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Rodents
In terms of number of species (though not necessarily in terms
of number of organisms–population–or biomass) rodents
make up the largest order of mammals, with over 40 percent of mammalian
species belonging to the order.
There are between 2000 and 3000 species of rodents, which are found
in vast numbers on all continents except Antarctica (they are the
only placental order other than bats, Chiroptera, to reach Australia
without human introduction), most islands, and in all habitats except
for oceans.
Most rodents are small; the tiny African pygmy mouse is only 6
cm in length and 7 grams in weight. On the other hand, the capybara
can weigh up to 45 kg (100 pounds) and the extinct Phoberomys pattersoni
is believed to have weighed 700 kg.
Rodents have two incisors in the upper as well as in the lower
jaw which grow continuously and must be kept worn down by gnawing;
this is the origin of the name, from the Latin rodere, to gnaw,
and dent, tooth. These teeth are used for cutting wood, biting through
the skin of fruit, or for defense. The teeth have enamel on the
outside and exposed dentine on the inside, so they self-sharpen
during gnawing. Rodents lack canines, and have a space between their
incisors and premolars. Nearly all rodents feed on plants, seeds
in particular, but there are a few exceptions which eat insects
or even fish.
Rodents are important in many ecosystems because they reproduce
rapidly, and can function as food sources for predators, mechanisms
for seed dispersal, and as disease vectors. Humans use rodents as
a source of fur, as model organisms in animal testing, for food,
and even in detecting landmines.
Members of non-rodent orders such as Chiroptera (bats), Scandentia
(treeshrews), Insectivora (moles, shrews and hedgehogs), Lagomorpha
(hares, rabbits and pikas) and mustelid carnivores such as weasels
and mink are sometimes confused for rodents.
The fossil record of rodents began after the extinction of the
dinosaurs 65 million years ago. By the end of the Eocene epoch,
beavers and squirrels appeared in the fossil record. They originated
in Laurasia, the joined continents of North America, Europe, and
Asia. Some species colonized Africa, giving rise to the earliest
hystricognaths. From there they rafted to South America, an isolated
continent during the Oligocene and Miocene epochs. By the Miocene,
Africa collided with Asia, allowing rodents such as porcupines to
spread into Eurasia. During the Pliocene, rodent fossils appeared
in Australia. Even though marsupials are the prominent mammals in
Australia, rodents make up almost 25% of the mammals on the continent.
Meanwhile, the Americas became joined and some rodents expanded
into new territory; mice headed south and porcupines headed north.
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