Mice and squirrels share a common ancient ancestry, as both animals belong to the largest order of mammals, Rodentia. This grouping signifies a common evolutionary origin tracing back tens of millions of years. Understanding their connection requires examining the anatomical features that define the order before looking at the specialized traits that separated their lineages.
Shared Characteristics of Rodents
All members of the Order Rodentia share a highly specialized dental structure. This shared feature is a single pair of continuously growing, chisel-like incisor teeth in both the upper and lower jaws. The unique construction of these teeth defines the entire group, giving the order its name, which derives from the Latin word rodere, meaning “to gnaw.”
The incisors are rootless, growing throughout the animal’s life, which necessitates constant gnawing to keep them worn down. These teeth are covered with a thick layer of hard enamel only on the front surface. Because the softer dentine on the back wears away faster, the differential wear creates a perpetually sharp, self-sharpening edge. A significant gap, known as a diastema, exists between these incisors and the grinding cheek teeth, as rodents lack canine teeth altogether.
Defining Traits and Ecological Niche
Despite their shared dental blueprint, mice and squirrels belong to distinct biological families, reflecting a major evolutionary split. Squirrels are classified in the Family Sciuridae, part of the Suborder Sciuromorpha, while common house mice and rats belong to the Family Muridae (or the closely related Cricetidae), part of the Suborder Myomorpha. This taxonomic divergence is apparent in their physical and behavioral traits.
Body size varies dramatically between the groups, driven by their ecological needs. Many species of mice are relatively small, often weighing less than 50 grams, while the squirrel family includes the mouse-sized African pygmy squirrel as well as large groundhogs and marmots that can weigh up to 8,000 grams. Differences are seen in their morphology and activity patterns, reflecting different niches.
Squirrels, particularly tree squirrels, possess long, muscular legs and a large, bushy tail used for balance, signaling, and warmth. Mice, in contrast, typically have long, sparsely furred or scaly tails used for balance or thermoregulation.
Most tree and ground squirrels are diurnal, meaning they are active during the day. In contrast, the vast majority of mice and rats are nocturnal or crepuscular, preferring to forage at night or twilight.
Understanding Evolutionary Divergence
The lineages that led to modern mice and squirrels separated very early in the history of the Rodentia order. The fossil record indicates that the order itself arose during the Paleocene epoch, over 56 million years ago. Molecular clock estimates suggest that the Sciuromorpha suborder, which includes squirrels, represents one of the earliest offshoots from the common rodent ancestor.
The ancestors of Sciuridae and Myomorpha diverged tens of millions of years ago, leading to their specialized paths. This separation resulted in the vast diversity seen today, a process known as adaptive radiation. Squirrels became successful in arboreal, ground-dwelling, and gliding niches, while the Muridae lineage underwent an explosive diversification, making it the largest family of all mammals.