Trichechus manatus, Linnaeus, 1758

Russell A. Mittermeier & Don E. Wilson, 2014, Trichechidae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 548-562 : 560-561

1.

West Indian Manatee

Trichechus manatus

French: Lamantin des Antilles / German: Karibik-Seekuh / Spanish: Manati del Caribe

Other common names: American Manatee: Antillean Manatee, Caribbean Manatee (manatus); Florida Manatee,

North American Manatee (/ atirostris)

Taxonomy. Trichechus manatus Linnaeus, 1758 ,

“Mari Americano.” Restricted by Thomas in 1911 to West Indies.

A subspecies from the east coast of the USA, the “Baker Manatee,” bakerorum named by Domning in 2005 that lived from North Carolina to Florida became extinct in the late Pleistocene. Two extant subspecies recognized.

Subspecies and Distribution.

T. m. manatus Linnaeus, 1758 — Greater Antilles and Gulf and Caribbean coasts of Mexico, Central America, and N South America (S to Alagoas and Sergipe states, Brazil).

1. m. latirostris Harlan, 1824 — SE USA, primarily Florida and Georgia, with seasonal movements to other states bordering the Gulf of Mexico and the Atlantic Ocean.

Descriptive notes. Total length 250-390 cm; weight up to 1620 kg. External appearance of the West Indian Manatee is identical to the West African Manatee (1. senegalensis ). Body shape is somewhat fusiform but bulkier and more rounded than many other species of marine mammals. Head is small, with no neck and no external ear pinnae Paired nostrils near the end of the snout open dorsally. Eyes are small. Pronounced expansion of upperlip region forms the oral disk, a prehensile, grasping organ. Skin is finely wrinkled and uniformly gray to brown, with variation due to organisms thatlive on their skin, and sparsely haired, with specialized sensory hairs that are most prominent on the dorsum. Head has denser sensory orofacial hairs around muzzle, and oral disk and lower lips have specialized brush-like bristle fields for grasping and manipulating food. Pectoral flippers have 3—4 nails that are used for bottom locomotion and food handling. West Indian Manatees are not sexually dimorphic, other than position of external genitalia close to rectum in females and closer to umbilicus in males.

Habitat. A wide variety of shallow marine, estuarine, and freshwater habitats that support forage plants, but seemingly limited to regions with periodic access to freshwater sources for drinking. Depths of typical habitats are 1-10 m. The same West Indian Manatees will use habitats ranging from seagrass beds to rivers with freshwater plants and tidal creeks where the only available food is emergent vegetation at high tide. Cold-water temperatures govern seasonal limits to distribution of manatees in both North and South America, roughly corresponding to a minimum water temperature of ¢.20°C. The West Indian Manatee has a low rate of metabolism and high thermal conductance, rendering individuals susceptible to a cold-stress pathological syndrome. Habitat has expanded in Florida to include areas with artificial warm water sources in winter (e.g. electric power plant effluents). The West Indian Manatee does not require wilderness (some occur in urban areas), and individuals become habituated to humans where they are not hunted. Females may seek out secluded, quiet areas to give birth.

Food and Feeding. The West Indian Manatee eats seagrasses, freshwater aquatic plants, mangrove leaves, and most physically accessible rooted, submerged, floating, and bank vegetation. The list of known aquatic food plants includes multiple species in four genera of seagrasses, two genera of mangroves, eleven genera of freshwater submerged plants, twelve genera of freshwater floating plants, 40 genera of emergent plants, and 39 genera of algae (probably ingested incidentally with vascular plants). There are anecdotal reports of the ingestion of various animals (mostly invertebrates). Two feeding modes are used: excavating when both shoots and rhizomes of seagrasses are ingested and cropping when leaves and stems of all plants are taken. Retention time for ingesta is slow at about six days. Daily quantity of food ingested is ¢.7% of body weight in adults, consumed in ¢.4-7 hours of feeding/day.

.

Breeding. Most of the information on reproduction in the West Indian Manatee is based on studies of the “Florida Manatee” (1. m. latirostris), but other than diffuse seasonality (with minimal birthing or mating in winter), breeding is likely to be very similar in the “Antillean Manatee” (71. m. manatus ). Estrous females attract groups of roving adult and subadult males in “mating herds” that persist for up to three weeks, can involve 20 or more males, and can include group movements of up to 160 km. More than one male can mate with a female in these groups, implying sperm competition. Adult females typically produce many corpora lutea/ovary/pregnancy (average 36). Some males produce sperm as early as two years old, but most males, like females, probably are not sexually mature until aboutfive years old. The smallest length at maturity is ¢.250 cm in both females and males. Mature males are not always in a continuous breeding condition. Gestation is unknown precisely but is in the range of 12-14 months. A single offspring is born (twins occur in 1-4% of births). Offspring nurse for 1-2 years before they are weaned, butthis varies with the individual. Adult females give birth, on average, every 2-3 years. The Florida Manatee shows diffuse seasonality in reproduction, with lowest reproductive activity in winter.

Activity patterns. The West Indian Manatee shows no evidence of circadian rhythms and is active both day and night, with intermittent periods of activity and rest. Lack of strict circadian rhythms is consistent with an absence of a pineal organ near the base of the brain. This pattern changes with environmental factors; in winter, the Florida Manatee shows activity patterns that include resting at warm water springs and industrial effluents during the coldest times of day, with some remaining at these refugia and foregoing feeding for up to a week during lengthy cold periods. The West Indian Manatee also becomes more nocturnal in areas where it is hunted in daylight or where daytime boat activity is high. The Florida Manatee spends ¢.20-25% of the 24hour day feeding, c.20-25% resting, c.10-15% “cavorting” (social behavior similar to the wrestling and jostling seen in mating herds but of lesser intensity), and ¢.30-45% of the day traveling.

Movements, Home range and Social organization. Florida Manatees show sexual differences in movements and wide individual differences in migratory behavior. During warm seasons, males spend more time traveling than females and generally cover longer circuits in their travels, presumably reflecting the search for estrous females. Traveling males can be solitary or move in small groups that vary in composition. Seasonal migrations are generally southward in early winter and northward in spring, with timing triggered by changes in water temperature (with individual variability in threshold temperatures). On the Atlantic coast, four patterns of migratory behavior were seen, and movements of individual manatees were consistent from year to year. In southern Florida, some manatees did not migrate. Long distance migrants moved 575-831 km one way; medium distance migrants moved one-way distances of 150-400 km, and short distance migrants moved 50-150 km. One male was an extreme case, with repeated seasonal movements between Florida and coastal states as far north as Rhode Island (2360 km). Travel was usually direct and rapid (25-87 km/ day) between origin and destination points, with a few stopover areas in between. West Indian Manatees have high year-to-yearfidelity to seasonal home ranges. Home ranges were widely overlapping at migratory endpoints. In the vicinity of Everglades National Park in south-western Florida, movements in winter were less pronounced than on the Atlantic coast. Offshore-inshore patterns of seasonal movement were more localized, with inshore areas used more heavily in winter. Manatees that fed at offshore seagrass beds in this region in summer moved inshore to sources of freshwater every 2-8 days. The Antillean Manatee in Puerto Rico also uses seagrass beds for feeding, with periodic travel to freshwater sources. Maximum linear movements were only ¢.50 km in this more thermally constant environment. The Florida Manatee is notterritorial; they are highly tolerant of conspecifics and are often seen in groups, but the groups are very dynamic in composition. Females with their current offspring are the only stable social unit, and this stable association ends at weaning. There is good evidence that migratory patterns and seasonal home ranges are learned by offspring from mothers through tradition. Mothers and young communicate using touch and by underwater contact vocalizations that sound to the human ear like squeaks, grunts, and groans. Sounds are single-note calls with multiple harmonics and overtones that typically span 1-18 kHz and 200-300 milliseconds duration, with duration varying with context up to 900 milliseconds. These sounds have distinctive individual qualities that allow individual recognition between mothers and offspring. Manatees of all ages and both sexes use sound to communicate, with young manatees vocalizing more frequently than adults. Mostly anecdotal observation has raised the hypothesis that Florida Manatees use a form of underwater scent communication. The recent discovery of anal glands (poorly known in other aquatic mammals) in Florida Manatees lends further support to this possibility.

Status and Conservation. CITES Appendix I. Classified as Vulnerable on The IUCN Red List, but each subspecies is classified as Endangered. The Antillean Manatee appears to have suffered declines in abundance throughoutits distribution due to hunting, incidental killing in gill nets set for fish, and other human-related mortality factors. The West Indian Manatee is legally protected in every country or territory in which it occurs, but conservation actions and law enforcement are variable among nations. The Florida Manatee has been the subject of intensive protection, research, and conservation. These efforts have been very strong over the past 40 years, and the population of this subspecies has shown marked growth during this time period. Recent estimates for the Florida subspecies are at least 5000 individuals. There are no technically reliable estimates for population size of the Antillean Manatee, but limited expert opinion suggests the subspecies may have fewer individuals than the Florida subspecies. Overall genetic diversity in the West Indian Manatee is not dangerously low, but there is substantial geographic structuring, resulting in low diversity in several regions, particularly Florida.

Bibliography. Bengtson (1981, 1983), Bills et al. (2013), Deutsch et al. (2003), Domning (2005), Domning & Hayek (1984), Etheridge et al. (1985), Garcia-Rodriguez et al. (1998), Hartman (1979), Hernandez et al. (1995), Hunter et al. (2010), Irvine (1983), Kendall et al. (2004), Kinnaird (1985), Larkin (2000), Larkin et al. (2007), Ledder (1986), Lefebvre etal. (2001), Marmontel (1995), Marsh et al. (2011), Marshall, Huth et al. (1998), Marshall, Kubilis et al. (2000), Marshall, Maeda et al. (2003), Moore (1951), Nourisson et al. (2011), O'Shea & Hartley (1995), O'Shea & Poché (2006), Ortiz et al. (1998), Ralph et al. (1985), Rathbun & O'Shea (1984), Rathbun, Powell & Cruz (1983), Rathbun, Reid et al. (1995), Reep, Marshall & Stoll (2002), Reep, Marshall, Stoll & Whitaker (1998), Reep, Stoll et al. (2001), Reich & Worthy (2006), Reid (2006), Reid et al. (1995), Reynolds & Rommel (1996), Reynolds et al. (2004), Stith et al. (2006), Thomas (1911), Tucker et al. (2012), Vianna et al. (2006), Whitehead (1977).

Figures

1. West Indian Manatee (Trichechus manatus), 2. West African Manatee (Trichechus senegalensis), 3. Amazonian Manatee (Trichechus inunguus)

Subspecies and Distribution. T. m. manatus Linnaeus, 1758 — Greater Antilles and Gulf and Caribbean coasts of Mexico, Central America, and N South America (S to Alagoas and Sergipe states, Brazil). 1. m. latirostris Harlan, 1824 — SE USA, primarily Florida and Georgia, with seasonal movements to other states bordering the Gulf of Mexico and the Atlantic Ocean.