Bottlenose Dolphins (marine mammals)


I. Genus and Species

Bottlenose dolphins are arguably the best known of all cetaceans. They figured prominently in the legends of the ancient Greeks and Romans and were described in the writings of Aristotle, Oppian, and Pliny the Elder.Public audiences have focused on these species (e.g., Caldwell and Caldwell, 1972; Leatherwood and Reeves, 1990; Reynolds et al, 2000), and a number of comprehensive review articles have been produced as well (Tomilin, 1957; Leatherwood and Reeves, 1982; Shane et al, 1986; Wells and Scott, 1999). The name Tursiops can be translated as “dolphin-like,” deriving from the Latin Tursio (“dolphin”) and the Greek suffix -ops (“appearance”). Two species of Tursiops, T. truncatus, the “common bottlenose dolphin,” and T. aduncus, the “Indian Ocean bottlenose dolphin,” are currently recognized (Rice, 1998), pending revisions based on recent genetic information.

Bottlenose dolphins are cosmopolitan in distribution and demonstrate a great deal of geographical variation in morphology. T. truncatus is found in most of the worlds warm temperate to tropical seas, in coastal as well as offshore waters; T. aduncus is limited to the coastal waters of the Indian Ocean and Western Pacific Ocean, from eastern Africa to Taiwan, southeastward to Australia (Fig. 1). They are recognizable by their generalized appearance—a medium-sized, robust body, a moderately falcate dorsal fin, and dark coloration, with a sharp demarcation between the melon and die short rostrum (Figs. 2 and 3). Adult lengdis range from under 2 m to about 3,8 m, vaiying by geographic location (Cockcroft and Ross, 1990; Mead and Potter, 1990; Wells and Scott, 1999). Body size appears to vary inversely with water temperature in many parts of the world (but not the eastern Pacific). Bottlenose dolphins are colored light gray to black dorsally and laterally, with a light belly (Fig. 3). A light blaze or brush marking is sometimes observed on dieir sides. A distinct cape may be visible or may be obscured when the color pattern is very dark. T. aduncus tends to be smaller than T. truncatus, has a proportionately longer rostrum, and develops ventral spotting at about the time of sexual maturity (Ross and Cockcroft, 1990).

Shading indicates the species range of the common bottlenose dolphin, Tursiops truncatus. It is not possible to be explicit regarding the range of the Indian Ocean bottlenose dolphin, Tursiops aduncus, because of uncertainties regarding the taxonomic status of bottlenose dolphins in the Indian Ocean, but the species generally inhabits the coastal waters of the Indian and Western Pacific Oceans, along the entire eastern coast of Africa, through the Red Sea and Persian Gulf, eastward as far as Taiwan, and southeastward to the coastal waters of Australia.

Figure 1 Shading indicates the species range of the common bottlenose dolphin, Tursiops truncatus. It is not possible to be explicit regarding the range of the Indian Ocean bottlenose dolphin, Tursiops aduncus, because of uncertainties regarding the taxonomic status of bottlenose dolphins in the Indian Ocean, but the species generally inhabits the coastal waters of the Indian and Western Pacific Oceans, along the entire eastern coast of Africa, through the Red Sea and Persian Gulf, eastward as far as Taiwan, and southeastward to the coastal waters of Australia.

Variation in size, coloration, and cranial characteristics associated with feeding have led to descriptions of at least 20 nominal species of Tursiops (Hershkovitz, 1966, Rice, 1998). Recognition of the polymorphic nature of Tursiops and the existence of clinical variation had led to general agreement for many years that Tursiops was a single-species genus. However, recent genetic, morphologic, and physiologic studies suggest that revision of the genus may be necessary to acknowledge significant differences between forms from different oceans, as well as differences between forms in inshore vs offshore habitats within ocean basins (Hersh and Duffield, 1990; LeDuc et al, 1999: Mead and Potter, 1995; Rice, 1998). Inshore bottlenose dolphins in the Atlantic and some other regions tend to be smaller, lighter in color, have proportionately larger flippers, and differ in hematologic and mitochondrial DNA features from offshore forms (Hersh and Duffield, 1990; LeDuc et al, 1999); however, eastern Pacific offshore bottlenose dolphins are smaller and darker than inshore forms. The taxonomic status of Tursiops is made even more confusing by observations of hybridization with several other odontocete species.

Recent genetic evidence suggests that T. aduncus is more closely related to pelagic Stenella and Delphinus species, in particular S. frontalis, than to T. truncatus (LeDuc et al, 1999). This interpretation has more than just taxonomic implications. It could change the previous focus on differences in morphology and social behavior that were thought to reflect the polymorphic nature and behavioral plasticity of a single species to a renewed exploration of the many similarities perhaps brought about by evolutionary convergence in adaptations to a coastal/estuarine environment or, alternatively, a retention of primitive characteristics (plesiomorphy).

Lateral view of an adult male common bottlenose dolphin.

Figure 2 Lateral view of an adult male common bottlenose dolphin.

II. Distribution and Habitat

Common bottlenose dolphins are found in the temperate and tropical marine waters of the world (Fig. 1). In the North Pacific, they are commonly found as far north as the southern Okhotsk Sea, the Kuril Islands, and central California. In the North Atlantic, they are seen inshore during summer months off New England, offshore as far north as Nova Scotia, and have been recorded off Norway and the Lofoten Islands. Bottlenose dolphins occur as far south as Tierra del Fuego, South

Ventral view of a common bottlenose dolphin.

Figure 3 Ventral view of a common bottlenose dolphin.

Africa, Australia, and New Zealand. Limits to the species’ range appear to be temperature related, either directly or indirectly, through distribution of prey. Off the coasts of North America they tend to inhabit waters with surface temperatures ranging from about 10 to 32°C. At the northern limit of the species’ range in the western North Atlantic, they are seasonally migratory, with a more southerly distribution in the winter.

Indian Ocean bottlenose dolphins are found in the coastal waters of eastern Africa from South Africa northward to the Red Sea and the Persian Gulf, and eastward to Taiwan and Australia (Rice, 1998), where they typically occur year-round.

Tursiops inhabits most warm temperate and tropical shorelines, adapting to a variety of marine and estuarine habitats, even ranging into rivers. Both species of bottlenose dolphins tend to be primarily coastal, but T. truneatus is also found in pelagic waters, near oceanic islands, and over the continental shelf, especially along the shelf break. In the Indian Ocean, T. truneatus tends to inhabit offshore waters, whereas T. aduncus is the more common coastal species.

III. Food and Feeding

The diets of common bottlenose dolphins have been described from many regions. A large variety of fish and/or squid forms most of the diets, although bottlenose dolphins seem to show a consistent preference for sciaenids, scombrids, and mugilids. Most fish prey are bottom dwellers, but some surface dwellers or pelagic fish are also represented in the diets. Differences in diets have been found where both inshore and offshore Tursiops ecotypes have been identified. In some cases, bottlenose dolphin groups feed in different areas depending on sex and size, with lactating females and their calves frequenting and feeding in the nearshore zone, adolescents feeding slightly farther offshore, and resting females and adult males feeding farther still.

Indian Ocean bottlenose dolphins also consume a variety of fish and squid (Cockcroft and Ross, 1990). Their prey includes fish with preferences for reefs or sandy bottoms and some inshore schooling fish.

IV. Predation

Sharks are probably the most important predators of bottlenose dolphins, although killer whales may also occasionally prey on them as well. Mutual tolerance during encounters between sharks and dolphins is probably typical, but as many as half of all bottlenose dolphins bear shark-bite scars as evidence of occasional encounters, depending on the region. Populations of Indian Ocean bottlenose dolphins in Australian waters are apparently subject to more frequent shark attacks than most populations of common bottlenose dolphins (Wood et al., 1970; Corkeron et al., 1987; Wells etal, 1987; Cockcroft etal, 1989a; Mead and Potter, 1990). In at least some areas, Tursiops appears to be a relatively minor and occasional part of the diets of sharks. Most wounds and scars from sharks tend to be found on the posterior and ventral regions of the dolphins, suggesting that the dolphins were ambushed from behind and below; some attacks may have been something other than a predation attempt (e.g., sharks defending a territory). The primary shark-predators of both T. truncatus and T. aduncus are the bull shark (Carcharhinus leucas), tiger shark (Galeocerdo cuvier), great white shark (Carcharodon carcharias), and dusky shark (Carcharhinus obscurus) (Wood et al, 1970; Corkeron et al, 1987; Connor et al, 1999). Observations of captive dolphins suggest that they may recognize certain species of sharks as potential threats.

Anecdotal accounts describe common bottlenose dolphins attacking sharks by butting them with their rostra or by striking them with their flukes (summarized by Wood et al, 1970). Defense may explain the apparently high survival rate indicated by the shark-bite scars on living dolphins. The relatively infrequent occurrence of shark-bite scars on young dolphins indicates either that the calves are well protected by their mothers or that attacks on young dolphins are generally fatal.

Stingrays have also been implicated in the deaths of common bottlenose dolphins (Wells and Scott, 1999). The dolphins were wounded externally or internally from ingestion of small rays, and deaths resulted from physical trauma, infection, or toxicosis.

V. Ranging Patterns

Coastal common bottlenose dolphins exhibit a full spectrum of movements, including seasonal migrations, year-around home ranges, periodic residency, and a combination of occasional long-range movements and repeated local residency (Shane et al, 1986; Wells and Scott, 1999). Much less is known about the ranging patterns of pelagic and Indian Ocean bottlenose dolphins. In some places, coastal dolphins living at the high-latitude or cold-water extremes of the species’ range may migrate seasonally, as is the case along the Atlantic coast of the United States. Long-term residency has been reported from many parts of the world and may take the form of a relatively permanent home range or repeated occurrence in a given area over many years. For example, the year-round residents of several dolphin communities along Florida’s west coast have maintained relatively stable, slightly overlapping home ranges during more than 30 years of observations, and through at least four generations; seasonal changes in habitat use may occur within the ranges (Wells and Scott, 1999). Home range bounds are often demarcated by physiographic features such as passes or abrupt changes in water depth. Some common bottlenose dolphins may use seasonal home ranges joined by a traveling range. At Shark Bay, Western Australia, a group of Indian Ocean bottlenose dolphins has returned to the same beach for more than 20 years to interact with humans (Connor et al, 1999).

Longer-distance movements have been reported for some coastal common bottlenose dolphins, including range shifts of several hundred kilometers in an apparent response to environmental changes such as an El Nino warm-water event and a 600-km roundtrip for several identifiable dolphins in Argentina. Average daily movements of 33-89 km, monitored through travel distances of as much as 4200 km, have been reported for bottlenose dolphins in offshore waters (Wells et al, 1999).

VI. Social Behavior

Common bottlenose dolphins are typically found in groups of 2-15 individuals, although groups of more than 1000 have been reported. In general, bottlenose dolphins in bays and estuaries tend to form smaller groups then those in offshore waters, but the trend does not continue linearly with increasing distance from shore (Wells et al, 1999). Group composition tends to be dynamic, with sex, age, reproductive condition, familial relationships, and affiliation histories apparently being the most important determining factors. Subgroupings may be stable or repeated over periods of years. Basic social units include nursery groups, mixed-sex groups of juveniles, and adult males as individuals or strongly bonded pairs or trios.

Indian Ocean bottlenose dolphins are also found in groups of variable size. Numbers observed off South Africa range from 3 to 1000, around a mean of 140, whereas off Australia, groups average about 10 individuals in Moreton Bay, and average about 5, ranging up to 22, in Shark Bay (Connor etal, 1999; Corkeron, 1990).

Dominance hierarchies have been observed for both common and Indian Ocean bottlenose dolphins in captivity, with large adult males dominating all other pool mates, females forming a less-rigid hierarchy, and with the largest females dominant over smaller animals. Aggressive behaviors, including contact and posturing, are used to establish and maintain hierarchies. In Shark Bay, Australia, coalitions of male T. aduncus fight with other male coalitions and aggressively herd females (Connor et al, 1999).

VII. Activity Patterns

Common bottlenose dolphins in the wild appear to be active both during the day and night, interspersing bouts of feeding, traveling, socializing, and idling or resting (Shane et al, 1986; Wells et al, 1999). The duration and frequency of activities are influenced by such environmental factors as season, habitat, time of day, and tidal state and by physiological factors such as reproductive seasonality. Bottlenose dolphins feed in a large variety of ways, primarily as individuals, but cooperative herding of schools of prey fish also occurs. Individual prey capture involves behaviors as diverse as high-speed chases with a pin-wheeling capture at the surface, “fish whacking” in which a fleeing fish is struck with the dolphin’s flukes and often knocked clear of the water, pushing fish onto shore and then partially beaching to capture them, and herding and perhaps disorienting fish with percussive leaps and tail lobs referred to as “kerplunking.” Many of these same activities and behaviors have been observed for T. aduncus as well.

VIII. Life History

More detailed life history information is available for T. truncatus than for T. aduncus. Analyses of dentinal and cemental growth layer groups in teeth (Hohn et al, 1989) have shown that female common bottlenose dolphins can live to more than 50 years, and some males have reached 40-45 years of age (Wells and Scott, 1999). Calves achieve most of their growth during the period of suckling, i.e., the first 1.5-2 years of life. Females typically reach sexual and physical maturity before males, leading to sexual dimorphism in some regions. Age at sexual maturity varies by region, but females usually reach sexual maturity at 5-13 years. Sexual maturity for males tends to occur at 9-14 years, often many years before they reach physical maturity (late teens) and achieve breeding status. Most breeding males in captivity are at least 20 years old (Wells et al, 1999).

Indian Ocean bottlenose dolphins often develop ventral spotting at sexual maturity, when males are >235 cm in length and females are >230 cm (Ross and Cockcroft, 1990). Sexual maturity may be attained at an older age for T. aduncus than for T. truneatus, with females producing their first calf at age 12 or older (Connor et al, 1999).

IX. Reproduction

Although births have been reported from all seasons, calving tends to be diffusely seasonal for both T. tmncatus and T. aduncus, with peaks during warmer months (Connor et al, 1999). Hormonal monitoring of captive common bottlenose dolphins indicates that females are spontaneous sporadic ovu-lators, ovulating repeatedly during a given season, whereas males may be active throughout the year with a prolonged elevation of testosterone concentrations over the months that different females may be ovulating. The reproductive life span for T. truneatus is prolonged; females up to 48 years of age have successfully given birth and raised young (Wells and Scott, 1999). Calves are born after a gestation period of about 1 year and range in length from about 84 to 140 cm depending on the geographic region. Calving intervals of 3-6 years are common for T. truneatus, whereas 4- to 6-year intervals are more common for T. aduncus (Connor et al, 1999).

Lactation is the primary source of nutrition for the first year of life and may continue for several more years. Solid food has been found along with milk in the stomachs of calves as young as 4 months old. Maternal investment for free-ranging T. truneatus calves typically extends for about 3-6 years, with separation often coinciding with the birth of the next calf. T. aduncus calves in Shark Bay, Australia, typically attempt to nurse for 3-5 years (Connor et al, 1999). Simultaneously pregnant and lac-tating females have been noted on occasion for both species.

X. Sound Production

Most of what is known about bottlenose dolphin sound production has resulted from detailed studies of T. tmncatus, but it is reasonable to assume that T. aduncus acoustic patterns are comparable. Bottlenose dolphins produce these categories of sounds: whistles, echolocation clicks, and burst-pulse sounds. Dolphins produce a large variety of whistles, including largely stereotypic “signature whistles” that are individually specific, hypothesized to be used to communicate identity, location, and emotional state (Caldwell et al, 1990). Once the signature whistle develops in neonates, it remains stable for at least many years, and probably for life. The signature whistles of many male calves are similar to the whistles of their mothers, whereas those of female calves are not. Dolphin echolocation involves the production of “clicks,” with peak frequencies of about 40-130 kHz (Au, 1993). Echolocation is hypothesized to be used in navigation, foraging, and predator detection, among other possible functions. Burst pulses (“squawks”) tend to be produced during social interactions.

XI. Fossil Record

Although no conclusive fossil evidence of the origin of Tursiops exists, fossil records extend back several million years (Barnes, 1990). The geographic distribution of fossils falls within the range of modem animals. Anatomic features suggest that Tursiops evolved from some ancestral group of extinct fossil Delphininae, perhaps related to the subfamily Stenoninae, which might have evolved from the Kentriodontidae.

XII. Human Interactions

Common bottlenose dolphins take advantage of human activities in order to facilitate prey capture in a variety of ways. In Mauritania and Brazil, dolphins regularly drive schools of mullet toward fishermen wading with nets in shallow water, and in many parts of the world dolphins collect discarded fish from behind shrimp trawls and small purse seines or steal fish from various types of fishing gears.

A. Live Maintenance

The first Tursiops were publicly displayed at the Brighton Aquarium in 1883, at the New York Aquarium in 1914, and have been a regular attraction at Marineland of Florida since 1938. Common bottlenose dolphins continue to be the most common dolphins maintained in captivity throughout the world. According to a May 2000 National Marine Fisheries Service inventory, 35 U.S. facilities held 392 common bottlenose dolphins. In addition, several hundred bottlenose dolphins, mostly T. truneatus but also including some T. aduncus, were held in at least 16 other countries. Within the United States, approximately 70% of the dolphins are held primarily for public display, whereas the remainder are used primarily for research or military purposes. Improved facilities and increased knowledge about the requirements for the care of dolphins have led to increasing success in the long-term maintenance of the animals, to the point where birth and survivorship rates at the better facilities approach and, possibly in a few cases, surpass those of wild populations (Wells and Scott, 1999).

B. Directed Fisheries

The largest of the historical fisheries for common bottlenose dolphins involved several countries surrounding the Black Sea, where dolphins were caught for oil, meat, and leather. Because of declines in dolphin populations, these countries have since outlawed the fishery. Directed takes still occur in other parts of the world, such as Peru, Sri Lanka, and Japan, for human consumption, to reduce the perceived competition with commercial fisheries, or for bait. Live-capture fisheries for dolphins for public display have existed for more than 100 years. More than 1500 Tursiops were removed from the waters of the United States, Mexico, and the Bahamas by 1980 for display, research, or military applications in many parts of the world (Leather-wood and Reeves, 1982). Although no bottlenose dolphins have been collected in U.S. waters since 1989, some small-scale, live-capture fisheries for T. truncatus continue in other countries such as Mexico, Cuba, and Japan, and some T. aduncus have been captured in recent years in Taiwan.

C. Incidental Fisheries

Incidental catches of small numbers of T. truncatus have been reported for several fisheries, including purse-seine fisheries for tunas, sardines, and anchovetas (Wells and Scott, 1999). In some cases, dolphins have been killed by fishermen to prevent damage to their fishing gear or stealing of the catch or bait by the dolphins (Leatherwood and Reeves, 1982). A large incidental take of T. aduncus has apparently occurred in the Taiwanese gill-net fishery off Australia, with an annual mortality perhaps exceeding 2000 animals (Northridge, 1991). Large numbers of Indian Ocean bottlenose dolphins have also been killed in nets set for sharks off the swimming beaches of South Africa and Australia. In the United States, entanglement in or ingestion of recreational fishing gear has resulted in dolphin mortality.

D. Habitat Alteration, Pollution, and Vessel Impacts

Although the impact of habitat alteration and pollution on dolphins has not been studied systematically, anecdotal accounts suggest that human-caused degradation may have led to declines in some dolphin populations (Wells and Scott, 1999). Extremely high concentrations of chlorinated hydrocarbon residues have been found in the tissues of Tursiops in many parts of the world, with males accumulating higher concentrations than females with age (O’Shea, 1999). Cockcroft et al. (1989b) reported that first-born calves of South African bottlenose dolphins (identified by the authors as T. truncatus) received 80% of their mother’s body burden of contaminant residues (poly-chlorinated biphenyls and dieldrin), perhaps leading to increased neonatal mortality, but also reducing levels of contaminants in the mothers. The accumulation of contaminants in tissues of males has reached levels that theoretically could impair testosterone production, and thus reduce reproductive ability. Preliminary findings suggest that even relatively low levels of PCBs and DDT metabolites can result in a decline in bottlenose dolphin immune system function. Responses to other human use of dolphin habitat through boating, dolphin feeding, and swimming with dolphins are receiving much current research attention, but it is clear that common bottlenose dolphins suffer mortality and serious injury from collisions with boats and that their behaviors change in the presence of vessels.

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