Encyclopedia of Marine Mammals. Editor: William F Perrin, et al., Academic Press, 2002.
Ecology is the study of the natural environment and of the relationships of organisms to each other and to their surroundings. From its natural history beginnings in the late 1800s, the field of ecology has blossomed into a broad discipline, encompassing empirical and theoretical research in fields as diverse as mathematics, conservation, physiology, geography, and behavior. The study of cetacean ecology is very much in its infancy, however. Until fairly recently, most of what was known about cetacean ecology was largely composed of anecdotes and observations handed down by early whalers (Herman Melville’s “Moby Dick” provides for classic examples). Studying whales or dolphins in their natural environment is a formidable challenge. This is due to the logistical constraints of attempting to study highly mobile, oceanic animals that spend nearly all of their lives underwater, as well as the political and legal constraints of working on protected species, which include most cetaceans.
Cetaceans are a diverse group that includes some 83 species. They range in size from less than 1 m long for a newborn vaquita (Phocoena sinus) to 33 m in an adult blue whale (Balaenoptera musculus); they occupy water whose temperatures range from—2 to over 30°C; they exhibit a diverse array of life history strategies. Consider the sperm whale (Physeter macrocephalus), which can remain beneath the water for over an hour and dive to depths of several thousand meters; the Indian river dolphin (Platanista gangetica), which inhabits fresh water so turbid it is functionally blind; beaked whales of the genus Mesoplodon, which are so pelagic and so elusive that new species are still being described; the gray whale (Eschrichtius robustus), which annually migrates some 15,000 to 20,000 km between breeding and feeding areas; and the bowhead whale (Balaena mysticetus), which uses its rostrum to break ice in the Arctic.
One of the challenges of ecology is to search for pattern within diversity. Despite their diversity of form, behavior, and habitat, all cetaceans have some key features in common that underscore the fact that they are secondary marine forms, derived from terrestrial ancestors. That they are all air-breathing, live-bearing homeotherms provides a unifying theme. This article provides an overview of cetacean ecology with the ultimate goal of identifying some unifying principles in the ways that cetaceans interact with each other and with their environment.
Where Do Cetaceans Live?
On a global scale, cetaceans have invaded a large proportion of the ocean’s habitats. They inhabit coastal waters up to and including the surf zone (gray whale; some populations of bottlenose dolphins, Tursiops truncatus; harbor porpoise, Phocoena phocoena; Commerson’s dolphin, Cephalorhynchus commersonii), neritic waters over continental shelves (long-beaked common dolphin, Delphinus capensis; Lagenorhynchus spp.; Cephalorhynchus spp.; Phocoena spp.), and the most oceanic of systems (sperm whale; Frasers dolphin, Lagenodelphis hosei\ beaked whales). They are found in tropical waters (pantropical spotted dolphin, Stenella attenuata), temperate seas (Risso’s dolphin, Grampus griseus), and polar oceans, up to and within pack ice (beluga, Delphinapterus leucas; bowhead whale). They utilize much of the water column, some being confined to relatively shallow depths (most dolphins and baleen whales), and others diving to thousands of meters (sperm whale, many beaked whales). They have also invaded several of the worlds major river systems (Ganges, Indus, Amazon/Orinoco, Yangtze).
Cetaceans in different habitats might be expected to show differential development of adaptations that reflect selective pressures of the environments in which they function. For example, species in polar seas must conserve heat and so bowhead whales have relatively large bodies, thick blubber layers, and short appendages. Deep-diving species (sperm and beaked whales) must conserve oxygen and might be expected to have large blood volumes, a high hematocrit, and a well-developed diving response. Species that forage in low-light conditions (night feeders, deep divers, species living in turbid rivers) should have well-developed echolocation abilities relative to those that function in habitats with greater light levels and better visibility. To date, these types of comparative studies have been rare and are a promising line of future investigation.
The geographic range of cetaceans runs from cosmopolitan to extremely local. For example, the killer whale (Orcinus orca) can be found throughout the world’ oceans and, with the exception of humans, is the most wide-ranging animal on earth. At the other extreme is the vaquita, a tiny porpoise that occupies a few hundred square kilometers in the northern Gulf of California. Why some species are habitat generalists and others specialists remains largely a mystery.
On a smaller spatial scale, we are beginning to identify those features that correlate with centers of distribution for some species and so possibly identify what may be called critical habitat. For example, some species associate with ice edges (beluga), some with continental shelf edges or seamounts (beaked whales), and some with shorelines (gray whale, bottlenose dolphin, harbor porpoise). For oceanic species, habitat preferences are often defined by less obvious features: physical and chemical characteristics of the water itself, which define water masses and current boundaries. So, for example, some species associate with cold-water currents (Heaviside’s Cephalorhynchus heavisidii, Commerson’s, Peale’s Lagenorhynchus australis dolphins). Blue whales in the eastern Pacific are found in relatively cool, upwelling-modified waters with high primary and secondary productivity. In the eastern tropical Pacific, pantropical spotted and spinner (Stenella longirostris) dolphins segregate from common dolphins according to thermocline depth and strength, sigma-t (a measure of seawater density computed from surface temperature and salinity), and surface water chlorophyll content. These differences are statistically significant, and these species-specific distribution patterns track oceanographic variation on a seasonal and interannual basis.
We know very little about why these species-specific preferences exist. Most often, habitat preferences are suggested to relate to prey abundance or availability, which in turn are determined by physical oceanographic patterns. In fact, a number of studies have linked general distribution and movement patterns of cetacean species (humpback, Megaptera novaengliae; fin, Balaenoptera physalus; long-finned pilot whales, Globicephala melaena; Atlantic white-sided, Lagenorhynchus acutus; bottlenose and common dolphins, Delphinus spp.) with those of their prey. In a very few instances, physical features have been directly linked with the processes that cause them to aggregate prey or increase productivity, and indirectly with cetacean distribution. This is a productive area for future investigation.
How Do Cetaceans Use Their Habitat?
Home range can be defined as an area of regular use that typically provides for all of an animal’s requirements. The size of these areas ranges from 125 km2 for bottlenose dolphins along the west coast of Florida, to 600 km2 for minke whales (Balaenoptera acutorostrata) off the west coast of north America, to thousands of kilometers for many baleen whales, which annually migrate between the tropics and polar seas (and can be thought to occupy two home range areas, separated by large distances). Particularly for odontocetes, very little is known about home range size, but there are indications that it can be quite large. Bottlenose dolphins along the coast of Mexico and California regularly traverse 900 km. Pelagic spinner and pantropical spotted dolphins have been tracked over 500 and 1800 linear km, respectively, in a single year.
Within these home ranges, many cetaceans have well-defined habitat use patterns. This is perhaps best exemplified in species that migrate. These individuals move from high latitudes in the winter to low latitudes in the summer, distances spanning thousands of kilometers in some cases. They feed in their high-latitude habitat, where waters high in nutrients combined with seasonal sun produce a strong bloom in productivity and superabundant food. They mate and calve in their low-latitude habitat, where little feeding occurs because prey are scarce.
Differential habitat use patterns have also been identified for some non-migratory cetaceans. For example, island populations of spinner dolphins use sheltered bays to rest during the daytime and move to deep waters offshore during the night to feed. Coastal bottlenose dolphins feed in some areas of their range preferentially, sometimes with a. seasonal shift, presumably in response to prey movement. Most knowledge of habitat use patterns comes from coastal or neritic species; little is known about how or if pelagic species differentially use their habitat.
Food, Feeding, and Foraging
One of the most striking differences between marine and terrestrial ecosystems has to do with the form of primary producers. Whereas terrestrial ecosystems tend to be dominated by large macroscopic plants that are long-lived and provide substantial resources to other organisms in the form of food and physical structure, macroscopic plants are almost completely absent in marine ecosystems. In contrast, marine primary producers are dominated by microscopic, short-lived plants. This has profound implications. Marine herbivores are dominated by small, often microscopic animals themselves, whereas the majority of large marine animals are carnivores, including all cetaceans. Because the vast majority of oceanic habitat is pelagic, i.e., without any benthos, primary producers, herbivores, and consumers tend to be patchy in space and time. Thus the form of marine primary producers affects community composition, resource distribution, and so food, feeding, and foraging of cetaceans.
What Do Cetaceans Eat?
Most of what is known about the food of cetaceans comes from data collected from dead animals, through directed fisheries, incidental mortality, or strandings. Prey of cetaceans fall into four general categories. The first prey type consists of small individuals that school at relatively shallow depths (surface to several hundred meters). These are primarily planktonic crustaceans (euphausiids, copepods, amphipods) and small fish [e.g., herring (Clupea), sardine (Sardinops), anchovy (Engraulidae), sandlance (Ammodytidae)]. They tend to occur in temperate or polar seas or in those tropical latitudes that are associated with unusually high productivity. They generally occupy low trophic levels, have small body sizes, and occur in dense aggregations. Accordingly, the cetaceans feeding on them capture multiple individuals simultaneously, have large body sizes, and have evolved filtering mechanisms (baleen) to strain prey items from the water. All mysticetes feed on this prey type.
The second prey type is composed of larger organisms that also school at relatively shallow depths (surface to several hundred meters) or migrate up to shallow depths during the night. This includes many pelagic fishes [e.g., hake (Merluccius), pollock (Pollachius, Theragra), myctophids (Myctophidae)] and schooling squids (Loligo, Dosidicus), which occur throughout the world’s oceans.Because these prey are larger, they generally occupy higher trophic levels and are captured individually. Their cetacean predators typically have smaller body sizes. They include all of the large-schooling dolphins (e.g., dusky, Lagenorhynchus obscurus; common, striped, Stenella coeruleoalba; spotted dolphins) and some small-schooling or solitary species (e.g., bottlenose, Commerson’s, Indian river dolphin). These cetaceans tend to have a high tooth count, pointed teeth, and pointed snouts, all adaptations for pursuing fast, individual prey.
The third prey type is composed of large, solitary squid (e.g., Gonatus). These are most often found in deep waters throughout the worlds oceans. Because of their size and solitary habits, they are captured individually. Cetacean predators of these prey include the sperm whales (Physeter, Kogia spp.), all of the beaked whales (Ziphiidae), and pilot whales (Globicephala spp.). They are deep divers and tend to have reduced dentition, rounded heads, and well-developed melons, the latter perhaps indicative of the importance of echolocation for prey detection in the dark depths.
The final prey type includes species at high trophic levels that are themselves top predators. These include predatory fishes [e.g., tunas (Scombridae), sharks, salmonids], marine birds, pinnipeds, and cetaceans, including the largest of whales [rorquals (Balaenoptera spp.) and sperm whales]. Few cetaceans are able to take these prey items. They include the killer whale and, possibly, false killer whale (Pseudorca crassidens), pygmy killer whale (Feresa attenuata), and pilot whales. Two distinct forms of killer whales occur in waters off the west coast of North America: those that take fish and those that take mammals and birds. There is some indication that these two forms are found in Antarctic waters, and perhaps throughout the world’s oceans.
How Do Cetaceans Capture Prey?
Cetaceans have two main types of feeding apparatus: baleen and teeth. Baleen is used for straining prey items from the water or, in the case of the benthic-feeding gray whale, from the sediment. Teeth are used for catching individual prey items. Species with a high tooth count use them to grasp individual prey; those with a low tooth count tend to be suction feeders (see later).
We know the most about prey capture strategies for cetaceans that feed on small prey that school at relatively shallow depths (the mysticetes). This is because it is relatively easy to observe these animals feeding in the wild. Mysticetes have baleen plates suspended from the roof of their mouths, which they use to strain prey items from the water. The number of baleen plates, their length, and the density of baleen fibers per plate vary between species and are correlated with prey size. Right whales (Balaenidae) and sei whales (Balaenoptera borealis) have the greatest number of plates with the finest filtering strands and feed mainly on tiny copepods. Blue whales and most other rorquals have an intermediate number of plates with coarser filtering strands and feed on larger prey items such as euphausiids and small fishes. Gray whales have the fewest number of plates with the coarsest strands and are largely bottom feeders, sifting benthic infauna from muddy substrate.
In addition to specializing on different prey sizes, baleen whales have specialized feeding methods that also correlate with the morphology of their baleen. “Skimmers,” the right whales, swim slowly with their mouths open through dense clouds of slow-swimming copepods. “Gulpers,” including most rorquals, lunge into dense schools of euphausiids or fishes with their mouths open, closing them rapidly to trap their prey. All rorquals have throat grooves that run along the ventral surface of the mouth and throat, which allow the buccal cavity to expand during a lunge, taking in huge quantities of water and, with this, prey. A variation of this type of feeding is used by humpback whales when they form “bubble nets”: streams of bubbles emitted from the blowhole as the whale swims in a circular pattern toward the surface. The bubbles form an ascending curtain, which concentrates prey inside. Most of these cetaceans are solitary feeders but they regularly aggregate in areas of high-prey density and, when prey are extremely dense, will feed cooperatively at times, through bubble-net feeding or in staggered echelon formations.
Cetaceans that feed on larger fish and squid that school at relatively shallow depths capture individual prey items and swallow them whole. High speed is important, as is vision. Typically these predators forage cooperatively, herding prey into tight aggregations and capturing them in turn. Acoustic signaling is presumably important for the coordination of schooling activities. Some cetaceans in this group feed as individuals, particularly those found in coastal areas. They show a wide range of prey capture behaviors, including slapping fish with their flukes and deliberately stranding themselves on the beach in pursuit of fishes.
Cetaceans taking large, solitary squid feed at depth, in partial to full darkness. For this reason, not much is known about how they capture prey. They probably do not feed cooperatively because their prey do not school and because most of these cetaceans occur in small schools and are slow swimming. Most have reduced dentition, and evidence indicates that they are suction feeders, using the gular muscles and tongue in a piston-like action to suck prey into their mouths. How they are able to get close enough to their prey to suck them in remains a mystery. One intriguing idea is that they are able to partially stun prey with echolocation bursts. To date, this hypothesis remains largely untested.
Cetaceans that prey on top predators show a wide range of prey capture methods: hunting as individuals when prey are small and cooperatively in groups when prey are large. For example, killer whales may take pinnipeds by beaching themselves intentionally to grab adults and pups from ROOKERIES but hunt cooperatively to take dolphins and large whales. Cooperative behaviors include prey encirclement and capture, division of labor during an attack, and sharing of prey.
How Do Cetaceans Locate Prey?
Most cetaceans are visual predators, at least in part. For odontocetes, echolocation is equally important in locating and targeting prey, more so than vision in some species. Although only confirmed for a handful of captive species, all odontocetes are assumed to be able to echolocate and to use this sense extensively when foraging. At present, there is no evidence that mysticetes have the ability to echolocate, although they do produce low-frequency sounds that travel long distances (hundreds of kilometers). The long wavelengths of these pulses cannot resolve fine features and are transparent to most schooling prey, so it is doubtful that they could be used to locate and target prey patches.
The effective range of vision and echolocation is a function of water clarity and the specific echolocation abilities of a species, but both are probably limited to distances on the order of hundreds of meters to a few kilometers. On a larger spatial scale, patchiness and variability in space and time are characteristic of most marine ecosystems and little is known about how cetaceans locate prey in such environments. Presumably, many species simply travel large distances in a continuous search. Here, schooling may increase the chances of encountering a patch (the more eyes and ears, the better), and dolphin schools have been observed moving through the water in wide line-abreast formations, apparently searching for prey.
However, there are circumstances under which prey occur predictably in space and time, and it is likely that cetaceans search for and exploit these opportunities. For example, oceanographic features (e.g., boundaries between currents, eddies, and water masses) increase prey abundance or availability by enhancing primary production, by passively carrying planktonic organisms, and by maintaining property gradients (e.g., fronts) to which prey actively respond. Topographic features (e.g., islands, seamounts) are also sites of prey aggregation. Therefore, a good foraging strategy is simply to locate these physical features, and many species of cetaceans (right, blue, fin, humpback, sperm, killer whales, spinner, Risso s, common, Atlantic spotted Stenella frontalis dolphins) have been found to associate with them.
Many species of cetaceans locate and associate with predictable point sources of prey. For example, killer whales aggregate around pinniped rookeries when young seals and sea lions are weaning. Rough-toothed dolphins (Steno bredanensis) associate with flotsam in the oceanic tropics, which serves to aggregate communities of animals at a wide range of trophic levels. A wide variety of cetaceans associate with fishing operations to take their discards or their target species.
And there are times when prey are more accessible than others. The pelagic community of fishes and invertebrates, which live at depth during the day but migrate to the surface at night, provides an opportunity for cetaceans to predictably locate prey near the surface, and some dolphins (spotted, spinner, dusky, common) are known to feed on organisms in this community at night.
By far the most important predator of cetaceans is the killer whale (Fig. 3). Its pack-hunting behavior allows it to take everything from the fastest dolphins and porpoises to the largest whales, including blue and sperm whales. Other predators known to occasionally prey on smaller or weakened individuals include large sharks, and possibly false killer, pygmy killer, and pilot whales. Polar bears (Ursus maritimus) take cetaceans along the ice edge.
The ecological significance of this predation pressure in the lives of whales and dolphins is difficult to assess, but it may be significant. Individual large whales often show signs of killer whale tooth rake marks on their flippers, fins, and flukes, and up to one-third of the bottlenose dolphins off eastern Australia bear shark bite scars, suggesting that they regularly encounter predators. It has been hypothesized that large whales may undergo their annual migrations in order to reach calving grounds in areas of lower killer whale densities (i.e., the tropics). Aggregative behavior is a common defensive strategy among prey species and it is possible that schooling evolved in dolphins primarily as a defense mechanism against predators (see later). These kinds of behavioral adaptations have cascading effects influencing not only distribution and abundance, but also social structure, timing and mode of reproduction, foraging strategies, and speciation patterns. Predation therefore could play a major role in shaping cetacean communities and life history strategies.
Like many other animals, cetaceans form aggregations for two main reasons: feeding and protection. Feeding can bring animals together as passive aggregations in areas of high resource abundance. Alternatively, animals may actively seek others to take advantage of benefits provided by other school members. Schools also serve to protect members from predation, by providing cover for individual members, by confusing predators with synchronized movements of many individuals, by reducing the probability of predation on any one individual, by increasing the chance of detection of a predator, and by providing for coordinated defense. Although occurring in large groups also increases the potential for social interactions, including reproduction, this may only be a secondary benefit of schooling.
The majority of cetaceans occur in schools, although there are some species that regularly occur solitarily or in very small groups of pairs or trios (many mysticetes, large male sperm whales, most beaked whales, Kogia spp., and river dolphins). Most schooling species have characteristic school sizes (although they can vary somewhat area to area). For example, rough-toothed dolphins typically occur in groups of 10-20, pilot whales occur in schools of dozens, and some oceanic dolphins (Stenella spp., Delphinus spp.) regularly occur in groups of hundreds or thousands.
School size correlates with feeding habits: species that form large schools are almost all shallow-diving species that feed mainly on schooling prey, whereas those that occur in school sizes of 25 or fewer tend to be (a) deep-diving species that feed mainly on larger squids or (b) coastal species feeding on dispersed prey. School size also correlates with predation pressure; large cetaceans, presumably subject to lower predation pressure than small species, occur only in small groups, whereas small cetaceans, subject to higher predation pressure, occur in schools whose size correlates with the openness of habitat: the more open, the larger the school size. School size should correlate with resource availability and will affect reproductive strategies, although the nature of these relationships remains largely unexplored.
Although most schools are monospecific, several species regularly occur in mixed-species schools. Some of these associations appear to be opportunistic: bottlenose dolphins, for example, have been recorded to occur with over 20 different species of whales and dolphins. Other associations appear to be more prescribed: spotted and spinner dolphins regularly occur together in mixed schools. Risso’s, Pacific white-sided (Lagenorhynchus obliquidens), and northern right whale (Lissodelphis borealis) dolphins are commonly found in association. The nature of these interactions (e.g., why these species-specific associations occur, how these species avoid competition) is unknown.
Communities and Coexistence
Studies of communities typically focus on identifying member species and their interactions and then address mechanisms for their coexistence. These kinds of studies comprise a large part of the ecological knowledge for many terrestrial species. In contrast, almost nothing is known about this aspect of the ecology of cetaceans.
We do know that there are regularly occurring species assemblages. For example, pantropical spotted and spinner dolphins are frequently found in mixed-species schools in association with yellowfin tuna (Thunnus albacares) and are accompanied by large and speciose flocks of seabirds; this association is particularly prevalent in the eastern tropical Pacific, as opposed to other tropical oceans. We know that there are variations in typical co-occurrence patterns. In the Gulf of Mexico, for example, five species of Stenella coexist in a relatively small area, more Stenella species than any other tropical ocean. The nature of the interactions between species in these assemblages, why they associate, and the reasons for variations in community membership patterns are almost completely unknown.
Coexisting species, particularly those that are closely related or have similar ecological roles, potentially compete for resources. An often cited example is the Southern Ocean, where the relative abundances of cetaceans, pinnipeds, and seabirds, all krill consumers, have been reported to have changed between pre- and postwhaling years. One plausible explanation is competitive release: the decrease in biomass of cetacean predators released a huge prey base of krill to pinnipeds and seabirds, both of which were able to increase in abundance. However, available data on prey biomass, predator consumption, and population status are largely lacking, so the purported changes are in question. In fact, there is little hard evidence to indicate the degree, or existence, of competition between ecologically similar cetaceans.
Ecological theory states that stable communities of coexisting species must differ in resource utilization in some way: prey species or size specialization, differential habitat use, or diel pattern. Such niche partitioning is fairly clear for cetaceans on a broad scale. For example, there are species that feed on fish and those that feed on squid. There are species feeding in shallow water and those that feed at depth. Some cetaceans feed at night and others during the day.
On a smaller scale, one of the best known examples of niche partitioning is for baleen whales. In this group, there is a fair degree of prey specialization that presumably allows for niche partitioning in areas of sympatry. Blue whales feed almost entirely on euphausiids; fin whales and humpbacks feed mainly on fishes but take euphausiids when they are abundant; and right whales and sei whales feed mainly on copepods. Odontocetes provide additional possible examples. Bottlenose, short-beaked common (Delphinus delphis), pantropical spotted dolphins, and harbor porpoises exhibit diet specialization among age, sex, and reproductive class, although this diet specialization could be due to differing energy requirements. Aside from these examples, very little is known about how or if cetaceans partition resources.
Ultimately, in order to understand community structure, the mechanisms by which species partition resources, not merely the presence of differences in resource use, are of principal interest. The question then becomes, given that there are differences, what mechanisms can explain them? Community ecologists have identified interference and exploitative competition, mutualism, morphological or physiological factors, and habitat structure as potential mechanisms for maintaining resource utilization differences. This is an area that remains almost completely unexplored for cetaceans and the communities in which they are found.
The Role of Cetaceans in Marine Ecosystems
What role do cetaceans play in marine ecosystems and what is their significance? Most cetaceans are apex predators. As such, they take tons of prey from the ecosystem. (Some estimate that cetacean consumption equals or exceeds that of fisheries in the Georges Bank, the continental shelves of the northeastern United States, the northwestern Mediterranean, and the Southern Ocean ecosystems.) In so doing, it seems likely that cetaceans affect the life history strategies and population biology of their prey, as well as organisms at other trophic levels that interact in various ways with these prey. Little is known about the details of these dynamics, although this may be the most significant way in which cetaceans impact marine ecosystems.
More specific effects have been documented. For example, benthic feeders such as gray whales alter habitat by regularly turning over substrate (between 9 and 27% of the benthos in the northern Bering Sea) and therefore significantly affect the species composition of benthic communities. Feeding cetaceans provide feeding opportunities for seabirds by driving prey to the surface, sometimes injuring or disorienting it; in one study, up to 87% of all feeding individuals from four seabird species in the Bering Sea associated with gray whale mud plumes. Large whales dying at sea may sink to the bottom and provide rare but superabundant food and habitat for deep-water species. There is evidence that mollusc communities may have specialized on these resources for the past 35 million years, and some speculate that whale carcasses may have been instrumental in the dispersal of hydrothermal-vent faunas. Feces of some cetaceans, particularly large whales in areas of low productivity, may play a significant role in nutrient cycling. Cetaceans are host to a variety or commensal or parasitic species; in some cases (Cyamid whale lice), these species are completely dependent on cetaceans through all life stages.
The field of cetacean ecology is very much in its infancy. Technological advances and heightened interest will undoubtedly bring greater insight to this discipline in the near future. However, any attempts to make ecological sense of cetaceans as marine organisms and to interpret their distribution patterns, foraging ecology, community structure, and role in ecosystems must take into account the fact that many cetaceans today exist as remnant populations that have been reduced drastically through anthropogenic effects: commercial exploitation, incidental mortality, and habitat destruction. This means that cetacean ecologists must also add conservation biology to the list of disciplines that will likely affect our search for ecological patterns.