Manatees have an approximately 1:1 sex ratio and low reproductive rates. Males mature at 2 - 3 years of age, while females first become pregnant at 3 - 5 years of age. Most produce a calf by Age 7 (USFWS 2001; Garcia-Rodriguez et al. 1998; Marmontel 1995). Females produce 1 calf every 2 - 3 years, with twins occurring approximately 2% of the time. Cows that lose calves tend to come into estrous faster than those feeding calves. Females continue to produce calves into at least their thirties (Haubold et al 2006; Marmontel 1995), with observations of some captive cows giving birth at older ages. Cohesive social interactions among manatees occur only in mating herds, which typically consist of a female in heat being pursued by courting bulls. Herds may persist up to 4 weeks with different males cycling in and out of the herd daily (Haubold et al. 2006; Rathbun 1999; Rathbun et al. 1995). Hartman (1979) observed one estrous herd on Florida's West coast consisting of a single female and as many as 17 mature bulls that constantly pursued her. Juvenile males in the population joined and exited the pursuit at various times as well. Bulls generally are tireless while courting a female, constantly embracing or mouthing her as she swims. The constant harassment often leads females to swim into very shallow water, where they are known to beach themselves to avoid the attention of males. Rathbun et al. (1995) suggested that older, larger males dominate access to females in mating herds and are responsible for the majority of pregnancies. Breeding is reported in all seasons, and successive copulation frequently occurs. However, peak sperm production (as analyzed in recovered carcasses) occurs primarily from March through November. Only 20% of adult males showed evidence of sperm production from December - February (Hernandez et al. 1995). Marmontel et al. (1997) calculated the gross annual recruitment rate of manatees in Florida as 8% using the following formula: (# females in population) X (% mature females) X (Annual pregnancy rate). Life tables reported in this study indicated approximately zero population growth with a net reproductive rate of 1.09, and a finite rate of increase of 0.005, which is not statistically different from zero (Marmontel et al. 1997). Further, the authors suggested that manatee survival in Florida did not follow the pattern observed in other large mammals species, but rather resembled survivorship curves calculated in exploited populations.
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