When does a rat grow fur? - briefly
Rats are born hairless; their first fur sprouts around the fifth to seventh day after birth, and a complete coat is typically present by two weeks of age.
When does a rat grow fur? - in detail
Rats are born hairless, with skin covered only by a thin, translucent membrane. The first coat of fur appears during the first week after birth. By 3–5 days, fine, down‑like hairs emerge on the head and trunk, providing minimal insulation. Between days 7 and 10, these initial hairs are replaced by a denser, coarser pelage that continues to thicken over the next two weeks.
At approximately 14–21 days, the juvenile coat reaches full coverage, including the tail, ears, and limbs. This period coincides with the weaning stage, when the pups begin to consume solid food and their metabolic rate increases, supporting rapid hair growth.
Species variations affect timing. Laboratory strains such as the Sprague‑Dawley typically follow the schedule above, while wild‑derived rats may exhibit a slightly earlier onset of fur development, with visible hair by day 5 and complete pelage by day 12. Larger species, such as the Norway rat (Rattus norvegicus), show similar patterns but may have a marginally longer maturation window.
Factors influencing hair growth include:
- Ambient temperature: cooler environments accelerate follicle activity, resulting in thicker fur earlier.
- Nutrition: adequate protein and essential fatty acids are required for keratin synthesis; deficiencies delay pelage formation.
- Hormonal status: thyroid hormones and growth factors regulate follicle cycling; hypothyroidism can prolong hairlessness.
After the initial juvenile coat, rats undergo a molt around 6–8 weeks of age, shedding the soft baby fur and developing the adult coat, which remains until the end of life unless altered by seasonal changes or health conditions.