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Re: sauropods: homotherm,heterotherm or gigantotherm?



--- Mike Taylor <mike@miketaylor.org.uk> wrote:

> Well.  If a sauropod was endothermic, it could
> gather food much more
> quickly than an ectothermic counterpart could.

++++++++++++++++++++++++++++++

Well, it WOULD gather food much more quickly. It would
have to, since it would use the food up much faster. I
don't know of any evidence to suggest any
physiological increase in food gathering ability,
being coupled with a switch to automatic endothermy.

____________________________________

  More
> important, it
> would have much more efficient digestion (recall
> that in general a ten
> degree difference in temperature doubles the speed
> of all chemical
> reactions).  Would these factors outweigh the
> increased food
> requirements of the endothermic version?  I don't
> know, and neither
> does anyone else, as no-one's ever published any
> numbers on this.

++++++++++++++++++++++++++++++++++++++++++

I think there's a misconception prevalent here. A 10
degree change in temperature will change digestion
speed, NOT digestive efficiency. A reptile that is
digesting a meal at 10 degrees less than the optimum
temperature, will still digest that meal much more
effectively than most mammals (possibly most automatic
endotherms in general). It will just go slower.

A good paper on this is:

Wang, T., Zaar, M., Arvedsen, S., Vedel-Smith, C.,
Overgaard, J. 2003 "Effects of Temperature on the
Metabolic Response to Feeding in _Python molurus_."
Comparative Biochemistry and Physiology Part A. vol.
133. pgs: 519-527.

The authors suggest that the selection for higher
temperatures during digestion, in reptiles, has more
to do with decreasing the time it takes, than it does
the actual amount absorbed.

Couple that with the fact (mentioned previously by
another poster) that most reptiles (especially large
ones) are rather good at keeping themselves in a near
(or complete) homeothermic state, and it would seem
that the reptiles hold the competive edge on
digestion.

I think that this is a point that is often forgotten
when comparing mammals to reptiles. In general,
reptiles seem to be far better at digestion than
mammals are (at least for carnivores). The only
possible exceptions to this are ruminants. As such,
they would obviously require less food than a similar
sized mammal.

Unfortunately all of my data for comparisons, are
between carnivores. Still, it is worth noting.

Large cats & wild dogs all consume b/w 30-50% of their
prey. (Auffenberg, 1981)

Crocodylians & Varanids (oras in particular) consume
90% of their prey. (Auffenberg again)

Most snakes consume 100% of their prey. 

Of this, 80-90% gets transformed into useful energy
(Molnar, 2004, Wang et al, above). 

The only person I know of, who actually took this into
account when comparing mammals to reptiles, was Walter
Auffenberg. By doing so, he was one of the first
people to show data that suggested an apparent
convergence of automatic endotherm metabolisms, with
bradymetabolic metabolisms with increasing size (going
from a 10 fold difference in an animal the size of
_V.rosenbergi_ to a 5 fold difference in
_V.komodoensis_). 

If dinosaurs had digestive systems more in line with
other reptiles, than mammals (at least. I'm still not
sure about the birds), then we would expect them to
require less food than a similar sized mammal. Judging
by the relative efficiency suggested by the
_Tyrannosaurus_ coprolite, it would appear to be that
way. 

___________________________________

> However, Greg Paul's 1998 paper _Terramegathermy and
> Cope's Rule in
> the land of titans_ (Modern Geology 23:179-217)
> makes a strong case
> from simple observation of extant and fossil
> critters that _only_
> endotherms can get big: apart from dinosaurs, whose
> metabolic strategy
> is still controversial, the only big terrestial
> animals (defined as
> "more than a tonne in mass") have been mammals,
> hence endothermic.
> The biggest tortoises and lizards (e.g. _Geochelone_
> and _Megalania_)
> may have approached a tonne, but we have no evidence
> that the
> significantly exceeded it.  In contrast, we have
> extant elephants
> around ten tonnes, extinct ones that were probably
> closer to fifteen
> tonnes, and twenty-tonne extinct rhinos.  Not to
> mention oddities like
> _Megatherium_, the giant ground sloth.  Paul's
> argument boils down to
> "If ectotherms can grow larger than endotherms for
> metabolic reasons,
> why don't they?" which I find pretty darned
> convincing.

+++++++++++++++++++++++++++++++++++

The biggest problem with that argument, is that all
the examples we have of "ectotherms" growing to such
large sizes, are fossil examples. Furthermore it
always seems that these fossil examples happen to then
become the subjects of questions on metabolism. 

For instance, rauisuchians reached _Allosaurus_ sized
levels. In the water we had dozens of large mosasaurs,
ichthyosaurs and plesiosaurs. We also had all the many
(seem to be coming out of the woodwork lately)
terrestrial crocodylomorphs that were, at least, on
par with the largest terrestrial cats today. Yet, all
of these critters have, at one point in time, been
considered to be automatic endotherms.

So it becomes a catch 22. One asks for evidence of
large growing bradymetabolic critters. Then, when the
evidence is presented, the question becomes: "well,
how do we know they weren't automatic endotherms too?"

I suppose one could ask why we don't have any extant
multitonne reptiles, but then one could ask the same
thing about all those "hyper-endothermic" birds people
are so fond of.

Also, in regard to the multitonne argument. I'd
suggest checking out these old threads from the
archives.

http://dml.cmnh.org/1995Nov/msg00367.html

and then later with:

http://dml.cmnh.org/1995Dec/msg00412.html

It features and exchange between GSP and former poster
Paul Willis, on the validity of stating that
_Megalania_ only weighed one tonne. 

Willis brings forth a variety of reasons why such a
statement should not be stated so matter-of-factly
(e.g. the fact that all we have are "SLPs" to go on).

He also goes on to report on newer material than GSP
had, which significantly upped the weight of
_Megalania_. 

Some of that material has been recently published, and
can be read about in Ralph Molnar's recent book on the
subject (Dragons in the Dust: The Paleobiology of the
Giant Monitor Lizard _Megalania_). He calculated that
the largest material suggested a 2.1 tonne (1.9 metric
tonne) animal. Furthermore, Molnar stated (pg. 127)
that: "The vertebrae in the collection suggest that
lizards this size weren't unusual, although it is only
to be expected that bias in the preservation would
favor the larger specimens." 

__________________________________________

> As if that weren't enough there is the histological
> evidence of Martin
> Sander's 2000 paper _Longbone histology of the
> Tendaguru sauropods:
> implications for growth and biology_ (Paleobiology
> 26:466-488), which
> seems to show pretty conclusively that sauropods
> grew _fast_.  In
> particular, one well-preserved femur of _Janenschia_
> (femur Ja2) was
> determined to have attained "sexual maturity" (i.e.
> a levelling off of
> growth) in 11 years, and maximum size around 26
> years.  Table 1 (on
> p. 468) shows that this femur was 127cm long, which
> is indicative of a
> mass around 20-30 tonnes.  So this thing was
> averaging a tonne of
> growth per year, or nearly 3 kg per day.  Show me an
> ectotherm that
> can do that.

+++++++++++++++++++++++++++++++++++++++

I do believe that falls in line with estimated growth
rates for the giant squid (_Architeuthis dux_), but
I'm not sure. Doesn't matter though, because I have a
better one. 

Young altricial birds grow at twice the rate of a
proportional sized sauropod (Erickson, et al 2001).
Yet this is also the time of their lives, when these
young birds are known to be ectothermic (see Chinsamy
& Hillenius [2004] for a substantial list of
references backing this up). 

It would appear that food availability, and nutrient
partitioning (rather than general metabolic rate) are
the prime factors in fast growth.

Jason


Refs:

Auffenberg, W. 1981. The Behavioral Ecology of the
Komodo Monitor. Gainesville University Press.

Chinsamy, A., Hillenius, W.J. 2004 "Physiology of
Nonavian Dinosaurs" The Dinosauria Second Edition.
Univ. of Cal. Press. pg: 643-659 (in particular: 658).

Erickson, G.M., Rogers, K., Yerby, S.A. 2001.
"Dinosaurian growth patterns and rapdi avian growth
rates" Nature. Vol. 412 pgs: 429-432

Molnar, R. 2004. Dragons in the Dust: The Paleobiology
of the Giant Monitor Lizard _Megalania_. Indiana
University Press.

Wang, T., Zaar, M., Arvedsen, S., Vedel-Smith, C.,
Overgaard, J. 2003 "Effects of Temperature on the
Metabolic Response to Feeding in _Python molurus_."
Comparative Biochemistry and Physiology Part A. vol.
133. pgs: 519-527.

"I am impressed by the fact that we know less about many modern [reptile] types 
than we do of many fossil groups." - Alfred S. Romer

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