Marine mammals have entered just about all ocean habitats, and several mighty rivers and inshore seas as well. Only the deep abyss is foreign to them, but— remarkably—elephant seals (Mirounga spp.), sperm whales (Physeter macrocephalus), and several other toothed whales can “easily” dive to depths that exceed 1000 m, where it is cold and dark and where the pressure is 100 times and more what we experience on land. Perhaps just as remarkable is the fact that some of these divers, the pinnipeds, are also able to live on land, where they mate, give birth, and molt.
Morphologic, physiologic, and behavioral adaptations to the environments of marine mammals are largely driven by their food, and the habitats of their prey. Although there are various ways that ecological adaptations can be divided, this article does so by several broad-based general habitat types: open ocean, semipelagic, coastal, and riverine feeding and breeding habitats and—for pinnipeds and the polar bear—their obligatory stint on land to breed.
I. Open Ocean
There are two major types of open ocean marine mammals: “surface dwellers” and “deep divers.”
A. Surface Dwellers
Most of the open ocean, or pelagic, smaller toothed whales and dolphins spend their entire lives within about 200 m of the surface. The near-surface environment is low in primary and secondary productivity except in latitudes higher than about 50° north and south of the equator. Therefore, these pelagic cetaceans travel great distances in search of food, often in large herds of hundreds to thousands. The large herds may be for better detection of prey, possible cooperative prey herding, and enhanced detection of predators such as deep water sharks and the larger cousins of dolphins, killer whales (Orcinus orca). All of these capabilities may be enhanced by several species traveling together, in so-called multispecies aggregations. An example in the eastern tropical Pacific (ETP), where a dolphin herd may travel over 1000 km in 1 week, is the co-occurrence of spinner (Stenella longirostris), pantropical spotted (S. attenuate!), and common (Delphinus spp.) dolphins. These dolphins are slim-bodied (or “sleek”), built for speed and long-distance endurance. They do not have the thick blood (packed with red blood cells) so characteristic of deep divers. Instead, they feed on sporadically encountered near-surface fishes and squid, or at night on animals that rise to within several hundred meters of the surface in association with the deep scattering layer (DSL). Their occurrence in large schools has another function: the school is the social, breeding, and calf care-giving unit, and these nomadic wanderers tend to be within their “complete” society at all times. Exceptions are when young males, for example, may form separate bachelor herds or bands or when adult males move among breeding herds (as in sperm whales).
While several species of baleen whales migrate through deep water, they tend to feed on rich areas of invertebrates and fishes that are found more often close to shore. However, others habitually feed in open ocean waters. As is the case for the surface-dwelling odontocetes, baleen whales most often feed within about 200 m of the surface, as none of them are exceptionally deep divers. Blue (Balaenoptera musculns), fin (B. physalus), sei (B. borealis), and Brydes and Eden’s whales (B. brydei and/or edeni) are good examples of oft-pelagic, near-surface feeders. Blue and fin whales tend to feed on euphausiid crustaceans, or krill; whereas sei and Bryde’s whales feed more commonly on shoals of fishes. All of them lunge through their food rapidly. The right (Eubalaena spp.) and bowhead whales (Balaena mysticetus) often surface-skim feed in the open ocean of productive high latitudes, whereas gray whales (Eschrichtius robustus) feed on ampeliscid (tube-dwelling) am-phipods. They do so in waters less than 200 m deep, both near shore and far away from land in the Bering, Chukchi, and Beaufort Seas. Although rorquals are built for speed so that they can lunge into food rapidly, right whales and gray whales tend to the more rotund body shape, or chunky.
Many pinnipeds also feed near the surface and, at times, up to several hundred kilometers from shore. The smaller true seals (such as ringed seals, Pusa hispida, for example) and all of the eared seals are not deep divers and therefore stay near the surface in those generally higher latitude waters where they find themselves in the open sea. Near-surface feeding pinnipeds are not likely to be out on truly oceanic seas further than several hundred kilometers from land. Northern fur seals (Callorhinus ursimis), however, are often found in deep pelagic waters of the north Pacific.
B. Deep Divers
Many of the larger toothed whales and a few true seals dive “deeply,” or below about 500 m. Sperm whales are likely to be the champion divers. They routinely feed at depths around 500 m on fishes and squid, but can also dive to 2,000 m and more in search of the larger truly pelagic squid. Although we know little of the dive capabilities of other deep-diving odontocetes— pilot whales (Globicephala spp.), 20 species of beaked whales (family Ziphiidae), dwarf and pygmy sperm whales (family Kogiidae), and the false killer whale (Pseudorca crassidens) are good examples—it is likely that all of them are capable of greater than 500-m dives as they feed largely on midsized deep water fishes (often of the family Myctophidae) and squids. Curiously, the largest dolphin-like (or delphinid) cetacean, the killer whale, appears to feed without diving deeply. It is possible, but remains unproved, that some smaller toothed whales can evade killer whales by diving down.
The champion pinniped divers are northern and southern elephant seals (Mirounga spp.) as well as the Weddell (Lep-tonychotes iveddellii) and probably several other large true seals. They can (but do not often) dive down to 1000 m and beyond. They feed on fishes and squid at these depths, but it has been surmised that at least some deep dives are “resting dives” as the animals conserve energy while their metabolism is largely shut down at depth. Such possible rest (or “sleep”) may even help them evade detection by predators such as most active sharks, who are not deep divers, and killer whales. Because elephant seals spend only about 15% of their time at the surface, it is not really correct to call them “divers.” Their life is underwater and they are indeed “surfacers” who come up only for life-sustaining air.
All deep divers have adapted physiologically and morphologically for the task. Blood and muscles have changed to hold as much oxygen as possible, and peripheral vasoconstriction and shutting off of nonvital body functions during a dive take place.
II. Semipelagic
Quite a few marine mammals habitually occur in the zone between shallow and deep water, often at the edge of the continental shelf or some other underwater feature. There is high productivity there, caused by upwelling or current systems as sea meets land, and it makes sense that this is a major point of aggregation. Sperm whales off Kaikoura, on the South Island of New Zealand, feed in such a zone near the deep Hikurangi Trench about 10 km out. However, the sperm whales are often within 1-5 km from shore, in productive waters 200 to 600 m in the deep, shore side of the trench. Blue whales of Monterey Bay, California, do so as well, as they take advantage of large stands of krill to enter the area in late summer. Dall’s porpoises (Phocoenoides dalli) are also found in some abundance in Monterey Bay, not as frequently in very shallow nor veiy deep waters, but on the edge of the productive Monterey Canyon. Dozens of species and hundreds of geographic examples could be cited as those that occur in such productive “neither nearshore nor open ocean” zones.
Several dolphins are “semipelagic” in another sense. They seek out deep productive waters in areas close to shore so that they can feed in the open sea yet retreat to the shallows, often into bays and inlets or onto expansive shoals during rest. Spinner dolphins of the tropical islands of the Pacific have such a habit. During the day, they rest and socialize within island bays and lagoons, even entering atolls through narrow passes in some areas. It is believed that nearshore rest is to avoid large oceanic swells and trade winds, as well as predation by large oceanic sharks. At night, these dolphins head out to sea, often only 1-5 km from land off these abruptly rising volcanic islands. The dolphins meet the DSL as it comes to within several hundred meters of the surface at night and thus have a food resource available that these only-average divers could not obtain during the day, when the DSL is 600 m or more below. Atlantic spotted dolphins (Stenella frontalis) appear to do the same, but have daytime rest over an expansive shallow area: the Grand Banks of the Bahamas, only 6-10 m deep.
Pinniped females that go on foraging dives in between nursing their young on land, such as Galapagos fur seals (Arcto-cephalus galapagoensis), also use the productive shelf and dropoff waters to feed while—in their case—needing to return to land to take care of their young.
III. Coastal
Many marine mammals can be termed “coastal,” and because all of the various taxonomic orders and suborders have coastal representatives, one to several examples of each group are given.
The most coastal baleen whale is undoubtedly the gray whale, for it feeds in shallow waters of the Bering Sea, usually but not always near coasts; travels on its immense migration from the Bering Sea to Baja California, Mexico—and back— along the coast; and mates and calves near and in coastal lagoons of the subtropics. It is likely that this rather slow cetacean hugs the coastline for safety (mainly, one surmises, for its young) against shark and killer whale predation. It probably also uses the coast to navigate. It would not be surprising, although present information is not clear on this point, if gray whales use the depth contours, rocky outcroppings of headlands, and other near-coastal features as signs of location as surely as we find our way to and from the supermarket. The coastline also allows them to find clouds of mysids, small aggregating invertebrates, among kelp beds, and to occasionally feed on stands of in-benthic invertebrates while on migration. A second “coastal” animal is the humpback whale (Megaptera novaeangliae), for it feeds in bays and inlets, breeds near islands, and only uses deep oceanic waters to get to and from these ends of migration. In the northeast Pacific, humpbacks feed in the fjord-like bays of southern Alaska and breed around the Hawaiian, and Mexican Revillagigedo, islands.
Odontocete cetaceans have many coastal representatives, with the best studied of them being the bottlenose dolphin (Tursiops truncatus). While separate populations of this highly adaptable species can exist in deep oceanic waters as well, it is the coastal form that has taken our fancy and makes for one of the better captive animals, presumably because it feels at home in small groups and with confines of cliffs; rocks, bayous, and channels. Bottlenose dolphins variably nose and poke their way among rocks to feed; feed on schooling fishes in the nearshore, at times trapping schools against a beach or cliff; feed on the bottom; and encircle prey as a cooperating group in the open coast sea. Dolphins of the Cephalorhytjchus genus of southern oceans tend toward coastal living, as do the humpback dolphins of the genus Sousa, harbor porpoises (Phocoena phocoena). and beluga whales of the arctic (Delphinaptenis leucas). Interestingly, these animals appear to have some form of fission-fusion society, traveling in subgroups of variable size from day to day. It is likely that they aggregate in small groups for greatest efficiency in hunting and that the social or breeding unit is all of the small groups of an area that get together at some time throughout a year, but never all at once. Most but not all coastal waters are turbid as well, and it may be that echolocation and communicative sounds are particularly well developed in these animals.
Many pinnipeds have coastal representatives, especially for the physically smaller species. California sea lions (Zalophus californianus) rest on the shore and feed in the coastal zone, hardly ever venturing further tlian several kilometers from land. Harbor seals (Phoca vitulina) and the two living but highly endangered tropical monk seals (Monachus spp.) do so as well.
Sea otters (Enhydra lutris), marine otters of Chile (Lontra felina), and the sirenians are all coastal shallow-water feeders. Otters feed on invertebrates on the bottom or on kelp-associated fishes. While many populations of sea and marine otters do not frequently haul out on land, they use kelp beds as resting stations and perhaps as a means to hide from sharks and killer whales. The West Indian manatee (Trichechus man-atus) and the dugong (Dugong dugon), the latter largely of the nearshore Indian Ocean, feed on sea grasses and are thereby restricted to the shallows.
IV. Riverine
While the term “marine mammals” is meant for mammals that take all or most of their sustenance from the sea, several species are included that have gone to a largely freshwater environment. Because these have close taxonomic affiliations to several other marine mammals, this inclusion makes sense.
There are several obligate river dolphins: the susu and bhu-lan (now listed as subspecies within one species, [Platanista gangetica]) of the Indian subcontinent; the baiji (Lipotes vexil-lifer) of the Yangtze River of China; and the boto or Amazon river dolphin (Inia geoffrensis) that also occurs in the Orinoco basin of South America. These dolphins live their lives in mighty rivers, feeding on invertebrates and fishes, generally in small groups numbering fewer than about six animals. Their eyes have adapted to the less saline environment, and their kidneys do not need to process the salty foods of the ocean. It is likely that they would not survive in salt water. The Amazonian manatee (Trichechus inunguis) is also restricted to the extensive freshwater system of the Amazon basin.
In addition to obligate river dolphins and the Amazonian manatee, there are several mammals that are facultative, those who have populations that occur in rivers and those who go in and out of rivers to the adjacent ocean. Of the first type are fin-less porpoises (Neophocaena phocaenoides) that occur throughout nearshore waters of southern Asia and a bit of the Indian Ocean, but have a thoroughly fresh water population in the Yangtze River. Recent work shows that the freshwater form has eyes, skin, and kidneys that are adaptively different from their ocean-going conspecifics. As well, the diminutive tucuxi dolphin (Sotalia fluviatilis) occurs nearshore along much of the tropical Atlantic Central and South American coast, but as separate populations in the Amazon River basin. Of the second type of marine mammals, where some go in and out of rivers as members of the same population, are Irrawaddy dolphins (Orcaella brevirostris), bottlenose dolphins, belugas, and West Indian and West African manatees (Trichechus manatus and T. senegalensis). To date, there are no well-defined morphologic or physiologic differences between those who frequent fresh waters more than others, and it is assumed that this wide salinity tolerance is itself an adaptation that allows exploitation of food resources in ecologically diverse realms. Belugas seem to enter rivers more often during a concentrated period of skin sloughing, or molt; these are the only whales known to molt.
Almost all pinnipeds are generally tied to the sea to feed, but a form of the harbor seal and the Asian Lake Baikal (fresh water) and Caspian Sea (somewhat salty) seals (Pusa spp.) occur in land-locked areas. They occur in remnants of areas that were once connected to oceans.
V. Life on Land
Polar bears do considerable feeding on land or ice, pinnipeds all need to come to land to give birth, and sea otters do so variably by population. Off California, sea otters give birth in the water, but are usually surrounded and buoyed by Macro-ctjstis sp. giant kelp fronds. While some pinnipeds, such as the walrus (Odobenus rosmarus) and Weddell seals, mate in water, most do so on solid land or ice, and all females need to come to solid substrates to give birth and to suckle their young. Indeed, a newborn pinniped (and polar bear) is not yet a marine mammal and would become overexposed rapidly and die if it were to be dunked into water. The natal pelt of most true or phocid seals is a downy fur, or lanugo, that holds insulating hair but is not waterproof; they have brown fat, a type of lipid that breaks down rapidly to generate heat; and they instinctively huddle near mother and each other to stay warm.
Pinnipeds have shortened and greatly changed fore and hind flippers, modified beautifully for swift and precise movement in water. However, they have had to compromise their morphology to keep a bit of it—so very necessary for procreation—available for life on land. It is now known that early cetaceans lived a similarly dual existence, with morphologic, physiologic, and likely behavioral compromises to survive in both realms. It is tempting to speculate whether pinnipeds, given another 20 million years of evolution, can make the same total transition to the sea.
