What should an adult rat weigh? - briefly
An adult laboratory rat usually weighs 250–350 g, varying by strain and sex. Wild adult rats are lighter, averaging 150–250 g.
What should an adult rat weigh? - in detail
Adult rats of the species Rattus norvegicus reach a stable body mass after 10–12 weeks. Male laboratory strains commonly weigh 300–500 g, while females average 250–350 g. The exact figure depends on genetic line: for example, Sprague‑Dawley males tend toward the upper end of the range, whereas Wistar females cluster near the lower end. Wild brown rats captured in urban environments typically range from 180 g to 300 g, reflecting limited food availability and higher activity levels.
Factors that modify mature weight include:
- Diet composition – high‑protein, calorie‑dense feed accelerates growth and raises final mass.
- Housing conditions – spacious cages reduce stress‑induced weight loss; overcrowding can suppress appetite.
- Health status – parasitic infections, respiratory disease, or metabolic disorders cause measurable weight decline.
- Sex and reproductive state – pregnant or lactating females gain 30–50 g above baseline.
Accurate assessment employs a calibrated digital scale with a precision of ±1 g. Weighing should occur at the same time of day, preferably after a brief fasting period, to minimize gastrointestinal variation. Repeated measurements over several days confirm a stable value and helps identify abnormal fluctuations.
In summary, a mature laboratory rat is expected to fall within 250–500 g, with males generally heavier than females; wild counterparts occupy a lower interval of 180–300 g. Deviations outside these limits warrant investigation of diet, environment, or health.