Sirenia

(Dugongs, sea cows, and manatees)

Class Mammalia

Order Sirenia

Number of families 2

Number of genera, species 3 genera; 5 species

Introduction

The sirenians are unique among mammals and marine mammals in that they are the only fully aquatic, herbivorous marine mammals. Their complete life cycle occurs in the water as do the life cycles of the cetaceans. Early explorers may have mistaken them for mermaids. Since sirenians eat water plants exclusively, they occur relatively close to coastlines where humans frequently are found in great numbers. Humans have exploited and over-exploited the sirenians range-wide and are responsible for the extinction of the Steller's sea cow (Hydrodamalis gigas). While the remaining sirenians (dugong and manatees) are protected range-wide by a variety of laws, humans continue to have a severe impact on them, primarily through habitat destruction. Given their incredible uniqueness we still know very little about their biology. So little, in fact, that it is difficult to develop detailed conservation strategies. The Florida manatee is probably the most intensively studied of all the living sirenians, yet the information gap is massive. For example, in 1979 Daniel Hartman estimated that the Florida manatee had a gestation period of "about a year." Yet nearly 25 years later (2003) that estimate is still the only estimate. The information gap for other sirenians is greater.

Evolution and systematics

The oldest sirenian fossils date back some 50 million years to the early Eocene along the Old World shores of the Tethys Sea. They probably evolved from primitive hoofed mammals with an ancestral line vastly different from that of the cetaceans. Their closest living relatives are the elephants (Proboscidea) with distant links to the hyraxes (Hyracoidea). While the seat of sirenian evolution was likely along what is now the eastern side of the Atlantic Ocean, the oldest fossil sirenians are from the western side of the Atlantic, namely Jamaica. The Dugongidae (dugongs and sea cows) evolved from the cosmopolitan subfamily Halitheriinae from the Eocene to the Pliocene. The subfamily Hydrodamalinae arose in the Miocene and was endemic to the North Pacific. The Dugonginae arose in the Pliocene and occurred in both Atlantic and Pacific regions. Apparently the manatees were able to out-compete the dugongs in the Atlantic. The modern manatees evolved from early dugongids in freshwater regions of northern South America in the Miocene. Expansion to Africa and North America occurred in the Pliocene or Pleistocene. The living and recent members of the order Sirenia are divided into two families, Trichechidae and Dugongidae. The Trichechidae contains three species in the genus Trichechus: T. manatus, the West Indian manatee; T. inunguis, the Amazonian manatee; and T. senegalensis, the West African manatee. The West Indian manatee is further divided into two subspecies: T. m. latirostris, the Florida manatee and T. m. manatus, the Antillean manatee. The Dugongidae has two subfamilies: Dugonginae and Hydrodamalinae. Sub-family Dugonginae has a single living species, the dugong (Dugong dugon). Likewise, the Hydrodamalinae contains a

single species, the Steller's sea cow, which is recently (1768) extinct.

Physical characteristics

The sirenians range in length from about 9.8 ft (3 m) for the Amazonian manatee to as much as 32.8 ft (10 m) for the extinct Steller's sea cow. Maximum body mass ranges from about 992 lb (450 kg) for the Amazonian manatee to more than 9,920 lb (4,500 kg) for the sea cow. All sirenians are nearly hairless. Their skin varies from relatively smooth (dugong) to rough (manatees) to rugose (sea cow). All lack hind limbs and vestigial pelvic bones can be found in the deep pelvic musculature. The forelimbs are short and flexible. The West Indian and West African manatees have nails, but they are absent in the Amazonian manatee and the dugong. The sea cow lacked phalanges (finger bones) and seems to have had some sort of hooked structure at the end of the flippers to provide a grasping surface. The tail is paddle-shaped in the manatees and fluke-shaped in the dugong and sea cow. External ears are absent and the eyes are small. Color varies from gray to brown and often depends on ectobionts (like algae) that may be growing on the skin. The manatees lack incisors and canine teeth, but have molars in both upper and lower jaws. These molars are replaced continually during the life of the animal. They erupt at the rear of each jaw quadrant, move forward as a result of forces generated during chewing, and fall out the front of the row as the roots are resorbed. They have been called "marching molars." The dugongs have tusks (incisors) and several peg-like molars in each row that are not replaced. Tusks generally erupt in mature males but rarely in females. Growth layer groups in dugong tusks can be used to estimate age. In manatees, growth layer groups in earbones are used for age estimation. The sea cow was toothless. All four species have "heavily whiskered faces" due to the stiff, thickened vibrissae (tactile hairs) around the lips. Sirenian bones are dense (osteosclerotic) and thick (pachyostotic) bones, particularly the ribs, that probably function in buoyancy regulation.

Distribution

The recent sirenians are distributed, with one notable exception, in tropical, subtropical, and temperate regions of the world. The exception is the recently extinct (1765) Steller's sea cow, which was found only in the frigid waters around Bering and Medney Islands in the northwestern Pacific Ocean. The manatees occur on both sides of the Atlantic Ocean. On they western side, they occur from Rhode Island (USA) southward along the U. S. coast to the Gulf of Mexico, Central America, many Caribbean islands, and along the South American coast to northeastern Brazil. The Amazonian manatee occurs only in the Amazon River basin, whereas the West Indian manatee is found throughout the described range but does not enter the Amazon basin. On the eastern side of the Atlantic the West African manatee occurs from Senegal southward to Angola. The dugong is distributed from the southeast coast of Africa northeastward along the Indian Ocean, Red Sea, Persian Gulf, the west coast of India, including Sri Lanka, and eastward to Indonesia, the Philippines, Papua New Guinea, the Caroline Islands, and Australia. The northern limit is around Okinawa and the Ryukyu Islands.

Habitat

The manatees and dugong occupy relatively shallow, warm coastal waters dominated by vegetation. The West Indian and West African manatees occur in coastal, estuarine, and riverine habitats, while the Amazonian manatee is found only in freshwater. There are good indications that that at least the West Indian manatee requires access to freshwater for drinking. Certainly Florida manatees appear to drink from various freshwater sources. The dugong occupies, to the best of our knowledge, a strictly saltwater habitat. In contrast to the manatees and dugong, the sea cow occupied an extremely cold, relatively inhospitable (compared to the tropics!) environment around a few islands in the northwest Pacific Ocean. This intertidal and subtidal habitat was dominated by macroalgae (kelp) upon which the sea cows fed. There is also some indication in the old literature that the sea cow had a preference for sites where freshwater streams entered the ocean.

Behavior

The sirenians, in general and to the best of our current knowledge (2003), may be considered as semi-social; the primary social unit is the female and her most recent calf. Breeding behavior is discussed below. Dugongs may be

found in foraging herds numbering in the tens or hundreds of individuals. Some dugongs have been documented to travel tens to hundreds of kilometers in the matter of a few days. Florida manatees also make relatively rapid, long distance movements. Many Florida manatees typically have a north-south migratory pattern in response to water temperature. Florida manatees also congregate in natural and artificial warm water refugia in the winter. Artificial refugia are those such as the outfalls of the cooling systems of electrical generating and other industrial activities. Some Antillean manatees may make seasonal movement to and from freshwater rivers. Sirenians make sounds with fundamental frequencies in the 3–10 kHz range. These sounds appear to function, in part, to maintain the cow-calf bond. Male dugongs on leks may use sound to attract females. Sirenians are not known to use echolocation and the full extent of their hearing capabilities is not known. Little is known about the behavior of Steller's sea cow.

Feeding ecology and diet

The manatees eat a rather wide variety of submerged, floating, emergent, shoreline, and overhanging vegetation. The dugong is restricted primarily to sea grasses. However, both manatees and dugongs have been noted to consume benthic invertebrates and some manatees have even eaten fish. The West Indian and West African manatees feed from the bottom to the surface and above while the Amazonian manatee is primarily a surface feeder. In contrast, the dugong is exclusively a bottom feeder. Manatees use their flippers and oral vibrissae to manipulate vegetation. Similarly, dugongs feed on all parts of the plants they consume, often uprooting the entire plant. Both manatees and the dugong have cornified plates on the anterior portions of their upper and lower jaws to crush vegetation and move it back for further crushing by the molars. The role of dugong tusks in feeding is unclear, since they only erupt in adult males. Manatees and dugongs have a single stomach compartment with an associated digestive gland followed by an equally capacious enlargement of the upper small intestine with its associated duodenal ampullae. The small and large intestines may reach lengths of 65.6 ft (20 m) each and are joined by a relatively small caecum. The sirenians are hind-gut digesters like the horse and use a variety of anaerobic microorganisms in the large intestine to break cellulose down into volatile fatty acids that can be absorbed by the gut. Food passage time is about one week. The sea cow was toothless and presumably crushed ingested kelp using the cornified plates on its upper and lower jaws. It apparently fed only on the surface and on algae growing on rocks exposed at low tide. Manatees studied in aquaria consume about 10% of their body weight in vegetation (wet weight) per day. In the field, both manatees and dugongs spend a large portion of their time feeding.

Reproductive biology

Little information is known about sirenian reproductive biology in general, but those facts that are available are outlined here. Manatees mature at two to 11 years of age. Females may be seasonally polyestrous. Gestation is thought to be about 12 months, but this has not been confirmed. Typically, a single calf is born that is 39.4–59 in (100–150 cm) long. Twins are rare. The typical calving interval is 2.5–3

years. The mating system has been described as "scramble competition polygamy or polyandry" or "scramble promiscuity." Individual estrous females are pursued by as many as 20 or more males. While males may mature at three to five years of age, they may not be able to secure mating rights until they are physically larger. In this case, size does matter. There is no pair bonding. Males play no role in care of the young.

Calves may be born at any time of the year, but there may be seasonal peaks in parts of the range.

Typically, dugongs produce single calves after a gestation period of about a year; the calf remains with the cow for more than a year. Male dugongs are not known to provide any parental care. Steller's sea cow reproductive biology is speculative. Steller wrote of family groups and suggested male-female pair bonding, which contrasts with the reproductive behavior of other living sirenians. Calves appear to have been seen at all times of the year, but may have been more common in autumn. This would suggest that mating occurred at most times of the year. Gestation period seems to have been at least one year, but may have been longer. Apparently only single calves were born. There are no data on age/size at sexual maturity or the degree of parental care. Steller's sea cows are thought to have been monogamous, but dugongs exhibit a variety of reproductive behavior, from scramble competition polygamy to lekking.

Conservation

The manatees are listed variously as endangered, threatened, or Vulnerable under international and national legislation. The Convention on Trade in Endangered Species of Flora and Fauna (CITES) lists the Amazonian manatee and

both subspecies of the West Indian manatee in Appendix I and the West African manatee in Appendix II. The IUCN lists all three species, including the subspecies, as Vulnerable. The U. S. Endangered Species Act (ESA) of 1973 lists both subspecies of the West Indian manatee and the Amazonian manatee as "endangered" and the West African manatee as "threatened." The African Convention for the Conservation of Nature and Natural Resources lists the West African manatee as "protected" under Class A. The Florida manatee is protected by the Florida Manatee Sanctuary Act in the State of Florida, USA. The dugong is listed in CITES Appendix I and is considered Vulnerable under IUCN criteria. It is listed as "endangered" under the U. S. Endangered Species Act. Steller's sea cow is extinct. In general, range-wide population estimates are not available for manatees or the dugong. In Florida, the Florida manatee population in 2003 was about 3,500 individuals. In Australia, the dugong population has been estimated to be over 80,000 individuals. Similarly, with the possible exception of Florida, population trends are unknown. While hunting of manatees and dugongs still occurs, it is probably at a very low level. However, other human activities are likely having significant impacts on manatee and dugong populations. Among these, habitat destruction is probably the most pervasive. Dredging, siltation from inland runoff, and human activities that result in salinity changes have drastic effects on sea grass beds. In Florida (and increasingly in other areas inhabited by manatees and dugongs), other human activities are directly responsible for killing manatees. Dozens of manatees are killed each year as a result of watercraft collisions, and they also are killed in flood control dams and navigation locks. Manatees become entangled in various kinds of fishing gear (crab pot float lines, monofilament fishing line, shrimp trawls) and are killed. Other fatalities have occurred when these animals have ingested fishing gear and various kinds of plastic. Dugongs in Australia have been killed as a result of entanglement in nets set to protect swimmers from sharks. The management of human population growth and associated activities in coastal and certain inland areas is essential for the protection of manatee and dugong habitat and for the long-term survival of the four species.

Significance to humans

Historically, manatees, dugongs, and the sea cow have been hunted by humans for food, hides, and bone. Hunting resulted in the extinction of Steller's sea cow, making it virtually the only marine mammal species eliminated by human activities. However, other sirenian species are close to extinction,

and this dubious record may not hold for very long. Manatees and dugongs are of cultural significance to indigenous peoples in various parts of the world. The ecological significance of manatees to humans is not clear. Manatees and dugongs recycle nutrients in sea grass beds and keep the plants in a continual state of regrowth. Many other species of value to humans (e.g. sea turtles, fish, shrimp) rely on sea grass beds for shelter. This aspect of "what good are they" is often over-looked when trying to convince people to protect manatees and dugongs. A substantial ecotourism industry has developed at Crystal River, Florida, where a large number of manatees spend the winter in the clear, warm waters of natural springs. Manatee observation platforms have been set up near the effluents of power plant cooling canals where manatees congregate.

Resources

Books

Anderson, P. K., and D. P. Domning. "Steller's Sea Cow." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Australian Parks and Wildlife Service. Management Program for the Dugong (Dugong dugon) in the Northern Territory of Australia: 2003-2008. Palmerston, Australia: Department of Infrastructure, Planning and Environment, 2003.

Dierauf, L. A., and F. M. D. Gulland, eds. CRC Handbook of Marine Mammal Medicine, 2nd ed. Boca Raton, FL: CRC Press, 2001.

Domning, D. P. "Desmostylia." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

——. "Sirenian Evolution." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Hartman, D. S. Ecology and Behavior of the Manatee (Trichechus manatus) in Florida. Lawrence, KS: American Society of Mammalogists, 1979.

Kaiser, H. E. Morphology of the Sirenia. Basel: S. Karger, 1974.

Marmontel, M., D. K. Odell, and J. E. Reynolds III. "Reproductive Biology of South American Manatees." In Reproductive Biology of South American Vertebrates, edited by William C. Hamlett. New York: Springer-Verlag, 1992.

Marsh, H. "Dugong (Dugong dugon)." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Odell, D. K. "Sirenian Life History." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

O'Shea, T. J., B. B. Ackerman, and H. F. Percival. Population Biology of the Florida Manatee. Washington, DC: U.S. Department of the Interior, National Biological Service, 1995.

Reynolds, J. E. III, and D. K. Odell. Manatees and Dugongs. New York: Facts On File, 1991.

Reynolds, J. E. III, and J. A. Powell. "Manatees." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Reynolds, J. E. III, and S. A. Rommel, eds. Biology of Marine Mammals. Washington, DC: Smithsonian Institution Press, 1999.

Rommel, S. A., D. A. Pabst, and W. A. McLellan. "Skull Anatomy." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Rommel, S. A., and J. E. Reynolds, III. "Skeletal Anatomy." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Rice, D. W. Marine Mammals of the World: Systematics and Distribution. Lawrence, KS: The Society For Marine Mammalogy, Special Publication Number 4, 1998.

Twiss, J. R., and R. R. Reeves, eds. Conservation and Management of Marine Mammals. Washington, DC: Smithsonian Institution Press, 1999.

Periodicals

Anderson, P. K. "Habitat, Niche, and Evolution of Sirenian Mating Systems." Journal of Mammalian Evolution 9, nos. 1-2 (2003): 55–98.

Deutsch, C. J., J. P. Reid, R. K. Bonde, D. E. Easton, H. I. Kochman, and T. J. O'Shea. "Seasonal Movements, Migratory Behavior, and Site Fidelity of West Indian Manatees Along the Atlantic Coast of the United States." Wildlife Monographs 151 (2003): 1–77.

Domning, D. P. "Bibliography and Index of the Sirenia and Desmostylia." Smithsonian Contributions in Paleobiology 80 (1996): 1–611.

O'Shea, T. J. "Manatees." Scientific American 271, no. 1 (1994): 50–56.

Organizations

Save the Manatee Club. 500 North Maitland Avenue, Maitland, FL 32751 United States. Phone: (407) 539-0990. Fax: (407) 539-0871. E-mail: [email protected] Web site: <http://www.savethemanatee.org>

Sirenian International, Inc.. 200 Stonewall Drive, Fredericksburg, VA 22401 United States. E-mail: [email protected] Web site: <http://www.sirenian.org>

Daniel K. Odell, PhD