Morphology question

[GeoTerraTestudo]

Why are the carapaces of radiateds and ploughshares so domed? I realize the carapaces of burrowing tortoises are rather compressed, but these two species have shells that are even higher than those of other, non-burrowing species? What do they need them for?

 

[Madkins007]

I believe I saw a comment somewhere in some article that the high dome made it easier for them to unflip, a useful element considering their home terrain.

 

[tortadise]

Yep. Also the breeding of the ploughshare and sometimes radiated the males will actually flip the female over with his large gular. The studies and observations of them in the wild indicate that larger gular scuted males tend to win the dominance battle over a female and continue to "fight" and go flip the female over and breed with her. Over the span of time I suppose the radiated evolved from the plough share with a smaller gular but still tries to "flip" the female over. But most captive kept Radiata tend to just use typical breeding style since they portray a "safe" less hostile environment or challenges that the wild Madagascar possess rather than an outdoor hand fed well cared for captive.

My thoughts on it. But I did read some observations in an article about that a while ago so it may be incorrect. Sounds reasonable to me.

 

[Tom]

I saw your question earlier, and I've been thinking about it. Don't have anything scientific, but I wonder if there is a thermal advantage. Heat inertia having to do with the volume to mass ratio. Or a greater mass with less surface area. Think pancake vs. radiated.

Whatever the answer, good question.

 

[tortadise]

quote='Tom' pid='433285' dateline='1333990067']
I saw your question earlier, and I've been thinking about it. Don't have anything scientific, but I wonder if there is a thermal advantage. Heat inertia having to do with the volume to mass ratio. Or a greater mass with less surface area. Think pancake vs. radiated.

Whatever the answer, good question.
[/quote]

That's a good thought too. It does get quite want where they are in the wild. But look at the parrot beaks and cape tortoises, as well as the geometric and tent tortoise. They are in super hot environments and don't portray the doming. Maybe we should set up an experimental expedition for a Madagascar and south Africa trip to put in a groups observation on this question.

 

[Yvonne G]

Maybe we should set up an experimental expedition for a Madagascar and south Africa trip to put in a groups observation on this question.

I'm in!

 

[GeoTerraTestudo]

Great responses! I have wondered about why chelonians that live on land in general tend to have more domed shells than their aquatic cousins. Even box turtles, which are a terrestrial pond turtle, have more domed shells than aquatic turtles.

Maybe part of the answer is that, in the water, it pays to have a flatter shell so that you can be more hydrodynamic. The exception to this would be map turtles, which have spine-like protrusions on their carapace. Even that is probably for swimming ability, though, since it probably functions like a dorsal fin, stabilizing the animal in fast-flowing rivers.

But that still doesn't answer why it pays to have a domed shell. Even other shelled animals, like snails, rolly-pollies, and armadillos, all have rounded shells. Why is that? Maybe because it provides for more room inside the body for the organs? I think there might be something else, though. Maybe the dome helps these animals resist attacks from predators? After all, that's why eggs and arches are rounded: to support weight on top of them. So maybe that's why both boxies and torties have domed shells (except pancake tortoises, which have to fit into tight crevices): to support weight on top of them.

But when you think about Madagascan tortoises, there are no large predators there. Maybe the extremely domed shells of babies help them resist the biting pressure of carnivores like the fossa? Also, what if it serves a display function? These tortoises, like most, have male-male combat. Maybe having a big, tall dome makes you look more intimidating to your rival? Like the spread fins and gills of a fish, the arched back and raised hair of a cat, or the antlers of a deer? I think the explanations for a higher dome that involve flipping and/or temperature make more sense. Here is a video of wild ploughshare tortoises fighting in Madagascar. The high dome of the shell does appear to prevent the loser from being flipped over completely onto his back:

[video=youtube]http://www.youtube.com/watch?v=nJvd_JL25Fc[/video]

 

[ALDABRAMAN]

Great responses! I have wondered about why chelonians that live on land in general tend to have more domed shells than their aquatic cousins. Even box turtles, which are a terrestrial pond turtle, have more domed shells than aquatic turtles.

Maybe part of the answer is that, in the water, it pays to have a flatter shell so that you can be more hydrodynamic. The exception to this would be map turtles, which have spine-like protrusions on their carapace. Even that is probably for swimming ability, though, since it probably functions like a dorsal fin, stabilizing the animal in fast-flowing rivers.

But that still doesn't answer why it pays to have a domed shell. Even other shelled animals, like snails, rolly-pollies, and armadillos, all have rounded shells. Why is that? Maybe because it provides for more room inside the body for the organs? I think there might be something else, though. Maybe the dome helps these animals resist attacks from predators? After all, that's why eggs and arches are rounded: to support weight on top of them. So maybe that's why both boxies and torties have domed shells (except pancake tortoises, which have to fit into tight crevices): to support weight on top of them.

But when you think about Madagascan tortoises, there are no large predators there. Maybe the extremely domed shells of babies help them resist the biting pressure of carnivores like the fossa? Also, what if it serves a display function? These tortoises, like most, have male-male combat. Maybe having a big, tall dome makes you look more intimidating to your rival? Like the spread fins and gills of a fish, the arched back and raised hair of a cat, or the antlers of a deer? I think the explanations for a higher dome that involve flipping and/or temperature make more sense. Here is a video of wild ploughshare tortoises fighting in Madagascar. The high dome of the shell does appear to prevent the loser from being flipped over completely onto his back:

[video=youtube]http://www.youtube.com/watch?v=nJvd_JL25Fc[/video]

Nice video.

 

[tortoises101]

Ploughshares and radiateds both live in areas with very dry ambient temperatures for most of the year. A domed shell provides more surface area and larger surface area:volume ratio reduces moisture loss.

 

[Tom]

Ploughshares and radiateds both live in areas with very dry ambient temperatures for most of the year. A domed shell provides more surface area and larger surface area:volume ratio reduces moisture loss.

Are you saying they lose moisture from their shells? Their skin?

 

[GeoTerraTestudo]

Ploughshares and radiateds both live in areas with very dry ambient temperatures for most of the year. A domed shell provides more surface area and larger surface area:volume ratio reduces moisture loss.

A lot of tortoise species live in hot, dry areas. Why would these two need shells that are more domed than everyone else's? Except for the burrowing species, they would probably all benefit from it. Maybe it's just that this lineage just happened to evolve it, while others by chance happened not to?

 

[HLogic]

Ploughshares and radiateds both live in areas with very dry ambient temperatures for most of the year. A domed shell provides more surface area and larger surface area:volume ratio reduces moisture loss.

The concept above is correct but the statement is not. The more spherical an object the smaller the surface area to volume ratio and thus the less area available for evaporation, heat transfer, etc. with respect to the volume within.

 

[tortadise]

I think it's an evolutionary anomalie. There's multiple species of tortoises that show no high doming that live in very similar habitats. The only difference is the soil from eastern south Africa and north western Madagascar are totally different. Perhaps this plays a part in bone structure. I've never compared x-rays or anatomical differences between a radiated and lets say a natals geometric or padloper. Perhaps the doming plays a role in enlarged hips or leg bones so they can dig better nest, having stronger legs. There's just so much that could go into play here.

Look at sulcatas and leopards, same regions one burrows one doesn't totally different shell structure. I shall investigate this further with science.

 

[tortoises101]

Ploughshares and radiateds both live in areas with very dry ambient temperatures for most of the year. A domed shell provides more surface area and larger surface area:volume ratio reduces moisture loss.

The concept above is correct but the statement is not. The more spherical an object the smaller the surface area to volume ratio and thus the less area available for evaporation, heat transfer, etc. with respect to the volume within.

Oops, spelling error. *facepalm*

 

[tortadise]

It happens. Especially to me ALL the time with an iPhone.

 

[tortoises101]

Ploughshares and radiateds both live in areas with very dry ambient temperatures for most of the year. A domed shell provides more surface area and larger surface area:volume ratio reduces moisture loss.

A lot of tortoise species live in hot, dry areas. Why would these two need shells that are more domed than everyone else's? Except for the burrowing species, they would probably all benefit from it. Maybe it's just that this lineage just happened to evolve it, while others by chance happened not to?

Tortoises living in dry areas which cannot burrow to the extent of gopherus are small species. Psammobates, chacos, padlopers are all small species living in dry areas. Their small size is their big advantage. Their small size allows them to seek out humid microclimates (under vegetation and rocks) to prevent dessication, which larger species sometimes cannot. Some behavioral adaptations have also been recorded in species like angulateds and tent tortoises. An interesting on is sticking their head through moist sand and filtering out the water beneath. The soil type of Madagascar is very dry and sandy in both the spiny thickets and Baly Bay region, and the lack of drinking water there IMO emphasizes the importance of a lower surface area:volume ration.

 

[Radiated]

what is IMO?

 

[GeoTerraTestudo]

what is IMO?

IMO = "In my opinion"