West Indian Manatee

The **West Indian manatee** (*Trichechus manatus*) or "sea cow" is a manatee, and the largest surviving member of the aquatic mammal order Sirenia(which also includes the dugong and the extinct Steller's sea cow).

The West Indian manatee is a species distinct from the Amazonian manatee (*T. inunguis*) and the African manatee (*T. senegalensis*). Based on genetic and morphological studies, the West Indian manatee is divided into two subspecies, the **Florida manatee** (*T. m. latirostris*) and the **Antillean** or **Caribbean manatee** (*T. m. manatees).* However, recent genetic (mtDNA) research suggests that the West Indian manatee actually consists of three groups, which are more or less geographically distributed as: (1) Florida and the Greater Antilles; (2) Mexico, Central America and northern South America; and (3) northeastern South America.

Both the Florida manatee and the Antillean manatee are endangered and have been of great conservation concern to federal, state, private, and nonprofit organizations to protect these species from natural and human-induced threats.

## Description

Like the other sirenians, the West Indian manatee has adapted fully to an aquatic life style, having no hind limbs. Pelage cover is sparsely distributed across the body, which may play a role in reducing the build-up of algae on their thick skin. The average West Indian manatee is about 2.7–3.5 m (8.9–11.5 ft) long and weighs 200–600 kg (440–1,320 lb), with females generally larger than males. The difference between the two subspecies of the West Indian manatee is that the Florida manatee is commonly reported as being larger in size compared to Antillean manatee. The largest individual on record weighed 1,655 kg (3,649 lb) and measured 4.6 m (15 ft) long. This manatee's color is gray or brown. Its flippers also have either three or four nails.

## Distribution and Habitat

As its name implies, the West Indian manatee lives in the West Indies, or Caribbean, generally in shallow coastal areas. However, it is known to withstand large changes in water salinity, so has also been found in shallow rivers and estuaries. It can live in fresh, brackish, and saline water. It is limited to the tropics and subtropics due to an extremely low metabolic rate and lack of a thick layer of insulating body fat. While this is a regularly occurring species along coastal southern Florida, during summer, this large mammal has even been found as far north as Dennis, Massachusetts and as far west as Texas.

The Florida manatee (*Trichechus manatus latirostris*), a subspecies of the West Indian manatee, is the largest of all living sirenians. Florida manatees inhabit the most northern limit of sirenian habitats. Over three decades of research by universities, governmental agencies, and NGOs has contributed to understanding of Florida manatee ecology and behavior. They are found in freshwater rivers, in estuaries, and in the coastal waters of the Gulf of Mexico and the Atlantic Ocean. Florida manatees may live to be more than 28 years old in the wild, and one captive manatee, "Snooty", has lived for over 60 years.

The greatest threat to the Florida manatee population is the potential future loss of warm-water habitat; vessel strikes are believed to be the greatest limiting factor to the speed at which the manatee population can recover from stochastic events, although more recent assessments (2013) offer more optimistic projections. Large concentrations of Florida manatees are located in the Crystal River and Blue Springs regions in central and north Florida, as well as along the Atlantic Coast, and Florida Gulf Coast.

The other subspecies of the West Indian manatee is sometimes referred to as the Antillean manatee (*T. m. manatus*). Antillean manatees are sparsely distributed throughout the Caribbean and the northwestern Atlantic Ocean, from Mexico, east to the Greater Antilles, and south to Brazil. They are found in French Guiana, Suriname, Guyana, Trinidad (but with a lack of recent sightings), Venezuela, Colombia, Panama, Costa Rica, Nicaragua, Honduras, Guatemala, Belize, Mexico, Trinidad, Cuba, Haiti, the Dominican Republic, Jamaica, and Puerto Rico. Historically, Antillean manatees were hunted by local natives and sold to European explorers for food. Today, they are threatened by loss of habitat, poaching, entanglement with fishing gear, and vessel strikes.

## Behavior and Food

The West Indian manatee is surprisingly agile in water, and individuals have been seen doing rolls, somersaults, and even swimming upside-down. Manatees are not territorial and do not have complex predator avoidance behavior, as they have evolved in areas without natural predators. The common predators of marine mammals, such as killer whales and large sharks, are rarely (if ever) found in habitats inhabited by this species.

Based upon their behavior, Bauer et al. (2010) suggests that manatees may obtain the characteristic of pheromonal communication like their relative, the elephant. Some scientists (Rathbun, Reid, Bonde, and Powell, 1995) have observed that manatees form long periods of mating herds when wandering males come across estrous females, which indicates the possibility that males are able to sense the estrogen or other chemical indicators. Manatees also eat other manatees' feces; it is assumed that they do this to gather information about reproductive status or dominance indicating the important role chemoreception plays in the social and reproductive behavior of manatees.

Manatees feed on about 60 plant species, which include sea grasses as their major food source. They also consume some fish and small invertebrates. While many manatees are known to eat a large quantity throughout the day, the amount they eat depends on their body size and activity level. Because manatees feed on abrasive plants, their molars are often worn down and are continually replaced throughout their life, so they are called "marching molars".

The West Indian manatee has a high casualty rate due to thermal shock from cold temperatures. During cold weather, many die due to their digestive tracts shutting down at water temperatures below 68 °F (20 °C). Many manatee deaths are caused by both large and small boats, mostly from their propellers. Manatees are also at a disadvantage because they are not able to quickly move away from an oncoming boat.

### **Vibrissae**

Manatees have sensitive tactile hairs that cover their bodies and faces called vibrissae. Each individual hair is a vibrissal apparatus known as a follicle-sinus complex. Vibrissae are blood filled sinuses bound by a dense connective tissue capsule with sensitive nerve endings that provides haptic feedback to the manatee.

Usually vibrissae are found on the facial regions of terrestrial and non-sirenian aquatic animals and are called whiskers. Manatees, however, have vibrissae all over their bodies. The vibrissae located in their facial region are roughly 30 times denser than the vibrissae on the rest of their body. Their mouth consists of very mobile prehensile lips which are used for grasping food and objects. The vibrissae on these lips are turned outward during grasping and are used in locating vegetation. Their oral disks also contain vibrissae which have been classified as bristle-like hairs that are used in nongrasping investigation of objects and food.

Research has found that manatee vibrissae are so sensitive that they are able to perform active touch discrimination of textures. Manatees also use their vibrissae to navigate the turbid waterways of their environment. Research has indicated that they are able to use these vibrissae to detect hydrodynamic stimuli in the same way that fish use their lateral line system.

## Reproduction

Although female West Indian manatees are mostly solitary creatures, they form mating herds while in estrus. Most females first breed successfully between ages of seven and nine; they are, however, capable of reproduction as early as four years of age. Most males reach sexually maturity by the time they are three or four. The gestation period is 12 to 14 months. Normally, one calf is born, although on rare occasions two have been recorded. The young are born with molars, allowing them to consume sea grass within the first three weeks of birth. On average, manatees that survive to adulthood will have between five and seven offspring between the ages of 20 and 26. When a calf is born, it usually weighs between 60 and 70 pounds and is between 4.0 and 4.5 feet long. The family unit consists of mother and calf, which remain together for up to two years. Males aggregate in mating herds around a female when she is ready to mate, but contribute no parental care to the calf.