The reproductive biology of the Florida manatee was summarized in the 2001 Recovery Plan (USFWS 2001 and references therein). Breeding takes place when one or more males are attracted to an estrous female to form an ephemeral mating herd (ranging in size from about 5 to 22 individuals). The highest frequency of mating herds is from February to July and mating herds can persist up to 4 weeks, with different males joining and leaving the herd daily. Permanent bonds between males and females do not form. During peak activity, the males in mating herds compete intensely for access to the female. Successive copulations involving different males have been reported. Some observations suggest that larger, presumably older, males dominate access to females early in the formation of mating herds and are responsible for most pregnancies, but males as young as three years old may produce sperm. Although breeding has been reported in all seasons, studies of reproductive organs from carcasses of males found evidence of sperm production in 94% of adult males recovered from March through November, but in only 20% of adult males recovered from December through February. Females appear to reach sexual maturity by about age five but have given birth as early as four, and males may reach sexual maturity at 3 to 4 years of age. Manatees may live in excess of 50 years and evidence for reproductive senescence (reduced reproduction rate with age) is unclear. The length of the gestation period is uncertain but is thought to be between 11 and 14 months. The normal litter size is one (with an equal sex ratio at birth); twins are rare (less than 2% of pregnancies) and the largest number of births occur between May and September. Calf dependency usually lasts one to two years after birth. Calving intervals vary greatly among individuals. They are probably often less than 2 to 2.5 years, but may be considerably longer depending on age and possibly other factors. Females that abort or lose a calf at birth may become pregnant again within a few months or even weeks.
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