RODENTS: EVOLUTION, CHARACTERISTICS, FEEDING, REPRODUCTION - BIOLOGY - 2025

The rodents are placental mammals belonging to the order Rodentia, characterized by having each upper and lower jaw a pair of front teeth that have root and continued growth. This huge group of animals includes rats, squirrels, marmots, beavers, and porcupines, among others.

Their way of moving around is varied, being able to walk in a quadruped way, run, climb, dig, jump, swim and even glide. The Siberian flying squirrel (Pteromys volans) can move from one tree to another by gliding, by extending the membranes that join its front and rear limbs.

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Rodents have a great cognitive capacity, they quickly learn, among other things, to recognize and avoid poisoned baits. Guinea pigs may learn the routes that lead them to find their favorite foods: fruits. Squirrels could easily locate their food, thanks to their spatial memory, also relying on their specialized sense of smell.

Although some species are considered as pests for humans, they can also perform ecological functions. In North America, the excavations made by prairie dogs when building their caves play an important role in the aeration of the soil and in the distribution of nutrients.

Behavior

Social

Rodents have a diverse range of behaviors related to social organization, feeding, defense, and mating.

Some rodents, when they locate food, take only small portions of it, to obtain information about its taste. If they like them, they return to the site in search of more, being able to transfer it to their burrow.

If the food is presented in large sizes, they break it into smaller pieces to be able to be introduced into the cave. Rodents are often thought to carry their food to the burrow for storage and use in times of scarcity.

However, research has made it possible to associate this behavior with being able to consume food in a safe place, away from the threat of predators or other diners of the same species.

Rodents are organized into groups that involve a certain territorial and hierarchical behavior. The males or the females, depending on the species, are usually territorial under situations such as the defense of the burrow, the feeding routes and the places where they build the nest.

Courtship

Before mating, male rodents court using ultrasonic vocalizations, at a frequency that cannot be picked up by the human ear. Research shows that these sounds are more than squeaks, they are "songs" with special rhythmic characteristics.

The male begins to emit them at the moment of catching the smell of a female's urine, which lets him know that she is sexually suitable to mate.

As part of the courtship, before copulation, the male rodent may gently bite the head or some parts of the female's body. You can also smell his urogenital area. The sexual act between members of this species does not exceed 20 seconds.

Evolution

Teething is the characteristic used to recognize the fossils of rodents, whose oldest record comes from the Paleocene, 66 million years ago. These fossils are found in North America, Europe, and Asia.

The difference between mammals and glires, a clade made up of lagomorphs and rodents, occurred in the late Cretaceous. Rodents are thought to have evolved on the Asian continent, where multituberculates, an extinct species of mammal, were affected by the Cretaceous-Paleogene extinction.

Due to this ecological vacuum, rodents were able to diversify. However, the multituberculates and rodents subsisted together for at least 15 million more years.

In the Eocene, rodents began to develop specific characteristics, giving rise to new species. At the end of this prehistoric period, the Histricognatos emigrated to Africa, so that later some of them arrived in South America, approximately 41 million years ago.

When the African continent was united to the Asian one, during the Miocene, African rodents began their spread through Asia and Europe. Some of these species were large. Primitive rodents arrived in Australia around 5 million years ago.

Taxonomy

• Animal Kingdom.
• Subkingdom: Bilateria.
• Infra-kingdom: Deuterostomy.
• Phylum: Chordates.
• Subfilum: Vertebrates.
• Infrafilum: Gnathostomata.
• Superclass: Tetrapoda.
• Class: Mammal.
• Subclass: Theria.
• Infraclass: Eutheria.

Order Rodentia

Suborder Anomaluromorpha

Most of the species in this group have patagium, an epithelial membrane found between the front and rear legs, similar to that found in true flying squirrels.

Its tail is characterized by having, on its ventral part, two bands of scales. The Zenker squirrel and the scaly-tailed squirrel are some of the representatives of this suborder.

Suborder Castorimorpha

These animals have a strong body constitution, varying in size from 12 to 30 centimeters. The males are usually larger than the females, almost doubling their weight. The color of their hair usually matches the tones of the habitat where they grow.

They have very large, pouch-shaped cheeks. Its eyes are small and its tail is short and with a lot of fur. Some examples are beavers and kangaroo mice.

Suborder Hystricomorpha

Their habitat is rocky deserts, they are medium-sized rodents. Her hair is long and silky-looking, usually in shades of brown. Some species are nocturnal and live in burrows.

Their diet is based on plant tubers and bulbs. Porcupines and guinea pigs belong, among other species, to this suborder.

Suborder Myomorpha

These can be grouped taking into consideration the characteristics of your jaws and molars. The medial and lateral masseter muscles can move forward, making it possible for them to gnaw. They are located in the different habitats of almost all continents, except Antarctica.

One of his favorite foods is seeds. Some animals in this suborder are the hamster, mice, and true rats.

Suborder Sciuromorpha

Its body is usually thin, having a bushy tail and large eyes. In some species, the hind limbs are longer than the front limbs, with 4 or 5 toes on each leg. These have pads and claws, which allow it to climb trees and grab its food.

Squirrels, representatives of this suborder, can climb down from trees by moving headlong.

General characteristics

-The senses

Some specimens have special calls to communicate, for example the alarm calls they make when they feel threatened. These vocalizations can become so specific that they have one for each predator. Furthermore, the timbre and tone of these indicate the urgency of the situation.

View

Rodents have two types of light receptors, therefore they are dichromatic. They are sensitive to ultraviolet rays, which are found at a high level during the day and at twilight. This is advantageous for those rodents that are active during those hours.

Touch

Rodents produce vibrations when they hit the ground with their feet or head. These waves are captured and interpreted by other animals of the same species, receiving warning signals or courtship.

The blind mole rat strikes the walls of the tunnels where it lives with its head in order to communicate with other neighboring mole rats.

Smell

The scent is used to demarcate the territories and also to recognize their relatives, having a special behavior for them, known as nepotism. Smell signals can come from urine, feces, or sweat.

-Sexual dimorphism

In some species the males are larger than the females, while in others the opposite is the case. Male-biased dimorphism occurs in ground squirrels and solitary mole rats, and female-biased dimorphism is present in jumping mice.

-Face

Its nose is short, with a rounded tip. The oral cavity is divided in two, the front part has the incisor teeth and the back part has the premolars and molars.

The upper lip is divided in such a way that the incisors are visible, despite the fact that the mouth is closed. The tongue is short, covered with small taste buds.

-Tail

The vast majority of rodents have tails, varying in shape and size. Some are prehensile, as in the harvest mouse, others are vestigial. Sometimes it can be separated from the body of the animal, allowing it to escape from the predator. It may happen that this tail, which was cut off, regenerates.

The tail can be used to communicate, as do moles, who strike it against the surface of the water.

-Size

Its size is variable. One of the smallest species is the swamp mouse (Delanymys brooksi), which measures 6 centimeters and weighs between 6 and 7 grams. The largest is the capybara (Hydrochoerus hydrochaeris), which weighs 65 kilos, measuring 134 centimeters long.

-Mandible

The lower jaw moves forward while gnawing and backward when it has to chew. It has a strong musculature, increasing its power to gnaw things of high hardness

-Extremities

The legs have claws, these being long in the digging species and sharp in the arboreal ones. The front limbs usually have 5 fingers, where an opposable thumb is included, while the rear ones have 3 or 5 digits. The elbow allows the limb great flexibility.

They are mostly plantigrade animals, involving walking on the palms and soles of the feet.

-Cheek bag

This organ is a particular morphological feature in kangaroo, hamster, and squirrel rats. They are two "bags" that can reach the ears of the animal, being able to be removed from the inside out to be cleaned. In the hamster, they are open in the mouth, while in the Geomyvoidea they open on the cheek.

Mice do not have this bag, but the elasticity in their cheeks allows them to stretch, fulfilling the same function.

Feeding

Rodents have a plant-based diet, which includes soft leaves, seeds, fibrous plants, grass, or roots. Others are carnivores, eventually consuming carrion.

They also eat insects such as small arthropods, larvae, or earthworms. The omnivorous diet of some rodents is composed of different plants and material of animal origin.

To obtain their food, the vast majority of rodents are opportunists, consuming the food they get in their path, while others are predators. The food can be consumed in the place where it is collected or taken to its burrow.

Digestive system

The digestive system is conditioned for a type of plant-based diet, although some species are omnivorous, carnivorous or insectivorous.

The stomach is single chamber. Some specimens of lemmings carry out the pre-digestion of food in a part of this organ, as occurs in ruminant animals.

Plant cells contain cellulose, a chemical element that is difficult for the body to process. In the case of rodents, the disintegration of cellulose molecules occurs in the cecum, thanks to the action of bacteria. The colon has folds that help with this action.

In the large intestine, the colon produces two types of stool, hard ones that contain non-reusable waste substances, and soft ones, called cecótrope, rich in nutrients that could not be completely disintegrated.

Many species of rodents are cecotrophs, since they consume their soft feces in order to take full advantage of the nutrients it contains.

Reproduction

The reproductive system in males and females is located in the back of the abdomen. Reproductive cells are found in the ovaries, in the case of females, and in the testes of the male. These are the eggs and sperm, respectively.

The organs that are part of the male reproductive system are the scrotum, testicles, epididymis, penis, prostate, and seminal vesicle.

The penis has an extraskeletal bone called a staff, which is not connected to the rest of the skeleton. This contributes to the mating process, allowing the erection of the penis to last longer.

The testicles can be located externally or within the abdominal cavity. In some species these have a seasonal decline.

The reproductive organs in the female are the ovaries, the fallopian tubes, the uterus, the vagina. The ovaries are inside an ovarian bag supported by a membrane called the mesovarium.

Females have a double uterus, joining the vagina distally. On the ventral part of this the clitoris is located. The vaginal opening to the outside of the body is protected by the lips of the vulva.

Mating

Once males and females reach sexual maturity, reproductive cycles begin. The litters begin to happen one after another, with a difference of 120 or 160 days, this is because the females are polyestric.

In the vast majority of rodents, ovulation occurs as a regular cycle, such is the case in brown rats. In other species it is induced during mating, as is the case in some specimens of mice.

During copulation, males of some species deposit a plug in the female genital opening. The function of this is to prevent the sperm from leaving the vagina, in addition to preventing other males from inseminating that female. This plug can be removed by the females, whenever they wish.

Gestation

Gestation can last between 22 and 24 days. During this stage, the females can live with the male, but when the time of delivery approaches, he moves away because the female becomes restless and skittish during the time of delivery.

If she feels stressed or something disturbs her, she can assume these stimuli as signals of threat, and can have extremely aggressive reactions, even with her own young.

Some groups of rodents are characterized by being highly fertile, where the female could give birth many times a year, the gestation is short and the litter is made up of numerous young.

Many members of the order rodentia are monogamous, where the male and female form a kind of bond. Others are polygamous, where the males monopolize and attempt to mate with multiple females.

Anatomy and morphology

Teeth

In all rodents the incisor teeth lack roots. These have an enamel layer on the front and softer dentin on the back. Its growth is constant.

While the incisors perform their movements when chewing food, which they do against each other, the dentin wears away, leaving the edge of the tooth very sharp, similar to that of a blade.

They do not have canines, which creates a space, called a diastema, between the incisors and the molars. Their number could range between 4 and 22, and may or may not have roots.

Its growth is continuous and frequently its crown is high, although some may have it low. The molars are specialized for grinding food.

The structure of the jaw joint ensures that the upper and lower incisors do not coincide when chewing, in addition to preventing the premolars and molars from contacting while the animal is gnawing.

Skull

In the skull of rodents, a great development of the lower jaw, incisor teeth and molars can be seen, giving it a unique appearance within mammals.

The eye socket is open at the back. The end of the zygomatic bone is very poorly developed or in many cases does not exist. The lacrimal foramen is always close to the eye socket. The zygomatic arch is located behind the premolars and molars.

The nasal bone is large, extending forward, being separated from the maxilla thanks to the incisor bone. They have a short palatal bone.

The parietal is much smaller than the intraparietal. The tympanic bull is large and is always present in rodents. In gerbils there is also a mastoidal bull, located in the posterior area of ​​the skull, in the form of a bulge.

The lower jaw, in its anterior part, is narrow and round in shape, contrary to the large and less rounded shape of its anterior part. This feature is typical of the order Rodentia.

Skeleton

The skeleton has a rounded constitution, with short front legs and slightly longer back legs. They are plantigrade and tailed, usually long. However, due to the habitat and the type of feeding, these structures may have specific characteristics, adapted to these needs.

The spinal column is made up of 7 cervical, 13 thoracic, 6 lumbar vertebrae, and a variable number of caudal vertebrae. The shoulder blade is narrow, with a long acromion. Some specimens have a clavicle, although in some it is not very developed or non-existent.

In the pelvis are inserted a large group of muscles, called hamstrings, with the distal insertion of these in the tibia. The pubic joint is long and bony in character.

The front legs have a noticeable separation between the ulna and the radius. In the hindquarters, the tibia and fibula grow together in those species that move by jumping, thus allowing to cushion the strong impact received by the upper joint.

The big toe may be underdeveloped or absent. In gerbils, the metatarsals of the hind legs are elongated, growing, in some species, together.

Habitat

Rodents are part of the most widespread mammals in the world, and can be found in all continental territories, except in Antarctica. These are the only placentals that have colonized, without human intervention, New Guinea and Australia.

Humans have made it easier for these animals to spread to remote places, such as oceanic islands. In this way rodents demonstrate their ease of adaptation to places of extreme cold, such as the tundra, and to arid deserts.

The species that live in arid places build shelters, to take shelter from the adversities of the environment. These can be from holes in trees, crevices in rocks, nests of leaves and sticks, burrows or complex networks of underground tunnels.

Some are arboreal, like porcupines, while other specimens, like mole rats, live almost exclusively under the ground. Other groups dwell on the land, having burrows to hide in.

Beavers and muskrats are considered semi-aquatic rodents, although the one that has most adapted to living in water is the water rat, which is located at the mouths of rivers, especially in the south of France.

References

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