The growth of common North American tree squirrels, such as the gray, red, or fox squirrel, is a rapid process. This accelerated development transforms a helpless newborn into an agile, independent adult in a matter of months. The timeline for growth is tightly packed, with specific developmental milestones occurring consistently across the first three months of life. This swift rate of change prepares the young squirrel for survival in a demanding environment.
Neonatal Development (Weeks 1–4)
A newborn squirrel, often called a kit or pup, enters the world in a completely undeveloped state, weighing only about 13 to 20 grams. At birth, these young are pink, entirely hairless, and possess sealed eyes and ears, making them blind and deaf. This defenseless condition makes them fully reliant on the mother for both nutrition and warmth within the nest, typically built in a tree cavity or a leafy drey.
During the second week, the skin begins to darken, and a thin layer of fine fur starts to appear. The lower incisor teeth begin to erupt around the end of the third week, marking the first preparation for solid food consumption. By the fourth week, thin fur covers most of the body, and the eyes and ears are preparing to open, though they remain closed. The kit’s weight increases significantly, with some gray squirrels reaching 90 to 135 grams.
Transitional Growth and Independence (Weeks 5–12)
The fifth week signals a dramatic shift as the eyes begin to open. This newfound vision quickly leads to increased activity and awareness of their surroundings within the nest. The upper incisors emerge around five or six weeks, and the cheek teeth follow shortly after, indicating a readiness for gnawing and chewing.
By six to seven weeks, the young squirrels are fully furred and visually resemble miniature adults, complete with a developing bushy tail. They begin to explore the nest and its immediate surroundings, with limbs and claws growing stronger for climbing practice. Weaning, the transition from mother’s milk to solid foods, begins around seven to eight weeks of age, though the mother may still provide milk for several more weeks.
Between ten and twelve weeks, the young leave the nest to become fully independent. They are now actively foraging, learning essential survival skills like burying food and evading predators through observation of their mother. The young typically achieve full independence and disperse from the natal area by the twelfth week.
Reaching Adult Size and Maturity
Even after achieving independence and leaving the nest around 12 weeks, the squirrel has not yet reached its full physical size or reproductive maturity. The young squirrel continues to grow, steadily increasing in weight and body length over the next several months. Most tree squirrels reach their full adult size and weight, which for a gray squirrel can range from 400 to 600 grams, by about nine months of age.
The final stage of growth involves reproductive development, which occurs later than physical growth. Sexual maturity is typically reached in the spring or summer following their birth, meaning they are approximately 10 to 12 months old. For gray squirrels, females generally become fertile around 15 months of age, while males may be reproductively mature slightly earlier, closer to 12 months. This delayed timeline ensures the young have the necessary size, strength, and survival experience before contributing to the next generation.