Shell Suits

hawksbill turtleThe colours and patterning seen on the shells of tortoises and other chelonians can be very appealing. Unfortunately, this has not always been of benefit to the reptile. Hawksbill turtles (Eretmochelys imbricata) have been hunted in the world’s oceans for their exquisitely mottled shell for thousands of years. The outer layers or scutes were sliced up and made into hair combs, furniture and other decorative objects - described as tortoiseshell. This became particularly fashionable during the Victorian era, threatening the survival of the species.

The structure of the shell of chelonians is remarkable. Whereas our backbone is largely fused right along its length to provide us with support, their neck and tail vertebrae are highly flexible. In contrast however, the length of their vertebral column encased in their shell is immovable, being incorporated into it. 

Shell structure

The strength and rigidity of the shell comes from an inner bony casing, on top of which are the often colourful outer scutes, comprised of keratin - similar to our own fingernails. Extra support is build into the shell because the joints in the bony layer do not correspond to those in the scutes above. There can be as many as ten bony plates in the centre of the carapace, covered by just a single scute.  At the outer edges of the shell where the bony sections are larger however, the situation is reversed, so that  a single bony plate may be overlaid by two scutes.  

The shell is very much alive, having both a blood supply and nerve endings.  As a result,  an injury to the shell is likely to result in bleeding, while the chelonian can feel when its shell is being touched. The scutes themselves grow from beneath, which is why a pattern resembling growth rings appears, being especially obvious in land chelonians. But do not be misled into believing that each ring constitutes a year of the reptile’s life, rather like the those in a tree. It is quite usual for a number of these rings to be apparent in the case of a young tortoise well before it is a year old.

gopher tortoiseWhat can be said with more accuracy however, is that broad spacing tends to indicate a period when the reptile was feeding well. On the other hand, the rings may become worn away in the case of species such as the gopher tortoises (Gopherus species) of North America, as the reptile rubs its shell on the sides of the passageway of its subterranean home. They also tend to become more obscured in older tortoises.

Red-eared sliders (Trachemys scripta elegans) and similar terrapins differ from their land relatives by shedding their scutes as they grow. This is why basking facilities are very important for this particular group of reptiles, enabling the shell to dry and encouraging the transparent scutes to detach cleanly from the new growth beneath.

Do not be tempted to pull the scutes off when they become loose at the edges as this could be damaging. They will be come away naturally in due course. Basking out of the water is also important for these terrapins because it seems to slow the development of algal growth on their shells. This can otherwise penetrate beneath the scutes, causing them to become deformed.

What the shell reveals

Much can be learnt about the lifestyle of a chelonian from the appearance of its shell.  Tortoises have particularly domed shells, compared with aquatic chelonians, and so have a larger lung capacity. This type of shell is also likely to be harder for a potential predator to seize in its mouth.  As a result, the slow-moving tortoise can retreat with relative safety into its shell and wait for the danger to pass.

pancake tortoiseThe only exception is the bizarre pancake tortoise (Malacochersus tornieri) which lives in rocky areas of East Africa, where its flattened body shape and lighter shell enable it to retreat under stones and escape predators. It can almost sprint in comparison with other tortoises. This alteration in shape would appear to be achieved at some cost however, since pancake tortoises only produce clutches comprised of single eggs, rather than a dozen or more.

In the case of aquatic chelonians, those which spend some time in water but also roam on land, like the Malay box turtle (Cuora amboinensis), can be recognised by the relatively domed shape of their shells. Species which are almost entirely aquatic have much flatter shells. This enables them to swim with far less effort, as the more streamlined shape of the shell encounters less water resistance.

The side-necks such as the New Guinea snake-necked turtle (Chelodina novaeguineae) fit into this category. The long neck of these turtles enables them to rest on the bottom and obtain air without difficulty by raising their nose to the water’s surface.  A similar strategy is used by the soft-shelled turtles, belonging to the genus Trionyx, although in this case, it is not the neck but rather the nose which is elongated, allowing them to snorkel unseen below the surface. 

Big heads

In the case of both the side-necks and the Chinese big-headed turtle (Platysternon megacephalum), the head is vulnerable to attack. Side-necks are forced to curl their long necks around the edge of the shell, tucking their head in here if danger threats. Their necks are simply too long to be drawn into the shell itself, while in the case of the Chinese big-headed turtle, the head is too big to be retracted. Instead, it is reinforced with bone, with additional protection from a tough scute on top.

The soft-shells have sacrificed the protection of a tough outer keratin layer during the course of their development, having leathery skin here instead. Beneath, there are bony struts rather than a solid casing of bone. This means their bodies are lighter so that swimming is easier, but they are far less well-protected against predators as a result.  Even so, soft-shells are a highly successful family, with representatives of the group present in North America, Africa and Asia. They compensate from this lack of protection by aggression and increased mobility, not hesitating to bite if they feel threatened.

jagged shell turtleOther turtles have evolved more passive ways of escaping danger, such as the bizarre cogwheel or spiny turtle terrapin (Heosemys spinosa) from rain forest areas of south-east Asia. The unusual name of these turtles stems from the sharp projections all around their shells, which are especially pronounced in young individuals. These are thought to be a deterrent to snakes, making it hard for them to swallow one of these turtles. In other species, such as the jagged shell turtle (Cyclemys mouhoti) (left), similar spikes are restricted to the rear of the shell, probably helping to protect against an unexpected ambush from behind.

Colouration

What to us may simply appear to be conspicuously attractive shell coloration can provide vital camouflage. The leopard tortoise (Geochelone pardalis)  is one of the most attractively patterned tortoises from the African continent, thanks to its contrasting cream and blackish brown shell markings. On a green lawn, it will be obvious, but in drier African grassland, this variable patterning breaks up the tortoise’s outline to the extent that it blends very effectively into the background. This is described by zoologists as “disruptive camouflage”.

matamataBeing dull can have its advantages too, in a survival sense. The African mud turtles (Pelusios species) are essentially blackish-brown - what colour could be better to merge into a pool? The same appliers in the case of the Alligator snapping turtle (Macroclemys temmincki), the largest freshwater species in North America. It simply lies in wait, luring its prey with its worm-like tongue, while further south, the mata mata (Chelus fimbriatus) of the Amazon region (right) also relies on camouflage, resembling a submerged piece of wood, remaining still and striking once prey comes within reach.  

The appearance and structure of the shell has not surprisingly played a key role in the survival of these ancient reptiles. What to us may simply be seen as pretty or bizarre can represent the difference between life and death for a chelonian in the wild.