• Basic rat statistics
• Definitions

Rats are extremely well studied animals, so we know an enormous amount about their basic biology. Here's a list of the Norway rat's basic biological data (Baker et al. 1979 and Weihle 1987, as reported in Pass and Freeth 1993; additional references noted below). For definitions of some of the terms and measurements used, see the next section.

Life span
Captivity	2-3.5 years	Pass and Freeth 1993
Wild	<1 year	Davis 1948
Weight
Birth	5-6 g	Pass and Freeth 1993
Weaning	30-55 g	Pass and Freeth 1993
Puberty	150-200 g	Pass and Freeth 1993
At 12 weeks: males	200-400 g	Pass and Freeth 1993
At 12 weeks: females	150-270 g	Pass and Freeth 1993
Adult: males	300-800 g	Pass and Freeth 1993
Adult: females	250-400 g	Pass and Freeth 1993
Developmental and life history events
Hair coat	9 days	Pass and Freeth 1993
Incisors erupt	8-10 days	Addison and Appleton 1915
Eyes open	12-14 days	Pass and Freeth 1993
Ear canal open	12-14 days	Pass and Freeth 1993
First leave nest to explore	14-16 days	Nelson 1995
First molars erupt	19 days	Addison and Appleton 1915
Second molars erupt	21 days	Addison and Appleton 1915
Weaning age	20-21 days (after 2nd molars erupt)	Pass and Freeth 1993
Faint yellow pigment appears on upper incisors	21 days	Addison and Appleton 1915
Faint yellow pigment appears on lower incisors	25 days	Addison and Appleton 1915
Descent of testes	15-50 days	Pass and Freeth 1993
Young rats first appear above ground	22-30 days	Adams and Boice 1983, Boice 1977
Third molars erupt	35 days	Addison and Appleton 1915
Puberty: males	39-47 days	Engelbrect J. T. et al. 2000, Chappel and Ramaley 1985, Korenbrot et al. 1977
Puberty: females	34-38 days	Engelbrect J. T. et al. 2000
Social maturity	160-180 days (5-6 months)	Adams and Boice 1983
Menopause (females)	450-540 days	Durbin 1966
Reproduction
Estrus cycle	4-6 days	Pass and Freeth 1993
Gestation	21-22 days	Pass and Freeth 1993
Litter size	6-14 offspring (varies with maternal age)	Pass and Freeth 1993, Mohan 1974
Maximum milk yield	12-14 days postpartum	Pass and Freeth 1993
Physiology
Rectal temperature	38-39º C	Pass and Freeth 1993
Heart rate	320-480 bpm (beats per min)	Pass and Freeth 1993
Blood pressure: diastolic	60-90 mm Hg	Pass and Freeth 1993
Blood pressure: systolic	75-120 mm Hg	Pass and Freeth 1993
Respiratory rate	85-110 breaths/min	Pass and Freeth 1993
Tidal volume	1.6 ml	Pass and Freeth 1993
Body surface area in cm2	9.1 x g0.66	Pass and Freeth 1993
Food consumption
Daily food consumption	5 g/100 g body weight	Pass and Freeth 1993
Daily water consumption	8-11 ml/100 g body weight	Pass and Freeth 1993
Urine
Urine output per day	5.5 ml/100g body weight	Pass and Freeth 1993
Urine osmolarity	1660 mOsm/kg of H20	Pass and Freeth 1993
Urine pH	7.5-8.5	Pass and Freeth 1993
Urine SG	1.04-1.07	Pass and Freeth 1993
Blood
Blood volume	5.6-7.1 ml/100 g body weight	Pass and Freeth 1993
Plasma volume	3.08-3.67 ml/100 g body weight	Pass and Freeth 1993
Red blood cell count	7-10 x 106cells/µl	Pass and Freeth 1993
Hemoglobin	11-19 gm/dl (dl = deciliter)	Pass and Freeth 1993
Packed cell volume	40.5-54%	Pass and Freeth 1993
Clotting time	2-5 min	Pass and Freeth 1993
Prothrombin time	8-4 sec	Pass and Freeth 1993
PTT	21.1 +- 3.7 sec	Pass and Freeth 1993
Fibrinogen	190 (150-230) mg/dl	Pass and Freeth 1993
Leukocyte count
Total	9(6-18) x 103/µl	Pass and Freeth 1993
Neutrophils	14-20%	Pass and Freeth 1993
Lymphocytes	69-86%	Pass and Freeth 1993
Monocytes	1-6%	Pass and Freeth 1993
Eosinophils	1-4%	Pass and Freeth 1993
Basophils	Rare	Pass and Freeth 1993
Platelets	500-1,000 x 103/µl	Pass and Freeth 1993

For those interested in a detailed summary of the timeline of neural development of the rat in utero, see Clancy et al. 2001.

• Basophil: type of white blood cell that releases anticoagulants during an inflammatory response. Involved in allergic reactions.
• Blood pressure, diastolic and systolic: Blood is pumped through arteries by contractions of the heart. When the heart contracts (beats), it sends a surge of blood through the arteries. The highest blood pressure during this surge is the called the systolic pressure. The heart relaxes between beats, and the lowest pressure during this time is called diastolic blood pressure. A blood pressure measurement is made up of both the systolic and diastolic pressure. It is normally recorded as the two pressures separated by a slash, with the systolic number first: systolic/diastolic (e.g. 120/80).
• Body surface area. Body surface area is difficult to measure. It is therefore estimated as: surface area (square cm) = 10 x (mass in g)^2/3. The prefactor "10" is empirically adjusted. The prefactor for rats is 9.1. Therefore, to find the surface area of a rat, plug its mass into the equation 9.1 x g^0.66. So, a 300 gram rat would have a surface area of 9.1 x 300^0.66 = 392.6 cm²
• Eosinophil: white blood cell that contains granules filled with chemicals that damage parasites. Involved in allergic reactions.
• Fibrinogen: protein in blood plasma that is necessary for blood coagulation.
• Hemoglobin: The iron-protein pigment in red blood cells that carries oxygen. measured as the amount of hemoglobin in grams (gm), per deciliter (dl) of whole blood. A deciliter is 100 mililiters.
• Leukocyte: white blood cell.
• Lymphocyte: type of white blood cell of crucial importance to the adaptive immune response. There are two types:B-cells and T-cells.
• Monocyte: large white blood cell that scavenges and destroys invading microorganisms. In the tissues, monocytes develop into macrophages.
• Neutrophil: type of white blood cell involved in defense against bacteria and fungi
• Osmolarity: Total concentration of solutes in a solution. Specifically, the osmotic pressure exerted by a solution across a semi-permeable membrane when compared to pure water. Measured in osmols (Osm). An osmol is an amount of substance that dissociates (breaks apart) in solution to form one mole of osmotically active particles (particles that can pass through the membrane). ref
• Packed cell volume: the ratio of the volume occupied by packed red blood cells to the volume of whole blood.
• pH: logarithmic scale used to describe the acidity or alkalinity of a solution. A pH below 7 is acidic, above 7 is alkaline (or basic), and exactly 7 is neutral.
• Plasma volume: the volume of plasma in the blood. Plasma is the noncellular portion of blood, consisting of water, inorganic salts, and organic molecules.
• Platelet: Smallest of circulating blood cells. Forms plugs on damaged areas and initiates blood clots.
• Prothrombin time: Time necessary for blood to clot. Measures activity of clotting factors I, II, V, VII and X.
• PTT: partial thromboplastin time. Test that measures clotting time in plasma. Focuses on a specific pathway in the blood clotting process.
• Puberty: males: defined here as the time of balano-preputial-separation (BPS). BPS is the androgen-dependent separation of the foreskin (prepuce) from the glans penis (balanus) (Korenbrot et al. 1977, Gaytan et al. 1988). The BPS is not synonymous with puberty, but it is an androgen-dependent event that is necessary for complete copulatory behavior, and is therefore a commonly used index of pubertal development. The BPS just precedes the appearance of mature sperm in the epididymis (Korenbrot et al. 1977). The BPS also precedes the ability to perform full copulatory behavior, because to ejaculate the male must be able to evert the penis from the sheath (Moltz 1975). If male rats are handled by humans every day from day 1 to day 21 (weaning), puberty is delayed by 2.6 days (from age 42.0 to 44.6 days), probably by delaying the LH peak, which in turn delays the androgen peak (Korenbrot et al. 1977).
• Puberty: females: defined here as the time of vaginal opening (VO), also called vaginal patency.
• SG: specific gravity. The ratio of the weight of a volume of liquid to the weight of an equal volume of water.
• Tidal volume: the volume of air inhaled/exhaled during normal breathing. This is not the total volume of the lungs, because the lungs always retain some air during the exhale.