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Aquatic mammal

Adapted from Wikipedia · Adventurer experience

A majestic humpback whale swimming gracefully underwater.

Aquatic mammals and semiaquatic mammals are animals that live partly or fully in bodies of water. This group includes many different 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 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. Even today, some of these animals still face dangers such as getting caught in fishing nets or hitting boats. Protecting aquatic mammals and their homes is important.

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. The first two molars had cusps in a straight row. These teeth are very similar to the teeth seen in mesonychians.

Another docodontan, the Late Jurassic Haldanodon, has been suggested to be a swimmer and burrower.

The tritylodontid Kayentatherium has been suggested to be semiaquatic.

Another lineage of Mesozoic mammals, the eutriconodonts, have been suggested to be aquatic animals.

Illustration of Prorastomus, a sirenian from the Eocene

However, two other eutriconodonts occur in marine deposits, implying that they died there and are thus truly aquatic mammals.

A metatherian, the stagodontid Didelphodon, has been suggested to be aquatic.

Cenozoic

An extinct genus, Satherium, is believed to be ancestral to South American river otters.

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).

Sirenians, along with Proboscidea (elephants), group together with the extinct †Desmostylia and likely †Embrithopoda to form the Tethytheria.

The oldest fossil of the modern platypus dates back to about 100,000 years ago, during the Quaternary period.

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. Pinnipeds are semiaquatic; they spend the majority of their time in the water, but need to return to land for important activities. In contrast, both otters and the polar bear are much less adapted to aquatic living. Likewise, their diet ranges considerably as well. While the number of marine mammals is small compared to those found on land, their roles in various ecosystems are large.

Marine mammals were first hunted by aboriginal peoples. They were also the target for commercial industry, leading to declines in populations of exploited species. After commercial hunting ended, some species have rebounded in numbers; conversely, other species are critically endangered. Other than hunting, marine mammals can be killed as bycatch from fisheries. 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.

Adaptations

Mammals usually live on land, but some spend time in water. They have special ways to stay safe and find food. They can't breathe underwater like fish, so they have special nose and throat parts to keep water out when they breathe. Some, like manatees, use sensitive whiskers to feel for food. 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 have thick skin and a layer of fat for protection. Their bodies help 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. 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.

Indian rhinoceroses mainly eat grasses but also eat leaves, small tree branches, fruits, and water plants.

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.

Hippopotamuses leave the water at night to walk up to 10 kilometers to find grass to eat.

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.

Interactions with humans

Fur from animals like beavers was important long ago. It was used to make special hats because the fur worked well.

Some animals, like the Indian rhinoceros, were hunted a lot. This made their numbers go down. Otters were also hunted for their fur, which has caused problems for them in some places.

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

Some animals are eaten by people. Moose meat is eaten in many places, 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.
An exhibit of a water opossum at the Swedish Museum of Natural History in Stockholm.

Related articles

Marine mammal
Amazonian manatee
Baikal seal
Capybara
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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