Squirrels are arguably one of the most popular rodents, and mammals for that matter. They range from small to large, colorful to dull, and cute to just annoying. Though some squirrels are considered pests, they play important roles in their ecosystem. They fertilize plants, feed on many other pests, and spread tree seeds. There are many types of squirrels; Ground Squirrels, Marmots, Tree Squirrels, Giant Squirrels and Pygmy Squirrels. Their habitats vary greatly, from high up in the mountains to parks in the middle of a busy city. Wherever a squirrel may be, its impact on the world around it can be seen. 
The body of a Tree Squirrel, and Flying Squirrel is designed for life in trees. They have long slender bodies and bushy tails. Tree Squirrels and Flying Squirrels have long and muscular limbs for climbing, but Flying Squirrels have flaps of skin connected to their forefeet that allow them to glide through the air. The bodies of Ground Squirrels are designed for life on the ground. They have short legs, and strong paws with claws for digging. The forefeet of squirrels have four long digits and a small thumb, and their hind feet have 5 digits. Claws, are found on all digits, except the thumb; which has a nail. The fur coat density of a squirrel depends on the climate of their habitat; denser fur coat for colder climates, thinner fur coat for warmer climates.
Squirrels vary greatly in size; the smallest being the African Pygmy Squirrel, and the largest being the Oriental Giant Squirrels (Genus Ratufa). They also very in color; some have two basic shades of solid brown or gray, while some have stripes of color on the coat. Some tropical species exhibit combinations of gray, white, maroon, yellow, orange, black, brown and red. The distinguishable characteristic of squirrels is their skull structure and primitive jaw structure. Their skulls are small and short, with a short snout, and an arched skull profile. Squirrels have four large incisors, with enamel that grows continuously throughout the life of the squirrel. Since these teeth are constantly used for gnawing, they are kept short, but the teeth can become a problem for squirrels in captivity.
The life of a squirrel begins at the conception of a litter of baby squirrels; which are called kits. The gestation period of a litter of kits is about 30 to 60 days, depending of the size of the species; shorter gestation for smaller squirrels, longer gestation period for bigger squirrels. A litter typically consists of two to four kits each, with about two litters in the life of the mother squirrel.  One litter of kits may be fertilized by one male, but some litters may have varying paternity; some kits could have different fathers, because reproduction in squirrels is polygynandrous, meaning a female or male squirrel has multiple partners. Some squirrels have been known to change fur color distinctly during mating season, to attract a mate. 
When the kits are born, they have no fur, closed eyes, and closed ears. All kits are born into a nest and live there from 26 days to 42 until they can live independently and are weaned, but development is different for each species. Some squirrels reach sexual maturity by 87 days old, others by the time they are three years old, hibernating squirrels tend to develop faster, and some are born at larger sizes, so the time to independence is much shorter. In some species, the females may aggregate, or come together to take care of the kits, and pass on their territory on to them, which greatly increases survival rate. In the wild, most young squirrels die in the first year of life. Most squirrels have a maximum longevity of 8 to 14 years in the wild, and up to 16 in captivity.
Squirrels can be found worldwide. They are native to most every land mass, with the exception of Australia, Madagascar, Greenland, Antarctica, and New Zealand. Ground Squirrels and Marmots find most of their population concentration in North America. Tree squirrels are most densely populated in North America and South America. Giant Squirrels are limited to the eastern hemisphere. Flying Squirrels are native to most parts of Asia, and Pygmy Squirrels are found exclusively in South America. In the 19th century, an attempt to introduce squirrels to Australia was made. Two species were introduced; the Eastern Gray Squirrel (Sciurus carolinensis) and the Northern Palm (Funambulus pennantii) Squirrel, however, only the Northern Palm Squirrel remains. 
Squirrels exist in a variety of habitats; Trees, burrows, rainforests, arctic tundras and suburban areas are only a few of the places you would expect to find a squirrel. Some squirrels can be found at high elevations, such as the Himalayan Marmot (Marmota himalayana) which can be found at elevations upwards of 16,000 feet., and some in your own backyard. Squirrels are major contributors in their ecosystem. Many squirrels consume seeds as food, which may act as a method of seed spreading for many plants. The red squirrel also has a profound effect on its ecosystem. It will eat mushrooms that give plants nutrients and water. After eating these mushrooms, the spores of the mushrooms may be spread to other areas through the feces of the squirrel. 
The Flying Squirrels
The Flying Squirrels are a group of squirrels known for their ability to glide through the air, at lengths of about 150 feet in one glide. Despite their name, "Flying Squirrels" is a misnomer because they are not actually able to fly as would a bird or bat, rather they glide through the air. Flying Squirrels are able to glide through the air due to a sheet-like membrane called a patagium, which stretches wrist to ankle. Flying squirrels are able to steer themselves with the aid of their long tails and limbs. Most Flying Squirrels are nocturnal and omnivorous, most make their home in trees, and live generally in densely wooded forests. Some domestic Southern Flying Squirrels (Glaucomys volans) have been known to be kept as house pets. Flying Squirrels typically live around 4 years in the wild, and up to 13 in captivity. 
Evolution and Squirrels
Scientists suppose that squirrels evolved around 50 million years ago. However, this estimate was upset by a recent find in China of a gliding squirrel that dated at around 125 million years old.
The Flying Squirrel
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