Taxonomy, the science of classifying organisms, confirms that squirrels are members of the Order Rodentia. This classification is based on shared anatomical characteristics, not superficial appearance. Rodentia is the largest group of mammals on Earth, and this grouping places squirrels alongside animals like mice, rats, and beavers.
The Defining Traits of Rodentia
The classification of any animal into the Order Rodentia is determined by its specialized dentition and the unique structure of its jaw. All true rodents possess a single, prominent pair of incisor teeth in both their upper and lower jaws. These teeth are described as aradicular or open-rooted, meaning they never stop growing throughout the animal’s life.
The incisors have a characteristic chisel shape because the hard enamel is only on the front surface, while the softer dentine is on the back. As the animal gnaws, the dentine wears down faster than the enamel, which naturally sharpens the tooth into an effective cutting edge. This continuous growth necessitates constant gnawing to keep the teeth worn down to a functional length.
Rodents lack canine teeth, which creates a significant gap, known as a diastema, between the incisors and the cheek teeth (premolars and molars). The jaw’s musculature is also specialized to facilitate the gnawing action. The masseter muscle is highly developed and attaches in ways that allow the lower jaw to move forward and backward during gnawing, a movement called propaliny.
Squirrels’ Specific Taxonomic Placement
Squirrels belong to the Order Rodentia, which is part of the larger classifications of Kingdom Animalia, Phylum Chordata, and Class Mammalia. Their definitive classification as rodents is based on the specialized incisors they share with all other gnawing mammals.
The most specific grouping for squirrels is the Family Sciuridae. This family includes familiar tree squirrels, ground squirrels, marmots, chipmunks, and flying squirrels. Sciuridae is organized into five subfamilies, including Sciurinae, which contains tree and flying squirrels, and Xerinae, which groups terrestrial species like marmots and prairie dogs. The family encompasses approximately 279 species across 51 genera.
Clarifying Common Misconceptions
Despite their classification, squirrels and other rodents are often confused with a few other small, gnawing mammals that appear outwardly similar. The most frequent point of confusion is with animals in the Order Lagomorpha, which includes rabbits, hares, and pikas. Lagomorphs share the trait of continuously growing incisors and a herbivorous diet, but their dental structure immediately separates them from rodents.
Lagomorphs, unlike rodents, have two pairs of upper incisors, with one pair situated directly behind the other. This arrangement, known as duplicidentata, is the primary anatomical difference separating the two orders. Other small mammals, such as shrews and moles, belong to the Order Eulipotyphla. These animals have a completely different dentition adapted for an insectivorous diet and lack the continuously growing incisors characteristic of Rodentia.