Squirrels are not marsupials; they are classified as placental mammals. This distinction places them in one of the three main evolutionary branches of the Mammalia class, alongside marsupials and the egg-laying monotremes. This biological classification, known as taxonomy, organizes life based on shared ancestry and physical traits. The key difference between squirrels and marsupials lies in their differing reproductive strategies.
Squirrels Belong to the Rodent Family
Squirrels are classified as placental mammals belonging to the Order Rodentia, making them rodents. This order is the most diverse group of mammals, encompassing over 40% of all mammalian species. Within Rodentia, squirrels belong to the Family Sciuridae, which includes a wide variety of species like tree squirrels, ground squirrels, and flying squirrels.
The defining feature of the Rodentia order is the presence of a single pair of continuously growing incisors in both the upper and lower jaws. These specialized teeth are worn down by constant gnawing, a behavior essential for processing tough plant materials such as seeds and nuts. The Sciuridae family is further characterized by traits like slender bodies, large eyes, and, in many species, a notably bushy tail.
Key Characteristics of Marsupials
Marsupials belong to the Infraclass Marsupialia, a group defined primarily by a unique reproductive process. They give birth to highly underdeveloped, or altricial, young after a very short gestation period, which can be as short as 12 days. This results in a neonate that is often no larger than a honeybee.
Following birth, the tiny offspring, sometimes called a joey, must crawl unassisted to a specialized external pouch on the mother’s abdomen, known as the marsupium. Inside the pouch, the young attaches firmly to a teat, where it remains for an extended period to complete its development. Most marsupial species, including kangaroos, koalas, and wombats, are concentrated in Australasia, while a smaller number, like the opossum, are found in the Americas.
How Reproduction Differs
The greatest biological separation between squirrels and marsupials lies in their reproductive anatomy and the developmental timeline of their young. Squirrels are placental, or eutherian, mammals, meaning their embryos develop inside the mother’s uterus for a long duration, nourished by a complex, long-lasting placenta. This structure facilitates continuous nutrient and waste exchange, allowing the young to be born in a relatively advanced, developed state.
Marsupials, in contrast, utilize a simpler, temporary yolk-sac placenta during their brief gestation, which limits the amount of time the embryo can spend in the uterus. Because they lack the complex placental structure of eutherians, the young are born extremely early. The marsupial strategy involves a heavy investment in prolonged and complex lactation after birth, whereas the placental mammal strategy involves a heavy investment in uterine development before birth.