How much should a five-month-old rat weigh? - briefly
A rat that is five months old typically weighs between 250 g and 350 g, with males usually closer to the upper limit. Females tend to be nearer the lower end of that range.
How much should a five-month-old rat weigh? - in detail
A five‑month‑old laboratory rat is typically at the end of its rapid growth phase and should weigh within a well‑defined range that depends on strain, sex, and husbandry conditions.
The most common strains show the following average weights at 150 days of age:
- Sprague‑Dawley: males ≈ 350–450 g; females ≈ 250–350 g.
- Wistar: males ≈ 300–400 g; females ≈ 200–300 g.
- Long‑Evans: males ≈ 300–380 g; females ≈ 210–280 g.
These values represent the central 95 % of a healthy population under standard laboratory feeding (approximately 20 g of pelleted diet per day) and optimal environmental conditions (22 ± 2 °C, 12‑hour light/dark cycle, enrichment). Individual animals may fall outside these limits due to genetic variation, litter size, or experimental manipulation.
Key factors influencing weight at this age:
- Sex – Males consistently exceed females by 30–40 % because of greater muscle mass and skeletal growth.
- Nutrition – Caloric restriction reduces final weight by 10–20 %; high‑fat or high‑protein diets can increase it by a similar margin.
- Health status – Chronic disease, parasitic infection, or stress can cause weight loss of 5–15 % relative to the expected range.
- Housing density – Overcrowding may limit access to food, leading to lower body mass.
Weight measurement should be performed with a calibrated analytical balance (accuracy ± 0.01 g). Animals are best weighed in the morning, after a brief fasting period of 2–3 hours, to minimize gastrointestinal content variability.
Monitoring weight weekly from weaning (21 days) to five months provides a growth curve that can be compared against standard reference charts for the specific strain. Deviations of more than 10 % from the predicted trajectory warrant veterinary assessment and possible adjustment of diet or environment.
In summary, a healthy five‑month‑old rat of common laboratory strains should fall within the 200–450 g window, with precise expectations determined by strain, sex, and husbandry practices. Regular, accurate weighing is essential for confirming normal development and detecting early signs of pathology.