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Aquatic mammal

Adapted from Wikipedia · Discoverer experience

A playful bottlenose dolphin riding the waves near a research boat.

Aquatic mammals and semiaquatic mammals are a diverse group of animals that live partly or fully in bodies of water. This group includes many different kinds of animals, such as marine mammals that live in oceans, and freshwater animals like the European otter. These animals are not a special group based on biology, but they all depend on living in or near water.

A common bottlenose dolphin (Tursiops truncatus)

Some aquatic mammals, like the Amazonian manatee and river dolphins, spend their whole lives in water. Others, like the Baikal seal, capybara, and hippopotamus, go in and out of the water. These animals have adapted in many different ways to living in water, and they eat many kinds of food, from plants to small fish.

In the past, many aquatic mammals were hunted for their fur, which was used to make warm coats and hats. This caused their numbers to drop sharply. Even today, some of these animals still face dangers such as getting caught in fishing nets or hitting boats. These challenges make it important to protect aquatic mammals and their homes.

Taxonomy and evolution

Aquatic mammals vary greatly in size and shape. They include animals that live in oceans, like whales and sea lions, as well as animals that live in freshwater, like otters and beavers. These mammals are not a single group but share a dependence on water environments.

Groups

This list covers mammals that live in freshwater, though it can include marine species. For a list of saltwater mammals, see List of marine mammal species.

Evolution

Illustration of Castorocauda lutrasimilis, a semiaquatic mammal from the Jurassic

See also: Evolution of cetaceans and Evolution of sirenians

Mesozoic

One of the first known proto-mammals similar to modern placentals was aquatic, the Jurassic mammaliaform Castorocauda. It seems to have been adapted to water much like a beaver, with teeth different in many ways from all other docodonts, presumably due to a difference in diet. Most docodonts had teeth specialized for an omnivorous diet. The teeth of Castorocauda suggest that the animal was a piscivore, feeding on fish and small invertebrates. The first two molars had cusps in a straight row, eliminating the grinding function suggesting that they were strictly for gripping and not for chewing. This feature of three cusps in a row is similar to the ancestral condition in mammal relatives (as seen in triconodonts), but is almost certainly a derived character in Castorocauda. These first molars were also recurved in a manner adapted to hold slippery prey once grasped. These teeth are very similar to the teeth seen in mesonychians, an extinct group of semiaquatic carnivorous ungulates, and resemble, to a lesser degree, the teeth of seals.

Another docodontan, the Late Jurassic Haldanodon, has been suggested to be a platypus or desman-like swimmer and burrower, being well adapted to dig and swim and occurring in a wetland environment.

The tritylodontid Kayentatherium has been suggested to be semiaquatic. Unlike Castorocauda and Haldanodon, it was an herbivore, being probably beaver or capybara-like in habits.

Another lineage of Mesozoic mammals, the eutriconodonts, have been suggested to be aquatic animals with mixed results. Astroconodon occurred abundantly in freshwater lacustrine deposits and its molars were originally interpreted as being similar to those of piscivorous mammals like cetaceans and pinnipeds; by extension some researchers considered the possibility that all eutriconodonts were aquatic piscivores. However, Zofia Kielan-Jaworowska and other researchers have latter found that the triconodont molars of eutriconodonts were more suited for a carnassial-like shearing action than the piercing and gripping function of piscivorous mammal molars, occluding instead of interlocking, and that Astroconodon's aquatic occurrences may be of little significance when most terrestrial tetrapod fossils are found in lacustrine environments anyway.

Illustration of Prorastomus, a sirenian from the Eocene

However, two other eutriconodonts, Dyskritodon and Ichthyoconodon, occur in marine deposits with virtually no dental erosion, implying that they died in situ and are thus truly aquatic mammals. Nonetheless, Ichthyoconodon may not be aquatic, but instead a gliding mammal. More recently, Yanoconodon and Liaoconodon have been interpreted as semiaquatic, bearing a long body and paddle-like limbs.

A metatherian, the stagodontid Didelphodon, has been suggested to be aquatic, due to molar and skeleton similarities to sea otters.

Cenozoic

An extinct genus, Satherium, is believed to be ancestral to South American river otters, having migrated to the New World during the Pliocene or early Pleistocene. The South American continent houses the otter genus Lontra: the giant otter, the neotropical river otter, the southern river otter, and the marine otter. The smooth-coated otter of Asia may be its closest extant relative; similar behaviour, vocalizations, and skull morphology have been noted.

The most popular theory of the origins of Hippopotamidae suggests that hippos and whales shared a common ancestor that branched off from other artiodactyls around 60 million years ago (mya). This hypothesized ancestral group likely split into two branches around 54 mya. One branch would evolve into cetaceans, possibly beginning about 52 mya, with the protowhale Pakicetus and other early whale ancestors collectively known as Archaeoceti, which eventually underwent aquatic adaptation into the completely aquatic cetaceans. The other branch became the anthracotheres, and all branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene without leaving any descendants. River dolphins are thought to have relictual distributions, that is, their ancestors originally occupied marine habitats, but were then displaced from these habitats by modern dolphin lineages. Many of the morphological similarities and adaptations to freshwater habitats arose due to convergent evolution; thus, a grouping of all river dolphins is paraphyletic. For example, Amazon river dolphins are actually more closely related to oceanic dolphins than to South Asian river dolphins.

Sirenians, along with Proboscidea (elephants), group together with the extinct †Desmostylia and likely †Embrithopoda to form the Tethytheria. Tethytheria is thought to have evolved from primitive hoofed mammals ("condylarths") along the shores of the ancient Tethys Ocean. Tethytheria, combined with Hyracoidea (hyraxes), forms a clade called Paenungulata. Paenungulata and Tethytheria (especially the latter) are among the least controversial mammalian clades, with strong support from morphological and molecular interpretations. That is, elephants, hyraxes, and manatees share a common ancestry. The ancestry of Sirenia is distinct from that of Cetacea and Pinnipedia, although they are thought to have evolved an aquatic lifestyle around the same time.

The humpback whale is a fully aquatic marine mammal.

The oldest fossil of the modern platypus dates back to about 100,000 years ago, during the Quaternary period. The extinct monotremes Teinolophos and Steropodon were once thought to be closely related to the modern platypus, but more recent studies show that platypodes are more related to the modern echidnas than to these ancient forms and that at least Teinolophos was a rather different mammal lacking several speciations seen in platypodes. However, the last common ancestor between platypodes and echidnas probably was aquatic, and echidnas thus secondarily became terrestrial. Monotrematum sudamericanum is currently the oldest aquatic monotreme known. It has been found in Argentina, indicating monotremes were present in the supercontinent of Gondwana when the continents of South America and Australia were joined via Antarctica, or that monotremes existed along the shorelines of Antarctica in the early Cenozoic.

Marine mammals

Main article: Marine mammal

Marine mammals are aquatic mammals that rely on the ocean for their existence. They include animals such as sea lions, whales, dugongs, sea otters and polar bears. Like other aquatic mammals, they do not represent a biological grouping.

Marine mammal adaptation to an aquatic lifestyle vary considerably between species. Both cetaceans and sirenians are fully aquatic and therefore are obligate ocean dwellers. Pinnipeds are semiaquatic; they spend the majority of their time in the water, but need to return to land for important activities such as mating, breeding and molting. In contrast, both otters and the polar bear are much less adapted to aquatic living. Likewise, their diet ranges considerably as well; some may eat zooplankton, others may eat small fish, and a few may eat other mammals. While the number of marine mammals is small compared to those found on land, their roles in various ecosystems are large. They, namely sea otters and polar bears, play important roles in maintaining marine ecosystems, especially through regulation of prey populations. Their role in maintaining ecosystems makes them of particular concern considering 25% of marine mammal species are currently threatened.

Marine mammals were first hunted by aboriginal peoples for food and other resources. They were also the target for commercial industry, leading to a sharp decline in all populations of exploited species, such as whales and seals. Commercial hunting lead to the extinction of Steller's sea cow and the Caribbean monk seal. After commercial hunting ended, some species, such as the gray whale and northern elephant seal, have rebounded in numbers, however the northern elephant seal has a genetic bottleneck; conversely, other species, such as the North Atlantic right whale, are critically endangered. Other than hunting, marine mammals, dolphins especially, can be killed as bycatch from fisheries, where they become entangled in fixed netting and drown or starve. Increased ocean traffic causes collisions between fast ocean vessels and large marine mammals. Habitat degradation also threatens marine mammals and their ability to find and catch food. Noise pollution, for example, may adversely affect echolocating mammals, and the ongoing effects of global warming degrades arctic environments.

Adaptations

Mammals originally lived on land, so those that spend time in water have special ways to stay safe and find food. They can't breathe underwater like fish, so they have special nose and throat structures to keep water out when they breathe. Some, like manatees, use sensitive whiskers to feel for food, while others, like toothed whales, use sound to find their way and locate prey.

Aquatic mammals move in different ways. Cetaceans, such as dolphins, have smooth bodies and move quickly with their tails. Sirenians, like manatees, can push themselves with their tails or walk on the ocean floor. Earless seals swim by moving their hind flippers side to side and use their front flippers to steer. On land, they move in a bouncing way but can quickly return to water where they feel safer.

The pygmy hippopotamus has four weight-bearing limbs, and can walk on land like a fully terrestrial mammal.

Some aquatic mammals, like beavers and otters, keep their fur oily and waterproof to stay warm. Others, like dolphins and seals, have lost their fur and instead have thick skin and a layer of fat for protection. The way their bodies are built helps them stay balanced in water—some need to be heavier to stay on the bottom, while others need to be lighter to swim freely. Their eyes also change to see better in water, especially in places with little light.

Certain aquatic mammals have special ways to hunt and navigate. Some can feel vibrations in the water from schools of fish, and toothed whales use sound to find prey or avoid obstacles.

Ecology

Further information: Keystone species

Beaver ponds help the environment in many ways. They store water during dry times and stop rivers from drying up. During floods, beaver dams slow down water, which helps protect the land around them. The dams also trap dirt, making the water cleaner for animals to drink and helping fish lay eggs. However, the slower water and lack of tree shade can make the water warmer. Beavers also help control tiny water creatures that keep algae in check.

Beavers eat wood from trees like aspen, cottonwood, willow, alder, birch, maple, and cherry. They also eat plants such as sedges, pondweed, and water lilies. Instead of hibernating, beavers store sticks and logs in their ponds to eat during the winter. Their dams flood nearby forests, giving them easy access to food like tree leaves and bark.

Indian rhinoceroses mainly eat grasses but also eat leaves, small tree branches, fruits, and water plants. They use their lips to grab grass and bend it down to eat.

Manatees move between flooded forests and deep lakes depending on the season. They eat plants found in these areas.

Moose eat new growth from trees like birch and aspen, as well as water plants like waterlily and water milfoil. They can bend small trees down to reach leaves and even stand on their hind legs to reach higher branches. Moose are good swimmers and often go into water to eat plants growing at the bottom of lakes.

Hippopotamuses leave the water at night to walk up to 10 kilometers to find grass to eat. They can eat up to 68 kilograms of grass each night. Pygmy hippopotamuses also leave the water at night but eat mainly ferns, broad-leaved plants, and fallen fruits because they live in thick forests where grass is rare.

The Amazon river dolphin eats many kinds of fish, including croakers, cichlids, tetras, and piranhas. They also eat freshwater crabs and river turtles. South Asian river dolphins mostly eat fish like carp, catfish, and freshwater sharks, as well as invertebrates such as prawns.

Small aquatic animals like desmans, shrews, and voles dive quickly to catch small fish and invertebrates. The giant otter shrew catches tiny crabs. The Lutrine opossum eats small birds, rodents, and invertebrates. Most water voles eat grass and plants near water, but some in England have been seen eating frogs' legs.

Interactions with humans

Fur from animals like beavers was very important in the past. It was used to make special hats because the fur worked well for that purpose.

Some animals, like the Indian rhinoceros, were hunted a lot, which made their numbers drop very low. Otters were also hunted for their fur, which has caused problems for their populations in some places.

Pollution and changes to their homes have also hurt some aquatic animals. For example, chemicals in lakes can make animals sick, and building dams can change the places where they live.

Some animals are eaten by people. Moose meat is eaten in many places and tastes good, but people should not eat certain parts of moose because they can be unhealthy.

Images

A beaver from the Grand River in Painesville, Ohio, swimming in its natural habitat.
A gentle Amazonian manatee swimming in its natural habitat.
A river dolphin named Inia at Zoo Duisburg, alongside a fish species called Pterygoplichthys gibbiceps.
A playful otter in Southwold, Suffolk, England.
A playful North American River Otter swimming in its enclosure at the Oregon Zoo.
A friendly capybara at Hattiesburg Zoo.
A curious platypus, known for its duck-like bill and beaver-like tail, swimming in an aquarium.
A hippopotamus resting in the water at the Memphis Zoo.
A majestic Indian Rhinoceros at the Cincinnati Zoo.
An exhibit of a water opossum at the Swedish Museum of Natural History in Stockholm.

Related articles

Capybara
Marine mammal
Amazonian manatee
Baikal seal
Hippopotamus
Body of water

This article is a child-friendly adaptation of the Wikipedia article on Aquatic mammal, available under CC BY-SA 4.0.

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