Trichechus inunguus, 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 : 562

3.

Amazonian Manatee

Trichechus inunguus

French: Lamantin de 'Amazone / German: Amazonas-Seekuh / Spanish: Manati 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, Colombia, Brazil, Ecuador, and Peru.

Descriptive notes. Total length 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 (7. manatus ) and the West African Manatee (71. 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 “varzea” (white water or muddy water) and “igapo” (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 Solimoes system of western Amazonia, manatees stay in the varzea 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 varzeas 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. They have 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 Amana, 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 ¢.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 varzea 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 varzeas 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, R.C. (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), Satizabal et al. (2012), Sousa-Lima et al. (2002), Vianna et al. (2006).

Figures

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

Distribution. Amazon River system including its estuary, Colombia, Brazil, Ecuador, and Peru.