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.
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: American alligator, Burmese pythons, deep south frosts, evolution, extinction, Gopherus polyphemus, Greenland Ice Core temperature data, hesperotestudo crassicutata, Hesperotestudo incisa, Pleistocene giant tortoise