Dec. 2002
Marine Mammals – Skulls and Bones
M-08
MATERIALS:
Pinniped skulls – Elephant seal, harbor seal, California sea lion
Dolphin skull
Sea Otter skull
Baleen sample
Whale vertebra (picture only for South County)
Marine Mammal Skeletons poster
Marine Mammal Feeding Methods poster
Echolocation poster
Picture of skull and living animal of each marine mammal represented in this kit
Tongs with brush representing baleen whale jaws
Ice tongs representing toothed whale and pinniped jaws
Marine Mammal Bone Clues
“Bone Detective” Photos
Included in the kit are resource materials for the docent to examine ahead of time. This
kit overlaps the Marine Mammals kit. If the docent is not familiar with the Marine Mammals kit
particularly the various characteristics and adaptations of marine mammals, a review of the
materials in that kit may be helpful. The background materials in the Skulls and Bones kit in
Forest and Foothills may also provide useful information especially on teeth structure.
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Marine Mammals – Skulls and Bones
M-08
OBJECTIVES:
To compare the various characteristics and adaptations of marine mammals through
studying their skull and bones.
SCIENCE THEMES:
Evolution, adaptation
PROCESS SKILLS:
Observing, comparing, relating
GRADE LEVELS: 4-8
FOCUS WORDS:
Mammal, skeleton, skull, cranium, pinniped, cetacean, vertebrate, vertebra, sagittal crest,
carnivore, canine, incisor, baleen, echolocation, vestigial
BACKGROUND INFORMATION
What is a skeleton?
A skeleton is a strong frame that supports and protects an animal’s body. All mammals, including
marine mammals, have the same general design of skeleton frame. The mammal skeleton is
internal, or inside the body, and includes: the skull, the spine, the ribs and four limbs.
Skull: The skull is the most complex part of the skeleton. The skull is a hard casing of fused
bones. It protects the brain and houses the sensory organs – the eyes, ears and nose. It provides
openings for breathing and eating as well as an anchor for the teeth.
Spine: The “backbone.” The main support for the body, where all the other parts of the skeleton
attach. A spine is made up of several vertebrae linked together in a long flexible column. The
spine identifies the animal as a vertebrate.
Ribs: A protective cage around the heart and lungs.
Limbs: Allows the animal to move around and explore its environment.
Muscles attach at all different parts of the skeleton providing the necessary forces for movement
of that part.
What are some basic characteristics of "marine" mammal skeletons?
Marine mammals are unique from other mammals in that they are adapted for a lifestyle primarily
in the ocean environment. Their body shapes (and thus the skeletal frame that helps define the
shape) are streamlined with minimal protuberances. These torpedo or submarine-like bodies
move efficiently through the water with minimal resistance. Most marine mammals don’t have
arms or legs, but flippers - another adaptation to optimize swimming. However, if you look at the
bone structure within their skeletons, they have bone structures that resemble the arm and leg
bones of humans. They are shaped a little differently however, flattened and shortened- to help
form the paddle-like flippers. Because whales live exclusively in the water, they have lost almost
all trace of their hind limbs (legs), but if you look closely at some whales’ skeletons, you can see
remnants of their leg bones. These “vestigial” leg bones, are evidence that millions of years ago
whales lived and walked on land, but then returned to the water probably because of food supply.
Dolphin and whale skulls are unique among mammals because their nostrils or “blowhole” is
positioned on top of their skulls, well removed from the snout. This important adaptation allows
them to breathe without emerging from the water. The rib cages of deep diving marine mammals,
particularly whales, tend to be more flexible as compared with other mammals. This allows for
tremendous expansion of the lungs, as well as the ability to collapse their lungs during deep dives.
A baleen whale’s rib cage is particularly flexible with few attachments to the sternum (breast
bone). A stranded baleen whale may suffocate easily when beached as the rib cage collapses
under the whale’s own weight.
What is a marine mammal’s skeleton made of?
A marine mammal’s skeleton is made primarily of bone, just like humans and other mammals.
Bone is a strong hard substance made with calcium and other minerals. Most marine mammals’
bones tend to be oilier than human bones because of all the blubber they store for warmth, food
reservoirs and buoyancy. Whale bones can also be spongier than land mammal bones, because the
bones do not have to be as strong as land mammals whose bones need to support them on land.
Because of the oil content and the surrounding marine environment, whale bones tend to
decompose more quickly. It is rare to find a completely intact whale skeleton.
Why study skulls and bones?
Bones last much longer than other parts of the body because they decompose or breakdown much
more slowly due to the minerals in the bone. Bones, or fossils of bones, are frequently the only
record left of animals that lived a long time ago. Bones, especially the skull, can tell us a lot about
an animal and it’s lifestyle. The bones tell us the approximate size and shape of the animal, what
types of food it might eat, what senses it may have relyed on most, any special adaptations it may
have to the environment (like the blowhole in the whale), and many other things, including even
how it might have died.
MARINE MAMMAL SKULLS
PINNIPED SKULLS – Elephant seal, Harbor Seal, California Sea Lion
Pinnipeds are excellent hunters and their skulls give us several clues to this skill. They are
carnivores, with strong canine teeth set in strong jaws to catch their diet of mainly fish. Pinnipeds
typically catch and swallow their food whole, so they don’t have very specialized “chewing”
teeth (molars). Sometimes the teeth posterior to their canines, are referred to as postcanines since
they are not highly specialized for chewing. The number of teeth in a pinniped is less than most
other mammals.
Pinnipeds skulls are shaped so their eyes and nose are near the top of their head so they can stick
them out of the water easily. Pinnipeds tend to have very large eyes to help them see their prey
underwater. Note the lacy bones (the turbinates) in the nasal cavities of the pinniped skulls. These
are the animal’s air filtering system. The turbinates are lined with mucous to warm the air when
the pinniped inhales, and to prevent salt and sand from entering the respiratory system.
ELEPHANT SEAL
This is a male skull. The male can be up to three times larger than the female, so the male skull is
typically much larger than the female skull.
Elephant seals are named for the mature male’s huge nose. At about age nine, the male’s snout
becomes greatly enlarged into what is called a proboscis. The proboscis is inflatable, and serves
as a resonating chamber for the male’s deep throated vocalizations made during the mating
season. The probiscis is made of soft tissue and is not part of the skull, but lies on the snout
portion of the skull.
This skull shows the massive, powerful jaws and huge canine teeth of the male. These teeth and
jaws are not only important to help them catch their diet of fish and squid, but they are also useful
for their mating battles. The males slash and bite at each other’s chests with their giant mouths to
establish who gets the female elephant seals.
HARBOR SEAL
California’s smallest pinniped, it has strong canine teeth to help catch and hold its diet of fish,
crab, octopus and squid.
Seal and sea lion skulls are very similar, and hard to tell apart. There are a few differentiating
features including no prominent ridge above the eye socket (no supraorbital process) on the seal,
which is present on the sea lion skull; and no bony ridge on top of the head (sagittal crest) on the
seal skull, present on the male sea lion skull. See if the students are able to tell the seal skull and
sea lion skull apart given these clues.
Seals move on land by “hopping” on their bellies looking more like giant caterpillars. They are
unable to rotate their hind flippers forward, like sea lions do, because of the strong tension of a
tendon running through a groove in the ankle bone. Seals use their hind flippers for power
swimming, and their front flippers for steering.
CALIFORNIA SEA LION
This is an adult male skull as can be identified by the bony ridge on the top of its skull called the
sagittal crest. The sagittal crest is where the jaw muscles attach at the top of the head. The adult
male has very powerful jaws. Male sea lions needs powerful jaws to battle for female sea lions.
A sea lion’s age can be determined by its teeth. They secrete a layer of cement on the insides of
their teeth each year. When the sea lion dies, you can look at a cross section of a tooth, much like
growth rings on a tree, to determine its age. Note the strong canine teeth that come in handy to
catch and hold fish and squid before they are swallowed.
A sea lion is able to “walk” on land due to its ability to rotate its hind limbs forward and use them
like long “feet”. When the sea lion swims his hind flippers are rotated backwards, behind his
body, so he can use them like a rudder. The front flippers provide the main power for swimming.
CETACEAN SKULLS – Dolphin, Baleen Whales
"The Marine Mammals Feeding Methods" poster shows the two different types of whales, toothed
and baleen, and their contrasting feeding techniques. There are also two different types of tongs
to illustrate the two different types of jaws and feeding techniques.
Whales are classified into two different groups based on their teeth: Toothed whales (odontoceti)
and baleen whales (mysticeti) The toothed whales eat primarily fish and squid, using their teeth to
catch and grip their prey, then swallow the food whole, not using their teeth for chewing. The
baleen whales are filter feeders, using the baleen like giant sieves. They draw in seawater through
their giant mouths, and spit it out back through the baleen, trapping krill, fish, and other small
ocean creatures inside the baleen plates.
The whale skull is probably the most modified among mammals, and is a massive skull. The skull
is “telescoped” with elongated jaws and a shortened braincase. One of the most striking features
is the position of the nostril which may be single (odontoceti) or double (mysticeti) nasel passage
which forms a single“blowhole.” The blowhole is typically placed on the highest point of the
head, well removed from the snout.
DOLPHIN
The dolphin skull is streamlined. It has a long narrow jaw, like the neck of a bottle (hence bottlenosed dolphin), with about 50 pair of round pointed teeth. Ask the students how they think the
dolphin uses his teeth. (These sharp peg-like teeth allow it to grip slippery fish and squid until the
prey can be swallowed whole. These teeth are not designed for the tearing and chewing of food
like ours.) One of the most striking features of a dolphin’s skull (and all cetaceans’ skulls) is the
position of its nostril. The two nasal passages merge into one that opens to the top of the skull as
one blowhole. Ask the students to find the blowhole and give a reason why this is such a good
adaptation for the dolphin (allows them to breathe without emerging from the water).
Sound is a very important sense to the dolphin. Sound travels well underwater, and allows the
dolphin to “see” long distances underwater with a process called echolocation. This is very
helpful in locating food especially in murky or deeper waters (Please see the "Echolocation"
poster for details on echolocation). Point out to the students how the dolphin skull is designed to
optimize these echolocation skills. The skull is concave on top (no “forehead”) to allow space for
the melon that then creates the dolphin’s forehead. When the sound waves return, the dolphin
receives the sound through its lower jaw, not an external ear. Show the students that the jaw is
hollow, filled with an oil of similar density to sea water, to optimize sound transmission. Also
note that the angle of the lower jaw is optimized to capture the maximum amount of returning
sound waves. Show how the jaw leads right up to the inner ear to deliver the sound waves. In the
whale family, to which the dolphins belong, only odontocetes (toothed whales) have shown the
ability to echolocate.
Note the position of the dolphin’s eyes. They are placed far to the sides. This gives them a wide
field of view allowing the dolphin to see forward, to the side and also somewhat to the rear. This
helps them survive in their watery environment.
SEA OTTER
The sea otter has strong jawbones to help them crush the shells of crabs, abalone, clams and sea
urchins. They have several different shaped teeth good for tearing, ripping and chewing their
food. The incisors, the front teeth, are good for scooping the meat out of shells.
Individual otters have their own favorite foods. Each otter may eat only two or three different
types of food. Sea otters’ teeth are often stained to match their favorite food. If a sea otter 's teeth
are tinted purple, he probably likes purple sea urchins. Otters that feast on crab and squid tend to
have whiter teeth. These feeding habits may also explain why several sea otters can live in one
area peacefully, since they are not always hunting the same food, they are not always in direct
competition.
The sea otter, unlike most of its other marine mammal counterparts, has paws, not front flippers,
and they use them to help catch and hold their food. The sea otter is also one of the few mammals
who uses tools, such as a stone, to help it to open its food.
The sea otter has a very flexible spine or backbone. This lets them reach all parts of their body so
they can clean their fur. Keeping their fur clean is very important in keeping them warm, because
otters have no blubber. Otters blow and pat small bubbles into their fur while somersaulting and
grooming to keep themselves warm and insullated. This flexible backbone also means they can
do all sorts of fancy tricks, performing forward and backward somersaults and twisting while
swimming in the water.
Note on the poster with the sea otter skeleton that the outside toe is the longest, and inside toe is
shortest, just the opposite of humans. With a large outer toe, an otter can spread its webbed feet
wider for swimming. This toe design does not work well for walking though, and would make us
wobbly!
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MARINE MAMMAL SKULLS AND BONES – Activity Description
The docent should inform the students that the skulls in this kit are not from real animals. They
are replicas molded from actual marine mammal skulls. The vertebra in the north county kit is a
real whale vertebra. These skulls and bones were chosen because they are representative of
marine mammals that can be found in and around the waters off the California coast.
This kit is primarily intended to be a hands-on kit that allows the students to handle and explore
the skulls and vertebra (with supervision). Each skull and the vertebra have a corresponding
labeled picture in the kit to help the docent identify the key components of that individual skull or
bone. If the docent is unable to transport every skull to the site, the pictures can be used as
teaching aides as well.
For all grades:
1. As a starting point, briefly review the definition of a marine mammal – a mammal (warmblooded, breathes air) who lives in and around the ocean. Ask the students to identify some
marine mammals that are common to California – gray whale, California sea lion, sea otter
etc… Marine mammals have several special adaptations that help them survive in the ocean
environment including structural adaptations that can be seen in their skeletons and skulls.
2. Discuss the definition of skeleton – a strong frame that supports and protects the animal, and
the
basic design of a mammal’s skeleton – skull, spine, ribs, 4 limbs. Discuss
some basic characteristics and adaptations of marine mammals as relates to their skeletons.
Marine mammals have streamlined bodies with flippers instead of arms and legs to help them
swim
efficiently in the water. Inside the flippers, the bones look very similar to ours. Whales do not
have
hind limbs, but do have vestigial legs bones. Whales have a blowhole on top of their head so
they do
not have to emerge from the water to breathe. Rib cages of deep diving marine mammals tend
to be
more flexible than terrestrial mammals to allow for compression with the increasing pressure
that
is experienced at great depths under the water.
See Background Information – Introduction for more details. Use the Skeletons poster to allow
the
students to see a variety of marine mammal skeletons and to help illustrate the adaptations of a
marine
mammal as contrasted with the human skeleton.
3. Using any one of the skulls as a guide, review the importance and basic parts of a skull.
Where does
the brain go? The cranium or braincase – the “box” of bones that surrounds the brain. Point
out that
the cranium is like an internal crash helmet that protects the brain. A bicycle helmet provides
added
protection for your brain. Have the students identify the different sensory organs the skull
houses eyes, ears, nose. Point out that the skull provides the openings for breathing and eating.
The docent may choose to review a few skulls in depth or briefly review as many skulls as
possible in
the allotted time. Each skull has relevant facts listed in the Background Information – Marine
Mammals – Skulls section. Each skull also has a corresponding labeled skull picture and a
photograph of a living animal. The Background Information facts are listed on the back of the
corresponding skull picture to assist the docent while teaching.
4. For each skull, be sure to review the teeth structure and feeding style of that marine mammal.
Discuss how all marine mammals depend on the ocean for their food. They must survive in a
more
“three-dimensional” environment than a terrestrial mammal. They are all excellent swimmers
who
need to dive and hunt in cold, dark water. They all catch fish or other creatures from the sea,
but may
use different methods to catch them. Pinnipeds and toothed whales catch their prey one at a
time
using powerful vice-like jaws. Toothed whales also have the special adaptation of
echolocation to
locate their prey. Baleen whales are filter feeders trapping lots of small prey at one time. Sea
otters
have paws, not front flippers. They use their paws and tools to help catch and open their prey.
To illustrate the different feeding styles the docent may use the Feeding Methods poster, baleen
sample and two different styles of tongs representing “jaws”: tongs with brush represent
baleen
whale jaws (baleen was commonly used to make brushes in the 19th century), ice tongs
represent
toothed whale and pinniped jaws that pierce and grip their prey before swallowing whole. Also
see
Background Information on each marine mammal for detailed information on diet and feeding
method.
5.
Discuss the whale vertebra. Ask the students what part of the body this comes from, and what
might
be its function? See Background Information – Whale Vertebra section and the whale
vertebra poster for details.
For older grades:
1. Marine Mammal Bone Clues: Review the information you have taught the students by
giving
them the clues listed at the back of this script. See if they
can identify the marine mammal, its skull, and its living picture.
2. Bone Detectives: What happened to these marine mammals? Let the students examine the
pictures of the skulls with unusual features. Have the students guess what has happened to
these marine mammals. (Answers on back of picture.) Point out that because bone survives
longer than any other part of the animal, it provides us some history and information about
that animal. The skull is the most informative part of the skeleton in telling us about an
animal and its lifestyle.
WHALE VERTEBRA
North county kit contains a real whale vertebra. South county kit has a picture of a whale
vertebra.
North County: Ask the students the differences they see, feel, and smell between the real bone of
the vertebra and the replica skulls. The real bone is dark and greasy because of all the blubber and
oil that was in the whale’s body.
Ask the student what part of the body this comes from, and what might be its function? A
vertebra is one of the many “building blocks” that make up the spine or the backbone. The blocks
or vertebrae stack up one on top of another. Each vertebra moves only a little in relation to the
one next to it, but over the whole row these movements add up making the spine flexible. Whales
are powerful swimmers, and their strong swimming muscles are attached to the vertebrae in the
spine. The caudal (tail) vertebrae of whales are strikingly different from terrestrial mammals. The
vertebrae are enlarged with extra bony processes called chevron bones that provide increased
muscle attachments to aid in lifting the flukes. The muscles in the tail portion of the spine are the
largest and most powerful in its body. Some whales and dolphins have fused neck vertebrae so
that the movements of the head and neck act as a unit assisting in the streamlining of swimming
movements.
The vertebrae also provide a protective canal for the spinal cord where the nerves providing
information to all parts of the body run.
For older grades:
Marine Mammal Bone Clues
(Clues for a dolphin)
My jaw is hollow.
I use sound waves to find my food.
I have peg-like teeth
I have a long bottle-shaped nose.
(Clues for a baleen whale)
My mouth is like a sieve.
I am very huge, but I eat tons of very small organisms (krill).
My nostrils are on top of my head.
(Clues for an male elephant seal)
I am a large marine mammal.
My jaws are massive and I use them to battle other males in my group.
I have a strange looking nose.
(Clues for male sea lion)
I can walk on all four flippers by rotating my hind flippers forward to act more like “legs”.
I have a huge ridge on the top of my skull.
I have small ear flaps on each side of my head.
(Clues for harbor seal)
I am California’s smallest pinniped.
I cannot rotate my hind flippers forward and use them like “legs”. I look more like a giant
inchworm or caterpillar when I move on land.
I do not have an external ear flap.
(Clues for sea otter)
I use my front lower teeth to scoop meat out of shells.
I am the smallest marine mammal.
I have paws instead of front flippers.
I can have purple teeth if I like purple sea urchins.
I have a flexible spine that lets me clean my fur everywhere on my body and lets me swim like an
acrobat.