The West Indian manatee (Trichechus manatus), also known as the North American manatee, is a large, aquatic mammal native to warm coastal areas of the Caribbean, from the Eastern United States to northern Brazil. Living alone or in herds, it feeds on underwater plants and uses its whiskers to navigate. It is divided into two subspecies, the Florida manatee (T. m. latirostris) in the United States and the Antillean manatee (T. m. manatus) in the Caribbean,[8][9] both of which are endangered and face pressure from habitat loss, pollution, and other human activity. The West Indian manatee is the largest living member of the sirenians (order Sirenia), a group of large aquatic mammals that includes the dugong, other manatees, and the extinct Steller's sea cow.
Manatees are herbivores, have developed vocal communication abilities, and are covered in highly sensitive whiskers (called vibrissae) that are used for feeding and navigation.[10] In breeding season, several males form mating herds around an individual female; on average, one calf is born to a female manatee every two to three years.[11]
In the 1970s, the West Indian manatee was listed as "endangered" in the United States under the Endangered Species Act, when there were only several hundred left.[12] The decades since have witnessed significant efforts to protect this species from natural and human threats, particularly collisions with watercraft.[13] In 2017, the United States changed the classification to "threatened", citing a substantial increase in the total population.[12][14]
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.[15] The largest individual on record weighed 1,655 kg (3,649 lb) and measured 4.6 m (15 ft) long.[16][17] Manatees are estimated to live 50 years or more in the wild,[11] and one captive Florida manatee, Snooty, lived for 69 years (1948–2017).[18]
Given that manatees are mammals, they breathe air, have warm blood, and produce milk. Like the other sirenians, the West Indian manatee has adapted fully to aquatic life, having no hind limbs. Instead of hind limbs, the manatee has a spatula-like paddle for propulsion in the water. Manatees have evolved streamlined bodies which lack external ear flaps, thus decreasing resistance in the aquatic environment. Pelage cover is sparsely distributed across the body, which may play a role in reducing the build-up of algae on their thick skin. Manatee skin is gray but can vary in coloration due to algae and other biota, like barnacles, that opportunistically live on manatees. Scar tissue on manatees is white and persists for decades, allowing for easy identification. The Florida manatee has three to four nails on each flipper.[19]
The West Indian manatee has a prehensile snout, like their relative the elephant, for grabbing vegetation and bringing it into their mouths. Manatees have six to eight molariform teeth in each jaw quadrant. These molariform teeth are generated at the back of the mouth and slowly migrate towards the front of the mouth, at a rate of 1–2 mm per month, where they then fall out. This tooth ‘conveyor belt’ provides unlimited tooth production which is beneficial for the manatee which feeds on vegetation four to eight hours per day and consumes 5-10% of its body weight per day. Manatees have 3–5 cm hairs that cover their whole body and provide somatosensory information. Manatee bones are dense and solid which allows them to act as ballast and promote negative buoyancy. This helps counteract the positive buoyancy which comes from their high fat content. These two buoyancy counterparts, along with air in the lungs, helps manatees achieve neutral buoyancy in the water. This makes breathing, foraging, and swimming easier for the manatee. Manatees are unique, compared to other mammals, in that they have a longitudinally oriented diaphragm that is split in half to form two hemidiaphragms. Each hemidiaphragm is capable of independent muscular contractions.[19]
The West Indian manatee inhabits mostly shallow coastal areas, including rivers and estuaries. Manatees can withstand large changes in salinity and are found in both freshwater and saltwater.[11] Manatees' extremely low metabolic rate and lack of a thick layer of insulating body fat limits them to locations with warm waters, including tropical regions.
The Florida subspecies is primarily found along the Florida coast, but has been spotted as far north as Dennis, Massachusetts, as well as along the Gulf Coast of Texas.[20] Concerning their forays inland, a manatee was once spotted in the Wolf River (near where it enters the Mississippi) in Memphis, Tennessee, in 2006: a distance of over 700 river miles from the Gulf of Mexico.[21] The Antillean subspecies has a sparse distribution throughout the Caribbean, ranging as far north as Mexico and as far south as Brazil.[22][23]
An analysis of mitochondrial DNA patterns indicates there are actually three primary geographic groups of the West Indian manatee: (i) Florida and the Greater Antilles; (ii) Mexico, Central America and northern South America; and (iii) northeastern South America.[24][23]
The seasonal distribution of the West Indian manatee varies with water temperatures. Temperatures below approximately 20 °C (68 °F) increase the risk of cold-induced stress or mortality for manatees.[11][25] Consequently, Florida manatees seek out warm-water refuges during the winter, concentrated primarily along the Florida peninsula.[26] Many of these refuges are artificial, created by the runoff from nearby power plants.[4] In contrast, the Antillean manatee is less susceptible to cold-induced stress due to the warmer waters across its range.
Florida manatees inhabit the most northern limit of sirenian habitats. There are four recognized subpopulations of the Florida manatee, termed the Northwest, Southwest, Atlantic Coast and St. John's River populations.[27] Large concentrations of Florida manatees are located in the Crystal River[28] and Blue Springs regions in central and north Florida. The Antillean manatee is sparsely distributed throughout the Caribbean and the northwestern Atlantic Ocean, from Mexico, east to the Greater Antilles, and south to Brazil. Populations can also be found in The Bahamas, French Guiana, Suriname, Guyana, Trinidad, Venezuela, Colombia, Panama, Costa Rica, Curaçao, Nicaragua, Honduras, Guatemala, Belize, Cuba, the Dominican Republic, Jamaica and Puerto Rico. It is possibly extirpated from the Turks and Caicos Islands, and definitively so from all the Lesser Antilles except Trinidad and Curaçao. Its presence in Haiti is uncertain.[14] A reintroduction was planned to take place in Guadeloupe, but was terminated early due to political issues.[29]
Because manatees evolved in habitats without natural predators, they lack predator avoidance behavior. The large size and low metabolic rates of manatees lends to their capacity for long and deep dives, as well as their relative lack of speed. Manatees are frequently solitary creatures, but they do aggregate at warm-water habitats during the winter and during the formation of breeding herds.[11] In spite of their docile demeanor, due to their large size the West Indian manatee has nearly no natural predators in its native environment. American alligators and American crocodiles have been suspected to on occasion take a manatee.[30][31] A dying manatee was discovered in Puerto Rico, wounded by a large shark bite thought to be inflicted by either a tiger shark or bull shark.[32] It is projected that large sharks may threaten manatees occasionally but rarely enter the manatee's brackish habitats, whereas marine mammals in more direct coastal oceanic waters are well known to be highly vulnerable to sharks and orcas.[33]
Manatees have been shown to form large mating herds when males come across estrous females, indicating that males may be able to sense estrogen or other chemical indicators.[34][35] Manatees can communicate information to each other through their vocalization patterns.[36] Sex and age-related differences are apparent in the vocalization structure of common squeaks and screeches in adult males, adult females, and juveniles.[37][38][39] This may be an indication of vocal individuality among manatees.[38] An increase in manatee vocalization after a vocal playback stimulus shows that they may be able to recognize another manatee's individual voice.[38] This behavior in manatees is found mostly between mother and calf interactions.[40] However, vocalization can still be commonly found in a variety of social interactions within groups of manatees, which is similar to other aquatic mammals.[40][citation needed] When communicating in noisy environments, manatees that are in groups experience the same Lombard effect as humans do; where they will involuntary increase their vocal effort when communicating in loud environments.[41] Based on acoustic and anatomical evidence, mammalian vocal folds are assumed to be the mechanism for sound production in manatees.[42] 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.[34]
Manatees are obligate herbivores that feed on over 60 species of aquatic plants in both fresh and salt water. Seagrass is a staple of the manatee diet, particularly in coastal areas.[43][44][45] In addition, when the tide is high enough, they will also feed on grasses, roots and leaves (including those of mangroves), as well as algae.[46][47][48] Manatees typically graze for five or more hours per day, consuming anywhere from 4% to 10% of their body weight in wet vegetation per day, though the exact amount depends on their body size and activity level.[49] Because manatees feed on abrasive plants, their molars are often worn down and are replaced many times throughout their lives, thus earning the nickname "marching molars." The molar teeth are similar in shape, but of varying sizes. Manatees do not have incisors; these have been replaced by horny gingival plates.[50] Some individuals may also inadvertently eat invertebrates (such as small aquatic insects and crustaceans) and will eat fish both in captivity and in the wild.[47][48]
Manatees are nonruminants with an enlarged hindgut. Unlike other hindgut fermenters, such as the horse, manatees efficiently extract nutrients, particularly cellulose, from the aquatic plants in their diet. Manatees have a large gastrointestinal tract with contents measuring about 23% of its total body mass. In addition, the passage rate of food is very long (about seven days).[51] This slow process increases the digestibility of their diet. It is suggested that chronic fermentation may also provide additional heat and is correlated with their low metabolic rate.[46]
Manatees have sensitive tactile hairs that cover their bodies and faces called whiskers or vibrissae. Each individual hair is a vibrissal apparatus called 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.[10]
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.[52][53]
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.[10]
Male manatees reach sexual maturity at 3–4 years of age, while females reach sexual maturity at 3–5 years of age.[54][11] Manatees appear capable of breeding throughout their entire adult life, although most females first breed successfully at ages 7–9. Breeding occurs in ephemeral mating herds, where several males aggregate around an estrous female and compete for access to her.[52] These mating herds can last up to 2 to 4 weeks. The estrous cycles of female manatees typically are approximately one month long. Females in captivity have been observed to skip cycles.[55] Larger males, presumably older, have been observed to dominate mating herds and are likely responsible for most successful copulation events.[56] However, between males and females there is a little sexual dimorphism where females are typically slightly larger than males.[57]
The reproductive anatomy of males consists of internal testes within the abdominal cavity, which are located posterior to the kidneys. In the Florida manatee, males experience seasonal changes in their spermatogenesis. They are active during the warm breeding season; during this time all stages of spermatogenesis can be seen. However, during the cold non-breeding season, they are inactive, and spermatogenesis is inhibited.[58]Recently there has been a study that shows support that males use chemosensation for the anal glands to locate females in estrous.[59]
The reproductive anatomy of females is most like that of elephants. Female manatees resemble elephants in their zonary placenta, bicornuate uterus, and corpora albicantia. The female manatees’ ovary gross morphology consists of an oval disk shape with a flat surface of the cortex. And since they are polyovular the ovulation scar is often challenging to identify.[55]
The gestation period in manatees lasts for 12–14 months, after which they give birth to one calf at a time or, rarely, twins.[56] When twins are born, they are fraternal twins and are dizygotic.[60] When a calf is born, it usually weighs 60–70 lb (27–32 kg) and is 4.0–4.5 ft (1.2–1.4 m) long. Manatees do not form permanent pair bonds, and the male contributes no parental care to the calf, which remains with the mother for up to two years prior to weaning. Female manatees have two axillary mammary glands located under their flippers. The lactation period for mothers and their calves last typically for one to two years. Prior to weaning, there is an increase in the hormone progesterone in the mother.[61]During this two-year period with their mother, calves learn how to locate warm water sources for winter and about migration routes for the summer seasons.[62] A single female can reproduce once every 2–3 years, which is referred to as the calving interval.[11] Wild manatees have been documented producing offspring into their late 30s, and a female captive manatee has given birth in her 40s.[4]
Captive breeding programs are not being conducted for the Florida manatee due to conservation concerns that are being addressed in their natural environments. Until these concerns are rectified, a captive breeding program will not be beneficial for this species.[63]