How much do rats weigh per year? - briefly
Rats generally add 10–15 g each month, so an adult laboratory or pet rat weighs roughly 300–400 g after twelve months.
How much do rats weigh per year? - in detail
Rats attain most of their body mass within the first three months after birth. Newborn pups weigh 5–7 g; by eight weeks they typically reach 150–300 g, depending on strain and sex. After this period growth slows dramatically, and adult weight stabilizes for the remainder of the year.
Key factors influencing yearly weight:
- Genetics: Laboratory strains such as Sprague‑Dawley average 250–300 g, while wild brown rats (Rattus norvegicus) often weigh 350–500 g due to varied diet.
- Sex: Males are 10–20 % heavier than females of the same strain.
- Nutrition: High‑calorie pellets can increase adult weight by up to 30 % compared with a low‑fat diet.
- Seasonality: Wild populations exhibit a 5–10 % increase in mass during colder months, attributable to fat deposition for thermoregulation.
- Health status: Illness or parasite load can reduce body mass by 15–25 % relative to healthy counterparts.
Typical weight trajectory for a laboratory rat:
- Week 0–2: 5–7 g (neonatal)
- Week 3–4: 20–30 g (juvenile)
- Week 5–8: 100–200 g (rapid growth)
- Month 3–12: 250–300 g (plateau)
For wild rats, the plateau may be reached later, around month 5, and final adult weight often exceeds 400 g. Throughout the twelve‑month period, weight fluctuations rarely exceed ±5 % of the established adult average, unless external variables such as food scarcity or extreme temperature intervene.