Colours of Wildlife: Leopard Tortoise, Part 2
Created | Updated Sep 28, 2014
Leopard Tortoise, Part 2: Tricks and twists of evolution
Still, the shell of a tortoise protects it in most circumstances. Without it, these slow reptiles would not still be surviving and flourishing all over the world. How this shell evolved is not yet fully understood. Something quite remarkable has happened along the course of this evolution. Regular reptiles, indeed all regular land-living vertebrate animals, have ribcages around mainly their hearts and lungs, to protect these, and on the outside of their rib cages they have their shoulder and hip girdles which support the bones and muscles of their limbs. Ribcages are flexible to allow the lungs to expand and contract as animals breathe. But for tortoises this set-up is quite different. The ribcage has become totally rigid, fused to the backbone, and supports the bony shell, the ribs themselves lining the inner surface of the shell, with hard, flattened bone derived from the skin lying over these to form the external shell. The ribcage also now no longer surrounds only the lungs and heart, it surrounds the entire body. Most strange of all, the pectoral and pelvic girdles which support the front and back legs, have moved to a position inside the rib cage! This presents an amazing evolutionary puzzle. What strange twists and turns did the limb girdles have to perform in order to end up inside the rib cage? And how was it possible for them to go through all these changes and contortions while still leaving the animals able to move through all the intermediate stages?
For clues, we need to look at the evolutionary history of tortoises and turtles. They are a very old group. It's not entirely clear which group of early reptile-like things turtles and tortoises evolved from. One of the earliest turtle ancestors known was Odontochelys, living about 220 million years ago, which did not yet have a full shell like its modern descendants. It did have flattened ribs overlaid with bony plates on its upper surface, but these were not yet tightly connected. It did have a well-developed bottom shell which in modern turtles is called the plastron.
Studying Odontochelys and the muscle arrangements in modern turtles and tortoises, it becomes clear that a neat trick has occurred. It is hard to move the scapula into the rib cage, with the configuration of most land vertebrates where the scapula is over and at the back of the forward part of the ribcage. In Odontochelys, the ribs retracted backwards, away from the scapula. From there, the ribs were able to expand sideways without the scapula getting in the way, and finally, the ribs moved forwards again, but this time going outside of the scapula, surrounding it. (Of course this process happened over many, many generations and a whole slew of species from Odontochelys to the first turtles with conventional modern-style shells around the end of the Triassic.) The muscle attachments that connect the scapula with the ribs as well as with the limbs all were retained, but some of the arrangements flipping inside-out so to speak along the way!
Today, leopard tortoises are still common and widespread. In South Africa, they are frequently seen in nature and game reserves, and often outside the reserves as well. They can survive anywhere there's a bit of vegetation to munch, and where humans will tolerate them. Fortunately, humans do generally tolerate them. Indeed they're a welcome sight to most people! Unlike most other wild animals, they can be approached closely, even touched, although of course it is best not to upset or pester them. A tortoise when closely approached will withdraw its head and limbs into its shell, but if you wait just half a minute or so, it will come out again, and if you remain quiet and a short distance away, will resume with its business. Even if it walks away, you can easily keep up with it.
Many people also keep these tortoises as pets. They can be kept semi-wild in a yard, free to roam around. So long as there are veggies to chew on, they will probably stay. They're also kept in just about every zoo or animal park over here. Here you see a picture of a few individuals I photographed in the Bester Bird and Animal Zoo in Pretoria. One of these tortoises is much bigger than the others, and its favourite pastime seems to be ramming into them and turning them on their backs! A tortoise turned on its back is not quite as helpless as it looks; it can indeed in time right itself again.
