Sea oats are important sand stabilizers at Grotto Beach, San Salvador, Bahamas. See other beautiful phenomena from the Bahamas.
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Introduction
The Florida manatee is one of the worldÕs rarest marine mammals, both in its evolutionary history, and lifestyle. However, this great mammal has been endangered, or facing the risk of imminent extinction, due to mostly human activities for nearly thirty five years. It is one of the most endangered marine mammals found in U.S. and Caribbean waters. In 1972, the manatee was added to the U.S. Marine Mammals Protection Act (MMPA) and the U.S. Endangered Species Act (ESA) in 1973 (Rathbun, 2000). Since then, not much has changed, as the manatee is still not viable or self-sustaining, and mortality rates annually are still on a generally upward trend. The number of individual manatees remaining in the world is now slightly over 100,000. This number is smaller than that of any other mammalian order (Nowak, 2003). While there are many groups in order to help save the manatees, these animals are still facing great risks as a species. The problems lie in that there are a wide range of groups, more than seventy, at work yet no consistent plan and little research is being done to help identify how to combat the most harmful and prominent factors contributing to their deaths. Until we learn more about manatee habitat protection, preventing human related deaths, and take legitimate and progressive action toward stopping it, the manatees will continue to suffer.
This topic applies to marine ecology because, although we know very little about the manatee and its contributions to the marine community, they are an integral part of the marine system especially for the Florida, Caribbean and surrounding waters. The effects of removing a single species may not be noticed immediately, but it will eventually disrupt the most efficient functioning of the community. This paper will address how the manatee evolved and how it lives now, the problems it faces as a species, the importance of manatees within the marine environment, the attempts made to secure its viability as a species, and what should be done in the future in order to ensure manatee survival.
Evolutionary Background
The official classification of the Florida manatee is as follows (Reynolds, 1991):
Phylum: Chordata (vertebrates and closely related invertebrates)
Subphylum: Vertebrata (vertebrates with backbones or spinal columns)
Class: Mammalia (mammal)
Order: Sirenia (manatees and dugongs)
Family: Trichechidae (manatees)
Genus and Species: Trichechus manatus (West Indian Manatee)
Subspecies: Trichechus manatus latirostris (Florida)
The Florida manatee, as shown in its classification, is a marine mammal and one subspecies of the siren order, which includes three species of manatee, one extant species of dugong, and one extinct species of dugong, StellerÕs Sea Cow. The sea cow, a massive animal occupying cold waters of the Bering Sea, was killed off completely in 1768 by Russian hunters, about twenty five years after its discovery (Reynolds, 1991). This species was largest of the sirens and also the only one who could survive in the cold, arctic waters. The three species of manatee are the trichechus inunguis, the Amazonian manatee, trichechus senegalensis, the West African manatee, and trichechus manatus, the West Indian manatee. The West Indian manatee also has two subspecies, separated by their location. They are the Florida manatee, trichechus manatus latirostris, and the Antillean manatee, trichechus manatus manatus.
This particular order of mammals is in fact a rarity of sorts as it has no evolutionary relationship to any other major group of living marine mammals. In fact, the Sirenians developed from a branch of four-legged land mammals nearly 55 million years ago during the early Eocene epoch (Zeiller, 1992). The manatee and its fellow siren the dugong are more closely related to the terrestrial elephants than the other two orders of marine mammals, Cetacea, which includes whales, dolphins, and porpoises, and Carnivora, which counting subgroups includes mammals such as seals, sea lions, walruses, sea otters and polar bears (Reynolds, 1991). The orders they are most commonly associated with are a group called Òsub-ungulatesÓ which are Òall regarded as evolutionary offshoots of a primitive ungulate [hooved] ancestral stockÓ (Reynolds, 1991). The other orders include those of the elephants, the hyraxes and the aardvarks. Commonalities between the manatee and these animals, particularly the elephant, include that they are all vegetarians, have virtually no hair on their bodies, have nails or hooves instead of the primitive claws, an upper lip very similar to the elephantÕs trunk it its look and functional abilities, and texture, thickness and color of the skin (Babinski, 2001). Traits that separate this species, from their terrestrial ancestors are the loss of the hind legs up to the pelvic bone, the presence of six neck vertebrates instead of the usual seven, and the horizontal movement of molar teeth replacing used, worn teeth in the front from behind apparently limitlessly (Nowak, 2003; Zeiller, 1992). Evolutionarily speaking, these animals changed from land to sea beings in a relatively short time frame; within the first ten or less million years of their appearance they had numerous identifiable genera. The earliest manatee-like mammals appeared in the Miocene, about 15 million years ago and by the early Pliocene, 5 million years ago, all Recent Siren species were flourishing.
Anatomy, Behavior, and Importance
Anatomy
The Florida manatee has a spindle-shaped body with flipper-like front limbs and the absence of rear limbs and the presence of a modified tail shaped like a flat rounded paddle. The absence of the rear limbs is present until the pelvic bone, which proves existence of these creatures as once terrestrial beings. They can reach up to 3-4 meters in length, about 13 feet, and can weigh as much as 3,500 pounds. Males typically weigh between ten and twelve hundred pounds and females typically two thousand (Dietz, 1992).
The manatee is also rare in that it has only six vertebrate in the neck, compared to usual seven in mammals. This serves no functional purpose, except that this deviation did not prove to be fatal in the evolutionary process for the manatee. Furthermore, there is almost a complete absence of less-dense bone in manatees and bone marrow cavities. While this has been attributed to the manateeÕs low metabolic rate or slower development, the extreme density of the bones does help to offset positive buoyancy, and also makes the animalsÕ skeletal system very strong (Reynolds, 1991). Eyes are small, and they lack the presence of an external ear. The upper lip is perhaps the most flexible and rapid moving muscle of the manatee. It is cleft, with each side moving independently of the other and is used to grab and manipulate vegetation when eating. The skin is generally gray or brown, rough texture and virtually hairless. The lifetime is unknown for certain, however estimates are between 40-70 years (Dietz, 1992).
Eating and Diet
The manatee, like all other sirens, is an herbivore and feeds on a wide variety of submerged, emergent, floating and shoreline vegetation. As the Florida manatee subspecies can migrate between fresh water and sea water without problem, there is great diversity among the types of plants it can consume. In salt water, the manatee feeds mostly on several different species of sea grass, including turtle grass, a type of Thalassia, manatee grass, and shoal grass. They also eat some types of algae, floating hyacinth, and often mangrove leaves and seedlings in salt water and freshwater (Hartman, 1979). They typically feed at depths around 4 meters (Nowak, 2003). These mammals have been called the lawnmower mammal of the sea for the amount of sea grass and vegetation they eat. They spend about six to eight hours a day grazing, usually for one to two hours at a time (Rathbun, 2000). As stated in The Enchanted Braid, the dugong, an animal only slightly larger than the related manatee, requires enormous amounts of sea grass for its diet and are known to eat about twenty six pounds of sea grass every day (Davidson, 1998). In general, they consume about 10 percent of their body weight each day in the summer and more during the winter (Dietz, 1992). In several cases, the manatees have been employed to help control water hyacinths and other aquatic weeds within the waterways. In fact this was how the manatee eventually came to exist in the Caribbean waters. In the 1800s, the West Indian manatee was transferred across the Atlantic and placed in the Caribbean to help control sea grass throughout the existing travel ways.
Breathing
While the manatee is a completely marine mammal and spends much of its time underwater, it does not have the anatomy to breathe underwater and thus must surface periodically. They have several adaptations which allow them to hold their breath and stay under water longer than the average terrestrial mammal such as having two separate lung cavities, each with its own diaphragm, and valved nostrils which close when the manatee dives. If at rest, manatees can stay under water as long as twenty minutes, but typically only stay on an average of 3-5 minutes (Nowak, 2003). Once they surface, manatees can retain about 90% of the air in their lungs. For perspective, humans retain only about 10% (Zeiller, 1992). This combination of biological advantages allows the manatee to function well within the marine environment as its anatomy also prevents it from leaving the water. It is during this surfacing that manatees are most susceptible to human-related injuries, by boat accident, intentional vandalism, or interference with man-made machinery such as dam locks.
Communication
The best known myth about the sirens is that they are mermaids. Although it is difficult to see the resemblance between the manatee or dugong and a mermaid, the connection has lasted through time. Despite the similar tail, the foremost reason for this myth is the sounds made were thought to resemble that of a beautiful, mythological creature. In fact, the name of the order, sirenia, comes from the supposed mermaid-like songs of the dugong and manatee, originating from the ancient myth of the sirens (Nowak, 2003). The sirens, according to Greek mythology, were beautiful bird women disguised as seductresses whose songs would tempt and lure sailors to their deaths. Their song was thought to resemble that of the manatees and dugongs.
Very little is known about manatee communication and its predictability. One of the main reasons for this is that manatees are solitary beings with only exceptions of mother calf relationships and during mating. They do hear very well, communicating audible to the human ear with, as one observer found, Òhigh pitched chirps that would sound far more appropriate emanating from sparrows than from 1,200-pound mammalsÓ (Zeiller, 1992). They do vocalize using echolocation, which is the use of sound waves to emit a sort of sonar sound and then listens to the echoes to give them information about location, range and the identification and distance of other objects (Dietz, 1992). According to mammologist David Hartman, manatees have been known to vocalize when under stress or frightened, in apparent irritation when disturbed by other manatees while feeding on the bottom, during mating or heightened sexual frenzy, during play, and while stretching (Hartman, 1979). Their sounds also become more frequent between mother and calf, and particularly when they feel threatened or when visibility is low. While there is no way to tell for sure without further investigation, Hartman also states that he believes Òthat manatees are able to recognize on another by sound Ð that is, by subtle variations in pitch, frequency, and timbre of individual vocalizations. It was obvious, for example, that cows [mothers] were selectively responsive to the calls of the calves and vice versaÉ It seems reasonable to assumeÉ that Ôvoice printsÕ characterize individual manateesÓ (Hartman, 1979). Thus, while further research still must be conducted, it seems likely that their vocalizations serve a purpose for communication, safety and identification. It has been suggested that the sounds made my boat motors can disrupt this communication.
Environment and Temperature Regulation
The natural habitats for the Florida manatee include near-shore coastal areas, estuaries, effluent warm water from industrial power plants and inland waterways. Because of their low metabolic rates, which is only 15%-22% of that predicted for a terrestrial mammal of a similar body weight, they are relatively poor producers of body heat, and thus do not adapt well to changes in water temperature (Reynolds, 1991). They are extremely sensitive to temperature changes and must migrate south when water temperatures reach below 20 degrees C, (68 degrees F). Severely cold weather, even for a few days, can kill manatees as it slows the metabolic rate even more and cause them to stop eating. Manatees who were found dead after periods of cold weather have often Òdepleted their body cavity fat, stopped feeding (no food in stomachs), and are dehydratedÓ (Hartman, 1979). Young manatees and calves are even more susceptible to this kind of trauma, as they often do not yet know migration routes.
In order to escape colder waters and weather fronts, manatees follow a north south migration pattern. In the summer months, manatees can be found as far north as Virginia and as far west as the Texas gulf (Rathbun, 2000). However in the winter months, they are rarely found farther north than the end of the Florida peninsula. The migrate to and from natural and artificial warm water refuges, including warmer southern waters, natural springs, and warmer water made from Port Everglade power plants, or pulp mills. Once the cold weather has passed, the manatees will return to their preferred feeding and habitat locations. This is one of the most important issues in manatee protection because as human actions are so essential to manatee survival, we must know and understand their migration patterns in order to better avoid them.
Endangered: Past, Present and Future
As early as the sixteenth century, humans used commercial exploitation of the West Indian manatee as well as extensive subsistence hunting. This was the beginning of the depletion of the population for the manatee. At that time and since then, they have been killed for their meat, and also died due to the draining of swamps, accidental and intentional drowning in fishing nets, accidents with boat collisions, interference with flood control structures and pollution. By the 1970s the problem with the Florida manatee was so apparent that the government felt the need to protect if by state law and federal law. The manatee is now protected by two major pieces of federal legislation, the Marine Mammal Protection Act of 1972, and the Endangered Species Act of 1973. These acts defined the manatee species as being under imminent risk for extinction. They are two of over seventy organizations that help contribute to protecting the manatees in Florida now, including the Florida Manatee Sanctuary Act of 1978, U.S. Fish and Wildlife Service (FWS), the Marine Mammal Commission (MMC), and the Save the Manatee Club, a grassroots organization which encourages the public to contribute to manatee conservation (Rathbun, 2000). These groups prohibit a great deal or potential problems for the manatees by outlawing activities such as Òkilling, injuring, capturing, harassing, or attempting any such activitiesÓ that are defined by changing the normal behavior for the animal (Reynolds, 1991). In fact even chasing and handling a manatee is illegal now in Florida, and any of the above activities will result in penalties up to $20,000 and or a year in prison (Reynolds, 1991).
While all of these methods are effective, there is still struggle to prevent the two most prominent causes for manatee death, which are due to collisions with boats and perinatal deaths, or those caused by or related to the loss of dependent calves. Since 1976, manatee mortality in Florida has averaged between 125-130 animals; however the problem lie in the facts that there are also only about 125-30 births per year and there are no truly accurate estimates on the size of the manatee population (Nowak, 2003). Also since this time, about thirty deaths every year are caused by boat related injuries and manatee mortality due to collisions with boats reached record levels starting in 1994. Almost all literature points to the sad fact that most manatees in Florida bear the scars from propellers and collisions with boats. As one observer noted, ÒIf there is one dramatically visible sign of manÕs effect on the manateesÉ it was the embarrassing amount of scars and mutilation visible on each animal. Forget the fact that every animal we saw that day, a total of thirty-seven, has scars. The sad part was that most of them had several scars. A few were even horribly disfigured. No Tail, a large male first seen at the spring in 1983, lost over 80 percent of his tail in a collision with a boat propeller. PhoebeÉ besides carrying a plethora of propeller slashes on the rear portion of her dorsal surface, she has also lost the use of her left front flipper which became entangled in a discarded monofilament lineÓ (Dietz, 1992). However gruesome these collisions may be, another issue perinatal death is far more complex. The causes of perinatal mortality are largely unknown, but they may be related to cold temperatures, premature births, and other natural factors or human-related causes such as pollution and death or nursing mothers due to collisions with boats (Hartman, 1979). Prevention of this cause of mortality can only be accomplished by further research, of which the funding and organization in recent years has been dismal.
So since the manatee protection programs have been established in Florida for well over thirty years, why then is the manatee still in such danger? Why, in 2007, did the Florida Fish and Wildlife Conservation Commission decide to keep the manatee on the state list under ÒendangeredÓ instead of down-listing to ÒthreatenedÓ? Down-listing would mean manatees were now only at a high risk for extinction as opposed to facing imminent risk for it. Supporters argue that the manatees would not lose protection because they would still be endangered on federal terms, and that manatee numbers are up to 2,800 from the 1,700 in the 1991 survey. However, the problem that the organizations have faced in truly protecting the manatees, is that there are so many groups and levels of distribution that there is not enough organization and delegation for the plans to have any true effectiveness. Because of this lack of organization there is a severe lack of reliable estimates of historical and current manatee numbers and Florida, which Òmakes it difficult to answer the question of whether the increased mortality rate is due to expanding manatee populations or expanding boat and human populationsÓ (Rathbun, 2000). Furthermore, Òthe research community in Florida has also been slow to initiate studies to demonstrate the effectiveness of the numerous regulations instituted to reduce boat-related injuries and deathÓ (Rathbun, 2000). Without this essential data, controversy continues over the effectiveness of each program versus another and meanwhile boat-related and other manatee deaths continue.
Based on current levels of organization, several changes need to be made before the manatees can truly be safe from human interference. First, the federal and state programs must come together to reach a common plan for action. Unlike the past, however, this process cannot afford to take five or more years. Next, the all programs need to collectively elect a strong leader who can provide authority to reach the common goal developed to meet the Òlowest common denominatorÓ of each groupsÕ plan, adequately promote an overall goal of manatee recovery, and fosters cooperation and communication between the groups (Rathbun, 2000). The groups should agree to meet regularly, all contribute to the costs, and be willing to constantly revise, or more appropriately, update the plan once more information has been gathered. Conservation of the manatee needs to be restructured from the strict conservative current recovery plan and toward one with more coordination, continuity, and stability. Conservation efforts should include maintaining the ecosystem in a desirable state; including safety factors in management decisions to allow for the fact that our knowledge is limited and fallible; formulating and applying conservation efforts that do not waste other resources, and ensuring that monitoring and assessment precede planned interactions with the animals (Reynolds, 1991). We need to rethink what conservation means, and how this can be done most effectively. We need to improve both the quality and safety of the manateeÕs habitat before we can expect them to recover. And we also need to recognize that it has taken almost 300 years to decrease the manatee and siren population so drastically, and it will take a long time as well for them to regain viability. No matter what we do, the most important thing to remember is that the manatee is more than a distant sea creature. It is an integral part of a system in nature, a system that we, too as humans are a part of. Until we come to recognize ourselves as essential to their survival, and they to our survival, there is little hope for the manatee.
References
Babinski, Edward T. (2001). Sirenian Evolution: Elephant to Sea Cow. Retrieved June 1, 2008, from http://www.edwardtbabinski.us/manatee/sirenian_evolution.html.
Davidson, Osha Gray. (1998). The Enchanted Braid. New York: John Wiley & Sons, Inc.
Dietz, Tim. (1992). The Call of the Siren: Manatees and Dugongs. Golden: Fulcrum Publishing.
Hartman, Daniel. (1979). Ecology and Behavior of the Manatee (Trichechus manatus) in Florida. Pittsburg: The American Society of Mammalogists.
Nowak, Ronald M. (2003). Dugong, Sea Cow and Manatees: Sirenia. WalkerÕs Marine Mammals of the World. Baltimore: Johns Hopkins University Press.
Rathbun, Galen B and Richard L. Wallace. (2000). The Florida Manatee. In Richard P. Reading and Brian Miller (Eds.), Endangered Animals: A Reference guide to Conflicting Issues (pp. 107-111). Westport: Greenwood Press.
Reynolds, John E. III and Daniel K. Odell. (1991). Manatees and Dugongs. New York: Facts On File.
Zeiller, Warren. (1992). Introducing the Manatee. Gainesville: University Press of Florida.
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