Rat Anatomy and Biology

Introduction Laboratory rat Pet rats Rattus norvegicus (“Norway rat”) Laboratory rat Pet rats Came from Asia to Europe at the beginning of 1700’s Rattus rattus (“black rat”) Many strains (inbred- brother/sister matings for 20 generations) and stocks (outbred) used in research Outbred stocks used as pets

Norway Rat Albino Wild

Black Rat (Rattus rattus) has faded away

Strains & Stocks Wistar, Sprague-Dawley, Holtzman (albino) Common outbred rat stocks: Wistar, Sprague-Dawley, Holtzman (albino) Long-Evans (“hooded”) Smart, sociable, docile, trainable Excellent pets if handled gently

Hooded Rat Wistar Wistar

Terminology Dorsal - the back or upper surface Ventral - the belly or lower surface Lateral - the side Anterior - the front or head end Posterior - the hind or tail end Medial - toward the midline of the animal Proximal - closer to the midline of the body Distal - farther from the midline of the body Superficial - near the surface Deep - a distance below the surface

External Features Vibrissae - These are groups of very long hairs found just behind the nose and above the eyes. They are tactile organs very useful to a nocturnal animal Ear - The long flexible fold of the ear is called the pinna. It is composed of cartilage covered with skin. Eyes - Notice that the eyes are placed anteriorly in the skull. The central area through which light enters the eye is the pupil. External nares - Look for this pair of openings near the tip of the snout. Air can be drawn through these openings into the respiratory system

Skeletal System The skeletal system supports the body against gravity and provides a protective encasement for some of the vital organs such as the brain and heart. There are two main parts to the vertebrate skeleton: -The axial skeleton, which is composed of the skull, mandible, vertebral column, ribs, and sternum; - The appendicular skeleton, which is composed of the limbs and the pectoral and pelvic girdles

Rat Skeleton

Most of the rodents share a dental formula of: 2 (I 1/1, C 0/0, P0/0, M 3/3) The incisors are open rooted and grow continuously; the molars don’t. Incisors will develop a yellow-orange color as the animal age.

Anatomy & Physiology Hypsodontic incisors (grow continuously) Brachiodontic molars Harderian gland- porphyrin production: oversecretion (chromodacryorrhea) when stressed or sick Brown fat- thermogenic

Porphyrin from Harderian Gland

Brown Fat Function: Stores Glycogen Regulate Temperature

Anatomy & Physiology Stomach anatomy (limiting ridge) prevents vomiting No gallbladder

Rat Stomach Rodents are monogastric with their Stomach divided in two areas; One glandular portion One nonglandular portion

Coprophagy Rats and mice eat their own feces. Important for obtaining vitamin B produced by bacterial action in the colon. Directly from anus – grid floor does not prevent. Preventing coprophagy decreases growth by 10-20%. Important in pharmacology with per OS administration.

Reproductive Anatomy Inguinal canals remain open Bicornuate uterus (duplex) 6 pair of mammary glands: 3 thoracic, 3 inguinal Sexing: urogenital distance (Males > Females Scent glands (male accessory sex gland)

Sexing Male Female

Reproductive Physiology Puberty ~ 2-3 mo. (strain variation) Maximal fertility 3.5-10 months of age Estrous cycle length: 4-5 days Copulatory plug left by male Can palpate pregnancy by 12 days of gestation Gestation length 19-23 days (avg=21)

Parturition Nest-building ~ 5 days prepartum 1.5-4 hrs prepartum = fluid discharge Parturition takes 1-2, to several hrs. Dystocia is rare Average litter size is 6-12 pups Cannibalism in rats means dam is stressed

Parturition and Neonatal Life Pups are altricial (helpless), hairless, and nidicolous (blind) In utero maternal antibody transfer and via milk until around day 21 of age

More rats!!! Females go into heat between about 10 and 24 hours after giving birth. Specifically, females go into heat on the first evening that is at least 10 hours after giving birth (Gilbert et al. 1985). This phenomenon of coming into heat shortly after giving birth is called postpartum estrus.

Timed Pregnacies Vaginal smears Groups of females housed with a male and monitored Rats usually mate at night After mating (post coitus) a vaginal plug forms and falls out in about 24 hrs. Place a dark paper on cage floor and look for the plug to confirm mating Then remove male

Rat Pup Development Female ~ 250-300g Male ~ 300-600g Incisors erupt at 6-8 days Fully haired by 7-10 days Eyes open at 10-12 days Weaning at 21 days Lifespan: 2.5 - 3 years Adult weights: Female ~ 250-300g Male ~ 300-600g

Behavior and Handling Docile, easily trained Nocturnal Coprophagic Males are more sociable than females mice Males prefer company but tolerate single housing

No musty odor like mice Males can be housed together Females can be housed together, post parturient females may fight among.

Housing & Husbandry Most rats are housed in shoebox cages composed of polypropylene (opaque) or polycarbonate material (clear) with a wire bar lid used to hold the water bottle and feed. Bedding is placed directly into the shoe box cage allowing the absorption of urine and the animal to burrow. This type of cage will hold 1-3 adult rats depending on the size of the cage

Housing & Husbandry Caging: “Shoe-box” Germ free SPF (specific pathogen free) Conventional Solid floor

Housing & Husbandry 10 ml water/100g BW/24 hrs Optimal temperature range: 65-79 °F Keep humidity >40% to prevent ringtail Water requirement: 10 ml water/100g BW/24 hrs Medicating water may reduce intake

Do not supplement: obesity Need hard diet to wear teeth down Pelleted natural ingredient diets are used to feed all rodents and are composed primarily of cereal grains which are supplemented with additional protein, vitamins and minerals Balanced all-in-one pelleted diet 5 g food/100g BW/24 hrs Do not supplement: obesity Need hard diet to wear teeth down

Commercial Diets