When does a rat become large? - briefly
A rat is regarded as large when its body length (excluding tail) reaches roughly 20–25 cm and its weight is between 300 and 500 g. These thresholds correspond to mature brown rats (Rattus norvegicus) under normal conditions.
When does a rat become large? - in detail
Rats reach a size that can be classified as large when they exceed typical adult dimensions for their species. For the common brown rat (Rattus norvegicus), an adult weight above 300 g (approximately 10.5 oz) and a body length (head‑to‑base of tail) longer than 250 mm (10 in) are generally regarded as large. The black rat (Rattus rattus) is considered large at a weight above 250 g and a body length exceeding 220 mm (8.7 in).
Growth to these dimensions follows a predictable pattern:
- Birth to weaning (0–21 days): Neonates weigh 5–7 g; rapid growth occurs as milk intake peaks.
- Juvenile phase (21–60 days): Weight increases by roughly 2–3 g per day; skeletal development accelerates.
- Early adulthood (60–120 days): Growth rate slows; most individuals attain 70–90 % of adult weight.
- Mature adult (120 days onward): Continued weight gain depends on diet, health, and genetics; some individuals surpass the large‑size thresholds.
Factors influencing attainment of large size include:
- Genetics: Selective breeding for size in laboratory strains can raise average adult weight by 20–30 %.
- Nutrition: High‑calorie diets, especially those rich in protein and fats, accelerate weight gain.
- Environment: Warm, low‑stress conditions reduce metabolic demands, allowing more energy to be allocated to growth.
- Sex: Males typically grow larger than females, often exceeding female weights by 10–15 %.
In captivity, rats provided ad libitum access to nutrient‑dense food can reach 400–500 g, far above the large‑size benchmark. In the wild, resource limitation and predation pressure keep most individuals near the lower end of the adult range, making truly large specimens relatively uncommon.
Therefore, a rat is deemed large when its weight and body length surpass the species‑specific adult averages, a condition achieved after roughly four months of age under favorable genetic and environmental circumstances.