Roz Wade, Learning Officer at the Museum of Zoology, writes:

Welcome to our first Mammalwatch post in Wildlife from your Window. We thought what better way to start than with a post about that terror of the bird feeder – the grey squirrel.

The grey squirrel, Sciurus carolinensis, is a familiar sight in our gardens. The body is around 25cm long, and the tail is bushy and almost the same length. As their name suggests, the fur is grey, but is tinged reddish-brown around the face and feet. They are agile animals, making excellent tightrope walkers and able to scurry through the treetops with ease.

Reds vs Greys

If you have been taking part in our Open Your Window Bingo, you may have noticed that a grey squirrel only scores one point. Why is that? Grey squirrels are an invasive species in the UK. They were introduced here from North America in 1876, and have since spread across the country, and have had a big impact on our wildlife, particularly our native red squirrels.

The red squirrel, Sciurus vulgaris, is smaller than the grey squirrel, has a russet-coloured coat, and tufts of fur on the tips of its ears. They live in both coniferous and deciduous woodland across Europe and Asia. Since the introduction of grey squirrels to the UK, our population of red squirrels has really suffered. Grey squirrels can outcompete red squirrels for food and habitats. They also carry a virus called squirrelpox which can be fatal to red squirrels. There are still some places in Britain where you can find red squirrels, and there are conservation efforts to protect these populations. Find out more about the invasive grey squirrel and efforts to protect red squirrels with Inside Ecology and the Wildlife Trusts.

Where do squirrels fit in the tree of life?

Squirrels are rodents. Rodents are a very successful group of mammals – there are more species of rodent than any other type of mammal alive today. Rodents have a large pair of incisor teeth top and bottom at the front of the jaws. These keep growing throughout life. Rodents have enamel only on the front surface of their incisors. The sharp, chisel-like shape of these teeth is formed by the softer dentine on the rear surface of the tooth wearing away more easily than the tough enamel on the front.

The incisors of rodents are gnawing teeth. They can be used to gnaw on hard foods such as nuts and seeds, but also for gnawing through wood, plastic, pipes and more. Beavers use their incisors to fell trees to build their dams.

Rodent skulls from left to right: American beaver, domestic guinea pig, brown rat

In squirrels, the incisors are quite chunky for gnawing into tough nuts and seeds.

Skull of a grey squirrel, Sciurus carolinensis. Notice the orange enamel on the front of the incisors. These incisors are big and tough to gnaw into hard nuts and seeds. You can also see the grinding molar teeth toward the back of the jaws

There are over 270 species of squirrel alive today in Africa, Eurasia and the Americas. They include tree squirrels moving along branches and tree trunks, flying squirrels gliding from tree to tree, and ground squirrels nesting in burrows. Our red and grey squirrels are both tree squirrels, living in forests and adapted to climb.

Animal Acrobats

Have you ever watched a squirrel in the woods and wondered just how does it climb so quickly up and down the vertical tree trunks? Tree squirrels, like our grey and red squirrels, have a number of adaptations for life in the trees. The long tail of a squirrel helps them to balance when travelling along branches. Sharp claws on their toes give squirrels added grip. Then to help them travel down tree trunks safely head first, tree squirrels have a specialised ankle joint that allows them to rotate the hind foot to point backwards. Take a look next time you see one in your garden clambering down a tree or fence post.

And finally…

Have you looked out of your window and seen a long, slender mammal with black fur bounding across the grass or through the trees? I know I have, thinking at first what a strange-shaped cat before realising it was a black squirrel. Black squirrels are melanistic grey squirrels – that means they are grey squirrels that produce more of the pigment melanin, giving their fur its dark hue. This is a trait that is passed on in the genes. There are large populations of black squirrels in North America. In the UK, they were first seen in Bedfordshire in 1912, but have since spread, with records in Cambridgeshire and Hertfordshire, and probably more widely than that. So take a second look when you see a peculiarly shaped black cat outside – it might be a black squirrel.