Vertebrates are animals with a backbone: mammals, birds, fish, amphibians and reptiles. Invertebrates are animals like insects, spiders, worms, crabs and woodlice, which do not have a spine; most have a tough exoskeleton, instead.
Almost all vertebrates have a similar structure to their skeleton.
- A long, flexible spine, supporting the skull
- Ribs forming a cage around the heart and lungs
- Four limbs which come off the spine, connected by the shoulder blades and pelvis.
The mammal skeleton can be divided into two sections: the 'axial skeleton' and the 'appendicular skeleton'. The axial skeleton is the central core: the skull, spine and ribcage; all the bones in the body's long axis. The appendicular skeleton is the bones which 'append' to the axial skeleton: the front and back legs, the shoulder blade and pelvic girdle.
Main photo shows a Grey squirrel's shoulder blade.
Grey Squirrel Skeleton
The skeleton will be examined, in the following order.
Click below, to go directly to a section.
In this presentation, I will focus on different parts of the skeleton. Search on Google for images of the whole skeleton of a squirrel. Click here, for example. Try here, to see a chimpanzee skeleton, another mammal, more like a human.
The average length of a Grey squirrel is 26cm, plus the tail which measures about 22cm. Grey squirrels weigh about 560g.
Red squirrels are smaller, at just 21cm and weigh 270g.
Grey squirrels are common all over the UK. Red squirrels have been driven out by Grey squirrels and are far more rare. In England Red squirrels only survive on islands such as the Scilly Isles, the Isle of Wight and Brownsea Island, as well as parts of northern England. Red squirrels still live in good numbers in Scotland and Ireland.
All the bones in these photos are from a Grey squirrel.
The main photo shows some of a Grey squirrel's tail bones.
Mammal skulls are all very different shapes. Over millions of years, the shape has adapted to reflect the lifestyle and diet of each animal species. Some animals have large eye sockets and others have very small ones; some have huge jaws and sharp teeth for killing their prey and others have jaws which are ideal just for chewing up grass.
The skull is formed of several pieces of bone which fuse together as the animal grows. In babies and very young animals, the sections are not yet sealed. This makes it easier for the mother to give birth.
Some animals like squirrels' skulls, have a solid, heavy skull, whereas others, like rabbits, have skulls which are light and full of holes. Squirrels need a solid skull in case they fall or crash into a branch. If their skull was fragile, this could be fatal.
The position of the eye sockets, indicates whether the animal is a predator or prey.
Prey animals like squirrels and rabbits, need to keep watch in all directions. Their eye sockets face out to the side, so they can see what is happening on either side and behind them. Squirrels also need to be able to look above them for hazards and to estimate distances, when they are leaping around in the trees. Squirrels have relatively large eyes, compared to other small mammals like mice and rats.
The brain is enclosed at the back of the skull. There is a large, oval hole at the base of the skull, where the spinal cord enters and connects to the brain. The hole is called the 'foramen magnum'.
The skull bones do not extend over the whole length of the nose. There is just cartilage, at the end. Feel yours!
Inside the nasal cavity are thin layers of bone, which would have been covered in scent cells, when the squirrel was alive.
Animals which rely on a strong sense of hearing, such as rabbits and hares, have large, upright ears. Squirrels have relatively small ears, which project up from the head.
Red squirrels have ear tufts which are most prominent in winter. They do not improve their hearing!
See the round hole, low down at the back of the squirrel's skull, in the main photo. This leads to the middle ear cavity.
The middle ear cavity and bones of the inner ear are contained within the ball-shaped bones at the base of the skull, on either side of the 'foramen magnum'. In squirrels, these are quite large. Squirrels have a very sense of good hearing.
Squirrels have a pair of jaw bones, which attach near the ears, at the back of the skull. They contain the lower teeth. Chewing muscles attach to the large, flat area on the jaw bones.
Animals have different types of teeth, depending on their diet. Squirrels eat nuts, acorns, flowers, fruit, roots and the seeds in pine cones, as well as raiding birds' nests for their eggs and young.
All rodents like rats, mice, voles and squirrels, have a large pair of incisors at the front of the mouth, which they use to nibble and gnaw at roots, tree bark and nuts.
Rodents have small molars further back in the mouth, for chewing and grinding up their food.
Squirrels have a tough orange layer of enamel on the front of their incisors and a softer layer behind. This gives their teeth a sharp, cutting edge. The incisors get worn down by use, so keep growing throughout the animal's life.
Squirrels have four pairs of molars on either side of the mouth.
Squirrels have a large gap between the front and back teeth. This is called the 'diastema'. They suck in their cheeks while they are gnawing to close off the back of their mouth. This prevents them choking on nut shells or chunks of tree bark, before it has been chewed.
Animals with a backbone, or 'spine', are called vertebrates. All mammals, birds, fish, amphibians and reptiles have a spine.
The spine is a series of small bones, called 'vertebrae', which link up to form a column in the centre of the animal's body. The spine connects at the top to the skull and extends to the tip of the tail. It is quite rigid, with some flexibility at the waist and in the tail.
The spinal cord runs along a tunnel, formed inside the small vertebrae. Nerves branch out from the spinal cord, through gaps between the bones.
In mammals, the shape of the vertebrae varies, depending on where they are in the spine. The different vertebrae are neck (cervical), chest (thoracic), lower back (lumbar), hip (sacral or pelvic) and tail (caudal).
The first two vertebrae in the spine, below the skull, are called the 'atlas' and 'axis'. They have special features which enable the animal to nod and twist its head.
The atlas, a ring-shaped bone, allows the head to nod.
The axis has a ball shape on which the head turns from side to side. This ball slots into a socket, on the atlas.
In squirrels there are five more neck bones, or 'cervical' vertebrae below the axis.
Then come the thirteen chest, or 'thoracic' vertebrae. Each of these connects to a pair of ribs. In many mammals, such as foxes and stoats, the thoracic vertebrae project backwards, away from the ribs. These bony projections form a ridge along the animal's back, for the back muscles to attach onto.
Below the thoracic vertebrae, are the six lower back or 'lumbar' vertebrae. They support the lower back and connect the upper body to the pelvis. In mammals, there is often more flexibility in this region, so the animal can bend at the waist.
The four hip or 'sacral' vertebrae form part of the 'pelvic girdle' and may even be fused with the bones of the pelvis, for extra stability. In the photo below, the sacral vertebrae are fused together to form one bone, inside the long 'pelvic' bones.
The spine continues for the whole length of the body, to the tip of the tail. The tail or 'caudal' vertebrae can vary in number, between twenty-four and twenty-six.
Muscles attach along the length of the spine, which help the animal to move. The shoulder blades and pelvic girdle attach the limbs to the spine.
Lumbar vertebrae, in the main photo.
Like all mammals, squirrels have a rib cage, which surrounds and protects the heart and lungs. Pairs of ribs attach between the thoracic vertebrae and project forwards to link up with the breastbone, forming a bell-shaped cage. The first ribs below the neck are the shortest; each pair of ribs is slightly larger than the ribs above.
Each rib has two sections: the 'vertebral rib' which attaches to the thoracic vertebrae and the 'sternal rib' which attaches to the breastbone, or 'sternum'. In the photo below, only the vertebral ribs are still attached to the spine.
Squirrels have thirteen pairs of ribs in total; eight pairs attach to the breastbone. Below these, three pairs attach not to the breastbone, but to the ribs above. There are two pairs of floating ribs, which do not attach at the front.
Main photo shows the first few ribs attaching to the chest/thoracic vertebrae underneath the neck/cervical vertebrae.
The small bones in the tail are an extension of the spine. Squirrels have twenty-four to twenty-six tail bones, or 'caudal' vertebrae.
Long tails help squirrels to balance in the trees, climbing fences or standing on their back legs.
The tail bones are different shapes, getting smaller and narrower towards the tip. The tail muscles attach to these vertebrae.
At the tip of the tail, the bones are long and thin.
In the middle of the tail, at the joint of each vertebra, there is a small T-shaped bone for added strength and stability.
Squirrels have a typical mammal shoulder blade, or 'scapula': a flat, triangular -shaped bone with a narrow ridge projecting out on one side. Large muscles for moving the front legs, attach to the wide surface of the shoulder blade.
The shoulder blades attach the fore legs to the spine, the central core of the body. The first fore leg bone, the 'humerus', attaches to a cup-shaped socket on the scapula, with a ball and socket joint. This type of joint gives the forelegs a wide range of movement; they can rotate forwards, backwards, out to the side and inwards, in front of the body.
Fore Legs & Feet
Squirrels run and leap around on all four legs. They use all their legs to climb trees and fences and clamber around in the branches. Sometimes they pause and stand on their two back legs, to check out what is happening.
Squirrels also use their front legs for collecting and manipulating food and nest material, burying food, digging, scratching and preening.
The bones in the front legs are a large bone, the 'humerus', connected by a hinge joint to two thinner bones, the 'radius' and 'ulna'.
The joints in the knees, fingers and toes are also hinge joints. These allow the legs, fingers and toes to bend and straighten.
Squirrels have four sharp claws on their front feet and five on their back. The claws help them grip onto the trees and food, as well as dig up buried acorns.
They have thick pads underneath, which cushion their toes when they land heavily against the tree trunks and branches or leap down from a height, onto the ground.
The hip bones, or 'pelvis', transmit the push from the hind legs to the rest of the body. The back legs drive the body forward.
The hind legs connect to the hips with a ball and socket joint. The ball shape projection on top of the thigh bone, or 'femur,' fits into the cup-shaped socket in the pelvis. This type of joint enables the hind legs to move backwards and forwards and swivel in underneath the body and out to the side.
The two bones of the pelvis link together, around the 'sacral' vertebrae', which are fused into one.
The pelvis lies horizontally along the spine and the legs extend at right angles to this central column. Muscles which move the legs and knees, attach onto the surface of the pelvis.
Hind Legs & Feet
The hind legs of a squirrel are longer than the front legs.
The thigh bone, or 'femur' is one of the strongest bones in the body, as it has to support the weight of the squirrel.
Below the femur, are the two narrower bones, the 'tibia' and 'fibula'. These are the longest bones in the body. The tibia is the weight-bearing shin bone. The fibula is thinner and does not support the body. It anchors the muscles for the ankle and foot.
The fibula is fused with the tibia at the base to form a bone which looks like the clasp of a brooch. The Latin word for brooch is 'fibula'.
Where the femur joins the fibula and tibia, is the knee joint. This is a hinge joint, which allows the bones to move only in one direction.
Animals which walk with the whole foot touching the ground like humans, are described as being 'plantigrade'. Often it is heavy, slow-moving animals which walk this way but all rodents, including fast-moving mice and shrews, are plantigrade. Humans, squirrels, rabbits, hedgehogs and badgers all walk with their whole feet touching the ground.
Squirrels have large back feet and smaller front feet. They push off with the back feet when they leap up into the trees. Rabbits and mice also have larger back feet than front feet.
Squirrels have a special adaptation for running down tree trunks! They can rotate their ankles 180 degrees, which means they can run headfirst down a tree with their back feet pointing upwards, for a better grip.
The main photo shows the ball on top of a squirrel's femur.
I am Susanna Ramsey and I have a unique collection of natural history objects relating to British Wildlife. Over the last ten years, I have assembled an extensive range of skulls, skeletons, bones, skins, feathers, wings, antlers, insect specimens and taxidermy, all from animals in the UK.
During 2010-2018, I took my Nature Collection into local primary schools to display and run workshops for the children, linking the exhibition to science topics in the National Curriculum such as Adaptations, Bones, Classification, Food Webs, Habitats, Life Cycles and Local Wildlife.
In 2018-2020, I worked with the leading schools' catalogue, TTS ,to create a range of Educational Resources for primary schools, nurseries, after school clubs and families. There are Look & Learn Cards for British Birds, Mammals and Minibeasts, a Food Webs Activity Pack, Classification Packs for Natural History & British Wildlife, Animal Discovery Bags for exciting wildlife trails, Playground Signboards and Identification Wheels. To find out more, click here or scroll down.
In the school workshops, children and teachers were always fascinated to see what is inside the animals which we see everyday in the garden and local park. On these web pages, I want to continue to share my enthusiasm for the skeletons of our local wildlife.
BRITISH WILDLIFE PRODUCTS
If you know children who are interested in nature, are a teacher, or would like to learn more about British Wildlife yourself, explore the range of British Wildlife products recently created by The Nature Collection and the leading schools' catalogue, TTS.
The Classification: Natural History pack features 40 small photos of animal skeletons, skulls, feathers, insect specimens and much more, all from The Nature Collection!
Photo shows the soft toys in the Mammal Discovery Bags.
The products are perfect for use in primary schools, nurseries, after school clubs, forest schools or at home with friends and family. Click on the links below to find out about each product.