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Oh really? More on dino metabolics



In reply to J Ruben's comments that my assertions on dino metabolics are
misleading and inaccurate. Oh really? Let's see on whom that shoe really
fits.

Ruben likes to cite various authorities when they support of his arguments.
Yet he is in the minority concerning avian-like air-sacs in theropods and
sauropods, because there is a broad consensus that they had them. For
example, a leading  authority on tetrapod lungs, Steven Perry, has repeatedly
asserted that these dinos probably did have air-sacs (try Form and Function
in Birds 4, 1989). Also, Brooks Britt in his PHD thesis (& 1994 SVP poster
session that I assume Ruben saw) shows that the pneumatic vertebrae of
pterosaurs, theropods, and sauropods do show the bird-like characterstics
indicative of pulmonary air-sacs. To this, add the fact that birds (incl
Archaeopteryx), theropods, sauropods & pterosaurs share shortened, stiffened
dorsal columns and long, hinge headed, mobile abdominal ribs (contrary to
Ruben's inaccurate assertion otherwise, take a look at a mounted sauropod
skeleton sometime JR) - the mobility of sauropod ribs is especially notable
because one might expect them to be fixed immobile like in ankylosaurs in
order to support the enormous belly, but instead they had the potential to
inflate abdominal air-sacs like in birds. As for the sternal problem, many
flightless birds have reduced breast plates, especially the giant armless
elephant birds in which the plate is very short, yet they breath with
air-sacs just fine thank you (the expanded sternum of birds is primarily an
anchor for flight muscles, any respiratory function is probably secondary).
That sauropods had solid limb bones in order to carry their mass is no more
significant than the solid limb bones of many diving birds. Whether dinosaur
air-sac systems were as fully developed as in flying birds is open to
question, but Ruben's airy dismissal (excuse the pun) of a strong connection
between the high oxygen capacity of air-sac ventilated lungs constitutes
biased and untestable arm waving in a desperate attempt to get rid of
inconvenient data. After all, no reptile needs or has them. (Ruben's
inadequate dismissal of  feathers as indicators of high aerobic metabolisms
is another casual dismissal of reality. Thousands of bird species have
feathers, thousands of mammal species have fur, but no reptile has either.
Why? Because insulation inhibits the absorbtion of solar radiation, as proven
by the low values of temperature increase in basking birds.) 

As for the problem of sauropod cardiac power, no reptile heart  produces the
extreme pressures (200+ mmHg) needed to pump blood up a few meters. Why?
Because they are too small, and do not have enough power to produce such high
pressures (this is true of croc double pump hearts). As for those those herp
heart experts, I can cheerfully report that I have talked with one who is
much less dismissive of my ideas than JR (the Morrison paper has been
accepted by the way). In fact, another of those experts (Seymour, 1976 Nature
262:207) noted that a tall sauropod's heart's "rate of oxygen consumption
would be high". Seymour was so appalled by the metabolic and blood pressure
consequences of tall sauropods that he argued they must have either been
aquatic (which sauropod trackways disprove), or held their necks horizontal
(which neck base articulations disprove in most sauropods). Two cardiac
specialists (1992 Lancet 340:534) also demonstrated the need for expanded
cardiac tissues in tall sauropods. Ruben's arm waving is all well and good,
but I would like to see him (and others who favor reptilian sauropods)
present some solid calculations showing how in a 100+ tonne sauropod, a heart
working at reptile levels (i.e. about the same power level as wee little 10
tonne elephant's heart) could oxygenate ~45 tonnes of muscles and many more
tonnes of organs, and pump blood up the four story neck. It is obvious that a
croc-bird heart as big or bigger than a whale's could power such a land
monster. The low power of small herp hearts may help explain why fully
terrestrial classic reptiles (such as monitors and tortoises) have never
exceeded much more than one tonne on land. Mammals and dinosaurs may
regularly exceed this size limit in part because they have/had the large,
powerful, oxygen hungry hearts needed to be really big and tall in the high
energy world of 1 gravity.    

Citing that an ora can sprint about as fast and far as a human is not at all
impressive, because A) humans are notoriously slow mammals, and B) this
represents brief ANaerobic activity, and tells us nothing about dinosaur
aerobiosis. What is interesting is that oras normally cruise along at a
typically reptilian plod of about 1 km/h. (Determined by a tedious  series of
timings of the beasts in DSC and TLC cable videos. Claims that big monitors
normally walk at 4 km/h have not been confirmed by a large statistical sample
so far as I know [wrote Auffenberg for his data but he does not have any]. In
videos oras moving fast are almost always engaged in brief bursts of
anaerobic activity around carcasses or in fights). Dinosaur trackways show
they regularly walked at the 3-10 km/h speeds typical of aerobically capable
birds and mammals. It is widely agreed on fairly good evidence that many
dinosaurs traveled long distances and even migrated at such speeds. No
reptile can walk so fast for long distances. In fact, the locomotary evidence
for high aerobic scopes in dinosaurs is much better than in advanced
therapsids. The latter all have short legs suggestive of rather limited
sustained speed-exercise potential. Dinosaurs had long, erect, digitigrade
legs that favored if not forced the high walking speeds that can only be
sustained by avian-mammalian level aerobiosis. Dinosaurs were built for the
high sustained activity seen in birds and mammals, not the short burst
activity/sustained low activity seen in reptiles.

Let us turn to Ruben's 1995 claim that Troodon grew no more rapidly than
Alligator, which is based on misleading manipulation of the data (his idea of
comparing growth in full grown dinosaurs to still growing reptiles not only
distorts the comparison, but is impractical because we do not now at what %
of final size most dinosaurs become fecund). The theropod may have grown to
50-70 kg in 3-5 years assuming bone rings were annuli (faster growth is
suggested if the deep set rings were multi-annular, as they are in some
mammals). According to the Avery Island Alligator growth data (the fastest
growing reptiles in the wild recorded), one male made it to 55 kg in 6 years,
and most males got to 70 kg in about 9 years (no one has ever bothered to
plot up this data, I did this spring and the results are most revealing).
Let's see, Troodon 50-70 kg in at most 5 years, gators 50-70 kg in 6-9 years.
Sure seems that the dino outgrew reptiles! Do farm-raised gators grow faster?
Yes, but no dinosaur was raised on a farm!!! Ruben neglected to note that
Reid 1987, 1990 has used bone rings to estimate that A) the ornithopod
Rhabdodon grew faster than crocs (to ~300 kg in 10-15 years), and B) a
sauropod died at a mass of about 8 tonnes at age 28-29. Consider
super-sauropods, which must have reached at least 30 tonnes in two to three
decades in order to maintain stable populations (according to Dunham et al
1989). They probably had to add kilograms each day. Elephants and whales do
grow so fast. Maximum reptile growth is measured in dozens of grams per day,
and it would take a reptile centuries to grow 10 tonnes or more. It is true
that a number of mammals grow no more rapidly than reptiles (some marsupials
and primates), and some primitive dinosaurs (Chinsamy) may also have had
reptilian growth rates. But all animals that grow faster than wild reptiles
do so because high aerobic metabolisms are involved: A) the baby has a high
activity metabolism and feeds itself the large amounts of food needed to grow
fast. B) the baby has a low metabolism, but is fed large amounts of food by
parents with high metabolic rates. C) the baby (reptile) has a low
metabolism, but has large amounts of expensive flesh thrown at it by an
endothermic farmer desperately trying to keep from losing his shirt raising
gators that even then grow too slowly to be highly profitable (see S. Grenard
1991 Handbook of Alligators and Crocodiles). We know mammals and birds grow
as fast as dinosaurs. Assertions that fully terrestrial reptiles can somehow
manage to grow as fast as a number of dinosaurs seem to constitute further
examples of unsubstantiated, untestable, speculative arm waving. The
inability of juvenile land reptiles to grow fast may be another reason why
only mammals and dinos with high metabolic rates grow gigantic on land (Ruben
is correct that sharp declines in metabolic rates upon cessation of growth
are improbable). 

Interesting thing. Bennett & Ruben have proposed a viable  aerobic expansion
hypothesis, and presented sound criteria for assessing the diagnostic value
of criteria. Then they appear to fail to apply their work logically. Ruben
waxes rhapsodic about respiratory turbinates when a number of birds and
mammals do not have them, yet he dismisses feather and fur as metabolic
indicators when the observed positive correlation between insulation and high
metabolic rates is perfect. He prefers high exercise aerobic scopes in short
legged, flat footed advanced therapsids rather than in long limbed dinosaurs.
    

There is a lot of room for debate about dinosaur metabolics, but blanket
assertions that there is not the slightest bit of reliable evidence for high
aerobic scopes are truly misleading and inaccurate. The evidence that most
dinosaurs had aerobic metabolic rates (but not necessarily thermoregulatory
controls) similar to those of birds and mammals is very good (so is the
evidence that primitive dinosaurs had an intermediate  metabolic system
different from both reptiles on the one hand, and other dinosaurs, birds and
mammals on the other). The ectothermic hypothesis in comparison is so weak
that the poor thing is all dressed up, but has no place to go. 

I could use references for the Bennett papers discussing the metabolics of
the racehorse goana and ora, perhaps I will learn something. (Rowe, certainly
feel free to forward this message to you know whom.)

GSPaul