Squirrel
Anatomy
Below are some interesting facts that I have learned about squirrel anatomy over the years that I have been following them...
Whiskers
Did you know that squirrels have whiskers on their arms and other places, as well as their nose?
Just like the ones on their nose, these extra long hairs are for sensing proximity to things that they cannot see (or see well). Their vision has a large radius for spotting predators, but they have a blind spot behind and underneath their skull (If a squirrel turns their back on you they are trusting you to look out for predators in the other direction). Under their arms is a perfect place to have some more sensors. However, they are not the only ones, they also have some on their back legs and tummy. Those other whiskers are usually harder to spot, but the above image also shows the latter.
Thumbs
Did you know that squirrels have adorable little thumbs?
They sometimes have little nails on them, but not claws. Set quite far back on their arm, they're less likely to touch messy surfaces when climbing or running around and thus they stay cleaner for holding and manipulating food. After eating they will vigorously lick their hands, making especially sure to have clean thumbs.
Ankles
Squirrels are one of a few animals that can rotate their back feet 180 degrees due to having double joined ankles. This allows them to run down trees as fast as they can run up them, and it allows escape predators with ease. In addition to this, their tail allows them to shift their centre of balance quickly, enabling them to balance, jump and cling to tree trunks much better than they would otherwise.
Tail
If you're familiar with Eastern Grey squirrel's white tipped tail fur (giving them a pretty glow), you might be curious how a red squirrel's tail gets it's colour. The above image shows how the winter colours of one red squirrel tail varies over its length. Although they're not all this bold and varied, plus they do change in the summer, they all end with the squirrel's main colour and have darker colours just underneath. Thus you will never see a red squirrel with a bright halo like the Eastern Greys. Since red squirrels can be quite grey in the winter this can be a reliable way to identify grey squirrels in locations that have both (like the UK & Italy) if you're struggling to identify them. Of course red squirrels will also have tufted ears in the winter, so this shouldn't be a problem.
Fur
It can be quite difficult to recognize your squirrels from winter to summer considering how much their coat changes. Their winter colours can vary, but they often go from a thick grey in the spring to a short and very deep red in the summer (with no tufted ears). The thickness of the tail fur will also change. The Winter fur will start to grow back around September. These changes are triggered by daylight hours rather than temperature (due to reliability). The thicker Winter fur is not only brighter, but it also scatters light more, making it more adaptable to various surrounding colours (patchy snow) and changeable lighting for camouflage. The squirrel's base colour comes from their food and dwellings. Pine trees in this locations are bright red at the top (where their nests tend to be), whereas at high altitudes Pine trees are darker and so follows the squirrels coat colour.
Body Fat
Unlike other types of squirrel, the Eurasian Red, or Sciurus Vulgaris (meaning "Common Shadow-Tail") cannot retain body fat. This is why they grow a thick winter coat. If you see a rather round adult red squirrel it will likely be a pregnant female (like the one above).
Young
These ridiculously adorable creatures are out of the nest and highly capable at 8 weeks old. Sadly their mortality rate is around 70% for their first year. They might look rather clueless, but the ones that remain will be stealthy little ninja survivors. No matter what time of the year they are born (can be any time), they will be complete with a winter coat, just in case. They will have their tufted ears and extra fluff, even if the adults have lost all of theirs already. Although they will also lose it pretty quick too.
Squirrels can have three to five kits (babies) at a time and will stay in the drey (nest) for about 8 weeks. Red squirrels are so territorial that the male and females won't nest together unless it's extremely cold. Only the mother will bring up the young and when they are out of the nest they will soon be on their own and treated as competition for food sources by the adults.
Bite
If you're wondering how a squirrel can break open a walnut or hazelnut so quickly, a rodent's jaw muscles are able to concentrate about 48 megapascals (7000 psi) of pressure on to the tip of their front teeth. Although localized, that's more than a Great White Shark or a Crocodile. Needless to say, they can cause a lot of damage if they're scared, so treat them with a lot of respect if getting this close. They have four back teeth on each side, top and bottom, totaling twenty including their four front ones. Like Beavers, those orange tinted front teeth are infused with iron for extra strength. These never stop growing, so they need to gnaw things often in order to keep them in check.
Food
As well as nuts, squirrels like to eat mushrooms, flowers, tree sap, seeds, fruit and vegetables. They will also eat birds eggs, although that is a less common occurrence. To see more specifics of what to and not to feed them see the section on food here.
Leucism
One interesting colour characteristic that we saw in Switzerland was some white patches of some of the squirrels. Although this can affect squirrels with red fur, it seems to be more common on ones with darker fur. One image I saw online from the area showed a dark squirrel with all four feet in white. This condition should not to be confused with Albinism, which affects the whole body and also turns the eyes red.
Speed
Squirrels enjoy posing for pictures... ok, you immediately knew that was bullshit didn't you? Their survival nature conditions them into frantically skittish creatures with no patience and lightning reactions. If you were to give them a coffee with sugar it's possible for them to break the light speed barrier. Getting images of them eating is easy, but capturing their more interesting action poses can be a real challenge. Even the most advanced modern mirrorless cameras are not fast enough to keep up with them most of the time.