by Randi J Task

Created on: March 28, 2009

In the past it was thought that the Rodents as an order of mammals was not related to our own order, the Primates, but that the bats were more related to Primates. Modern molecular work reveals that Rodents and Primates share a closer kinship than bats and that squirrels as quite primitive rodents have a closer link to ourselves as do tree shrews. Small, typically tree dwelling (arboreal) omnivorous mammals with a high degree of intelligence. None are truly solitary, some are highly social, they often share their territory with others for protection or in overlapping territories. Most squirrels operate by day and sleep in leafy nests, tree holes or in burrows.

Rodents comprise about 40% of all mammals and the squirrels contain the most number of species in all the rodents, now about 275 in over 50 genera. Squirrels are broadly divisible into tree, ground and flying squirrels and they all share a bushy tail, high intelligence, four functional digits on the hand and five on the foot. Five families of land mammals have spread to all the major continents - the cat family, dog family, raccoon family, rabbit family and squirrels. Traditionally the squirrel family was divided between ground, tree and flying squirrels. Today, the family is divided into several more subfamilies in which the ground and flying squirrels form subsets of tree squirrel ancestors.

Tree squirrels are specialized in having a flap of skin between their limbs, sometimes supported by a special extension in the hands. Many are nocturnal and their teeth are more complex. Ground squirrels are among the largest of rodents and can grow up to 8kg in the Himalayas and Asia - their tails are much smaller and they look something more akin to beavers. They of course include prairie dogs, ground hogs though don't all stem from the same lineage. Mostly they live in burrows for protection and often become large in the cold climate in which they live.

Squirrels are most prolific in south east Asia - most squirrel species come from there. Many are brightly colored (reds, bold blacks and yellows) and include giant squirrels that can approach 2-3kg and up to a metre long. They can have patterns like stripes as found in chipmunks and are often very vocal. They are the nearest to birds in size and behaviour. But squirrels also tend to be a lot smarter, they have a much larger brain to body ratio than expected.

In colder northern climates, tree squirrels, being unable to hibernate, have to stash winter stocks of food - hence their habits of burying or caching nuts.

As with many mammals, the female does most of the rearing of young with the males not playing a big role.

Being smart, when introduced beyond their natural boundaries they can spread, at the expense of the original wildlife of a region. The Grey squirrel in the UK is famously or infamously now replacing the smaller, native, Eurasian red squirrel.

At the moment there is a resurgence in research on squirrels as Ancient rodents that evolved in North America and spread to Asia. They are important food for many larger predators and play a key role in forest regeneration in northern temperate regions given many squirrels forget or never collect some of their buried seeds which grow into new forest.

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