RATS: CHARACTERISTICS, TAXONOMY, HABITAT, NUTRITION - BIOLOGY - 2024

The rats (Rattus) are rodents are a genus of placental mammals. Their geographical distribution is very extensive, however they are native animals of the temperate zone of continental Asia and the region of Australia and New Guinea.

On average, these animals are medium in size. The length of its tail depends on the habits it performs; if they are terrestrial it is shorter than if they were arboreal. This organ is important to maintain balance when climbing or in the event that they swim in a body of water.

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The Murids, the family to which rats belong, appeared in the Eocene, approximately 34 million years ago. Modern Murid species evolved during the Miocene and expanded geographically during the Pliocene, between 5.3 and 1.8 million years ago.

The genus Rattus emerged from the Muridae around 3.5 and 6 million years ago, in India, China, and Southeast Asia. Subsequently, the members of this group suffered two speciations. The first divergence occurred 3.5 million years ago and separated the taxonomic branch that developed in New Guinea.

Within this first group, the first species to separate was Rattus norvegicus, 2.9 million years ago. The last were Rattus rattus and Rattus tanezumi 400,000 years ago.

The second divergence of the genus Rattus began around 1.2 million years ago, and may still be in development.

Zoonosis

Rats are animals that act as vectors of many infectious diseases, transmissible to other animals and to humans. Due to its close coexistence with man, the contagion figures are alarming. Some of these conditions are:

Rat bite fever

This is caused by the bacteria Streptobacillus moniliformis and Spirillum minus. In the rat, these microorganisms are commensal microbiota, but in humans they cause a terrible infection. Symptoms could be acute fever and lymphadenopadia.

Its transmission is by animal bites and by ingesting food contaminated with urine and rat feces.

Leptospirosis

It is an infection caused by the Leptospira bacteria. It could lead to kidney failure that could lead to death. Its contagion is through the consumption of food that has been in contact with the excrement of rats.

General characteristics

Size

The size may vary. One of the smallest species is the Vietnamese Osgood rat, which can be 12 to 17 centimeters long. However, the largest species, the Bosavi woolly rat, measures about 82 centimeters.

Teeth

The teeth of rats are heterodonts and decodonts. Its incisors are long, presenting a chisel shape. Its dentition has two incisors, which grow continuously, and three molars. They do not have canines and premolars.

Head

It is solid and pointed, covered with fine and short hairs. In it her eyes and prominent ears stand out.

Extremities

Its legs have very sharp claws. The forelimbs are short, with four fingers on each leg, the thumb is rudimentary. The hind limbs are longer, with five toes, each with fleshy pads.

Fur

The vast majority have a short, dense and soft-textured coat. In other species the hair is thicker, woolly and longer.

The color is also variable. The basic pattern on the back is yellowish brown, often speckled from dark brown to black.

The Sikkim rat has brownish-brown upper areas and white underside of the body. The Himalayan field rat (Rattus nitidus) has brown dorsal area, gray underparts, and white legs.

Tail

The tail is covered with a short, fine fur. In some species these hairs are longer towards the tip, which makes it appear that they have a tuft on the tail.

It is an excellent tool that helps the rat to control its jumps. It also works as a balance beam, when walking on cables or while swimming.

The senses

The senses are highly developed, especially hearing, smell and taste. They are unable to distinguish colors. Sight is not one of their primary senses, which is demonstrated by the fact that blind rats continue their lives normally.

Taxonomy and classification

Classification

Due to the geographic extension of the genus, adaptation to each environment has given rise to a local or regional speciation. This causes the members of this group to be classified into five different groups.

• Norvegicus group. This includes only the species Rattus norvegicus, which could have originated in northeast China.
• Rattus group. Made up of 20 species native to tropical and subtropical Asia, including peninsular India, Taiwan, some islands of the Philippines, Celebes, and Southeast Asia. Specimens of this group live in mountain rain forests and in agricultural fields.
• Australia-New Guinea Group. It is made up of 19 species, native to New Guinea, Australia and the Moluccas. Its habitat is sandy areas, open grasslands, and other herbaceous areas.
• Xanthurus group. This group comprises five species, native to Sulawesi and the nearby island of Peleng. In these localities they live in jungle-type formations, at different altitudes.
• There is a group made up of eleven species, whose relationships with the rest of the specimens of the genus are not defined. They occupy rainforests from peninsular India to the Philippines and Southeast Asia.

Taxonomic hierarchy

Animal Kingdom.

Subkingdom Bilateria.

Chordate Phylum.

Vertebrate Subfilum.

Tetrapoda superclass.

Mammal class.

Subclass Theria.

Infraclass Eutheria.

Order Rodentia.

Suborder Myomorpha.

Family Muridae

Subfamily Murinae

Genus Rattus

Habitat

Rats, in their natural habitat, are nocturnal. The exception to this is the brown rat, as it is an active animal both during the day and at night. They are terrestrial species, although there are some arboreal ones. Climbers have a long tail and fleshy pads on their broad hind legs.

This group of rodents hides among the roots of large trees, in understory canopies, and in canopy trees.

Species with shorter tails and small rear footpads tend to live on land. The vast majority of these animals can swim, even preferring to eat food found in water.

Some rats can dig burrows. They are also capable of building their nests under rocks or in rotten tree trunks. They are skilled at finding crevices, caves in rocks or in abandoned houses, where they can take refuge.

Distribution

Rats are found worldwide. Some examples are the paddy rat, found in Southeast Asia, the Australian rat, which lives in eastern Australia, and the brown rat, which lives on all continents except Antarctica.

House rats like areas with warm climates, while brown rats prefer temperate ones.

The genus Rattus is generally cosmopolitan, they like to live where the human being is. Two of the species in this genus, Rattus rattus and Rattus norvegicus, can be found almost anywhere on the planet, except at the poles.

Although rattus are widespread worldwide, their original distribution was Indo-Asia. Members of this genus were dispersed throughout Europe during the Crusades in the 13th century.

They arrived to the American continent on the ships that carried the conquerors to fulfill their feats of conquest, in the middle of the 17th century.

Nutrition

It is often thought that all rats can eat whatever they can get. The truth is that the diet varies by species and habitat. The brown rat, which lives in the mountainous regions of the Canary Islands, is an omnivorous animal, however it prefers shrimp, mussels, amphibians, fish, rabbits, and eels.

Many species that inhabit the rainforest, such as the Sulawesian white-tailed rat and the Hoffman's rat, eat only fruits and seeds. Others, such as the Philippine forest rat (Rattus everetti), in addition to plant species, eat some insects and worms.

Those found in rice fields and tropical scrublands, such as the paddy rat (Rattus argentiventer) and the Malayan rat (Rattus tiomanicus), feed on insects, slugs, and other invertebrates.

Digestive system

Digestive system

The digestive system of the genus Rattus is formed by the alimentary canal and the digestive glands.

Alimentary canal

It is a tubular structure that goes from the mouth to the anus. It is demarcated in different regions, each with specific functions.

- Mouth. It is a transverse opening that is protected by two soft, moving lips. The upper lip has a cleft in the middle. The oral cavity is formed by the palate, on the floor of which is the tongue.

In both jaws are the teeth, which help to mince, grind and chew the food eaten.

- Pharynx. This connects the oral cavity with the esophagus. The dorsal part is known as the nasopharynx and the ventral part is called the oropharynx.

- Esophagus. It is a long tube that runs through the thoracic region and, behind the diaphragm, opens towards the stomach.

- Stomach. This organ is a muscular and glandular sac. In it are substances, such as hydrochloric acid, which are responsible for gastric digestion.

- Intestine. This is divided into the small and large intestine. The duodenum and ileum are part of the small intestine. In the large intestine there are two areas: the colon and the rectum.

- Year. It is the final part of the alimentary canal, whose opening to the outside is called anus. It has a muscle called the sphincter.

Digestive glands

There is a group of glands that are involved in the digestive process. Among them are the salivary, gastric, and intestinal glands.

Digestive process in rats

Digestion begins in the mouth, with the enzymatic action of saliva and with the partitioning and chewing of food. Once these reach the stomach, where they are stored, the physical and mechanical breakdown of the ingested material occurs.

In addition to this, the hydrochloric acid found in the stomach, performs the enzymatic breakdown of proteins. After this gastric digestion, the food mass reaches the large intestine. There, in the cecum, cecal fermentation occurs by microbes and bacteria.

These organisms process the fibers to generate fatty acids and vitamins, which are used by the animal's body.

Reproduction

Females of this genus are continuous polyestric. Your reproductive system is made up of the ovaries, which produce eggs, ovarian ducts, vagina, and uterus. In rats, this muscular organ has two horns, connected to each other by the vagina.

They have twelve mammary glands, six in the thorax and six in the abdomen. The first estrus is between 40 or 75 days old.

The descent of the testes in males of the genus Rattus occurs between 15 and 50 days of life. The other male sex organs are the penis, scrotal sacs, epididymis, and prostate.

Rats reach sexual maturity at approximately two or three months of age, and can produce up to 12 litters per year. In some species, reproduction occurs throughout the year, while in others it is limited to the wet seasons or the summer months.

Mating and gestation

Ovulation and the estrous cycle are closely related and can be modified by external factors. The pineal gland could be affected in its functioning by changes in light intensity or by some situations that could stress the female.

This alteration could delay or accelerate the production of eggs and therefore mating.

The male, as part of the courtship, may bite the female on the head and body. If she meets heat, she may do a kind of "dance", moving forward and spinning. This is a signal to the male, indicating that he is ready for mating.

The vast majority of males will show interest in the female and will attempt to smell and lick the female's genital and anal region.

The gestation period lasts between 21 and 26 days, having annually up to twelve litters. Each of these usually has 8 or 9 offspring, but it could be the case of having more than 15 in a single birth.

Delivery

The birthing process takes about 1 hour, with a new calf being born every 5 to 10 minutes. Once they are born, the mother will remove the yolk sac and lick them. The female ingests each of the placentas and umbilical cords.

Behavior

Rats generally form groups. Depending on the species, there may be only one dominant male, as is the case with brown rats. In others, there are several males with shared leadership.

Females can raise their offspring alone or collectively. They usually share the burrow with several females in the reproductive stage. Each of these has separate nests.

In a female burrow there may be one or more males. Depending on the density of the group, males may restrict others from mating with their females.

The social system of the males will depend on the number of members that inhabit the burrow. If they are few, the male rats fiercely defend the territories they inhabit, mating almost exclusively with the females that are in it. Each male has a separate home.

A rat in its territory attacks an intruder, but if that same rat enters another's territory, it surrenders to the attack of the resident male.

In places with high population density, the territories are difficult to control, then despotism prevails. In this system, one rat becomes dominant, while the others are socially subordinate.

References

1. ITIS (2018). Rattus. Recovered from itis.gov.
2. Guy Musser (2018). Rat. Encyclopedia britannica. Recovered from britannica.com.
3. Alina Bradford (2015). Facts about rats. Lives cience. Recovered from livescience.com.
4. Wikipedia (2018). Rat. Recovered from en.wikipedia.org.
5. The Johns Hopkins University (2018). The rat. Recovered from web.jhu.edu.