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Author Topic: Do the shells of tortoises have an impact on their ability to heat their bodies?  (Read 6002 times)

Offline greensleeves

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Reptiles are ectothermic (cold-blooded). As such, they rely on external heat to warm their bodies up before they can become active.

Of course all animals need to defend themselves against predators, but there are many different types of defence mechanism which can be employed (speed, burrowing, camouflage, claws, flight etc etc). Why did tortoises of all things - cold-blooded reptiles - evolve a hard shell for defence, which presumably is counter-productive physiologically in that it seemingly drastically reduces the surface area available for heating up their bodies through exposure to the sun's rays? Is there an efficient conduction of heat through the shell? I wouldn't have thought so, in which case all warming would have to occur through just the head and feet.

Maybe that's why tortoises walk so slowly - because they can't heat up their bodies fast enough to get a move on!
« Last Edit: 23/02/2011 15:39:26 by greensleeves »


 

Offline Don_1

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Why did tortoises evolve a shell?

For protection against predators? No. I don't think so. I rather think that the protection the shell offers was a bonus.

Being attacked by a predator would not cause an evolutionary change in the body. Itís a bit like claiming that birds grew wings because they kept falling out of trees. If it were the case, then wouldn't all prey animals have evolved some sort of bite protection? Take a look at the giant tortoises of the Galapagos. These tortoises have no predators, yet they still have their shell. Surely if the shell developed as a means of protection against predators, it would have become vestigial by now, if not disappeared altogether on these tortoises.

Proganochelys quenstedti is probably the earliest fully shelled and beaked tortoise dating to around 210 million years ago. Prior to this only a half shelled, toothed turtle named Odontochelys semistestacea dating to around 220 million years ago is known of. The Scutosaurus, dating to around 250 million years ago, may have been the ancestor of these. They had bony scutes under the skin, so, to my mind, this also discounts the theory that the shell evolved as protection.

Whether the scutes migrated to the surface or the outer skin became surplus to requiremments, who knows? But it would seem reasonable that the legs, which were underneath the Scutosaurus' body, migrated to the side, if the animal spent much of its time in water (which may have been the case, to help support its body) and to allow the evolving shell to develope into the best shape.

So what was the primary purpose of the shell? For me there is a clue in the modern testudines.

Your assumption that the shell is a barrier to heating is quite wrong. In fact the shell will warm quite quickly. Even on a fairly overcast day, the shell can absorb the sun's rays very effectively. The domed shape also gives a good area and even when the sun is quite low in the sky, with just a little tilt, the tortoise can present it's entire shell surface to the sun, thus making the most of the rays at all times. Of course this also works in reverse, so when out of the sun, the tortoise can quickly loose heat. I would argue that this fast absorption and dissipation of heat, makes the tortoise a very effective and fast body heat controller.

This is not the only advantage the shell gives the tortoise. The other great advantage is water retention. The shell makes a good barrier to water loss through evaporation. Tortoises can go very long periods without the need for water. Again, this works both ways. If Scutosaurus did spend much time in water, the scutes under the skin may have helped prevent too much water being absorbed into the skin, making an already heavy animal even heavier. The water retention ability could have been a bonus in that it allowed Scutosaurus to go for long periods without the need for water, where suitable drinking water may have been scarce.

To conclude, I think the primary object of the shell was heat collection with water retention as a secondary purpose and protection as a bonus, which some tortoises later evolved further so they could retract their head, legs and tail into the shell and the Hingebacks going as far as to be able to close the shell for almost total protection against predators.

It could well be that the tortoises efficient heat exchanger/water jacket gave them the edge in surviving the great extinction of the Permian and the Cretaceous-Tertiary extinction and stood them in good stead to survive until today.

As for tortoises being slow, well when your quite well armoured, you donít need to move too fast when a predator comes around. But then the Gopher can get up quite a speed when it needs to scurry down its burrow when brush fire sweeps across the land and marine turtles are pretty nifty too.
 

Offline greensleeves

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A well considered reply Don! You obviously know something about tortoise shells! I'm very aware that many anatomical features today do serve different or additional purposes to those for which they originally evolved - the best known example being feathers - so yes, I can see how the shell may not have evolved primarily as a defensive coat of armour, even though that may seem like its most obvious function today.

Don is it conjecture or proven that the shell does allow efficient transfer of heat to the body? As efficient as bare, scaly skin? If you're right, then that would certainly remove the main drawback I suggested to the existence of a shell in a cold-blooded creature, and I agree in those circumstances the domed shape would greatly increase the surface area available for heat transfer.

But you say that tortoises, or their ancestors, evolved round about the time of the Permian-Triassic boundary and during the Triassic Period. Given that these were among the hottest periods in Earth's history, I wonder if the other quality you mention - that of water retention - may not have been more important in the original development of the shell?
« Last Edit: 23/02/2011 20:48:23 by greensleeves »
 

Offline Don_1

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Sorry, I've been a tad on the busy side of late and only had the time to drop in for a few moments now and then, but I will endeavour to respond ASAP.
 

Offline Don_1

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Goodness gracious! I never did get around to answering this. How time flies. Still, better late than never.

Having a couple of tortoises, I can tell you that they can warm up pretty quick. Faster than scaly skin? Perhaps not in the case of flat scaly skin, but where the scales protrude, throwing some shade on the adjacent scales and the skin underneath, it may be so. Even though their early evolution took place during a particularly warm period in Earth's history, what you must consider, is that reptiles cannot absorb ambient heat. So even on a hot day they cannot warm without the Sun's rays. Since extreme heat leads to much evaporation, clouding would have trapped heat but limited those all important Sun's rays. Being able to warm quickly, before cloud reduced the effect of the Sun's rays, would be quite an advantage. Tortoises can still warm in quite heavy hazey conditions.

Could the primary cause of developing a shell have been for water retention? Yes, I suppose that might be possible. But again I would look to the modern testudines and ask, if that were the case, why did the freshwater aquatic chelonian species, such as the modern Red-Eared Slider Trachemys scripta elegans and it's ancestors retain their shell's after taking to water?

In further support of my theory that the shell evolved primarily as a aid to heat capture, I look to another modern testudine, the Red Footed Tortoise Chelonoidis carbonaria, which inhabits the forests of Sth America. Being a ground dwelling reptile in a forest requires a good ability to absorb the Sun's rays quickly and efficiently as and when they break through the forest canopy. The retention of the shell would seem to be a good plan for the Red Foot, but since they inhabit both dry forest and rain forest, if water retention were the main objective, those in the rain forests might have lost their shell and become a different species.
 

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