At what age do rats begin to grow? - briefly
Rats experience the most rapid increase in body size during the first three weeks after birth, reaching juvenile proportions by that time. By five to six weeks of age they attain near‑adult weight and length.
At what age do rats begin to grow? - in detail
Rats exhibit a rapid increase in body mass during the first three weeks after birth. Neonates weigh approximately 5–6 g and achieve 15–20 g by the end of the first week. This phase is characterized by exponential growth of skeletal muscle, adipose tissue, and internal organs, driven primarily by high levels of growth hormone and insulin‑like growth factor‑1.
Around post‑natal day 21 (three weeks), rats are weaned from maternal milk and transition to solid food. The weaning period coincides with a marked acceleration in somatic growth; average body weight reaches 50–70 g, and tibial length increases by roughly 30 %. Nutrient intake, particularly protein and essential fatty acids, becomes a decisive factor in determining the growth trajectory during this interval.
Puberty initiates between five and six weeks of age. Gonadal maturation triggers a secondary growth spurt, especially in males, whose body weight can rise from 150 g at six weeks to 250–300 g by eight weeks. Females typically attain 200–250 g within the same timeframe. Sexual maturity, confirmed by the presence of spermatozoa in males or estrous cycling in females, occurs around eight to twelve weeks, after which growth rate declines sharply.
By twelve weeks, most laboratory strains have reached a plateau in body mass: Sprague‑Dawley and Wistar rats stabilize at 300–350 g, while smaller strains such as the Long‑Evans may plateau near 250 g. Skeletal growth ceases as epiphyseal plates close, and organ size reaches functional adulthood. Subsequent weight fluctuations reflect changes in adiposity rather than linear growth.
Key developmental milestones
- Day 0–7: Birth weight ~5–6 g; rapid increase to 15–20 g. Primary reliance on maternal milk.
- Day 14–21: Weaning; body weight 50–70 g; tibial length expands ~30 %.
- Week 5–6: Onset of puberty; secondary growth spurt; males 150 g, females 130 g.
- Week 8–12: Sexual maturity; weight plateau at strain‑specific adult values; epiphyseal closure.
Growth is modulated by genetics (strain differences), sex, environmental temperature, and dietary composition. Accurate assessment relies on regular body‑weight measurements and linear dimensions (nose‑to‑tail length, tibia length). Once the adolescent phase concludes, further size changes are minimal, marking the end of the growth period.