Young mice, often called pups, are born in a state of complete dependency. Their journey from birth to self-sufficiency is a rapid and complex process, marked by significant physical and behavioral transformations.
Early Development and Growth
Newborn mouse pups are altricial, meaning they are born blind, deaf, and hairless, relying entirely on their mother for survival. At birth, they appear bright pink and may have a visible milk spot, indicating recent feeding. Their ears are initially sealed, appearing as small nubs, and their eyes remain closed.
Around day 2, their ears start to develop, becoming fully detached from the head by day 4. Fur growth begins around day 5 or 6, initially as a light fuzz that thickens and covers their bodies by day 10. Eyes begin to open around day 11 and are fully open by day 12 to 14. Teeth also start to erupt around day 11, allowing them to begin nibbling solid food by day 12.
Nesting and Maternal Care
Mouse mothers construct nests in secluded and protected locations to house their offspring. Nesting is an instinctive behavior that intensifies a few days before birth and continues for approximately two weeks postpartum. The quality of the nest directly correlates with pup survival, with dome-shaped nests offering better protection than cup-shaped or flat ones.
Maternal care involves continuous nursing, grooming, and keeping the pups warm within the nest. The mother’s milk provides nutrition and plays a protective role in developing the pups’ immune systems. It contains components like the complement system, which helps shape the gut microbiome of the pups, making them less susceptible to certain bacterial infections. This mechanism is independent of antibodies and supports early life health.
Transition to Independence
Weaning, the process of transitioning to independence for young mice, begins around three weeks of age. During this period, pups gradually shift from relying solely on their mother’s milk to incorporating solid foods into their diet.
As they develop, juvenile mice begin exploring their surroundings and learning survival skills. This includes foraging for food independently and developing strategies to avoid predators. Mice are considered fully independent after 21 days, entering the juvenile stage which lasts until about six weeks of age when they reach sexual maturity.