Thoughts on Turtle Anatomy

We cooked the snapping turtle yesterday with very good results, which I will detail in a later entry. Meanwhile, I have some thoughts on disassembling the turtle that I want to jot down here. Prepare for extensive amateur zoology geekery.

I just don't understand the animal's nervous system. People like to say that reptiles keep moving after they are dead because 'they have a primitive nervous system,' but that doesn't really explain anything. Nor do I think that that the word 'primitive' is necessarily a useful or accurate term.

How does the turtle manage to maintain so many basic functions for hours after its entire brain has been removed? Perhaps the better question is 'why can't mammals do this?' I suppose that the heart keeps beating because nothing has stopped it from beating. It beats of its own volition without need of instructions from the brain in the first place. Digestive functions probably continue for the same reason. The movement of the limbs is harder for me to understand. Is it possible that some degree of the movement of the turtle's limbs under normal conditions is locally automatic in response to direct stimulus?

The three-chambered heart seemed large in proportion to the overall size of the turtle. The lungs were slight enough in their deflated state to overlook. The animal had surprisingly few muscles overall. Legs, tail, neck. Entire groups of muscles that I have always taken for granted in quadrupeds were simply missing. The liver appeared healthy, was of a rich reddish-brown color, and was partially fused to the stomach.

A tremendous amount of water was contained in the turtle. There was a sort of gel-like membrane beneath the skin and surrounding many organs, which I would estimate was at least 90% water. Perhaps this helps the animal to regulate its buoyancy?

The digestive system was strikingly simple and efficient. The stomach consists of a long white tube which reminded me in appearance of the large intestine of some mammals. The contents included five or six crayfish (the more recent of which had been swallowed whole), one dace of about two inches in length, and several small stones. Along the length of the stomach, which was roughly six inches, the state of the contents varied rapidly. The crayfish at the top was in very good condition while everything at the opposite end was mashed into tiny pieces, well-digested, and only identifiable by the tips of the claws. In only a few inches of tissue the organ appears to do very thorough work.

I wonder whether the stones were swallowed deliberately in order to aid digestion or if those were accidentally scooped up with the crayfish?

Fat content was minimal. I would estimate that the turtle had no more than 1 or 2% fat. What little fat was present was of a yellowish-orange color, very soft, and easily removed. I suppose that this low fat content is probably owed to the fact that it is April and the turtle was only recently out of hibernation.

The skin was tough and strong and difficult to remove. I noticed that my knives dulled very quickly while working on the snapping turtle. There are a number of small, hard scutes on the skin of the upper parts of the limbs. They appear to be hardened, raised scales. I do not understand the purpose of these projections. They are too sparse to work as armor and not sharp enough serve as weapons. They are not positioned in such a way that would aid grip during mating. Maybe these are luxury organs that signal overall health to potential mates?

'Hey look, I'm so good at obtaining food that I can spare all of this protein and calcium to make pointless scutes on my skin that don't even do anything. My DNA is awesome! Come get some!'

The scutes on the top of the tail are also strange, resembling the tail of an alligator. These are thick and numerous enough to possibly function as armor.

Altogether this is a very impressive design. The snapping turtle is basically just a very simple eating machine that can move into almost any water that isn't fast-moving and eat pretty much any small thing that moves. It can root along the bottom and dig up food there, or allow its rock-like appearance to facilitate an ambush. The animal has pared away almost everything that is not essential. During bad stretches in terms of climate and environment, it doesn't take much for these animals to eke out a living. The only excess is perhaps those little scutes on the limbs, which would probably be easily reduced or eliminated in response to change.

Its easy to see why turtles have been around for around 220 million years. The body plan just works through good times and bad times. They pre-date most dinosaurs and all snakes and crocodiles. Snapping turtles (specifically their family, Chelydridae) first appear in the fossil record about 70 million years ago. They survived massive extinction events that wiped out most of the other species in the world. Snappers evolved in a world that they shared with T. Rex, plesiosaurs, and the tiny mammals that would eventually evolve into humans. They are impressive, simple, and fascinating animals.


[Photo used courtesy of BlackburnPhoto under Creative Commons license 2.0]