Phylum: Chordata
Class: Mammalia
Order: Cetacea
Suborder: Odontoceti
Family: Delphinidae
Genus: Lagenorhynchus
Species: Lagenorhynchus obliquidens
Figure
1. Spyhopping
Source: Sylvestre 1993
San Francisco State University
Department of Geography
Geography 316: Biogeography
In progress 12/10/2001
The Biogeography of
Pacific White-sided Dolphin (Lagenorhynchus
obliquidens)
by Ka Man Chan, student in Geography 316, Fall 2001
| Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Cetacea Suborder: Odontoceti Family: Delphinidae Genus: Lagenorhynchus Species: Lagenorhynchus obliquidens |
Figure
1. Spyhopping |
The Pacific white-sided dolphin was named by Thomas Nicholas Gill, a librarian of the Smithsonian Institution, in 1865. Based on three skulls collected in San Francisco ten years earlier, he classified this species Lagenorhynchus obliquidens. The genus name Lagenorhynchus is derived from the Greek lagenos, meaning "bottle", and rhynchus, meaning "snout", while the species name obliquidens comes from the Latin words obliquus (slanting) and dens (teeth) (May 1990, Würtz 1998).
Description of Species Habitat Social Behavior Feeding Breeding Distribution Evolution Controversy
Figure 2. Its side and back |
| The Pacific white-sided dolphin is a fast acrobatic swimmer, that likes to perform somersaults. It has a short but well-defined beak, conical head, sickle-shaped dorsal fin, and a pair of short flippers. Each jaw consists of 21-28 pairs of small, acute, and cone-shaped teeth, which is a key difference between dolphins and porpoises (Evans 1987, Brust 1990). Its coloration is distinctive. The streamlined body has a black back, pearl-white belly, and gray sides. Light-gray stripes, or the “suspenders”, stretch along both of its sides. This species has a distinctive bicolored dorsal fin varying from dark in the front to light gray on the rear, sometimes with white patches (Leatherwood and Reeves 1983, Kreitman and Schramm 1995). The rubbery skin is smooth and firm, and it helps for gliding through water with less effort. A layer of fat storage, or blubber, lies underneath the skin to keep them warm and afloat (Brust 1990). An adult Pacific white-sided dolphin grows between seven to eight feet and weighs about 300 pounds. Generally, males are slightly larger than females. Studies show that the maximum size differs in various geographical areas (Leatherwood and Reeves 1983, Klinowska 1991, Kreitman and Schramm 1995). |
Figure 3. Porpoising |
These dolphins, as well as the order of Cetacea, were once land creatures. Throughout million years of evolution, the animals have adapted completely to the water (Brust 1990). Land has now become a barrier. Pelagic Pacific white-sided dolphins are restricted to the temperate Northern Pacific Ocean. They seem to like ocean bottoms with ridges and slopes, which are excellent areas for prey (Leatherwood and Reeves 1987). Although these dolphins are considered to be deepwater marine mammals, occasionally they can be found 100 miles away from the shore (Kreitman and Schramm 1995). Shifting northward and southward, or inshore and offshore, depends on water temperature change. Compared with large whales, these dolphins travel relatively short distances. They can also migrate hundreds of miles seasonally for prey availabilities (Leatherwood et. al 1988, Brust 1990).
Pacific white-sided dolphins are gregarious. Groups typically consist of 5 to 100 individuals, but they can form large schools up to a thousand individuals during migration (Evans 1987). The schools generally include all ages and both genders and occasionally mix with other species; common dolphins and Risso's dolphins are their favorite company (Patent 1987, Leatherwood et. al 1988, Sylvestre 1993). Group sizes and shapes change depending on context. The numbers of herds tend to be smaller in shallow water. When traveling, the dolphins may either swim close together or form a line side by side. When feeding, they divide into sub-groups of between 10 to 20 individuals (May 1990). Researchers suggest that schooling helps the community with finding food, mates, and predator defense (Evans 1987). |
Figure 4. Diving |
Feeding:
Pacific white-sided dolphins are nocturnal carnivores; they feed primarily
on squid and small fish, such as anchovies, hakes, herrings, and sardines. These fish prey
on microscopic animals called zooplankton. To avoid the sunlight, the zooplanktons remain
in the deep ocean during the day. When these tiny animals rise to the surface to eat
phytoplankton at night, some of them become the fish's and squid's prey. Consequently, the
fish and squid are fed upon by the dolphins (May 1990). Like other dolphin species,
Pacific white-sided dolphins use sound waves, called echolocation, to find food and
navigate through the environment. Instead of speaking, they make a clicking sound, and the
clicks travel and bounce back to them. From the echoes, they are able to determine an
object's direction and distance (Brust 1990, National Marine Mammal Laboratory 2001). The
dolphins just use their teeth to grasp food; in fact, they swallow the whole fish without
chewing. Adults can consume approximately 20 pounds of food per day (Sylvestre 1993,
National Marine Mammal Laboratory 2001).
| Researchers believe that dolphins are "highly unlikely" to be monogamous (Connor 1994). This species' sexual maturity age remains unknown, but studies show that the reproductive maturity of other species of the same family ranges from 3 to 16 years old (Evan 1987). Maturity is determined by their growth in length. When males reach at 5.6-6 feet, and females reach 6-6.3 feet, these dolphins are biologically ready for reproduction. Mating usually occurs in late summer and early fall. Gestation period last for 9-12 months, with an average of 10 months. Female dolphins usually give birth to a single calf once two or three years ( National Marine Mammal Laboratory 2001). During labor, the mother remains near the water surface. Normally, the calf's tail comes out first, and it swims to the surface for its first breath (Carwardine 1995, Würtz 1998). Newborns are about 3 feet long and weigh 30 to 50 pounds. The calf is fed on milk and is nurtured for about a year by its mother (Leatherwood et. al 1988). |
Figure 5. A cute couple |
Distribution:
The cosmopolitan Delphinidae family can be found all over the
pelagic world, except for the polar seas. This continuously distributed Lagenorhynchus
genus mainly inhabits cool waters (Nowak 1997). One out of six Lagenorhynchus members,
Lagenorhynchus obliquidens, is an example of circumboreal distribution.
Pacific white-sided dolphins inhabit primarily deep offshore, temperate waters of the
North Pacific, north of the tropics and south of the Arctic (21°
N to 60°N).
The population is divided into two stocks; one is in the northeastern Pacific, and the
other one is in the northwestern Pacific. Separating the two stocks is an area with low
population density along the southern Aleutian Islands (Klinowska 1991).
This species’ range is considerably extensive. The eastern part
of the range extends southward to Baja California and northward to the Gulf of Alaska. The
population was estimated to be between 30,000 to 50,000 in 1993 (Sylvestre 1993). In the
western part of the range, it extends from Kril Islands to the entire Pacific coast of
Japan. They have been reported near Cedros Island and Natividad Island in Baja California
year around. Small numbers can also be found near Monterey, California to Gordo Bank
offshore of Baja California throughout the year (Leatherwood et. al 1988). They appear to
be the most abundant dolphinids in the temperate northeastern Pacific, and the second or
third most common species in southern California in winter (Klinowska 1991).
The reasons for seasonal migrations in most areas are not yet known. However, studies have found that Pacific white-sided dolphins tend to move near the shore of southern California during the winter and spring, and then move northward and out to the ocean during the summer and fall. These seasonal movements may be related to the availability of warm waters and prey (Leatherwood et. al 1988, Sylvestre 1993, Nowak 1997).
Map of Distribution:
|
| Dolphins were regarded as fish by Greek philosophers. Until 1758, when
Linnaeus rectified this mistake, he classified them as mammals (Coffey 1977). Unlike fish,
dolphins use lungs to breathe. They are warm-blooded, with a steady body temperature
regardless of the surrounding temperature. Their calves are nurtured in mothers’ womb
and fed with milk after birth (Brust 1990). Dolphins, whales, and porpoises belong to the order of Cetacea which is comprised of three suborders: Archaeoceti (ancient whales), Mysticeti (baleen whales), and Odontoceti (toothed whales). Tracing the evolution of cetaceans, most scientists agree that cetaceans and ungulates (hoofed mammals), such as sheep, cows, and pigs, descend from the same ancestor. Evidence of this has been found by Boyden and Gemeroy at Rutgers University, New Jersey in 1950 (Würtz 1998). Later, Sarich and Lowenstein, researchers at University of California Berkeley and U.C. San Francisco respectively, studied proteins in blood serum and found a striking relationship between cetaceans and hippopotamus (Würtz 1998); however, DNA studies have not yet confirmed this discovery (Gulf of Marine Aquarium 1999).
|
Figure 6. Evolution: from Proteutheria to Odontoceti
|
The origin of cetaceans traces back to the early Paleocene (65 million years ago) (Würtz 1998). It used to be believed that cetaceans evolved from insectivores (Coffey 1977, Leatherwood and Reeves 1987). Now, scientists believe that the order Cetacea and Artiodactyla, the progenitor of hoofed animals, shared a common ancestor-Condylarthra. The family Mesonychidae of Condylarthra order inhabited the riverfronts of Tethys Sea, which is today’s Mediterranean Sea and Persian Gulf. Their dentition indicated that this family included herbivores and carnivores. At the beginning, these terrestrial animals preyed on mollusks and slow-moving fish. As the growing population increased resource competition, their teeth developed to be capable of catching fast-swimming fish as well. Towards the end of Paleocene, the mesonychids gave rise to archaeocetes, the true cetaceans (Gakin 1982, Evans 1987, Würtz 1998). Fossils found in Pakistan indicate a link between mesonychids and archaeocetes. About 52 million years ago, Ambulocetus natans adapted to aquatic life and was able to move on dry land as well. It swam by paddling its webbed back feet and undulating its long tail. Its movement on dry land resembled seal and sea lions (Würtz 1998). The Protocetidae is the earliest family of archaeocetes; they were not physiologically well adapted to the ocean. Paleontologists discovered some fossil remains of Pappocetus in Nigeria and Egypt in the 1960’s. Pappocetus was comparatively small (8 ft). The nostrils were still in the front, and the inner ears had not yet developed to adapt to live in water. But, their various teeth forms suggested different levels of adaptation to food in the water. Scientists believe that until the Eocene (50 million years ago), the protocetids had completely adapted to aquatic life based on the fossils of primitive Pakicetus found in Pakistan in 1980 (Würtz 1998). During the Oligocene (38-25 million years ago), the Archaeoceti diversified into two suborders of the modern cetaceans: Odontoceti and Mysticeti. The body became more streamlined, and a dorsal fin had developed. Unlike the previous suborders, nostrils retreated further back along the head. By the late Oligocene, the teeth of odontocetes evolved into cone shapes and became homodont (Evans 1987). |
 Figure
7. Evolution: from Mesonychidae to Delphinidae |
The Squalodelphidae, the primitive dolphins family, appeared in the early Miocene (23 million years ago). Fossil records suggested that they inhabited in Europe, North and South America, and the freshwater of Australia. Most of them were about three meters in length with long snouts and lots of homodont teeth (Evans 1987).
The family Delphinidae came in the late Miocene (11 million years ago). According to the fossils found mostly in Europe, scientists suggest that this is the most diverse cetacean family, which includes today's common dolphins, killer whales, and Pacific white-sided dolphins (Evans 1987).
Today, after 65 million years of monophyletic evolution, cetaceans have branched off into 9 families, 38 genera, and 76 species. Pelagic dolphins are distributed all over the ocean from the tropics to the temperate regions, while freshwater dolphins can be found in Gange, Indus, or Amazon river systems.
Between 1960 and 1990, 4,600 porpoises, dolphins, and small whales were caught for research, military, or display purposes. The Pacific white-sided dolphin was the first dolphin to be drafted into the US Navy. In 1960, researchers at the Naval Ordnance Test Center at China Lake, California, obtained a dolphin from a park in Los Angeles and named her Notty. The US Navy was interested in Notty's streamlined body, and it hoped to find out some ideas to improve the torpedo design (May 1990). Training and working with the dolphins have continued since
Arguments about captivity are extreme. Some argue that it is inhumane while others argue that it is good for educational purposes; people get a chance to see the animals in person. That certainly brings more profits to marine parks and aquariums. However, 75 percent of the captive dolphins are females and most of them are immature, this affects the population in a long run (May 1990). Besides, they lost their abilities to hunt food. If they are released back to the ocean, they will be less likely to survive.
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