Ear seals
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New Zealand fur seals |
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| Gray , 1825 |
The eared seals (Otariidae) are a family usually large seals to which the fur seals and sea lions are counted which form large at numerous rocky coasts of the oceans colonies. They can move around better on land compared to the dog seals .
The family was given its scientific name in 1825 by the British zoologist John Edward Gray , which is derived from the generic name Otaria ; In the past, different species of ear seals were placed in this genus, today only the maned seal ( Otaria flavescens ).
features
height and weight
Male eared seals of the New Zealand sea lions ( Phocarctos hookeri ) can grow up to 3.50 meters, while females of the Galápagos fur seals ( Arctocephalus galapagoensis ) sometimes only reach a body length of one meter. The weight varies depending on the species and gender between 25 kilograms for some female fur seals and more than 500 kilograms for males of the Steller sea lions ( Eumetopias jubatus ). A clear sexual dimorphism is noticeable : in the northern fur seals ( Callorhinus ursinus ), for example, the males are about four and a half times as heavy as the females. Differences in size and weight are a result of the animals' special modes of reproduction.
hide
The slender, spindle-shaped body is covered by a uniformly brown fur, which in the fur seals is supplemented by a thick undercoat. It contributes to thermal insulation through the air bubbles it contains in the water and is renewed at regular intervals. First the hair of the undercoat and then the protruding awn hairs are replaced; Unlike the other seal family, the dog seals, this process is largely continuous, so that never all hair is shed at once. The layer of fat under the skin is usually thinner in ear seals than in dog seals.
Skull and teeth
The skull of the ear seals is very similar to the bear skull. As there, the large wing of the wing bone ( os sphenoidale ) is traversed by a canal through which the internal carotid artery ( internal carotid artery ), which supplies the brain, runs. The mastoid process ( processus mastoideus ) of the temporal bone ( os temporale ), to which strong muscles attach, is clearly defined as in bears, but clearly separated from the ossified capsule of the middle ear ( bulla tympanica ). The joint head for the upper head joint ( articulatio atlanto-occipitalis ), the condylus occipitalis , is offset far upwards. The eye sockets ( orbita ) are only incompletely separated from one another; The frontal bone ( os frontale ) has two well-developed processes on its posterior and upper edge , which are referred to as the postorbital process and the supraorbital process. The two halves of the lower jaw in the ear seals are not firmly attached to the symphysis ( symphysis mandibulae ). In the males, the skull is somewhat differently proportioned than in the females and in many cases has a conspicuous crest .
The ear seal dentition has three upper and two lower incisors on each side ; except for the outermost upper incisor, they are all two-humped. Behind each is a well-developed canine , followed by four upper and lower conical pre-grinding teeth . While there is one molar per side in the lower jaw , the number in the upper jaw varies between one and three, depending on the type; all premolar and molar teeth are homodont , that is, they are of the same shape. The structure of the teeth can be summarized in the tooth formula 3/2 1/1 4/4 1–3 / 1.
Axial skeleton
The spine shows clear reinforcements in the area of the cervical spine and the approximately 15 thoracic vertebrae, which can be understood as an adaptation to the strong mechanical loads emanating from the front fins used for locomotion. It ends in a short tail. The breastbone ( sternum ) of the ear seals is ossified at the handle ( manubrium sterni ).
Fins
In contrast to the dog seals, ear seals have fins that are also of great use when moving on land. The muscular front fins, which have a smooth, leathery surface, are very long, especially in the fur seals, and each have five toes with rudimentary nails. The toe length decreases from the outside in. The shorter hind fins can be turned under the body and used when moving, which the dog seals cannot do. The three middle toes have well-developed toenails, while the two outer nails are mostly largely degenerated .
Breathing, circulation, genital organs, genetics
In most species, the right lung is enlarged. The key for the Heat control arteriovenous anastomoses , cross-connections between arteries and veins that allow increased blood supply to the outer layers of the body and a faster heat exchange, exist with sea lions only in the fins, which they therefore occasionally wet land with urine, the evaporative cooling for Take advantage of cooling.
In contrast to the dog seals, the testicles of the males are located in an external scrotum . The karyotype of the taxon comprises 2n = 36 chromosomes .
Sense organs
As the German name of the family suggests, all animals have externally visible ears. They are usually about five centimeters long, of a cartilaginous consistency and taper to a point on the outside. In contrast to the dog seals, there are basically no whiskers ( vibrissae ) above the eye socket .
distribution and habitat
Ear seals are common in polar, temperate, and subtropical seas; however, they achieve their greatest abundance of individuals in the seas of the Arctic and Antarctic . They can be found in arctic waters on the north coast of Siberia and North America, in the Pacific on the west coast of North and South America from Alaska to Mexico and from northern Peru to Cape Horn as well as on the east coast of north Asia from Japan to northeast Russia , on the south island of New Zealand and on the Galápagos -Islands . In the Atlantic , they occur on the South American east coast from Tierra del Fuego to southern Brazil and on the south and southwest coast of South Africa and Namibia . Important colonies in the Indian Ocean are on the south coast of Australia . There are also numerous isolated islands in the waters around Antarctica .
Like all seals, ear seals spend a large part of their life in sea waters close to the coast; they are less often found in the brackish water of the mouth systems of large rivers or even in the fresh water of these rivers themselves. Mating and young rearing take place on land, especially on rocky islands and isolated beaches, where there are no land-dwelling predators. Ice-dwelling species are, unlike the dog seals, not known; Eared seals also generally prefer slightly warmer water temperatures.
Locomotion
Eared seals only use their powerful front fins for propulsion in the water, with which they row , as it were ; the hind fins, on the other hand, are passively stretched backwards when swimming. The speeds achieved in this way are up to 27 kilometers per hour.
Unlike dog seals, ear seals can move around well on land. On uneven terrain, they are able to escape a running person; Trained animals are even able to climb ladders.
When moving, the main weight rests on the laterally extended front fins, which are bent by 90 degrees at the "wrist" so that the fin parts corresponding to the hand lie flat on the ground. The same applies to the hind fins, which are brought ashore under the body and are oriented so that the toes point forward.
Way of life
All ear seals are very social animals and often stay in groups; they have developed a large repertoire of sounds for communication. This is particularly noticeable in the mating season, when the animals gather in large colonies. This is not least due to the comparatively small number of suitable rearing sites for the young animals, so that the seals concentrate at those sites that are available. At least the females are philopatric in many cases , so they keep returning to their birth colony.
nutrition
Most ear seal species are generalists, so their food spectrum is relatively wide. The Antarctic fur seals ( Arctocephalus gazella ), which have specialized almost entirely in krill , are an exception . In the case of the other species, besides krill, the prey usually consists of small schooling fish, squid and various crustaceans . Some species of sea lions have expanded this spectrum to include birds such as penguins or the juveniles of other seals. In contrast to the dog seals, ear seals mostly dive only in shallow waters, although some species have been shown to reach depths of more than 100 meters.
Reproduction
With all ear seals, the males, called bulls, watch over a harem of females (cows) in the colonies. The bulls always arrive first at the mating sites. Here they fight for places, which often happens in bloody fights that sometimes lead to the death of individual animals. Weaker bulls are pushed to unattractive places on the edge of the colony or inland, while the strongest males get the best places on the bank. When the females arrive, the territories are determined, the strongest bulls can now watch over up to 80 cows. Nevertheless, they have to actively defend their claimed territory and use threatening gestures to prevent neighbors from expanding their territory; in occasional fights, young animals are regularly trampled to death. Since leaving their territory temporarily would be tantamount to giving up completely, the males have to forego food altogether for up to ten weeks and live on their fat reserves during this time. This factor is believed to have contributed significantly to the significant differences in weight and height between the sexes. Because of the extreme demands of conquering and defending a territory, most bulls are only able to maintain their status for two or three years and are then ousted by younger animals.
Initially, however, the cows give birth to the young they conceived the previous year two to three days after their arrival. Their gestation period is usually about 11 to 12 months, but can last up to 18 months in Australian sea lions ( Neophoca cinerea ); almost always only one young is born. When it is born, it has a particularly thick coat that protects against cooling and is known as a lanugo . It is only after two to three months that it is replaced by the adult fur. Newborns are immediately able to swim and can move around bumpy on land within half an hour.
About a week later, the cows mate with the bull in their territory, whose aggression is now at its peak. Cows that want to leave a territory are often prevented from doing so by force. Only after mating can they leave the colony and look for food. They return to shore at regular intervals to look after their offspring.
The baby is now suckled by its mother over a period of four to six months. Communication between her and her child takes place primarily through sounds: every young animal has its own characteristic call with which it responds to utterances by the mother and which the mother can hear from the noise of a larger group of animals. The identity of the offspring is confirmed based on the smell.
In contrast to the dog seals, the mother-child relationship is maintained over a longer period of time. Some young animals even receive irregular milk after their mother has given birth to the next cub. It is even known from Steller's sea lions ( Eumetopias jubatus ) that cows can suckle three young animals from three consecutive years at the same time. In these cases the youngest animal is always in the worst starting position and often dies due to a lack of food.
Eared seals can reach an age of more than 20 years.
Danger
While sea lions and fur seals have arguably been hunted throughout human history, this has only threatened entire populations in the last few centuries. The persecution of the South American fur seal began in the 16th century, but the systematic elimination of entire colonies mostly extended to the following centuries. From 1786 to 1867, an estimated 2.5 million northern fur seals were killed on the Pribilof Islands in the Bering Sea , while the Antarctic fur seals were almost extinct by the end of the 19th century. Two species, the Guadalupe fur seal and the Juan Fernández fur seal , were even considered extinct for a long time until they were rediscovered in 1954 and 1965, respectively. Only protective measures introduced towards the beginning of the 20th century improved the situation. At the beginning of the 21st century, the threat comes less from the still permitted hunts, which are still permitted to a limited extent in the Aleutian Islands , for example . Rather, the animals are most at risk from marine pollution and fishing.
Trace elements such as copper and selenium or organic compounds such as butyl tin , polychlorinated biphenyls (PCB) and dichlorodiphenyltrichloroethane (DDT) accumulate in the fatty layers of the animals in particular . While the exposure in males is age-dependent, it decreases in lactating females, which suggests that the corresponding substances in breast milk are passed on to the young animals. Since numerous species with widely dispersed habitats off Japan, California, Alaska or Siberia are affected, the problem is not local.
The industrially organized fishing industry has depleted numerous fish stocks at such a rapid rate that it is difficult for seals to find enough food. For populations of Steller's sea lions in Alaska, the reason for the demonstrable decline in the number of individuals is most likely due to the fact that many young animals simply starve to death. A second influence is fishing near seal colonies, which often leads to the animals becoming entangled in the nets and dying as bycatch . However, at least this problem is increasingly being reduced by appropriate safety gears and practices.
The current endangerment situation can be found in the red list of the International Union for Conservation of Nature (IUCN), the world nature conservation organization :
- three species, the Australian sea lion ( Neophoca cinerea ), the Galápagos fur seal ( Arctocephalos galapagoensis ) and the Galápagos sea lion ( Zalophus wollebaeki ) are therefore endangered ( endangered );
- two species are listed as endangered ( Vulnerable ), these are the northern fur seal ( Callorhinus ursinus ) and the New Zealand sea lion ( Phocarctos hookeri );
- two other species are considered to be Near Threatened , these are the Juan Fernández fur seal ( Arctocephalos phillippi ) and the Steller sea lion ( Eumetopias jubatus ).
In addition, there are eight species in these Red Lists that are currently assessed as Least Concern .
The Japanese sea lion ( Zalophus japonicus ) is considered to be extinct ( Extinct ).
Tribal history
The sister group of the ear seals are probably the walruses (Odobenidae), with which they are united in the taxon Otarioidea.
Many of the anatomical-physiological features mentioned, such as the retention of strong front fins, once led to the assumption that ear seals were more primitive than dog seals; However, this can be considered obsolete: Apart from the fact that modern taxonomy completely dispenses with the division into primitive and modern organisms, the oldest fossils of both groups are roughly the same age.
Fossil forms
The fossil records of the ear seals go back to the Miocene . Two genera are known from this age, which are clearly recognizable as ear seals based on their fin anatomy, but whose relationship to taxa living today is unclear. Pithanotaria from the North American Pacific coast is the oldest known ear seal. It lived in the Middle Miocene, about 12 to 13 million years ago, and was barely one and a half meters long, about the size of a Galápagos fur seal. Thalassoleon is then known from the late Miocene , which was also native to the Pacific coast of North America; this genus was already significantly larger and already shows the typical gender dimorphism of the group. Both species probably took advantage of the large food supply in the extensive coastal kelp forests .
Members of the genus Callorhinus and Arctocephalus as well as the genus Zalophus appeared in the lower Pliocene . All other genera living today are fossilized only since the Pleistocene and evolved from Arctocephalus- like predecessors.
Although the history of the distribution of the ear seals will probably never be elucidated in detail, the rough features can be identified with some probability today: According to this, the origin of the ear seals, like the seals as a whole, lies on the American North Pacific coast. From there, their range expanded to the north and west, until they were finally found on both sides of the North Pacific. Only when the land connection between North and South America was established a few million years later did the animals spread south along the west coast of South America.
From there they made their way around Cape Horn into the South Atlantic, following the east coast of America northward until tropical waters apparently prevented their further expansion. Following the Antarctic Circumpolar Current, other populations probably reached South Africa and isolated islands in the southern Indian Ocean. This first radiation was probably followed by a second from sea lion-like ear seals, which in turn took its starting point in the North Pacific. After a short time, some populations crossed the equator and, like their fur seal-like predecessors, spread around the southern tip of America and into the Atlantic. New Zealand and Australia were probably settled from South America.
Evolutionary adjustments
The polygynous mode of reproduction, in which a male mates with several females, probably developed due to the amphibious way of life: Because the females are dependent on giving birth to their offspring on land, they gather because of the often limited space and the need to staying close to the shore, in a confined space. There they can then be monopolized by the most aggressive and largest males. For this reason and because of the necessary food reserves, a large "fighting weight" was central to the evolutionary success of a male. The gender-specific differences can therefore very likely be explained by sexual selection .
Systematics
There is no great doubt that ear seals are a natural family group, i.e. include all descendants of their common ancestor. The family is commonly divided into seven genera with 15 recent species. Informally, the first two are referred to as fur seals and the other five as sea lions.
- Genus Northern Fur Seals ( Callorhinus )
- Northern fur seal ( Callorhinus ursinus )
- Genus southern fur seals ( Arctocephalus )
- Guadalupe fur seal ( Arctocephalus townsendi )
- Juan Fernández Fur Seal ( Arctocephalus philippii )
- Galápagos fur seal ( Arctocephalus galapagoensis )
- South American fur seal ( Arctocephalus australis )
- Sub-Antarctic fur seal ( Arctocephalus tropicalis )
- Antarctic fur seal ( Arctocephalus gazella )
- New Zealand fur seal ( Arctocephalus forsteri )
- South African fur seal ( Arctocephalus pusillus )
- Genus Eumetopias
- Steller sea lion ( Eumetopias jubatus )
- Genus Otaria
- Maned Seal ( Otaria flavescens )
- Genus Zalophus
- California sea lion ( Zalophus californianus )
- Galápagos sea lion ( Zalophus wollebaeki )
- Japanese sea lion ( Zalophus japonicus ) †; this species is considered extinct.
- Genus Neophoca
- Australian sea lion ( Neophoca cinerea )
- Genus Phocarctos
- New Zealand sea lion ( Phocarctos hookeri )
A taxonomic division into fur seals (Arctocephalinae) and sea lions (Otarriinae) is considered out of date, as it is based on only one characteristic, the hairiness, which is very pronounced in fur seals and rather sparse in sea lions. In 2009 a comprehensive molecular genetic analysis of all ear seals was carried out. It was confirmed that the northern fur seal is a basal species that separated from the main evolutionary line of the eared seals about 11 million years ago. Most of the remaining species diversified from one another 7 to 4 million ago.
The following cladogram shows the results of the study:
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For the last six species, it was proposed to introduce “Arctophoca” as a new generic name.
Individual evidence
- ↑ Eumetopias jubatus , Neophoca cinerea , Arctocephalus galapagoensis , Zalophus wollebaeki , Callorhinus ursinus , Phocarctos hookeri , Arctocephalus philippii and Arctocephalus townsendi in the Red List of Threatened Species of the IUCN . Retrieved January 21, 2010.
- ↑ Alaska Fisheries Science Center / NOAA) Tom Gelatt (National Marine Mammal Laboratory, Katie Sweeney: IUCN Red List of Threatened Species: Steller Sea Lion. February 4, 2016, accessed April 19, 2020 .
- ↑ Zalophus japonicus in the IUCN Red List of Threatened Species . Retrieved January 21, 2010.
- ↑ a b Takahiro Yonezawa, Naoki Kohno & Masami Hasegawa: The monophyletic origin of sea lions and fur seals (Carnivora; Otariidae) in the Southern Hemisphere. Genes. 441 (1-2): 89-99. doi: 10.1016 / j.gene.2009.01.022. PMID 19254754 Researchgate.net
- ↑ Annalisa Berta, Morgan Churchill: Pinniped Taxonomy: evidence for species and subspecies. Mammal Review. 42 (3): 207-234. September 2011, doi: 10.1111 / j.1365-2907.2011.00193.x
literature
- Ronald M. Nowak: Walker's Mammals of the World. 6th edition. Johns Hopkins University Press, Baltimore 1999, ISBN 0-8018-5789-9
- Malcolm C. McKenna, Susan K. Bell: Classification of Mammals. Above the species level. Columbia University Press, New York 2000, ISBN 0-231-11013-8
