Manatees, aquatic herbivores, also known as "sea cows", these herbivores usually spend up to eight hours a day grazing on seagrass and other marine or freshwater vegetation, eating up to ten percent of their body weight in aquatic vegetation each day. While people may see many manatees gathered together at winter warm water refuges during winter months, during the rest of the year manatees are semisocial as they travel around the state's waterways in search of food, mates, or places to rest. Except for mother (cow)/calf pairs, manatees do not need to travel together, although they do socialize when other manatees are encountered. Females (cows) reach sexual maturity between three and five years and give birth to their first calf between the ages of four and seven years old, while males (bulls) reach sexual maturity between five and seven years. During the breeding season, bulls gather and pursue a cow to form a mating group and breed at different times. Bulls are not part of the family unit and will leave a cow alone after her breeding period is over. Manatees can breed and give birth throughout the year; however, birthing usually peaks in the spring. Females have a 13-month gestation time, and have a low reproductive rate, giving birth to an average of one calf every three to five years. The calf will stay with the mother for up to two years.