Delphinidae

(vertebrate zoology) A family of aquatic mammals in the order Cetacea; includes the dolphins.

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(Delphinidae)

Class: Mammalia

Order: Cetacea

Suborder: Odontoceti

Family: Delphinidae

Thumbnail description
Medium to large fully aquatic carnivores, characterized by a fusiform body, a head with a projecting beak carrying homodont teeth, well-formed eyes, lacking external ears, a single blowhole on top for respiration, pectoral appendages reduced to flippers, loss of pelvic appendages, and a horizontal tail consisting of two flukes for propulsion

Size
4.5–30 ft (1.4–9.0 m); 117–12,000 lb (53–5,600 kg)

Number of genera, species
17 genera; 34 species

Habitat
Oceans, bays, estuaries, and rivers

Conservation status
Endangered: 1 species; Lower Risk/Conservation Dependent: 5 species; Data Deficient: 17 species

Distribution
Worldwide in all pelagic and coastal marine waters from the Arctic ice edge to the Antarctic ice edge and including a number of rivers

Evolution and systematics

The cetaceans appear to have been derived from ancient ungulate stock. The earliest whales are described as archaeocetes, which evolved from mesonychian condylarths. The mesonychids were ungulate ancestors that were primarily terrestrial. Fossil archaeocetes from about 52–42 million years ago (mya) have been found in Africa, North America, Pakistan, and India. Recent fossil findings have identified a "walking whale" of the genus Ambulocetus as a clear example of the transition from terrestrial to aquatic life. One of the more obvious evolutionary developments in the transition from archaeocetes to modern cetaceans was the movement of bones in the skull as the nasal openings migrated to the more effective position on top of the cetacean's head. This telescoping resulted in elongated premaxillary and maxillary bones of the skull, creating a rostrum or beak. Baleen whales (Mysticetes) and toothed whales (Odontocetes) appear to have diverged about 25–35 mya. Three families of archaic, extinct dolphins are known from the Miocene. These led to the current family Delphinidae, which is now the most diverse of the cetacean families. The first modern dolphins appeared in the fossil record from about 11 mya.

Comparisons of the fossil record, morphological features of existing animals, and genetic data have led to a variety of controversial descriptions of the phylogenetic relationships between cetaceans and other mammals, and within the odontocetes.

While there is general agreement regarding ties between cetaceans and ungulates, the nature of this relationship remains unresolved. It has been suggested that artiodactyls (even-toed ungulates) such as the hippopotamus may be their closest living relatives, but available data are in conflict. Within the Cetacea, dolphins are distinguished by the loss of the posterior nasal sac, and a reduction of the posterior end of the premaxilla skull bone. Distinctions between dolphins and other groups of small cetaceans such as river dolphins and porpoises involve comparison of a variety of skull features. In more general terms, dolphins differ from porpoises in that the dolphins tend to have a larger and more falcate dorsal fin as compared to the lower, more triangular porpoise fin; dolphins tend to have a longer, more clearly demarcated beak; and dolphin teeth are conical in shape as compared to the spatulate teeth of porpoises.

The classification of dolphins is undergoing much revision with the advent of genetic analysis techniques and the increased efforts by scientists to collect small genetic samples from specimens from around the world. In 2003, 17 genera, 34 species, and 16 subspecies of dolphins were recognized. Though subfamily designations must be considered tentative pending additional study, one proposed scheme identifies five subfamilies: Delphininae (Delphinus, Lagenodelphis, Lagenorhynchus, Sousa, Stenella, and Tursiops), Globicephalinae (Feresa, Grampus, Globicephala, Pseudorca, and Peponocephala), Lissodelphinae (Cephalorhynchus and Lissodelphis), Orcininae (Orcinus and Orcaella), and Stenoninae (Sotalia and Steno). A

number of hybrid dolphins have been identified in captive breeding situations and in the wild, serving to further blur the distinctions between species. Some of these hybrids have been both viable and fertile.

Physical characteristics

All dolphins have essentially the same fusiform body shape, streamlined for efficient movement through a dense water medium. The dolphins possess flippers as their pectoral appendages, with the bones of the hand and arm modified into a solid wing-like structure, articulating with the shoulder, and serving as control surfaces for maneuvering. Pelvic appendages are essentially nonexistent, reduced to small, internal pelvic bones. The vertebral column, with its variably fused cervical (neck) vertebrae and prominent processes for attachment of the strong musculature used for propulsion, tapers toward the tail, where a pair of fibrous horizontal fins form flukes. Contrary to the baleen whales, which have two nostrils, dolphins have a single blowhole on top of their heads for respiration. Variation exists among the dolphins in terms of robustness, the presence of a clearly demarcated projecting beak, numbers and size of teeth, and the presence of a dorsal fin. In general, body shape of the smaller dolphins seems to roughly grade from more slender to more robust, with greater mid-body girth, moving away from the equator. Dolphin teeth are homodont, meaning that all of the teeth in a dolphin's jaws are alike in structure. The pointy teeth are designed for grasping individual prey items, rather than for chewing. The cone-shaped teeth differ in size and number, depending on the prey of the different dolphin species. The number and size of the teeth, in turn, influence the size of the beak and shape of the mouth of each species. For example, common dolphins (Delphinus delphis) may have more than 250 small teeth in long, forceps-like jaws for capturing small schooling fish and invertebrates, whereas killer whales (Orcinus orca) have about 50 large teeth for capturing large fish and removing large pieces from a variety of marine mammals. A fibrous dorsal fin located near the middle of the back is found on all but two species of dolphins, the northern and southern right whale dolphins (Lissodelphis spp.). Dorsal fins vary from species to species in height and shape, but in general they serve to stabilize the swimming dolphins, as radiators for cooling the internal reproductive organs, and as weapons. As a secondary benefit to researchers, the dorsal fins of many dolphins are individually distinctive based on shape and natural notch patterns, providing a means of reliable identification for observing individuals over time. A few small hairs occur on dolphin beaks at the time of birth, but these are soon lost. Males and females look quite similar in most dolphin species, though there may be differences in body and/or appendage size in some. Female dolphins typically have a genital and an anal opening in a single ventral (belly) groove, with a nipple located in a mammary slit on each side of the genital opening. Males typically have a genital opening in a ventral groove anterior to the separate groove for the anus.

Dolphins are medium- to large-sized aquatic mammals. They range from the tiny and endangered Hector's dolphin (Cephalorhynchus hectori) at 4.5 ft. (1.4 m) and 117 lb (53 kg) to adult male killer whales at 30 ft (9 m) and 12,000 lb (5,600 kg). Gender differences in adult body size (sexual size dimorphism) occur in some dolphin species. In general, the females of some of the smaller species (Cephalorhynchus spp.; the tucuxi, Sotalia fluviatilis) are slightly larger than the males, whereas the males of the largest species (killer whales, false killer whales [Pseudorca crassidens], pilot whales [Globicephala spp.]) tend to be much larger than the females. In some cases, features that might affect performance in battle or chasing females, such as dorsal fin height (for example, killer whales), tail stock height, or fluke span are disproportionately larger for males than for females. In most of the dolphin species, males and females are of similar size or males are somewhat larger.

Dolphins exhibit a tremendous range of species-specific color patterns, including shades of black, white, gray, brown, orange, and pink. Countershading occurs for most dolphin species. Presumably this pattern of light bellies and dark backs provides camouflage for dolphins avoiding predators or approaching prey. Dolphins living in clear water tend to have more striking and complex color patterns on their sides than do dolphins living in the murky waters of estuaries or rivers. One hypothesis for this difference is that dolphins inhabiting clearer waters can use flashing of color patterns in much the same way as birds in coordinated flocks to signal changes in direction, or for other social displays.

Distribution

Dolphins are distributed worldwide through marine waters. Killer whales have been termed cosmopolitan in their distribution because they are found along the ice edge in both arctic and antarctic waters, and in many areas in between. Though most genera have representatives in each of the major ocean systems of the world, individual species tend to be more limited in their distributions. Tucuxi are found only in coastal waters and rivers of South and Central America, the Irrawaddy dolphin (Orcaella brevirostris) only in inshore and riverine Indo-Pacific waters, and all four species of Cephalorhynchus are found only in the southern hemisphere.

Habitat

Dolphins are entirely aquatic, meaning that they must find food, mate, produce and rear young, and avoid predators in water. Access to the water's surface for air, availability of prey, predator abundance, and temperature constraints are among the major factors determining habitat use by dolphins.

Dolphins can be found in all available marine habitats. These habitats are truly three-dimensional, and water depth and physiography are important habitat features. Dolphins occur in greatest abundance where resources are most available. In open-ocean or pelagic habitats, this tends to be near islands or seamounts, where nutrient-rich waters carried in deep currents are brought to the surface and support an extensive ecosystem, or where different water masses meet. Similarly, upwelling is a wind- and current-driven phenomenon that creates highly productive areas along the continental slope. Estuaries, where rivers meet marine waters, are extremely productive, and support large numbers of dolphins.

Suites of adaptations are associated with patterns of habitat use. For example, oceanic dolphins of the genera Stenella and Delphinus are among the most streamlined of the dolphins.

With small appendages and many fused cervical (neck) vertebrae, they are designed to maintain a rigid body and move at high speed through pelagic waters free of obstacles. Their long beaks and numerous tiny teeth facilitate the capture of small prey found in rich patches in the open ocean habitat. In contrast, some of the more coastal species such as bottlenosed dolphins (Tursiops spp.) and Irrawaddy dolphins are more flexible and/or have larger appendages for maneuvering around bottom and shoreline features in more restrictive habitats. Most killer whales are found near shorelines, where their large appendages facilitate maneuvering around ice floes, shore features, and highly maneuverable, sometimes amphibious, prey (killer whales sometimes slide onto beaches to capture pinnipeds). Their large size helps them to chase and capture large, fast-swimming prey (for example, blue whales, Balaenoptera musculus) in more open habitats. The blackfish, including pilot whales (Globicephala spp.) and related species, are among the deepest diving dolphins, diving hundreds of feet (meters) in search of squid and fish. They lack projecting beaks, tend to be mostly black (the deepest parts of their habitat lack light), have rounded or bulbous heads, and heavily muscled tailstocks that would facilitate reaching depth and returning to the surface quickly after a prolonged foraging dive. The tucuxi and Irrawaddy dolphin frequent rivers; tucuxi are found several thousand miles (kilometers) up the Amazon River in Peru.

Behavior

Dolphins exhibit a wide range of sociality, reflecting the diversity of morphological variations and habitats of the family. Few of the species include solitary individuals as a common feature of the social system. Group size varies from species to species. Among the smaller dolphins, inshore and riverine species such as bottlenosed and tucuxi dolphins tend to form small groups, typically of fewer than 10–20 individuals. In more open habitats offshore, common dolphin and spinner and spotted dolphin (Stenella spp.) groups may number in the thousands. Such variability in the size of groups is likely related to the abundance and distribution of prey (rich, patchy prey offshore, more evenly distributed, predictable prey inshore), and to exposure to predators (dolphins in open habitats are likely more vulnerable to detection and attack by predators than in shallow inshore waters where predator-approach options are more limited). Spreading out in long formations, large groups of dolphins increase the probability of finding fish schools. Similarly, larger groups can work together to better detect and avoid or defend against predators. Larger dolphins such as the pilot whales and killer whales form groups of intermediate size, typically involving fewer than 100 individuals. Because they are larger than most potential predators, predation pressure on adults is minimal, and it is likely that available resources place the primary constraints on group size.

Group composition and cohesion are also quite variable among the dolphins. Many of the smaller dolphin species live in groups of fluid composition, a situation referred to as fission/fusion. Swimming associates may change from day to day, but repeated associations over time are common, especially involving individuals sharing a home range and that are of the same gender, and similar age and reproductive status. In some cases such as male bottlenosed dolphins, small numbers of individuals may be close associates for many years. Some of the larger dolphin species (pilot whales, killer whales) maintain much more stable groups, consistent with the term pod. In these species, the groups may consist of one or more closely associating maternal lineages of several generations, and many of these individuals may remain together for years or decades. In the case of killer whales, it has been suggested that this long-term stable, multi-generational grouping facilitates highly coordinated prey capture such as when the group works together to attack a large baleen whale.

Communication between dolphins remains incompletely understood. Dolphins may signal one another by changing their orientation to show more or less of their high-contrast color patterns to others, often by rolling slightly from one side to another. While visual signals may be important to dolphins living in clear water, many dolphins inhabit turbid waters where vision is limited to a few feet (meters) at best.

Acoustics play a premiere role in the lives of dolphins. Dolphins have exceptional hearing, and can hear frequencies nearly 10 times higher than humans. They are also capable of producing three classes of sounds within this broad range of hearing. Dolphins lack vocal chords. Instead, they produce sounds by cycling air past tissues in the nasal region. Dolphins often produce "burst pulse" sounds, which sound like squawks, when socializing with one another. They also produce sonar, or echolocation, clicks to investigate their environment. These broadband clicks bounce off objects in the environment, and the dolphin is capable of interpreting, with a high degree of discrimination, the echoes returning through the animal's lower jaw. The dolphin can adjust the rate of production of these rapidly repeated clicks to allow the echo to return between clicks. Clicks are very directional signals, projected forward in a narrow beam from the dolphin's melon. Dolphins also produce frequency-modulated tonal whistles, which are much less directional. Individual bottlenosed dolphins tend to produce one specific whistle more often than others. This is termed its signature whistle, and playback experiment results suggest that they are used at least as identifiers or contact calls, presumably to maintain group cohesion in murky or dark water. Dolphins responded much more strongly to recorded signature whistles of kin or close associates than to those of less familiar individuals. Variations in the pitch or other features of these whistles may carry additional information about the emotional state of the producer. As a variation on this identifier theme, killer whales produce calls that are pod-specific, rather than individual-specific, and remain unchanged for several decades. Captive dolphins have been taught to understand artificial language, but they have yet to initiate acoustic responses or to clearly demonstrate the nature of a true language of their own.

Some dolphin communication may combine visual and acoustic modes, or involve other senses. For example, "jaw pops" involve opening and closing the mouth in a threatening manner and producing a loud popping sound. Tail slaps, when the flukes are slapped sharply on the water's surface also produce a loud report. Dolphins are very tactile animals, and many affiliative interactions involve physical contact between various body parts. Socio-sexual displays are also an important part of the dolphins' behavioral repertoire. Dolphins are apparently able to taste, but not smell. Though it remains to be demonstrated conclusively, it is possible that dolphins may communicate such things as reproductive readiness through production of chemicals that can be tasted by others.

Dolphins exhibit a variety of ranging patterns. Knowledge of the ranging patterns of dolphins inhabiting pelagic and continental shelf waters is limited by the inherent difficulties of conducting research in these regions. However, available information suggests that dolphins typically do not range through entire ocean basins, but instead occupy generally definable regions within basins. This pattern becomes more evident where geographical features help to define ranges. For example, scientists working from oceanic islands have been able to repeatedly identify individuals of a variety of dolphin species, including spinner dolphins and pilot whales, over periods of years in the waters near these islands. Along shorelines and in enclosed bays, residency is frequently noted. Some killer whale pods residing in Puget Sound and near Vancouver Island have been observed repeatedly in the same waters for nearly three decades; some of these pods have also been observed as far away as Alaska and California. Atlantic spotted dolphins and bottlenosed dolphins inhabiting the Bahama Banks have been identified repeatedly for nearly two decades. Tucuxi dolphins in bay waters of southern Brazil have been observed repeatedly for nearly a decade. Probably the best-known ranging patterns are those of inshore bottlenosed dolphins. In nearly every study around the world, at least a few, if not most, of the dolphins have been determined to exhibit residency in an area for at least part of the year and over multiple years. The longest-term study, in Sarasota Bay, Florida, continues to identify four generations of year-round residents after more than 33 years of observation. In places where the animals are living at the extremes of the species' range, seasonal migrations have been noted. For example, bottlenosed dolphins along the Atlantic seaboard of the United States move as far north as New Jersey in summer, but migrate at least as far south as North Carolina as waters cool. Territoriality, in terms of defended areas, has rarely been described for dolphins.

Dolphin activities tend to occur in bouts. Dolphins are nearly constantly on the move, surfacing to breathe, diving, and traveling from one location to the next or milling in one area. They intersperse bouts of travel with foraging, socializing, play, and/or rest, or some combination of these activities. As voluntary breathers, dolphins must remain conscious at all times. Thus, they do not sleep in the same way as humans. It is believed that they decrease their overall activity level and rest one hemisphere of the brain at a time. In some species, the degree of synchrony of group members can be very high. Different species of dolphins exhibit different levels of aerial activity. Spinner dolphins and right whale dolphins engage in frequent leaps, whereas other dolphins such as bottlenosed or tucuxi tend to be more subdued.

Feeding ecology and diet

Dolphins are carnivorous, with most eating fish and/or squid. In contrast to the batch-feeding baleen whales, dolphins typically capture prey one item at a time. Some dolphins may eat other invertebrates such as shrimp, and others, especially killer whales and false killer whales, may eat other marine mammals. Killer whales are known to prey upon sea otters, pinnipeds, porpoises, dolphins, and baleen whales. In the vicinity of Vancouver Island, different pods of killer whales specialize on different prey. The residents feed primarily on fish, while the transients emphasize marine mammals in their diet. Dolphins typically consume about 5% of their body weight in food each day.

Dolphins use a variety of techniques to find and capture prey, ranging from individual hunting to coordinated, cooperative efforts involving entire dolphin groups. Dolphins find prey visually and acoustically. Some leaps performed by dusky dolphins (Lagenorhynchus obscurus) prior to feeding are believed to be for the dolphins to locate bird flocks that may be flying over fish schools. Dolphins use both passive and active acoustics to find prey. Many fish and marine mammals produce sounds that can be heard by the dolphins, and allow them to locate the prey. Dolphins also use their echolocation to actively search for and zero in on prey. The idea has been proposed that dolphins can produce echolocation clicks of sufficient strength to stun prey, but little evidence exists to suggest that this approach is used widely. Inshore dolphins often hunt prey individually, especially when feeding on nonschooling fish inhabiting seagrass meadows, reefs, or other seafloor features. In open ocean habitats, large schools of foraging dolphins such as common dolphins may spread across broad areas in a line-abreast formation to search for schools of fish or squid, and then converge on the schools once they are found, presumably as a result of some acoustic cue. Working cooperatively, dolphins will circle prey schools, condensing them and driving them to the surface where that barrier further limits their escape. Dolphins then pass through the densely packed prey and grab individuals. The extreme case of cooperative feeding involves killer whale pod members working together to subdue large baleen whales, much like wolf packs attacking large ungulates.

Dolphins exhibit a variety of specialized foraging behaviors. Some dolphins in Australia place sponges on their rostra presumably to aid in prey capture. Killer whales in Argentina and bottlenosed dolphins in several locations engage in strand feeding, in which they slide onto beaches after prey. Bottlenosed dolphins also engage in "kerplunking," a behavior that involves driving their flukes and tailstock through the water's surface, creating a large splash and bubbles that may flush prey from cover. They also engage in "fishwhacking," which involves striking fish with flukes, often sending them soaring, stunned, through the air. Atlantic spotted (Stenella frontalis) and bottlenosed dolphins use a behavior known as "crater feeding" to dig into sandy seafloors in search of buried prey. Dolphins around the world have also learned to take advantage of human fishing efforts, including obtaining fish lost or discarded from trawlers and seiners, and working to drive schools of fish toward artisanal fishermen working with cast or seine nets from shore. In the latter case, the barriers provided by the fishermen and the confusion from their fishing activity may enhance the dolphins' prey-capture efficiency. Many of these specialized foraging and feeding behaviors are believed to provide evidence for the cultural transmission of knowledge through dolphins' societies.

Typically, dolphin prey are eaten intact, swallowed head first. If a fish is too large to take in this way, or if it has dangerous spines that could injure the dolphin if ingested, the dolphin may break it into smaller pieces by tossing it, rubbing it on the seafloor, or in some cases, working with another dolphin to tear the fish apart. When feeding on large marine mammals, killer whales often work together to restrain the large prey while they bite off pieces of the animal.

Reproductive biology

Behaviors for courtship and mating for reproduction are not well understood for most dolphins. Sexual behaviors are used in several contexts, including developing and maintaining dominance and other social relationships, in addition to reproduction. Males and females of all ages, from the time they are several weeks old, engage in sexual behavior with members of the opposite and the same gender, and sometimes with their own close relatives. The vast majority of these sexual interactions are social in nature, but they confuse the issue of research that tries to identify those involved specifically in reproduction. Sexual behaviors include stroking with flippers and flukes, rubbing the genital region, inserting fin tips or beaks into the genital slits, and intromission of the fibro-elastic penis. Dolphins typically mate belly-to-belly, but mounting may involve males approaching at a different angle to the female's body. Copulations generally are brief, lasting less than a minute, but they may be repeated. Dolphins have large testes and exceptionally high sperm counts, facilitating multiple copulations.

Data for defining mating systems are difficult to collect for dolphins, but genetic studies are now allowing some of the first dolphin paternity testing, and continued work should clarify understanding. Available evidence suggests that monogamy is not a practice in which dolphins engage. Bottlenosed dolphin paternity tests indicate that females may use different sires for subsequent calves. For the better-studied dolphins, associations between breeding males and females tend to be brief, lasting days to weeks, and one male or male coalitions may associate with one receptive female at a time, sometimes battling with other males for access to the female. Males may move between female groups during a breeding season. This pattern has been referred to as serial polygyny or promiscuity.

Dolphin reproduction can occur anywhere within the animals' range, but calf rearing may lead to shifts in habitat use to more protected or productive areas, or to the creation of nursery subgroups of mothers with calves inside of larger dolphin groups. Reproductive seasonality may dictate where reproduction occurs for species that move over large areas or migrate seasonally. Seasonality tends to be most evident in environments that experience strong seasonal variations in water temperature or other environmental factors such as flooding cycles for riverine species. For example, most bottlenosed dolphins have well defined breeding seasons that vary with latitude, but births, after a 12-month gestation, tend to occur as temperatures warm and food is abundant. Similarly, tucuxi dolphins give birth during flood stage in the Amazon River, when food is most abundant, after a 10-month gestation.

Sexual maturity occurs between five and 16 years of age for dolphins, depending on the species. Larger species tend to mature later, and females tend to mature before males. Dolphins usually produce a single calf after gestation periods of 10–15 months, again depending on the species. The calf is born tail-first, and once the mother snaps the umbilical cord, the calf swims to the surface for its first breath. Following expulsion of the placenta, the calf begins nursing from the nipples on each side of the mother's genital slit. Nursing bouts are brief, but repeated often, as the calf receives milk that is very rich in fat. Calves may begin to capture small prey on their own when they are only a few months old, but may not be fully weaned from milk until they are up to 3.5 years old, and bottlenosed dolphins and pilot whales more than seven years old have been found with lactating mothers or with milk in their stomachs. The association between mother and calf may last for one or more years beyond nutritional weaning, suggesting the importance of learning and protection. Killer whales carry this pattern to an extreme, with adult female and male offspring remaining within the mother's pod. For all dolphins examined to date, it seems that mothers are fully responsible for calf rearing; paternal investment is limited to insemination.

Conservation status

The conservation status of dolphins varies by species, sub-species, and population. According to the IUCN Red List, only Hector's dolphin is considered Endangered, due primarily to incidental mortality in fishing gear. Of the 34 dolphin species, 17 are Data Deficient and five are Lower Risk/Conservation Dependent. Given the lack of information available for 67% of the taxa, one should not draw too much solace from the current listing of only a single species as endangered. Data on population sizes and numbers of losses from specific human activities are lacking for many species.

Dolphins face many threats from human activities. The degree of risk from these threats varies from site to site. Few countries actively hunt dolphins in directed fisheries. Japan continues to harvest striped dolphins (Stenella coeruleoalba) in fisheries that involve driving schools ashore, harpooning, or crossbows. The dolphin population is in decline, and continued hunting is unsustainable. As numbers of striped dolphins decline, 10 other species of dolphins are being hunted in their place. International efforts to halt this fishery have not been successful. In Peru, Sri Lanka, the Philippines, and elsewhere, incidental catches of dolphins in fishing gear have led to the development of directed fisheries for thousands of dolphins for meat, using purse seines, harpoons, gillnets, and explosives. Efforts to regulate some of these fisheries have had little effect on takes due to difficulties with monitoring and enforcement.

Other directed dolphin fisheries around the world involve commercial collection of bottlenosed dolphins and other species for oceanarium displays, research, and military activities. The vast majority of dolphins collected from the wild are bottlenosed, but smaller numbers of killer whales, false killer whales, Pacific white-sided dolphins (Lagenorhynchus obliquidens), Commerson's dolphins (Cephalorhynchus commersonii), and rough-toothed dolphins (Steno bredanensis) have also been collected for public display and interactive programs. These fisheries typically involve relatively small numbers of individuals, but when removals are concentrated in small areas and emphasize young females, locally resident populations can be placed at risk. Data indicate that removal of individual bottlenosed dolphins from stable communities can adversely impact the remaining community members in terms of calf survivorship or availability of appropriate social associates. Modeling efforts have shown that the existing captive population of bottlenosed dolphins in North America, if breeding is managed appropriately, is sufficient to sustain itself with appropriate genetic diversity for decades without the need for new genes from the wild. Commercial collection of dolphins has been banned or restricted in a number of countries, but Cuba, Japan, and some African nations, among others, still engage in this activity.

Of much greater concern is the issue of dolphins killed incidentally in nets set for fish. One of the best known examples involved hundreds of thousands of pantropical spotted (Stenella attenuata) and spinner dolphins killed each year in the tuna seine net fishery in the eastern tropical Pacific Ocean from the 1950s into the 1990s. Currently, fewer than several thousand dolphins are killed each year in this fishery. This turnaround occurred as a result of changes in equipment and approaches, and increased efforts by the fishing crews to reduce mortalities, driven in large part by public outrage and pressure over the situation. Dolphins continue to be killed in large numbers in fishing nets in many parts of the world. Trawls, seines, and gillnets are responsible for dolphin mortalities. Efforts to make nets more reflective to dolphin echolocation, or the use of noisemaking "pingers" to alert dolphins to the presence of nets have met with mixed success. In the southeastern United States, most states have banned the use of large gillnets in inshore waters, but North Carolina continues to allow such fishing, leading to numbers of deaths of bottlenosed dolphins in excess of what scientists consider to be sustainable. Efforts are underway through a federal "take reduction team" negotiation process to bring stakeholders together to arrive at a solution that considers the needs of the animals as well as those of the humans involved.

Other kinds of fishing activity also impact dolphins. Baited hooks on long-lines set for swordfish and other species kill or injure dolphins such as pilot whales. Monofilament fishing line and lures and hooks used in recreational fishing kill or injure dolphins through entanglement and ingestion. Crab trap float lines also entangle and drown dolphins.

Among the most insidious, worldwide threats to dolphins are environmental contaminants. More than 10,000 chemicals have entered the environment as a result of human activities. Many of these persistent toxic chemicals enter the dolphins' environment through airborne deposition and runoff, and work their way up through the food chain into dolphins through the fish they eat. Many of these chemicals bind with lipids, and as a result, they accumulate in fatty tissues such as blubber and are transferred to young through fat-rich milk. Chemicals such as PCBs and DDT metabolites and other pesticides have been found in some species at levels of great concern. Based on recent population declines correlated with measured levels of contaminants in blubber samples, killer whales in Puget Sound have recently come under scrutiny for endangered species status. Similarly, concentrations of contaminants in bottlenosed dolphin blubber from the southeastern United States have been measured at levels in excess of those of concern for human health, and evidence is mounting for the role of these contaminants in first-born calf mortality and in the decline of immune system function. While some chemicals have been banned as knowledge of their effects on health and reproduction become known, some will persist in the environment for decades to come; additionally, other chemicals are now emerging as potential sources of concern.

Habitat degradation and loss can take many forms for dolphins. Shoreline alteration and dredge-and-fill operations can decrease productivity of the prey that supports dolphin populations. For riverine species such as Irrawaddy dolphins and tucuxi, dams across rivers can isolate subpopulations, or prevent access to critical resources. Pollution from sewage, garbage, and other wastes can threaten dolphin health. Heavy fishing can lead to competition with fisheries for prey. Marine construction and vessel operations introduce noise that can lead to disturbance of normal activities, or interference with communication or biologically important sounds for the dolphins. High-energy pressure waves from explosives or military activities may injure or kill dolphins. Boat collisions also cause dolphin deaths and injuries in areas where high levels of boat traffic occur.

Significance to humans

Dolphins have figured prominently in human lives for thousands of years. They are depicted in ancient Greek and Roman artwork, are incorporated into mythology, and appear in early writings. Their relationships to humans have long been considered special, with numerous ancient and contemporary accounts of dolphins saving humans lost at sea. The dolphins' smile (actually a fixed fact of anatomy rather than an expression of emotion), their endearing (mostly trained) antics in oceanaria, films, and television, reports of complex social behavior, the ease with which they can be trained to perform complex behaviors, their problem-solving capabilities, and early (false) claims that they could likely communicate in English have supported the idea in many peoples' minds that these creatures should be considered superior to other animals, on an elevated plane with humans.

As more is learned about dolphins, people have begun to develop an appreciation for the animals because of how well they are adapted to life in the sea, without the need to credit them with supernatural powers. Today, millions of people worldwide visit marine parks to see, feed, and/or swim with captive dolphins, or view wild dolphins on dolphin-watching tours, creating a multi-billion dollar industry. It has been argued that this increased familiarity with the animals encourages action to protect them. Others argue that holding dolphins in captivity is cruel exploitation. Increased interest in dolphins is creating conservation issues in some cases, as people begin to feed, swim with, and otherwise disturb wild dolphins.

Dolphins have been and are now used by humans for other purposes besides entertainment and education. In some places, they assist artisanal fishermen, or are harvested as an inexpensive source of protein. They are studied to understand their exceptional sonar and diving capabilities, and to evaluate their cognitive abilities. Dolphins have also been used by the military in the United States and the former Soviet Union to search for weapons, assist divers, and perform surveillance tasks.

Species accounts

Resources

Berta, A., and J. L. Sumich. Marine Mammals: Evolutionary Biology. San Diego: Academic Press, 1999.

Leatherwood, S., and R. R. Reeves. The Sierra Club Handbook of Whales and Dolphins. San Francisco: Sierra Club Books, 1983.

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[Article by: Randall S. Wells, PhD]