The common question of whether a squirrel is a rodent or a mammal stems from a misunderstanding of how biologists organize the living world. These terms do not represent mutually exclusive categories, but rather different levels of biological grouping. Squirrels are classified as both a mammal and a rodent, reflecting the broad traits they share with many animals and the specialized traits that link them to a smaller group. Understanding this dual identity requires looking into the structured system.
The Hierarchical Structure of Life
Scientists use a system called Linnaean taxonomy to organize all organisms into a nested hierarchy, starting with broad categories and moving toward increasingly specific ones. This framework includes levels such as Kingdom, Phylum, Class, and Order, where each category is contained within the one above it. A simple way to think of this structure is that all apples are fruit, but not all fruit are apples.
In the case of the squirrel, its classification starts with the Class Mammalia, which is a large and inclusive grouping. Within that broad Class, all members are then sorted into smaller, more specialized groups, one of which is the Order Rodentia. Therefore, every animal belonging to the Order Rodentia is automatically a member of the Class Mammalia, but the reverse is not true. This nested structure explains how the squirrel can simultaneously fulfill the requirements of two different biological groups.
Why Squirrels Are Classified as Mammals
Squirrels are classified into the Class Mammalia based on fundamental biological traits shared by all mammals. One of the most defining characteristics is endothermy, which means squirrels are warm-blooded and can internally regulate their body temperature regardless of the external environment. This ability is supported by a four-chambered heart that efficiently separates oxygenated and deoxygenated blood.
Squirrels also possess a body covering of hair or fur, which provides insulation necessary for maintaining their stable internal temperature. Female squirrels have mammary glands that produce milk to nourish their young after giving birth, a trait from which the Class Mammalia derives its name. Furthermore, all mammals, including squirrels, have three unique bones in the middle ear—the malleus, incus, and stapes—which transmit sound vibrations.
The Defining Characteristics of Rodents
A squirrel is classified as a rodent because it belongs to the Order Rodentia, a group defined almost entirely by a specialized dental structure. Rodentia is the single largest order of mammals, containing approximately 40% of all mammalian species, including mice, beavers, and porcupines. The primary feature that unites this diverse group is the presence of a single pair of upper and lower incisors that grow continuously throughout the animal’s life.
These incisor teeth are rootless and open-ended, meaning they never stop growing, which necessitates constant gnawing to wear them down. The teeth have a hard layer of enamel only on the front surface, with softer dentine material on the back. As the squirrel gnaws, the softer dentine wears away faster than the hard enamel, which naturally creates a sharp, chisel-like edge that is perpetually self-sharpening. This unique dental arrangement also results in a significant gap, known as a diastema, between the incisors and the cheek teeth, as rodents lack canine teeth altogether.