Snapping Turtle

Snapping Turtle West Virginia 200716
Female Snapping Turtle just after crossing a road to find a good place to lay eggs in a location remote from predation and near water where hatchlings might survive. 

Common Name: Snapping Turtle, Common Snapping Turtle – The name refers to the prominent toothless beak that has a powerful, snapping bite for capturing prey and for defense. The term “common” is sometimes added to distinguish this species from the larger and more fearsome alligator snapping turtles that inhabit the Gulf Coast northward along the Mississippi River and its tributaries.

Scientific Name: Chelydra serpentina – The genus is from Chelydros, the Greek word for an amphibious serpent or a tortoise. Serpens is the Latin word for a creeping animal, usually taken to mean a snake (serpent). The leitmotif of snake may be due to the snapping turtle’s unusually long tail.

Potpourri:  Snapping turtles have an unsavory reputation as aquatic aggressors, lurking in the depths of freshwater ponds to lop off the extremities of innocent waders.  The resultant chelonaphobia, a form of zoophobia … the unreasonable fear of animals … applies to those who see turtles as terrible, preventing those afflicted from getting into the water in the first place. Turtle phobia can only apply to snapping turtles … box turtles hide form intruders in their armored sanctuaries and painted turtles slip into the water when approached. There is at least a modicum of  rational apprehension of water immersion due to the possible presence of large, aggressive marine predators like some sharks that (rarely) attack humans with nightmarish consequences exaggerated by cinematic jaws. But in spite of the University of Maryland motto, there is no reason to “fear the turtle.” There is no record of anyone ever being killed by a snapping turtle and the incidence of injury of any kind is vanishingly small, mostly on land due to improper handling. While snapping turtles do bite with a bone crushing finality, it is no more or less that many other animals which are larger and more mobile. Nothing to fear but fear itself.

Snapping turtles are in a separate family (Chelydridae) in the turtle order (Testudines) of the reptile  class (Reptilia) with kindred crocodiles, snakes, and lizards. They are among the oldest of all animal groups, having evolved from the earliest reptiles about 200 million years ago, long before the age of the related and now extinct dinosaurs. That they survived the Cretaceous-Paleogene (K-Pg) extinction 66 million years ago with their avian cousins as the thunder lizards perished en masse along with three fourths of all living things is testimony to the resilient “intelligent design” of natural evolution.  Testudinal structure is a case study in  the random course of genetic mutation  that has no plan, but which rarely but inexorably succeeds by repetitive trial and error. Turtles are unlike any other reptile in having a carapace exoskeleton, a horny toothless beak, and the bones and ligaments of locomotion located inside the rib cage. [1] Their abrupt appearance in the fossil record absent a gradual transition through stages of partial shell hybrid variants has been a perennial issue with paleobiologists. Specifically, how could the ribs that had always been the vertebrate organ cage become body armor?

A turtle shell seems to be a relatively simple structure with an arched top comprised of polygonal scales called scutes with a flattened bottom plastron as foundation. However, complexity is biology’s handmaiden ― there are about sixty separate bones growing in synchrony to form the whole. The rib bones in turtles grow straight through the muscle in which they would become embedded in most vertebrates until they reach the dorsal (back) tissue that is known as the carapacial ridge. Here they release bone morphogenic proteins (BMPs) and hedgehog proteins that convert the nearby tissue cells into bone, filling in the spaces around the fifty-odd segments like mortar in the brick wall of the carapace. The nine bones of the plastron follow a different path ―no ribs and no tissue ossification. Here the bone cells expand independently, fusing together like those that form the brain-encasing skull except they encase the body. [2] It is feasible that the mutation in the bone forming cells occurred randomly, imparting an almost immediate enhancement survival benefit in the form of a fortress palladium. The turtle as battle tank succeeded and the mutation was passed on as a  cladistical advantage.  While we should not fear the turtle, the whole body shield renders the turtle fearless, like their teenage ninja mutant namesakes. The snapping turtle especially so.

Snapping turtles are apex predators of North American freshwater habitats with almost nothing to fear until humans came along. They are seldom seen as they spend the majority of their time hidden in the mud and ooze at the bottom of a pond or lazy river where quiescence prevails. There they lurk as a cryptic mud-colored mound until a proximate  potential meal appears. Alligator snapping turtles (Macroclemys temmincki) take this one step further with a worm-like appendage that is anchored to the bottom of their maw as lure to passing fish. The coup de grace is administered by the snapping beak deployed at the business end of the long, flexible neck that can extend outward some thirty centimeters, about two-thirds of the length of the shell. As observations of snapping turtle predation are limited by the black of their lagoon habitat, dietary preference research requires capture and dissection. Not surprisingly, slow bottom dwellers like crayfish, catfish and diving beetles are among the more likely menu alternatives, but toads, tree frogs, muskrats, and even waterfowl are occasional entrées. Snapping turtle predation of pond ducks and migrating geese is one of the bones of contention concerning their presence in water habitats, particularly those that qualify as private property. A study conducted in 1943 found that less than one percent of the stomach contents of 470 snapping turtles consisted of bird remains. While other studies have asserted avian losses exceeding ten percent, the general vilification of snapping turtles contributes to some exaggeration of decimation. [3]

The lore of snapping turtles as malevolent monsters of the deep is a matter of American acculturation. Comics and cartoons frequently depict barefoot boys jumping out of ponds with a turtle snapped onto a toe. The perception of snapping turtles as hurtful is due primarily  to their pugnacity when encountered on land, the only time that they are readily observable.  Any animal will trend toward wariness and become aggressively defensive when away from its natural habitat home, water in this case. An inquisitive human intruder only makes things worse. However, they only lunge and snap when cornered, and will only bite when proffered something that is bite-sized, like a twig (or a finger perhaps).  A second fear factor is the ominous appearance of the nutcracker beak,  a formidable weapon designed to do real damage. This is an evolutionary trade-off that favored powerful jaws for bone-crunching carnivores … turtles that feed mostly on plants and invertebrates get by with much less. [4]  Association with the alligator snapping turtle with the size and strength to  sever a finger renders all of their kith and kin suspect. Rumors abound of a human finger being found inside one caught by trappers for alligator snapper stew, a popular dish in bayou country. One confirmed case of finger loss involved a group of intoxicated bar patrons and a bet as to whether anyone could stick their finger into an alligator snapping turtle’s mouth and pull it out before it snapped shut. [5] No prestidigitation there. In their natural aquatic environment, snapping turtles are docile, noted mostly for their curiosity which can result in a slight bump as they investigate swimmers or boats snout first. When approached in the wild in their home waters, they do not even bite, much less snap.

Snapping turtles are well suited to riparian wetlands and boggy marshes. A prime location has an average water depth of about two feet so that the occupant can sit on the bottom and poke its head above the surface for an occasional gulp of air. These are reserved for the larger males that predominate ― might makes right is more than an aphorism in the wild. They have been observed in the same location for stretches of up to ten years, when they eventually are displaced by a younger, stronger, or larger rival.  Life consists of  walking along the bottom with deliberation eating the aquatic plants that typically comprise over fifty percent of their diet. One study found that over 90 percent of the contents of the stomachs of 278 individuals was plant material. [6] Snapping turtles have superior vision both above and below water with an optical range that extends to directly overhead. The larger males are mostly sedentary, ensconced in the mud waiting for their unwitting faunal prey … the more spry, younger turtles are more likely to hunt for food. Smaller males and females occupy less desirable and more marginal waterways that contribute to a density that can be as many as 30 adult turtles  in a single acre. The bucolic regimen of lounging in the water with plenty of food comes to an end in winter when hibernation in a frozen pond is the only option and spring when the hormones of procreation mandate their expiation in the sexual union of new life.

The tilt of the Earth’s axis creates the seasons in succession consequent to the annual orbit of the sun. In the temperate latitudes, winter’s dark and cold shadows are an existential risk for turtles that live in water that freezes. One of the attributes of the perfect pond is having a depth that is below the freeze line with adequate vegetation to provide oxygenated (aerobic) water for survival.  These goldilocks spots become the hibernacula not only for the resident, staked-claim dominant males but also for guest turtles that can congregate  in a small area sometimes stacked vertically. There they live in suspended animation for up to six months at the northern reaches of their range  with no food and no air with body temperatures just above freezing.  Hibernation is one of the most compelling cases for evolution, the shutdown of all but essential activity a matter of physiological adaptation. Dissolved oxygen that still persists early in winter is absorbed through the skin and the membranes of the mouth to sustain the metabolism of the slowly pulsating heart. When the oxygen runs out, sugar and fat breakdown continues for a time causing an increase in acidity that eventually becomes  life threatening. In one laboratory experiment in a water tank held at near freezing with bubbling nitrogen to maintain anaerobic conditions, turtles survived for four months, their blood PH dropping for a neutral 8.0 to a near lethal 7.1. Southern populations of turtles not accustomed to hibernation only survived for one month in the same experiment. [7] Practice, even among turtles, makes perfect. When spring comes to the hibernaculum pond and the ice melts, the dominant male chases everyone else out of the water and they all set off looking for someone to mate with … like spring break at the beach.

Snapping Turtle Hatchling Columbia 200523
Snapping Turtle Hatchling making its way to water and safety.

Male snapping turtles are aggressive and sex does not appear to be consensual since females attempt to flee when pursued. This is an anthropocentric perception as it would be at best difficult to determine what constitutes normal behavior for a male or female snapping turtle; they are all perpetually gruff by human standards. Females are much more peripatetic than males, travelling several miles in search of a location that will provide some survivability benefit to the 22 to 62 eggs that will be laid. Prime nesting sites are mostly upstream of home ponds on sandy banks adjacent to water into which the vulnerable hatchling turtles can rapidly move. It is along these watercourse causeways  that the males wait in ambush.  Since females can store sperm for several years, neither copulation nor nest building to lay eggs is necessarily an annual event. In any given year, 72 percent of females deposit their eggs at a nesting site that is almost always the same site used previously. This is not a particularly good strategy, as up to 90 percent of the nests are destroyed on the first night by egg-eating foxes, racoons, skunks, opossums, and coyotes. [8] But it works well enough, as snapping turtle populations are, at least for now, stable.

The journey of snapping turtles, particularly females, over long distances through sometimes dense forested areas to the same location every other year or so raises questions about navigation. This conundrum pertains to animals in general … migratory birds travelling thousands of miles over continental spans to return to nesting grounds and salmon seeking their birth stream after years foraging in the open ocean, among others. There are several candidates that could plausibly be involved in geographic positioning. Transits made above ground could use the positions of the sun, moon, and stars augmented in some cases by landmarks where available and perhaps by wind direction, temperature, and air  composition. Waterborne transits could use marine parameters like salinity, suspended solids, acidity, and currents  for orientation. Much more likely, however, is the one parameter that is universal and does not depend on temperature, elevation, or cloud cover ― magnetism. Lines of magnetic flux vary on the macroscopic scale of the north and south poles and on the local scale by the presence of magnetite.  The ability to use the magnetic field as a sensor is called spontaneous magnetic alignment (SMA) and appears to use two types of magnetoreception mechanisms. The first is a magnetite-based (MBM) and is thought to be the basis for a geographical map (where am I?) of different field strengths and inclinations. The second type is more esoteric in that it involves complementary light inputs. Known as the radical-pair mechanism (RPM), it involves a linkage between photopigments and magnetism that hypothetically provides directional inputs (where do I go?).   Snapping turtles have been experimentally shown to respond to variations in radiofrequency (RF) signals that emulate magnetism. [9] At this point, there is more theory than fact, but one thing is certain. Turtles and other animals can find their way back against insuperable odds to the same place and it can’t be luck or magic.

Even though snapping turtles are ubiquitous and listed as being of “least concern” in species data bases, there is reason to consider conservation measures now before it is too late.  This is especially true in Canada, where many of the  empowered First Nations peoples refer to  North America “Turtle Nation” and believed that a turtle allowed the earth to be created on its back. This is not as implausible as it sounds. Western lore includes the notion that the flat earth rested on the back of a turtle swimming in an endless sea. This is not all that far removed from the “modern” scientific ever expanding universe that began with a big bang containing dark matter and dark energy that have not yet been defined. It surely can have nothing to do with turtles, though, unless they are dark matter. The life cycle of turtles is stressed. Snapping turtles become sexually mature only after a decade or so, live for a long time, and rely on frequent reproductive events that result in high mortality rates for embryos and hatchlings to maintain the population. It has been estimated that the probability of an embryo surviving to reproduce in adulthood is 0.1 percent (1 out of every 1,000). The concern is that with this population dynamic, the loss of an adult can result in serious and possibly irreparable harm. People are the problem.  The human horde encroachments include habitat destruction for more houses, road kills along the paved accesses to the new houses, and increases in predator populations like racoons that share human habitats. Adding insult to injury are more direct assaults including fishing bycatch, the killing of snapping turtles by people who kill snakes for the same reason, and legal and illegal “harvesting.” As an example, between 1996 and 2006, the US Fish and Wildlife Service recorded over one million snapping turtles shipped overseas in the illegal wildlife trade. Yes, Virginia, the enemy is us and Santa Claus has nothing to do with it.

 

References:

  1. Behler, J. and King, F. National Audubon Society Field Guide to Amphibians and Reptiles, Alfred A. Knopf, New York, 1979, pp 425-437.
  2. Pennisi, E. “Neural Beginnings for the Turtle’s Shell” Science, 13 February 2004, Volume 303, Issue 5660, pp 951.
  3. http://www.virginiaherpetologicalsociety.com/reptiles/turtles/eastern-snapping-turtle/eastern_snapping_turtle.php
  4. Herrel, A. et al. “Evolution of bite performance in turtles”. Journal of Evolutionary Biology. 25 October 2002, Volume 15 Issue 6 pp 1083–1094. https://onlinelibrary.wiley.com/doi/full/10.1046/j.1420-9101.2002.00459.x
  5. Gibbons, J. W. “Can a Snapping Turtle bite off a finger?”24 June 2018 Savannah River Ecology Laboratory, University of Georgia http://archive-srel.uga.edu/outreach/ecoviews/ecoview180624.htm
  6. Cameron, M. Committee on the status of Endangered Wildlife in Canada (COSEWIC) Assessment and Status Report, “Snapping Turtle, Chelydra serpentina” 2008 at http://publications.gc.ca/collections/collection_2009/ec/CW69-14-565-2009E.pdf
  7. Heinrich, B. Winter World, Harper Collins, 2003, pp 145-156
  8. Kynast, S. “Snapping Turtles” Tortoise Trust Web Site at http://www.tortoisetrust.org/articles/snappers.htm
  9. Landler, L. et al “Spontaneous Magnetic Alignment by Yearling Snapping Turtles: Rapid Association of Radio Frequency Dependent Pattern of Magnetic Input with Novel Surroundings”. PLoS ONE 10(5). 15 May 2015. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4433231/
  10. Management Plan for the Snapping Turtle (Chelydra serpentina) in Canada [Proposed]. Environment and Climate Change Canada (2016).  Species at Risk Act Management Plan Series. Ottawa: Ottawa, Environment and Climate Change Canada  https://www.registrelep-sararegistry.gc.ca/virtual_sara/files/plans/mp_snapping%20turtle_e_proposed.pdf

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