The most obvious feature of any tortoise or turtle is it's shell. This is the animals primary defence mechanism against would-be predators , which has remained almost unaltered by two hundred million years of evolution. This basically is an extension of the rib cage , which unlike most vertebrates is housed on the "outside" rather than being inside the body .
The shell is made up two halves, the underneath known as the plastron and the top half known as the carapace. Both parts are fused together at the sides by a "bridge".
The whole shell of a tortoise is actually made up of numerous small bones which are covered by seperate plates of keratin called "scutes". As a tortoise grows , extra layers of keratin are added underneath the existing layer , causing "growth rings". Unfortunately contrary to popular belief an animal cannot be accurately aged by counting these rings. However they can tell us approximately how many spurts of growth the animal has had, thus we could also gague what type of seasonal changes the animal has in it's natural enviroment. Abundant vegetation means more food, which relates to more growth. Sparce vegetation due to extreme climatic conditions would normally mean little food , leading to little or no keratin growth, i.e. when a tortoise enters a state of aestivation or brumation.
Very old tortoises often have extremely worn scutes, giving an almost completely smooth appearance to their shells .
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 paralell to, and at either side of the vertebrals
The "marginals" - which flank the costals and attatch 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, giving the tortoise the appearance of sporting a custom body kit !
The scutes of the plastron are also seperately categorised, of which there are two scutes in each catergory. Starting from the head, moving down to the tail we have ; The "gular", the "humeral", the "pectoral", the "abdominal", the "femoral" and finally the "anal". (Some species have an intergular scute between the two gulars).
Some tortoises and turtles have a flexible "hinge" on their plastron which they can use for extra protection from predators by clamping the carapace and plastron firmly shut, while some females of other species have a much less flexible plastron, but nevertheless flexible enough to move slightly, aiding the tortoise in 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 is as follows ;
Skull and lower jaw mandible (consisting of many small bones which are fused together)
Cervical vertebrae (neck bones)
Dorsal vertebrae (a rib branches off each dorsal vertebrae, which are fused to the carapace)
Humerus (upper fore-leg bone)
Radius and Ulna (lower fore-leg bones)
Carpals (wrist bones of front legs)
Phalanges (digit bones)
Scapula and Coracoid (bones of the pectoral girdle)
Femur (upper rear leg bone)
Fibula and Tibia ( lower rear leg bones)
Tarsals (ankle bones of rear legs)
Metatarsals (bones of the feet)
The muscular system in tortoises is quite different to most other vertebrates. Muscles which are usually used to flex and twist the backbone in nearly all animals are almost completely obsolete in tortoises and turtles, due to the spine being rigid. However they have enormously well developed muscles in their extremely flexible necks, allowing them to retract into the shells.
They also have well developed leg and tail muscles and of course posess considerably powerful muscles in their lower mandibles - if you have ever tried to prize open a reluctant tortoise's mouth, or happened to get a finger trapped at feeding time, then you will have certainly "felt" the full force of these muscles in action.
Although the tortoise has the same digestive organs as most other vertebrates, it has adapted to cope extremely well in severe conditions were 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.
It can achieve this by means of a "hindgut system" which effectively is 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 the valuable water in the bladder and only expelling the waste product in the form of insoluble uric acid crystals, which has a look similar to tooth-paste when passed.
The main difference of tortoise respiration compared to ours is the volume of CO2 it can contain in it's blood . Normally when you hold your breath, the CO2 in the blood makes you want to start breathing again, but tortoises are much more tolerable of this , allowing them to inhale much less frequently. Why ? because if you startle a tortoise it's first reaction is to retract into it's shell and the only way a tortoise can do this is by making room by emptying it's lungs. Thus a frightened tortoise will consequently remain for some time with almost empty lungs. It is the same CO2 tolerance which allows turtles to hold their breath while diving.
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 an animal's carapace varies in accordance with it's 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. While Turkish Testudo Ibera, for example, are extremley melanistic enabling them to absorb more heat.
Also a tortoise's carapace incorporates tiny pores which help to trap in the radiant heat. It is therefore worth noting at this point that an owner should never use any oils on their pet's shell, as this will significantly hinder it's thermoregulation capabilities.
The tortoise's heart pumps blood to all the vital organs and muscle groups, just as ours indeed does, but a large amount of blood is also effectively sent underneath the carapace to "warm up" before continuing to circulate around the body.
An external basking temperature range of between 25 to 35*c is needed to allow the animal to internally thermoregulate it's body temperature to the 30*c required for optimum metabolic efficiency.
As human beings we always tend to be rather anthropomorphistic as far as our pets are concerned. The truth is tortoises unfortunately don't see, hear, smell or think the same as us at all, infact I don't think a human can probably comprehend what life must really be like from a tortoise's perspective .
Tortoises are extremely sensitive creatures. Despite popular belief, they can feel the slightest touch to their skin and to their 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. I can only imagine this to be as painfully similar to having one's tooth drilled without any anesthetic.
Fortunately most people now know better, but if you are in any doubt about how sensitive the carapace is, then I would recommend quietly creeping up to a sleeping tortoise and very gently stroke it's shell............ I bet you it wakes up !
At the time of writing there has been little study on the effectiveness of a tortoises eyesight. We know that tortoises have good all around vision due to having their eyes at the side of the head as oppose to having binocular vision like humans, but we don't 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 particular 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.
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 , they are by no means deaf........
Try clapping your hands near a tortoise and watch the reaction ! The ears themselves have no external auricle and can probably be best described as simple ear "flaps" or "scales" which are located behind the animals eyes towards the rear of the head.
This is the primary sense that a tortoise uses and is considerably more acute than most owners probably realise. A tortoise relies heavily on "smell" for it's daily activities including finding food, finding a mate, finding appropriate nesting areas, smelling for predators, etc . Infact a tortoise uses smell for everything it does !
I once remember a male Testudo Ibera of mine almost cracking it's shell due to continually "ramming" a stone ornament in the garden, this went on for minutes until it was removed..............It hadn't gone mad, it had just confused the ornament for a real tortoise, because an hour earlier another of my collection had sat next to the statue and had obviously left a scent trail which prompted this territorial behaviour.
Despite their strange appearance and "clumbsy" looking way of rambling around, tortoises are in fact very agile , incredible diggers and even better climbers. This is due in part to their excellent sense of balance.
I would imagine the sense of balance becomes even finer as the animal 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 obviously varies from one individual to another. Some of my own stock are nothing short of "mountaineers", being able to ramble through extreme terrain and scale enormous heights, in comparison to their size !