The life of a mouse progresses rapidly, with significant transformations occurring in a very short period. This quick development is a characteristic of the species. Each phase of their life, from birth to old age, is marked by distinct physical and behavioral changes that offer insight into their biology.
The First Month: Newborn and Weaning Stages
A mouse begins its life completely dependent on its mother. At birth, pups are hairless, deaf, and their eyes are sealed shut. They rely on their mother for nourishment and warmth, as they cannot regulate their own body temperature. A litter consists of five to eight pups, though it can be as large as twelve.
The second week marks a period of rapid change. By day four, the ears have developed, and by day six, a fine layer of fur begins to grow, which is fully developed by day 10. Around day 14, their eyes open, and they begin to gain mobility and explore the immediate vicinity of the nest.
Between the third and fourth weeks, pups enter the weaning stage, transitioning from milk to solid food. They also increase their mobility and mimic the social behaviors of their mother and littermates. By the end of the first month, they are largely independent.
The Juvenile and Young Adult Mouse
Following weaning, a mouse enters its juvenile period at approximately four weeks of age. Lasting until about eight weeks, this stage is characterized by high energy, curiosity, and playful behavior. Mice develop motor skills, learn to forage, and reach sexual maturity, with some females able to reproduce as early as five to six weeks old.
In the young adult phase, from about eight to twelve weeks, growth stabilizes and juvenile energy subsides. The mouse reaches its full physical size during this time. Social hierarchies that began forming in the juvenile stage become more established.
The Adult and Senior Mouse
From three months to about a year and a half, a mouse is in its prime adulthood. Its weight is stable, and it maintains peak physical condition. The behaviors of an adult mouse are settled and routine, focused on nesting, foraging, and reproduction. For house mice indoors, this stage can extend to three years due to a stable environment.
As a mouse surpasses 1.5 years of age, it enters its senior stage, and signs of aging become apparent. Indicators include gradual weight loss, a decline in activity, and more time spent sleeping. The fur may become thinner or scruffy, and some older mice develop a clouding of the eyes.
This later stage can also bring age-related health issues, such as tumors and respiratory problems. These conditions affect the mouse’s quality of life, and a veterinarian can provide guidance on management. The focus shifts to ensuring the animal’s comfort as its life cycle ends.