Flying squirrels are placental mammals, specifically rodents, belonging to the infraclass Eutheria, which includes most mammals on Earth. The confusion often arises because of the superficial physical similarity they share with the sugar glider, an Australian marsupial. Although both animals glide through the air, their fundamental biological classification and reproductive strategies place them in completely different mammalian groups. The defining difference lies not in how they move, but in how they reproduce and develop their young.
Flying Squirrels are Placental Mammals
Flying squirrels are members of the tribe Pteromyini, nested within the family Sciuridae, which includes all other true squirrels, such as gray squirrels and chipmunks. Their taxonomic placement is within the order Rodentia, making them closely related to mice, beavers, and porcupines. There are over 50 species of flying squirrels worldwide, including the North American northern and southern flying squirrels (Glaucomys sabrinus and Glaucomys volans).
As placental mammals, flying squirrels are defined by their reproductive biology, which includes a relatively long gestation period. The female develops a chorioallantoic placenta, an organ that facilitates a prolonged exchange of nutrients, oxygen, and waste between the mother and the developing fetus inside the uterus. For species like the southern flying squirrel, gestation lasts approximately 40 days, allowing the young to complete a significant portion of their development in utero.
The young are born in a relatively advanced state compared to marsupials, although they are still helpless and naked at birth. They are weaned after about two months and are fully independent around 120 days of age.
The most notable physical feature of flying squirrels is the patagium, a furry membrane of skin that stretches from the wrists of their forelimbs to the ankles of their hindlimbs. This specialized membrane allows them to glide, not fly, by launching themselves from high points in trees. The tail acts as a stabilizer and brake, permitting them to steer and control their descent across distances that can exceed 150 feet in a single glide.
Defining the Characteristics of Marsupials
Marsupials belong to the infraclass Metatheria. The primary characteristic of marsupials is their very short gestation period, which results from their reproductive tract structure and the primitive nature of their placenta. Most marsupials develop only a simple yolk sac placenta, incapable of sustaining the fetus for an extended period inside the womb.
The young are born at an incredibly underdeveloped, almost embryonic stage, often after a gestation lasting only a few weeks. The newborn, sometimes referred to as a “joey,” is tiny, blind, hairless, and has poorly developed hind limbs. The forelimbs are strong enough to allow the neonate to crawl immediately after birth from the mother’s birth canal to her abdominal pouch, or marsupium.
Once inside the pouch, the underdeveloped young attaches to a nipple, where it remains for an extended period to complete its growth and development. Other unique anatomical features include two separate vaginae and uteri in females and a bifid, or double, penis in males.
Marsupials are primarily found in Australia, New Guinea, and the Americas, with well-known examples including kangaroos, koalas, and the Virginia opossum.
Why the Confusion Exists: Convergent Evolution
The common confusion between flying squirrels and marsupials stems from the existence of gliding marsupials, such as the sugar glider (Petaurus breviceps) and the greater glider (Petauroides volans). These Australian animals are marsupials, distant relatives of the kangaroo, yet they possess a patagium and glide through the forest canopy in a manner nearly identical to the flying squirrel. This remarkable similarity in form and function between two entirely unrelated species is a classic example of convergent evolution.
Convergent evolution occurs when different species independently evolve analogous structures or traits as a result of adapting to similar environmental pressures. Both the flying squirrel and the sugar glider are small, nocturnal, arboreal mammals living in dense forests, where gliding provides a significant survival advantage for efficiently traveling between trees to find food and escape predators. The similar ecological challenge led to the independent evolution of the patagium in both lineages.
The two groups of mammals—Eutheria (placental mammals) and Metatheria (marsupials)—diverged from a common ancestor over 100 million years ago. The sugar glider and the flying squirrel developed the same gliding membrane, demonstrating that physical function does not determine taxonomic group. Classification is based on reproductive mechanisms: the flying squirrel relies on a complex placenta for internal development, while the sugar glider relies on its pouch for post-birth development.