Cover photo shows a Field vole skull next to a common daisy, for scale. The skull is 2.3cm long x 1.3cm wide.
Introduction to Rats, Mice, Voles and Shrews
Main photo shows pelvic bones from a Field vole inside a pea pod, to show their size. They are 1.9cm long x 0.9cm wide.
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.
Very Small Mammal Skeletons
The skeletons will be examined, in the following order.
Click below, to go directly to a section.
- Jaw Bones
- Rib Cages
- Shoulder Blades
- Fore Legs & Feet
- Hind Legs & Feet
In this presentation, I will focus on different parts of the skeleton. Search on Google for images of the whole skeletons. Try here, to see a squirrel skeleton, an example of a rodent. Try here, to see a chimpanzee skeleton, another mammal, much more like a human.
The main photo shows two Field vole ribs, about 1cm long and less than 0.1cm wide.
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. This makes it easier for the mother to give birth.
Voles and shrews have skulls which are paper-thin, incredibly fragile and full of holes. It is very rare to find a complete shrew skull inside owl pellets.
The skulls of mice and rats are very similar. The rat skull is almost twice as big as the mouse.
The skull of a Grey squirrel, another rodent, is similar, even higher than the rat skull; it has much larger, more-rounded eye sockets.
Prey animals like mice and voles, need to keep watch all the time. Their eye sockets face out to the side, so they can see what is happening on either side and behind them. Mice, rats and voles have large beady eyes.
Despite their tiny size, shrews are fierce predators. They have small eyes, which face forwards. They find most of their prey by sense, using their long whiskers to feel along the tunnels in the undergrowth and to search in the soil for earthworms and centipedes.
The brain is enclosed at the back of the skull.
The bones of the skull do not extend over the whole length of the nose. There is just cartilage, at the end.
All these animals have long snouts to sniff out danger and food. Shrews have a long pointed snout. Noses on voles are blunter than on mice.
Animals which rely on a strong sense of hearing, such as mice, have upright ears.
See the round hole, low down at the back of the mouse's skull, in the photo below. This leads to the middle ear cavity.
Two bulbous shaped bones underneath the skull, at the back, contain the middle ear and inner ear bones.
Main photo shows the jaw bones of a Brown rat, 2cm long x 2cm wide.
Mammals have a pair of jaw bones, which attach below the ears, to the back of the skull. They contain the lower teeth. The animal's chewing muscles attach to the large, flat area at the back of the jaw bones.
Jaw bones are relatively solid and are often well preserved inside owl pellets. Sometimes the teeth are still in place but often they have fallen out.
A mouse's jaw bone is shorter and the gap between the front teeth and molars, is smaller.
Main photo shows the red-tipped incisors projecting from the skull of a Common shrew!
Animals have different types of teeth, depending on their diet.
- Field voles are herbivores eating grass roots, plant shoots, fruit and fungi.
- Wood mice are omnivores with a more varied diet eating seeds, grains, nuts and buds, as well as feeding on dead animals, caterpillars, worms, centipedes and snails.
- Rats too, are omnivores eating just about anything: cereals, nuts, food scraps, birds' eggs, nestlings, lizards, smaller mammals, insects, fish and amphibians.
- Shrews are insectivores, eating mainly insects and spiders, but also earthworms, slugs and snails.
All rodents, which include rats, mice, voles and squirrels, have a huge pair of upper and lower incisors at the front of the mouth, which they use to nibble and gnaw at roots, tree bark and nuts.
There is a tough, orange layer of enamel on the front of their incisors and a softer layer behind. This creates a sharp, cutting edge as the back of the teeth wear away more slowly than the front. The incisors keep growing throughout the animal's life.
Rodents have sets of small molars further back in the mouth, for chewing and grinding up their food.
Rodents 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.
Shrews are insectivores and have a very different arrangement of teeth. Their teeth have a red layer on the tips: this indicates the presence of iron, for added strength to crunch up the insect exoskeletons. They have long incisors and jagged molars, with spikes pointing up into the mouth. Their teeth do not keep growing throughout their life.
Moles are also insectivores. They have similar teeth to shrews teeth, without the red tips. The Common shrew skull is 1.9cm long whereas the Mole skull is almost twice the size, at 3.5cm. The extra weight and size, where larger jaw muscles can attach, eliminate the need for the Mole's teeth to be reinforced with iron.
Tooth Root Holes
When analysing the bones in owl pellets, scientists look closely at the jaw bones and skulls, to identify which small mammals' bones are in the pellets. Often the molars fall out and only the root holes left by the 'cheek teeth', remain. These are diagnostic.
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, in mammals, at the waist and in the tail.
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).
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.
The spinal cord runs along a tunnel, formed inside the small vertebrae. Nerves branch out from the spinal cord, through gaps between the bones.
The first two vertebrae in the spine 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, lies above the axis: it allows the head to nod. The axis has a ball shape on which the head swivels. This ball slots into a socket, on the atlas.
In mammals there are five more neck bones, or 'cervical vertebrae' below the axis, making a total of seven cervical vertebrae.
The thoracic vertebrae each connect to a pair of ribs. In many mammals, such as rats and stoats, the thoracic vertebrae project out away from the ribs. These bony projections form a ridge along the animal's back; the back muscles attach to these additional surfaces.
These mammals have thirteen thoracic vertebrae.
Lower Back/Lumbar Vertebrae
Below the thoracic vertebrae, are the lower back or 'lumbar vertebrae'. These support the lower back and connect the upper body to the pelvis. There is often more flexibility in this region, so the animal can bend at the waist.
These small mammals have six lumbar vertebrae.
The 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 above, the sacral vertebrae are fused together, inside the pelvic bones. There are four sacral vertebrae.
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, for different mammal species. Rats and mice long, flexible tails. They generally have twenty-six to twenty-eight caudal vertebrae.
Like all mammals, squirrels have a rib cage, which surrounds and protects the heart and lungs. Pairs of ribs attach to some of the cervical and 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 true rib has two sections: the 'vertebral rib' which attaches to the vertebrae and the 'sternal rib' which attaches to the breastbone, or 'sternum'.
Rodents have thirteen pairs of ribs in total; seven pairs attach to the breastbone. Below these, three pairs attach not to the breastbone, but to the ribs above. There are three pairs of floating ribs, which do not attach at the front.
Main photo shows tail bones of a Field mouse, with a tiny, connecting bone at each joint.
The small bones in the tail are an extension of the spine. Rats, voles and mice have twenty-six to twenty-eight tail bones, or 'caudal vertebrae'.
Long tails help animals to balance climbing trees, walls or fences or when standing up on their two back legs.
The tail bones are different shapes, getting smaller and narrower towards the tip. The tail muscles attach to these vertebrae.
In shrews and mice, in the middle of the tail, where the vertebrae join, there is an extra, little bone for added strength and stability.
Main photo shows the shoulder blade of a Field vole, against a common daisy.
A shoulder blade, or 'scapula', attaches each of the front legs to the spine. It joins onto the top bone in the leg, the 'humerus', 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.
The muscles for moving the front legs, spread over the wide surface of the shoulder blade.
Rats, mice and voles have a typical mammal shoulder blade, or 'scapula': a flat, triangular -shaped bone with a narrow ridge projecting upwards. The shrew's shoulder blade is slightly narrower.
The shoulder blades of mice, voles and shrews are incredibly thin, almost transparent.
Fore Legs & Feet
Main photo shows a Harvest mouse on a Teasel.
All these mammals 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.
They 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 follow the same pattern as for all mammals: a long 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.
Shrews have five toes on their front feet, whereas rats, mice and voles have four. They all have five toes on the longer, back feet.
Tiny claws help them grip onto the plants and trees as they clamber around. They also use their claws to dig up worms and insects and hold their food.
Main photo shows the pelvis of a Field vole, still attached to the spine.
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 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.
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.
It is possible to differentiate between males and females by looking at the shape of the pelvic bones. Females have a thinner line on the outer edge of the triangle: from left to right in the photo below, perhaps numbers 2, 6 and 7 were from female voles. The pelvic bones are thinner because females have to give birth. Imagine how tiny the young are, which pass between these two bones!
The two bones of the pelvis link together, around the 'sacral vertebrae', which are fused into one, in some animals.
Hind Legs & Feet
Main photo shows the tiny, transparent hind-foot bones of a Field vole, from underneath.
The hind legs of these small mammals are usually slightly longer than the front legs. They push off from their large back feet and hind legs.
The thigh bone, or 'femur' is one of the strongest bones in the body, as it has to support the animal's weight.
Below the femur, are the two narrower bones, the 'tibia' and 'fibula'.
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 to form a bone which looks like the clasp of a brooch. The Latin word for brooch is 'fibula'. In voles and shrews, the bones are fused half way down the tibia.
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 shrews, mice, rats and voles are also, all plantigrade.
All these animals have large back feet and smaller front feet. They push off with the back feet when they leap up into the undergrowth or trees.
These feet are all tiny and incredibly fragile, with the toe bones just a few millimetres long.
The Collector, Photographer & Author
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 & Body Structure, 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 scroll down or click here.
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!
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 more about each product.