The Pacific White-sided Dolphin:

Lagenorhynchus obliquidens

Selena Healey

Marine Mammals 342-001

Professor Tudge

October 4th, 2016

Abstract

Lagenorhnychus obliquidens, the Pacific White-sided Dolphin, is a widely spread species

with a high population endemic to the temperate ocean waters of the Pacific Ocean (IUCN). This

species is largely located in the norther pacific, but does reach as far south as Mexico and

Taiwan (Berta, 2015). There appear to be two genetically distinct groups of Pacific White-sided

Dolphin located along the western coast of the United States, but other than slight differences in

size between these two populations, very few differences can be discerned between the two

(Allen, 2011). This species shares the standard characteristics of the family Delphinidae, with

slight adaptations that have made this species more competitive in its local regions. This species

is best characterized by its large scythe shaped dorsal fin and distinctive pattern colorations and

is easily identifiable to researchers and tourists alike (Berta, 2015). This is an extremely

energetic and playful species that exhibits an array of surface behaviors like porpoising and bow

riding, as well as aerial somersaults (Allen, 2011). The Pacific White-sided Dolphin, compared

to most other mammals of its size, has a considerably higher daily energy requirement and preys

on fish and cephalopods to meet these energy needs (Rechsteiner, 2013). The interactions that

this species has with its prey and predators makes its existence in the pacific ecosystem

incredibly important in maintaining proper population balances.

Introduction

The Pacific White-sided Dolphin is a widely abundant species located throughout the

temperate ocean waters of the Pacific. This species encompasses a large range and has

documented populations reaching from southern Alaska to northern Mexico on the eastern half

of the Pacific, and as far south as Taiwan in the western half of the Pacific (IUCN). This wide

and consistent population spread makes this species particularly relevant to several countries.

This species plays important roles in ecosystem regulation that can affect the fishing practices of

many nations, and so monitoring these populations proves very important. This is a very social

species that interacts extensively with other species of dolphins and marine mammals, making it

further impactful on ecosystems. In this paper, numerous aspects of this species will be explored,

from the classification and evolutionary rise of the Pacific White-sided Dolphin, to the general

biology and anatomy, and so on. By exploring different aspects of this species a better

understanding of the requirements and necessity of this animal in the Pacific ecosystem will

become apparent.

Systematics and Classification

The Pacific White-sided Dolphin is known under many different names in the scientific

and general community. Some of its common names, besides “Pacific White-sided Dolphin,”

include the “Pacific Striped Dolphin, White-striped Dolphin, and Hook-finned Porpoise,”

depending on regional terminology (Allen, 2011). Its official taxonomic classification is:

Animalia, Chordata, Mammalia, Cetartiodactyla, Delphinidae, Lagenorhynchus obliquidens

(IUCN). Being of the class Mammalia, the Pacific White-sided Dolphin shares all of the

common mammalian characteristics. It is warm blooded, gives birth to live young, and nurses

them on milk produced in the female animal’s mammary glands. The order Cetartiodactyla

groups together two sub-orders, the Artiodactyla or “Even-toed ungulates,” and the Cetaceans,

which includes all modern species of whale and dolphin. These orders are so grouped because

modern molecular data supports the theory that the Artiodactyla are the most recent common

terrestrial ancestors to the now aquatic Cetaceans (Figure 1) (Gatesy, 2013). Further under this

classification, the Pacific White-sided Dolphin is known as an Odontocete and falls within the

family Delphinidae. Odontocetes are the Cetacean sub-order that includes all of the toothed

whales, which physiologically different from the Myticetes or “Baleen Whales”, and within the

toothed whales, dolphins are classified under the family name “Delphinidae” (Figure 2). Finally,

the binomial name for the Pacific White-sided Dolphin is Lagenorhynchus obliquidens (Figure

3). This name was given to this species in 1865 by W.P. Trowbridge an engineer, naturalist, and

surveyor of the pacific coast in the mid-1800’s (Department of Vertebrate Zoology). The literal

translation of the Greek name of this species is “slanted-tooth flask beak” in relation to this

dolphin’s physiological traits (Allen, 2011). From this scientific name arises an additional

common name that is used very prominently in the regions where these dolphins are found. The

name “Lag” is taken from the beginning of Lagenorhynchus, and is frequently used to identify

this species because it is considerably shorter than and of the other common names (Allen,

2011).

Evolution

The Pacific White-sided Dolphin shares it evolutionary history with all of the other

Cetacean species. This specific species is believed to have evolve less than 3 million years ago

(Figure 3) (Slater, 2010). Modern day Cetaceans arose originally through the adaptation of

terrestrial mammals to an aquatic lifestyle approximately 53-54 million years ago (Figure 2).

Through the use of morphological data, it is currently believed that the Artiodactyla, or “hoofed

ungulates,” are the most recent common terrestrial ancestors of modern Cetacean species. This

links hoofed animals like bovine and equestrians, with the modern myticete and odontocete

cetaceans. Following this evolutionary divergence, several key species of arcaeocetes, or

“ancient whales,” evolved and brought rise to important modern physiological traits. The

development of elongated skulls with incisor and molar teeth running parallel to each other in the

jaw is one such trait that can easily be traced through the various cetacean predecessors. The

evolutionary distinction between toothed whales (Odontocetes) and Baleen whales (Myticetes)

occurred over 35 million years ago, separating out the evolutionary drives that would lead to the

development of a wide diversity of toothed whales and dolphins (Figure 2).

Distribution

The Pacific White-sided Dolphin prefers to inhabit the temperate ocean waters across the

North Pacific, as well as the Sea of Japan, Southern Okhotsk Sea, the Southern Bearing Sea and

Southern Gulf of California (IUCN) (Figure 4). Despite its wide geographical spread, the

majority of the research collected on this species has been on the Eastern Pacific populations.

This species also inhabits continental slope waters along the Eastern and Western coasts of the

Pacific, as far south as Baja California, Mexico and Southern China, respectively (IUCN). They

are known to move along the coast of North America, traveling the California Currents, and will

migrate in accordance to food availability, following their prey north and south as the seasons

change (Allen, 2011). There are two documented subgroups of this species that are genetically

distinctive, existing along the western coast of the United States, divided by their regionality

(Allen, 2011). There is considerable difference in the maximum size that these two distinct

subgroups can achieve, and the occurrence of individuals of these two types seems to be

dependent on the northern or southern origin of the individual dolphin, with the larger of the two

subgroups being located in the more southern Pacific waters (Allen, 2011). Though the Pacific

White-sided Dolphin is more comfortable in the open ocean where they have a near consistent

distribution (Ferrero, 1996), they have also been known to travel inwards through inshore

passages (Allen, 2011).

Anatomy

The Pacific White-sided Dolphin shares many characteristics with other dolphin species,

with slight evolved traits that set them apart from other Delphinids. One very unique attribute of

this species, and many other odontocetes, is the elongation and asymmetric disfiguration of the

skull. This trait results from a “reduction of the posterior end of the premaxilla” and the “loss of

the posterior sac of the nasal passage,” causing the skull to appear twisted to the right side

(Berta, 1999). Another anatomical trait that is shared with other Delphinids is a short blunt beak

with homodont teeth. In dolphin species the shape of the nose or “beak” can be either long and

cylindrical, or short and blunt; the Pacific White-sided Dolphin falls into the latter of this

category (Berta, 2015). Delphinids are also known for their fins: a dorsal fin rigid along the

back, a notched tail fluke, and a set of symmetrical flippers. All of these traits can be seen in the

Pacific White-sided Dolphin (Figure 6).

While the Pacific White-sided Dolphin sports all of the common identifiable traits, many

of them have undergone special adaptations, distinguishing this species from others. Most

notably is the shape of the dorsal fin possessed by this species. The Pacific White-sided Dolphin

is known for, and easily identified by, the large hooked/scythe shaped dorsal fin that is located

approximately halfway down its back (Berta, 2015). This species as a result can be classified

very quickly by observers as soon as the dorsal fin breaks the surface of the water. The front fins

are also larger proportionally on the Pacific White-sided Dolphin and tend to be more rounded in

shape than the fins seen in sister species (Allen, 2011). These dolphins have also developed a

large brain that is uncharacteristic for a dolphin species. The brain size for this species is 4.5

times the brain size expected for a land mammal of similar body mass, making the Pacific

White-sided Dolphin’s brain one of the largest in the mammalian world compared to its weight

(Allen, 2011). Another species specific trait is the 23-32 pairs of identical, cone shaped teeth that

label classify the Pacific White-sided Dolphin as a homodont, which literally means “same

teeth,” (Berta, 2015).

Another species specific trait is the body coloration seen on the skin of these dolphins. At

its most basic, the Pacific White-sided Dolphin displays a countershading pattern with a black

body and white belly (Berta, 2015). On top of this basic coloration are a series of markings that

set these animals apart from others, in the areas they are endemic to. Along the flanks and dorsal

fins of this animal it is common to find dark gray blaze patches, with light gray thoracic patches,

black lips and black eyes (Figure 7) (Tsutsui, 2001). Though there is a general trend to the

markings of this species, individuals will show variations of this pattern, and in some regions

there are completely anomalous document skin patterns. In one study, conducted in a small

region in Hokkaido, Japan, fourteen individuals were documented as having skin patterns that

differed greatly from the general population, a majority of which displayed large patches of

white on the dorsal side of the animal (Tsutsui, 2001). These patterns have actually aided in the

collection of research on various populations because they clearly differentiate individuals from

one another. Further, these markings prevent accidental misidentification of this species as

another species, such as the Common Dolphin or the Dall’s Porpoise, which inhabit overlapping

geographic areas and appear similar at a distance were it not for the stark marking differences

(Allen, 2011).

There is very little sexual dimorphism seen within Pacific White-sided Dolphins (Allen,

2011). For the most part, males and females appear the same externally. Most anatomical

differences are found in the reproductive systems, instead of the visual traits. Subtle

dimorphisms can occur between male and female individuals, but these differences are better

seen as trends than to the naked eye. For example, adult male Pacific White-sided Dolphins

average a length between 5.75 and 8.25 feet in length, while adult female Pacific White-sided

Dolphins average a length of 7.5 feet (Berta, 2015). Since they are longer in length when fully

grown, male Pacific White-sided Dolphins are also heavier than females, and weigh in at the top

of the species weight spectrum around 330 pounds (Allen, 2011). Other than the differences in

size, which can vary depending on region, there is no way to determine the sex of an individual

from a population without using catch and release methods.

Behavior

The Pacific White-sided Dolphin, like many other Delphinidae species, are extremely

curious, intelligent, and social creatures. This species is known to gather in extremely large

groups, or “pods,” which have been documented reaching numbers of over one thousand

members, though under most circumstances pods do not exceed 50 members (Allen, 2011).

Little is known about the social structures that form within these large groups; more research in

this area could reveal novel aspects of the social interactions that occur within this species. The

Pacific White-sided Dolphin has also been documented interacting outside of their species,

joining pods containing other species of dolphin, such as the Risso’s Dolphin, the California Sea

Lion, and the Northern Right Whale Dolphin (IUCN). It is believed that this intermingling is

done strategically to benefit all of the dolphins within the pod, regardless of their species. The

more individuals searching for food, the faster and more likely it is that the whole pod will be

able to locate a food source. There is also speculation as to whether these large groups serve

defensive purposes, deterring predators like Great White Sharks and Orca Whales, both of which

have been known to prey on the Pacific White-sided Dolphin (Allen, 2011). These large groups

are also known to migrate together as a part of their search for food sources (Berta, 2015).

Between members of a pod, there are several ways that information is communicated.

Many dolphin species use whistles to communicate between individuals; this is not a behavior

that has been exhibited heavily amongst Pacific White-sided Dolphins (Berta, 2015). This

species instead relies more heavily on their echolocation and sonar clicking to gather and share

information, with individuals possessing their own signature voices (Berta, 2015; Allen, 2011).

This communication technique is used extensively when foraging for food or traveling, to

maintain contact with the rest of the pod (Allen, 2011). These dolphins also rely heavily on their

sight, maintaining line-of-sight on the other members of their pod. Studies have shown that this

is the primary means of maintaining contact with each other during the night, so as not to attract

predators with vocalizations (Allen, 2011).

The Pacific White-sided Dolphin is more active during the day than at night, and shows

peaks of activity around the times of dawn and dusk (Allen, 2011). This is a very lively species,

displaying many various surface behaviors during the day. One of the most common surface

behaviors that this species will exhibit is bow riding, the act of surfacing alongside of a vessel

moving at sea and riding the wakes created as a result (Allen, 2011). The attraction that this

species has to marine vessels has made them an extremely popular focus of tourist sight-seeing

excursions along the western coast of the United States which, coupled with their innate

playfulness, has made this species an outstanding ambassador for marine life (Allen, 2011).

Besides bow riding, the Pacific White-sided Dolphin is also known to frequently “porpoise,”

throwing its body out of the water in an arc that sometimes completely clears the animal of the

water (Berta, 2015). They are also known to be voluntarily acrobatic, performing frontwards and

backwards flips above the ocean’s surface (Berta, 2015).

General Biology

The average weight of a Pacific White-sided Dolphin calf at birth is 33 pounds (Berta,

2015). Calves will rely on their mothers for up to three years before becoming completely

independent (Berta, 2015). After birth, studies have found that it takes a male dolphin calf 10 to

11 years in order to reach sexual maturity (Ferrero, 1996). Female calves require less time to

sexually mature than their male counterparts, and reach full sexual maturity within the range of 8

to 11 years of age (Ferrero, 1996). Upon reaching sexual maturity, individuals of this species are

capable of copulating most effectively on an annual basis. This reproductive seasonality is seen

within both sexes as a result of peaking female progesterone levels and male testosterone levels

from June to August (Robeck, 2009). The average gestation period is approximately 11 to 12

months long, with mothers giving birth the summer following fertilization (Robeck, 2009). The

Pacific White-sided Dolphin lives a lengthy life, with the oldest recorded female dolphin living

to the age of 46 years old, and the oldest recorded male dolphin living to the age of 42 years old

(Allen, 2011). If gender is ignored, the average lifespan for any individual dolphin will range

from 36 to 40 years of age (Berta, 2015).

There are three major threats to the life of a Pacific White-sided Dolphin: predators,

parasites, and fishing nets. The major predators of the Pacific White-sided Dolphin include the

Great White Shark and the Orca Whale, the later which, in the 1980’s, aggressively hunted the

Pacific White-sided Dolphin off the shores of British Columbia, Canada, by trapping them

within bays (Allen, 2011). Today, one of the most common causes of death in this species results

from parasitic infections. Every year a considerable number of dead dolphins are found washed

ashore, where the cause of death was from a nematode or trematode infection (Allen, 2011). The

third risk posed to the lives of the Pacific White-sided Dolphin is a result of fishery actions.

Though commercial fishing of this species was made illegal in 1993, a number of individuals

still lose their lives when indirectly caught in driftnets, gill nets, and trawls that are set out by the

fishing industry for other animals (Berta, 2015). Though the combination of these factors pose a

sizeable risk to the Pacific White-sided Dolphin population, a consistent population is still

capable of existing. Today, it is estimated that some 900,00 – 1,000,000 Pacific White-sided

Dolphins exist within the waters of the Pacific Ocean (IUCN). This healthy population size has

secured this species as one of “least concern” according to the International Union for

Conservation of Nature.

The diet of the Pacific White Sided Dolphin includes most notably species of

cephalopods and school fish (Berta, 2015). A wide range of prey species are eaten depending on

availability, including salmon, anchovies, herring, hake, Pollock, capelin, rockfish, Pacific

sardine, shrimp, and market squid (Allen, 2011). This species has a relatively high energy

requirement, requiring more energy per individual than most land animals of similar size

(Rechsteiner, 2013). The energy requirements of individuals change throughout their lifespan,

generally increasing with age through adolescence, when the energy requirements reach their

peak to properly sustain growth (Rechsteiner, 2013). Juvenile dolphins actually require higher

energy intake than any other age group, including adults when they are not reproducing

(Rechsteiner, 2013). The Pacific White-sided Dolphin relies on its speed and agility extensively

when foraging and hunting for prey, reaching impressive bursts of up to 17mph to catch escaping

prey (Allen, 2011). The homogeneity of the dolphin’s dentition, with their conical shape, creates

and excellent means of catching and holding onto slippery fish and squid species (Allen, 2011).

Conclusion

The Pacific White-sided Dolphin is a very important species for several reasons. As

previously mentioned, this species plays an active role as an animal ambassador for marine life

in the Pacific Ocean through their playful curiosity of tourism vessels (Allen, 2011). The casual

nature with which this species can approach humans makes in important for conservation efforts

of all marine life, especially their own. Additionally, this species plays a crucial role in

maintaining the ecosystem in the Pacific Ocean. Being both a predatory and prey species, this

animal regulates the abundance of key fish species, while also providing a food source to large

predators. Maintain this species is very important for the future of many other interacting

species.

With such high population estimates, upwards of one million individuals, there is very

little threat of extinction of this species at this time (Allen, 2011). This does not mean however

that preventative measures should be ignored in the conservation of this species. Actively

pursuing means of maintaining the population of the Pacific White-sided Dolphin is very

important to sustain this beautiful, lively, intelligent species, as well as the ecosystem of the

Pacific Ocean. Though hunting of this species by humans has not been practiced for many

decades, humans still pose a considerable risk to this species (IUCN). Mortality of the Pacific

White-sided Dolphin since the 1970’s has increased, as a result of accidental capture of this

animal as “bycatch” when performing wide scale fishing practices in search of other oceanic

species (Allen, 2011). Fishing nets of various types are one of the leading causes of death in this

species, as discussed in a previous section. This danger can be avoided through the use of more

careful fishing techniques and better designed fishing equipment. The ban of gill-net and drift

net fishing practices of this dolphin’s prey, like salmon and squid, has seen a considerable

decrease in threat since 1993 (Berta, 2015). Continuing to improve fishing practices through

seasonal allowances and the conscientious design of fishing nets will go a long way in protecting

the Pacific White-sided Dolphin from human threats.

Appendix

Figure 1: A phylogenetic tree portraying the evolutionary relationship between modern day Artiodactyla and Cetaceans. The

shared ancestors for the many living Artiodactyla and Cetacean species are shown at each point of branching in this tree, slowly

leading towards the evolution of modern Cetaceans. The scale at the bottom of this tree indicates the approximate time of

evolutionary divergence in millions of years from modern day. This image was taken from “A phylogenetic blueprint for a

modern whale. Molecular Phylogenetics and Evolution” by J. Gatesy et al.

Figure 2: A phylogenetic tree displaying the evolution of modern whales from the Archaeocetes (the ancient whales) and their

predecessors. The Family Delphinus is indicated with a dotted line leading to an picture of a dolphin near the top of the figure. The branching seen at the orange indicator “O,” in the middle of the image, marks the divergence of Mysticetes (Baleen Whales)

from Odontocetes (Toothed Whales). At the bottom of the figure a scale indicates the time of evolutionary divergence in millions

of years from modern day. This image was taken from “A phylogenetic blueprint for a modern whale. Molecular Phylogenetics

and Evolution” by J. Gatesy et al.

Figure 3: A phylogenetic tree showing the relationship between modern cetaceans. The Pacific White-sided Dolphin

(Lagenorhynchus obliquidens) is noted using a large red arrow. Around it are several other existent species from the same

genus. The time of divergence in millions of years from modern day is indicated using the time scale at the bottom of the figure.

This image was taken from: Diversity versus disparity and the radiation of modern cetaceans by G. Slater et al.

Figure 4:The golden orange shaded region on this map indicates the known distribution of the Pacific White-Sided Dolphin. The

image was taken from the IUCN Red List website (http://www.iucnredlist.org/details/11145/0).

Figure 5: The basic, shared anatomy characteristic of all dolphins. This image was taken from:

https://commons.wikimedia.org/wiki/File:Dolphin_anatomy.png

Figure 6: A diagram showing the physical characteristics of adult Pacific White-sided Dolphins. Traits unique to this species

and those shared with other species of dolphins can be seen within this image, such as the beak shape, dorsal fin size and shape,

and fluke shape. The coloration and size of these dolphins are also displayed in this image. This image was retrieved online

from: http://press.princeton.edu/birds/shirihai/pacifics/index.html by H Shirihai et al.

Figure 7: The coloration markings of the average Pacific White-sided Dolphin. The markings located on the face and dorsal fin

prove the easiest means of identifying this species in the field since there are no other species in the same regions that carry this

specific set of colorations. This image was taken from the journal article “Pacific white-sided dolphins (Lagenorhynchus

obliquidens) with anomalous colour patterns in Volcano Bay, Hokkaido, Japan” by S. Tsutsui et al.

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