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Re: [reopening the can]



Dave wrote:

> I have been quiet for a while now on the bioenergetics issue, but with
> Dann's post on ectothermy/endothermy and Archaeopteryx it seems like an
> opportune moment to make a few remarks.  Dann has his facts straight as far
> as I can tell, but the specific examples cited may tend to mislead.
> 
> The basic difference between an ectotherm and an endotherm is
> physiologically quite a profound one, a fact not always appreciated by
> those who discuss the subject.  It is true that the metabolic rates of
> endotherms are generally much higher than those of ectotherms.  But resting
> metabolic rate varies greatly with body size, and this must always be
> factored in when discussing same.
> 
> The critical difference is that when the temperature drops, an ectotherm's
> metabolic rate decreases.  An ectotherm's core temperature can fluctuate
> tremendously without doing it harm because ectotherm proteins are designed
> to withstand these fluctuations without denaturing.  An endotherm is in a
> sense just the opposite.  When its thermostatic mechanism detects a
> temperature drop, its metabolic rate INCREASES.  Its core temperature stays
> relatively constant, as it must.  Otherwise its proteins, which are highly
> temperature sensitive, will denature and it will be very dead.  
> 
> Of course there are animals that have "hybrid" systems (although not that
> many), and there are variations on the two basic themes.  But the
> commitment to endothermy is a major one, necessitating the abandonment of
> the highly energy-efficient ectothermic life style.  This is probably why
> no crocodilian, lizard, or turtle that we know of has ever made the switch,
> despite a few million years of opportunity.  Similarly, no endotherm that
> we know of has ever switched back to ectothermy.  
============================================================================================================================================
I write:

What about that whole ectothermic naked mole rats thing? Was that ever settled?
 
======================================================================
Back to Dave:

> Generally, endotherms have much higher resting metabolic rates than
> ectotherms of similar size.  This is because endotherm body temperatures
> tend to be much higher than the surrounding environment, and maintaining
> the thermal gradient requires a lot of energy.  I suspect that the reason
> endotherms have such high body temperatures is probably related to the
> function of their proteins, although I cannot claim to have seen any
> literature on the subject.
> 
> Any time we see evidence of specialized insulation, we should be instantly
> suspicious.  No living ectotherm has blubber.  No living ectotherm has
> feathers.  No living ectotherm has fur.  Why?  Because cover a snake with a
> blankie and you have not increased its body temperature one iota.  You
> can't conserve heat that isn't there.  

============================================================================================================================================
Now I come in with:
 
Actually Leatherback sea turtles have many layers of fat and blubber. They have 
it so they can deal with the extreme cold of the Arctic, which they go up to to 
catch the jellyfish they love so much.

White Pointers (Great Whites) also have some blubbery layers to deal with the 
cold waters that contain their preffered meal of elephant seals.

Also many pythons are known to shiver on their eggs to increase the 
temperature. Bees shiver their wings to warm and cool off hives.

There is some degree of internal heating their.
======================================================================
Dave continues:

> Archaeopteryx was an endotherm.  This hypothesis is one that I am quite
> confident of, for all kinds of reasons.  Rather than launch into a long
> list of them, I will simply point out that all living ectothermic tetrapods
> have sprawling or partially erect postures.  Far from developing a fully
> erect posture, many such animals have evolved smaller legs, even to the
> point of leglessness.  Even fast, relatively active ectotherms, such as
> whiptail lizards, have relatively short legs that sprawl to the side.  Why?
>  Because ectothermy by its very nature involves long periods of immobility.
>  The most common question we get from people in the zoo about reptiles is
> "Are they alive?  Why don't they move?"  Ectotherms are energy conservers.
> Why evolve an erect posture if you're going to spend most of your time
> sitting on your bum?  If you have a sprawling posture and break a leg bone,
> no big deal.  You can still walk and even run.  But if you have an erect
> posture and break a leg, you're in trouble.
> 

===========================================================================================================================================

I follow up with:

Explain semi-erect. I've heard this term used alot with crocodilian gates but 
can't figure out what they are talking about. 

When I look at a crocodilian standing and walking, I see it's back legs fully 
upright and underneath the body. The only time they are not is when the animal 
is sliding into the water or floating lazily along a riverbank. Are they 
refering to the fore legs. That I can understand since they are only semi-erect 
in that half pushup type stance, but not the rear legs.

As for ectotherms developing shorter and shorter legs, first your just citing 
reptiles on this. Frogs have very large back legs used for hopping considerable 
distances. With lizards you have two different types of leg development. The 
short legs that belong to burrowing animals and the long legs that belong to 
arboreal and especially semi aquatic animals. 

Far from sitting on their bums all day, these lizards not only run but they are 
bipedal when doing it. In the case of Basilisks and frilled dragons they only 
go down on all fours when eating or in trees.

Now I'm not sure what you meant by that busted leg bit. First off even lizards 
with a sprawling gate don't drag their bellies. Some of the smaller species do, 
but they are the ones that evolved the short legs. A collared lizard with a 
busted leg is indeed in some trouble. They rely on those legs for fast getaways 
and for helping cool down.
============================================================================================================================================

Back to Dave:

It may well be asked, how will endothermy ever evolve from ectothermy?  I
> believe it does require an unusual set of circumstances, which is why it
> has happened only two or perhaps three times to my knowledge in the whole
> history of evolution, all of which occurred at more or less the same time,
> the Triassic.  What was so special about the Triassic?  I don't know, but I
> suspect that it has something to do with four-chambered hearts and stamina.
>  Something must push the animal to develop a partially erect posture, which
> preadapts it to some degree to endothermy.  I think crocodilians might have
> evolved endothermy in time, except that they became aquatic.  It is
> interesting that most have maintained a partially erect posture in spite of
> their aquatic habits.
> 
> Best regards,
> 
> Dave
===========================================================================================================================================
And now I'll just finish up with:

Extant crocodilians adapted to the water, but many of the rest went on to 
totally terrestrial lifestyles. Some became herbivores, others may have been 
tree climbers. Even the little croc _Gracilosuchus_(sp?) became a bipedal 
animal. Crocodilians and their group the Crocodylatarsia were extremely diverse 
in habits and habitats. 

Now the reason that extant crocs still have the semi-erect?(that still needs 
explaining) stance is because they are not totally aquatic animals. Many like 
_Crocodylus palustris_(the mugger) and _Crocodylus niloticus_ make long treks 
from water hole to water hole. 

The living genus _Paleosuchus_(dwarf caimans) have become increasingly more 
terrestrial with _Paleosuchus palpebrosis_ spending almost all of it's time out 
of the water.

Not to mention the fact that _Crocodylus johnsoni_(Johnstons's croc) have been 
seen galloping. 

With the exception of _Gavialus gangecticus_(The Gharial) which has relatively 
weak limbs, the crocodilians are far from just aquatic animals.

Now one more thing, a rant really:

One thing that always got on my nerves when watching documentaries on large 
reptiles like crocs and komodos was when the narrator would say that "Due to 
their cold-bloodedness, these animals spend most of the day sitting around and 
moving in and out of the water" 

The main thing that is wrong here is that they are forgetting that both the 
Komodo dragon and the crocodilians are top predators and a top predator is the 
most lazy animal you'll meet. 

Lions and cheetahs. Wolves and Wild dogs. All these are endothermic animals 
that are top predators. And all these animals spend omost of their lives 
asleep. Just moving from sun to shade and relaxing the day away. Only when they 
are hungry do they ever do any serious movement. 

That makes sense. Whether your a croc taking out a zebra or a hyena chasing a 
Impala, catching large prey is a tiring task, so it makes more sense to 
conserve your energy until you need it, and since your a top predator you don't 
have to worry too much about getting preyed upon by another animal. Ectothermy 
doesn't make crocs lazxy. being top predators is what does that.

Now for real comparisons in the category of ecto endo we should look at 
_Anolis_ and _Geckonidae_ . All the little lizards and other small reptiles 
that spend their time catching prey and avoiding predators. These animals are 
much more active than a lion or Hyena. 

The emerald tree monitor eats five to six times a day and needs to be fed 
daily. Animals like these make me doubt the "limits" of an ectothermic 
physiology.

Indeed if Endothermy was so much greater than ectothermy, than mammals wouldn't 
be the least numerous animals about. 

The way I look at it, ectothermy just means controlling body temperature from 
the outside, while endothermy means controlling temperature from the inside. 
Two different solutions to the same problem.

Archosaur J        


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