Marine mammals, which include seals, whales, dolphins, porpoises, manatees, dugongs, marine otters, walruses, and polar bears, form a diverse group of 129 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the aquatic environment for feeding. The level of dependence on the aquatic environment for existence varies considerably with species. For example, dolphins and whales are completely dependent on the marine environment for all stages of their life, whereas seals feed in the ocean, but breed on land.
Marine mammals can be subdivided into four recognized groups; cetaceans (whales, dolphins, and porpoises), pinnipeds (seals, sea lions and walruses), sirenians (manatees and dugongs), and fissipeds, which are the group of carnivores with separate digits (the polar bear, and two species of otter). 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. While the number of marine mammals is small compared to those found on land, their total biomass is large. They play important roles in maintaining marine ecosystems, especially through regulation of prey populations. These two factors make them an integral component of the marine environment. This is of particular concern considering 23% of marine mammal species are currently threatened.
- 1 Taxonomy
- 2 Diversity, distribution and habitat
- 3 Anatomy and physiology
- 4 Threats
- 5 See also
- 6 References
- 7 Other references
- 8 External links
Mammals have returned to the water in at least nine separate evolutionary lineages: Cetacea, Sirenia, Desmostylia, Pinnipedia, Ursus maritimus (polar bear), Kolponomos (marine bear), Thalassocnus (aquatic sloth), Enhydra lutris (sea otter) and Lontra feline (marine otter); the eutriconodont Ichthyoconodon might have also been marine in habits. Four of these lineages are extinct (Desmostylia; Kolponomos; Thalassocnus, Ichthyoconodon). Despite the diversity in morphology seen between groups, improving foraging efficiency has been the main driver in the evolution in these lineages. Today, marine mammals belong to one of three orders: Cetartiodactyla, Sirenia, or Carnivora.
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. Use of Order Cetartiodactyla, instead of Cetacea with Suborders Odontoceti and Mysticeti, is favored by most evolutionary mammalogists working with molecular data and is supported the IUCN Cetacean Specialist Group and by Taxonomy Committee of the Society for Marine Mammalogy, the largest international association of marine mammal scientists in the world. Some others, including many marine mammalogists and paleontologists, favor retention of Order Cetacea with the two suborders in the interest of taxonomic stability.
Within the Order Sirenia are the manatees and the dugongs. Order Carnivora contains the pinnipeds (sealions, walruses and seals), the polar bear (Ursus maritimus), and the two otters (Enhydra lutris and Lontra feline).
Classification of living species
- Order Cetartiodactyla
- Cetacea (unranked)
- Mysticeti (baleen whales)
- Odontoceti (toothed whales)
- Family Physeteridae (sperm whale) = 1 species
- Family Kogiidae (pygmy and dwarf sperm whales = 1 genus; 2 species
- Family Monodontidae (narwhal and beluga) = 2 genera; 2 species
- Family Ziphiidae (beaked whales) = 6 genera; 21 species
- Family Delphinidae (oceanic dolphins) = 17 genera; 38 species
- Family Phocoenidae (porpoises) = 2 genera; 7 species
- Family Platanistidae (South Asia river dolphin) = 1 species
- Family Iniidae (boto) 1 species
- Family Pontoporiidae (franciscana) = 1 species
- Order Sirenia (sea cows)
- Order Carnivora (carnivores):
- Suborder Pinnipedia (sealions, walruses, seals)
Diversity, distribution and habitat
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. The variation observed in range size is a result of the different ecological requirements of each species and their ability to cope with a broad range of environmental conditions. There is a high degree of overlap between marine mammal species richness and areas of human impact on the environment which is of concern.
Anatomy and physiology
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. Marine mammals are adept at thermoregulation using dense fur or blubber to reduce heat loss; as well as circulatory adjustments to conserve their body temperature (counter-current heat exchangers); torpedo shaped bodies, reduced appendages, and large size to prevent heat loss.
Most marine mammals are hypoosmotic and as a result they are constantly losing water to the surrounding environment. They have evolved a number of mechanisms to overcome this, but most retain their water by using highly efficient kidneys, that can concentrate urine. Marine mammals are able to dive for long periods of time. Both pinnipeds and cetaceans have large and complex blood vessel systems which serve to store oxygen to support deep diving. Other important reservoirs include muscles, blood, and the spleen which all have the capacity to hold a high concentration of oxygen. Other features include bradycardia (reduced heart rate), and vasoconstriction (shunts most of the oxygen to vital organs such as the brain and heart) also assist with extended diving and oxygen deprivation.
If oxygen is depleted, marine mammals can access substantial reservoirs of glycogen that support anaerobic glycolysis of the cells involved during conditions of systemic hypoxia associated with prolonged submersion. Sound travels differently through water therefore marine mammals have developed a number of ways to ensure effective communication, prey capture, and predator detection. The most notable adaptation is the development of echolocation in whales and dolphins. Lastly, Marine mammals have evolved a number features for feeding, which are mainly seen in their dentition. For example, the cheek teeth of pinniped and odontocetes are designed specifically to capture fish and squid. In contrast, Mysticetes have evolved baleen plates to filter feed plankton and small fish from the water.
Marine mammals were hunted by coastal aboriginal humans historically for food and other resources. These subsistence hunts occur in Canada, Greenland, Indonesia, Russia, the United States, and several nations in the Caribbean. Under the terms of the 1986 moratorium on whaling, the International Whaling Commission (IWC) allows whaling carried out by aboriginal groups if it occurs on a subsistence basis; however hunts in Canada and Indonesia are conducted outside the authority of the IWC. The effects of these are only localised, as hunting efforts were on a relatively small scale. Later, commercial hunting was developed and marine mammals were heavily exploited. This led to the extinction of the Steller's Sea Cow and the Caribbean monk seal. Today, populations of species that were historically hunted, such as blue whales (Balaenoptera musculus musculus and B. m. brevicauda), and the North Pacific right whale (Eubalaena japonica), are much lower compared to their pre-exploited levels. 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.
Despite the fact commercial whaling is generally a thing of the past since the passage of the International Whaling Commission’s moratorium on commercial whaling, a number of marine mammals are still subject to direct hunting. There are only two nations remaining which sanction commercial whaling: Norway, where several hundred northeastern North Atlantic minke whales are harvested each year; and Iceland, were quotas of 150 fin whales and 100 minke whales per year are set under objection to an ongoing moratorium established by the International Whaling Commission in 1986. Japan also harvests several hundred Antarctic and North Pacific minke whales each year under the guise of scientific research. However, the illegal trade of whale and dolphin meat is a significant market in some countries. Seals and sealions are also still hunted commercially in some countries, including Canada, Greenland, Iceland, Norway, Russia, Finland and Sweden.
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. While larger cetaceans are capable of dragging nets with them, the nets sometimes remain tightly attached to the individual and can impede the animal from feeding sometimes leading to starvation. Abandoned or lost nets and lines cause mortality through ingestion or entanglement. 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. In particular, fast commercial vessels such as container ships can cause major injuries or death when they collide with marine mammals. Collisions occur both with large commercial vessels and recreational boats and cause injury to whales or smaller cetaceans. The critically endangered northern right whale is particularly affected by vessel strikes. Tourism boats designed for whale and dolphin watching can also negatively impact on marine mammals by interfering with their natural behavior.
Habitat loss and degradation
Habitat degradation is caused by a number of human activities. Marine mammals that live in coastal environments are most likely to be affected by habitat degradation and loss. 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. For example, extensive shellfish aquaculture takes up valuable space used by coastal marine mammals for important activities such as breeding, foraging and resting.
Competition/conflict with fisheries
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.
Competition/conflict with aquaculture
Shellfish aquaculture takes up space so in effect creates competition for space. However, there is little direct competition for aquaculture shellfish harvest. On the other hand, marine mammals regularly take finfish from farms, which creates significant problems for marine farmers. While there are usually legal mechanisms designed to deter marine mammals, such as anti-predator nets or harassment devices, individuals are often illegally shot.
Contaminants that are discharged into the marine environment accumulate in the bodies of marine mammals when they are stored unintentionally in their blubber along with energy. 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. For example, these can cause disruptive effects on endocrine systems; impair the reproductive system, and lower the immune system of individuals, leading to a higher number of deaths. Other pollutants such as oil, plastic debris and sewage threaten the livelihood of marine mammals.
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. Underwater noise is generated from shipping, the oil and gas industry, research, and military use of sonar and oceanographic acoustic experimentation. Acoustic harassment devices and acoustic deterrent devices used by aquaculture facilities to scare away marine mammals emit loud and noxious underwater sounds.
Global climate change
Two changes to the global atmosphere due to anthropogenic activity threaten marine mammals. The first is increases in ultraviolet radiation due to ozone depletion, and this mainly affects the Antarctic and other areas of the southern hemisphere. An increase in ultraviolet radiation has the capacity to decrease phytoplankton abundance, which forms the basis of the food chain in the ocean. The second effect of global climate change is global warming due to increased carbon dioxide levels in the atmosphere. Raised sea levels, sea temperature 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.
- Pompa, S.; Ehrlich, P. R.; Ceballos, G. (2011-08-16). "Global distribution and conservation of marine mammals". Proceedings of the National Academy of Sciences 108 (33): 13600–13605. doi:10.1073/pnas.1101525108.
- Jefferson, T. A. , Webber, M. A. & Pitman, R. L. (2009) Marine Mammals of the World A Comprehensive Guide to their Identification London ; Burlington, MA: Academic ISBN 978-0-12-383853-7. 7–16
- Kaschner, K.; Tittensor, D. P.; Ready, J.; Gerrodette, T.; Worm, B. (2011). "Current and Future Patterns of Global Marine Mammal Biodiversity". PLoS ONE 6 (5): e19653. doi:10.1371/journal.pone.0019653.
- Schipper, J.; Chanson, J. S.; Chiozza, F.; Cox, N. A.; Hoffmann, M.; Katariya, V.; Lamoreux, J.; Rodrigues, A. S. L.; Stuart, S. N.; Temple, H. J.; Baillie, J.; Boitani, L.; Lacher, T. E.; Mittermeier, R. A.; Smith, A. T.; Absolon, D.; Aguiar, J. M.; Amori, G.; Bakkour, N.; Baldi, R.; Berridge, R. J.; Bielby, J.; Black, P. A.; Blanc, J. J.; Brooks, T. M.; Burton, J. A.; Butynski, T. M.; Catullo, G.; Chapman, R.; et al. (2008). "The Status of the World's Land and Marine Mammals: Diversity, Threat, and Knowledge" (PDF). Science 322 (5899): 225–30. doi:10.1126/science.1165115. PMID 18845749.
- Uhen, M. D. (2007). "Evolution of marine mammals: Back to the sea after 300 million years". The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology 290 (6): 514–22. doi:10.1002/ar.20545. PMID 17516441.
- Geisler, Jonathan H.; Uden, Mark D. (2005). "Phylogenetic Relationships of Extinct Cetartiodactyls: Results of Simultaneous Analyses of Molecular, Morphological, and Stratigraphic Data". Journal of Mammalian Evolution 12 (1–2): 145–160. doi:10.1007/s10914-005-4963-8.
- Graur, D.; Higgins, G. (1994). "Molecular evidence for the inclusion of cetaceans within the order Artiodactyla" (PDF). Molecular Biology and Evolution 11 (3): 357–364. PMID 8015431.
- Agnarsson, I.; May-Collado, LJ. (2008). "The phylogeny of Cetartiodactyla: the importance of dense taxon sampling, missing data, and the remarkable promise of cytochrome b to provide reliable species-level phylogenies". Mol Phylogenet Evol. 48 (3): 964–985. doi:10.1016/j.ympev.2008.05.046. PMID 18590827.
- Price, SA.; Bininda-Emonds, OR.; Gittleman, JL. (2005). "A complete phylogeny of the whales, dolphins and even-toed hoofed mammals – Cetartiodactyla". Biol Rev Camb Philos Soc. 80 (3): 445–473. doi:10.1017/s1464793105006743. PMID 16094808.
- Montgelard, C.; Catzeflis, FM.; Douzery, E. (1997). "Phylogenetic relationships of artiodactyls and cetaceans as deduced from the comparison of cytochrome b and 12S RNA mitochondrial sequences". Molecular Biology and Evolution 14 (5): 550–559. doi:10.1093/oxfordjournals.molbev.a025792. PMID 9159933.
- Spaulding, M.; O'Leary, MA.; Gatesy, J. (2009). "Relationships of Cetacea -Artiodactyla- Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution". PLoS ONE 4 (9): e7062. Bibcode:2009PLoSO...4.7062S. doi:10.1371/journal.pone.0007062. PMC 2740860. PMID 19774069.
- Cetacean Species and Taxonomy. iucn-csg.org
- "The Society for Marine Mammalogy's Taxonomy Committee List of Species and subspecies".
- Berta, A & Sumich, J. L. (1999) Marine mammals: evolutionary biology. San Diego: Academic Press ISBN 0-12-093225-3
- Pfeiffer, Carl J. (1997). "Renal cellular and tissue specializations in the bottlenose dolphin (Tursiops truncatus) and beluga whale (Delphinapterus leucas)" (PDF). Aquatic Mammals 23 (2): 75–84. Retrieved 2014-04-25.
- Lockyer, Christina (1991). "Body composition of the sperm whale, Physeter cation, with special reference to the possible functions of fat depots" (PDF). Journal of the Marine Research Institute 12 (2). ISSN 0484-9019. Retrieved 2014-04-25.
The significant levels of carbohydrate, probably mostly in the form of glycogen, in both blubber and muscle, may represent an instant form of energy for diving via anaerobic glycolysis.
- Hochachka, P.; Storey, K. (1975). "Metabolic consequences of diving in animals and man". Science 187 (4177): 613–621. Bibcode:1975Sci...187..613H. doi:10.1126/science.163485. ISSN 0036-8075. PMID 163485.
- Whitehead, H., Reeves, R. R. & Tyack, P. L. (1999) Science and the conversation,protection,and management of wild cetaceans (eds) J. Mann , R. C. Connor, P.L Tyack & H Whitehead in Cetacean societies : field studies of dolphins and whales. Chicago : University of Chicago Press ISBN 0-226-50340-2
- Clapham, P. J.; Young, S. B.; Brownell, R. L. (1999). "Baleen whales: Conservation issues and the status of the most endangered populations". Mammal Review 29: 37–62. doi:10.1046/j.1365-2907.1999.00035.x.
- "History of Whaling". The Húsavík Whale Museum. Retrieved May 16, 2010.
- "Modern Whaling". The Húsavík Whale Museum. Retrieved May 16, 2010.
- Baker, C. S.; Cipriano, F.; Palumbi, S. R. (1996). "Molecular genetic identification of whale and dolphin products from commercial markets in Korea and Japan". Molecular Ecology 5 (5): 671–685. doi:10.1111/j.1365-294X.1996.tb00362.x.
- Perrin, W. F. (1994) "Status of species" in Randall R. Reeves and Stephen Leatherwood (eds.) Dolphins, porpoises, and whales: 1994–1998 action plan for the conservation. Gland, Switzerland: International Union for Conservation of Nature and Natural Resources
- Hall, M. A. (1998). "An ecological view of the tuna—dolphin problem: impacts and trade-offs" (PDF). Reviews in Fish Biology and Fisheries 8: 1–34. doi:10.1023/A:1008854816580.
- Anderson, Paul K. (2001). "Marine Mammals in the Next One Hundred Years: Twilight for a Pleistocene Megafauna?". Journal of Mammalogy 82 (3): 623–629. doi:10.1093/jmammal/82.3.623. JSTOR 1383601.
- Wursig, Bernd and Gailey, Glenn A. Marine Mammals and Aquaculture: Conflicts and Potential Resolutions in Robert R Stickney and James P. McVey (eds) Responsible marine aquaculture. Wallingford, Oxon; New York: CABI. ISBN 0-85199-604-3
- Constantine, R.; Brunton, D. H.; Dennis, T. (2004). "Dolphin-watching tour boats change bottlenose dolphin (Tursiops truncatus) behaviour". Biological Conservation 117 (3): 299–307. doi:10.1016/j.biocon.2003.12.009.
- Rosen, D. A.; Trites, A. W. (2000). "Pollock and the decline of Steller sea lions: Testing the junk-food hypothesis". Canadian Journal of Zoology 78 (7): 1243–1250. doi:10.1139/z00-060.
- McAlpine, D. F.; Stevick, P. T.; Murison, L. D. (1999). "Increase in Extralimital Occurrences of Ice-Breeding Seals in the Northern Gulf of Maine Region: More Seals or Fewer Fish?". Marine Mammal Science 15 (3): 906–911. doi:10.1111/j.1748-7692.1999.tb00857.x.
- Hutchins, J. (1996). "Spatial and temporal variation in the density of northern cod and a review of hypoth-eses for the stock's collapse" (PDF). Canadian Journal of Fisheries and Aquatic Science 53 (5): 943–962. doi:10.1139/cjfas-53-5-943.
- Baker, J. R.; Jones, A. M.; Jones, T. P.; Watson, H. C. (1981). "Otter Lutra lutra L. Mortality and marine oil pollution". Biological Conservation 20 (4): 311–321. doi:10.1016/0006-3207(81)90017-3.
- Harwood, J. (2001). "Marine Mammals and their Environment in the Twenty-First Century". Journal of Mammalogy 82 (3): 630–640. doi:10.1644/1545-1542(2001)082<0630:MMATEI>2.0.CO;2. JSTOR 1383602.
- Madronich, S.; McKenzie, R. L.; Björn, L. O.; Caldwell, M. M. (1998). "Changes in biologically active ultraviolet radiation reaching the Earth's surface". Journal of Photochemistry and Photobiology B: Biology 46: 5–19. doi:10.1016/S1011-1344(98)00182-1.
- Simmonds, M. P.; Isaac, S. J. (2007). "The impacts of climate change on marine mammals: Early signs of significant problems". Oryx 41: 19. doi:10.1017/S0030605307001524.
- "Arctic Sea Ice | NRDC". www.nrdc.org. Retrieved 2015-12-08.
- A Bayesian Network Modeling Approach to Forecasting the 21st Century Worldwide Status of Polar Bears
- Perrin WF, Wursig B and Thewissen JGM (2009) Encyclopedia of Marine Mammals Academic Press. ISBN 9780080919935.
- The Society for Marine Mammalogy The largest organization of marine mammalogists in the world.
- Introduction to the Desmostylia Museum of Paleontology, University of California – extinct group of marine mammals