Baby hippopotamuses, which are more commonly known as calves, are born underwater and typically weigh in somewhere between 50 and 110 pounds and are about 4 feet long at birth. Because they are mammals that are born underwater, the newborn hippo must swim to the surface in order to take its first breath. After they're born, hippo calves can rest on their mother's backs when they don't have the strength to swim, and they get into the water and dive under the surface to nurse.
Hippo calves take about 6 years to mature, with female hippos beginning reproductive puberty at around 3 or 4 years old. Typically, a female hippopotamus will give birth to only one calf at a time, though it is possible for them to become pregnant with and deliver twins. Hippos have 8-month gestation periods, and they will usually not ovulate again for a period of about 17 months, which is roughly the same amount of time it takes to nurse and wean a calf. These animals, which are the third largest mammals on land, focus on rearing a single, robust calf over a period of years rather than giving birth to multiple litters of weak young as some other mammals do.