shell anatomy

Here are some interesting facts about turtles and their anatomy and physiology…

Turtles cannot climb out of their shell – their shell is part of their skeleton! As turtles evolved, their ribs, sternum and spine fused to form a hard shell.  Since a turtle’s shell is made of bone, a fractured shell is as painful as a broken leg.

The shell is covered by ‘scutes’, which are modified scales. There are nerves and blood vessels in the scutes, and they will bleed if damaged.

The top part of the shell is called the carapace. The bottom part, which covers the belly, is called the plastron. These two halves are joined along the sides at the “bridge”.

There is a lot of variation in the size and shape of turtle shells. Some turtles, like the Blanding’s Turtle, can pull themselves entirely into their shell. Snapping Turtles on the other hand are not very well protected by their shell. They cannot pull themselves inside their shell and their plastron is very small. This leaves them vulnerable to predators when they travel over land.

Some turtle’s shells are not even bony at all. Softshell turtles have a leathery shell instead of a bony one.

Turtles are ectotherms, which has far reaching effects on their care.  Since they are unable to regulate their own body temperature, they need to be provided with the temperature range that is ideal for their particular species.  This is called their ‘POTZ’ or Preferred Optimal Temperature Zone.  This is the temperature range that allows all physiological functions; eating, breeding, moving.  They will not be able to do any of these if not kept at this temperature range. Even their immune system depends on this.  Injured turtles that are not active, are generally kept in a warm room – heat lamps are not a good idea as they will not be able to move away from them and may actually over heat. Their slow metabolic rate means that they are slow to do everything, from healing, to recovering from anesthesia.

While most air-breathing vertebrates use a diaphragm to bring air in and out of their lungs, turtles have no diaphragm. This makes sense, since their rigid shell prevents them from expanding and contracting their chest. The lack of a diaphragm prevents them from coughing effectively, and makes them prone to respiratory issues.  It also means that they need to use pectoral muscles to aid in respiration. Movements of the head/neck and legs, are utilized to bring air into the lungs for aerobic respiration.  Once anesthetized, independent respiration is difficult, and so you always will have to ‘breathe’ for them.  Anesthesia can be challenging, since some of the approaches used in other animals, cannot be used for them.

Turtles have such incredible physiological abilities,  we as humans can learn a great deal from them.  Since they are ectotherms, they cannot function in very cold weather, and therefore need to hibernate.  This allows them to survive the cold winter months without using much energy at all.  Some can even survive full freezing and then thawing! They can survive in almost zero oxygen levels and can switch to anaerobic respiration easily.  They also have auxiliary measures for absorbing oxygen. During hibernation, they also further lower their already low Metabolic Rate.

Turtles can utilize anaerobic metabolism exceedingly well.  To refresh your memory, anaerobic metabolism is the creation of energy through the combustion of carbohydrates, in the absence of oxygen.  In humans, a classic example of this occurs in the long distance runner, or extreme sprinter.  When there isn’t enough oxygen, glucose and glycogen can’t be fully broken down to carbon dioxide and water, and lactic acid is produced instead. In humans this can cause muscle pain, as there is insufficient oxygen to keep up with the muscle’s demands for energy.  In the case of turtles, switching to anaerobic metabolism in low oxygen levels does result in a large buildup of lactic acid, but they are extremely efficient at buffering it, so it does not cause them issues.

Their auxiliary respiratory areas enable turtles to absorb any  oxygen present, via alternatives to the lungs; water flowing over their vascular cloacal (the vestibule where the gastrointestinal, reproductive and urinary tracts exit ) lining accomplishes this oxygen uptake.

From a medical point of view, their superior (although slow) healing abilities make treatment of turtles very rewarding…. they can even regenerate nervous tissue.  Another very useful and phenomenal adaptation turtles have, is the ability of females to store sperm – for years sometimes.  For a species that isn’t particularly social, this is very useful indeed!

Unlike birds and snakes, turtles do  have a bladder.  However, the outflow of the urinary system, reproductive system and digestive system all exit via the same vestibule, called the ‘cloaca’.

Turtles are unique in their ability to survive long periods of  food deprivation.

They can survive situations  that would easily kill a mammal or bird.  Due to their slow metabolism they will not catabolize muscle and fat quickly, and so can withstand a long time without food.  They are the complete opposite to birds, who lose condition extremely fast.  Injured turtles often will not eat for many weeks, and show no weight loss.  In addition, most of our species do not eat on dry land, and so will not eat until they can be placed in water.

Turtles are also very tolerant to blood loss – their low metabolic rate translates into slow bleeding (unlike birds!) and better ability to compensate.  They also have lower packed cell volumes than mammals normally, and can tolerate amounts so low that would require a blood transfusion in other species.

Likewise, turtles can survive  injuries that would require euthanasia in any other species; you cannot use the same criteria for triage in turtles as you would in any other species. Their healing ability truly is remarkable, but they do need a lot of time to accomplish this!

All in all, turtle rehabilitation is extremely rewarding – not only are we carrying out a conservation function, but the percent that can be saved after traumatic injuries is much higher than in other species.