Discussion question: grassland tortoises

[GeoTerraTestudo]

I have been pondering something, and I would love to hear your ideas on this. As we all know, there are many arid and semi-arid tortoise species out there, occupying desert, scrub, and grassland habitats. Africa has the sulcata and the leopard tortoise; Eurasia has the Testudo species; and North America has the Gopherus species. These animals all have fascinating adaptations, but what really impresses me is that the Russian tortoise (T. horsfieldi) ranges farther north into colder climates than any other tortoise in the world. Here is a passage from Lagarde et al. (2001):

"...annual activity (3.5 months) is strongly constrained by the harsh environment ... The Uzbek continental climate limits the annual activity period of T. horsfieldi to only 3 months. Within this 3-month period, ambient temperatures fluctuate strongly each day (from –10 to +45°C), further limiting activity to a total of 360 h per year. Consequently, steppe tortoises can exploit trophic resources only during 16% of the full food-availability period. In our population, steppe tortoises are clearly more constrained by unfavourable environmental temperatures than by food availability."

This is particularly amazing when you consider that Mediterranean tortoises (the other four Testudo species) require about 1000 hours of sunshine per year; that's about 3x as much time for activity than what Russian tortoises get! Nevertheless, this tortoise species is well suited to the steppes of Central Asia. So here's my question: if one of the Testudo species was able to adapt to the Central Asian steppes, then why weren't any of the Gopherus species able to do the same in North America's steppes (also known as prairie, or the Great Plains)? There is a box turtle that inhabits the Great Plains (the ornate box turtle), but no gopher tortoises. Isn't that strange? I have some thoughts on why that might be, but I'm really curious to hear what you think. Why are there no northern tortoises in North America, the way there are in Eurasia?

Lagarde, F., X. Bonnet, B.T. Henen, J. Corbin, K.A. Nagy, and G. Naulleau. 2001. Sexual size dimorphism in steppe tortoises (Testudo horsfieldi): growth, maturity, and individual variation. Can. J. Zool. 79: 1433–1441.

 

[Terry Allan Hall]

Interesting question...

 

[ascott]

Pondering here...will get back to you....hmmmmmm?

 

[EricIvins]

I only have experience with Florida Gopher Tortoises, but I can tell you one thing........They are very soil type specific.......Only one soil type really........

The problem is, that soil type is very condusive for Human consumption.......

 

[ascott]

I believe EricIvins has hit that nail bang on....we have the EXACT epidemic taking over the Desert Tortoise here in California......and in greedy portions.

 

[GeoTerraTestudo]

I agree with the above comments, but I have some more thoughts on the subject, too.

To my knowledge, the ancestor of today's Gopherus tortoises came from the northern Great Plains ... however, this was some 2 million years ago when the climate was milder, so this still does not explain why Central Asia has a northern grassland tortoise, while North America does not.

Here's my hypothesis. According to Lockwood et al. (1994), rainfall in the Great Plains is heavier during the spring, but in Central Asia it is heavier during mid-summer. This, in turn, leads to a greater biomass of insects (grasshoppers) in North America. Maybe having more insect prey and less vegetation is the reason why the Great Plains are home to Terrapene box turtles instead of Gopherus tortoises.

Lockwood, J.A., L. Hong-Chang, J.L. Dodd, and S.W. Williams. 1994. Comparison of Grasshopper (Orthoptera: Acrididae) Ecology on the Grasslands of the Asian Steppe in Inner Mongolia and the Great Plains of North America. Journal of Orthoptera Research, No. 2: 4-14.
http://www.jstor.org/stable/3503601

 

[Terry Allan Hall]

I agree with the above comments, but I have some more thoughts on the subject, too.

To my knowledge, the ancestor of today's Gopherus tortoises came from the northern Great Plains ... however, this was some 2 million years ago when the climate was milder, so this still does not explain why Central Asia has a northern grassland tortoise, while North America does not.

Here's my hypothesis. According to Lockwood et al. (1994), rainfall in the Great Plains is heavier during the spring, but in Central Asia it is heavier during mid-summer. This, in turn, leads to a greater biomass of insects (grasshoppers) in North America. Maybe having more insect prey and less vegetation is the reason why the Great Plains are home to Terrapene box turtles instead of Gopherus tortoises.

Lockwood, J.A., L. Hong-Chang, J.L. Dodd, and S.W. Williams. 1994. Comparison of Grasshopper (Orthoptera: Acrididae) Ecology on the Grasslands of the Asian Steppe in Inner Mongolia and the Great Plains of North America. Journal of Orthoptera Research, No. 2: 4-14.
http://www.jstor.org/stable/3503601

Makes sense.

 

[ascott]

Hi Geo...so here is my theory
The gopherus here in the states have very basic requirements for survival...very basic...they need shelter for the harsh heat/cold...they require occasional rainfall...they require source of food hence the rain provides for the little moisture required for that food to grow.....they require each other for well, you know here is where I believe there in falls the evolutionary downfall if you will...if there is a disruption in their "normal" food type it can take a tortoise several months for their body to develop a new food into viable nutrition...therefore if there is huge amount of rainfall each and every year they will constantly be encountering new food supply as well as fighting the increased amount of wetness in the earth and that coupled with extreme cold temps could lead to respiratory issues,etc (and well no one there to give them antibiotic injections) also they mature sexually at around 15-20 years of age and with their high hatchling failure rate the two alone can be devastating alone...but wait I have more..also then add in us humans and our love for open desert space to off road...to build...to use for commercially cheap property (in some cases free) so their habitat is further desturbed...plant loss hence their need to eat other not normal plants to survive which takes us back to it takes their system so long to make new food viable to their nutritional needs....and with heavier rainfall the insect population increases which starts the predator food chain to increase in those areas as well....
The central asia tortoise has alot of the same obstacles...except they are smaller...they have evolved to thrive out of the burrow for much shorter time...their diet also includes carrion and they also consume their own as well as other creatures "poop" and will consume bone particals along with their grazing....they mature sexually at about 5-7 years of age which is much shorter than gopherus...

I have to say once again that these are just my opinion based on my personal research....just my opinion

 

[GeoTerraTestudo]

Hi Geo...so here is my theory ... The central asia tortoise has alot of the same obstacles...except they are smaller...they have evolved to thrive out of the burrow for much shorter time...their diet also includes carrion and they also consume their own as well as other creatures "poop" and will consume bone particals along with their grazing....they mature sexually at about 5-7 years of age which is much shorter than gopherus...

I have to say once again that these are just my opinion based on my personal research....just my opinion

Yes, but Texas tortoises are about the same size as Testudo tortoises (around 5-8 inches long), and yet they are found only in the arid southwestern zone of Texas, and have never evolved a way to inhabit the green grasslands to the north. Their cousins are excellent diggers, but Texas torts just scrape out shallow pallets and stay in the warmer parts of Texas. Why is that? Maybe it's because the green pastures of the Great Plains are deceptively poor in nutritional value for a tortoise. Horsfield's tortoise survives by feeding on forbs (weeds), which are much easier to digest than grass. Could it be that rainfall patterns in America's steppes do not allow as much of a forb biomass to grow there compared to Asia? Maybe this has prevented Gopherus from colonizing America's grassland, the way Testudo has colonized the grasslands of Central Asia.

Or maybe it's just something lacking in Gopherus, which is basal and more closely related to Manouria than any other genus. Manouria species living at altitude do have some cold tolerance, but not as much as the Russian tortoise. Maybe there is some constraint preventing Gopherus from developing cold tolerance the way Testudo has. I don't know, but I'm very curious.

 

[ascott]

Hi Geo....yes the texas tort is smaller than say the gopherus species found in ca. az etc....I know that the texas tort has been known to be a meat eater vs the ca/az desert tort does not...I wonder if the texas tort is smaller due to their poor vegetation options as well...as you mentioned they dig pallets in the soil in the grasslands (which use to be rich in variety of nutritious vegetation before almost all turned into agricultural fields) while theymature sexually near 12-15 years of age their life span is only near max of 50-60 years...and as the desert tortoise they are very specific to specific areas....the similarity between the texas and desert tortoise is that they seem to hold fast/thrive in specific areas and when those areas are desturbed in such a way that they can not keep up....well, they don't..they end up on our endangered/threatened to extinction lists.... there are studies done that show the texas torts that linger on the outskirts of the grasslands are much healthier..problem is that those areas will to be consumed and made part of the low nutrition grasslands...so I would say that I agree with your speculation about the grassland being low to no nutrional value...heck here in the Mona e desert it has unfortunately been invaded by the cheat grass, red broke in such a way that there are studies going on that show the desert tortoise as well as rabbits and other desert dwellers are consuming large amounts and getting no nutritional value...so here comes another cycle of studies that will likely show another obstacle for these guys....

Again....these are strictly my shared opinion

 

[Terry Allan Hall]

This is very interesting, guys.

 

[GeoTerraTestudo]

I thought of another possible reason why there might not be a Great Plains tortoise, the way there is a Russian steppe tortoise. In his book, Mediterranean Tortoises, Brian Pursall states that there are parts of the Mediterranean Basin that could support tortoises but don't because their soils are poor in limestone (calcium carbonate). Tortoises need a lot of calcium for their shells, so they need to live on limestone soils where the plants will take up a lot of calcium, which will in turn be consumed by the tortoise. Portugal, he states, has the same climate as Spain, but whereas Spain does support Iberian tortoises, Portugal does not, and this is because Portuguese soil contains less calcium than Spanish soil.

Could that be true for the Great Plains? Prairie soil is good for agriculture, but maybe it is calcium-poor. Box turtles can live there, but being omnivorous, maybe they can get their calcium through the animals they eat, whereas tortoises rely completely on vegetation for their nutrition. I guess the way to answer this would be to find out how soil in Kansas compares to soil in Kazakhstan in terms of its calcium carbonate content. You would think Kansas soils would have high levels of calcium because, like the Mediterranean area, the Great Plains is the site of a former inland sea from the Cretaceous Period. However, if enough silt has accumulated over the eons, then maybe that limestone layer is too far down. I'm not sure, but it would interesting to find out.

 

[CactusVinnie]

"The Uzbek continental climate limits the annual activity period of T. horsfieldi to only 3 months. Within this 3-month period, ambient temperatures fluctuate strongly each day (from –10 to +45°C), further limiting activity to a total of 360 h per year."

Hi all!

I think it's a mistake in interpreting those values- if taken literally, Uzbekistan should be a hell on earth. And that is only Springtime!!
It rather means that during those favorable interval- largely, from March/April to June/July- the extremes are btw. -10 to 45*C. The coldest March morning fall to -10*C, the hottest day in July to 45*C, all in the active season of the tortoises. Not unlike, for instance, some parts of KS, CO or UT, where those temps at those times may be the same. They don't come out earlier due to cold, they don't stay later due to heat and lack of food.

 

[CactusVinnie]

Interesting aspects, but I am pretty sure that the tortoises would naturalise in the Great Plains, esp. Horsfields., given the opportunity. Many carbonatic soils there, and in some regions of the Rockies too. West of Continental Divide is a lot of limestone, so I speculate that the Desert Gopherus is limited by other factors (climatic) to spread further north, than lack of calcium. Also, Russians would be the most appropriate species for that area too- it climbs up to 2300m in Pakistan and Afghanistan.

 

[Madkins007]

Remember- we DID have tortoises here in Nebraska- a largish one Stylemys nebrascensis[\i] (http://www.bhigr.com/store/product.php?productid=448 if anyone has a spare $95, I would LOVE one of the replica fossil shells!) There is a site called Toadstool Geological Park (part of the Oglala National Grassland) that features several tortoise shell fossils from roughly 30-40 million years ago, and is not too far from the Hudson-Meng Bison Kill site. The soil is varied, but the presence of limestone suggests good calcium levels 'back then'.

There certainly has been climatic change since then, but another key issue is that there have also been several volcanic events. Much of Nebraska at one point or another was covered with ash from Yellowstone and other regional volcanoes. Thick layers of ash would have been a real problem for tortoises while other fauna could have avoided it in many ways.

I have often thought it would be interesting to see what would happen if you got several acres of suitable soil, plants, etc. in Western Nebraska and let a few hundred Russians loose. I suspect that in a few generations, we would have a strain that was pretty acclimated to our climate.

 

[Jacqui]

Well we do know they can survive Nebraska winters outside.

 

[Tiger Cowboy]

It's an interesting question, but there are so many variables I don't think it's possible to get an answer. The very nature of evolution would make it impossible to answer.

What you are looking for is convergent evolution, two species from separate but comparable geographical areas and different taxa having similar characteristics to fill a similar niche in the ecosystem, such as some of the boas and pythons. It might be that another species filled the niche that another Gopherus species could have occupied.

It's possible that the environment is different enough that a tortoise just wouldn't fit. I would like to point out that the great plains we know today are VERY different from the plains of even ~150 years ago. Back then there were bison in vast numbers that would munch and trample huge areas, and there were the Rocky Mountain locusts (largely forgotten now but a large deal back when) that decimated huge areas of the plains, and other species that were introduced or have since gone extinct. Maybe not so great for a tortoise.

 

[Terry Allan Hall]

Interesting aspects, but I am pretty sure that the tortoises would naturalise in the Great Plains, esp. Horsfields., given the opportunity. Many carbonatic soils there, and in some regions of the Rockies too. West of Continental Divide is a lot of limestone, so I speculate that the Desert Gopherus is limited by other factors (climatic) to spread further north, than lack of calcium. Also, Russians would be the most appropriate species for that area too- it climbs up to 2300m in Pakistan and Afghanistan.

Agreed...T. horsfieldii could easily colonize quite a bit of the USA, given the opportunity.

 

[Jacqui]

. It's possible that the environment is different enough that a tortoise just wouldn't fit. I would like to point out that the great plains we know today are VERY different from the plains of even ~150 years ago. Back then there were bison in vast numbers that would munch and trample huge areas, and there were the Rocky Mountain locusts (largely forgotten now but a large deal back when) that decimated huge areas of the plains, and other species that were introduced or have since gone extinct. Maybe not so great for a tortoise.

Or you could look at the bison, the huge range fires, and the locusts in a positive light. These would all cause regrowth and it would be regrowth that would be better for the tortoises then the older plants. Along with regrowth would be the opportunity for other plants to come into their own until the old growth was back. Those other plants might even be a better quality food for the tortoises.

The bison might stomp on the tortoises, but they could survive that, based on the box turtle I use to have that had a clear cow hoof shell mark.

 

[redbeef]

consider more than just climate and those kind of conditions: if there is something that would compete w/a horsfield 'type' for resources in the great plains area, than that organism may be (or may have been) filling part or all of the 'small tortoise ecological niche' leaving no room for the n. american tortoises to adapt to live in that area/niche.

also, i don't know a lot about the range of the horsfield tortoises aside from country: uzbekistan, afghanistan, part of Iran etc...is it really inhabiting only steppes in these countries or is it in more rugged foothill type terrain? they kind of live in all sorts of terrain, aside from grasslands, don't they?