Manatees
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Caribbean manatee ( Trichechus manatus ) |
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| Sirenia | ||||||||||||
| Illiger , 1811 | ||||||||||||
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The manatees (Sirenia) are an order of herbivorous mammals with four species still alive today. They are counted to the superior order of the Afrotheria ; Among the animals still alive today, the elephants are their closest relatives. Besides whales and seals , manatees are the third major taxon of marine mammals ( marine mammals ). Unlike seals, they do not have adequate limbs to move about on land. In contrast to the whales, manatees are always near the coast or even in fresh water and often in very shallow water.
features
External anatomy
Manatees are massive animals with a cylindrical body. The recent species reach body lengths of 2.50 to four meters, Steller's manatee ( Hydrodamalis gigas ), which was exterminated in the 18th century within only 27 years of its discovery, even grew to 8 meters long. The weight of the recent species varies between 250 and a maximum of 1500 kilograms. The front legs of the animals are transformed into fins , the rear legs are completely regressed. A back fin as with most whales are not, the tail is transformed into a horizontal fin. A remodeled skin muscle , the dorsal panniculus carnosus muscle , forms the main stroke muscle of the caudal fin. The shape of the caudal fin is the most obvious external distinguishing feature between the two recent families. While fork-tailed manatees have a crescent-shaped fluke , it is circular or spade-shaped in the round-tailed manatees.
The muzzle is clearly set off from the head and blunt. It is surrounded by hard whiskers . The nostrils are on the top of the muzzle. Compared to the torso , the head is relatively large, but with a weight of only 250 to 350 grams in relation to body size, the brain is one of the smallest that can be found among mammals.
The skin is very thick and wrinkled, with the epidermis of manatees still alive today living in tropical waters being very thin. Steller's manatee, on the other hand, had a very dense epidermis with a thickness of up to 7.5 centimeters as an adaptation to the polar waters, to which it owes the name "bark animal". The fur of the manatee is limited to a few bristles in the area of the mouth opening and individual hairs on the trunk, embryos , on the other hand, still have a full coat, and even newborns have significantly more hair than the adult animals.
Construction of the skeleton
As with the whales , the manatees also suffered from severe pachyostosis , i.e. an increase in the thickness of the bones of the skeleton, as well as a compression of the bone substance by reducing the Haversian canals and the medullary cavity . The skeleton, and thus the entire animal, became heavier and the static buoyancy in the water was reduced. At the same time, the bones are less flexible and break more easily. The skull has a very elongated snout region ( rostrum ) formed by the premaxillary , which in the Dugong is also bent downwards in front. The zygomatic arches are very wide and are relatively high on the skull. The very massive masticatory muscles with the large masseter muscle insert into these . The nostrils are shifted very far back on the dorsal side of the skull. The back part of the skull, which is made up of the brain and temples , is comparatively small.
The dentition is different in the individual taxa. The incisors of the round-tailed sea cows have receded; in the dugongs the first incisor in the males forms a short tusk , in the females it remains in the jaw . The canines are completely absent in all recent species. The gear change takes place as in the elephant horizontally ( horizontal change of teeth ), it has evolved independently but in both groups. The molars ( premolars and molars ) grow out of the jaw one after the other and are worn on the front edge. The teeth of the fossil members of the core group are still completely preserved, and so only a normal change of teeth was possible. The front part of the palate is lined with horn plates that presumably help with eating. The short tongue is also horny.
The number of vertebrae varies depending on the species. The manatees are the only group of mammals besides the Hoffmann two-toed sloth ( Choloepus hoffmanni ) to have only six cervical vertebrae , the dugong and the extinct Steller's manatee have seven cervical vertebrae. This is followed by 17 ( Trichechus , Hydrodamalis ) or 19 ( Dugong ) thoracic vertebrae and two ( Trichechus ) or four to five ( Dugong ) lumbar vertebrae . The rudiments of the basin are not or only by a band with the spine connected, corresponding to only a sacral vertebrae available. The tail consists of 22 to 24 ( Trichechus ) or 28 to 29 ( Dugong ) caudal vertebrae .
The pool is down to a rudiment completely reduced, it is a clip of the seat leg , which is embedded in muscle tissue. The rear extremities are completely absent. The front extremities are transformed into paddle-like fins. In the shoulder , the collarbone (clavicle) is reduced, and the shoulder blade (scapula) can be triangular ( trichechus ) or sickle-shaped ( dugong ). The hand has five bony finger rays that are embedded in muscles, and all joints, unlike those of the whale's fins, are flexible.
Internal anatomy
In the manatees, as in other mammals, the lungs occupy the entire space above the diaphragm . However, this is very much stretched in the horizontal plane and extends to just before the pelvic rudiments, so that the lungs are in the back area. Due to this position, the buoyancy generated by the air-filled lungs is distributed over the horizontal plane of the animals, which enables them to lie stable and swim in the water. The heart lies near the head between the lungs and, like that of the elephant, has a deep incision between the two ventricles at the apex of the heart. This makes it two-lobed - a feature that is only found in them and the proboscis and is the reason for their relationship ( autapomorphy ).
The gastro-intestinal tract consists of a single-chamber stomach followed duodenum (the duodenum), which has a large bulge, the ampulla ulcer, as well as an adjoining intestine , approximately represents 20 times the body length of the animal. The stomach and the ampoule are mainly used to store the ingested and very well chewed food, the actual digestion takes place in the subsequent intestine. Food takes an average of five days to be digested and eliminated.
The female ovaries are located near the abdominal wall. The uterus has two horns (uterus bicornis), which means that the two halves are separated by a septum. The testicles of the males are also in the abdomen, the penis lies under the abdominal skin in its own penile fold. The muscles of the penis attach to the ischial rudiment of the pelvis.
distribution and habitat
The distribution areas of the manatees living today do not overlap and are sometimes very far apart. The only species of fork tailed manatee (Dugongidae) still alive today , the dugong ( Dugong dugon ), is found exclusively on the coasts of the Indian Ocean , including the Red Sea , and the south-western Pacific Ocean . The types of manatees (Trichechidae) live one in the Gulf of Mexico off the coasts of Florida and the southeastern United States, the coasts of Central America and the Caribbean islands and the northern coasts of South America ( West Indian manatee , Trichechus manatus ), next to the territory of the Amazon in South America ( Amazon Manatee , Trichechus inunguis ) and finally on the coasts of West Africa between Senegal and northern Angola and in the river systems there such as the Niger and other West African rivers ( African Manatee , Trichechus senegalensis ).
While all species still alive today live in tropical waters, the habitat of the extinct Steller manatee was in the polar waters of the Bering Sea .
Way of life
Very little is known about both the way of life and the social behavior of the manatees. They usually live individually or in small family groups, sometimes larger groups with several hundred animals are formed. There are hardly any social ties with the exception of the mother-child relationship, which lasts about two years. A day-night rhythm is not pronounced, these animals can be active both day and night. Communication is primarily acoustical and tactile . So-called mother-child duets occur between mother and child, which take place in a frequency range of 600 to 6,000 Hertz .
Manatees always move slowly, drifting and swimming. Adult manatees come to the surface every one to five minutes to breathe. Longer dives can take up to 20 minutes. Besides humans, manatees have very few natural enemies. In the marine areas, these include mainly larger sharks and the killer whale , in the rivers mainly crocodiles and in South America also the jaguar .
nutrition
Manatees have a predominantly vegetable diet; their diet consists of seaweed, algae and other aquatic plants as well as leaves of mangrove trees that they can reach . Manatees need around 90 kilograms of plant-based food in a day, they are busy eating an average of six to eight hours a day. While the manatees mainly feed on the surface of the water and the freshwater species mainly graze on water hyacinths and grass islands from above, dugongs only eat on the seabed. Steller's manatee mainly fed on seaweed .
It is unclear to what extent they also eat animal food. Probably unintentionally they also consume small invertebrates with the plant-based food , which supply the animals with protein . There are reports that captive animals have enthusiastically eaten fish. In Jamaica , Caribbean manatees have been observed pulling fish from nets and eating them.
Reproduction and development
The manatees have neither a temporary mating season nor a specific mating behavior. The female ovulates several times a year and mates with several males in the water, with no rivalry fights. The zygote is embedded centrally in the uterus . The embryo or fetus is supplied via a belt placenta (placenta zonaria). The young animal is born after around 12 to 14 months of gestation in the water and actively swims directly to the surface of the water. At this point it weighs between 10 and 30 kilograms. During the following 18 months, the young animal is suckled by the mother, after which it remains in the direct vicinity of the mother for a few more months. Manatees reach sexual maturity at six to ten years of age, manatees in total reach an age of around 40 years and dugongs one of 60 years.
Tribal history
The first known manatee-like fossils date from the early Eocene of Hungary and are around 50 million years old. They were four-legged herbivores that could still move on land, but probably already lived mainly in shallow water. Manatees were very successful in the next millions of years, as demonstrated by countless fossil finds from the fringes of the Tethys . Fossils were found mainly on the coasts of today's North America and Europe as well as North and East Africa, India , Pakistan and Java . The hind legs of the animals had soon receded, but a horizontal caudal fin developed.
During the Eocene, the manatee families of the Prorastomidae (†), the Protosirenidae (†) and the fork-tailed manatees were formed. The manatees also emerged at the end of the Eocene or only in the Miocene (about 23 million years ago) , depending on the doctrine . There is no trace of the first two families mentioned in the Oligocene (23 to 34 million years ago), so that since then there have only been the more recent families of the fork and manatee. In the Miocene and Pliocene (until about 2 million years ago) manatees were much more common and diverse than they are today. Presumably, the climate change of the Pleistocene with its ice ages was responsible for the fact that they are only a residual group with a few species today.
Systematics
Manatees have land ancestors in common with the trunk animals and accordingly form the sister group of these animals. The taxon that can be formed from these two groups is called Tethytheria , since this group evolved on the edge of the Tethys. The monophyly of Tethytheria is justified by a number of features, including the absence of sweat glands, which suggests a semiaquatic ancestor of the earliest elephants and manatees.
As next of kin of Tethyteria are Hyrax discussed, this discussion is not yet complete. Together with these and some extinct taxa, they form the taxon of the Paenungulata , which, based on molecular genetic data, are classified as part of the Afrotheria . The following relationships result within mammals:
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Two families can be distinguished within the manatee :
- the fork-tailed manatees (Dugongidae) now comprise only one living species, the dugong ( Dugong dugon ). Until about 250 years ago there was another species, which is now extinct, Steller's manatee ( Hydrodamalis gigas ).
- the manatees (Trichechidae), also called Manatee comprise three species in a genus, the Caribbean Manati ( Trichechus manatus ), the Amazon Manati ( Trichechus inunguis ) and the African Manati ( Trichechus senegalensis ). There are indications of another species in a tributary of the Brazilian Rio Aripuanã , a “pygmy sea cow” with a body length of about 1.30 meters, but scientific confirmation has not yet been obtained.
Hazard and protection
All types of manatees were hunted for meat by the inhabitants of the coasts of their range. This is especially documented for the Indians of the North and Central American coasts. The meat was used as food and the skin and other parts of the body for other purposes. William Dampier , who became known as a British privateer and traveler, described the Caribbean manatee from the Gulf of Mexico and the rivers of Panama in his 1681 travel reports . There he also described the hunt for the animals by the Miskito and the subsequent use of the meat as food and the coarse skin as oars and horse whips. However, no excessive hunting is known; the hunt was usually carried out for current needs. In contrast, Steller's manatees have been pursued by seal hunters from their discovery and killed in large numbers. The last animals disappeared in 1768, only 27 years after their discovery by Georg Wilhelm Steller .
Today all four living species are listed as endangered by the IUCN . However, the greatest danger today is no longer from hunting, but, especially for the Caribbean manatee, from sport boats, which can cause serious injuries to the animals with their screws when driven over. For this reason, protected areas were created especially off the US coasts in the Gulf of Mexico and marked with clearly visible signs; Motorboat traffic is not allowed in these areas.
Another threat is the intrusion of humans into their living space; Due to their metabolism, manatees need an immense amount of aquatic plants to cover their energy needs and, associated with this, a corresponding water quality, which decreases more and more as their refuge areas are opened up. The rivers in South America and Africa in particular are becoming more and more clouded and contaminated with environmental toxins, plant-rich retreats are becoming rare.
Manatees in Mythology, Art, and Literature
Time and again the manatees are associated with the sirens or mermaids in Greek mythology . However, since no manatee species live in the Mediterranean Sea and thus in the vicinity of the Greeks, this connection is excluded. Rather, even in the times of the Babylonians, who had access to the distribution area of the dugongs in the Red Sea , there were descriptions of fish people, including the god Oannes and the goddesses Atargatis and Derketo , who were also found among the Greeks in the form of the Nereids and Tritons .
The first connection between the manatees and the mythological sea creatures was apparently created by Christopher Columbus , who came across Caribbean manatees in the Gulf of Mexico and described them as mermaids. It is assumed that this association was primarily due to the teats being almost in the chest and the face that appeared human from a distance due to the frontal eyes. In fact, from a distance manatees can be mistaken for bathing people, but the siren singing does not match the manatees. In 1493 Columbus noted in his logbook that the sirens of the Caribbean were less beautiful than those of Horace .
Jules Verne took up the description of the manatee as a mermaid in his work 20,000 Leagues Under the Sea , in which the protagonists encounter a giant female dugong and identify him as a mermaid. In the novel, the dugong is hunted and harpooned, then drags the boat (a dinghy of the Nautilus ) after it and then attacks and destroys the boat. The Dugong is also described in The Mysterious Island as an aggressive and dangerous animal that attacks a dog, but then becomes the victim of a larger sea creature itself.
The well-known cryptozoologist Bernard Heuvelmans tried to explain the representations of the manatees as mermaids and wrote in 1990:
"Since the oars tail manatee has a pair of chest-sized teats - like her cousin, the elephant, and humans too - and her body tapers to a fish-like tail, she has always been regarded as the fascinating mermaid on both sides of the Atlantic, despite her ( in our eyes) ugly face - and because of the same signs she was considered cannibal and was suspected of the worst crimes. "
Above all, Steller's manatee appears repeatedly in books and stories after its extinction. For example, in his story The White Seal from the Jungle Book , Rudyard Kipling describes how the main character Kotick meets a group of grazing giant manatees that lead him to a beautiful beach. Jeremias Gotthelf used the following picture in his book Uli the tenant : "Now he came up the Glungge again, like a hunted manatee drives through the reeds". In his work Kraft und Stoff, Ludwig Büchner uses the extermination of Steller's manatee as an argument to deny the purposeful action of nature.
Movie
The last paradises: the mysterious world of the manatees . Documentation, 2004, 45 min., A film by Hans Jöchler, production: Bayerisches Fernsehen .
Individual evidence
- ↑ See Stern, May 25, 2005
- ^ Search for "Sirenia" in the IUCN 2008 Red List of Threatened Species . Accessed February 2, 2009.
- ↑ Bernd Heuvelmans: The Metamorphosis of Unknown Animals into Fabulous Beasts and of Fabulous Beasts into Known Animals. in: Cryptozoology. 1990, 9, 1-12. Translation after Richard Ellis: Sea Monsters - Myths, Fables and Facts. Birkhäuser, Berlin 1997. ISBN 3-7643-5422-4
- ↑ Jeremias Gotthelf, Walter Muschg (Ed.): Works. Vol. 2. Uli the tenant. Diogenes, Zurich 1978. ISBN 3-257-20561-9
- ↑ Ludwig Büchner: Kraft und Stoff, empirical-natural-philosophical studies. In a generally understandable presentation . Theodor Thomas publishing house, Leipzig 1976
- ↑ Film synopsis with trailer and from BR ( Memento from August 7, 2004 in the Internet Archive )
literature
- Martin S. Fischer: Sirenia, manatees. In: W. Westheide, R. Rieger: Special Zoology. Part 2. Vertebrate or skull animals. Spektrum Akademischer Verlag, Munich 2004, ISBN 3-8274-0307-3 .
- Ronald M. Nowak: Walker's Mammals of the World . Johns Hopkins University Press, Baltimor 1999, ISBN 0-8018-5789-9 .
- J. Ripple, D. Perrine: Manatees and Dugongs of the world. Voyagour Press, Stillwater 1999. ISBN 0-89658-528-X .
- Ann Forsten, Phillip M. Youngman: Hydrodamalis gigas. In: Mammalian Species . The American Society of Mammalogists, New York 1982, 165 (pdf; 278 kB). ISSN 0076-3519
- Sandra L. Husar: Trichechus senegalensis. In: Mammalian Species . The American Society of Mammalogists, New York 1978, 89 (PDF; 407 kB). ISSN 0076-3519
- Sandra L. Husar: Trichechus inunguis. In: Mammalian Species . The American Society of Mammalogists, New York 1977.72 (pdf; 411 kB). ISSN 0076-3519
- Sandra L. Husar: Trichechus manatus. In: Mammalian Species . The American Society of Mammalogists, New York 1978.93 (pdf; 642 kB). ISSN 0076-3519
- Sandra L. Husar: Dugong dugon. In: Mammalian Species . The American Society of Mammalogists, New York 1978, 88 (pdf; 861 kB). ISSN 0076-3519
Web links
- Sirenian International (English)
- Fragile sea giants. Manatees have extremely brittle bones , Wissenschaft.de , March 21, 2005
- "Sleek? Well, No. Complex? Yes, Indeed. " New York Times , August 29, 2006, with audio slide show (2:37 min.)
- "The manatee is big, fat and lazy - but not stupid" , Süddeutsche Zeitung , August 29, 2006, short version of the NYT article
