Squirrels and rats are related, classified biologically as cousins within the animal kingdom. Both animals belong to Rodentia, the largest and most varied order of mammals on Earth, accounting for approximately 40% of all mammal species. Their shared membership in this biological group places them on the same branch of the evolutionary tree, confirming a distant common ancestor. This classification signifies that despite their obvious differences, their fundamental body plan is rooted in a single, successful lineage. To understand their relationship, it is necessary to examine the specific scientific levels that connect and then separate them.
Shared Ancestry: Membership in the Rodent Order
The primary feature uniting squirrels and rats is the specialized dental structure required for their lives as gnawing mammals. All members of the Order Rodentia possess a single, prominent pair of incisor teeth in both the upper and lower jaws. These front teeth are aradicular, meaning they are rootless and continue to grow throughout the animal’s entire lifespan. This continuous growth necessitates constant gnawing to keep the teeth worn down to a functional size.
The incisors are self-sharpening due to the tooth’s composition; the front surface is covered in hard enamel, while the back is softer dentin. As the rodent chews, the softer material wears away faster, leaving a perpetually sharp, chisel-like edge that is highly effective for cutting through hard materials. Furthermore, rodents lack canine teeth, creating a large, toothless gap known as a diastema between the incisors and the cheek teeth.
The Great Divide: Different Suborders and Families
While the shared dental structure places them in the same Order, the evolutionary split that created their distinct forms occurs at the next levels of classification: the Suborder and Family. Rodentia is divided into five suborders, largely based on the structure of the masseter muscle and jaw arrangement. This muscular and skeletal difference dictates how they chew and gnaw, separating the “squirrel-like” rodents from the “rat-like” rodents.
Squirrels belong to the Suborder Sciuromorpha and the Family Sciuridae, which includes tree squirrels, ground squirrels, and marmots. Rats, conversely, belong to the Suborder Myomorpha and the Family Muridae, a group that also encompasses mice and gerbils. Muridae is the largest family of mammals, containing over 700 species. The separation at the suborder and family level indicates their relationship is akin to second or third cousins, having diverged millions of years ago.
Observable Differences in Anatomy and Lifestyle
The taxonomic split is visibly reflected in the animals’ physical characteristics and daily habits, which are adaptations to their separate evolutionary paths. The most immediate difference is the tail structure. Squirrels possess a large, bushy tail often used for balance while climbing, as a blanket for warmth, or as a parachute-like aid when falling. Rats, particularly common species like the Norway or roof rat, have a long, thin, and largely hairless tail that serves primarily for thermoregulation and balance on the ground.
Their preferred habitats and activity patterns are also highly distinct. Squirrels are generally diurnal, meaning they are active during the day, and many species are arboreal, spending significant time in trees. Rats are predominantly terrestrial and nocturnal, venturing out mainly at night, and they are often fossorial, specializing in burrowing and underground living. This nocturnal lifestyle is supported by a reliance on whiskers for navigation, contrasting with the squirrels’ sharper daytime vision.
Finally, their diets differ significantly. Squirrels tend to be specialized foragers focused on nuts and seeds. Rats are highly generalized omnivores that will consume a vast array of plant and animal material, reflecting their ability to thrive in almost any environment, including those created by humans.