The Florida Manatee, Trichechus manatus latirostris
The Florida manatee, Trichechus manatus latirostris, one of two subspecies of the West Indian manatee, is native to Florida’s coastal waters. The other subspecies is the Antillean manatee, Trichecus manatus manatus, found in Mexico and the Caribbean. Manatees are semi-social herbivores and have no predators. Since 1991, statewide aerial surveys have been conducted during the winter to estimate the size of manatee populations. Recent estimates of the Florida manatee population range from 3300 to 4000 (Reep & Bonde, 2006).
Manatees are part of the mammalian order Sirenia, which includes three additional species: the West African manatee, T. senegalensis, the Amazonian manatee, T. inunguis, and the dugong, Dugong dugon (U.S. Fish and Wildlife Service, 2001). The fossil record of the Sirenian line dates back approximately 50 million years, with origins in the Amazonian region, prior to the uplift of the Andes Mountains. Ancestors of present day manatees have been living in Florida waters for millions of years, and their fossils are commonly found throughout Florida. While the family of present day West Indian manatees evolved in the Caribbean region about two million years ago, the modern Florida manatee appeared more recently, following the last glacial period, approximately 12,000 years ago (Reep & Bonde, 2006).
Figure. The Florida manatee (photo: USGS – Sirenia Project).
Extent of Florida Manatees
Today, Florida is at the northern extent of the range for manatees. However, in summer, manatees often roam northward along the Atlantic Coast to Georgia and the Carolinas. They have been observed as far north as the Chesapeake Bay and New York (Haubold et al., 2006). In winter, manatees return to warmer waters. They retain a clear memory of resource patterns within their environment and will return to the same site year after year to seek refuge from cold winter temperatures (Reep & Bonde, 2006). Scientists have identified four distinct groupings of manatees in Florida that are based on the geographic configuration of resources: the Upper St. John’s River, the Atlantic Coast, the Southwest, and the Northwest (Haubold et al., 2006).
What is a Manatee?
Florida manatees are large, heavy-boned, “fusiform” shaped animals —their head and body comprise one streamlined form —about 10 feet in length, and weighing approximately 2200 lbs at maturity. They have thick, dark grey, rubbery skin with sparse hairs except around the snout, where the hairs form bristles (U.S. Fish and Wildlife Service, 2001). Their forelimbs and tail are flattened, paddle-like structures. They have no hind limbs. Their lungs are very large and aid the manatee in maintaining neutral buoyancy as they graze underwater. Their valve-like nostrils are located on the top of their snout, which is downturned, providing better control for feeding on aquatic vegetation. The snout itself is a muscular upper lip that, together with the bristles covering the lips, sense and manipulate vegetation for efficient feeding. Manatee teeth are specially designed to crush vegetation that may be covered with sand. As the sand wears away the molars, new teeth are continually produced in the back of the jaw, pushing the older teeth forward in a “conveyer belt” fashion (Reep & Bonde, 2006). The eyes of manatees are very small, are forward looking on the sides of the head, and can be closed or protected by a membrane. Manatee ears are tiny, and located on the sides of their head behind the eyes. Their hearing is not very acute, and extends over a narrow band of low frequency sounds (U.S. Fish and Wildlife Service, 2001).
Manatee Life Cycle
A manatee’s life can span fifty to sixty years. They reach sexual maturity at two to five years of age and bear one calf after a gestational period of about 12 to 14 months. Calves stay with their mothers for one and a half to two and a half years, learning the patterns of the environment and nursing frequently. A female manatee gives birth about every two and a half to three years, if the calf survives to weaning age (Haubold et al., 2006; Reep & Bonde, 2006). Manatee deaths in Florida have been reported and recorded since 1974. The FWC maintains statistics on the causes of death of all reported manatee carcasses. Causes of death are classified as: human-related, including “watercraft,” “flood gate/canal lock,” and “other human;” naturally occurring, including “perinatal,” “cold stress,” and other “natural;” and “undetermined.” Another category for “unrecovered” counts the number of manatee carcasses that were sighted but not recovered for necropsy. The “other natural” category includes “manatee deaths resulting from infectious and non-infectious diseases, birth complications, natural accidents, and natural catastrophes (such as red tide poisoning)” (Fish and Wildlife Research Institute, 2007a). Red tide dinoflagellettes (Gymnodinium breve) can cause sudden high numbers of manatee deaths. For example, in 1996 Lee County experienced a red tide event that killed numerous manatees, with 37 deaths reported in March and April alone, where normally between 1 and 5 deaths are reported in this category. Monthly and yearly counts of manatee mortality by county and cause of death since 1974 can be viewed and downloaded from the FWRI website (http://research.myfwc.com/manatees/). A summary of manatee mortality statistics between 1974 and 2000 is presented in the Manatee Recovery Plan, 3rd Revision (U.S. Fish and Wildlife Service, 2001)
Figure. Female manatee nursing a calf (photo: USGS – Sirenia Project).
Manatees live in the shallow waters of estuaries and coastal areas, and are as commonly found in urban canals as in remote shoals and inlets. They are herbivores that feed on a wide variety of aquatic vegetation. While their diet consists mainly of seagrasses, they will eat other plants, including mangroves and salt marsh grasses. Therefore, manatees are considered generalists with respect to their diet and habitat (Reep & Bonde, 2006). In the summer months, from April through October, they disperse widely, in areas where aquatic vegetation is plentiful.
Despite their massive size and thick skin, manatees are very sensitive to cold water temperatures. During winter months, typically from November through March, when water temperatures drop below 68º F, they congregate at warm water aggregation sites. These sites include natural springs, power plants, other industrial discharges, dredged canals and basins, and areas at the southern end of the state. With the advent of power plants, industrial plants, and extensive canal development, manatees have become habituated to many artificial warm water sources—a significant issue in the overall status of manatees. They exhibit strong fidelity to these sites, remembering their locations and returning to them year after year. This behavior is not only exhibited with respect to warm water sources, but other resources as well, including foraging areas and freshwater sources (Reep & Bonde, 2006)
Figure. Group of three manatees feeding on seagrass
(photo: USGS – Sirenia Project).
Threats to Manatees
Manatees have no natural predators. Environmental threats to their survival include cold temperatures, extreme storms, and red tide events. Other threats stem from human interactions with their environment. When seagrass beds are scarred by boat propellers or water pollution diminishes seagrass production, manatees in turn feel the effects. Changes to power plant technology that reduce the availability of warm water in winter months could greatly influence manatee survival. Likewise, flow reductions in natural springs due to increased groundwater withdrawal could negatively affect manatees (Haubold et al., 2006). Collision with boats, however, is the most significant human related threat to manatee survival (U.S. Fish and Wildlife Service, 2001). Accidents that do not result in the death of a manatee (sub-lethal) are also very significant. More than 1000 manatees have been photographed with scars from boat impacts, and 97% of these bear scars from more than one collision (Haubold et al., 2006). Other human related causes of manatee mortality include being crushed in locks or between the hulls of boats and canal sides. Ingesting litter and entanglement in fishing nets and gear is also a serious problem for many marine animals, including manatees.
Figure. Two manatees under a boat propeller
(photo: USGS – Sirenia Project).
Figure. Manatee with a rope embedded in its right flipper
(photo: USGS – Sirenia Project).