Pleistocene Rattlesnakes

Too many people have an irrational fear of snakes.  One of my neighbors was once frightened by an harmless garter snake a car had flattened like a tortilla in front of her house.  I have peeved more than 1 neighbor over the years by defending a rattlesnake’s right to life.  In my opinion rattlesnakes are an interesting member of the local fauna; less dangerous than big dogs, speeding cars, and lightning.  The odds of getting bitten by a rattlesnake are exceedingly small.  Rattlesnakes do not exist to bite people.  Instead, they want to eat small animals, mate, and stay warm.  They want larger animals, such as humans, to leave them alone.  The majority of snake bite victims stupidly handled the serpent and would have never been bitten, if they would have left it alone.  There is no reason to fear snakes, but they should be respected and not treated like a pet or a toy or as proof of faith in God.

Rattlesnakes are not considered endangered, but they were formerly more common prior to human settlement.  The first white men to explore Kentucky were constantly blundering into them. ( See: https://markgelbart.wordpress.com/2012/04/20/excerpts-from-the-journal-of-an-expedition-to-kentucky-in-1750/ ) Habitat loss and direct destruction during rattlesnake roundups greatly reduced their population.

Genetic evidence suggests the rattlesnakes first diverged from other pit vipers about 22 million years ago.  This divergence probably occurred in western North America because there are more species in that region than anywhere else.  There are 30 species of rattlesnakes, and all of them live in America.  Traditionally, herpetologists categorized 27 species in the crotalus genus and 3 in the sistrurus genus, but the most recent genetic analysis suggests 1 of the sistrurus species should be considered a crotalus.

There are 3 species of rattlesnakes that range throughout southeastern North America today–the eastern timber (Crotalus horridus), the eastern diamondback (C. adamanteus), and the pygmy (Sistrurus miliarus).  Evidence from the fossil record shows all 3 of these species have lived in the region since at least the late Pliocene over 2 million years ago.  However, the eastern timber rattlesnake is presently absent from peninsular Florida, but it did live there during the mid-Pleistocene.  This species probably became extirpated from the peninsula of Florida during a time of high sea levels when  most of the state was  inundated by ocean, and for some undetermined reason it has failed to recolonize its former territory.  I think its ecological niche is now occupied by the other 2 species of rattlers which are better adapted to flat land habitat.  Eastern timber rattlers prefer rocky deciduous woods where they can seek thermal refuge under crevices between boulders and the ground, while pygmies and diamondbacks seek out gopher tortoise and rodent burrows.

Timber Rattlesnake Crotalus horridus, Pennsylvania, Gravid females basking - Stock Image

Eastern timber rattlers (and copperheads?) in an hibernaculum.

Eastern timber rattlesnake range map.  During the mid-Pleistocene they lived in peninsular Florida but have failed to recolonize this region following interglacial sea level rise.

The Eastern diamondback rattlesnake is the largest species of crotalus.

Eastern diamondback range map.

Sistrurus miliarius barbouri

Pygmy rattlesnakes reach a length of only 2 feet long.

File:Sistrurus miliarius distribution.png

Pygmy rattlesnake range map.

There are many species of interesting western rattlesnakes including the prairie, sidewinder, Mojave, and Mexican small-headed.  The genes of the latter species (C. intermedius) reveal a past history of alternating isolation and reunion between populations.  Currently, populations of Mexican small-headed rattlesnakes and closely related species are isolated from each other by desert.  They inhabit pine/oak montane forests at high elevations.  But during Ice Ages the pine/oak forests expand into the desert and populations isolated during interglacials reconnect.

The composition of rattlesnake venom varies regionally and evolves over time in response to environmental changes and evolving defense adaptations among prey species.  An environment that changes from one with an abundance of prey to one of scarcity may lead to rattlesnakes with more potent venom.  Rattlesnake venom has a slew of toxins that can damage nerves, muscles, and blood.  This makes it difficult to manufacture anti-venom that will work.

References:

Bryson, Robert

“Ephemeral Pleistocene Woodlands Connect the Dots for Highland Rattlesnakes of the Crotalus intermedius Group”

Journal of Biogeography 2011

Murphy, Robert et. al.

“Phylogeny of the Rattlesnakes (Crotalus and Sistrurus) Inferred from Sequences of Fine Mitochondrial DNA Genes”

Biology of the Vipers 2002

 

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