The genus Rattus comprises a rather large group of more than 56 species of which the most common as pets are Rattus rattus and, above all, Rattus norvegicus. The former is also called “black rat” or “roof rat” and is usually very dark coloured, smaller and more slender than Rattus norvegicus. Rattus norvegicus is commonly known as the culvert or sewer rat. Both species originated in Asia and reached Europe at different times: the first to arrive, a very long time ago, was the black rat, which was joined by its cousin Rattus norvegicus only more recently in the Middle Ages, following the caravans. Despite its name, Rattus norvegicus colonised Scandinavia only half way through the 1700s and then reached the New World on ships. Thanks to its ability to adapt it is a cosmopolitan species that has dominated, from an ecological point of view, over Rattus rattus which is nowadays much less prevalent than Rattus norvegicus. In the 14th century Rattus rattus was responsible for the Black Death pandemic, although its only fault was to be parasitized by Xenopsylla cheopis, the flea carrying Yersinia pestis, which was the aetiological agent of this devastating plague. Nowadays the rat is no longer considered only a pest, but is often reared as food for snakes and raptors and widely used in animal experiments as well as being, fortunately, much appreciated as a pet (Figs. 1 and 2).
Classification
Regno: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Classe: Mammalia
Ordine: Rodentia
Sottordine: Sciurognathi
Famiglia: Muridae
Sottofamiglia: Murinae
Genere: Rattus
Specie: Rattus rattus e Rattus norvegicus
THE RAT IN THE WILD
In nature, Rattus rattus prefers fairly isolated environments and is a strictly arboreal species that adapts well to life high on roofs or in attics; it is a good jumper. Rattus norvegicus, in contrast, is exclusively terrestrial and, as a consequence of being an omnivore, lives in ditches, rubbish tips, basements, barns and public gardens. It nests in long, shallow tunnels with many exits, but also inside woodpiles, under floors and in haystacks. This rat is an excellent swimmer and can travel many kilometres in this way. Rattus rattus is a very good climber, while Rattus norvegicus has the distinction of being able to ascend sewer pipes by leverage of its splayed legs on the walls of the pipe. In nature rats are predominantly nocturnal animals although this pattern can be modified to daytime activity in areas with a high density of nocturnal predators such as owls and foxes. The social group is formed of several males and many females and the hierarchy is determined on the basis of fights, the outcome of which initially depends mainly on physical strength and size, but subsequently age also becomes important so the dominant male, even if older and less robust, will still be able to maintain his previously earned status without too much effort. The search for food ranges over an areal extending from a few score meters to some kilometres, depending on the availability of food and the presence of predators. Rats communicate through a vast range of sounds, ultrasounds and body positions. They are omnivores and adapt well to using any organic waste left over by humans but also the green parts of plants, seeds, roots, invertebrates, molluscs, birds’ eggs and nestlings as well as the young of other rodents: indeed, the rat also practises cannibalism, eating sick or dead members of its own colony. In the wild it shows a certain tendency to neophobia and is, therefore, wary about unknown, new foods, but soon learns to identify them, in part by acquiring information from the behaviour of other rats; likewise, rats easily learn to recognize unusual objects in their own environment and can, therefore, avoid traps. The neophobia is much less evident, being almost absent, in animals in captivity, particularly if they trust the person offering them a new type of food.
ANATOMICAL AND PHYSIOLOGICAL FEATURES
It is worth starting with a small note of clarification: the rat is not a big mouse and a mouse is not a small rat. For the layman the only difference between these two animals is their size, whereas they are actually two zoologically distinct species. Rattus rattus is smaller and more slender than Rattus norvegicus. The brown rat, besides being a pet, is widely used as food for snakes and as a laboratory animal: according to statistics published every 3 years by the Ministry of Health, in Italy alone more than 250,000 rats are used every year for in vivo experimental studies. There are some strains of Rattus norvegicus that have been particularly selected by man: the albino, the nude rat (both athymic and not), the curly-haired rex and the tailless manx. Furthermore, genetically modified animals have been selected for having particular characteristics of use in experiments, such as hypertension, obesity, polycystic kidneys and many others.
The rat is anatomically very similar to the mouse, differing essentially by the absence of a gallbladder and body size and proportions. The body length is extremely variable and can reach 30 cm, to which the length of the tail must be added, which can be as much as 85% of the length of the body. The rat weighs about 300-500 g, although specially bred obese strains can reach or even exceed 1 kg. There are five digits on the anterior and posterior paws (the thumb is minimal) all with nails. The average lifespan is 3 years, the body temperature is about 37.5 to 38.5°C and the animal has 42 chromosomes. The rat has a very fast metabolism, reflected by its heart rate, which ranges between 300 and 500 beats/minute, and respiratory rate of 70-115 breaths/minute. Despite having a hibernating gland, the rat does not hibernate.
Skin
The skin of the rat is covered by hair, whereas the palms of the hands, the soles of the feet, the nose and lips are hairless. There are sparse, bristly hairs on the tail and so this appendage cannot be considered truly glabrous. The palmar and plantar surfaces of the limbs have footpads containing sweat glands. The digits have tapered claws except the thumb, which has a true nail. The surface of the nose has nasal creases with tactile function. There are glands on the surface of the head and neck, while the hibernating gland, always present in this region and easily confused with adipose tissue, is involved in fat and carbohydrate metabolism and thermogenesis in the rat. There are usually six mammary glands on each side of the abdomen, although a disparity of one or both of the mammary rows (5 or 7 mammary glands) is not uncommon (Fig. 3). There is abundant brown fat between the scapulae.
Skeleton
The skeleton of the rat is extremely flexible and compressible in order to facilitate movement in even very small spaces. The splanchnocranium is very well developed, in contrast to the neurocranium (Fig. 4), which is rather small. The sternum has six sternabrae and the vertebral formula is C7; T13; L6; Cocc 27-30. The anterior limb has a clavicle and scapula with acromion; there are five metacarpal and metatarsal bones, each followed by a digit with three phalanges, except the thumb which has only two segments. The ulna and radius are well separated, as are the tibia and fibula, while the pelvic girdle is narrow and elongated.
Cardio-circulatory and respiratory apparatuses
The heart is close to the left thoracic wall (which makes intracardiac blood sampling easy) and is situated between the 3rd and 5th intercostal spaces. The rat’s heart, besides being vascularised by coronary arteries, also receives extra-coronary blood supply from arteries derived from the internal carotid arteries. The systolic blood pressure (maximum) is 115-116 mmHg while the diastolic pressure (minimum) is 90 mmHg. The left lung has only one lobe, whereas the right lung is divided into four lobes (cranial, medial, caudal and accessory).
Digestive tract
The rat is an omnivore and adapts well to an “occasional” diet; it has a proportionally very long digestive tract, about 1 metre long or even more, in order to digest even nutritionally very poor foods. The upper lip is divided into two by a philtrum. The tongue has a median sulcus and is anchored to the floor of the mouth by two frenula. The dental formula is: I 1/1; C 0/0; Pm 0/0; M 3/3 for a total of 16 teeth. At the level of the insertion of the oesophagus in the lesser curvature of the stomach there is special fold which makes it almost impossible for the rat to vomit. The stomach is entirely protected by the rib cage and divided into two clearly distinct parts with a proventricular part lined by keratinised mucosa and another glandular part. The small and large intestines are very similar, both in diameter and because they lack taeniae coli, and do not, therefore, have the typical sacculations and pouches of the gut (Fig. 5). The liver is large and rests on the diaphragm; it is divided into three main lobes (left, right and quadrate) which have subdivisions (Fig. 6). The rat does not have a gallbladder and the biliary ducts from the lobes of the liver drain into the hepatic duct. The pancreatic ducts open into the hepatic duct before this joins the duodenum.
Genito-urinary system
In the adult the kidneys are located beneath the last ribs and, therefore, in physiological conditions they are not palpable. In contrast, the bladder protrudes significantly from the pubis into the abdominal cavity and, given its large capacity, is easily palpable when full. However, since rodents have the habit of urinating when handled, this procedure is not always simple. The adrenal glands are some distance from the renal blood vessels, which simplifies their removal. The nephrons are very superficial and, therefore, readily accessible. In the male the testicles are oval-shaped and descend into two distinct scrotal sacs at around 14-21 days of life. The prostate is formed of four lobes. The vesicular glands are situated dorso-laterally to the bladder, while the bulbo-urethral glands are located close to the transition from the pelvic urethra to the penile urethra. Rats also have coagulation glands that collaborate in the production of the seminal fluid. The penis contains a cartilage called the “penile bone” and at rest has a flexure, which distends during erection of the organ. The prepuce is rather cranial and contains the preputial glands. The inguinal canal remains patent and the testicles can be retracted into the abdomen by the contraction of specific muscles. In the female the ovaries are situated at the level of the last lumbar vertebrae and are surrounded by abundant adipose tissue, which often makes them difficult to identify. The uterus is double, with a double cervix. The urethral opening is independent and opens at the base of the clitoris. When sexing the animal, if the testicles have been withdrawn into the abdomen, the differentiation can be made by observing the ano-genital distance which is greater in the male than in the female (Figs. 7 and 8).
Sensory organs
The eyes, which are spherical and slightly protruding, are located laterally; the third eyelid is very small. The orbit houses the Harderian gland, which secretes a porphyrinic and, consequently, red-coloured substance. This secretion is almost never seen in healthy animals, but accumulates in cases of stress or pathological states, giving rise to typical “red tears” (Fig. 9). The ears of the rat are fairly large, but proportionally smaller than those of the mouse.
REPRODUCTION
During her life the female rat can give birth on average up to seven to ten times or even more, with the litter size ranging from six to 14 pups and in exceptional cases even up to 20. Reproductive performance (fertility, fecundity and prolificacy) starts to decrease after 12 months of life and the menopause occurs after 450-500 days of life.
The testicles descend into the scrotum between 20 and 50 days of life, while the vaginal ostium opens after 35 days. Both males and females reach puberty at about 50-60 days of life; however, it is advisable to avoid mating rats until about 65 days of life or until they weigh at least 250 g in order to allow them to reach a more mature state of body development. The rat has continuous polyoestrus with a reproductive cycle of 4-5 days and heat lasting 12 hours. The Whitten and Bruce effect, described in mice, also exists in rats, although it is less significant. The cycle can be influenced by an increasing photoperiod, which is considered optimal when illumination is present for 12-16 hours. Too long a photoperiod can compromise fertility even after only 3 days of exposure to light, which is worth taking into account in conditions of artificial illumination. Pharmacological synchronisation of oestrus in a group of females can be induced by administration of progesterone or its synthetic analogues in order to obtain anoestrus following lysis of the corpus luteum and PMSG to induce ovulation. The rat has a fertile oestrus about 48 hours post-partum and in the case of mating at this time could have a gestation lasting 3-7 days longer than normal because of the delay in uterine implantation of the embryos. Following mating, a vaginal plug, produced by the male sex glands, occupies the space between the vulva and the cervix.
Reproduction can be carried out in conditions of monogamy (1 male and 1 female), polygamy (1 male and 2 females) or in a harem (1 male and 6 females). The male can remain with the female or females throughout gestation and even in the post-partum period, but in order to avoid irritation to the pregnant does and to the future pups, it is better to separate the reproducers, putting the male in a cage on his own. The pregnant does can deliver and rear their pups in a colony or the groups can be housed separately such that each doe cares for her own pups (Fig. 10). If, however, the intention is to exploit the post-partum heat, the reproducers must be kept together: in this way the male himself will perceive the time of oestrus and mount the female at the appropriate time. Alternatively, if the reproducers have been separated, the female must be taken to the male at the time that the heat is predicted, but in this case the success rate is lower.
Gestation lasts 21-23 days and mammary gland development becomes evident after the first 14 days. The pregnancy can be diagnosed by palpation of the foetuses or by weighing the mother to detect the increase in weight. If the post-partum oestrus is not exploited, the reproductive cycle starts again 2-4 days after weaning. Pseudopregnancy is uncommon in this species. At birth the pups weigh, on average, 5-6 g (Fig. 11); they open their eyes after 10-12 days and are weaned at 21 days (when they weigh 40-50 g). The lactating doe feeds the pups numerous times during the day (Fig. 12). The various phases of the reproductive cycle can be demonstrated by staining reproductive tract tissue with May Grunwald Giemsa:
- Pro-oestrus: many nucleated epithelial cells and few keratinised cells
- Oestrus: numerous keratinised cells
- Meta-oestrus: a few keratinised cells and an increased white cell count
- Di-oestrus: numerous leukocytes
