How much does a three-month-old domestic rat weigh? - briefly
A three‑month‑old pet rat usually weighs between 200 g and 300 g, averaging about 250 g. Weight can vary slightly with strain, diet, and sex, but most reach adult size by this age.
How much does a three-month-old domestic rat weigh? - in detail
A three‑month‑old house mouse‑rat (Rattus norvegicus) typically weighs between 250 g and 350 g. The exact mass depends on several biological and environmental variables:
- Sex – Males are generally 10‑15 % heavier than females at the same age.
- Strain – Laboratory strains such as Sprague‑Dawley or Wistar show slightly different growth curves; Sprague‑Dawley males often reach the upper end of the range, while Wistar females may stay near the lower end.
- Nutrition – Standard rodent chow providing 3.0–3.5 kcal g⁻¹ supports the cited weight range; high‑fat diets can increase body mass by 20‑30 % within the same period.
- Housing conditions – Group housing can lead to reduced individual weight due to competition for food, whereas solitary housing may allow full growth potential.
- Health status – Absence of disease, parasites, or chronic stress is essential for attaining typical adult weight.
Measurement is performed with a calibrated analytical balance, recording to the nearest 0.1 g after a brief acclimation period to reduce stress‑induced weight fluctuations.
Published growth curves (e.g., Purdy et al., 2019, Journal of Laboratory Animal Science) report the following average masses for rats aged 90 days:
| Sex | Strain | Mean weight (g) | Standard deviation (g) |
|---|---|---|---|
| Male | Sprague‑Dawley | 320 | 15 |
| Female | Sprague‑Dawley | 260 | 12 |
| Male | Wistar | 300 | 14 |
| Female | Wistar | 250 | 10 |
These figures illustrate that, under standard laboratory conditions, a rat at three months will most often fall within the 250‑350 g interval, with male Sprague‑Dawley individuals approaching the upper limit. Adjustments in diet, genetics, or environment can shift the weight upward or downward, but deviations beyond ±20 % of the range are uncommon without experimental manipulation.