Primeval Monster Gators (Alligator mississippiensis)

How big can an American alligator get?  Early Colonial explorers and naturalists claimed these reptiles could grow to as large as 20-25 feet long.  William Bartram wrote that he saw individuals of this length during his trip through Florida in 1774, but he’s notorious for miscalculating distances.  I believe Bartram had poor eyesight even before he was struck with scarlet fever a few years later.  The largest alligator on record was a specimen killed on Marsh Island, Louisiana in 1890.  It measured 19 feet, however, this record was never verified because the beast was too heavy to drag away from shore.  It would have weighed an estimated 2200 pounds.  The largest verified alligator was a specimen measuring 17.5 feet, killed in Everglades National Park.  Modern day state records have recently been set in Arkansas, Mississippi, and Texas, but these all measured slightly under 14 feet.  Large alligators of this size are rarer today than during the pre-European settlement era because hunters kill most gators that exceed 6-8 feet.

lufkin record alligator

A nearly 14 foot long alligator killed in Texas.  A fossil of a Pleistocene-aged alligator was from an animal estimated to be 7 feet longer than this one.

If alligators formerly did reach over 20 feet in length as Bartram and others claimed, it occurred to me there might be some fossil evidence of this.  Alligators are one of the most common large vertebrate species found in Florida fossil sites.  Data from the University of Florida Museum indicates alligator fossils have turned up in probably over 100 fossil sites in Florida, and this doesn’t even include the many found by hobbyists.  Alligators have been a common component of Florida’s fauna for millions of years.  The American alligator has undergone little morphological change for nearly 5 million years.  Its evolutionary predecessor, Olsen’s alligator, was smaller and did differ in skull morphology enough to be considered a separate species.  One alligator specimen found at Haile, an early Pleistocene fossil site (~2 million BP),  represented an animal estimated to be 21 feet long when it was alive.  A gator this size likely weighed 2500 pounds.  This specimen certainly supports the veracity of early explorer’s claims of seeing alligators that were 20-25 feet long. 

maxilla of Alligator mississippiensis

Fossil alligator maxilla from Sarasota, Florida.  This specimen dates to the late Pleistocene.

Pleistocene alligators did not necessarily top the food chain.  During droughts, big gators often travel overland looking for deeper water holes.  When in this vulnerable situation, even large gators could have fallen prey to big cats such as jaguars or saber-tooths.  In some areas of South America, the alligator-like caiman is an important item of a jaguar’s diet.

Zombie Gators

Some Florida lakes have become so polluted from fertilizer run-off that large algal blooms occur.  Gizzard shad thrive in these polluted waters, while populations of other species of fish decline.  Alligators can safely eat gizzard shad as long as there are other species of fish in the water.  However, when alligators are forced to subsist on just gizzard shad they enter a zombie-like state known as ataxic neuropathy.  This is a fancy name for becoming paralyzed.  Gizzard shad flesh has thiaminase–an enzyme that breaks down thiamine, an important B vitamin.  This prevents gators from digesting thiamine without which their nervous system ceases functioning.  The zombie-like state is followed by death, unless the gator is given thiamine.

Zombie Alligators

A zombie gator.  Alligators fed on a diet of nothing but gizzard shad suffer a nutritional deficiency that paralyzes them.  They can be revived with thiamine, a B vitamin.

gizzard shad fish

Gizzard shad (Dorosoma cepedianum).  Gators can safely eat this fish as long as they have other species of fish in their diet as well.  A diet of just gizzard shad will poison them.

Bartram’s Battle Lagoon

In 1774 William Bartram traveled on the St. Johns River, Florida.  He witnessed an incredible fish migration that attracted hundreds of large alligators.  A fish migration as he described no longer occurs on this scale at this geographic locality.  Few, if any places left on earth, provide such a spectacle.  It was probably a migration of gizzard shad.  This migration attracted largemouth bass which fed upon the shad, while gars preyed on the bass.  Alligators ambushed them all.  Bartram saw this migration on the part of the St. John’s River that flows between Lake George and Lake Dexter.  (There are 2 Lake Dexters in Florida.  This is the one located in Ocala National Forest.)  Below the next image is Bartram’s account.  (Incidentally, Bartram refers to largemouth bass as trout.  Old timers still refer to bass caught in brackish waters as “green trout.”)

Click to view larger image

Lake Dexter is the middle lake.  The part of the St. John’s River between Lake George in the distance and Lake Dexter in the middle is where Bartram witnessed a massive fish migration that attracted hundreds of gators.

It was by this time dusk, and the alligators had nearly ceased their roar, when I was again alarmed by a tumultuous noise that seemed to be in my harbour, and therefore engaged my immediate attention.  Returning to my camp, I found it undisturbed, and then continued on to the extreme point of the promontory, where I saw a scene, new and surprising, which at first threw my senses into such a tumult, that it was some time before I could comprehend what was the matter; however, I soon accounted for the prodigious assemblage of crocodiles at this place, which exceeded every thing of the kind I had ever heard of.

How shall I express myself to convey an adequate idea of it to the reader, and at the same time avoid raising suspicions of my want of veracity.  Should I say, that the river in this place from shore to shore and perhaps near a half a mile above and below me, appeared to be one solid bank of fish, of various kinds, pushing through this narrow pass of St. Juans into the little lake, on their return down the river, and that alligators were in such incredible numbers, and so close together from shore to shore, that it would have been easy to have walked across on their heads, had the animals been harmless.  What expressions can sufficiently declare the shocking scene that for some minutes continued, whilst this mighty army of fish were forcing the pass?  During this attempt, thousands, I may say hundreds of thousands of them were caught and swallowed by the devouring alligators.  I have seen an alligator take up out of the water several great fish at a time, and just squeeze them betwixt his jaws while the tails of the great trout flapped about his eyes and lips, ere he had swallowed them.  The horrid noise of their closing jaws, their plunging amidst the broken banks of fish, and rising with their prey some feet upright above the water, the floods of water and blood rushing out of their mouths, and the clouds of vapour issuing from their wide nostrils, were truly frightful.  This scene continued at intervals during the night, as the fish came to pass.  After this sight, shocking and tremendous as it was, I found myself somewhat easier and more reconciled to my situation, being convinced that their extraordinary assemblage here was owing to this annual feast of fish, and that they were so well employed in their own element, that I had little occasion to fear their paying me a visit.”–William Bartram, 1774

References:

Ross, J.P. et. al.

“Gizzard Shad Thiaminase Activity and its Effect on the Thiamine Status of Captive American Alligators (Alligator mississippiensis)”

Journal of Aquatic Animal Health 2009

http://www.flmnh.ufl.edu/vertpaleo/fossilspeciesAlligatormississippiensis.htm

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