The common house mouse, known scientifically as Mus musculus, is a small rodent often found in close proximity to human environments. Understanding the typical size of an adult house mouse can be helpful for identification.
The Typical Size of an Adult Mouse
A full-grown house mouse measures between 7.5 to 10 centimeters (3 to 4 inches) from its nose to the base of its tail. The tail itself can be as long as or slightly longer than its body, ranging from 5 to 10 centimeters (2 to 4 inches) in length, contributing to a total length of 12.5 to 20 centimeters (5 to 8 inches) for an adult mouse. An adult house mouse is quite light, weighing between 11 to 30 grams (approximately 0.4 to 1 ounce).
When is a Mouse Considered Full Grown?
Mice develop rapidly, reaching full physical size and sexual maturity quickly after birth. Newborn mice are born blind, hairless, and completely dependent on their mother. Their ears fully develop around the fourth day, and fur begins to appear by about six days.
Their eyes open around 13 to 14 days after birth, and they are weaned from their mother at 21 days (three weeks of age). At this point, juvenile mice can begin foraging for food independently. Female house mice can reach sexual maturity and begin reproducing as early as five to six weeks of age, while males mature around eight weeks. A mouse is considered “full grown” or an adult once it reaches this stage of physical and reproductive maturity, which is generally around six weeks old.
Mouse Versus Rat: Key Size Differences
Distinguishing a full-grown mouse from a young rat can be challenging due to some overlap in overall size. However, several physical characteristics differentiate them beyond just length or weight. A house mouse has a relatively smaller, more pointed snout, and its ears are proportionally larger compared to its head size. These ears may also appear sparsely haired.
In contrast, a young rat has a blunter, larger head relative to its body, with smaller ears that are more proportionate to its head. The tail of a mouse is thin, long, and covered with fine hairs, often being as long as or longer than its body. A young rat’s tail, however, is thicker, scaly, and almost hairless. Additionally, young rats have larger feet compared to their body size than mice do. Another differentiating factor is their droppings; mouse droppings are small (around 3-6 mm) with pointed ends, while rat droppings are larger (around 12-18 mm) and can be more blunt or capsule-shaped.