Re: [erect crocodile problems]

[archosaur@usa.net]

Dave writes:
> The man asked to be convinced, so here goes.
> 
> When a physiologist measures the resting metabolic rate of a crocodile,
> (s)he does not make it stand up to do so.  Standing up is not considered a
> resting position for a reptile.  A crocodile on its feet consumes
> considerably more energy than a resting crocodile.  Although it can
> maintain such a position for a long time at warmer temperatures, it soundly
> defeats the greatest advantage of ectothermy in the process, namely energy
> efficiency.
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A couple things.

One: I'd be interested in knowing where I could find this informatiosn. Can you 
cite any works on it?

And two: Just because standing up is not generally considered 
a usual postition for a reptile, doesn't mean it isn't. Many
lizards (pet iguanas for example) will rest in a postion not unlike 
a dog sitting down. 

Plus, as you stated below, top predators (terrestrial ones anyway)spend most of 
their time resting. Crocs are top predators. So it's makes sense for them to 
conk out most of the time. 
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Dave:

> The only reason a terrestrial vertebrate can take advantage of the energy
> efficiency of ectothermy is because it can flop down on its belly most of
> the time.  Greg Paul has said previously on this mailing list that a fully
> erect posture probably forces elevated aerobic exercise capacity.  I would
> take out the probably.  All vertebrates with fully erect postures, large
> and small, have high performance cardiovascular systems, high surface-area
> respiratory systems, and physiological core temperature regulation.
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Now this is a problem that has bothered me for a long time.

Other than, the forelegs, how is a crocodile not standing erect?

>From the alligator and caiman skeletons I've seen, I don't see 
any reason for them not to be able to stand erect (hind legs)

Documentaries show it. Even the living animals that I've seen
show it. 

I just don't understand where this semi-erect bit comes from.

If it is the forelegs, then are ceratopsians and stegosaurs
considered semi-erect. 

Besides that, crocodiles have large lungs and a muscular diaphram. 
What is missing from this picture to say that they can't have
an erect stance? 
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Dave:

> The problem becomes even worse if we give the crocodile an enormously long
> neck.  Its low-performance cardiovascular system will be incapable of
> pumping blood to its head.  It is striking and rather hilarious to extract
> blood from the tail of a snake or lizard and see what a difference it makes
> when you elevate the heart. 
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I don't think I even want to know about that snake lizard
experiment. 

At any rate, crocodilians are different from all other
reptiles in that they have a four chambered heart. Just
like birds and mammals. 

Although they are not crocodiles, how does one explain 
Tanystropheus and all those other prehistoric reptiles with 
enormous next compared to body size. 

Even today there are chelonians with very long necks. 

Back to crocs.
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Dave:

> Sophisticated stay mechanisms, columnar limb structure, and other
> adaptations of large mammals have never enabled them to escape the need for
> endothermic metabolisms.  There is only so much energy savings to be had
> without producing such a rigid structure that the animal can barely move.
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Is it even worth mentioning poikilothermic, bats, mole
rats, and hummingbirds? 

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Dave:

> Yet in the final analysis, it does not matter how many hours a day you
> spend on your feet.  Big cats lay around most of their lives, yet have
> never evolved ectothermy.  This is because a fully erect posture forces a
> walking speed beyond that which can be sustained by ectothermy.  Abundant
> trackway evidence tells us that dinosaurs routinely walked at speeds of
> 5-10 kph.  Yet a green iguana will become exhausted within 17 min at a
> speed of <0.5 kph.
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Can I see the proof for that one too? I'd be interested
in reading it. 

Still, if you were going to use a lizard comparison how about 
a bearded dragon, hydrosaur, or collared lizard? At least
they are on the ground. 

Komodo monitors (a favorite example on this list) make daily forages from 1-5 
miles a day. They cover alot of ground. 

Crocodilians traverse vast distances in order to get from
one water hole to the next.

The best examples seem to be C.palustris, the marsh crocs in india
who have been reported to make 15 mile treks from one water
hole to the next.

Then theres Paleosuchus, which seem to have once again started the 
transition from water dweller to land dweller in crocodilians.
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Dave:

> The semantic morass that has been created in the field of bioenergetics in
> my view serves only to confuse scientists and laymen alike, and cloud what
> is really a reasonably clear-cut issue.  Many ectotherms have relatively
> high thermal optima, manage to keep their body temperature within fairly
> narrow limits during active periods, and have higher metabolic rates than a
> given endotherm.  None of this should distract us from the fact that an
> ectotherm's metabolic rate decreases with decreasing temperature, while an
> endotherm is just the opposite.  None of this should distract us from the
> fact that we can plot the resting metabolic rates of terrestrial
> vertebrates against their body sizes and produce two very distinct clusters
> of data, which do not overlap.  Birds and mammals form one cluster,
> ectotherms another.  
>  
> Selection for energy efficiency might indeed be expected to push towards
> ectothermy.  The problem is that endothermy is such a huge physiological,
> molecular, and morphological commitment  that it is very difficult to
> reverse.  There is no evidence that this has ever happened on this planet,
> despite a few million years of opportunity.  Selection for energy
> efficiency in hummingbirds and bats is so strong that it has resulted in
> the evolution of torpor states.  But NOT ectothermy.
> 
> Best regards,
> 
> Dave

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You're right. Hummingbirds and bats are not ectothermic at
all. But they are poikilothermic. Their body temperature
drops with the environment. 

You stated that many living ectotherms (seem to only use herps in your
examples, but nonetheless) keep body temperatures higher
than most endotherms. These temperatures are also higher
than their environment too.

A white pointer and mako shark are both endothermic
when moving, but poikilothermic and tachymetabolic 
at a more rested state. 

Same for leatherback sea turtles, moths, bees, other
insects, crocodilians, komodo monitors and so on.

Plus crocodylatarsians have spawned numerous "un-crocodile like"
animals, like Gracilosuchus and Pristichampsids. 

I'm not stating that they evolved endothermy and then lost
it. I'm just saying that they never needed it.


Archosaur J

More fuel for the fire huh?    


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