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RE: MY TRIP TO THE ZOO



A replier to the original version of this message commented that he too
remembered crossing the water covered ford across Rock Creek Park to the
Washington Zoo in the post WW II era. I forgot to note that when I did so it
was in my Grandfathers pink Chevy Belair. 

Canning made some statements on reptile energetics and growth that I am not
sure are or are not correct. First, he seems to state that the aerobic
capacity of oras is higher than their "relatives". As far as I know no one
has measured the aerobic exercise capacity of oras - partly because they are
rare, partly because they are big nasty suckers that no one wants to work
with. It is true that many varnaids as well as some other lizards have higher
aerobic capacity than other reptiles, how true this is of oras, especially
the grown ups, I am not sure. 

Canning says that captive reptiles can grow 4 to 6 times faster than in the
wild. I have heard the same, but as far as I know such data has never been
published in terms of complete growth curves (Coulson et al 1973 for
instance, present a captive gator growth curve about twice the maximum
observed in the wild). (One also has to be careful to distinguish between
comparing captive growth to maximum and normal natural growth rates). If any
new refs are available they would be appreciated.

Canning says that the main factor associated with rapid growth of captive
reptiles is environmental temperatures high enough to allow body temperatures
to be high and stable. This is a tricky issue. The wide variation on captive
ora growth is largely due to food consumption (I am not sure whether
temperature is a factor), and farm raised gators are fed very large amounts
of food in addition to being kept nice and toasty.  If Canning can provide a
source for evidence of rapid reptilian growth without throwing lots of food
at them it would be useful. Note that the growth rates of crocodilians may be
the highest that nonmarine reptiles can achieve in the wild. This is because
the juveniles can energy efficiently swim in search of food, thus achieving a
high food intake/energy expenditure ration. Land reptiles that have to walk
inefficiently in search of food may not be able to grow as fast, and the oras
suggest that this is so. (Because marine ectotherms always forage very energy
efficiently, they can and do grow more rapidly than terrestrial ectotherms.)
 

Turning to Canning's comments on aerobically capable yet inertially
homethermic ectothermic dinosaurs. First, even small dinosaurs much too small
to be inertially homeothermic, yet their trackways appear to consistently
record speeds (2-10 km/h) far above those achieved by reptiles of similar
size (I am not aware of any data proving [via a speed frequency data set
rather than off-the-cuff observation] that in the wild any reptile regularly
cruises at a speed much above 1 km/h, any well documented contrary info would
be appreciated). Even sauropods were regularly walking at speeds too high for
a giant reptile to power aerobically no matter how warm and stable its body
temp might be. While at the zoo I talked to a physiologist, who agreed that
if dinosaurs cruised at 3-10 km/h then their aerobic capacity almost
certainly had to be above reptilian levels (unless they constantly used
anaerobic power and constantly excreted the toxic by-products, which would be
fantastically energy inefficient). There is a widespread consensus that
dinosaurs migrated, and combined with the high walking speeds it is probable
that the exercise energy budgets of dinosaurs were at least two to four times
higher than those observed in reptiles. 

Canning seems to be suggesting that such aerobically capable dinosaurs could
still be ectotherms. But is this true? If the daily energy budgets and heat
production produced by exercising dinosaurs is higher than in reptiles, they
may have been producing enough internal heat that they were not dependent
upon environmental heat. (This is fully compatible with small dinosaurs being
insulated.) If so then these dinosaurs were endotherms, not ectotherms. Some
mammals have aerobic exercise capacity not far above the reptile level, but
they are insulated endotherms. This brings us to the subject of resting
metabolic rates. There are reasons to believe that in vertebrates increasing
the aerobic exercise capacity above the reptile level requires boosting the
resting metabolism above the reptile level too. It has to do with poorly
understood and complex constraints of the centralized respiro-circulatory
system and "leaky" tachyaerobic cell membranes. If this is true - and there
some speculation that dinosaurs somehow managed to combine high maximal with
low minimal aerobic capacity - then the probabiity that dinosaurs were
regularly walking around as fast as ratites suggests that their resting
metabolic rates were well above the reptilian maximum. If so then dinosaurs
were certainly endothermic.   

On growth. Canning suggests that inertially homeothermic dinosaurs could grow
faster than modern reptiles. The problem with this is obvious. As far as we
know all dinosaurs started out as wee little hatchlings of at most a few
kilograms. At the same time inertial homeothermy does not kick in until body
mass reaches a few hundred kilograms. Even then temperature stability is
achieved only on the daily cycle, if winter is cool then body temperature
will be too. Ergo, if baby dinosaurs were ectothermic then their body
temperatures would have fluctuated daily, and growth rates would be naturally
reptilian and slow. It takes wild reptiles at least a few decades to grow a
few hundred kilograms (Webb et al, Aust. Wildl. Res. 1978 5:385). The biggest
extinct nonmarine reptiles were super-crocs that reached something like ten
tonnes. Their main growth may have occurred at age 30-100 when they became
large enough to stay internally warm, and grew at a moderate rate from less
than a tonne to full size (living hidden in water, and free of the fear of
falling, super-crocs may have regularly lived for a century or more). We can
compare reptilian growth to those observed in some dinosaurs.  

Varricchio has used bone rings to tentatively estimate that Troodon grew to
60 kg (my estimate) in 3-5 years. Varricchio notes that the rings may be
multiannual in which case growth was faster.  The fastest growing wild
reptiles I know of are gators (McIlhenny 1934 Copeia:80 - if any as more
recent data showing faster growth in wild reptiles please let me know). At
the most the gators reached 10 kg in three years, and 30 in five. (One male
reached 56 kg in six years, but it was still in the midst of its fastest
juvenile growth to an adult mass of a few hundred kilograms, so this is not
comparable to the troodonts which had already experienced a period of slow
adult growth before they stopped growing and expired). Troodonts seem to have
been growing at least twice and perhaps six or more times faster than
reptiles. Clearly Troodon could not have been an inertial homeotherm, so its
fast growth is better ascribed to some form of homeothermy and high food
consumption due to a high energy budget.  

Reid (Modern Geology 1987 11:133) has estimated that a sauropod was about
28-29 years old when it died at 8 tonnes (my estimate) based on what look
like good bone annuli. If correct, then this beast was growing as fast as the
biggest elephants. Since reptiles take ten years to reach just 200 kg, it is
unlikely that inertial homeothermy can explain this very high rate of growth.


The biggest problem (excuse the pun) for applying reptilian energetics to
growing dinosaurs are the super-sauropods. Since they almost certainly
reached 100 or more tonnes in 30 to 100 years (as I explained earlier this
month, it is unlikely that animals living in predator rich environments where
crippling falls

 are possible are unlikely to live over sixty to eighty years), they were
growing as fast as whales. I suggest that those who wish to apply reptilian
growth to dinosaurs sit down and plot (non-log) the known growth curves for
living whales, elephants, and reptiles, and then plot the probable maturation
times and sizes of super-sauropods and the biggest extinct reptiles. I have
done so, and the results should give advocates of reptilian sauropods pause
(indeed it is ironic that while sauropods are often thought to be among the
best candidates for low metabolic rates, there extreme size and growth rates
may have required that they have very high metabolisms). Arguing that some
form of reptilian energetics can explain how sauropods grew at the same
fantastic pace as whales is a highly dubious proposition. 

Bone ring counts have been used to plausibly calculate slow, reptilian gorwth
curves for some theropods and prosauropods by Chinsamy and Reid. However,
some mammals and birds grow slowly, the bone microstructure of these
dinosaurs differs from that of reptiles, and it is possible that the rings
were multiannul rather than annual and growth was faster than it first seems.
Some early birds have reptile-like bone annuli, but the juvenile bone has
been lost in these thin walled elements and the preserved rings may just
indicate slow adult growth (which is observed in some marsupials, and male
elephants). I know of no dinosaur or early bird that exhibits a fully
reptilian growth pattern, and some dinosaurs cannot be distinguished from
mammals and birds in this regard.  

I would be interesting if Canning makes some comments on the above
observations. 

GSPaul