All mammals that have adapted to life in the sea are referred to as marine mammals . This includes the whales (including dolphins ), seals , manatees and sea otters . These groups are not closely related; their respective ancestors were land mammals. While the seals and sea otters still partly live on land and can move around there, whales and manatees can no longer do so.
The adaptations to the marine way of life include several convergent characterizations. In all of these groups, for example, the front extremities are transformed into paddle-like fins, while the hind legs are either lost completely (whales, manatees) or are combined in the water to form a uniform tail fin (seals). In addition, there is almost complete hair loss, the formation of extensive subcutaneous fatty tissue and changes in the respiratory system in order to be able to carry out long dives.
Depending on the definition, the polar bear can also be counted among the marine mammals as the largest living land predator because it is highly adapted to the marine habitat.
Anatomical and physiological features
The sea represents a habitat that differs significantly from the land and places special demands on the organisms living in it. Correspondingly, organisms that have moved from land to sea in the course of evolution have various adaptations in their body structure and physiology , without which this way of life would not be possible.
With a few exceptions such as the sea otter , mammals that have adapted to marine life tend to have an enlarged body compared to their nearest recent and fossil relatives. This is especially true for the whales, where the blue whale is the heaviest known animal in the history of the earth with a length of over 30 meters and a body mass of up to 200 tons. But the manatees are also significantly larger compared to the snakes and among the seals the elephant seals and walruses reach significantly larger body sizes and weights than other predators on the land such as the bears . This increase in size is probably mainly due to the loss of restrictions on land caused by buoyancy in the water and thus the reduction in gravity . There is also the problem of thermoregulation and metabolic rates, which are reduced by the fat layer and, above all, by the larger body: Smaller animals of the same temperature require significantly more energy for their heat balance and metabolism in relation to their body size than larger ones.
Most land animals can swim more or less well, but in animals living permanently in the water, this basic skill is often supplemented by changes in limbs and other parts of the body. Many of these animal species form webbed feet between the toes and fingers , which allow a greater displacement of the water. Webbed mammals are found in the Australian platypus , for example , various rodents such as the beaver , insectivores such as the desmanes or even predators such as the otters . Also for locomotion in the water and at the same time for steering, some of these animals have a modified oar tail, such as the beaver; in addition, all aquatic mammals are streamlined .
A further development of the swimming extremities is the transformation of the entire hand and legs into fins . In these, the fingers and toes are not only connected by a thin skin, but are completely integrated into a muscular paddle and no longer recognizable as individual rays. This form of transformation has only formed as a convergence within the mammals among the three groups of marine mammals : the seals, the manatees and the whales. All these groups have moved from life on land to a more or less complete life in seawater, only a few species within the whales and seals specialized in freshwater life in a further step.
Among the marine mammals, the remodeling of the extremities of the seals is the least advanced, especially in order to enable locomotion on land. The front legs here represent a fin in which the five finger rays are completely present. The first finger beam is clearly elongated and strongly developed, the length then decreases evenly. The rear extremity also contains all five rays, here the two outer ones are clearly elongated. The webbing that extends beyond the finger rays is supported by cartilaginous tissue. The locomotion and thus the use of the fins is different in the two large groups of seals: While the ear seals use the front fins as drive organs and use the hind legs only for control and support, the dog seals use the folded hind limbs for propulsion and support it with their front legs .
In the manatees, the front legs form completely closed paddles, in which no rays can be seen from the outside. These paddles can be moved in both the elbow and the wrist. The hind legs are gone, and the tail has been replaced with a large oar, the fluke . The latter is the main driving force behind the animals' very slow swimming style; the front fins form oars for steering in the water.
The whales are most clearly adapted to marine life. The hind limbs are also completely absent in them, and the front legs are transformed into large, paddle-shaped fins ( flippers ). In contrast to the manatees, the joints are no longer present except for the shoulder joint. While the outer fingers are greatly shortened, the middle fingers II and III are very long and have a significantly increased number of finger bones, namely up to fourteen ( hyperphalangia ). The tail of all whales forms a horizontal fluke, which is designed as a two-part caudal fin and provides the propulsion of the animals. For stability in the water, whales have an additional fin, the dorsal fin . This is an unpaired structure in the center of the back that is only supported by connective tissue and cartilage and is immobile.
Mammals are lung breathers , which means that they need the oxygen in the air to breathe . No mammal is able to breathe in water because there are no gills . For this reason, all mammals living in the ocean must regularly come to the surface to breathe. However, especially the whales are able to dive for a very long time without needing new oxygen. This is possible due to some physiological processes and changes in the lungs and other organs.
The whale's nostrils are on the top of the head, so there is no need to fully emerge to breathe. With each breath, the animals completely exchange between 80 and 90 percent of the contents of their lungs (in land mammals this proportion is only around 10 to 15 percent). The lungs have a double network of capillaries , which is otherwise only found in manatees and platypus, and allows twice the amount of oxygen to be extracted from the air we breathe. Compared to land mammals, only a very small part of 9 percent of the total oxygen is stored in the lungs, the major part of around 82 percent is bound in the myoglobin of the muscles , the rest circulates in the blood . In comparison, the lungs of land mammals contain around 34 percent of the oxygen and the muscles only 25 percent. So that the oxygen can also be transported effectively, the whales have two to three times the relative amount of blood per kilogram of body weight as land mammals. When diving, the oxygen demand is also reduced, the blood circulation and the heart's activity slow down. In addition, some organs and body regions are decoupled from the bloodstream. The special adaptations allow the whales to stay under water for a very long time. Some furrow whales dive for up to 40 minutes, sperm whales for up to 90 minutes and duck whales for up to two hours. Sperm whales can reach depths of over 3,000 meters.
In seals, it is primarily modifications of the circulatory system that allow longer dives. In spite of the reduced heartbeat when diving, the blood flow is mainly ensured by flexible vein walls and cavities. The nose and larynx are closed during the dive and the blood flow to the body is also reduced. A significantly higher amount of blood than in terrestrial mammals and an increased proportion of the blood pigment hemoglobin also allow significantly higher oxygen uptake from the breath. The Weddell seal and the southern elephant seal in particular can dive for up to an hour and reach depths of up to 66 meters.
Manatees breathe about every two to five minutes and reach maximum dive times of about 20 minutes. Like the whales, they can very effectively take in oxygen via a double capillary network and their metabolism is very slow. The heart rate is also significantly lower than in animals of comparable size and can be lowered even further.
Especially for mammals as homoiothermic , ie "warm-blooded" animals, the problem arises in seawater that effective thermal insulation must exist. With the exception of some seals, marine mammals do not have the fur that takes on this function in terrestrial mammals, as it is more of a hindrance in the water. Instead, all marine mammals have developed a very thick layer of fat that lies beneath the skin. This subcutaneous fatty tissue isolates the animals from the cold sea water and ensures that the body can maintain a constant temperature. In some whales, this layer, which they call a bubbler , can be up to 70 centimeters thick.
Especially in the fur-free seals, the body surface temperature is kept in the range of the water temperature by a reduced blood flow in order to lose as little heat as possible to the water. The fur of the fur seal, on the other hand, has air pockets that have an insulating effect on the outside.
The situation is similar with the sea otter, which is the only marine mammal that does not have an insulating layer of fat. Here, the thermal insulation is provided by an extremely dense fur with air pockets. Sea otters have up to 100,000 hairs on one square centimeter of fur.
Due to the high salt content in sea water, marine organisms that have a significantly lower salt concentration in their body fluids must be able to excrete excess salt. Too much water must not be given off. For mammals, this means that their kidneys must be able to produce highly concentrated urine . For this reason, the seal's kidneys are lobed and have around 150 to 300 renal lobes ( renculi ). The kidneys of the whales even have a few thousand renculi and are elongated organs on the spine .
From an inverse perspective, marine mammals must be able to extract (fresh) water from seawater. Because although (fresh) water is constantly lost through the skin via osmosis and seawater swallowed with food has a high salt concentration, these animals manage to maintain a significantly lower salt content in the body. It is characteristic of biological life to maintain the order of the body against the "self-running" processes of inanimate nature, which gradually increase entropy - disorder - in larger systems.
Fish in the sea have the same ability, fish in fresh water do the opposite.
The food options in the sea can be built on different bases. As with land mammals, marine mammals also have herbivores and carnivores. The Manatees are herbivores whose diet of sea grass and filamentous algae is that they graze. For this purpose they have large molars for chopping up food.
Most seals and toothed whales hunt for fish and other marine animals that they can catch by mouth. For this purpose, the representatives of both groups have uniform, pointed teeth, which are necessary to hold the prey in place and which form a fish trap in some toothed whale species. Other toothed whales such as beaked whales and various dolphins specialize in hunting cephalopods , especially squids . They have a reduced number of teeth with large teeth that they can use to hold prey in place.
Most modified are the whales that feed on krill crabs . These crabs live in huge schools and are the basis of life for the baleen whales . In the baleen whales, to which the largest living animals in the world belong, a basket of beards was formed to replace the teeth to catch the crabs . When they prey, the whales take huge amounts of water and crabs into their mouths and filter the water masses as they squeeze out through the beards, the crabs get caught in the bark sieve and can be swallowed.
The four groups of marine mammals come from very different fields of relationship within the mammals. So the seals developed within the predators as a sister group of the bears . The closest relatives of the manatees are found among the elephants and the snakes . The relationship of the whales has not yet been finally clarified. According to current knowledge, they are descended from even-toed ungulates . Your closest relatives living today are likely the hippos . Sea otters are a genus of otters that belongs to the marten family . Nevertheless, marine mammals have characteristic similarities that distinguish them from land mammals. Examples are the streamlined shape, paddle-shaped upper limbs, closable nostrils and a layer of fat that protects against cooling down. This convergent development is a result of adapting to life in the open sea.
Similar adaptations to marine life can also be observed in other non-mammalian land vertebrates . The penguins within the birds as well as sea turtles and the extinct ichthyosaurs , sea crocodiles and plesiosaurs have also developed a streamlined shape and fins. These animals are also dependent on oxygen in the air and had to be adapted accordingly to allow longer diving times. The thermoregulation, on the other hand, only plays a role in the penguins, which solve this with a corresponding lower plumage and a layer of fat. As cold-blooded animals, reptiles adapt to their surroundings and live mainly in warmer marine areas.
Reasons for the habitat change
Competition with other land dwellers and / or the use of new resources can be seen as possible reasons for the change of habitat back from land to sea . For example, there was possibly a food supply in the sea for the early forms of today's marine vertebrates that they could use better than fish (e.g. krill ). By gradually adapting and developing key features such as webbed feet, changed body shape and other features, these animals were able to advance further into the sea than their competitors in the course of evolution . In this way, special transition species were formed that represent the ancestors of today's marine life.
- William Gearty, Craig R. McClain, Jonathan L. Payne: Energetic tradeoffs control the size distribution of aquatic mammals. Proceedings of the National Academy of Sciences (PNAS), March 26, 2018; online 201712629 doi : 10.1073 / pnas.1712629115 .
- Wilfried Westheide, Reinhard Rieger (Hrsg.): Special zoology. Part 2: Vertebrate and skull animals , Spektrum Akademischer Verlag, Munich 2004.