Rodents are a diverse group of small to medium-sized mammals characterized by a single pair of constantly growing incisors in both the upper and lower jaws. They typically have robust bodies, short limbs, and long tails. As members of the order Rodentia, they constitute about 40% of all mammalian species, including some well-known animals like mice, rats, squirrels, prairie dogs, porcupines, beavers, guinea pigs, and hamsters that are native to all continents except Antarctica and some oceanic islands.
Despite once being classified with rodents due to their continuously growing incisors, rabbits, hares, and pikas are now placed in a separate order, Lagomorpha, though they share a common ancestor with rodents, forming the clade Glires. However, both groups differ in their dentition and jaw structure.
Description
Size
Rodents vary in size, with the largest being the greater capybara (Hydrochoerus hydrochaeris) which weighs as much as 146 lbs (66 kg). However, most rodents do not weigh more than 3.5 oz (100 g).
While in some rodents, like mole rats and pocket gophers, the males are larger than the females, this sexual dimorphism is quite the opposite in others, like chipmunks and jumping mice, where the females are slightly larger.
Distinguishing Features
Rodents are distinguished from other mammals by their ever-growing, sharp, and open-rooted incisors. Most species possess up to 22 teeth (lack canines or premolars) and have a prominent gap or diastema between the incisors and the cheek teeth.
They have a specific jaw muscle called the masseter, which plays an important role in mastication (chewing) and accounts for about 60% to 80% of the total muscle mass. Eastern gray squirrels have a particularly large and deep masseter that helps in efficient biting.
Some rodents, such as kangaroo rats, hamsters, chipmunks, and gophers, have cheek pouches extending from the mouth to the shoulders. These pouches help store food for later use.
Their forelimbs have five digits, with an opposable thumb, whereas the hindlimbs have only three to five digits. They also have claw-like nails on their digits.
Depending on the species, most rodents also have tails of varying sizes (and textures, like furry or bald). Some, like the Eurasian harvest mouse, have prehensile tails with grasping ability.
They have well-developed sensory organs for smell, hearing, and vision. Some nocturnal rodents, like rats, possess eyes sensitive to ultraviolet light. They also have sensitive whiskers or vibrissae to sense their surroundings.
Taxonomy
Thomas Edward Bowdich (1821) coined the term Rodentia for the group containing all rodents. It is derived from the Latin word rodens, which means gnawing or eating away at something.
Initially, lagomorphs were considered rodents because of their continuously growing teeth, but they were later assigned a separate order (Lagomorpha) because they possess an extra pair of incisors in the upper jaw and are evolutionarily different. Presently, both rodents and lagomorphs form a clade called Glires.
Brandt (1855) had previously proposed three rodent suborders: Sciuromorpha, Hystricomorpha, and Myomorpha. However, a recent American Society of Mammalogists classification lists around 2,200 species under four suborders, 34 families containing over 481 genera.
Family: Sciuridae (Squirrels, including chipmunks, prairie dogs, marmots)
Evolution and Fossil Records
The earliest rodent fossils date back to the Paleocene Epoch (66 to 56 million years ago), around the extinction of the non-avian dinosaurs.
It is believed that these mammals evolved in Asia after the multituberculates (order Multituberculata) were washed off in the Cretaceous–Paleogene extinction event (~66 million years ago). Such ecological void helped rodents diversify in the future.
A few million years after the Cretaceous-Paleogene boundary, rodents diverged from lagomorphs (comprising rabbits, hares, and pikas.
Beavers (Castor) first emerged in Eurasia during the late Eocene epoch and later migrated to North America by the late Miocene epoch.
Around this time, the hystricognaths entered Africa, having originated in Asia around 39.5 million years ago. Some fossil records also suggest that New World hystricognaths (also called caviomorphs) had crossed the Atlantic Ocean to reach South America.
Nesomyids (family Nesomyidea) shifted from Africa to Madagascar some 20 to 24 million years ago. Around 20 million years ago, families like Muridae had emerged.
By the Early Miocene, when Africa collided with Asia, rodents, like porcupines, began spreading into Eurasia.
Rodents, like Old World rats and mice, first arrived in Australia around 5 million years ago via Indonesia.
Around 3 million years ago, in the Piacenzian Age, the New World porcupines (family Erethizontidae) migrated from South to North America following the Great American Interchange. However, members of the family Sigmonodontinae shifted to the south and diversified greatly.
Distribution and Habitat
Rodents are abundant in all continents except Antarctica. Some rodents, such as Polynesian rats, have been anthropogenically introduced to distant islands like Hawaii and Easter Island. However, they have firmly colonized Australia and New Guinea without human intervention.
Being one of the most widespread groups of mammals, rodents are found in almost all terrestrial habitats, ranging from snowy tundra to blazing hot deserts.
While tree squirrels and New World porcupines are arboreal and live in trees, others, like mole rats, gophers, and tuco-tucos, live in underground burrows. They are also found in agricultural lands.
Beavers and muskrats are semi-aquatic, whereas the earless water rat (Crossomys moncktoni) is completely aquatic, living in cold, fast-flowing streams of the mountains.
Diet
Although most of these mammals are herbivorous and feed on seeds, stems, leaves, flowers, and roots, some small rodents, like field voles, consume insect larvae, worms, fungi, fish, or meat.
Shrewlike rats specialize in soft-bodied invertebrates, whereas the Australian water rat feeds on crustaceans, mussels, snails, frogs, and water birds (sometimes their eggs too). In contrast, the grasshopper mouse often consumes other species of mice. Sometimes, the Texas pocket gopher feeds on its feces (coprophagy).
Behavior
Rodents are diurnal (active during the day), nocturnal (active at night), or sometimes active both day and night. They continuously wear their teeth down to prevent them from overgrowing and piercing their skulls.
Feeding and Digestion
Different groups of rodents have evolved different feeding techniques based on their diets.
To avoid emerging above the ground, the Texas pocket gopher uses its jaws to pull plant roots into its underground burrows.
The African pouched rat forages above the ground and crams food in its cheek pouches until they bulge. Having acquired food, it returns to its burrow and removes any unwanted material from its diet.
Agouti (Dasyprocta) breaks open capsules of the Brazil nut fruit and stores excess seeds for later use.
These mammals absorb nearly 80% of ingested food through highly efficient digestive systems. They first soften the food in the stomach and then pass it on to the caecum for bacteria to reduce it to the simplest, most absorbable forms.
Locomotion
Rodents move from one place to the other using a variety of locomotory techniques, like walking, burrowing, running, climbing, hopping, gliding, and swimming.
Most rodents usually walk on both the palms and soles of their feet (plantigrade); however, some, like agouti, are digitigrade and walk only on the digits of their limbs.
Scaly-tailed squirrels and flying squirrels use their parachute-like membranes (stretch between the forelimb and hindlimb) to glide from tree to tree.
Eusociality
While prairie dogs live in colonies, the edible dormouse and the pocket gopher live solitary lives (though they aggregate in the breeding season). Surprisingly, naked mole rats have even been found to form castes within their colonies.
The tendency to form families is most common in large rodents.
Brown rats form colonies of six females guarded by a single male.
Beavers live in units comprising a pair of adults, along with their old and young offspring.
Ground squirrels form colonies based on female kinship, while their males disperse after weaning and lead a nomadic life.
The black-tailed prairie dog forms colonies in burrows that spread across hectares of land. These burrows are occupied by territorial family groups known as coteries, comprising adult males and females, their nonbreeding yearlings, and the current year’s offspring.
Both naked and Damaraland mole rats establish underground colonies of up to 80 individuals. In Damaraland mole rats, the colonies comprise a single reproductively active male and female. The other individuals remain sterile only until they wish to establish their own colonies.
Communication
All rodents use visual, olfactory, auditory, and tactile cues to communicate with their conspecifics.
Visual
Rodents make perfect use of their UV sensitivity using the two types of light-receptive cones in their retina (short wavelength blue and middle wavelength green). For example, when alarmed, degus stand up on their hind limbs, using their exposed bellies to reflect UV light to alarm other members. They also stand on all four legs to become less visible to their predators.
Voles, mice, rats, and degus may also use their urine to mark their trails or territories since the urine strongly reflects UV light.
Olfactory
These mammals scent-mark using urine, feces, and glandular secretions. These marks provide information regarding an individual’s identity, sex, health, dominance, and reproductive status.
These olfactory cues help rodents recognize close relatives and exhibit nepotism, in which they show preference to their kin.
Auditory
Depending on the species, rodents emit various alarm calls to alert their conspecifics and ward off predators. Social species produce a broader range of vocalizations than solitary ones, with some, like Kataba mole rats, using as many as fifteen different calls.
Tactile
The Middle East blind mole rat bangs its head against the tunnel where it lives. This act generates seismic underground signals that help the rat communicate with its group members.
Some rodents, like the banner-tailed kangaroo rat, drum their feet on the ground when facing danger. Such foot drumming signals the attacker about the rodent’s alertness, thereby compelling it to refrain from attacking. Similarly, Cape mole rats also use ground vibrations in intra-specific communication during male-male competition and courtship.
Reproduction and Life Cycle
Mating
Rodents, like prairie voles, are monogamous, where males and females form permanent pair bonds that last their lifetime. Others are polygamous, having multiple mates throughout their lives.
Males of polygynous species (those that mate with multiple females) usually attempt to monopolize their mates defensively or non-defensively. In yellow-bellied marmots, California ground squirrels, Columbian ground squirrels, and Richardson’s ground squirrels, the males own resource-rich territories to attract potential females. In the case of marmots, these males vehemently guard their territories against invading males and rarely lose in battles.
In species with a non-defense strategy, like Belding’s ground squirrels, the males roam around in search of females to monopolize.
In addition, species like white-footed mice are strictly promiscuous, with females having multiple male partners, whereas giant kangaroo rats can alternate between monogamy, polygyny, and promiscuity.
Birth and Parenting
Rodents are viviparous (produce live young) and give birth to either altricial or precocial young. While altricial young (found in squirrels and mice) are blind, hairless, and underdeveloped, precocial ones (found in guinea pigs and porcupines) are relatively well-developed, furry, and have eyes.
Females with altricial young give birth by sitting or lying down in elaborate nests, and they maintain these nests until the young are weaned. However, precocial mothers barely invest in such nest-building and usually give birth by standing.
While altricial young emerge in the direction in which their mothers give birth, precocial young emerge behind their mothers. Alticials always receive more attention and care from their mothers, whereas precocials are relatively independent and get weaned in days. However, the latter signal their mothers using specific calls.
In some rodents, like mice and black-tailed prairie dogs, the young are nursed by non-parental individuals (alloparenting or cooperative breeding). Belding’s ground squirrels build communal nests where mothers usually nurse non-related young along with their own.
Sometimes, due to stress, like resource limitations, some rodents (such as California ground squirrels) resort to infanticide shortly after giving birth. Sometimes, they also cannibalize or eat their own offspring. Similarly, instances of feticide are also found in some rodents, like Alpine marmots.
Adaptations
Male rodents possess pheromones like non-volatile major urinary proteins (MUPs) that attract potential female mates.
In the Cape ground squirrel, a promiscuous rodent, the male’s testes are as long as 20 percent of its head-to-body length. Such anatomical trait helps produce a higher number of sperm and increases the chances of inseminating the females.
Ecological Importance
Different rodent species occupy multiple niches and play important ecological roles.
When prairie dogs burrow into the soil, they aerate and increase its organic content. Due to improved soil quality and subsequent better foliage, some large herbivores, like bison and pronghorn, usually feed near prairie dog colonies.
Burrowing rodents usually feed on the fruiting bodies of fungi, unknowingly dispersing their spores through feces and spreading their population.
Beavers play a crucial hydrological role by altering the paths of streams and rivers, thereby creating new, extensive wetland habitats. Studies have found that these rodents have caused a 33% rise in the number of herbaceous plants in riparian areas (the interface between land and rivers).
ReferencesArticle last updated on 15th January 2025
