The Pleistocene Box Turtle (Terrapene carolina putnami)

I self torture myself every Wednesday: I go in my backyard and do 150 pullups, jogging in place between sets, and I follow this exertion with yardwork such as cutting the grass with a scythe or chopping firewood with a dull axe.  (Lawnmowers and chainsaws are noisy and unpleasant and I use neither.)  Three weeks ago while torturing myself, I discovered a male box turtle (Terrapene carolina) sitting on the edge of a shallow burrow consisting of leaf litter and pine straw.  I didn’t think this would be sufficient cover to protect a cold blooded reptile from forthcoming frosts.  Yet, this turtle was still alive after the first frost of the year.  I perused the scientific literature and learned from 1 study that the average depth of a box turtle’s winter burrow in Ohio is only 2 inches, though some burrow as deep as 7 inches.  Snow further insulates them.  Box turtles range as far north as Maine and apparently they can survive when their body temperatures fall below freezing.  However, winter freezing is their leading cause of mortality.  Hard winters diminish their populations to some degree.

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A male box turtle that may spend the winter in my backyard.  Male box turtles have red eyes.  Females have yellow eyes.

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Note the burrow into the leaf litter and pine straw behind the turtle. Despite their terrestrial existence, box turtles are more closely related to aquatic turtles than land tortoises.

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Also note how shallow this burrow is.  Box turtles can survive hard freezes covered by just 2 inches of leaf litter and/or soil.

Various climatic fluctuations during the Pleistocene isolated populations of box turtles.  These changing conditions resulted in the regional variations in box turtle morphology evident today.  There are 6 recognized subspecies of eastern box turtle: the eastern (T. c. carolina), the Florida (T.c. bauri), the 3-toed (T.c. trungus), the Gulf Coast (T.c. major), the Mexican (T.c. mexicana), and the Yucatan (T.c. yucatan).  Moreover, there was a large extinct species (T.c. putnami), known from Pleistocene fossil sites.  All of these subspecies readily hybridize on the borders of their ranges.  A late herpetologist, William Auffenberg, and others hypothesized T.c. putnami preferred warm coastal grasslands, while T.c. carolina inhabited forested landscapes.  Accordingly, fossil remains of T.c. putnami could be used as evidence that a particular fossil site was of an interglacial age, and the presence of T.c. carolina was evidence a specific site was of glacial age.  But some remains of T.c. putnami were found at the Jones Creek fossil site in Missouri which dates to a climatic phase when jack pine and grass dominated the local flora.  Jack pine no longer occurs south of Michigan, and its presence in Missouri is evidence of a cooler climate than that of today.  I don’t believe Dr. Auffenberg’s hypothesis is valid.  T.c. putnami was widespread during the Pleistocene and likely well adapted to both glacial and interglacial conditions.

Partial carapace of the extinct subspecies of box turtle, T. carolina putnami.  Too bad the colors have faded away.

A recent comprehensive study of box turtle morphology and genetics determined that there was no evidence for the existence of the Gulf Coast subspecies, T.c. major.  Instead, the authors of this paper suggest box turtles found on the Florida panhandle, thought to be this subspecies, are descended from hybridization between the eastern subspecies, T.c. carolina, and the extinct subspecies,  T.c. putnami.  This would explain the larger size of Gulf Coast box turtles compared to other subspecies.  For some undetermined ecological reason the once widespread Pleistocene ecomorph of box turtle still occured as a relic population in this region, and the survivors hybridized with another now more common subspecies.

Box turtles are amazing survivors.  They are omnivorous, capable of eating any plant matter such as fruit, flowers, and fungi; and animal matter including insects, worms, snails, birds’ eggs, and carrion.  And they can survive scarcity of food because they have slow metabolisms.  They can live to be 100 years old and can regenerate when severely burned.  Females can mate once and lay fertile eggs for 4 years.  Box turtles survived millions of years of climatic fluctuations and still live and can be found in my backyard.

References:

Austin, Jones; et al

“Morphological and Molecular Evidence Indicate that Gulf Coast Box Turtles (T.c. major) are not a Distinct Evolutionary Lineage in the Florida Panhandle”

Biological Journal of the Linnean Society 2011

Claussen, Denis; et al

“Hibernation in the Eastern Box Turtle, Terrapene Carolina

Journal of Herpetology 25 (3) 1991

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