... must... sleep... ;-)