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Dolphins are mammals, not fish. What makes a dolphin
a mammal?
- They breathe air directly into their lungs.
- They have hair at some point during their life cycle.
- They are warm-blooded.
- They give birth to live young.
- They nurse their young.
The ocean can be a very harsh place for a mammal to
live. However, dolphins are uniquely adapted for the
marine environment. How have dolphins and
other marine
mammals come to live in an aquatic world?
Scientists believe evolution holds the key to the answer.
Evolution
Inside their pectoral fins, dolphins have a skeletal
structure similar to a human arm and hand. They have
a humerus, complete with a ball and socket joint. They
have a radius and ulna, as well as a complete hand structure,
including five phalanges, or finger bones. This is one
of the many internal physiological structures leading
scientists to believe that dolphins and whales evolved
from a terrestrial ancestor.
There is currently a debate as to which ancient group
of animals cetaceans evolved from. One theory suggests
that cetaceans are
descendants of the Mesonychid, an animal that lived about
55 million years ago. This terrestrial mammal ranged
from the size of a small dog to the size of a large bear
and for some reason started to frequent the shallow water
areas of the Tethys sea. The Tethys Sea was a body of
water located just north of Africa, which stretched from
the modern day Mediterranean to beyond India. Why would
this terrestrial animal start inhabiting shallow water
areas? It is possible that the Mesonychid started spending
progressively more time in the shallow waters of the
Tethys Sea foraging for fish and other marine life. If
competition for food were high, the Tethys Sea might
have provided a previously unexploited resource. The
Mesonychid may have also been forced into shallow waters
due to an increase in surrounding predators or because
of a climatic change. We may never know for sure, but
we can speculate based on animal behavior we see today.
Another theory, based on DNA, molecular, and genetic
studies, suggests that cetaceans share a common heritage
with that of the hippopotamus. This theory would place
cetaceans in the group Artiodactyl, composed of even
toed ungulates (deer, sheep, cows, pigs, hippos, etc.)
New skeletal discoveries seem to back this claim, giving
possible morphological evidence as testimony. Scientists
have recently discovered ancient whale skeletons in Pakistan
with well-preserved anklebones displaying features similar
to that of the Artiodactyl group.
In either case the terrestrial ancestor of the cetacean
quickly adapted to its new niche and the evolutionary
process continued at a rapid rate. When animals move
into unoccupied niches, evolutionary radiation tends
to act swiftly, as it appears to have done in this case.
Ambulocetus arrived several million years
later. This next evolutionary link displayed subtle changes,
resembling a terrestrial animal less and a marine mammal
more. The legs on the Ambulocetus were shorter
and more paddle-shaped. However, it probably still spent
some time on land. The nostrils of this cetacean ancestor
had also migrated to the top of the snout, probably to
facilitate more efficient breathing. Ambulocetus expended
less energy breathing because it only had to poke its
nostrils above water as opposed to its whole head.
The oldest and most primitive whales were the Protocetids,
a family of archaic whales or Archaeocetes. These whale
ancestors arrived around 50 million years ago, during
the Eocene Epoch. Due to the locale of their remains,
it appears that the earliest phases of cetacean evolution,
including the Protocetids, were confined to the Tethys
Sea. The jaw of this animal is much more slender and
elongated than that of the Ambulocetus. The
shape of Protocetus is not entirely clear because
skeletal structures discovered are not complete. It appears
that they still had hind limbs, and that their life style
may have been amphibious, rather than fully aquatic.
About 38 to 45 million years ago, Basilosaurus,
the largest known advanced Archaeocete, appeared. The
skeletons of these animals have been found as far south
as New Zealand and Antarctica. The spread of this early
cetacean would indicate its successful adaptation to
aquatic life. Basilosaurus was, in fact, fully
aquatic. The forelimbs of Basilosaurus were
elongated flippers/paddles. As they elongated, the cartilage
between the phalanges fused, preventing the joints from
curling. The sinuses in the skull base of the Basilosaurus enlarged,
a feature seen also in early toothed and baleen whales,
suggesting a link to modern day whales. The hind limbs
of Basilosaurus became virtually non-existent.
At this point, the only function that the hind limbs
might have served would have been for alignment in mating,
or for pushing off of the bottom of the ocean.
Following Basilosaurus, there were many other
evolving forms that led to the modern-day cetacean. Baleen
whales and toothed whales diverged from a common ancestor
around 35 million years ago. The family Delphinidae made
its appearance around 11 million years ago, while the
Bottlenose Dolphin emerged only about 2-5 million years
ago. Through evolution, dolphins have made significant
adaptations to become the aquatic marvels that you see
today.
Taxonomy
Dolphins and whales belong to the order Cetacea. In
fact, dolphins are small toothed whales. The order Cetacea
contains two sub orders: Mysticete and Odontocete.
Mysticetes include all of the baleen whales. Baleen
whales do not have teeth. Instead they have fibrous plates
called baleen which they use as sieves to trap their
food. They swallow large mouthfuls of water and plankton,
krill, or fish, and push the water out through the baleen
plates. The plankton, krill, or fish are too big and
become trapped in the fibrous interior of the baleen
plates. The whale then uses its tongue to gather and
swallow all of the trapped food inside its mouth.
Odontocetes include all of the toothed whales. Toothed
whales catch and swallow their prey one by one, grasping
them with their teeth. The largest Odontocete is the
sperm whale.
The Mysticete sub order only contains four families.
The Odontocete sub order has ten families, including
the family Delphinidae, to which dolphins belong. It
is believed the Odontocete sub order contains many more
families than the Mysticete sub order for several reasons.
Odontocetes eat a larger variety of foods, and are smaller
in size than most Mysticetes, so they are able to fill
more niches in their ecosystems.
Skeleton
The skull of the Mysticete is bow-shaped, while the
Odontocete skull is compact and has a parabolic dish
shape. The Mysticetes' bow-shaped skull is adapted to
accommodate its baleen. The skull shape also suggests
that Mysticete probably do not possess any echolocation
capabilities. The thickness of the Odontocetes' skull
is adapted for its unique feeding requirements. The compact,
thick skull provides a large base for muscles to anchor,
allowing for the power needed to catch prey. The parabolic
dish shape of the Odontocete skull is believed to play
a large part in the echolocation abilities of the Odontocete.
The spinal cord of the Odontocete is much thicker than
that of the Mysticete relative to size. This seems to
again be related to feeding practices. Mysticetes do
not have to move quickly to catch their prey. They feed
on slow moving creatures such as plankton or krill. Mysticetes
also have no natural predators to outrun. As they do
not need to move quickly, they do not require an extensive
muscle mass. Odontocetes, however, have predators they
must outrun as well as prey they must chase and catch.
Both of these activities require a large muscle mass.
Once again, a larger bone surface area would accommodate
a more extensive muscle mass.
Dolphins vs. Porpoises
Dolphins and porpoises are both small-toothed whales
and are separated into two distinct families. Dolphins
belong to the family Delphinidae, while porpoises belong
to the family Phoecinidae. There is only one difference
that is consistent between a dolphin and a porpoise:
the shape of their teeth. All cetaceans in the family
Delphinidae have conical shaped teeth. All cetaceans
in the family Phoecinidae have flat, spade-shaped teeth.
There are a few other differences
that are generally true, but not in every case. Porpoises
generally have a blunt, rounded snout, while dolphins
have an elongated, protruding rostrum. In general,
porpoises are much smaller than dolphins and have a
small triangular-shaped dorsal fin, as opposed to the
dolphin’s larger sickle-shaped
dorsal fin.
Hair
Bald is beautiful, especially
on a dolphin. However, being mammals, even dolphins
have hair at one point in their life. When they are
born, dolphins have whiskers on their rostrum. A dolphin’s
whiskers are about one quarter inch long, and will
fall out shortly after birth, as a result of water
pressure. Even so, the follicular pits remain open.
One can see these follicular pits on the rostrum of
any dolphin. They probably serve some sort of sensory
function, like the detection of speed or current. Some
great whales and one type of river dolphin retain whiskers
their entire lives.
Reproduction
One of the characteristics of mammals is the ability
to bear live young and nurse them for a period of time.
Dolphins put a large amount of energy into raising their
young, which stay with them an average of three to six
years. (More information about the reproductive and maternity
cycles of the Bottlenose dolphin may be found in the
Dolphin Research Center Maternity Information File.)
Respiratory System
Dolphins breathe air directly into their lungs via
the blowhole. Dolphins and whales can also use their
blowhole to create sounds. Odontocetes have only one
nasal opening, or blowhole, at the surface of the skin.
They do have two nasal passages underneath the skin,
but the septum does not rise all the way to the surface.
Mysticetes have two nasal openings, or blowholes, at
the surface of their skin. The septum rises all the way
to the surface of a Mysticetes skin. Whalers could actually
identify whales from afar by their blow. Odontocetes
have a blow that shoots straight up, while Mysticete
blows fountain out. In a sperm whale, only the left nasal
passage rises to the surface. The right nasal passage
circles around inside the head and is speculated to be
associated with sound production.
Cetaceans do not breathe through their mouths at all.
In fact, the trachea and esophagus are completely separated
with the aid of an organ called the goose beak. The goose
beak is an evolutionarily modified larynx that bridges
the gap between the nasal passages and the trachea, and
is designed to keep anything but air out of the lungs.
It is a cartilaginous organ that sits unattached, but
tightly fit, inside the sporacular channel found in the
skull underneath the blowhole.
Sometimes it is necessary
to administer fluids and medications to a dolphin via
a stomach tube. The elasticity of a dolphin’s
throat tissue, combined with the separation of trachea
and esophagus make the tubing process quite easy. This
process is quite uncomfortable for a human due to the
epiglottis and a gag reflex. Humans and other mammals
have a gag reflex to protect their lungs from swallowing
foreign objects into the trachea from the mouth. Dolphins
have no gag reflex, since the trachea and esophagus
are completely separated. So, this procedure does not
bother a dolphin.
Skin
The core of a dolphin’s body is well insulated
by a thick layer of blubber. Generally, a blubber layer
is anywhere between ¾” to 1¼” thick.
The thickness is related to the type of environment the
dolphin lives in. Colder waters require a thicker blubber
layer. The blubber is actually the modified third layer
or “hypodermis” of a dolphin’s skin.
Skin is made up of three layers: an epidermis (top layer),
a dermis (middle layer) and a hypodermis (bottom layer).
The surface layer, or epidermis, of a dolphin’s
skin is ten times thicker than any land mammal, and serves
as protection against the elements. Additionally, the
entire top layer of skin sloughs off every two to four
hours, helping the dolphin stay streamlined, moist and
free of external attachments while traveling through
the water.
Upon close examination of
a dolphin’s skin, micro
dermal ridges are visible. Micro dermal ridges are small
ridges that trap water molecules at the surface of the
skin. This allows the dolphin to pass through its environment
with less resistance because liquid moves more easily
past another liquid than past a solid object. Micro dermal
ridges allow a dolphin to travel more like a liquid through
water with less resistance. A famous sail boat competitor
used the idea of micro dermal ridges to his advantage
to win the World Cup. He had Dupont design a skin for
the bottom of his boat based on a dolphin’s micro
dermal ridges. This technology increased his boating
speed by a half-knot. Many wet suit companies use the
principle of micro dermal ridges as well.
In addition to micro dermal
ridges, a dolphin has very pliant skin. When a dolphin
reaches a certain speed while traveling through water,
the typical smooth flow of water becomes more turbulent.
A dolphin’s skin will actually
ripple at accelerated speeds to counter the effect of
turbulence as they travel through the water.
Temperature Regulation
How does a dolphin keep warm in the water without hair?
Water conducts heat away from the dolphin's body twenty-five
times more efficiently than air. There are two ways that
dolphins keep warm: through insulation provided by their
blubber and through thermoregulation.
All mammals are warm blooded, which means they can
regulate their own internal body temperature. If you
look closely, you can see many veins running through
the tail flukes of a dolphin. These veins are important
for maintaining the internal body temperature of a dolphin.
The tail, dorsal fin, and pectoral fins are great places
for a dolphin to release heat because they contain no
blubber. Inside the flukes, dorsal fin, and pectoral
fins are a bunch of arteries surrounded by a network
of veins. Along the surface of the skin, in these areas,
are more veins and smaller capillaries. Cetaceans use
this unique circulatory arrangement to deal with thermoregulation.
The process is called counter-current heat exchange.
During the winter, when a dolphin needs to keep warm,
arteries leading from the core of the body to the cooler
flippers, flukes and dorsal fin, are surrounded by veins
leading from these same areas. In this way, blood returning
from cooler areas to the body is warmed and heat loss
is minimized (cool venous blood traveling next to the
hot artery is warmed).
In the summer, a dolphin keeps cool by increasing blood
flow out to the extremities. The outgoing arteries expand
with the increased blood flow. The veins and capillaries
along the surface of the skin also expand, allowing heat
to be released through the surface of the skin. Because
the veins surrounding arteries cannot conduct such a
large flow of blood back into the body, some blood must
return along the peripheral veins close to the surface
of the skin. The heat from the blood is lost through
the surface of the skin and transferred into the environment.
Some of the blood returning from these areas is still
cold when it enters the core of the body; thus, cooling
the animal down.
Motion
While skin and other parts
of a dolphin’s body
contribute to its graceful movement, the tail flukes
provide the power. The tail is the strongest part of
the dolphin’s body. All marine mammals move their
tails up and down, rather than side to side like a fish.
The upstroke is believed to be the more powerful stroke
for a dolphin. When a dolphin moves its tail down, the
tips of the tail bend upward, creating less surface area
to push with. When a dolphin moves its tail up, the tips
of the tail remain ridged and flat, creating maximum
surface area to propel with. Another clue indicating
a more powerful upstroke than downstroke is the muscle
movement. When a dolphin moves its tail upward, the large
muscle mass along its back must contract, and contracted
muscles are stronger.
Diving
The maximum recorded & published
depth achieved by an Atlantic bottlenose dolphin during
a dive exceeded just over 990 feet by a Navy-trained
dolphin named Tuffy (Ridgway et al. 1969).
Wild coastal bottlenose dolphins spend the majority
of their time in water that averages 7-10 feet deep.
Offshore animals are believed to frequent much deeper
depths. Diving depths are directly related to the needs
(food in particular) of a dolphin. Pressure changes
and oxygen conservation are of primary concern during
diving. Dolphins have special ways of dealing with
these issues. They can hold their breath for an average
of 7.25 minutes. When a dolphin respires, it replenishes
over 70-90% of its lung capacity. Humans only replenish
about 17% at a time. When diving, a dolphin slows its
heartbeat as low as 12 beats per minute. Blood flow
is reduced to a minimum to conserve the energy and
oxygen of the animal. Dolphins have a flexible ribcage
with a large number of free-floating ribs, which aid
on deep dives. At a certain depth, high pressure causes
a dolphin’s lungs to collapse. A flexible ribcage
facilitates this process.
Where does a dolphin store its oxygen when it dives?
The darkness of the muscle indicates a high oxygen concentration.
Dolphins store oxygen in their muscles with the help
of a blood protein called myoglobin. In fact, dolphins
have a higher blood volume than most terrestrial animals.
Dolphins can store 41% of oxygen intake within their
muscle tissue. Humans can only store 19% of oxygen intake
within their muscle tissue. Because oxygen is already
stored within the muscle tissue, dolphins can slow down
their blood circulation to a minimum.
Blood Pressure
While diving a dolphin’s heart rate drops as
low as 12 beats per minute, conserving oxygen while submerging.
When resurfacing, a dolphin’s heartbeat can skyrocket
to 120 beats per minute. If this fluctuation occurred
in a human, he/she would probably have a stroke. A dolphin,
however, is able to handle the quick change in blood
pressure because of a special adaptation called the retia
mirabilia. The retia is a tissue found underneath the
ribcage, between the blowhole and dorsal fin area. It
consists of a dense mass of blood vessels that act like
a sponge. The arteries in a dolphin feed into the retia,
rather than going directly to the brain. The diverted
blood flow saturates the vessels within the retia, like
a sponge, when a dolphin’s heart rate is high.
The retia then controls the flow of blood to the brain,
maintaining a consistent flow, no matter how much blood
is contained in its vessels. Thus, the retia acts as
a buffer, protecting against a surge of blood during
high blood pressure, and against a lack of blood flow
during reduced heart rates.
Digestive System
A dolphin has a three chambered stomach, similar to
an ungulate (cow or deer), pointing further to its evolution
from a terrestrial ancestor. Since dolphins do not chew
their food, the mastication of their meal is taken care
of in their first or fore stomach. The majority of digestion
is processed in the main stomach or second chamber. The
last section of their stomach, the pyloric stomach, takes
care of the remainder of their digestion prior to the
contents emptying into the intestinal region.
Renal System
In making the change from terrestrial to aquatic living,
cetaceans needed a way to accommodate for the higher
salinity of their environment. Unlike human kidneys,
which are just two singular renules (or balls), dolphins
have two kidneys with multiple renules. These renules
all function as separate kidneys which help filter out
the higher amount of salt content they must deal with
in their daily environment.
Dolphins also have a very small bladder, which causes
them to urinate frequently. Dolphins can be taught to
give a urine sample on command for medical purposes.
Teeth
A dolphin is born with 88
to 100 teeth. This original set of teeth will last
a lifetime. The dolphin's smile contains a gap where
you might expect to find front teeth. This gap allows
the baby to latch on to the mother’s
nipple without biting her.
Scientists have found that every year a dolphin lays
down a new layer of enamel inside its teeth, causing
a ring to form inside the tooth. Therefore, one of the
ways we can determine the age of a dolphin is by counting
the enamel rings found inside the teeth, one for each
year of its life. This is similar to counting the rings
inside a tree to determine its age.
Dolphins seem to live an average of 25 years, although
some dolphins have lived into their early 50s. As a dolphin
gets older, it becomes very difficult to count rings
in their teeth since they are so tightly compressed.
Vision
Dolphins have very good vision
both above and below water. Researchers have determined
that dolphins are able to see objects as far away as
at least 12 to 18 feet in air and around 9 feet underwater
(Mass & Supin
1995; Herman et al. 1975; Pepper & Simons 1973).
Scientists are still unclear, however, whether or not
dolphins have color vision. The number of rods and cones
contained in a dolphin’s eye are believed to facilitate
a greater range of light intensity rather than color.
When looking at things below
the surface of the water, an oily glandular secretion
helps protect a dolphin’s
eyes from the elements. The corneas and lenses of a dolphin’s
eyes are shaped to see light through water. The ocular
muscles, however, seem to bend the lenses to accommodate
above-water viewing.
A dolphin’s pupil is shaped like a horseshoe
when it is fully dialated. In extremely intense light,
a dolphin’s pupil constricts down to two tiny openings,
in essence a “double pupil” (Herman et al.,
1975). Instead of dealing with very intense light focused
on one single point of the retina, the double openings
allow for two smaller points of lesser intensity to be
used instead. The smaller the opening through which light
passes, the greater the distance far-away objects remain
in focus. Parallel light rays produce a perfect image
without the necessity of focusing, therefore, the “double
pupil” helps a dolphin have acute vision both in
air and underwater during the day. This same structure
may also play a part in a dolphin’s depth perception.
Dolphins also have very good night vision with the
aid of a beautiful blue/green tissue called the tepetum
lucidum. This tissue gathers all available light in a
low light situation and reflects it back through the
lens allowing for better vision. Cats, dogs and a variety
of other animals have a tepetum lucidum allowing for
night vision.
References
Through millions of years of evolution, dolphins have
made significant adaptations to survive in an environment
that is typically very hostile toward mammals. Through
time, they have become one of the most well equipped
and efficient creatures of the sea. The following references
are good sources for further information regarding the
incredible physiology of cetaceans.
Berta, A. and J. L. Sumich. 1999. The Evolutionary
Biology of Marine Mammals. Academic Press: San
Diego, CA.
Bryden, M. M. and R. Harrison, ed. 1986. Research
on Dolphins . Oxford University Press: New York,
NY.
Herman, L. M., ed. 1980. Cetacean Behavior .
John Wiley and Sons, Inc.
Hoelzel, R.A., ed. 2002. Marine Mammal Biology. An
Evolutionary Approach. Blackwell Science, Ltd. Malden,
MA.
Leatherwood, S. and R. R. Reeves, ed. 1990. The
Bottlenose Dolphin. Academic Press: San Diego,
CA.
Norris, K. S. 1991. Dolphin Days: The Life and
Times of the Spinner Dolphin. Norton: New York,
NY.
Pryor, K. and K. S. Norris, ed. 1991. Dolphin Societies:
Discoveries and Puzzles . University of California
Press: Berkeley and Los Angeles, CA.
Reynolds, III, J. E. and S. A. Rommel. 1999. Biology
of Marine Mammals. Smithsonian Institution Press:
Washington and London.
Reynolds III, J.E., R.S. Wells, and S.D. Eide. 2000.
The Bottlenose Dolphin: Biology and Conservation.
University Press of Florida: Gainesville, FL.
Ridgeway, S. H. and R. Harrison, ed. 1994. Handbook
of Marine Mammals, Volume 5: The First Book of Dolphins.
Academic Press: San Diego, CA.
Ridgeway, S. H. and R. Harrison, ed. 1999. Handbook
of Marine Mammals, Volume 6: The Second Book of
Dolphins and the Porpoises. Academic Press: San Diego,
CA.
 Copyright © 2007 Dolphin Research Center
Information Can Be Used with Proper Citing
Dolphin Research Center, 58901 Overseas Highway, Grassy Key, FL 33050-6019
305-289-1121 www.dolphins.org
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