Breeding occurs throughout the year and peak months for birth vary geographically. The exact length of gestation is unknown, but it is presumed to be about 1 year. Single calves are the norm and twins are rare. Parturition takes place in shallow water, and newborn calves are able to swim immediately to the surface for their first breath of air. Newborn calves are about 100-120 cm long and weigh 20-35 kg. Newborns cling to the mother's back and ride from the surface to grass beds along with the feeding mother. Young suckle underwater beneath the mother in an inverted position. Lactation lasts approximately 18 months, but young are known to eat grass at 3 months. Young may remain with the mother for a year. Sexual maturity is reached in both sexes by an age of 9-10 years, though it can occur as late as 15 years.