How much does a laboratory rat weigh? - briefly
A typical adult laboratory rat weighs between 250 g and 350 g, depending on strain and age. Females are generally lighter than males, often falling near the lower end of this range.
How much does a laboratory rat weigh? - in detail
Laboratory rats typically belong to the species Rattus norvegicus and are bred for research. Adult individuals weigh between 250 g and 500 g (0.55–1.10 lb), with most standard strains falling in the 300–350 g (0.66–0.77 lb) range. Weight varies with sex, age, strain, diet, and health status.
Male rats are generally heavier than females. In common outbred strains such as Sprague‑Dawley and Wistar, mature males average 350–400 g, while females average 250–300 g. In the Long‑Evans strain, typical adult weights are 300–350 g for males and 250–300 g for females. Fischer 344 rats tend to be lighter, averaging 250–300 g for males and 200–250 g for females.
Age‑related growth follows a predictable pattern:
- Birth: 5–7 g (0.01–0.02 lb)
- 3 weeks: 50–100 g (0.11–0.22 lb)
- 6 weeks: 150–250 g (0.33–0.55 lb)
- 8–10 weeks (young adult): 250–300 g (0.55–0.66 lb)
Weight gain accelerates during the juvenile phase and stabilizes after sexual maturity, usually around 10–12 weeks of age.
Strain‑specific reference ranges (average adult male weight) are often listed in laboratory animal manuals:
- Sprague‑Dawley: 350 g
- Wistar: 350 g
- Long‑Evans: 325 g
- Fischer 344: 275 g
Measurement protocols require calibrated analytical balances with a precision of at least 0.1 g. The animal is weighed in a clean cage or a restraining device, and the cage weight is subtracted to obtain the net mass. Scales should be tared before each session, and readings are recorded to the nearest gram.
Dietary manipulation directly influences body mass. Standard chow maintains weights within the normal range, whereas high‑fat or high‑calorie regimens can raise adult weights to 500 g or more, creating an obese phenotype used in metabolic studies.
Environmental factors such as temperature, housing density, and stress levels also affect weight. Cooler ambient temperatures increase metabolic demand, potentially reducing body mass, while overcrowding can lead to weight loss due to competition for food.
In summary, a typical adult laboratory rat weighs between 250 g and 500 g, with most standard strains averaging around 300–350 g. Precise weight values depend on sex, strain, age, nutrition, and housing conditions, and accurate measurement follows strict laboratory protocols.