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RE: MY TRIP TO THE ZOO (long)
>Canning made some statements on reptile energetics and growth that I am
>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
Yes, sorry that was ambiguous - I should have said varanids
have a higher aerobic capacity, the "relatives" being most other
lepidosaurs. I don't think V.komodoensis has been examined;
the handful of species that have been measured have significantly
greater aerobic capacity than the average lizard. I would be surprised if
oras were different. My point about thermal stability of large adult
varanids however does come from field research on V. komodoensis and
>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
>new refs are available they would be appreciated.
Unfortunately most of this data comes from reptile breeders, and if
it is published it is usually in obscure, non-peer reviewed journals.
You might dismiss it as 'anecdotal', but anecdotes are not worthless.
I have seen an 8 month old Boa constrictor almost 2 metres in length.
If you are familiar with this species you'll know it's a large, bulky
snake - they breed at sizes smaller than that one. I would have guessed
this snake was at least 3 years old, but the guy that bred and showed me
this was totally trustworthy.
>Canning says that the main factor associated with rapid growth of captive
>reptiles is environmental temperatures high enough to allow body
>to be high and stable. This is a tricky issue. The wide variation on
>ora growth is largely due to food consumption (I am not sure whether
Here comes another anecdote: last year I was asked to look after about
30 hatchling lacertids as part of a conservation project. After 4
months 90% of them had reached adult size (based on the literature)
of a 2-2.5 year old. Now, I don't feed them that much. They get 3 or
4 insects each a day, and 1 or 2 days a week they get no food at all.
They do have a thermal gradient which allows easy thermoregulation, and
thermostatic underfloor heating which comes on at 23 degrees C air
temperature. It's not just total food intake that is important but
also assimilation efficiency, which it is reasonable to assume will
depend on achieving a temperature which is optimal for enzyme efficiency.
Admitedly, there are other factors like no predators to
avoid (although their own social interactions ensure their lives are
not stress-free) and a good supply of UV - no cloudy days.
Now, I haven't and won't publish this because:
a) I originally expected to have them for a week or two.
b) This is a someone else's conservation project, not my
c) I had no idea their growth would be so dramatic (I have since
learned that others have had similar results).
d) I've got my own research to do..
If I'm ever asked to do this again, and have the time, I shall make the
measurements, do the stats and publish.
>Turning to Canning's comments on aerobically capable yet inertially
>homethermic ectothermic dinosaurs. First, even small dinosaurs much too
>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
>cruises at a speed much above 1 km/h, any well documented contrary info
The available data shows that lizard walking speeds are well within, not at
the maximum of, their aerobic capacity (there are exceptions); I think we
would agree that dinosaurs are locomotory specialists with their erect
stance and stride, but more importantly see my general point about the
comparison between the two dinosaur metabolism models below.
>On growth. Canning suggests that inertially homeothermic dinosaurs could
>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
>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.
This is true. Under the inertial homeothermy model hatchlings
would have had to behaviourally thermoregulate until they reached
sufficient bulk for a degree of inbuilt thermal stability. From the
monitor data I would say this is contributory at 50kg or so.
I'm afraid I
skipped this last time due to time pressure, and because my thoughts on
this are a bit tortuous and speculative, but here goes:-
The growth rate of modern lizards, snakes (probably crocs) is not directly
restricted by their thermal metabolism, but more by their behaviour; they
could be more active, they could eat more, they could grow faster if they
'wanted' to. Natural selection has not favoured such behaviour patterns.
Squamate reptiles commonly live about 20 years, giving 16 or 17 breeding
seasons (assumming annual breeding), and indeterminate growth and a
correlation of fecundity with size makes the later years more productive
than the early years.
By going all out for maximum growth rate the best they could achieve is 1,
maybe 2, extra breeding seasons, and not particularly productive ones at
that. However, the increased risk of predation due to maximal activity
means that, on average, their lifetime reproductive potential is reduced,
not increased, by fast growth. They do exhibit a lot of variation in
growth rates however; if it was advantageous, natural selection could
increase growth rate as the genetic variation is there to allow this.
Dinosaur ecology is very different from lizard ecology. They have a lot
more growing to do from egg to adult in any case, and selection must have
applied in the reverse direction, selecting for the fastest growth rates,
for the inertial homeotherm model to be credible.
To give a real example, I'm afraid I can't think of anything better than
theraphosid ('tarantula') spiders. They all start off about the same size,
they all end up about the same size, but tree dwellers commonly go from egg
to adult in 9-12 months whilst burrowers take 6-10 years to do the same.
This correlates with habitat, not taxonomy, and unlike lizards seems
genetically fixed - exactly the same pattern occurs in captivity. You can
take a tree-dweller and a ground dweller, feed them the same quantities of
food and it still happens. Either their metabolism is radically different
(which I doubt) or the genes for fast growth rate have been selected for in
To avoid being misunderstood, I am NOT trying to claim that presumed
dinosaur growth rates are all within the lizard range; I think the
comparison is largely meaningless as lizards are specialists adapted
to a totally different way of life than dinosaurs.
Finally, I've not answered all of Greg's points because of the length of
his and my mails, and I want to reiterate my more important general point.
The two competing models are i) bird-like metabolism vs. ii) inertial
homeotherm metabolism (not lizard metabolism, not like ANY living
vertebrate. Demonstration that dinosaurs could outperform crocodiles and
monitors in many respects is not proof that were all bird-like, unless you
accept, which I obviously don't, that lizards represent the ultimate in
There is also a semantic problem of what constitutes an endotherm (greater
than 50% body heat from internal sources? Does even this necessarily imply
a bird-type basal metablic rate at sauropod size?).
I think the differences in dinosaur behaviour predicted by the two models
are frequently exagerated. There are only 2
important ones that I can identify with any confidence. Endotherms would
have had greater stamina at maximum speed, and a greater food & water
intake. This last factor would have had profound ecological implications
with respect to carrying capacity, etc. Other factors which are frequently
quoted such as body temperature, parental behaviour, nesting, brooding,
social behaviour, and walking & running speeds are compatible with either
model; the dinosaur as lizard model has been untenable for decades.
There is nothing in the inertial homeotherm model which implies that no
dinosaurs were endotherms; I wouldn't be at all surprised if some,
particularly smaller theropods were, plus maybe polar forms together
with pterosaurs, icthyosaurs maybe. I haven't even touched on why I'm
sceptical of endothermy for larger species but I've gone on too long