Rat

1. Biology and husbandry of the rat
Adapted from “The Biology and Medicine of
Rabbits and Rodents”, 3rd ed, JE Harkness and JE
Wagner, eds
And “Rats – Biology & Husbandry”, Florida
International University

2. Origin and description
 Rattus norvegicus
 Wide variety of ecologic and genetic varieties
and strains available
 Certain health-related conditions are
dependent upon the variety of rat used
– respiratory disease
– retinal degeneration
– cancer

3. Anatomic and physiology
 Like other laboratory rodents have a baculum or os
penis
 Dental formula: 1/1 (incisors), 3/3 (molars)
 Continuously erupting incisors (hypsodontic)
 Open inguinal canal; divided stomach; large cecum;
articulated mandibular symphysis; pectoral and
inguinal mammary glands extending dorsally; diffuse
pancreas; no gallbladder
 Female has separate urethral and vaginal openings

4. Mammary gland distribution

5. Anatomic and physiology
 Omnivorous
 Brown fat in cervical (neck) location between
scapulae
– young rats
– thermogenic
 Pigmented lacrimal gland (Harderian gland)
– porphyrin-rich material secreted
– “red tears” - normal

6. Anatomy and physiology
 Long tail, rasp-like, may constitute 85% of body
length, the scales overlap. Usually longer in
female than in male. Important role for heat
loss and used as a balance organ.
 Body covered by hair except on nose, palms,
lips, and soles. Hair is divided into guard hairs
and under hair

7. Anatomy and physiology
 Brown fat plays a major role in thermogenesis
during exposure to cold
 Can modify the effects of various chemical
agents upon the body.

8. Anatomy and physiology
 Incisors remain sharp by wearing the soft
dentin and forming chisel like sharp enamel
edge
 Malocclusions can develop.
 Rat develop human-like cavity lesions.

9. Anatomy and physiology
 The cecum has a fermentation-like function (Vit B
Production) for digestion of cellulose
 In germ-free animals it is susceptible to torsion.
 Has prominent mass of lymphoid tissue in its apical
portion.

10. Anatomy and physiology
 The liver has four lobes: median or cystic lobe,
right lateral lobe, left lateral lobe, and caudate
lobe.
 Rats have ability to regenerate liver after partial
hepatectomy.

11. Anatomy and physiology
 Nostrils can close underwater.
 The lungs are divided into a single left lobe and
the 4 right lobes- cranial (anterior), middle,
caudal (posterior), and post-caval lobes.

12. Anatomy and physiology
 The testes descend at 30-40 days with the
inguinal canal which remains open.
 The testicles can be retracted into the
abdomen or left in the scrotal sac.

13. Anatomy and physiology
 There are 6 accessory sex glands:
– vesicular glands,
– coagulating glands,
– preputial glands,
– ampullary glands,
– cowper’s glands,
– prostate glands (2 pairs),
– There is one pair of bulbourethral glands (not within
the pelvic cavity)

14. Anatomy and physiology
 The female has a bicornuate uterus. The
horns are fused distally but each horn has its
own ostium internum, externum, and cervical
canal.
 Mammary tissue is limited until the first
pregnancy, when it increases significantly
before parturition.

15. Breeding
 Reach puberty at 50 to 60 days
 Mate at 65 to 110 days dependent upon strain
 Estrous cycle lasts 4 to 5 days with a 12 hour
estrous period
 After mating a white, waxy copulatory plug is
present in vagina for 12 to 24 hours
 Mammary development in 14 days

16. Breeding
 Fertile, postpartum estrous within 48 hours of
parturition
 21 to 23 day gestation
 Lactation will prolong gestation
 Pseudopregnancy uncommon, but lasts up to 13 days
 Nest builders; 6 to 12 young per litter
 Eyes open and hair present at about 1 week
 Weaned at 21 days

17. Breeding
 The female’s first estrus occurs at about 5
weeks (35 days) of age. Allowing the female to
fully mature (100-120 days) before breeding
usually means better reproductive performance
and healthier offspring.

18. Breeding
 Rat pups are born weighing 4.5-6 g, eyes will
open at 14-17 days and ears at 2.5-3.5 days.
They are hairless (fully haired 7-10 days), and
with no erupted teeth (incisor erupt at 8-10
days).
 Bedding should not be changed for 5-7 days
following parturition and the pups should be
disturbed minimally.

19. Breeding
 Females with poor milk production will have
pups with dark pink, cyanotic, or wrinkled skin.
Those with high milk production will have pups
with light pink skin and will spend relatively
short periods of time with the pups (about 1 hr
duration/feeding) and then will move away from
them to rest and cool her body temperature.

20. Clinical pathology
 Cardiovascular:
 Heart rate = 300-500 beats per minute
 Systolic blood pressure = 116 mm Hg
 Diastolic blood pressure = 90 mm hg
 Blood Volume = 6 ml per 100 grams of body
weight
 Respirations: = 85 breaths per minute
 Renal: Proteinuria is normal in the rat.

21. Public health concerns
 Allergies to dander and urinary proteins
 Many zoonotic diseases seen in wild rats are
not a problem in research colonies
– Leptospirosis, streptococcal infections,
salmonellosis, cestodiasis (tapeworms), Korean
hemorrhagic fever, sylvatic plague, St. Louis
encephalitis, Rat-bite fever

22. Sexing
External genitalia
Of male rat
P = penis
S = scrotal sac
A = anus

23. Sexing
External genitalia
of female rat
U = urethral orifice
V = vaginal orifice
A = anus

24. Behavior
 Communal – several males and females can
be housed together; fighting is rare
 Young are raised communally; shared nursing
 Burrowers; nocturnal; year-round breeders in
captivity
 Lifespan is generally 3 years
 Breeding age up to 12 months then decreasing
litter size until senescence (450 to 500 days)

25. Behavior
 Laboratory rats are docile, very adaptable,
curious animals, that sleep during the day and
are active during dark cycles.
 Frequent handling will increase docility, but
infrequent or rough handling will cause fear
responses.

26. Restraint
 Grab base of tail only
 Grasp around back and ribs with thumb and
forefinger immediately behind the mandibles

27. Housing
 bedding
– paper, wood chips, shavings, ground corn cob, sawdust
 temperature between 65o and 79o F
 relative humidity between 40% and 70%
 12:12 dark:light cycle
 change bedding as often as necessary to control odor
and keep rats clean and dry
– 1 to 3 per week

28. Housing
 Metal cages with mesh (2 wires per inch) floors
or plastic cages with solid floors
 40 sq inches floor space per adult (300 g) rat
 155 sq inches floor space for female with litter
 Cages should be 7 inches high
 Bedding should be nonallergenic, dust-free,
absorptive, nontoxic, clean

29. Feeding and watering
 Clean, fresh, pelleted rodent diet free choice
 Sipper tubes or automatic watering system
 20 to 27% crude protein
 300 g rat will consume 5 g feed and 10 ml
water per 100 g body weight per day

30. Senses
 Hearing is highly developed. Rats can hear up to 80
kHz in the ultrasonic range as opposed to people that
can hear up to 17 kHz. The maximum sensitivity occurs
from 15 to 25 kHz.
 •Sight is poor, with the retina composed almost entirely
of rods. They are blind to long wavelength (red) light
and have no color vision. They can see adequately in
dim light.
 •Smell is highly developed. Pheromones play an
important role.

31. Photoperiod
 The estrus cycle is very sensitive to constant light.
Exposure to light for 3 days may cause persistent
estrus, hyperestrogenism, polycystic ovaries, and
endometrial hypertrophy or metaplasia.
 •Light levels of about 30 ft-candles about 1.0 m above
the floor appears to be sufficient for animal care
 Light intensity also affects reproduction with the
greatest weight gains of pups seen at low light levels.
 Dark/Light Cycle: 12:12 hr.

32. Space requirements

33. Strain differences
 Some strains grow much larger than others (i.e. Long-
Evans grow at a faster rate than Fisher rats).
 •Life span is strain dependent (genetically predisposed
infections and tumors are important variables).
 Specific pathogen free (SPF) rats get bigger and live
longer than conventional rats.
 •Males continue to grow late into life while females
usually stop growth at 85-100 days.

34. Strains
 ACI August Copenhagen Irish, Congenital genitourinary
anomalies, Prostatic Adenocarcinoma
 BB/WOR Biobreeding Wooster Type 1 Diabetes Mellitus
 BN Brown Norway, Myeloid leukemia, hydronephrosis
 BUF Buffalo, Spontaneous autoimmune thyroiditis
 F344 Fischer 344, Long-term xenobiotic toxicity, gerontology,
esophageal and bladder carcinoma
 LEW Lewis, Allergic encephalitis/arthritis, Multiple sclerosis,
Myasthenia gravis.

35. Strains
 Lou/C Louvain, Plasmacytomas
 SHR Spontaneous Hypertensive, Hypertension,
myocardial infarction
 WF Wistar Furth, Mononuclear cell Leukemia,
Transplantation
 WKY Wistar Kyoto, Normotensive control for SHR,
Immunology
 ZF Zucker Obese (Fatty) Obesity

36. Strains
 LE Long Evans, Behavior, Nutrition
 SD Sprague-Dawley, Aging, Nutrition, Oncology,
Toxicology
 WIST Wistar, Aging, Nutrition, Oncology, Toxicology

37. Strains
 Brattleboro, Neurogenic diabetes insipidus
 Gunn Crigler-Najjar syndrome (Nonhemolytic
jaundiced rat)
 RNU Athymic Nude, Transplantation
 ZDF Zucker Diabetic Fatty, Type 2 Diabetes
Mellitus