The Water-Serpent-Turtle

A recent expedition on a hot, sunny afternoon in Dixie Plantation near Charleston, South Carolina would have been far less interesting without a run-in with a beast that is normally hid in the depths of ponds.  Snapping turtles of the family Chelydridae would be well termed the kings of freshwater turtles, and this encounter reinforced the impression.  Our local representative is the common snapping turtle, Testudines Chelydridae Chelydra serpentina.  I always like to make sense of tongue-twister scientific names, so with my nifty ‘Dictionary of Word Roots and Combining Forms’ (it’s worth getting – check it out here: http://www.amazon.com/Dictionary-Word-Roots-Combining-Forms/dp/0874840538), I could see this name translates to ‘turtle water serpent water serpent serpent.’ Why is this creature known as a water serpent?  It’s easy to speculate that this refers to their long tail, which in hatchlings can be longer than the carapace but in adults is still surprisingly long.  In adulthood, as evident in the turtle I encountered, the tail is also ridged with huge scutes that give this animal a very prehistoric look. 

 This is not the individual I encountered, but you can see the tail length in this picture!

 The sawtooth tail is very prehistoric looking.

Another possible explanation is its biting force, which may conjure up thoughts of old water serpent legends.  Snapping turtles bear by their very name a reputation of being fiercely capable biters, but what makes them so is not as obvious.  Turtles posses anapsid skulls and therefore have no fenestrae available to their jaw muscles for bulging while contracted.  This represents a limitation on muscle size and, subsequently, bite force.  The inner ear of turtles is enlarged when compared with similarly sized reptiles, further reducing available space for powerful jaw muscles.  Another limitation to most turtles is that their head must be small enough to retract into their shell.  Many turtles are able to circumvent the first of these limitations – the anapsid limitation – with deep notches on the posterior dorsum of the skull, giving room for muscle bulges.  These are known as otic or temporal notches.   

But this solutions presents a new problem: the posterior location of the adductor muscles requires that they reach forward towards the lower jaw to do their work.  This puts them nearly parallel to the bone they pull on, resulting in very low efficiency and strength.  Try the kinetics out for yourself by shutting a door from an arm’s length to one side of the door nob instead of straight in front of it.  The difference in the force you will generate is immense.  Chelydrid turtles have an anatomical solution:  the trochlear process.  Trochlea is Greek for pulley, and Chelydrids use a specially positioned bone as a pulley to deliver the force from the back of their heads to the jaws in the most efficient manner possible.  Instead of allowing the adductor muscle to run strait from the back of the head to the lower jaw, Chelydrid skulls position the muscle to keep it high in the skull as it runs forward with the support of a bone protruding out underneath the muscle.  At the position where the muscle has run far enough forward to be nearly directly above the mandible, this bone ends and the muscle runs vertically down to connect to the mandible.  

Combined with a huge head size not designed to retract into the shell housing one heck of an adductor muscle, this mechanism allows snapping turtles to do just that, and do it very well.  Vervost et al. (2011) measured the maximum bite force of 10 inch common snapping turtles at 30 pounds.  It might not seem like a lot, but when considering that force concentrated on the edge of a turtle’s beak, it must be similar to getting your hand slammed in a very sharp door.  But those were 10 inch turtles, and the turtle I encountered was probably 30% larger, while the record common snapper was measured at nearly 20 inches.  This may correlate to an exponential increase in jaw pressure as well.

As I hold the turtle, it gapes and displays those sharp jaws to try to warn me off.

This all begs the question, of course, of why this turtle needs such force.  It is certainly useful in intimidating perceived threats, as I experienced very closely.  But this can’t all be about defense, for other large turtles are effective at defense by keeping their head small enough to retract into the shell.  The logical place to turn is toward this animal’s feeding habits, but information on them seems very elusive.  From my research, most authors cite vegetation, worms, and fish as dietary sources, an explanation I find less than satisfactory.  One study (Punzo 1975) investigated stomach contents from 59 C. serpentina and found snake, turtle shell, and frog fragments, as well as unidentified bones.  It also seems possible that fish larger than can be swallowed in one piece may be eaten, necessitating jaw pressure great enough to cleave a large fish in half.  In any case, we can surmise that Chelydrids need their substantial jaw pressure to efficiently capture, kill, and perhaps dismember their prey.  More research in this area of diet would be helpful in elucidating this issue. On a side note, it is thought that snapping turtles, as sit and wait predators, target slow moving bottom-feeding fish that are 'trash fish' for fisherman, thus tipping the community in favor of other species and actually improving sport fishing.

One of the most fascinating characteristics of this creature’s natural history involves its migrations.  Consulting the Peterson’s field guide, we can see that common snappers occur all the way up into Canada, where their watery habitats freeze over for months at a time.  Snappers can bury themselves in the bottom of a pond and never take a breath for this entire time, reaching a state of true hibernation while their body respires anaerobically.  At times, some oxygen can be gained cutaneously through the neck, but other individuals have a different solution.  Common snappers have been found to travel as much as four kilometers from their summer haunts in ponds to flowing water where the oxygen content is at a maximum and ice may not form at all. 

Another migratory feat this creature can undergo is for the sake of finding a good nest site, and females have been found to travel five kilometers for the purpose.  If you have seen the bulk of this animal and the comparatively short legs, the significance of such a trek won’t be lost on you.

Among the fascinating characteristics of this species, the one great impression I have from my encounter is that of the power of this animal.  I carefully lifted the turtle, supporting its weight with my hand under the plastron while stabilizing the animal by holding its tail.  It literally jerked its tail so hard, slamming my hand against the marginal scutes, that the force of the blunt impact drew blood.  Meanwhile the strong legs and feet raked sharp, talon-like claws over my other hand, forcing me to let the animal down more than once.  On the ground, this individual was so intent on defending itself that it would not even back into the water, instead actually coming toward its assailant (me) in an effort to drive me back from its personal space.  The thrusting neck and snapping jaws were quite effective at intimidating me, and I left with a very healthy respect for the power of this animal and its vitality in defending itself.  Once directed towards the water, however, the turtle plunged in and happily left me to myself, dispelling any questions of it having aggressive intent.  Based on the known growth rates for this species, it's likely the individual I encountered was between 15 and 25 years old.  I can see why, and I wish this survivor many more years.

All in all, I think the common snapping turtle, Chelydra serpentina, has well earned its name as the water-serpent-turtle.

Sources:

Fred Punzo.  1975. Studies on the Feeding Behavior, Diet, Nesting Habits and Temperature Relationships of Chelydra serpentina osceola (Chelonia: Chelydridae)

Journal of Herpetology.  9:2; 207-210

Harvey Pough. 2004. Herpetology, Third edition.

http://www.tortoisetrust.org/articles/snappers.htm

Vervust, B., Brecko, J. and Herrel, A. (2011), Temperature effects on snapping performance in the common snapper Chelydra serpentina (Reptilia, Testudines). J. Exp. Zool., 315A: 41–47. doi: 10.1002/jez.650