How Do Skeletons Affect Animal Lifestyles?
These museum collection objects illustrate how different types of skeletons help define how an organism looks and lives.
A skeleton is a supportive framework for the body of an animal. Skeletons provide protection and support, while allowing for movement and growth. Skeletons of different animals can look quite different: A skeleton may be inside an animal’s soft body (internal), outside an animal’s soft body (external), or a combination.
How do skeletons support and protect a creature while allowing the animal to grow and to move? What does each type of skeleton mean for an animal’s lifestyle? Click on each object below to compare and contrast different types of skeletons.
Internal and External Skeletons
All backboned animals (vertebrates), including us, have internal skeletons (see the internal skeleton of the toad). Most animals without backbones (invertebrates) have external skeletons (see chiton, insect, sea biscuit), although some invertebrates have internal skeletons, too, although not made of bone like ours. They may be made of cartilage (like what connects our bones), cuticle (fingernail material), keratin (hair material), or even silica (ingredient of glass).
Then, you have the odd cases of animals for which you could argue that the skeleton is both internal and external! For example, a turtle skeleton is both the internal bones and the external shell, while a glass sponge skeleton is a framework around which web-like soft tissue is spread.
Whether internal or external, skeletons can be made of different materials. They can be bone, cartilage, or in the case of shells, calcium carbonate. Insect exoskeletons are made of chitin, which is similar to our fingernail material. Some sponges have skeletons made of glass.
Rigid and Flexible Skeletons
Comparing the skeletons across animals reveals a tradeoff between support and flexibility for movement.
Thinking about the human skeleton, our bones are rigid, but where we need flexibility we have cartilage (e.g., in ears and knees). Our bones are connected at joints that allow for movement, whether walking, swimming, or other types. Cartilage in our joints absorbs shock and allows efficient movement. Insects, which have hard exoskeletons, can move as well, thanks to jointed legs.
Flexible skeletons are important for animals that have to bend, either for swimming (eels) or withstanding waves (sea fans). Rigid skeletons are important for animals that have to sustain a lot of weight or grow really large (elephants).
Something with a hard skeleton and no joints is not likely to move around much or at all. Many such animals (like glass sponges) anchor to whatever substrate they live on and just stay there. Some (e.g., sea biscuit, chiton) can stick little soft “feet” out of their skeletons that allow them to move around slowly.
When we think of skeletons, we usually think of bones. But, many animals, such as earthworms, jellyfish, and leeches, have hydrostatic skeletons. Picture a body like a balloon. Muscles running along the length of the balloon contract to make it shorter and wider. Circular muscles surrounding the balloon contract to make it longer and skinnier. Compare it to compressing a balloon from two different directions at right angles to each other. As long as air doesn’t leak out, squeezing the balloon in one direction will make it get fatter in another. An advantage of this type of skeleton is that it makes for a very flexible body.
Animals with hydrostatic skeletons commonly have multiple chambers, such as the segments of an earthworm and the body of a sea anemone. Because each chamber can expand and contract independently, the worm can make complex, burrowing movements and the sea anemone can rapidly contract when it feels threatened. Even in animals with a stiff skeleton, hydrostatic pressure may be used in parts of their bodies that need flexibility. The tentacles of an octopus, the legs of a spider, and the trunk of an elephant rely on what are called muscular hydrostats. Instead of fluid-filled cavities, clumps of muscles compress and elongate.
What might be a disadvantage of a hydrostatic skeleton?
Cartilage is a connective tissue that is firm but flexible. Only cartilaginous fishes (sharks, skates, rays, and chimaeras) have a fully cartilaginous skeleton. Half as dense as bone, cartilage gives these fishes the buoyancy and flexibility they need to maneuver in the water.
However, all back-boned animals (vertebrates) have cartilage as a structural element of their bodies. Vertebrates start out as embryos with cartilaginous skeletons like sharks. As development proceeds, cartilage is gradually replaced with bone to varying degrees. Adult mammals, for example, typically have cartilage just in places where elasticity is needed, such as joints between bones, ribs, nose, and ears.
People have three types of cartilage: Elastic is the type in the ear and it is very flexible; fibrous, which is tougher, is found in the spine; and hyaline (articular), which is firm and is found in our knees as adults. When the skeleton first develops it is all hyaline cartilage, most of which then ossifies into bone by our 20s.
Our bony skeleton provides not only the rigidity for walking upright, but also sites for muscles and ligaments to attach. A cartilage skeleton is too soft for those roles. Animals like sharks whose entire skeleton is made of cartilage must instead support their bones and ligaments on thick, tough skin.
Why are our bodies composed of a variety of structural materials?
Growth and Replacement
How animals deal with growing, given the constraints of a skeleton, varies. Generally, an internal skeleton has to grow because it cannot be replaced. Think about how we and other vertebrates have bones that grow. Once we reach adult size, growth of both our soft bodies and our bones stops.
Some external skeletons grow, as they do in snails and turtles. Others are replaced as the animal grows. For example, insects literally grow out of their exoskeletons, splitting them down the middle to reveal newly-formed ones beneath. A third group of organisms has a combination of growth and replacement. Sponges, which shed skeletal parts and make new ones as they grow, are one example.
Scientific Name: Rhinella marina
Is this animal’s skeleton internal or external? Rigid or flexible? How does the skeleton influence this animal’s behavior, or lifestyle?
Catalog Number: EO63137-DSP (view complete catalog record)
Taxonomy: Animalia, Chordata, Vertebrata, Tetrapoda, Amphibia, Anura, Bufonidae
Discipline: Vertebrate Zoology
Description: This object is part of the Education and Outreach collection, some of which are in the Q?rius science education center and available to see.