When do rats stop growing? - briefly
Rats reach their final body size by roughly five to six months of age, with skeletal growth completing around that time. After this period, weight may fluctuate, but length and overall dimensions remain stable.
When do rats stop growing? - in detail
Rats experience a rapid increase in body mass during the first three weeks after birth, reaching approximately 70‑80 % of adult weight by day 21. Growth slows markedly after this period; most individuals stop gaining significant weight between five and six weeks of age. Sexual maturity appears around six to eight weeks, while skeletal ossification and final length are typically completed by ten to twelve weeks. At this point, weight plateaus and further increases are limited to minor fluctuations in body condition.
Key developmental milestones:
- Birth: 5–7 g, hairless, eyes closed.
- Day 7‑10: eyes open, fur develops, weight ~15 g.
- Week 3: weaning, weight ~70 g (≈80 % adult).
- Week 5‑6: growth rate declines, weight stabilizes near 100 g for females, 120 g for males.
- Week 8‑10: sexual maturity, testes descend in males, estrous cycles begin in females.
- Week 10‑12: skeletal growth completed, body length reaches adult measurement.
Growth cessation is governed by genetic programming, hormonal regulation (growth hormone, IGF‑1, sex steroids), and environmental factors such as nutrition and housing density. Males generally attain a slightly larger final mass and may extend the plateau phase by a few days compared with females.
Weight curves obtained from regular weighing (daily during the first month, then weekly) provide the most reliable indicator of growth completion. Radiographic assessment of epiphyseal plate closure offers a definitive measure of skeletal maturity.
For laboratory settings, selecting rats at eight to ten weeks ensures adult physiology without the confounding effects of ongoing growth. Pet owners should expect full size by three months and can use weight stabilization as a practical sign that the animal has reached maturity.