Added February 2004. Last updated 18 January 2011: updated page with further information, including Reproduction section, and Bibliography.

Turtles, tortoises and terrapins

An anatomical guide

Introduction

This page is intended to be a guide to the various parts of the chelonian anatomy. Firstly, it will help you to know what different books and guides are talking about when they use words like scute, marginal or plastron. Secondly, if you have a problem with your pet chelonian, knowing the parts of the body may help you to explain the problem more quickly to a veterinarian (assuming he or she knows anything about reptiles, which is by no means always the case).

DISCLAIMER: all information given here is given in good faith, having been researched from what we hope and believe are reliable sources (see Bibliography). Reading this guide should help you to understand tortoises and turtles better, but it will not qualify you to carry out any of the procedures normally performed by a vet! We accept no responsibility for any loss, damage or injury arising from such cases. Remember, if you have a turtle, tortoise or terrapin pet, the best thing you can do is (a) read up everything you can about the species (singular or plural) that you have, (b) be conscientious in your maintenance of the animal(s) through proper feeding, cleaning, etc, and (c) make sure you know of a suitable veterinarian who can be reached if problems do arise.

The shell

The most obviously distinctive part of any tortoise, turtle or terrapin, and which marks it out immediately as a different reptile to lizards, snakes and others, is the shell.

This is divided into two parts: the flat or flattish underneath (the plastron), and the top part, which is usually domed to a lesser or greater degree (the carapace).

The carapace is divided up into three rows of large scales, or scutes, which run around the shell. The top row, which is situated more or less in the centre of the carapace, consists of six (usually) vertebrals. Below the first vertebral is the scute directly above the neck, the nuchal. Running around the shell on either side of the nuchal are the marginals, which contact the sixth vertebral at the back of the shell. The remaining scutes (those between the marginals and the vertebrals) are the costals.

The plastron is divided in a different manner. It consists of a left and right half, each of which is divided into six (usually) plates. The plates are symmetrical, ie there is a similar plate on both sides. At the front of the plastron below the neck (and sometimes head) are situated the foremost plates, the gulars. These are followed by the humerals, the pectorals, the abdominals, the femorals and finally, the anals.

The lines of contact between different scutes or plates are known as sutures.

Patterns, markings and colours on the shell may also help to identify different species, although it should be noted that coloration in chelonians is much less varied than among squamates (snakes and lizards).

Limbs

All chelonians have four limbs. A further differentiation between different species is the shape of the limbs, whether or not they carry claws, and if so, how many. Webbing may be present on the limbs of some aquatic species, and in the case of marine turtles, two or all limbs may be modified into paddles.

Although maybe not strictly speaking a limb as such, the tail (its presence or absence, and its length) is also used in the description of chelonians.

Head

Although the head appears very similar in virtually all species of chelonian, there are again subtle differences. Some species have heads that cannot be retracted at all into the shell: some may compensate for this with extra protection on the head itself, or by simply being more belligerent generally. Others draw their heads in either directly backwards or sidewards.

Bones

In much zoology, vertebrate species are not defined on the basis of their outward shape and appearance but on their bone structure. Chelonians have a vocabulary of their own in this area, especially regarding the structure of the shell, which is made up below the surface of a large number of bony plates. As a rule, these correspond roughly (though not exactly) to the areas covered by the scutes.

The shell

The bones of the carapace are as follows. The proneural is the plate situated behind, but greater in size than, the nuchal. Behind this a row of eight (usually) neurals form the top of the shell, laying roughly beneath the vertebrals. They are associated with the spines of the vertebrae below. Behind these in turn lay two suprapygals (one behind the other) and the pygal at the back. On each side of the neurals is a row of eight pleurals, covered roughly by the costals. These are associated with the ribs. Finally, running around the bottom of the carapace between the proneural and the pygal on either side is a row of eleven peripherals. In addition, a typical carapace has an axillary buttress between the third peripheral and the first pleural, and an inguinal buttress between the seventh peripheral and fifth pleural. These are smallish plates.

The bony plates of the plastron are divided in a similar manner to the scutes, ie into pairs made up of left and right plates. Thus from the front of the plastron we find the left and right epiplastron; the left and right hyoplastron; the left and right hypoplastron; and at the rear, the left and right xiphiplastron. In side-necked chelonians (Pleurodira) there is an additional single plate in the front centre of the plastron, the entoplastron; this is completely surrounded by the epiplastrons at the front and the hyoplastrons at the back. Along each side of the plastron, running along the hyoplastron and hypoplastron, is the bony structure of the bridge, which effectively joins the plastron to the carapace on each side. There may be an axillary notch at the front of the bridge.

Some species have flexible areas, or hinges, in the plastron or occasionally the carapace, allowing one or both ends to be closed up. See the different species accounts for more details.

The chelonian skull

The skull of chelonians is one of the features that sets them apart from the other living reptile groups. Snakes and lizards are diapsid, that is to say, they have two openings on each side of the skull. Chelonians are anapsid, which means that they have no temporal openings, but rather indentations on the posterior or lower edge. These indentations either extend out from the back edge of the skull (as in the Cryptodira and Pelomedusidae) or up from its lower edge (in the Chelidae).

The skull in contrast to other reptilian families is short, massive and strongly arched [Grzimek]. Another feature of the skull that sets it apart from squamates is that the quadrate is firmly connected to adjacent skull bones, whereas the quadrate of lizards and snakes is moveable.

Vertebral column

The vertebral column has 8 cervical, 10 trunk and 18-33 tail vertebrae. The form of the cervical vertebrae is very variable and usually related to whether the animal is a cryptodire or pleurodire. Cryptodires have two sacral vertebrae (19 and 20) with broadened ribs that meet the ilia of the pelvis, whereas in pleurodires the pelvic girdle is rigidly connected to the dermal carapace by the ilia dorsally and the pubic and ischial bones ventrally, and the sacral region of the vertebral column is less distinct.

Other skeletal parts

The pectoral girdle has become greatly modified, but the pelvic girdle is more primitive. Another difference between the Cryptodira and the Pleurodira is that in the latter the pelvic girdle is rigidly connected to the plastron as well as the carapace.

Internal organs

As might be expected, the structure of chelonians has caused some modification to the internal organs.

Circulatory system

The organs of the circulatory system (eg the heart) have remained basically the same as in other reptiles.

Respiratory system

The lungs are more unusual. These are divided into simple chambers and lay directly beneath the carapace. They cannot be expanded by the action of the rib muscles (due to the rigidity of the shell) so special paired lung muscles are attached to a sort of diaphragm: these muscles press the air out of the lungs when they contract. In a relaxed state the lungs are filled with air - the opposite of what occurs in most other animals. (Think of how you breathe in air when your chest expands). Furthermore at least some aquatic turtles can use the technique of moving air from one part of the lungs to another to displace their centre of gravity, thus using the lungs essentially rather like a swim bladder. Lung problems such as disease cause the loss of this capability. Deep-diving marine turtles have further modifications in that their lungs are embedded in bony chambers formed by projections from the inner wall of the shell.

Apart from the lungs, other parts of the body assist in the exchange of gas (and hence oxygen intake) in the chelonian body. Depending upon the species, this can be pharyngeal tissues in

Digestion

The chelonian stomach often contains stones which are picked up deliberately by both aquatic and terrestrial species. This behaviour is also known in crocodilians and birds and seems to facilitate the breaking down of food, although one might consider that in the cases of both aquatic turtles and crocodilians it may also be to counter buoyancy.

The small intestine in aquatics is relatively short (as these are primarily carnivores, by and large) but very long in land tortoises, in whom it may be several times their body length and digest up to 30% of cellulose consumed [Grzimek].

Excretion

The hindgut and urinary and reproductive ducts all open into the cloaca, as is common in reptiles.

Senses and nervous system

The brain of the chelonian is small but fairly highly developed, especially in the areas of vision, smell and equilibrium. In the brain itself there is no parietal opening but the pineal body is relatively large. Chelonians have a reputation for reasonable intelligence among reptiles and some are are able to learn their way around a maze about as quickly as a rat (Larson et al, 2001).

The eyes have very good resolution and are protected by thick eyelids. Chelonians can see colour: their range of visual wavelengths is shifted towards the red end of the spectrum so they see a small range of infrared waves invisible to the human eye. Among other capabilities they can distinguish between different shades of colour and aquatic turtles underwater can discern a man approaching on the shore. The retinae of aquatic turtles are adapted for vision in cloudy surroundings (ie the underwater environment of a river or pond). Cheloniidae (marine turtles) and the Diamond-Backed Terrapin Malaclemmys terrapin, both of which live in saltwater or brackish environments, have specialised tear ducts to help control the body's salt content.

The sense of smell is the other extremely strong sense. Chelonians tend to sniff (literally) at most food. The Jacobson's Organ of chelonians is analogous to that of other reptiles but constructed differently.

By contrast, there is discussion over whether chelonians can hear anything at all or are completely deaf, like snakes. They do have a hearing mechanism, but anecdotal tales of tortoises coming at their owner's call probably owe more to the animal seeing the owner, or associating the appearance of the owner with feeding time.

Reproduction

All chelonians, like crocodilians but unlike some lizards and snakes, are oviparous (egg-layers). Courtship varies between persuasive and rather rough (Testudo males may repeatedly butt the shells of females, while some tortoise males may fight for the right to breed). The act of mating is normally the only time that tortoises utter a sound, a grunting or roaring: otherwise all chelonians are mute. Fertilisation is internal, as in all reptiles. As with other reptiles, the determination of sex in incubation may be determined by temperature within the nest, at least within most chelonian families. Once the nest is constructed or more usually dug and the eggs laid, the female abandons them. Marine turtles are constrained to return to land to lay their eggs after mating at sea: this is the only time that they return to land, spending the rest of their lives at sea.

Daily and seasonal activity cycles

Typical daily activity times vary among species, but as a rule those chelonians which are primarily or wholly terrestrial are mainly diurnal, although a few tortoises such as Indotestudo elegans or Gopherus species are more crepuscular. This may have something to do in the case of the latter with the high daytime temperatures of their desert environment. Most aquatic turtles are also diurnal, but the snake-necked chelids and the matamata are crepuscular and/or nocturnal. Marine turtles are diurnal and sleep drifting on the waves at night.

Seasonal activity varies with the species and more especially with the geographical location. For example, chelonians in temperate or cold parts of the world, such as the northerly parts of America or most of Europe, usually hibernate in the winter or at least reduce their daily activity, whereas in the deserts or hot savannah chelonians may instead aestivate (a similar period of inactivity during hot weather) during the dry season. In tropical humid areas there is usually little need for either hibernation or aestivation unless there is a marked dry spell. Hibernating animals usually reduce their food intake in autumn and finally stop a couple of weeks or so before finding or making a place to hibernate. Many choose empty burrows, but some tortoises simply dig themselves into the earth and many aquatic animals likewise settle down into the mud at the bottom of their watery environment. Agrionemys horsfieldi has long tunnels which it settles into during both the harsh winter and the broiling dry summers of its steppe environment. For many hibernating animals, hibernation is a vital part of their reproductive cycle since it allows the body to prepare for mating the following spring. Aestivation is usually a less intense matter than hibernation, but aestivating animals tend to look for similar places to spend this period.

Bibliography

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