thumb|A [[humpback whale (Megaptera novaeangliae)|alt=A humpback whale swimming]]

thumb|A [[leopard seal (Hydrurga leptonyx)|alt=A black-speckled seal with a light-gray underside and a dark-gray back, sitting on rocks, its mouth agape showing sharp teeth]]

Marine mammals are mammals that rely on marine ecosystems for their existence. They include animals such as cetaceans, pinnipeds, sirenians, marine otters, sea otters and polar bears. They are an informal group, unified only by their reliance on marine environments for feeding and survival.

Marine mammal adaptation to an aquatic lifestyle varies considerably between species. Both cetaceans and sirenians are fully aquatic and therefore are obligate water 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. Sea otters tend to live in kelp forests and estuaries. In contrast, the polar bear is mostly terrestrial and only goes into the water on occasions of necessity, and are thus much less adapted to aquatic living. The diets of marine mammals vary considerably as well; some eat zooplankton, others eat fish, squid, shellfish, or seagrass, and a few eat other mammals. While the number of marine mammals is small compared to those found on land, their roles in various ecosystems are large, especially concerning the maintenance of marine ecosystems, through processes including the regulation of prey populations. This role in maintaining ecosystems makes them of particular concern as 23% of marine mammal species are currently threatened.

Marine mammals were first hunted by aboriginal peoples for food and other resources. Many 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 led to the extinction of the Steller's sea cow, sea mink, Japanese sea lion and Caribbean monk seal, and possibly the Baiji. After commercial hunting ended, some species, such as the gray whale and northern elephant seal, have rebounded in numbers; conversely, other species, such as the North Atlantic right whale, are critically endangered. Other than being hunted, marine mammals can be killed as bycatch from fisheries, where for example they can become entangled in nets 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 degrade Arctic environments.

Taxonomy

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Classification of extant species

  • Order Cetartiodactyla
  • Suborder Whippomorpha
  • Family Balaenidae (right and bowhead whales), two genera and four species
  • Family Cetotheriidae (pygmy right whale), one species
  • Family Balaenopteridae (rorquals), two genera and eight species
  • Family Eschrichtiidae (gray whale), one species
  • Family Physeteridae (sperm whale), one species
  • Family Kogiidae (pygmy and dwarf sperm whales), one genus and two species
  • Family Monodontidae (narwhal and beluga), two genera and two species
  • Family Ziphiidae (beaked whales), six genera and 21 species
  • Family Delphinidae (oceanic dolphins), 17 genera and 38 species
  • Family Phocoenidae (porpoises), two genera and seven species
  • Order Sirenia (sea cows)

Evolution

thumb|left|Illustration of †[[Prorastomus, an early sirenian (40 mya)|alt=A furry, streamlined mammal swimming through the water with toes visible on each foot, similar to those of an elephant. All limbs are thrust backwards or underneath the animal.]]

Marine mammals form a diverse group of 129 species that rely on the ocean for their existence. They are an informal group unified only by their reliance on marine environments for feeding. Despite the diversity in anatomy seen between groups, improved foraging efficiency has been the main driver in their evolution. Based on molecular and morphological research, the cetaceans genetically and morphologically fall firmly within the Artiodactyla (even-toed ungulates). The term "Cetartiodactyla" reflects the idea that whales evolved within the ungulates. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. Under this definition, the closest living land relative of the whales and dolphins is thought to be the hippopotamuses.

Sirenians, the sea cows, became aquatic around 40&nbsp;million years ago. The first appearance of sirenians in the fossil record was during the early Eocene, and by the late Eocene, sirenians had significantly diversified. Inhabitants of rivers, estuaries, and nearshore marine waters, they were able to spread rapidly. The most primitive sirenian, †Prorastomus, was found in Jamaica, unlike other marine mammals which originated from the Old World (such as cetaceans). The first known quadrupedal sirenian was †Pezosiren from the early middle Eocene. The earliest known sea cows, of the families †Prorastomidae and †Protosirenidae, were both confined to the Eocene, and were pig-sized, four-legged, amphibious creatures. The first members of Dugongidae appeared by the middle Eocene. At this point, sea cows were fully aquatic. The lineages of Otariidae (eared seals) and Odobenidae (walrus) split almost 28 mya. Phocids (earless seals) are known to have existed for at least 15 mya, and molecular evidence supports a divergence of the Monachinae (monk seals) and Phocinae lineages 22 mya.

Polar bears are thought to have diverged from a population of brown bears, Ursus arctos, that became isolated during a period of glaciation in the Pleistocene or from the eastern part of Siberia, (from Kamchatka and the Kolym Peninsula). The oldest known polar bear fossil is a 130,000-to-110,000-year-old jaw bone, found on Prince Charles Foreland in 2004. The mitochondrial DNA (mtDNA) of the polar bear diverged from the brown bear roughly 150,000 years ago. meaning that the polar bear might not be considered a species under some species concepts.

In general, terrestrial amniote invasions of the sea have become more frequent in the Cenozoic than they were in the Mesozoic. Factors contributing to this trend include the increasing productivity of near-shore marine environments, and the role of endothermy in facilitating this transition.

Distribution and habitat

thumb|Marine mammal species richness: A) All species (n = 115), B) [[toothed whales (n = 69), C) baleen whales (n = 14), D) seals (n = 32), based on data from 1990 to 1999]]

Marine mammals are widely distributed throughout the globe, but their distribution is patchy and coincides with the productivity of the oceans. Species richness peaks at around 40° latitude, both north and south. This corresponds to the highest levels of primary production around North and South America, Africa, Asia and Australia. Total species range is highly variable for marine mammal species. On average most marine mammals have ranges which are equivalent or smaller than one-fifth of the Indian Ocean. Most cetaceans live in the open ocean, and species like the sperm whale may dive to depths of in search of food. Sirenians live in shallow coastal waters, usually living below sea level. However, they have been known to dive to to forage deep-water seagrasses. Sea otters live in protected areas, such as rocky shores, kelp forests, and barrier reefs, although they may reside among drift ice or in sandy, muddy, or silty areas.

Many marine mammals seasonally migrate. Annual ice contains areas of water that appear and disappear throughout the year as the weather changes, and seals migrate in response to these changes. In turn, polar bears must follow their prey. In Hudson Bay, James Bay, and some other areas, the ice melts completely each summer (an event often referred to as "ice-floe breakup"), forcing polar bears to go onto land and wait through the months until the next freeze-up. In the Chukchi and Beaufort seas, polar bears retreat each summer to the ice further north that remains frozen year-round. Seals may also migrate to other environmental changes, such as El Niño, and traveling seals may use various features of their environment to reach their destination including geomagnetic fields, water and wind currents, the position of the sun and moon and the taste and temperature of the water. possibly to prevent predation by killer whales. The gray whale has the longest recorded migration of any mammal, with one traveling from the Sea of Okhotsk to the Baja Peninsula. During the winter, manatees living at the northern end of their range migrate to warmer waters.

Adaptations

thumb|left|The labelled, anatomy of a [[dolphin showing its skeleton, major organs, and shape of the body]]

Marine mammals have a number of physiological and anatomical features to overcome the unique challenges associated with aquatic living. Some of these features are very species-specific. Marine mammals have developed a number of features for efficient locomotion such as torpedo-shaped bodies to reduce drag; modified limbs for propulsion and steering; tail flukes and dorsal fins for propulsion and balance.

Sound travels differently through water, and therefore marine mammals have developed adaptations to ensure effective communication, prey capture, and predator detection. These sounds are reflected by the dense concave bone of the cranium and an air sac at its base. The focused beam is modulated by a large fatty organ known as the 'melon'. This acts like an acoustic lens because it is composed of lipids of differing densities.

Marine mammals have evolved a wide variety of features for feeding, which are mainly seen in their dentition. For example, the cheek teeth of pinnipeds and odontocetes are specifically adapted to capture fish and squid. In contrast, baleen whales have evolved baleen plates to filter feed plankton and small fish from the water.

Ecology

Dietary

thumb|An [[orca hunting a Weddel seal|alt=An orca sticking the front end of its body out of the water onto a floating piece of sea-ice to grab a dark-brown seal with light-brown-spots]]

All cetaceans are carnivorous and predatory. Toothed whales mostly feed on fish and cephalopods, followed by crustaceans and bivalves. Some may forage with other kinds of animals, such as other species of whales or certain species of pinnipeds. One common feeding method is herding, where a pod squeezes a school of fish into a small volume, known as a bait ball. Individual members then take turns plowing through the ball, feeding on the stunned fish. Coralling is a method where dolphins chase fish into shallow water to catch them more easily. Orcas and bottlenose dolphins have also been known to drive their prey onto a beach to feed on it. Killer whales have been known to paralyze great white sharks and other sharks and rays by flipping them upside down. Other whales with a blunt snout and reduced dentition rely on suction feeding. Though carnivorous, they house gut flora similar to that of terrestrial herbivores, probably a remnant of their herbivorous ancestry.

Baleen whales use their baleen plates to sieve plankton, among others, out of the water; there are two types of methods: lunge-feeding and gulp-feeding. Lunge-feeders expand the volume of their jaw to a volume bigger than the original volume of the whale itself by inflating their mouth. This causes grooves on their throat to expand, increasing the amount of water the mouth can store. They ram a baitball at high speeds in order to feed, but this is only energy-effective when used against a large baitball. Gulp-feeders swim with an open mouth, filling it with water and prey. Prey must occur in sufficient numbers to trigger the whale's interest, be within a certain size range so that the baleen plates can filter it, and be slow enough so that it cannot escape.

thumb|left|[[Sea otters have dexterous hands which they use to smash sea urchins off rocks|alt=A sea otter floating on the water on its back holding a sea urchin with one hand and a rock in the other]]

Otters are the only marine animals that are capable of lifting and turning over rocks, which they often do with their front paws when searching for prey. The sea otter may pluck snails and other organisms from kelp and dig deep into underwater mud for clams. Under each foreleg, sea otters have a loose pouch of skin that extends across the chest which they use to store collected food to bring to the surface. This pouch also holds a rock that is used to break open shellfish and clams, an example of tool use. The sea otters eat while floating on their backs, using their forepaws to tear food apart and bring to their mouths. Marine otters mainly feed on crustaceans and fish.

Pinnipeds mostly feed on fish and cephalopods, followed by crustaceans and bivalves, and then zooplankton and warm-blooded prey (like sea birds). They typically hunt non-schooling fish, slow-moving or immobile invertebrates or endothermic prey when in groups. Solitary foraging species usually exploit coastal waters, bays and rivers. When large schools of fish or squid are available, pinnipeds hunt cooperatively in large groups, locating and herding their prey. Some species, such as California and South American sea lions, may forage with cetaceans and sea birds. Polar bears hunt primarily at the interface between ice, water, and air; they only rarely catch seals on land or in open water. The polar bear's most common hunting method is still-hunting: The bear locates a seal breathing hole using its sense of smell, and crouches nearby for a seal to appear. When the seal exhales, the bear smells its breath, reaches into the hole with a forepaw, and drags it out onto the ice. The polar bear also hunts by stalking seals resting on the ice. Upon spotting a seal, it walks to within , and then crouches. If the seal does not notice, the bear creeps to within of the seal and then suddenly rushes to attack. A third hunting method is to raid the birth lairs that female seals create in the snow.

thumb|A [[dugong feeding on the sea-floor|alt=A dugong with its mouth on the sandy seafloor, leaving a noticeable cloud which hovers near the bottom. There are two yellow fish with black stripes near its mouth, and there are grasses poking out of the seafloor]]

Sirenians are referred to as "sea cows" because their diet consists mainly of seagrass. When eating, they ingest the whole plant, including the roots, although when this is impossible they feed on just the leaves. A wide variety of seagrass has been found in dugong stomach contents, and evidence exists they will eat algae when seagrass is scarce. West Indian manatees eat up to 60 different species of plants, as well as fish and small invertebrates to a lesser extent.

Keystone species

Sea otters are a classic example of a keystone species; their presence affects the ecosystem more profoundly than their size and numbers would suggest. They keep the population of certain benthic (sea floor) herbivores, particularly sea urchins, in check. Sea urchins graze on the lower stems of kelp, causing the kelp to drift away and die. Loss of the habitat and nutrients provided by kelp forests leads to profound cascade effects on the marine ecosystem. North Pacific areas that do not have sea otters often turn into urchin barrens, with abundant sea urchins and no kelp forest. Reintroduction of sea otters to British Columbia has led to a dramatic improvement in the health of coastal ecosystems, and similar changes have been observed as sea otter populations recovered in the Aleutian and Commander Islands and the Big Sur coast of California. The polar bear is the apex predator within its range. The evolutionary pressure of polar bear predation on seals probably accounts for some significant differences between Arctic and Antarctic seals. Compared to the Antarctic, where there is no major surface predator, Arctic seals use more breathing holes per individual, appear more restless when hauled out on the ice, and rarely defecate on the ice.

Killer whales are apex predators throughout their global distribution, and can have a profound effect on the behavior and population of prey species. Their diet is very broad and they can feed on many vertebrates in the ocean including salmon, rays, sharks (even white sharks), large baleen whales, and nearly 20 species of pinniped. The predation of whale calves may be responsible for annual whale migrations to calving grounds in more tropical waters, where the population of killer whales is much lower than in polar waters. Prior to whaling, it is thought that great whales were a major food source; however, after their sharp decline, killer whales have since expanded their diet, leading to the decline of smaller marine mammals. A decline in Aleutian Islands sea otter populations in the 1990s was controversially attributed by some scientists to killer whale predation, although with no direct evidence. The decline of sea otters followed a decline in harbor seal and Steller sea lion populations, the killer whale's preferred prey, which in turn may be substitutes for their original prey, now reduced by industrial whaling.

Whale pump

[[File:Oceanic whale pump - journal.pone.0013255.g001.tiff|thumb|"Whale pump" – the role played by whales in recycling ocean nutrients

Whales defecate at the ocean's surface; their excrement is important for fisheries because it is rich in iron and nitrogen. The whale feces are liquid and instead of sinking, they stay at the surface where phytoplankton feed off it.

Upon death, whale carcasses fall to the deep ocean and provide a substantial habitat for marine life. Evidence of whale falls in present-day and fossil records shows that deep-sea whale falls support a rich assemblage of creatures, with a global diversity of 407 species, comparable to other neritic biodiversity hotspots, such as cold seeps and hydrothermal vents. Deterioration of whale carcasses happens through a series of three stages. Initially, moving organisms, such as sharks and hagfish, scavenge soft tissue at a rapid rate over a period of months to as long as two years. This is followed by the colonization of bones and surrounding sediments (which contain organic matter) by enrichment opportunists, such as crustaceans and polychaetes, throughout a period of years. Finally, sulfophilic bacteria reduce the bones releasing hydrogen sulphide enabling the growth of chemoautotrophic organisms, which in turn, support other organisms such as mussels, clams, limpets, and sea snails. This stage may last for decades and supports a rich assemblage of species, averaging 185 species per site.

Interactions with humans

Threats

Due to the difficulty to survey populations, 38% of marine mammals are data deficient, especially around the Antarctic Polar Front. In particular, declines in the populations of completely marine mammals tend to go unnoticed 70% of the time.

Exploitation

thumb|Men killing [[northern fur seals on Saint Paul Island, Alaska, in the 1890s|alt=A group of seal hunters surround a small group of sea lions with their clubs in the air]]

Marine mammals were hunted by coastal aboriginal humans historically for food and other resources. These subsistence hunts still occur in Canada, Greenland, Indonesia, Russia, the United States, and several nations in the Caribbean. The effects of these are only localized, as hunting efforts were on a relatively small scale. Because whales generally have slow growth rates, are slow to reach sexual maturity, and have a low reproductive output, population recovery has been very slow.

A number of whales are still subject to direct hunting, despite the 1986 moratorium on commercial whaling set under the terms of the International Whaling Commission (IWC). There are only two nations remaining which sanction commercial whaling: Norway, where several hundred common minke whales are harvested each year; and Iceland, where quotas of 150 fin whales and 100 minke whales per year are set. Japan also harvests several hundred Antarctic and North Pacific minke whales each year, ostensibly for scientific research in accordance with the moratorium.

thumb|left|Historical and modern range of northern sea otters|alt=Alaskan sea otters inhabit most of the Aleutian Islands and the Pacific Northwest, Asian sea otters inhabit the islands around Kamchatka Peninsula and those that stretch between there and Japan (excluding in the Sea of Okhosk) and California sea otters inhabit the coast of southern California. Their former range follows the coast of southern California north, into the Aleutian Islands, without any gaps in between.

The most profitable furs in the fur trade were those of sea otters, especially the northern sea otter which inhabited the coastal waters between the Columbia River to the south and Cook Inlet to the north. The fur of the Californian southern sea otter was less highly prized and thus less profitable. After the northern sea otter was hunted to local extinction, maritime fur traders shifted to California until the southern sea otter was likewise nearly extinct. The British and American maritime fur traders took their furs to the Chinese port of Guangzhou (Canton), where they worked within the established Canton System. Furs from Russian America were mostly sold to China via the Mongolian trading town of Kyakhta, which had been opened to Russian trade by the 1727 Treaty of Kyakhta.

Commercial sealing was historically just as important as the whaling industry. Exploited species included harp seals, hooded seals, Caspian seals, elephant seals, walruses and all species of fur seal. The scale of seal harvesting decreased substantially after the 1960s, after the Canadian government reduced the length of the hunting season and implemented measures to protect adult females. Several species that were commercially exploited have rebounded in numbers; for example, Antarctic fur seals may be as numerous as they were prior to harvesting. The northern elephant seal was hunted to near extinction in the late 19th century, with only a small population remaining on Guadalupe Island. It has since recolonized much of its historic range, but has a population bottleneck.

Polar bears can be hunted for sport in Canada with a special permit and accompaniment by a local guide. This can be an important source of income for small communities, as guided hunts bring in more income than selling the polar bear hide on markets. The United States, Russia, Norway, Greenland, and Canada allow subsistence hunting, and Canada distributes hunting permits to indigenous communities. The selling of these permits is a main source of income for many of these communities. Their hides can be used for subsistence purposes, kept as hunting trophies, or can be bought in markets.

Ocean traffic and fisheries

thumb|right|The remains of a [[North Atlantic right whale after it collided with a ship propeller|alt=A right whale sliced on both sides after colliding with a boat. A large amount of its flesh is visible as well as the intestines floating in the water]]

By-catch is the incidental capture of non-target species in fisheries. Fixed and drift gill nets cause the highest mortality levels for both cetaceans and pinnipeds, however, entanglements in long lines, mid-water trawls, and both trap and pot lines are also common. Tuna seines are particularly problematic for entanglement by dolphins. By-catch affects all cetaceans, both small and big, in all habitat types. However, smaller cetaceans and pinnipeds are most vulnerable as their size means that escape once they are entangled is highly unlikely and they frequently drown. Marine mammals also get entangled in aquaculture nets, however, these are rare events and not prevalent enough to impact populations.

Vessel strikes cause death for a number of marine mammals, especially whales. Tourism boats designed for whale and dolphin watching can also negatively impact on marine mammals by interfering with their natural behavior.

The fishery industry not only threatens marine mammals through by-catch, but also through competition for food. Large-scale fisheries have led to the depletion of fish stocks that are important prey species for marine mammals. Pinnipeds have been especially affected by the direct loss of food supplies and in some cases the harvesting of fish has led to food shortages or dietary deficiencies, starvation of young, and reduced recruitment into the population. As the fish stocks have been depleted, the competition between marine mammals and fisheries has sometimes led to conflict. Large-scale culling of populations of marine mammals by commercial fishers has been initiated in a number of areas in order to protect fish stocks for human consumption.

Shellfish aquaculture takes up space so in effect creates competition for space. However, there is little direct competition for aquaculture shellfish harvest.

Noise pollution from anthropogenic activities is another major concern for marine mammals. This is a problem because underwater noise pollution interferes with the abilities of some marine mammals to communicate, and locate both predators and prey. Underwater explosions are used for a variety of purposes including military activities, construction and oceanographic or geophysical research. They can cause injuries such as hemorrhaging of the lungs, and contusion and ulceration of the gastrointestinal tract. The second effect of global climate change is global warming due to increased carbon dioxide levels in the atmosphere. Raised sea levels, rising sea temperatures and changed currents are expected to affect marine mammals by altering the distribution of important prey species, and changing the suitability of breeding sites and migratory routes. The Arctic food chain would be disrupted by the near extinction or migration of polar bears. Arctic sea ice is the polar bear's habitat. It has been declining at a rate of 13% per decade because the temperature is rising at twice the rate of the rest of the world. By the year 2050, up to two-thirds of the world's polar bears may vanish if the sea ice continues to melt at its current rate.

A study by evolutionary biologists at the University of Pittsburgh showed that the ancestors of many marine mammals stopped producing a certain enzyme that today protects against some neurotoxic chemicals called organophosphates, including those found in the widely used pesticides chlorpyrifos and diazinon. Marine mammals may be increasingly exposed to these compounds due to agricultural runoff reaching the world's oceans.

Protection

thumb|Signatory countries of the [[International Whaling Commission (IWC)]]

The Marine Mammal Protection Act of 1972 (MMPA) was passed on October 21, 1972, under president Richard Nixon to prevent the further depletion and possible extinction of marine mammal stocks.

The Act was updated on 1 January 2016 with a clause banning "the import of fish from fisheries that cannot prove they meet US standards for protecting marine mammals". The requirement to show that protection standards are met is hoped to compel countries exporting fish to the US to more strictly control their fisheries that no protected marine mammals are adversely affected by fishing. Of the agreements made, three of them deal with the conservation of marine mammals: ACCOBAMS, ASCOBANS and the Wadden Sea Agreement. In 1982, the United Nations Convention on the Law of the Sea (UNCLOS) adopted a pollution prevention approach to conservation, which many other conventions at the time also adopted. There are 23 member states.

The Agreement on the Conservation of Small Cetaceans of the Baltic and North Seas (ASCOBANS) was adopted alongside ACCOBAMS to establish a special protection area for Europe's increasingly threatened cetaceans. and an environmental agreement (a type of international law) the International Convention for the Regulation of Whaling which controlled commercial, scientific and subsistence whaling.

The Agreement on the Conservation of Seals in the Wadden Sea, enforced in 1991, prohibits the killing or harassment of seals in the Wadden Sea, specifically targeting the harbor seal population.

The 1973 Agreement on the Conservation of Polar Bears between Canada, Denmark (Greenland), Norway (Svalbard), the United States and the Soviet Union outlawed the unregulated hunting of polar bears from aircraft and icebreakers, as well as protecting migration, feeding and hibernation sites.

Various non-governmental organizations participate in marine conservation activism, wherein they draw attention to and aid in various problems in marine conservation, such as pollution, whaling, bycatch, and so forth. Notable organizations include the Greenpeace who focus on overfishing and whaling among other things, and Sea Shepherd Conservation Society who are known for taking direct-action tactics to expose illegal activity.

As food

thumb|[[Pilot whale meat (bottom), blubber (middle) and dried fish (left) with potatoes, Faroe Islands|alt=The whale meat is dark purple and shredded like jerky, the blubber is a pale-pink color and in slices, the dried fish is a light-brown color and ripped into slices, and the potatoes are light-yellow and cut into thin slices.]]

For thousands of years, indigenous peoples of the Arctic have depended on whale meat and seal meat. The meat is harvested from legal, non-commercial hunts that occur twice a year in the spring and autumn. The meat is stored and eaten throughout the winter. The skin and blubber (muktuk) taken from the bowhead, beluga, or narwhal is also valued, and is eaten raw or cooked. Whaling has also been practiced in the Faroe Islands in the North Atlantic since about the time of the first Norse settlements on the islands. Around 1,000 long-finned pilot whales are still killed annually, mainly during the summer. Today, dolphin meat is consumed in a small number of countries worldwide, which include Japan and Peru (where it is referred to as chancho marino, or "sea pork"). In some parts of the world, such as Taiji (in Japan) and the Faroe Islands, dolphins are traditionally considered food, and are killed in harpoon or drive hunts.

There have been human health concerns associated with the consumption of dolphin meat in Japan after tests showed that dolphin meat contained high levels of methylmercury. Similar concerns exist with the consumption of dolphin meat in the Faroe Islands, where prenatal exposure to methylmercury and PCBs primarily from the consumption of pilot whale meat has resulted in neuropsychological deficits amongst children.