More than 5,400 mammal species roam the planet, running the gamut in size and appearance—from tiny bumblebee bats and  hundred-foot-long blue whales— to spindly-fingered aye-ayes and scaly pangolins. That astonishing diversity is also on display in how mammal mothers undergo pregnancy and birth, from the domestic dog bearing pups from different fathers in one litter to the swamp wallaby carrying babies in two separate uteruses. 

Intelligent, long-lived species often have lengthy pregnancies, which allows ample time for fetal development. The African elephant has the longest pregnancy, at a whopping 22 months. 

“The baby elephant is born very complete, and it can walk long distances,” says Thomas Hildebrandt, a veterinarian at