Female manatees reach sexual maturity at age 3 but do not breed successfully until 6 to 10 years of age, while males are sexually mature at age two. Manatee mating herds are made up of sexually mature males and females, with more males than females, and many males court one female within a breeding herd for from a week to one month, with only persistent males succeeding. Manatees reproduce very slowly: females carry the young up to 13 months before giving birth and usually produce a single calf with each pregnancy. Although calving occurs year round, there is generally a two- to five-year span before females reproduce again. Calves remain dependent on their mothers for up to two years, nurse under water, and are able to begin nibbling on aquatic vegetation a few weeks after birth.