West Indian manatees were known by the indigenous people of the Atlantic shores and coastal rivers of the tropical and subtropical Americas for many centuries prior to European settlement. Manatees figured prominently in traditional practices and folklore in many of these areas, and skeletal remains are well known from New World archaeological sites. Christopher Columbus encountered them in his journeys to the Caribbean during the 1490s, and Linnaeus gave West Indian manatees their current scientific name in 1758 at the very beginning of the modern system of binomial zoological nomenclature.
Size, shape and distinctive characteristics
The West Indian manatee is a large, slow-moving fusiform but stout marine mammal with a gray to tan coloration which can vary in color due to symbiotic algae that sometimes grow on the skin. The skin is sparsely studded with tactile hairs. There are no color markings. They have a distinctive prehensile muzzle with bristles and numerous tactile hairs, external nares near the tip of the snout, no external pinnae to the ears, and two long paddle-like flexible pectoral flippers that are used for manipulation of food, for sculling, and for "bottom-walking." The flippers have nails. There are no hindlimbs. Propulsion is primarily via the spatulate-shaped tail, which is unique to manatees (the dugong tail is similar in shape to tails of cetaceans). The skeleton includes heavy, dense, pachyosteosclerotic bones that help serve as ballast. The dentition is composed of an indeterminate number of supernumerary molars that are continually replaced through forward progression as they wear down from their abrasive diet of plants. They are hindgut digesters, and process large quantities of vegetation daily. Typical adults are about 3 m long with a body mass of 500-700 kg, although larger individuals up to 1,650 kg have been reported from Florida in recent years. They show no obvious sexual dimorphism other than placement of the external genitalia (in relation to distance from the anus).
The West Indian manatee has two subspecies, the Florida manatee (T. m. latirostris) and the Antillean manatee (T. m. manatus). Florida manatees occur along coasts and in rivers of Florida, with seasonal extensions into nearby states. Antillean manatees use similar habitat of larger islands of the Caribbean, along the Gulf and Caribbean coasts of Mexico and Central America, and northern and eastern South America to Sergipe, Brazil.
Ecology and Behaviour
The ecology and behavior of West Indian manatees is based largely on diet and life history, and may vary by region. Florida manatees show seasonal movement patterns related to winter temperatures, when many move to the warmer southern third of the peninsula. The migratory distance on the Atlantic Coast can be as far as 830 km one way, with movement rates of about 30 km/day (ranging up to 87 km/day). Florida manatees also gather seasonally around natural and artificial warm water sources in winter. Unlike other marine mammals, the herbivorous manatees have low metabolic rates and a low capacity for retaining body heat. This forces winter migration southward, or to alternate sources of warm water. Aquatic plant productivity is also limited in winter.
Individual Florida manatees show high fidelity to their seasonal ranges and return to the same areas year after year, probably based on traditional learning by calves from mothers. There is no obvious social structure other than bonds between females and their calves. Manatees communicate using underwater vocalizations in the 1-18 kHz range that have individually distinct components. Females and young recognize each other by these vocalizations. Adult males travel more frequently than females in their daily movements, presumably in search of receptive females. Groups of up to 22 males will form mating herds that focus on presumably estrous females for periods ranging up to 4 weeks. Females have been observed to copulate with more than one male during these events.
Seasonal movements of West Indian manatees have not been studied as intensively in more tropical areas. In such areas where studies have been undertaken, there are no strong seasonal fluctuations in sightings and few records of major long distance moves like those seen in Florida. There may be seasonal movements corresponding to wet and dry seasonal cycles of major rivers in the tropics, but this is not well known. West Indian manatees commonly drink at freshwater sources throughout their range, and may make regular movements to near shore springs or fluvial habitats for such purposes.
West Indian manatees utilize tactile, visual, and auditory senses in orientation and communication. The tactile hairs and bristles on the prehensile muzzles and lips probably allow fine scale discrimination of food plants and their parts, and the sparse hairs on the dorsum and sides probably detect minor pressure changes due to movement of water. These animals frequently engage in muzzling each other's skin, seemingly wrestling with each other underwater. Small groups of manatees can engage in this "cavorting" behavior intermittently for hours at a time. Although not poor, visual senses are of limited value in dark or turbid water and at night. Hearing ability is good, but they do not use high frequency sounds for orientation. Manatees do not naturally exhibit strong circadian rhythms in activity or behavior.
The life history of West Indian manatees is known mainly from studies of carcasses and sight-resight analyses of long-term records of distinctively scarred Florida manatees. The databases from these approaches are substantial and reliable, and have allowed development of excellent population dynamics models. Life history parameters of the species elsewhere are poorly known, but are probably grossly similar. Maximum longevity in Florida manatees numbers in decades, with wild individuals aged at 59 years by histological analysis of bone from carcasses, distinctive wild manatees sighted as adults for over 36 years, and captives still living at age 61. The mating system is promiscuous, with multiple males forming mating herds in pursuit of a single estrous female for several days to a few weeks. Adult females give birth to a single calf after a gestation period in the range of 12-14 months (but not precisely known). Timing of parturition is diffusely seasonal, with more births in the spring and summer months and few in winter. Twin births occur occasionally (unlike cetaceans or pinnipeds). Sex ratios at birth are 1 male to 1 female. Calves nurse for at least a year and sometimes for up to two years before they are weaned. Female Florida manatees can become sexually mature as young as 3 years old, although most first give birth at age 4 to 5. Intervals between births are about 2.5 to 3 years. Survival rates in Florida vary by region, with the highest annual adult survival estimated at 0.96 in the region with the fastest growing population. Annual population growth in this population was 1.06, but estimates were lower and even negative in other regions. Overall, elasticity analysis of life history-based population growth modeling shows that the key to maintaining stable or growing populations is high adult survival (this is generally true for most species of marine mammals), followed by subadult survival, with calf survival and breeding rates having lower contributions to population growth.
Unlike many other species of marine mammals, the herbivorous West Indian manatee has a low metabolic rate and high thermal conductance. Natural mortality factors impacting the Florida population are unusually cold winters, poisoning by dinoflagellate blooms caused by the Florida red tide organism, and possibly intense storms and occasional predation. Disease does not appear to play a major role as an agent of mortality. Natural mortality factors in other areas inhabited by West Indian manatees are poorly known. Anthropogenic causes of death overshadow natural factors throughout the range of West Indian manatees, and present major conservation issues. Population genetics research suggests regional differentiation within the full range of West Indian manatees, with lower diversity in the Florida subspecies. However, genetic diversity in Florida has not yet reached a crisis stage.
West Indian manatees eat a wide variety of aquatic plants in marine, estuarine, and freshwater habitats. They seem to prefer submerged, floating, and emergent vegetation, in that order. In marine areas they subsist mostly on seagrass leaves and rhizomes. They will feed on mangroves, and will eat bank-growing vegetation such as salt-marsh grass along tidal creeks. In freshwater systems they also will eat terrestrial plants growing along banks in addition to purely aquatic freshwater vegetation. The list of plant and algae species reported in the diet of wild West Indian manatees includes about 100 different genera. They also obtain nutrients from various epibionts growing on these plants.
Global abundance of West Indian manatees has not been accurately assessed. This is due to a number of factors. In most areas this species is found in dark or turbid water, and occurs in irregularly shaped bands of suitable habitat paralleling coastal, lagoon, and riverine shorelines. This has hampered development of statistically reliable abundance estimation techniques that adjust for detectability. Secondly, financial resources have been limited. West Indian manatees occur in waters of about 20 nations, many of them in the developing world. Even qualitative assessments have been lacking in some of these countries, particularly recently. These nations are also subdivided into various geographic and political administrative entities with differing responsibilities for managing and assessing natural resources. Furthermore, most attempts to determine the status of manatees have relied on aerial surveys that provide number of sightings as a crude index of "relative abundance" and help discern differences in local distribution; surveys also gauge status (stable, declining, increasing) primarily based on interviews with knowledgeable local people. Even in Florida, past counts made at warm water refuges in winter have statistical deficiencies that are only partially satisfied by standardization and intensive replication, and until recently had not attempted to estimate detectability. These synoptic surveys suggested a "minimum number" of at least 3,800 Florida manatees in 2009, a three-fold increase over the past 30 years that is also consistent with growth trends estimated by modeling of life history parameters. Elsewhere in the species range, population estimates boil down to expert opinion, even if based on fairly recent surveys of relative abundance, distribution, and status. Numbers of sightings of West Indian manatees in these other countries are all lower than those made in Florida, and usually number in the dozens. See also "IUCN Status" below.
The two subspecies of West Indian manatee are assessed separately by the IUCN. The Antillean subspecies is listed as Endangered (C1) because expert opinion suggested a population of less than 2,500 mature individuals with a likely decline of more than 20% over the next two generations (estimated at about 40 years in an unexploited population) without effective conservation. The Florida manatee is also listed as Endangered C1.
West Indian manatees face a number of difficult conservation issues that differ regionally with degree of economic development. In many areas inhabited by Antillean manatees the primary threat is intentional killing for both subsistence and local black markets. Manatee meat is relished in such areas, and parts of the carcasses are also believed to have medicinal qualities in local folklore. Bones are used as medical and magical potions, and sometimes are crafted into ivory-like objects. Incidental take by artisanal fisheries through entanglement and drowning in nets is also a major issue for this subspecies. Because of the traditional value given to manatee meat and body parts, this by-catch is usually sold or consumed locally, helping to perpetuate demand. Habitat loss and degradation through conversion of coastal zones and adjacent lowlands for agricultural and industrial purposes, and urbanization of coastal settlements, are growing threats. As tropical areas become developed, ship traffic and recreational boating have increased, and the number of reports of boat-killed Antillean manatees is growing. Wounding and killing in collisions with boats has long been the major issue for Florida manatees. Population viability modeling of life history parameters has shown that despite considering other known threats to this subspecies, conservation through reduction of the number killed by boats is by far the most effective means to achieve increased rates of manatee population growth in Florida. In addition to habitat degradation, other anthropogenic sources of mortality for the Florida subspecies include killing in floodgates and locks (there have been some recent technological breakthroughs that now seem to be curtailing this source of mortality), and entanglement in nets and lines. Florida manatees die during cold winters, and future diminution in the availability of warm water refuges is a major issue that has recently gained a sense of urgency. Power plants that provide artificial warm water are aging and some have become inoperable, and natural freshwater springs are suffering reduced flows because of demands put on groundwater resources by an increasing human population. Water quality is also suffering at some of the natural springs, and plant communities have shifted to less valuable species. Climate change will likely bring further threats that are difficult to predict, but may follow with increased sea level, shifts in ocean currents, more intense storms and greater runoff affecting water clarity, quality and coastal vegetation. More frequent harmful algal blooms (Florida red tides have killed many manatees in recent years) and increased proliferation of disease organisms are likely with climate change. The distribution and seeming tameness of some Florida manatees attests to the species lack of need for isolated wilderness reserves, a hopeful element in the face of ever-growing human populations.
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