What are Rodents?

Rodents belong to the taxonomic order "Rodentia".
This order includes Mice, Rats, Beavers, Guinea Pigs, Capybaras and Squirrels.  Rodentia is the single largest group of mammals in the world with around 2016 species recognised from 28 families. Most of the non flying mammals are rodents. They are native to every continent in the world, excluding Antarctica and New Zealand.  With this they have adapted to a range of environments ranging from ground-dwelling, arboreal, semi-aquatic and burrowers.

Rodents are now found in all continents except Antarctica. The main characteristics of rodents are:

  • Rodents have a single pair of incisors in each jaw, and the incisors grow continually throughout life. They have no canines and typically only a few molars at the rear of the jaws.
  • Rodents gnaw with their incisors by pushing the lower jaw forward, and chew with the molars by pulling the lower jaw backwards.
  • Breeding is both fast and prolific
  • They adapt quickly to environmental changes
  • Mostly, they are small animals, hence able to hide from predators.(The Capybara is the largest, weighing up to 100 pounds)
  • Rodents have large prominent incisors (teeth), which grow continually (both on top and bottom).  The  enamel on these teeth is very hard, enabling them to gnaw
  • They have a large gap beside the incisors (instead of canine teeth) allowing them to manoeuvre the incisors
  • Many have the ability to hibernate in extreme cold or estivate in hot conditions
  • Some of them, like the Rat-kangaroo and jerbos have developed long hind legs that allow them to jump in a similar way to Kangaroos. This is convergent evolution as these groups are not related taxonomically to Marsupials kangaroos (Family Macropodidae).

Some of the families included in this order are:

Family Aplodontidae
This family includes one single genus and species, the Mountain Beaver (Aplodontia rufa).  This animal is endemic to north-western areas of North America.  It is the most primitive of the rodents alive today and lives in humid forests.  The Mountain Beavers are restricted to areas of heavy rainfalls due to their primitive kidneys which cannot produce concentrated urine.  They are large, heavy set rodents weighing up to 1.5kg.  They feed on forbs and bark and live in colonies which can create a complex structure of burrows.

Family Sciuridae
This family contains two subfamilies - the tree squirrels and ground squirrels, and the “flying squirrels”.  There are around 273 species existing today from 50 genera.  They are distributed across most of the globe, except for Australia, Madagascar, southern South America and some deserts.  The tree squirrels (including the Grey Squirrel) are generally diurnal and arboreal, building dens in tree cavities and nests on branches.  They have thick bushy tails, large ears and sharp claws. Ground squirrels are terrestrial and burrow using their shorter and muscular forearms.  In cooler climates, both tree squirrels and ground squirrels hibernate.  The flying squirrels have a flying membrane (patagium) between their limbs to assist with gliding from tree to tree.  Most of the flying squirrels are nocturnal.

Family Castoridae
This family includes the North American Beaver (Castor canadensis) and the European Beaver (C. fiber).  The family name is related to the glands on the cloaca of the animals which produces a pheromone used to mark territory.  These animals are well adapted to a semi-aquatic lifestyle, with large bodies (weighing up to 35 kg), long overfur (guard hairs) and thick underfur for insulation.  They are characterised by their flattened, spatula-like tail which is used to propel them through the water and to ward off threats.  Their diet consists of bark, leaves and roots of trees.  They are able to carry large tree stems in their mouths while swimming due to the high position of their nostrils and the position of the epiglottis which is above the soft palate allowing air to travel directly from the nostrils to the trachea and lungs.  They usually live in family groups, working together to build “lodges” and food stores to help them survive the winter.

Family Heteromyidae
This family includes the kangaroo rats, kangaroo mice and pocket mice.  There are 59 species and 6 genera in this family.  They are found in north, central and south America in deserts and grasslands.   Some members, such as the kangaroo rats have hind legs adapted for bipedal jumping, the pocket mice are quadrapedal.  All members have cheek pouches used to carry food back to the burrow for storage.  They generally feed on seeds and plant parts.  Most members burrow and create complex tunnel systems with multiple openings.   Kangaroo rats and pocket mice can survive without water for long periods of time.

Family Dipodidae
This family contains 15 genera and 51 species.  It includes species such as the Meadow Jumping Mouse (Zapus hudsonius), Beach Vole (Microtus breweri) and Pygmy Hairy-footed Gerbil (Gerbillurus paeba).  Members of this group are distributed across the northern hemisphere through Asia, Africa and North America.  There is great diversity in this group, especially in the structure of the hind foot and life history.  The jumping mice have an elongated hind foot designed for jumping, whereas the birch mice have a simple hind foot.  Members of the group can be terrestrial and semiarboreal. Most family members are omnivorous feeding on seeds and insects.

Family Erethizontidae. 
There are 12 species from 4 genera within this family which contains the New World Porcupines, including the North American Porcupine (Erethizon dorsatum).  Members of this family are also distributed through Central America and northern Argentina.  They occupy a variety of habitats including grasslands, deserts, forests and woodlands.  They are large, heavy-set rodents, weighing up to 18kg.