Procaviidae Elephantidae Dugongidae (dugongs) Trichechus manatus (West Indian manatee) Trichechus senegalensis (African manatee) Trichechus inunguis (Amazonian manatee; freshwater species) Hippopotamidae Mysticeti (baleen whales) Odontoceti (toothed whales, except river dolphins) Ruminantia Perissodactyla Pholidota Feliformia Canidae Ursus maritimus (polar bear) all other ursids Enhydra lutris (sea otter) Lontra felina (marine otter) †Neogale macrodon (sea mink) all other mustelids Otariidae (eared seals) Odobenidae (walruses) Phocidae (earless seals) The term "marine mammal" encompasses all mammals whose survival depends entirely or almost entirely on the oceans, which have also evolved several specialized aquatic traits.
This term can include: the greater bulldog bat (Noctilio leporinus), the fish-eating bat (Myotis vivesi), the arctic fox (Vulpes lagopus) which often scavenges polar bear kills, coastal gray wolf (Canis lupus) populations which predominantly eat salmon and marine carcasses, the North Ronaldsay sheep (Ovis aries) which normally eats seaweed outside the lambing season, the Eurasian otter (Lutra lutra) which is usually found in freshwater but can be found along coastal Scotland, and others.
The sea otter evolved initially in northern Hokkaidō and Russia, and then spread east to the Aleutian Islands, mainland Alaska, and down the North American coast.
In comparison to cetaceans, sirenians, and pinnipeds, which entered the water approximately 50, 40, and 20 mya, respectively, the sea otter is a relative newcomer to marine life.
In temperate and tropical areas, they haul-out on to sandy and pebble beaches, rocky shores, shoals, mud flats, tide pools and in sea caves.
[44][45][46] Sound travels differently through water, and therefore marine mammals have developed adaptations to ensure effective communication, prey capture, and predator detection.
In contrast, other marine mammals—such as whales, dolphins, porpoises, manatees, dugongs, and walruses—have lost long fur in favor of a thick, dense epidermis and a thickened fat layer (blubber) to prevent drag.
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.
[65] The role of sea otters in maintaining kelp forests has been observed to be more important in areas of open coast than in more protected bays and estuaries.
[79] Several animal species, particularly Arctic foxes (Vulpes lagopus) and glaucous gulls (Larus hyperboreus), routinely scavenge polar bear kills.
[41] 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.
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.
[95][96] Japan also harvests several hundred Antarctic and North Pacific minke whales each year, ostensibly for scientific research in accordance with the moratorium.
[100] The scale of seal harvesting decreased substantially after the 1960s,[101] after the Canadian government reduced the length of the hunting season and implemented measures to protect adult females.
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.
Developments such as sewage marine outfalls, moorings, dredging, blasting, dumping, port construction, hydroelectric projects, and aquaculture both degrade the environment and take up valuable habitat.
[47] For example, extensive shellfish aquaculture takes up valuable space used by coastal marine mammals for important activities such as breeding, foraging and resting.
[47] Contaminants that are found in the tissues of marine mammals include heavy metals, such as mercury and lead, but also organochlorides and polycyclic aromatic hydrocarbons.
[120] 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,[121] including those found in the widely used pesticides chlorpyrifos and diazinon.
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.
[137][138] Today, dolphin meat is consumed in a small number of countries worldwide, which include Japan[139][140] and Peru (where it is referred to as chancho marino, or "sea pork").
[141] 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.
[142] The Faroe Islands population was exposed to methylmercury largely from contaminated pilot whale meat, which contained very high levels of about 2 mg methylmercury/kg.
The study of about 900 Faroese children showed that prenatal exposure to methylmercury resulted in neuropsychological deficits at 7 years of ageRinged seals were once the main food staple for the Inuit.
Captive life is also stressful due to the requirement to perform circus tricks that are not part of wild killer whale behavior, as well as restricting pool size.
They also oppose using sea lions for entertainment, claiming the tricks performed are "exaggerated variations of their natural behaviors" and distract the audience from the animal's unnatural environment.
[170] Bottlenose dolphins and California sea lions are used in the United States Navy Marine Mammal Program (NMMP) to detect mines, protect ships from enemy soldiers, and recover objects.
The sea lion team retrieved test equipment such as fake mines or bombs dropped from planes usually out of reach of divers who would have to make multiple dives.
The Navy states that this is to prevent them from ingesting harmful objects, but conservation activists say this is done to reinforce the trainers' control over the dolphins, who hand out food rewards.