Analogy: Squirrels and Sugar Gliders
Beyond being cute and cuddly, flying squirrels and sugar gliders have many striking
similarities: big eyes, a white belly, and a thin piece of skin stretched between their
arms and legs, a trait which helps them "glide" and remain stable when leaping from
But sugar gliders and flying squirrels also have some key differences. Most
importantly, they reproduce and bear their babies in fundamentally different ways:
- Flying squirrels are placental mammals. Placental mammals spend a long time
developing inside the mother's body being nourished by a placenta before they are
- Sugar gliders are marsupial mammals, like kangaroos. Marsupial mammals may only
spend a short time developing inside the mother's body and are very tiny when born.
After birth, a baby marsupial crawls into its mother's pouch and is nourished by her
milk as it continues to grow and develop.
Flying squirrels and sugar gliders are only distantly related. So why do they look
so similar then? Their gliding "wings" and big eyes are analogous structures. Natural selection independently adapted both lineages for similar lifestyles: leaping from
treetops (hence, the gliding "wings") and foraging at night (hence, the big eyes).