Diversity
The genus Stenella consists of 5 species of dolphins: S. attenuata (pantropical spotted dolphin), S. longirostris (spinner dolphin), S. coeruleoalaba (striped dolphin), S. frontalis (Atlantic spotted dolphin), and S. clymene (Clymene dolphin).(do et al., 2015). They are more commonly known as the spotted, striped, and spinner dolphins. Stenella dolphins are a part of the family Delphinidae which consists of dolphins, killer, whales, pilot whales, and their relatives. Dolphins are part of the suborder Odontoceti better known as the toothed whales. They are also part of the bigger order Cetacea which is comprised of all marine mammals.
The genus
Stenella
can be found in tropical, subtropical, and temperate waters (do et al., 2015, Moreno
et al., 2005). They can be seen surfacing, and have been found as far down as 5,000m.
All the species can be found in groups-known as pods-of sizes of 1 individual to hundreds
of individuals. They can grow up to lengths of 195 cm-235 cm (76in-92in), and will
weigh on average 72 kg-80kg (158lbs-176lbs). They will typically have varying beak
lengths as well. These species are typically carnivorous and will feed on a variety
of small organisms from the pelagic and benthopelagic waters, such as lanternfish,
squid, shrimp, and fishes.
Geographic Range
These 5 species of dolphins are found in the Pacific, Atlantic, and Indian oceans.
None are found in the polar regions of the oceans. Of the 5 species of dolphins in
genus
Stenella
, 2 of the species:
S. clymene
and
S. frontalis
are endemic to the Atlantic ocean, Gulf of Mexico, and Caribbean (do et al.,2015,
Moreno et al., 2005). The other 3 species:
S. Attenuata
,
S. longirostris
, and
S. coreruleoalba
are found throughout all 3 of the mentioned oceans. They are commonly found in warmer
regions of the oceans.
- Biogeographic Regions
- indian ocean
- atlantic ocean
- pacific ocean
Habitat
These species live in tropical, subtropical, and temperate waters; mostly in the deeper
open ocean waters, but some species will move to shallower waters closer to shore.
They typically will be found in the water column in a range from 20m-200m in shallower
waters, and 700m-5,000m in the deeper waters. They are found in waters with temperatures
that range typically 18°C-22°C (64°F-72°F), but can be anywhere from 10°C-22°C (50°F-72°F).
- Habitat Regions
- temperate
- tropical
- saltwater or marine
- Aquatic Biomes
- pelagic
Systematic and Taxonomic History
These dolphins are part of the greater superorder " Cetartiodactyla " which split into the two major orders: " Artiodactyla " which consists of the ruminants, peccaries, pigs, hippos, llamas, and camels; and " Cetacea " which consists of porpoises, whales, and dolphins. It is estimated that order " Cetacea " diverged from the Artiodactylates in the early Eocene period, about 60 Mya (Theordor, 2016). After their divergence from " Artiodactyla " the Cetaceans split into two main suborders the " Odontocetes "-the toothed whales and " Mysticetes "-the baleen whales around 33.8 Mya in the mid to late Eocene period (Marx and Fordyce, 2015). Furthermore, within suborder "Odontocetes" all dolphins are grouped under family " Delphinidae " which was estimated to first have appeared during the early Miocene period, about 23-16 Mya (Encyclopedia Britannica, 2021).
In 1866, John Gray used the name "
Stenella
" to define several species of dolphins as a subgenus of the genus "
Steno
". "
Stenella
" has now since been defined as its own genus in the family "
Delphinidae
". The genus "
Stenella
" groups together the 5 species of dolphins- "
S. attenuata
", "
S. clymene
", "
S. coeruleoalba
", "
S. frontalis
", and "
S. longirostris
". The species "
S. attenuata
" is split into two subspecies: "
S. a. attenuata
" and "
S. attenuata graffmani
", and the species "
S. longirostris
" is split into three subspecies: "
S. l. longirostris
", "
S. longirostris centroamericana
", and "
S. longirostris orientalis
". In the late 1800s other names were used to define this group, but are no longer
used today and some of the names are still used to define a few specific species within
this genus.
Physical Description
Stenella
dolphins are smaller in size compared to the other cetacean relatives. They have
a streamlined, torpedo shaped body, with a singular dorsal (back) fin, two pectoral
fins (one each on either side of its body), and a horizontal plane tail. This body
type is best to move quickly and easily through the water. The dolphins coloration
is dark grey almost black on their dorsal side, and light grey/white shading on their
ventral (belly) side. The countershading between the two colors is distinct near the
head and the two colors fade into each other near the tail. These species will typically
have spots that are white light grey in color appear all over back and appear more
often behind (anterior) to the dorsal fin(s). These Spots can appear as speckles,
spots, or stripes. They can also have some spots or none at all on their ventral side.
As newborns they have no spots, and coloring is a dark purple/grey (Perrin, 1969).
As they reach maturity coloring becomes more distinct grey and distinct differences
between the dorsal and ventral sides. The two colors come together in a downward-curving
shape along the body (Best, 1968). They can have dark patches or bands of color around
the eyes, blowhole, fins, and tail. All 5 species of
Stenella
dolphins typically grow to a length of 127cm-190cm(50in-75in)in females and 155cm-235cm(61in-93in)
in males. Some species grow larger than others. They're beak(or snout) length can
range from 11cm-20cm(4in-8in). They can weigh around 80kg(176lbs) or more. The males
are typically larger than the females of these species.
- Other Physical Features
- endothermic
- bilateral symmetry
- Sexual Dimorphism
- male larger
Reproduction
Dolphins are social animals. They can live in pods as small as a few individuals and
as large as hundreds of individuals. They can be found living solitary lifestyle,
but are most often found in pods. These social groups usually consist of a larger
number of females than the number of males; typically with a ratio around 60% females
to 40% males(Perrin et. al.,1976). In these pods there will be foraging individuals
that hint for prey, and other individuals that care for the young. It takes several
years for species of
Stenella
dolphins to reach sexual maturity. In females sexual maturity is reached between
7-10 years of age, and for males it is reached between 7-12 years of age. Once they
are reproductively mature they are polygamous, meaning they will have multiple different
mates over the course of they're lifetime. As individuals get older they're reproduction
rate decreases.
- Mating System
- polygynandrous (promiscuous)
- cooperative breeder
When dolphins want to mate they will engage in playful activities with each other
such as chasing each other, and will begin making lots of noises such as whistles,
clicks, and burst-pulse signals(Silva Jr. et. al.,2005). The males will begin to gently
tap their fins to the females fins and belly, and then rub, nudge, and gently bite
the female. To impregnate the female the male will move below the female with his
belly up, so that the two are belly to belly; the act of reproducing is quick, and
will only last a few seconds to a few minutes. There are two primary seasons for mating
and calving(birth)- the spring and the fall. However these reproductive peaks can
vary from year to year, and high reproductive peaks can occur during the summer as
well. Reproduction occurs in 3 stages: calving, lactation, and a period of inactivity
or estrus(Perrin et al.,1976).
- Key Reproductive Features
- iteroparous
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- sexual
- viviparous
- post-partum estrous
A female will be in the gestation(pregnancy) period for 11 months-1 year, and then
will give birth to live young in the spring or fall. After giving birth females will
lactate for typically 9 months, but calves can wean from lactation anywhere from 6
months to 19 months. It takes calves about 1-3 years to reach the juvenile stage and
full growth. While calves are nursing they tend to stay close to their mothers. Females
can be pregnant and lactating at the same time, and will give birth to only 1 calf
per pregnancy.
- Parental Investment
- precocial
- female parental care
-
pre-independence
-
protecting
- female
-
protecting
- extended period of juvenile learning
Lifespan/Longevity
Stenella
dolphins have a lifespan of anywhere from 20 years to upwards of more then 50 years
in the wild. The species
S. clymene
has a lifespan of around 25 years,
S. attenuata
has a span of 40 years(National Geographic 2022),
S. coerulalba
has a lifespan of up to 58 years(NOAA Fisheries, 2022),
S. frontalis
has a span of greater than 50 years(NOAA Fisheries, 2022), and
S. longirostris
has a span of 20-25 years(NOAA Fisheries, 2022). In captivity these species of dolphins
have roughly the same lifespan but can live a few years longer.
Behavior
Stenella
dolphins will live in pods of individuals consisting of both males and females, and
within these pods the individuals are organized by sex, age, and breeding status.
They are an extremely social species, and will interact with other Cetaceans (whales
and dolphins), as well as other marine animals, and sometimes interact with humans.
S. coeruleoalba
tend to be less social and interact with other organisms less frequently than the
other 4 species of
Stenella
dolphins. During the day they will stay near the surface and come up to the surface
to breathe, and at night they dive down to feed. They will swim in zig zag or back
and forth patterns moving between shallow and deeper waters, as well as engage in
periods of rest. During these rest periods the pods will swim close to one another
in a slow back and forth swaying as one group movement. The act of rest is typically
engaged in multiple times a day for around 3-5 hours at a time(NOAA Fisheries, 2022).
These dolphins are very playful, and both calves and adults will engage in playtime
with each other. play consists of jumping out of the water and flipping or spinning,
as well as "roto-tailing"-jumping high above the water and rotating their tail.
S. coeruleoalba
most often "roto-tail" compared to its sister species. The dolphins will also use
spinning to communicate with each other, or to remove ectoparasites from their bodies(NOAA
Fisheries, 2022). Play also consists of blowing bubbles or shooting water from the
blowholes, mock chasing schools of fishes, and playing with pieces of seaweed. A dolphin
will pick up a piece of seaweed and place it on its flipper, and then toss it back
and forth between their flippers, beak or tails. Most often a single dolphin will
play with the seaweed but sometimes two or three dolphins will all pass a singular
piece of seaweed back and forth between each other. More commonly a single piece of
seaweed will be shared between individuals who each in turn play with the seaweed.
Also they will frequently swim up to boats and swim in the bow waves or surf the wake
of these boats (Jefferson, 2003; NOAA Fisheries, 2022).
- Key Behaviors
- natatorial
- diurnal
- motile
- nomadic
- social
- dominance hierarchies
Communication and Perception
These species of dolphins will communicate using a wide range of auditory cues and
sounds, as well as using some visual cues, and frequent use of echolocation. Their
main source of communication is auditory sounds and acoustic signaling. These acoustic
signals will be in patterns that will rise and fall in a synchronous manner. They
frequently make a variety of sounds including whistles, clicks, click trains (repetitions
of multiple clicks), squeaks, squeals, squawks, barks, chirps, and more. They use
acoustic signaling for mating, hunting and prey spotting, as well as predator avoidance.
Dolphins also use echolocation for perception of their environment. Echolocation is
used to scan the environment, find prey, avoid predators, find other dolphins, and
detect objects from a distance. They perform echolocation by sending out a series
of clicks and the created sound waves travel until bouncing off an object and then
come back to the dolphin. They use the sound waves to determine how far away an object
is. This helps them locate where predators, prey, other dolphins, boats, and other
objects may be.
- Other Communication Modes
- choruses
- vibrations
- Perception Channels
- visual
- tactile
- acoustic
- echolocation
- vibrations
- chemical
Food Habits
These dolphins will feed mostly at night on prey from the mesopelagic (200m-1000m)
waters that vertically migrate to the surface at night. The pods of dolphins will
engage in cooperative hunting for their prey. They are able to catch their prey by
surrounding them, and then taking turns swimming at and grabbing the surrounded prey.
Their diet consists of a wide variety of small fishes, mollusks, crustaceans, cephalopods,
and other small invertebrates. Typical prey for the dolphins are octopuses, and squids,
shrimp, lantern fish, conger eels, flounder, jacks, and needlefish. These dolphins
will have preference for some prey over others, but tend to feed on any potential
prey they might come across.
- Primary Diet
-
carnivore
- piscivore
- eats non-insect arthropods
- molluscivore
Predation
Stenella
dolphins are considered one of the top marine predator species, and do not have very
many predators. Common predators of these dolphins which are known to harm or kill
them are sharks, killer whales, and other whales such as the false killer whale, pygmy
whale, and short finned pilot whale(Perrin et. al., 1987). Humans have also been known
to harm or kill these dolphins, and some people will hunt them for food. Individuals
that are more at risk within the pods of these dolphins are pregnant females, lactating
females, and young claves and juveniles. Sharks and whales will come up to and harass
the dolphins and attempt to subdue and separate their prey from the pod. if prey has
been subdued will attempt to kill it by biting and removing chunks of flesh from their
prey. Adult dolphins will try to protect their calves by engaging in group chasing
of the nearby predator and harassing and pestering them until the predator leaves.
Ecosystem Roles
Stenella
dolphins among most other cetaceans (whales, dolphins, porpoises) fulfill the predatory
ecological niches and are considered secondary, and tertiary consumers in aquatic
trophic levels. Each species fulfills different ecological niches based on the water
temperature and depth they spend the most time in, and as well as based on factors
affecting the distribution and abundance of their prey (Amaral et. al., 2015). These
dolphins will engage in habitat partitioning because of differences in environmental
and ecological requirements and spatial separation between the coexisting sympatric
species(Amaral et. al., 2015). These dolphins will also be host to a variety of ectoparasites
such as barnacles, whale lice, and flukes; as well as host to various internal parasites
such as
Phyllobothrium
,
Halocercus
,
Monorygma
,
Nasitrema
,
Pharurus
, and various parasitic worms. Some sea birds will follow the dolphins and will eat
the fish and small organisms that have been brought up to the surface and uneaten
by the dolphins. They will also form commensal relationships with some species of
fish and will play with them and mock chase rather than preying upon them.
Stenella
dolphins fill a variety of marine ecological niches.
- Ecosystem Impact
- creates habitat
- Seabirds (i.e. seagulls)
- Remoras
- Phyllobothrium
- Halocercus
- Monorygma
- Nasitrema
- Pharurus
- barnacles
- flukes
- whale lice
- parasitic worms
Economic Importance for Humans: Positive
Stenella
dolphins have a strong positive relationship with humans. Primarily, they provide
large tourist attractions for people to interact with them. People can go out in boats
to have close viewings of pods of these dolphins, and can even get in the water and
swim and play with them. The dolphins also provide a source of food to various groups
of native people living near to the areas inhabited by them. Aside from tourism, pods
of these dolphins are also researched and studied.
- Positive Impacts
- food
- ecotourism
- research and education
Economic Importance for Humans: Negative
There are no known adverse effects of Stenella dolphins on humans.
Conservation Status
At this time,
Stenella
dolphins are known to be of least concern when considering their conservation status.
Their populations are continuing to grow and thrive. Despite this there are still
threats affecting these species of dolphins-both natural and human caused. They are
hunted and can this can lead to decreases in populations. They can become entangled
in fishing nets, trawls, hand harpoons, and other fishing gear of commercial fishing
boats, which can lead to injury or death(NOAA Fisheries, 2022). Also outbreaks of
epizoonotic diseases can occur in their populations and nearby populations of other
dolphins, and these outbreaks can lead to dramatic decreases in their population numbers.
Also, noise pollution in the ocean can disturb and interrupt their echolocation and
sound perception behaviors,, which can make predator avoidance and prey hunting more
difficult. Loud enough noises can cause hearing damage, and temporary or permanent
hearing loss. Marine debris polluting the waters can be ingested by the dolphins,
or they can become entangled by them; which can lead to illness, injury, or death.
Even too much interaction and encounters of tourism can lead to inadequate periods
of rest which could lead to negative effects on fitness overtime (NOAA Fisheries,
2022).
Additional Links
Contributors
Abigail Ross (author), Colorado State University, Audrey Bowman (editor), Colorado State University.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Atlantic Ocean
-
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Pacific Ocean
-
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- temperate
-
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
- tropical
-
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
- saltwater or marine
-
mainly lives in oceans, seas, or other bodies of salt water.
- pelagic
-
An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).
- polygynandrous
-
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
- cooperative breeder
-
helpers provide assistance in raising young that are not their own
- iteroparous
-
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
- seasonal breeding
-
breeding is confined to a particular season
- sexual
-
reproduction that includes combining the genetic contribution of two individuals, a male and a female
- viviparous
-
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
- young precocial
-
young are relatively well-developed when born
- female parental care
-
parental care is carried out by females
- natatorial
-
specialized for swimming
- diurnal
-
- active during the day, 2. lasting for one day.
- motile
-
having the capacity to move from one place to another.
- nomadic
-
generally wanders from place to place, usually within a well-defined range.
- social
-
associates with others of its species; forms social groups.
- dominance hierarchies
-
ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates
- visual
-
uses sight to communicate
- acoustic
-
uses sound to communicate
- choruses
-
to jointly display, usually with sounds, at the same time as two or more other individuals of the same or different species
- vibrations
-
movements of a hard surface that are produced by animals as signals to others
- visual
-
uses sight to communicate
- tactile
-
uses touch to communicate
- acoustic
-
uses sound to communicate
- echolocation
-
The process by which an animal locates itself with respect to other animals and objects by emitting sound waves and sensing the pattern of the reflected sound waves.
- vibrations
-
movements of a hard surface that are produced by animals as signals to others
- chemical
-
uses smells or other chemicals to communicate
- food
-
A substance that provides both nutrients and energy to a living thing.
- ecotourism
-
humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.
- carnivore
-
an animal that mainly eats meat
- piscivore
-
an animal that mainly eats fish
- molluscivore
-
eats mollusks, members of Phylum Mollusca
- endothermic
-
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
- bilateral symmetry
-
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
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