Exploring Tortoise Anatomy: A Detailed Guide

Tortoises (and their water-loving relatives, the turtles) have unquestionably strange body composition. They have thick, elephant-like legs, a parrot-like beak, and, of course, large shells. They’re arguably the most oddly constructed vertebrates in the entire animal kingdom. 

The unique composition of tortoise anatomy obviously work for them – after all, tortoises have been walking around on the planet for tens of millions of years. As keepers, it’s our job to learn about the unique anatomy of tortoises so we can better understand them and provide the care they require and deserve. 

In this article, I’ve detailed the anatomy of tortoises, including their internal and external features and how this affects their day-to-day lives. 

First Thing’s First: Let’s Talk About That Shell

Many people believe that tortoises “wear” their shells, much like hermit crabs do. But this is a misunderstanding because they are in fact connected. Their shells are part of their bodies and thus they cannot slip out of their shell as is often portrayed in cartoons. 

There are essentially three parts of the shell: the dorsal (or upper) section is called the carapace, the ventral (or lower) section is called the plastron, and the area that connects the two is called the bridge.  

The carapace is essentially the tortoise’s rib cage. But instead of looking like the rib bones of most other animals, tortoise ribs are flattened and somewhat plate-like. Each rib touches the ribs adjacent to it, and they all combine to form the carapace. 

Like the ribs of other animals, the tortoise’s ribs (and therefore the carapace and greater shell) are connected to the tortoise’s spinal column. 

The plastron bones are also flattened, but they fuse with the tortoise’s pelvis and sternum. And unlike the bones of the carapace, which are clearly modified ribs, scientists debate the exact bones from which the plastral and bridge bones are derived.

Externally, the shell bones are covered in a substance called keratin, which is similar to the material that makes up human fingernails. The keratin grows as thin, platelike scales called scutes. The scutes vary in shape and number, but most tortoises have 13 scutes on the top of the carapace, 12 scutes on the plastron, and 22 scutes around the margin of the carapace.

Interestingly, the scutes tend to overlap the seams of the shell bones, which helps provide additional strength.   

The Rest of the Tortoise’s Skeletal System

The rest of the tortoise’s skeletal system possesses a number of characteristics that keepers would find relatively familiar. 

For example, they possess the same basic types of arm and leg bones that humans do. Their front legs each include a humerus, radius, and ulna, while each rear leg includes a femur, tibia, and fibula. These bones are, however, much shorter and slightly curved in tortoises, and they’re also much thicker relative to their length. 

Tortoises also have neck vertebrae that are roughly similar to those of most other animals, except that they are more flexible and retract farther than most. This allows tortoises to withdraw their heads into their shell when confronted with predators. At the opposite end of the vertebral column, tortoise tails also contain bones.  

Tortoise skulls lack the holes (termed fenestra) that many other reptiles, such as snakes and lizards, possess. This has the effect of making the skulls less flexible but more robust. Though some prehistoric tortoises possessed teeth, no modern tortoise does. Instead, they have a bony beak. 

Perhaps the most unusual aspects of the tortoise’s skeletal system are the shoulder bones and pelvis. Both components are highly modified, but most importantly, they’re located within the rib cage – a characteristic without equality in the animal kingdom. 

Appendages: The Legs, Feet, and Tails of Tortoises

Tortoise legs are very strong, thick, and pillar-like. This helps the animals carry their massive shells while still possessing the flexibility to move in the ways they need to. 

Tortoise feet appear as largely undifferentiated extensions of the legs. Scientists called the feet elephantine, thanks to the way they resemble the feet of elephants. The foot and toe bones are found within these rounded feet, with only the nails visible on the outside.

Tortoise tails are fairly short, though they often differ in length by species and sex. Generally speaking, males have much longer tails than females. The cloacal opening of the tail is also positioned closer to the tail tip in male tortoises. 

The Tortoise’s Unusual Head

Tortoise heads are fairly rectangular in shape – they’re often described as blocky.

The eyes are located on the sides of the head. Each eye is relatively large (especially in hatchlings) and possesses an eyelid. This distinguishes tortoises from some other reptiles, such as snakes, and it gives them a very endearing appearance. 

It’s worth noting that tortoise heads do narrow slightly toward the front. This provides them with some degree of forward-facing vision. However, they don’t have a great deal of binocular vision, as there is only a small amount of overlap between the visual field of each eye. 

Tortoises do not have a distinct nose like many animals. Instead, they merely possess two small nostrils located at the apex of the snout. 

The only externally visible ear structure is the membrane-like tympanum, which is analogous to a human eardrum. It vibrates when stimulated by sound, ultimately transferring the vibration to the liquid contained within the middle ear, located immediately beneath it. 

The tortoise’s beak is covered with keratin and forms a structure called a rhamphotheca. 

Processing Food: The Digestive Tract of Tortoises

The digestive tract of tortoises largely mirrors that of other reptiles. As food is consumed, it passes from the mouth and through the esophagus before arriving at the stomach. From there, it passes through the intestines, where secretions from the pancreas and liver aid in digestion. 

However, there are some differences from this point. Tortoises possess a chamber-like structure called the cloaca. The cloaca is the final receptacle for the products of the digestive system, as well as those originating in the urinary tract and reproductive system. 

Waste accumulates in the cloaca until it is eventually expelled. 

Processing Information: The Central Nervous System of Tortoises

The tortoise’s nervous system is essentially similar to that of most other vertebrates. It consists of the brain, spinal column, and nerves. These nerves detect stimuli which are then passed back through the nervous system to the brain, and they also receive signals passed from the brain to the muscles. 

It is worth noting that while tortoises are generally not considered especially intelligent by vertebrate standards, they do possess fairly impressive brain-to-body-weight indices. 

One very important thing tortoise keepers must understand is that nerves do extend into a tortoise’s shell. This means that tortoises can feel when their shell is touched. This can be a good thing at times, as some tortoises enjoy having their shells touched or lightly scratched. But it also means that tortoises can feel painful stimuli through their shells. 

Oxygen and Carbon Dioxide Processing: The Respiratory Systems of Tortoises

The respiratory system of tortoises is largely similar to that of most other vertebrates. Air is drawn in via the nostrils, and then it passes through a structure called the glottis before entering the trachea. 

However, the trachea connects to the branching bronchi more quickly than occurs in many other animals. This is thought to be an adaptation that allows the tortoise to withdraw its head into the shell more effectively. Once in the paired bronchi, air continues into the large, spongy lungs of tortoises. 

Another important distinction between the respiratory systems of tortoises and some other vertebrates is the lack of alveoli. Tortoises lack these structures, and instead have honeycomb-like lungs. This makes their lungs less efficient than the lungs of many other animals. 

Additionally, tortoises lack the muscular diaphragm many other animals possess. This, combined with the presence of a rigid shell, means that they have to draw air into the lungs and force air out in a different manner. 

Believe it or not, they use their legs to do so! 

When a tortoise needs to inhale, its legs move outward slightly. This reduces the pressure in the body cavity, thereby pulling air into the lungs. When they need to exhale, they pull their legs in slightly, which causes the opposite effect.  

This is another way in which the respiratory systems of tortoises are not as efficient as those of many other animals. However, because tortoises only need to breathe about four to eight times per minute, it serves their purposes well. 

The Circulatory System

The circulatory system of tortoises is connected to and works in conjunction with the respiratory system, as occurs in most other animals. By and large it also resembles that of other animals, except that tortoises have a three-chambered heart, rather than the four-chambered heart mammals possess. 

Instead of having two atria and two ventricles, tortoises only have a single ventricle. However, the ventricle is subdivided internally, and some species are thought to be able to keep oxygenated and deoxygenated blood separate despite not having separate ventricles.  

Filtering Blood: The Urinary and Renal System of Tortoises

Tortoise blood is filtered by two kidneys, which are flattened and positioned against the underside of the tortoise’s carapace. The kidneys remove toxins from the bloodstream, as well as the byproducts of protein metabolism. The resulting mixture contains watery urine and a paste-like substance called urea. Both substances are passed through paired structures called ureters and then to the urinary bladder, where they’re stored until the tortoise urinates. 

The bladder of tortoises is quite large and bifurcated when full, which enables it to contain quite a volume of urine. Tortoises also have accessory urinary bladders, which can also contain urine. 

The Reproductive System

The reproductive systems of tortoises exhibit a number of differences from the reproductive systems of other animals. 

Male tortoises differ markedly from squamates (an evolutionary group containing snakes and lizards), in that they do not possess paired intromittent organs. Instead, tortoises have a single penis like most other vertebrates. 

However, tortoises carry their penises internally inside the cloaca. They only evert them when attempting to mate. The exact size and shape of a tortoise’s penis varies by the species, but some are quite large – sometimes shockingly so. They also bear unusual shapes, which are sometimes likened to an “opening flower.” 

Tortoises engage in internal fertilization, so during mating, the male tortoise will insert his everted penis into the cloaca of a female tortoise, where sperm will be deposited.  

Female tortoises have paired ovaries, like most other vertebrates. Eggs are produced here, and then released into two structures called oviducts. Here, the eggs are fertilized and coated in calcium to create their hard shells. 

When the eggs are ready to be deposited, they’re released from the oviducts and travel to the cloaca. They’re then expelled, typically into a hole in the ground the female tortoise has pre-prepared. 

Citations

• British Chelonia Group – Tortoise Respiratory Disease
• Murray State University – Urogenital System of Tortoises
• Nature – Anatomy of the Tortoise