Aquatic and semiaquatic mammals

I've mentioned elsewhere that AAT/H proponents are typically extremely coy about what aquatic animals we're supposed to resemble; they're also somewhat idiosyncratic about what they say constitutes an "aquatic" or "semiaquatic" mammal. Marc Verhaegen has often claimed that non-aquatic mammals such as rhinoceros and mountain beavers are aquatic, which is nonsense. Others claim pigs are; again nonsense. Many times they don't really like to say at all, although they don't spend much time looking at actual aquatic and semiaquatic mammals, preferring vagueness, merely claiming that humans share hair and fat characteristics with "aquatics" or that such characteristics are found in "an aquatic environment". But if you take the step of looking at what animals they have to be talking about they get very defensive, because if you do you end up thinking "how much like a seal am I really?".

The general AAT/H vagueness about degree of water use has followed this general arc:

  1. a fairly vague statement by Hardy (less aquatic than otters for maybe 10 million years);
  2. virtually nothing beyond "as aquatic as an otter" or "less aquatic than an otter" for the next forty years (except for Hardy in 1978 making his earlier statement far more explicit (now 5-6 hours in water every day for 20 or so million years); okay, it's completely impossible -- longer than hominids of any kind have existed -- but at least he said what he was thinking);
  3. Algis Kuliukas' formulation of the "more aquatic than apes" phrase (sometimes specifically chimps); his definition is "...that water has acted as an agent of selection in the evolution of humans more than it has in the evolution of our ape cousins..." (the full definition is on the page about him on this site). This is so vague that's it's meaningless.

I mean really, water has acted as an "agent of selection" in the evolution of wolves and black bears more than it has in the evolution of our ape cousins, and wolves and black bears aren't semiaquatic nor were their ancestors.

That can be contrasted with typical scales of aquaticness used by actual zoologists. For instance, when I was recently making a list of semiaquatic and aquatic mammals one source I used had this as a note for a table of aquatic and semiaquatic mammals:

NOTE: Habitat: M, marine; C, coastal; F, freshwater. Degree to which aquatic: 1, obligately aquatic; 2, mostly aquatic, return to land for reproduction and resting; 3, moderately aquatic, fully capable of terrestrial locomotion, may forage on land

From Table 1.4 by Henry Pihlström, "Introduction: On Becoming Aquatic" in Sensory Evolution on the Threshold: Adaptations in Secondarily Aquatic Vertebrates edited by J. G. M. Thewissen and Sirpa Nummela Publisher: University of California Press

Or consider the explanation set forth in the "Methods" section of a 2001 paper by Bininda-Emonds et al.

("Flippers versus feet: comparative trends in aquatic and non-aquatic carnivores", Journal of Animal Ecology, 2001, 70:386-400): "We consider aquatic carnivores to be those species in which the aquatic habitat inevitably plays a key role in the life-cycle of an individual..."

Because the AAT/H claims that aquatic and semiaquatic living creates selection for hairlessness and fatness, I thought it would be useful to have a list of all the aquatic and semiaquatic mammals, and I'm also going to look at how many times aquaticism has evolved in aquatic and semiaquatic mammals and how many are hairless and/or fatty. The list totals 203 species; for my purposes it's broken down as follows:

  • 112 hairy; 75 not including seals and walrus
  • 91 hairless (84 cetaceans, 5 sirenia, 2 hippos)
  • 125 "fatties" (84 cetaceans, 5 sirenia, 36 seals and walrus)
  • 35 of the hairy ones tropical (or in the case of 2 or 3 species ranging into the tropics or subtropical areas)
  • 4 of the species listed are now (relatively recently) extinct

    The reason I've broken it down that way is the claims I've seen used over the years by AAT/H proponents and what they've said when their claims get critiqued. Two of the basic claims are that natural selection (and pointedly not sexual selection or genetic drift or the demands of thermoregulation) and a watery environment is behind humans being "hairless" and relatively fat. (They don't accurately describe human hair and fat characteristics but that's another mark against their ideas.) AAT/H proponents generally do this without saying what aquatic and/or semiaquatic mammals they're talking about; typically they just claim water use does the deed. Typically they also refuse to say how much water use this takes, sometimes going to comical lengths to not say. Looking at this list gives you an idea of just what amount of water use it takes to, say, lose body hair.

    "Fatties" refers to generally a good deal more body fat than is "average" among wild mammals such as primates, bovidae, most carnivores, and such; the average more or less seems to be around 5-10%. Naturally this varies a lot seasonally and with the health of the animal and how its food supply is, how much predation pressure they face, etc. so it is approximate, but we're looking for year-round "fatties" here. For this purpose we're also ignoring the many fatty non-semiaquatic mammals -- such as badgers, bears, pigs, camels, and hedgehogs -- just as AAT/H proponents virtually always do.

    When you point out that semiaquatic mammals are often hairy, AAT/H proponents sometimes claim that of course our early ancestors were tropical and only cold weather semiaquatic mammals are hairy. This is not true, so to point this out in the full list I've marked the tropical semiaquatic mammals, or those that range into the tropics or subtropics. Contrary to the common AAT/H claim, except for cetaceans, sirenia, and hippos they're all hairy.


    How many times has aquaticism or semiaquaticism arisen among extant mammals?

    I've got the full list of all aquatic and semiaquatic mammal species at the bottom of this page, and it's pretty long, but first there's the question of how many times hairlessness evolved among aquatic and semiaquatic mammals. Since AAT/H proponents typically do their best to ignore hairy aquatic and semiaquatic mammals entirely they never really get around to this question, but it's interesting. What makes it interesting is that there are two reasons the AAT/H claim that water use begets hairlessness falls down.

    The first is the degree of water use; even if you assume that water use rather than body size and thermoregulation is behind all these species' hairlessness you have only species which have been aquatic for tens of millions of years and which are highly aquatic, except for the hippos. With hippos it's generally acknowledged that the well known physics of body volume vs. surface is behind their hairlessness. For those who aren't aware of how this works, it's the same principle that a radiator works on; all those fins on a radiator dissipate heat because there's more surface area to volume. Large animals with stocky bodies tend to retain heat, and they're helped by losing that hair. Examples are hippos, rhinos, and elephants. Further evidence that this is why they're relatively hairless is the ice age far north versions of the elephant and the rhino, the wooly mammoth and the wooly rhinoceros. Of course cetaceans and sirenia are also almost all very large and have relatively small limbs in the form of flippers; they'd have a big problem with getting rid of heat if they weren't in the water virtually all the time, and in fact when these animals are transported or beached a major problem for them is overheating. Whether their ancestors lost body hair because of swimming (which seems especially unlikely in the case of slow-moving sirenia) or just body size is not known.

    The other question is how commonly hairlessness is connected with aquaticism at all. Even if you just add up how many aquatic and semiaquatic mammal species are hairless, the total is less than half. But it's just not possible that each and every present-day aquatic and semiaquatic mammal independently evolved aquaticness. We know that many of them have aquatic ancestors and have evolved from a long line of aquatic ancestors, and this is especially true for those which are hairless; it's incredibly unlikely that their aquatic ancestors were hairy for tens of millions of aquatic years and then after diverging they all became hairless. And of course even if that incredibly unlikely scenario were the case it would hurt rather than help the AAT/H argument. So the question is how many times has aquaticness evolved, and in the context of the AAT/H claims, how many times for hairless mammals and how many times for hairy mammals.

    So here's the answer:

    Almost certainly no more than 3 times for hairless mammals as opposed to almost certainly at least 29 times for hairy mammals.

    This means, contrary to the common AAT/H claim, comparisons with other mammals do not support the idea that living a semiaquatic lifestyle likely to make you "hairless". It's actually rare that hairlessness and aquaticism are correlated. The AAT/H claim already falters badly just by the existence of many hairy semiaquatic mammals. It falters even more from the existence of hairy tropical semiaquatic mammals. It falters even more when you see that among aquatic and semiaquatic mammals the only hairless ones are either very large and almost certainly hairless due to the physics of thermoregulation, or are fully aquatic and have been for tens of millions of years, longer than hominids of any sort have existed. Or they are both very large and fully aquatic for tens of millions of years. When you then go further than merely counting the hairy and non-hairy species and instead look at how many times aquaticism arose in mammals, and you separate that list into hairy and not hairy, it's just devastating. No wonder none of the idea's proponents has done this obvious test.

    Another AAT/H claim brought down by examining relatively easy to find facts is the idea that water makes mammals "fatties. This is because the aquatic "fatties" are all cetaceans, sirenia, and pinnipeds, all aquatic for tens of millions of years.

    I've had to make some educated guesses regarding some of the hairy semiaquatic mammal groups and whether closely related mammals diverged before or after becoming semiaquatic, and whenever this was necessary I decided to to help the AAT/H case by erring on the side of assuming fewer cases of semiaquaticism evolving in mammals which remain hairy. For instance the European and North American minks are different genuses, but it seems likely enough they diverged after becoming semiaquatic. I've lumped them together as one instance of semiaquaticism arising. The muskrat and the round-tailed muskrat, despite the common names, are fairly different animals, and it seems likely enough they did evolve semiaquaticism separately, but they are the closest related among the Arvicola and in this list I assume the opposite: that they diverged after becoming semiaquatic, so they too are also lumped together as one instance of semiaquaticism arising. There are notes after those groups of species I've done this for.


    Here's the lists which show how often aquaticism and semiaquaticism arose in mammals; each instance of aquaticism or semiaquaticism arising is marked with a "+" preceding the species or group of species:

    Non-hairy Aquatic or Semiaquatic Mammals

    Aquaticness evolved very probably no more than 3 times, as each group branched out from a mammal which had habits etc. very much like the present-day mammals in the group and all mammals in the group are similar in habits etc.

    + Cetaceans (whales, dolphins, porpoises)
    + Sirenia (manatees, dugongs, sea cows)
    + Hippopotamidae (hippopotamus)


    Hairy Aquatic or Semiaquatic Mammals

    Aquaticness evolved at least 29 times -- marked "+" if the group has an evolutionary past distinct from others in the list and/or has closely related members which are not semiaquatic, esp. if a majority of the members of the group are not semiaquatic.

    In those groups marked with a note there is some possibility that semiaquaticness evolved separately in the groups mentioned but for the sake of the estimate of how many times semiaquaticness arose I've assumed they did not; this makes the possible error on the side of fewer cases of semiaquaticism evolving in mammals which remain hairy.

      Monotremata
    +Ornithorhynchidae (platypus)
      Didelphidae
    +Didelphinae (water opossum)
      Soricomorpha
      Soricidae (shrews)
    +Soricinae (red-toothed shrews)
       Nectogale (elegant water shrew)
       Neomys (Eurasian water shrew)
       Nectogalini (Old World water shrews)
       Chimarrogale (Asiatic water shrews)
      note: it's possible but certainly far from definite that Nectogale, Neomys, and Nectogalini diverged from a semiaquatic ancestor; they're all in Nectogalini but so are many non-semiaquatic shrews
      Tenrecidae
    +Potamogalinae (otter shrews)
    +Limnogale (aquatic tenrec)
      Talpidae
    +Talpinae (Pyrenean desman)
    +Scalopinae (star-nosed mole)
      Carnivora
      Canidae
      Caniformia
    +Speothos (bush dog)
      Mustelidae
    +Lutrinae (otters)
    +Neovison (North American minks)
    +Mustela (European mink)
      note: I had previously listed Neovision and Mustela as one instance of becoming aquatic, given what I thought was the possibility that North American and European minks may have diverged after becoming semiaquatic. I've learned since from Henry Pihlström (personal communication) that molecular data shows they "are not particularly closely related; they belong to independently evolved semiaquatic lineages", and that in fact based on MtDNA "nowadays many authors feel that the American minks need to be separated into a different genus" (Henry Pihlström, personal communication).
      Ursidae
    +Ursus maritimus (polar bear)
      Viverridae (civets, genets)
      Viverrinae
    +Genetta; aka Osbornictis (aquatic genet)
      Hemigalinae
    +Cynogale (otter civet)
      Herpestidae (mongoose)
    +Atilax (marsh mongoose)
    +Pinnipeds (seals and walrus)
      Artiodactyla
      Bovidae
    +Reduncinae (lechwe)
    +Bovinae (sitatunga)
      Rodentia
      Hystricomorpha
    +Myocastoridae (nutria)
    +Caviidae (capybara)
    +Thryonomyidae (cane rats)
      Castorimorpha
    +Castoridae (beavers)
      Myomorpha
      Cricetidae
      Arvicolinae
    +Arvicola (water voles)
    +Ondatra (muskrat)
      Neofiber (round-tailed muskrat)
      note: Ondatra and Neofiber may have evolved aquaticness separately but they are the closest related among the Arvicola; this list assumes they diverged after becoming semiaquatic which may be incorrect
      Sigmodontinae
    +Neusticomys (fish-eating rats)
      Ichthyomys (crab-eating rats)
      Antomys (fish-eating rat)
      Rheomys (Central American water mice)
      note: it's possible Neusticomys, Ichthyomys, Antomys, and Rheomys diverged from a semiaquatic ancestor
    +Chibchanomys (water mice)
      Holochilus (the web-footed rats)
      Nectomys (the Neotropical water rats)
      Oryzomys (marsh rice rat)
      Amphinectomys (Ucayali water rat)
      note: it's possible but certainly far from definite that Chibchanomys, Holochilus, Nectomys, Oryzomys, and Amphinectomys all diverged from a semiaquatic ancestor; they're all in Ichthyomyini but so are many non-semiaquatic rodents
    +Scapteromys
      Muridae
      Murinae
    +Colomys (African water rat)
    +Nilopegamys (amphibious rat)


    Full list of aquatic and semi-aquatic mammals

    Compiled from ICUN Red List and ADW (Animal Diversity Web, University of Michigan Museum of Zoology, plus extras from Table 1.4 by Henry Pihlström, "Introduction: On Becoming Aquatic", in Sensory Evolution on the Threshold: Adaptations in Secondarily Aquatic Vertebrates edited by J. G. M. Thewissen and Sirpa Nummela, Feb. 2008, University of California Press

    List totals 203 species

    Note: * = tropical (or somewhat, with explanation -- ie. range extends into tropical or subtropical regions)

    Carnivora

    Polar bear (Ursus maritimus)
    Sea otter (Enhydra lutris)
    Marine otter (Lontra felina)
    Eurasian otter (Lutra lutra)
    Hairy-nosed otter (Lutra sumatrana) *
    Spotted-necked otter (Hydrictis maculicollis) *
    Smooth-coated otter (Lutrogale perspicillata) *
    North American River Otter (Lontra canadensis) * (ranges from far north down to Gulf coast to the southern tip of Florida)
    Southern river otter (Lontra provocax)
    Neotropical river otter (Lontra longicaudis) *
    Giant otter (Pteronura brasiliensis) *
    African clawless otter (Aonyx capensis) *
    Oriental small-clawed otter (Aonyx cinerea) *
    American mink (Neovison vison) * (ranges from far north down to Gulf coast to the southern tip of Florida)
    Sea mink (Neovison macrodon) status: extinct since 1894, northeastern North America, hunted for fur
    European mink (Mustela lutreola)
    Otter Civet (Cynogale bennettii) *
    Bush dog (Speothos venaticus) * (see notes at bottom of list)
    Aquatic genet (Genetta piscivora) *
    Marsh mongoose (Atilax paludinosus) *

    Bovidae

    Kafue Lechwe (Kobus leche kafuensis Haltenorth) (antelope) *
    Sitatunga (Tragelaphus spekii) (antelope) *

    Monotremes

    Platypus (Ornitho rhynchus anatinus)

    Didelphimorphia (American marsupials)

    Water Opossum (Chironectes minimus) * (can also be found at high elevations)

    Soricomorpha

    Star-nosed mole (Condylura cristata)
    Water shrew (Sorex palustris)
    Pacific water shrew (Sorex bendirii)
    Pyrenean Desman (Galemys pyrenaicus)
    Aquatic Tenrec (Limnogale mergulus)
    Nimba otter-shrew (Micropotamogale lamottei) *
    Rwenzori Otter Shrew (Micropotamogale ruwenzorii)
    Giant Otter Shrew (Potamogale velox) *
    Sumatran Water Shrew (Chimarrogale sumatrana) *
    Malayan Water Shrew (Chimarrogale hantu) *
    Himalayan Water Shrew, Elegant Water Shrew (Chimarrogale himalayica)
    Bornean Water Shrew (Chimarrogale phaeura) *
    Japanese Water Shrew (Chimarrogale platycephalus)
    Chinese Water Shrew (Chimarrogale styani)
    Elegant Water Shrew (Nectogale elegans)
    Transcaucasian Water Shrew (Neomys teres)
    Mediterranean Water Shrew (Neomys anomalus)
    Eurasian Water Shrew (Neomys fodiens)

    Rodentia

    Capybara (Hydrochoerus hydrochaeris) *
    American Beaver (Castor canadensis)
    European beaver (Castor fiber)
    Nutria or coypu (Myocastor coypus)
    Greater cane rat (Thryonomys swinderianus) *
    Peruvian Fish-eating Rat (Neusticomys peruviensis)
    Venezuelan Fish-eating Rat (Neusticomys venezuelae) *
    Montane Fish-eating Rat (Neusticomys monticolus)
    Musso's Fish-eating Rat (Neusticomys mussoi)
    Oyapock's Fish-eating Rat (Neusticomys oyapocki) *
    Tweedy's Crab-eating Rat (Ichthyomys tweedii)
    Crab-eating Rat (Ichthyomys hydrobates)
    Pittier's Crab-eating Rat (Ichthyomys pittieri)
    Stolzmann's Crab-eating Rat (Ichthyomys stolzmanni)
    Ecuador fish-eating rat (Anotomys leander)
    Mexican water mouse (Rheomys mexicanus)
    Goldman's water mouse (Rheomys raptor) -- may be slightly less aquatic than other Rheomys
    Underwood's water mouse (Rheomys underwoodi)
    Las Cajas ichthyomyine (Chibchanomys orcesi)
    Chibchan water mouse (Chibchanomys trichotis)
    Web-footed Marsh Rat (Holochilus brasiliensis) * (wide-range)
    Chaco Marsh Rat (Holochilus chacarius)
    South American Water Rat (Nectomys squamipes) *
    Trinidad Water Rat (Nectomys palmipes) *
    Magdalena Nectomys (Nectomys magdalenae) *
    Western Amazonian Nectomys (Nectomys apicalis) *
    Swamp Rat (Scapteromys tumidus)
    Argentine swamp rat (Scapteromys aquaticus)
    Australian water rat (Hydromys chrysogaster)
    Earless Water Rat (Crossomys moncktoni)
    Eurasian Water Vole (Arvicola amphibius)
    Southern Water Vole (Arvicola sapidus)
    Water Vole (Microtus richardsoni)
    Round-tailed muskrat (Neofiber alleni) * (south Florida)
    Muskrat (Ondatra zibethicus)
    Ucayali water rat (Amphinectomys savamis) *
    Alfaro's Rice Water Rat (Sigmodontomys alfari) *
    Marsh Rice Rat (Oryzomys palustris) (Gulf coast and to southern tip of Florida)
    African water rat (Colomys goslingi) *
    Amphibious rat (Nilopegamys plumbeus)

    Hippopotamidae

    Hippopotamus (Hippopotamus amphibius) *
    Pygmy hippopotamus (Choeropsis liberiensis) *

    Pinnipeds (seals and walrus)

    Crabeater Seal (Lobodon carcinophaga)
    Bearded Seal (Erignathus barbatus)
    Hooded Seal (Cystophora cristata)
    Northern Fur Seal (Callorhinus ursinus)
    South American Fur Seal (Arctocephalus australis)
    New Zealand Fur Seal (Arctocephalus forsteri)
    Galápagos Fur Seal (Arctocephalus galapagoensis) *
    Antarctic Fur Seal (Arctocephalus gazella)
    Juan Fernández Fur Seal (Arctocephalus philippii)
    Afro-Australian Fur Seal (Arctocephalus pusillus)
    Guadalupe Fur Seal (Arctocephalus townsendi) * (along Baja Mexico coastline)
    Subantarctic Fur Seal (Arctocephalus tropicalis)
    Caspian Seal (Pusa caspica)
    Ringed Seal (Pusa hispida)
    Baikal Seal (Pusa sibirica)
    Spotted Seal (Phoca largha)
    Harbour Seal (Phoca vitulina)
    Ross Seal (Ommatophoca rossii)
    Harp Seal (Pagophilus groenlandicus)
    New Zealand Sea Lion (Phocarctos hookeri)
    Australian Sea Lion (Neophoca cinerea)
    Californian Sea Lion (Zalophus californianus)
    Japanese Sea Lion (Zalophus japonicus) status: extinct
    South American Sea Lion (Otaria flavescens)
    Galápagos Sea Lion (Zalophus wollebaeki)
    Steller Sea Lion (Eumetopias jubatus)
    Northern Elephant Seal (Mirounga angustirostris)
    Southern Elephant Seal (Mirounga leonina)
    Weddell Seal (Leptonychotes weddellii)
    Grey Seal (Halichoerus grypus)
    Ribbon Seal (Histriophoca fasciata)
    Leopard Seal (Hydrurga leptonyx)
    Mediterranean Monk Seal (Monachus monachus) * (Mediterranean coast, Europe and northern Africa)
    Hawaiian Monk Seal (Monachus schauinslandi) *
    Caribbean Monk Seal (Monachus tropicalis) status: extinct *
    Walrus (Odobenus rosmarus)

    Cetaceans (whales, dolphins, and porpoises)

    Bowhead Whale (Balaena mysticetus)
    Common Minke Whale (Balaenoptera acutorostrata)
    Antarctic Minke Whale (Balaenoptera bonaerensis)
    Sei Whale (Balaenoptera borealis)
    Bryde's Whale (Balaenoptera edeni)
    Blue Whale (Balaenoptera musculus)
    Omura’s Whale (Balaenoptera omurai)
    Fin Whale (Balaenoptera physalus)
    Arnoux's Beaked Whale (Berardius arnuxii)
    Baird's Beaked Whale (Berardius bairdii)
    Pygmy Right Whale (Caperea marginata)
    Commerson's Dolphin (Cephalorhynchus commersonii)
    Chilean Dolphin (Cephalorhynchus eutropia)
    Heaviside's Dolphin (Cephalorhynchus heavisidii)
    Hector's Dolphin (Cephalorhynchus hectori)
    Beluga (Delphinapterus leucas)
    Long-beaked Common Dolphin (Delphinus capensis)
    Short-beaked Common Dolphin (Delphinus delphis)
    Southern Right Whale (Eubalaena australis)
    North Atlantic Right Whale (Eubalaena glacialis)
    North Pacific Right Whale (Eubalaena japonica)
    Pygmy Killer Whale (Feresa attenuata)
    Short-finned Pilot Whale (Globicephala macrorhynchus)
    Long-finned Pilot Whale (Globicephala melas)
    Risso's Dolphin (Grampus griseus)
    North Atlantic Bottlenose Whale (Hyperoodon ampullatus)
    Southern Bottlenose Whale (Hyperoodon planifrons)
    Indo-pacific Beaked Whale (Indopacetus pacificus)
    Boto (Inia geoffrensis)
    Pygmy Sperm Whale (Kogia breviceps)
    Dwarf Sperm Whale (Kogia sima)
    Fraser's Dolphin (Lagenodelphis hosei)
    Atlantic White-sided Dolphin (Lagenorhynchus acutus)
    White-beaked Dolphin (Lagenorhynchus albirostris)
    Peale's Dolphin (Lagenorhynchus australis)
    Hourglass Dolphin (Lagenorhynchus cruciger)
    Pacific White-sided Dolphin (Lagenorhynchus obliquidens)
    Dusky Dolphin (Lagenorhynchus obscurus)
    Baiji (Lipotes vexillifer)
    Northern Right Whale Dolphin (Lissodelphis borealis)
    Southern Right Whale Dolphin (Lissodelphis peronii)
    Humpback Whale (Megaptera novaeangliae)
    Sowerby's Beaked Whale (Mesoplodon bidens)
    Andrew's Beaked Whale (Mesoplodon bowdoini)
    Hubbs' Beaked Whale (Mesoplodon carlhubbsi)
    Blainville's Beaked Whale (Mesoplodon densirostris)
    Gervais' Beaked Whale (Mesoplodon europaeus)
    Ginkgo-toothed Beaked Whale (Mesoplodon ginkgodens)
    Gray's Beaked Whale (Mesoplodon grayi)
    Hector's Beaked Whale (Mesoplodon hectori)
    Strap-toothed Whale (Mesoplodon layardii)
    True's Beaked Whale (Mesoplodon mirus)
    Perrin's Beaked Whale (Mesoplodon perrini)
    Pygmy Beaked Whale (Mesoplodon peruvianus)
    Stejneger's Beaked Whale (Mesoplodon stejnegeri)
    Spade-toothed Whale (Mesoplodon traversii)
    Narwhal (Monodon monoceros)
    Melon-headed Whale (Peponocephala electra)
    Sperm Whale (Physeter macrocephalus)
    Finless Porpoise (Neophocaena phocaenoides)
    Irrawaddy Dolphin (Orcaella brevirostris)
    Australian Snubfin Dolphin (Orcaella heinsohni)
    Killer Whale (Orcinus orca)
    Spectacled Porpoise (Phocoena dioptrica)
    Harbour Porpoise (Phocoena phocoena)
    Vaquita (Phocoena sinus)
    Burmeister's Porpoise (Phocoena spinipinnis)
    White-flanked Porpoise (Phocoenoides dalli)
    South Asian River Dolphin (Platanista gangetica)
    Franciscana (Pontoporia blainvillei)
    False Killer Whale (Pseudorca crassidens)
    Guianian River Dolphin (Sotalia fluviatilis)
    Indo-Pacific Humpbacked Dolphin (Sousa chinensis)
    Atlantic Humpbacked Dolphin (Sousa teuszii)
    Pantropical Spotted Dolphin (Stenella attenuata)
    Clymene Dolphin (Stenella clymene)
    Striped Dolphin (Stenella coeruleoalba)
    Atlantic Spotted Dolphin (Stenella frontalis)
    Spinner Dolphin (Stenella longirostris)
    Rough-toothed Dolphin (Steno bredanensis)
    Shepherd's Beaked Whale (Tasmacetus shepherdi)
    Indo-pacific Bottlenose Dolphin (Tursiops aduncus)
    Common Bottlenose Dolphin (Tursiops truncatus)
    Cuvier's Beaked Whale (Ziphius cavirostris)

    Sirenia

    South American Manatee (Trichechus inunguis)
    West Indian Manatee (Trichechus manatus)
    West African Manatee (Trichechus senegalensis)
    Dugong (Dugong dugon)
    Steller's Sea Cow (Hydrodamalis gigas) status: extinct


    Others not aquatic enough to be called semi-aquatic, like fishing cat; proboscis monkey; several species of mouse deer which regularly dive into water and "swim" (or rather walk) underwater, remaining there for some time (plus/minus 1 hour, albeit coming up for air during that time, breathholding for 3-4 minutes at a time) to escape predators (Greater Oriental ChevrotainTragulus napu, White-spotted Chevrotain Moschiola meminna, and Water ChevrotainHyemoschus aquaticus), they remain underwater for some time; tapirs; babyrussa (not all species of babyrussa are relatively hairless as is commonly thought); swamp rabbits and marsh rabbits (both of which are species of North American cottontail); pacas (2 species, which often try to escape predators by swimming)

    Bush dog notes: "It is adapted to a semi-aquatic life amongst the forest.."; "In captivity Speothos has been found to enter water readily and to swim well -- 'with otter-like ease', according to Hershkovitz (1969)"; "They are semiaquatic and can 'dive and swim underwater with great facility'. (Nowak, 1999, Walker's Mammals of the World." ICUN does not call this species semiaquatic but besides the above several academic papers also point out that it is semiaquatic.

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