Pleistocene Megafauna Seasonally Migrated Back and Forth Between North Florida and Central Georgia

The ratio of strontium isotopes in the soil above the Florida aquifer is markedly different from that within it.  Herbivores that eat plants in these 2 different geographical regions ingest different ratios of strontium isotopes.  Scientists are able to place numerical values on the strontium isotope ratios in fossils of Pleistocene megafauna found at several  sites in north Florida to determine whether or not they spent some part of every year in central or even north Georgia.  This map isn’t exactly accurate–the strontium isotope values in Florida soils are also found much farther north into Alabama well above the Florida aquifer there.

On our modern anthropogenic-dominated earth, large mammal migrations are rare.  Wildebeest herds still follow their ancient migration patterns in east Africa, and barren ground caribou seasonally travel across Alaska and northern Canada, but man has forever ended the hundreds of other large mammal migrations that used to occur all over our once pristine planet.  Networks of roads, suburbs, cities, and farmlands put a mask over the landscape of southeastern North America, so  it is hard to imagine large herds of mammoths, mastodons, horses, and bison seasonally migrating throughout the region.  But scientists have found evidence that the first 3 once did.

The isotopic ratio of the element strontium, which is found in soil everywhere, varies geographically.  Strontium isotopes are absorbed by plants and then animals when they ingest the plants.  Scientists can determine where an animal resides by the ratio of strontium isotopes in an animal’s bone.  For the below referenced study, scientists determined the ratio of strontium isotopes for central Georgia and north Florida by looking at the strontium isotope ratios in local rodent teeth, plants, and groundwater.  They compared those values with strontium isotope ratios found in mastodon, mammoth, horse, deer, and tapir fossils excavated from several sites in north Florida, including Aucilla River and Sloth Hole.  From the numerical values they obtained, they were able to determine which individuals stayed in Florida and which seasonally migrated into central or even north Georgia.  (They couldn’t determine if the megafauna migrated between north and south Florida because strontium isotope ratios in the soil have the same value within the whole state.)

During the Last Glacial Maximum (~28,000 BP-~15,000 BP), mastodons seasonally migrated between north Florida and central Georgia, but mammoths did not.  However, prior to the LGM (>30,000 BP), mastodons did not migrate from Florida to Georgia.  Some undated mammoth specimens showed evidence that those individuals did seasonally migrate to central Georgia. These specimens may date to before the LGM.  The environmental changes that differentiate these 2 time periods may explain the change in migratory habits of these 2 species.  Mastodons were a semi-aquatic animal that lived in flooded swamps and marshes, and they mostly ate twigs, aquatic plants, and fruit.  Mammoths were an upland animal that fed mostly upon grass.  During the LGM the climate was dry and favored grass over trees, while wetlands were more scarce.  Accordingly, mastodons were forced to wander farther in search of favorable habitat.  Conversely, before the LGM, the climate was moist and favored trees over grass, while wetlands were abundant.  Under these conditions, mammoths were forced to travel longer distances looking for suitable habitat. 

The proboscideans are long-lived animals.  When the great beasts overgrazed the range, older individuals in the group remembered where to go to find fresher plant growth.  Recall the old saying–elephants never forget.  A good memory was critical to the survival of this species.  They had to remember where to go to find greener pastures.  Mastodons likely migrated up river valleys.  Mammoths may have followed these routes too, but were less tied to water.

The scientists only looked at the strontium isotope ratios of 3 horses.  Based on this scant sample size, before the LGM, horses migrated long distances between Florida and Georgia, but during the LGM they did not, indicating grass was plentiful all across the landscape then. 

Of the 12 white tail deer specimens, only 1 showed evidence of long distance migration.  White-tail deer have relatively small home ranges, but occasional individuals do disperse long distances.  The 2 tapir specimens didn’t migrate.

The Paleo-Indians probably used knowledge of migration patterns to ambush and wipe out whole herds of megafauna.  The need to migrate to find greener pastures was a fatal flaw for the great beasts when confronted with an increasing population of Homo sapiens.

Bull mastodons fighting for dominance.  I was unable to find an illustration on google images of Pleistocene megafauna migrating through an environment that would have looked like Florida and Georgia then.  Most illustrations of Pleistocene megafauna migration show them moving through icy tundra.  There was never icy tundra in Georgia and Florida.

Reference:

Hoppe, Kathryn; and Paul Koch

“Reconstructing the Migration Patterns of Late Pleistocene Mammals from Northern Florida, USA”

Quaternary Research 68 2007

http://www.es.ucsc.edu/~pkoch/pdfs/Koch%20papers/2007/Hoppe%20&%20Koch%2007%20QR%2068-347.pdf

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