The Extinct Pleistocene Giant Tortoise (Hesperotestudo crassicutata) Must Have Been Able To Survive Light Frosts

Illustration of the extinct giant tortoise that lived in the southern parts of North America.  It grew as large as the Galapagos Island tortoises but was more closely related to the much smaller extant gopher tortoise.

Scientists often use the presence of giant tortoise fossils as a proxy for past temperatures.  They conclude that because giant tortoises can not survive freezing temperatures than they must have lived during a time when the region was completely frost free.

Hesperotestudo crassicutata scute

Photo of part of a tortoise shell or scute from a specimen found in Texas.

Three species of closely related land tortoises lived in southeastern North America: a giant species (Hesperotestudo crassicutata) that grew as big as modern day Galapagos Island tortoises, an intermediate-sized species (Hesperotestudo incisa), and the gopher tortoise (Gopherus polyphemus) which is still extant.  It has occurred to me that the two larger species must have been able to survive light frosts, otherwise they would have become extinct when Ice Ages began.  Here are 5 reasons why I have come to this conclusion and disagree with the scientific consensus that the presence of tortoise fossils indicates warmer winters in this region than those of today.

1. The giant Pleistocene tortoise existed for at least 2 million years.  Within this vast time span, there must have been climatic phases, or at least events of crazy weather, that led to frosts in the deep south.  Today, frosts occur as far south as

Look at how much average temperatures fluctuated before the Holocene (~11,000 BP) when it’s assumed once a decade frosts began occurring in south Florida.  Notice also how much lower average temperatures were previous to the Holocene.  It doesn’t make sense the frosts in the deep south just began occuring 11,000 years ago.  They must have occurred before then.

south Florida at least once a decade.  It doesn’t make sense that these once a decade frosts just began to occur ~11,000 years ago and were absent for the previous 2 million years.  It just seems improbable that frosts began to occur in the deep south during the Holocene, a time of relative climatic stability, but didn’t occur during the Ice Ages which were times of dramatic climatic fluctuations (as the above chart shows) and generally of cooler climates.  If it’s true that giant tortoises couldn’t survive in an environment of light frosts, than that means they were extirpated in the southeast every time there was a frost.  They could only recolonize the south from enclaves in central America or what’s now Mexico, but that would mean a geographical corridor in the deep south must have remained frost free for thousands of years at a time–an unlikely climatic scenario, even during warm interglacials.

2. Scientists believe giant tortoises couldn’t escape the cold because they didn’t dig burrows.  This is a shaky assumption.  The only surviving species of giant tortoise lives on islands near the equator where there are no frosts.  As I discussed with my first point, Hesperotestudo did evolve in a region that must have had occasional light frosts, and therefore to survive, it must have evolved adapatations to escape the cold.  Moreover, Hesperotestudo is not the same species as extant giant tortoises, and we have no knowledge of its behavior patterns.  It’s closest living relative, the gopher tortoise, has a deeply innate instinct to dig burrows, and I see no reason for the assumption that giant tortoises didn’t also dig burrows.  Sea turtles dig deep pits to lay their eggs, proving that size is no obstacle to digging deep holes.

Gopher tortoises dig extensive burrow systems. The giant Pleistocene tortoise was closely related to the gopher tortoise.  There is no reason for the assumption that they did not also dig burrows which would have helped them survive frosts.

3. There is no evidence of tropical plants or pollen in the Pleistocene fossil record of the deep south.  If winters were warmer than those of today, and frost free, there should be fossils of tropical species of plants.  Instead, for example, a study of fossil plants from a site in the Aucilla River in north Florida, dating to the Pleistocene, found almost the exact same species that exist in the region today.  No tropical species were found.  Only 3 species outside their present day region were discovered here–osage orange, wild squash, and hazlenut. All three are temperate species, and the latter prefers cooler temperatures than exist today here.

4. Fossils of extant mammal species tend to be on average of individuals larger than those of the same species found in the region today.  According to Bergmann’s Rule, this indicates cooler climates and precludes warmer winters.

5. The prolonged freeze of 2009/2010 in south Florida caused a high mortality rate of the invasive Burmese python but did not cause their complete extirpation.  It seems reasonable to suppose that eventually, large reptiles that are maladapted to occasional frosts, would through selective pressure evolve to have an adapatation that enables them to seek thermal refuges.  And in fact, there are 2 clades of Burmese pythons with differing behavior patterns in their responses to frosts: the majority of the ones imported for the pet trade come from southeast Asia, and they’re naive to frost; but another population of this species occurs in temperate regions, and they’ve learned to seek refuge and hibernate during colder times of the year.

Like the northern population of Burmese pythons, and the American alligator, the giant Pleistocene tortoise was likely an animal of the subtropics that extended its range into southern temperate regions during warmer climatic stages.  And like pythons and alligators, selective pressures chose those individuals that took action to escape frost.  Alligators know to escape frost by moving into deep water, while caimans and crocodiles and southern Burmese pythons continue basking in subfreezing temperatures which leads to their deaths.  Like the alligator, Pleistocene giant tortoises must have survived frosts by moving to thermal enclaves such as burrows they dug themselves, the dens of other species, caves, hot springs, or under upturned tree roots.  How they survived frost is a subject for conjecture, but I have no doubt that somehow they must have.


Tags: , , , , , , , , ,

16 Responses to “The Extinct Pleistocene Giant Tortoise (Hesperotestudo crassicutata) Must Have Been Able To Survive Light Frosts”

  1. Mark Says:

    Mark, I agree with your hypothesis that they burrowed. i’m curious as to how many species could have benefitted by having immense burrows dug by such a large tortoise also. This could be a gold mine of dependent species. Interesting stuff.
    The alligators that are in north Alabama, just south of the Huntsville airport, have been there for 60+ (?) years now, and each time we have an extremely cold winter, people joke about it killing them off. It hasn’t happened yet. They make due, even using their heads to crack the ice to breathe if necessary. They still breed, and they are spreading.

  2. markgelbart Says:

    Wow, I didn’t know their were gators as far north as Huntsville.

    Fossils of extinct species of armadillos have been found in association with giant tortoise fossils in a collapsed cave system in north Georgia. I’m sure they shared space.

    The fossils from the north Georgia site, though, I hypothesize date to an interglacial that was warmer than the present.

  3. The Paleoindians Probably ate more Turtle than Megafauna « GeorgiaBeforePeople Says:

    […] Most turtle species reproduce so rapidly that paleoindian exploitation had little effect on their numbers.  But it’s likely paleoindian overhunting (if killing a slow-moving giant tortoies can be called hunting) probably did wipe out the 2 Pleistocene species of giant land tortoises–Hesperotestudo crassicutata, and Hesperotestudo incisa. See my article–… […]

  4. Some Giant Ground Sloths Dug Long Burrows « GeorgiaBeforePeople Says:

    […] Many extinct and extant organisms used or even depended upon ground sloth burrows.  The fossil remains of a glyptodont were found in 1 ground sloth burrow.  Glyptodonts were physically incapable of digging their own.  Giant tortoises too probably made use of sloth burrows, possibly explaining how this frost sensitive species survived as far north as Bartow County, Georgia during the Ice Age. (See…). […]

  5. Myles Traphagen Says:

    The Pleistocene was a period of more equitable temperatures. The period has been typically characterized by having a rather even distribution of temperatures and less seasonal fluctuation than the Holocene. The range between highs and lows was likely pretty small. The presence of continental glaciers prevented the hard frosts that we see today, much like the effects we see in marine climates that rarely freeze that are adjacent to inland continental climates that freeze regularly. So to specifically address your hypothesis that the lower average temperatures of the Pleistocene resulted in harsher, colder temperatures, well this may not be true. Average temperatures are quite a different matter than minimum and maximum low temperatures. p.s.I can’t see the Holocene temperature graph. I really like your site! Myles

    • markgelbart Says:

      The graph I show in my essay disproves your first sentence. The Pleistocene was not a period of more equable temperatures. Temperatures fluctuated far more wildly than they do today.

      How would the presence of glaciers prevent hard frosts?

      That makes no sense at all. The glacier exists because of permanent frost. There is no way that areas anywhere close to a glacier were frost free.

      • Myles Traphagen Says:

        What is the source for that graph? I’ve never seen a Pleistocene temperature graph.

      • markgelbart Says:

        I found it on google images.

        Just click on google images and type in Pleistocene temperature graph. There are hundreds of graphs based on data from ice cores taken from the Greenland Ice Sheet and Antartica, as well as mud cores and foraminifera analysis taken from the ocean bottom.

        The data suggests that temperatures in southeastern North America during the Ice Age were much cooler than modern day summers but winters were only slightly cooler than they are today. So in a way, temperatures were more equible though depressed compared to those of the present, and they fluctuated wildly. I’m certain frosts commonly occurred during stadials.

  6. The Lack of Pleistocene Crocodylus acutus Fossils in Southeastern North America and its Climatic Implications | GeorgiaBeforePeople Says:

    […] (Gopherus polypherus), a burrow dweller, also dug burrows where they could escape frosts.  (See:😉  Or they utilized burrows dug by giant ground sloths. (See […]

  7. The Lubbock Lake Fossil Site | GeorgiaBeforePeople Says:

    […] widely held assumption that giant tortoises couldn’t survive subfreezing temperatures. (See:😉  The giant tortoises were closely related to gopher tortoises, a species that does dig burrows; […]

  8. The Pleistocene Ridge and Valley Reptile Corridor | GeorgiaBeforePeople Says:

    […] light frosts by digging burrows or by using burrows dug by ground sloths and pampatheres. (See:😉  Moreover, red bellied turtles and southern toads do live in regions that experience light frosts […]

  9. Erik Ringdal Says:

    There is a large tortoise today digging deep: The sulcata of Africa, reaching up to +200 pounds.

  10. Erik Ringdal Says:

    I like the idea of a burrowing giant tortoise. A pollen analysis might show more about the climate back then. One thing more: The name crassiscutata from what I know means rough scales, and it certaily has them. I assume this indicates burrowing behaviour. Can the anatomy of the front legs be compared to other Gophers plus the sulcata?
    Regards Erik Ringdal

    • markgelbart Says:

      I found an anatomical comparison between extant and extinct southeastern extant tortoises in an old bulletin of the Florida Museum of Natural History. I think the author was Auffenberg. He studied crassicutata more than any other scientist. I’m sure anyone with access to the pertinent material could compare crassicutata to sulcata. That’s my complaint. There are no scientists currently studying the extinct tortoises.

  11. Erik Ringdal Says:

    Is there a scientist called Dick Franz who at least from time to time studies this subject? I stumbled upon him on the net, describing a journey in 2010 to the Southwest, taking pictures and measuring museum tortoises. I think he is in his sixties, perhaps living in Florida.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: