How are mice and rats born? - briefly
Mice and rats are viviparous mammals; females undergo a gestation of roughly 19–23 days and then deliver litters of 5–12 (mice) or up to 14 (rats) live offspring. The newborns are altricial, hairless and blind, requiring maternal care for several weeks before weaning.
How are mice and rats born? - in detail
Mice and rats reproduce through internal fertilization, producing litters after a short gestation. The reproductive cycle begins with the female’s estrous phase, which lasts 4–5 days; ovulation occurs at the end of proestrus. Mating typically takes place during the dark period, when males pursue receptive females and achieve copulation within a few minutes.
Fertilization occurs in the oviduct, after which the zygote travels to the uterus. Implantation takes place around day 4–5 of pregnancy. Gestation lasts approximately 19–21 days in mice and 21–23 days in rats. During this period, embryos develop rapidly: organogenesis is completed by day 14, and the fetus gains weight and fat reserves in the final days.
Birth is a coordinated process:
- Labor onset: uterine contractions begin, often triggered by hormonal changes (decrease in progesterone, increase in prostaglandins).
- Delivery: the dam assumes a prone position, pushes each pup out individually; each pup is born encased in a thin membrane that the mother removes with her teeth.
- Post‑natal care: the mother licks the newborns to stimulate respiration, cleans the umbilical cords, and initiates nursing within minutes.
Typical litter sizes range from 4 to 12 pups in mice and 6 to 14 in rats, though variations depend on strain, age, and nutrition. Neonates are altricial: eyes remain closed, ears are folded, and they rely entirely on maternal warmth and milk. Growth is rapid; weight doubles within the first week, and weaning occurs at 21 days for mice and 28 days for rats.