HMW 4 - Species accounts: West Indian Manatee (Trichechus manatus)

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Linnaeus, 1758
1. West Indian Manatee Trichechus manatus
French: Lamantin des Antilles / German: Karibik-Seekuh / Spanish: Manatí del Caribe
Other common names: American Manatee; Antillean/Caribbean Manatee (manatus); Florida/North American Manatee (latirostris)
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 subspecies recognized.
Subspecies and Distribution.
T. m. manatusLinnaeus, 1758 – Greater Antilles and Gulf and Caribbean coasts of Mexico, Central America, and N South America (S to Alagoas and Sergipe states, Brazil).
T. m. latirostrisHarlan, 1824 – SE USA, primarily Florida and Georgia, with seasonal movements to other states bordering the Gulf of Mexico and the Atlantic Ocean.
Descriptive notes. Head–tail 250–390 cm; weight up to 1620 kg. External appearance of the West Indian Manatee is identical to West African Manatee (T. 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 upper lip region forms the oral disk, a prehensile, grasping organ. Skin is finely wrinkled and uniformly gray to brown, with variation due to organisms that live 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 c.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 fresh-water 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 c.7% of body weight in adults, consumed in c.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” (T. 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” (T. 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 about five years old. The smallest length at maturity is c.2·5 m 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, but this 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 c.20–25% of the 24-hour 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 c.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-year fidelity 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 c.50 km in this more thermally constant environment. The Florida Manatee is not territorial; 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 throughout its 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. (2012), Deutsch et al. (2003), 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 et al. (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 & Poche (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), Tucker et al. (2012), Vianna et al. (2006), Whitehead (1977).
2. West African Manatee Trichechus senegalensis
French: Lamantin d’Afrique / German: Westafrika-Seekuh / Spanish: Manatí de África Occidental
Other common names: African Manatee,Sea Cow
Taxonomy. Trichechus senegalensis Link, 1795, Senegal.
This species is monotypic.
Distribution. Coastal areas and large inland rivers of West Africa from the Senegal River at the Mauritania–Senegal border S to the Longa River in Angola. They occur as far as 2000 km from the ocean in the Inner Niger Delta of Mali, up to 75 km off the continental shore in the shallows and mangrove creeks of the Bijagos Archipelago of Guinea-Bissau, and as far E as Lake Tréné in Chad; formerly in Lake Chad itself.
Descriptive notes. Head–tail up to 350 cm; weight 460 kg. Very little morphometric data are available for the West African Manatee, although eighteen presumed adults measured in Ivory Coast averaged 260 cm in body length. The West African Manatee appears indistinguishable in external appearance from the West Indian Manatee (T. manatus). Eyes appear to bulge outward more than in the West Indian Manatee or the Amazonian Manatee (T. inunguis), but this trait has not been investigated thoroughly on an anatomical basis.
Habitat. Shallow coastal waters, estuaries and lagoons; mangrove swamps; flooded agricultural fields; and rivers as far inland as depths and rapids permit. Habitats used by the West African Manatee are comparable to those of the West Indian Manatee in their breadth. The West African Manatee is euryhaline, but it is unknown if it requires periodic access to freshwater. Relationships between temperature and limits to the distribution of the West African Manatee have not been well established, but a water temperature of 18°C has been suggested to be a lower limit of tolerance.
Food and Feeding. The West African Manatee eats plants from at least 38 genera, invertebrates, and fish from a diversity of aquatic habitats. Many more food plants are likely to be revealed in the diet with further study. Methods of feeding and anatomical specializations seem identical to those of the West Indian Manatee. In some areas, West African Manatees feed on cultivated grains and crops in flooded fields, and they can be regarded as agricultural pests. Feeding occurs primarily at night in areas with histories of hunting by humans.
Breeding. In some areas, the West African Manatee has been reported to mate when rainy-season water levels are rising, which may result in parturition and early lactation coinciding with periods of increased aquatic plant productivity. Births of singletons (c.100 cm in length) have been reported. Anecdotal accounts of groups of up to 15 individuals suggest mating herds, as in the West Indian Manatee, but there are no published details on breeding and reproduction of the West African Manatee.
Activity patterns. There is little information available for this species, but in Ivory Coast are reported to feed for 4–6 hours daily, largely at night to avoid human hunters.
Movements, Home range and Social organization. There is little information available for this species, although some studies are underway. In Ivory Coast West African Manatees can move several kilometers between resting places and nocturnal feeding sites, which can be repeatedly used on successive nights. Seasonal movements occur in response to changing water levels in wet and dry seasons. West African Manatees are largely solitary but feeding and mating aggregations can occur. The breeding system involves an estrous female followed by mating herds of males similar to those better documented for West Indian Manatees.
Status and Conservation. CITES Appendix I. Classified as Vulnerable on The IUCN Red List. West African Manatees are protected by law in all nations in which they occur, but enforcement is sometimes lax, and exemptions occur for cultural purposes in some countries. The Nigerian government has allowed permits for killing of 1-2 manatees annually for a wrestling festival, and captures for symbolic uses also have been permitted in Niger. Illegal hunting can result in far more deaths, with an estimated total of over 2500 West African Manatees killed annually by poachers in Sierra Leone, Nigeria, and Cameroon combined. Many poaching methods are used in West Africa. As examples, manatees are lured into box traps with cassava, speared from platforms on stilts, entangled in nets, shot by harpoons on baited triggers, and trapped by fences on outgoing tides. Many hunting and trapping practices for manatees are known throughout West Africa, and incidental capture in fishing nets is an additional major threat that in some areas has a greater impact than direct hunting. Deterioration and loss of habitat are increasing with rapid human population expansion in many West African countries. West African Manatees occur in areas of extreme human poverty, where food is scarce, and their economic value can reach US$ 400 in some areas. They are a source of aquatic bush meat and oil, which adds to the concern that their populations may be depleted. The Convention on Migratory Species includes a Memorandum of Understanding for the conservation of small cetaceans and manatees in West Africa, with signatories including 16 West African countries. Many indigenous cultures hold West African Manatees in high esteem, and manatees and their body parts are subjects of many traditional beliefs.
Bibliography. Allsopp (1969), Awobamise (2008), Dodman et al. (2008), Domning (1982), Domning & Hayek (1986), Grigione (1996), Husar (1978), Issa (2008), Keith & Collins (2007), Kouadio (2012), Marsh et al. (2011), Marshall et al. (2003), Moore et al. (2010),Powell (1996), Reeves et al. (1988), Sikes (2010), Silva & Araújo (2001), Vianna et al. (2006).
3. Amazonian Manatee Trichechus inunguis
French: Lamantin de l’Amazone/ German: Amazonas-Seekuh / Spanish: Manatí del Amazonas
Other common names: Natterer Manatee,South American Manatee
Taxonomy. Manatus inunguis Natterer, 1883, Borba, Rio Madeira, Brazil.
There appears to be no geographically based genetic structuring of the Amazonian Manatee in Brazil, whereas some genetic structuring is evident between Amazonian Manatees in the Colombian and Peruvian Amazon Basin. Monotypic.
Distribution. Amazon River system including its estuary in Colombia, Brazil, Ecuador, and Peru.
Descriptive notes. Head–tail up to 300 cm; weight 450 kg. The Amazonian Manatee appears to be the most derived of the three extant species. It is the smallest and is more slender in appearance than the other two species. There are no nails on flippers. Skin appears smoother than in the West Indian Manatee (T. manatus) and the West African Manatee (T. senegalensis), and it tends more to blackish or dark gray in color. A large white or pink irregular blaze marking occurs on stomachs of many individuals, presumably providing disruptive countershading from below. Phalanges are longer relative to the humerus compared with the West Indian Manatee and the West African Manatee. Teeth are more specialized for a diet of grasses, which have a high content of abrasive silica. Teeth are smaller and have reduced occlusal areas (functionally increasing length of the enamel ridge per unit area), and have more complex patterns of lophs and enamel foldings than the teeth of the other two species of manatees. The Amazonian Manatee has specialization of the orofacial region and corresponding hairs, as in the West Indian Manatee.
Habitat. River channels, both “várzea” (white water or muddy water) and “igapó” (black water) lakes, and flooded forests. The Amazonian Manatee is found in freshwater throughout the Amazon River and its tributaries, although it may reach brackish water in the lower Amazon. When water levels are low in the central Amazon Basin, manatees will aggregate in deeper lakes where they may fast for prolonged periods and fall prey to hunters, sometimes being killed by the hundreds during exceptional dry periods. In the Rio Solimões system of western Amazonia, manatees stay in the várzea lakes and surrounding flooded forest during high water to feed on abundant macrophytes. They migrate to deeper “rias” (long narrow lakes formed by river valleys) with low food availability for the low-water period to avoid the possibility of the várzeas drying out completely and exposing them to predators and hunters.
Food and Feeding. Amazonian Manatees eat a wide variety of freshwater plants, especially emergent and floating vegetation. It has the lowest degree of rostral deflection in the family Trichechidae, corresponding with the predominant position of food plants in the upper water column. At least 63 species in 36 genera of vascular plants have been recorded in the diet, including many true grasses and other plants found in “floating meadows.” The more complex cusp patterns and lophs of the teeth of the Amazonian Manatee compared with the other extant manatee species suggest a special adaptation to the high silica content of grasses. About 8% of body weight/day is consumed in captivity, and a high proportion (45%–70%) of their fibrous diet is digestible because of symbionts and anatomical specializations of the digestive system, common to manatee species. Availability and composition of vegetation change seasonally with the predictable annual rainfall pattern in the Amazon region. The Amazonian Manatee will fast in lakes or rias during low-water periods when the only available plant material is detritus. In Lago Amanã, Amazonas State, Brazil, it has been calculated that the Amazonian Manatee may fast for up to seven months at its low basal metabolic rate, living on fat stores.
Breeding. Gestation is thought to be 12–14 months, and litter size is one. In Brazil, matings and births occur in December–July, with seasonally rising water levels and enhanced aquatic plant productivity. During prolonged dry seasons, breeding may be delayed.
Activity patterns. The Amazonian Manatee can undergo prolonged periods of fasting in the dry season. No circadian rhythms are known, with feeding and equivalent movement rates occurring day and night. Captive manatees spend c.33% of the day feeding, 17% resting, and 50% swimming.
Movements, Home range and Social organization. In the western Amazon, home ranges of the Amazonian Manatee included areas used only seasonally, with relatively short (on the order of 100 km or less) migrations between them. Home ranges in the várzea lakes at high water in the western Amazon contained seven times the amount of aquatic macrophytes than areas used in rias during the low water season. Surface reduction in flooded areas of várzeas was 4·5 times greater than that of flooded areas of rias. Movement patterns of manatees are unknown in the lower Amazon, where tidal cycles and other major habitat differences occur. Plasticity in movement patterns based on traditional learning is likely. No detailed information on social organization of the Amazonian Manatee is available, but similarity with the other species of manatees may be suspected (shared overlapping home ranges, scramble promiscuity in mating behavior, and traditional learning of movement patterns). Two captive manatees showed care-giving behavior by assisting a disabled companion with rising to breathe. Young Amazonian Manatees may remain with mothers for up to two years before becoming independent. Vocalizations are similar to those of the West Indian Manatee and are individually distinctive in captive individuals.
Status and Conservation. CITES Appendix I. Classified as Vulnerable on The IUCN Red List. The Amazonian Manatee is protected by national laws in all countries within its distribution, although subsistence hunting by some native groups is allowed in some areas. Illegal hunting is the main threat to populations of Amazonian Manatees, including traditional hunting by harpoon and more recent use of specially designed nets and other apparatus. Future habitat degradation is also a major concern. The Amazonian Manatee currently does not show evidence of low genetic diversity.
Bibliography. Arraut et al. (2010), Best (1981, 1982, 1983), Cantanhede et al. (2005), Colares & Colares (2002), Domning (1980, 1982), Domning & Hayek (1984, 1986), Marmontel et al. (2002), Marsh et al. (2011), Marshall et al. (2003), Montgomery et al. (1981), Rosas (1994), Satizábal et al. (2012), Sousa-Lima et al. (2002), Vianna et al. (2006).