The mammal known to have the longest pregnancy is the African elephant. An elephant's gestation period lasts approximately 22 months, making it the longest of any known mammal. This extended pregnancy allows the developing fetus to grow to a sufficient size, as elephants typically give birth to calves that can weigh 220 to 250 pounds at birth. The lengthy gestation period is crucial for developing the intricate social structures and physical adaptations needed for survival in the wild. After this long period, the mother usually gives birth to a single calf, although twins can occur on rare occasions, especially in captivity.