The most obvious feature of any tortoise is the shell. This is the tortoises primary defence mechanism against would-be predators. The shell has remained almost unaltered by two hundred million years of evolution. The shell is basically an extension of the rib cage, which unlike most vertebrates is housed on the “outside” rather than inside the body.
The shell is made up of two halves, the underneath known as the plastron and the top known as the carapace. Both parts are fused together at the sides by a “bridge”.
The whole shell of the tortoise is made up of numerous small bones which are covered by separate plates of keratin called scutes. As a tortoise grows, extra layers of keratin are added underneath the existing layer, causing “growth rings”. Contrary to popular belief, a tortoise cannot be accurately aged by counting these rings. However they can tell us approximately how many spurts of growth the tortoise has had, thus we could also gauge what type of seasonal changes the tortoise has in its natural environment. Abundant vegetation means more food, which relates to more growth. Sparse vegetation due to extreme climatic conditions would mean little food, leading to little or no keratin growth.
Very old tortoises often have extremely worn scutes, giving their shells an alost completely smooth appearance.
The scutes of the carapace are split into five categories;
- The Nuchal – the scute directly above the head
- The Supracaudal – the scute directly above the tail
- The Vertebrals – a single line of scutes which run centrally from the head to the tail
- The Costals – run parallel to, and at either side of, the Vertebrals
- The Marginals – flank the Costals and attach to the “bridge”
The Marginal scutes have a large influence on the overall shape of a tortoise’s shell. In some species, most noticeably Testudo Marginata, the Marginal scutes are extremely flared.
The scutes of the plastron are also separately categorised, of which there are two scutes in each category. Starting from the head moving down to the tail we have;
- The Gular
- The Humeral
- The Pectoral
- The Abdominal
- The Femoral
- The Anal
Some tortoises have a flexible “hinge” on their plastron which they can use for extra protection from predators by clamping the carapace and plastron firmly shut. Some females of other species have a much less flexible plastron, but nevertheless flexible enough to move slightly to aid her egg laying duties.
The skeleton of a tortoise is made up of two parts; the exoskeleton (carapace and plastron) and the endoskeleton (internal bones). The endoskeleton consists of two main groups; the appendicular skeleton (limb bones and girdles) and the axial skeleton (ribs, vertebrae and skull).
A very brief description of the bones;
- Skull and lower Jaw Mandible – consisting of many small bones fused together
- Cervical Vertebrae – neck bones
- Dorsal Vertebrae – a rib branches off each dorsal vertebrae, which are fused to the carapace
- Humerus – upper foreleg bones
- Radius and Ulna – lower foreleg bones
- Carpals – wrist bones of front legs
- Phalanges – digit bones
- Scapula and Coracoid – bones of the pectoral girdle
- Femur – upper rear leg bones
- Fibula and Tibia – lower rear leg bones
- Tarsals – ankle bones of rear legs
- Metatarsals – bones of the feet
The Muscular System
The muscular system in tortoises is quite different to that of most other vertebrates. Muscles which are usually used to flex and twist in the backbone in nearly all animals are almost completely obsolete in tortoises due to their spine being rigid. However, they have enormously well-developed muscles in their flexible necks, allowing them to retract into their shells.
They also have well developed leg and tail muscles, and possess considerably powerful muscles in their lower mandibles – if you have ever tried to pry open a reluctant tortoise’s mouth then you will have “felt” the full force of these muscles in action.
The Digestive System
Although the tortoise has the same digestive organs as most other vertebrates, it has adapted to cope extremely well in severe conditions where food and water conservation is at a premium.
The tortoise can extract and assimilate moisture and nutrients from food items which to the human eye look completely “dried up” and would be of no nutritional benefit to most other living creatures. Tortoises can achieve this by means of a “hindgut system” which is effectively like having two digestive tracts, the latter of which reabsorbs any moisture from the waste products already produced by the former. Arid habitat tortoises can also effectively split up their urinary waste in the kidneys, storing valuable water in the bladder and only expelling the waste product in the form of insoluble uric acid crystals. The crystals have a similar look to toothpaste when passed.
Respiration
The main difference between a tortoise’s respiration and ours is the volume of CO2 they can contain in their blood. Normally when we hold our breath, the CO2 in the blood makes us want to start breathing again, but tortoises are much more tolerable of this, allowing them to inhale less frequently. If you startle a tortoise, its first reaction is to retract into the shell and the only way a tortoise can do this is by emptying its lungs. A frightened tortoise will consequently remain for some time with almost empty lungs whilst in this state.
Circulatory System and Heat Exchange
Tortoises, like other reptiles, are cold blooded. This means they need to seek an external active heat source to keep their body at an optimum temperature range, enabling their vital organs to function properly. Tortoises do this by positioning their carapaces toward the sun (or an artificial radiant heat source in captive situations), a practice which has continued from long before evolution had even considered creating a mammal.
The colouration or “melanism” of a tortoise’s carapace varies in accordance with its geographical surroundings i.e. tortoises from extremely hot places like parts of Egypt and Morocco tend to be lighter in colour, thus reflecting some of the searing heat. Turkish Testudo Ibera, for example, are extremely melanistic, enabling them to absorb more heat.
A tortoise’s carapace incorporates tiny pores which hep to trap in the radiant heat. It’s worth noting that an owner should never use any oils on their pets shell, as this will significantly hinder its thermoregulation capabilities.
Just as ours, a tortoise’s heart pumps blood to all the vital organs and muscle groups, but a large amount of blood is also effectively send underneath the carapace to “warm up” before continuing to circulate around the body.
An external basking temperature range of between 25-35°c is needed to allow the animal to internally thermoregulate its body temperature to the 30°c required for optimum metabolic efficiency.
The Senses
Touch
Tortoises are extremely sensitive creatures. Despite popular belief, they can feel the slightest touch to their skin and shells. It was once thought that a tortoise’s carapace was void of any nerve endings, and as such horrific acts were often carried out and even recommended by media and literature of that time. This included drilling holes through the shells and tethering the animals.
Sight
At the time of writing, there has been little study on the effectiveness of a tortoise’s eyesight. We know that tortoises have good all-around vision due to having their eyes at the side of their head as oppose to having binocular vision like humans, but we do not know how sensitive or acute their vision actually is.
It is thought that tortoises certainly use their eyes to catch movement but perhaps have difficulty picking out detail. Some tortoise owners insist that their pet is fond of certain colours, often red. Although whether it is an actual colour preference or whether the animal is merely associating it with a favourite food item is open to debate.
Hearing
Numerous publications have tried to give the impression that tortoises are virtually deaf, although it is fair to say that their hearing is significantly different to ours and perhaps less sensitive to high frequency sounds, but they are no means deaf.
The ears themselves have no external auricle and can be best described as simple ear “flaps” or “scales” which are located behind the tortoise’s eyes towards the rear of the head.
Smell
This is the primary sense that a tortoise uses and it considerably more acute than most owners realise. A tortoise relies heavily on scent for its daily activities including finding food, finding a mate, finding appropriate nesting areas, smelling for predators etc. A tortoise uses smell for everything it does.
Balance
Despite their strange appearance and clumsy looking way of rambling around, tortoises are in fact very agile. They are incredible diggers and even better climbers; this is due, in part, to their excellent sense of balance.
The sense of balance becomes even finer as the tortoise matures. Hatchlings observed in captive situations always notoriously seem to end up on their backs, while adults seem to be sturdier on their feet, although this does vary from one individual to another.