Mutants in our Midst
How Anatomical Mutations Affect the
Skeleton and Musculature of Frogs
The Body
 Look around you! Every animal has certain
anatomical similarities.
 Among species, the arms and legs attach and
move similarly.
 Individuals of a species all move in similar
fashions.
 Frogs jump; whereas, humans walk.
 While humans can also jump, can you imagine how
tired you’d be if all you did was hop around all day?
 These motions are determined by our anatomy.
The Body…
 Sometimes, though, there are problems
with the development of the body that
result in what scientists call anatomical
mutations.
 Before discussing mutations, however,
we should get some background on the
body in general.
Bones! Bones! Bones!
 Almost all animals have bones—many
fish, all birds, all reptiles and amphibians
and all mammals, but why?
 Bones serve many purposes, like
providing structure and stability in the
body like the spine. Bones like the ribs
and the skull protect vital organs.
Bone Composition
 Bones are made of a very unique blend of
tissue that allows for the strength needed to
support the body and the mobility needed
to move the body.
 Bones are living tissue within a hardened
mineralized matrix.
 Bones are composed of two different
materials:
 Minerals, like calcium, that provide strength
 The osteoid protein matrix that provides
flexibility and elasticity
The Materials of Bones
 Bones are covered by a dense membrane
called the periosteum.
 This carries the blood vessels to the inner bone
layers.
 It also provides a protective sheath to the bone
tissue.
Inner Layers of Bone
 The inner layers of the bone are the
compact and spongy (cancellous)
bone.
 Compact bone is the outermost layer,
comprised of hardened rods surrounding
the vessels in the inner bone. (Black
arrow)
 Cancellous or spongy bone is very strong,
too, despite its name. It is a network of
interlocking cylinders. (Green X)
 Bone marrow is the innermost layer of
bone.
 Here, your blood cells are produced.
Bones and Muscles
Bones are also where many
muscles attach. These muscles
are called skeletal muscles. They
are responsible for mobilizing your
bones. Unlike smooth and
cardiac muscle, which regulate
internal functions of the body like
digestion and breathing, the
skeletal muscles are in charge of
voluntary action—like walking,
talking, swimming, etc.
Bones as Levers
 In the body, bones act as levers, amplifying
the movement of muscles into movement
that we can use.
 Muscles contract and relax to cause
movement.
 Muscles work in pairs—one contracting
while the other relaxes. This opposition
force allows for most of the movement we
think of in the body.
Joints as Fulcrums
 If bones are the levers of the
body, then the joints are the
fulcrums.
 These joints act as the fixation
and stabilization of the bones
as levers.
 The placement of the fulcrum
and effort of the muscle
depend on the classification of
the lever.
Levers—A Review
 Levers are a classification of
simple machines.
 They consist of a rigid bar
fixed at a fulcrum that
modifies the force and
motion applied. The load or
resistance is the opposition
force to the effort.
 There are three types of
levers determined by where
the fulcrum, effort and load
are located.
Images Courtesy of
Types of Levers
First Class Lever
Force/Effort
Images Courtesy of
Load/Resistance
Fulcrum
First class levers are like see-saws. The fulcrum is in the center with
the load or resistance on one side and the force or effort on the other.
They give the advantage of speed and strength, depending on the
fulcrums location between the two forces.
In the body an example of a first class lever is the base of the skull
which rests on the spine (fulcrum) and can move up and down (like
nodding your head “yes”).
Second Class Lever
Force/Effort
Images Courtesy of
Fulcrum
Load/Resistance
Second class levers have the fulcrum at one end with the resistance
in the center and the force at the end.
The bones of the foot (tarsal bones) are an example of a second
class lever, where the joints in the metatarsal bones (mid-foot) act as
the fulcrum and the contraction of the calf muscles act as the effort.
Second class levers have the advantage of strength, as in the tarsal
bones where these bones as levers must amplify the contraction of
the calf muscle to lift the entire weight of the body.
Third Class Lever
Images Courtesy of
Force/Effort
Fulcrum
Load/Resistance
The third class lever has the fulcrum on the end like the second class
lever, but the force is in the center with the load or resistance on the
end.
Most of the levers in the body are third class levers. They are adapted
for speed of movement rather than brute strength.
The ulna of the elbow joint is an example of a third class lever. The
force (the biceps muscles contracting) is located in the center, between
the weight of the forearm (load) and the fulcrum (elbow joint that fixes
the ulna to the humerus).
Anatomy
 Now that you understand how bones and
muscles work, let’s look at how they are
similar and different between species.
 First, take a look at the skeletal structure
of a human. Look at the bones that are in
your body, and then compare it to the
pictures of the skeleton of a frog.
 How are they similar? How are they
different?
The Human Skeleton
Image courtesy of
The Frog Skeleton
Bone differences
 Humans are bipedal and move and stand
on only two legs.
 Frogs are quadrupedal, meaning they
move and stand on four legs.
 There are other differences in the bones of
humans and frogs, like the proportions.
 Let’s examine this mathematically!
Proportionality
 The most apparent difference when you look
at the two skeletons might be that the frog
seems to have much longer legs for its body
size than the human.
 Each species has a typical set of ratio of
bone lengths that give the characteristic
appearances of the body.
 This mathematical approach to anatomy is
has been a popular aspect of science for
many years.
Mathematical Studies of Anatomy
The Vitruvian Man
 The Vitruvian Man is perhaps the most well-known
example of the synthesis of math and anatomy.
 The Roman architect Marcus Vitruvius Pollo created
a system that described the proportions of the human
body a series of fractions.
 1500 years later, Leonardo da Vinci created the
drawing that later became known as the Vitruvian
Man, which showed the intrinsic geometry of the
human body—with arms straight out to the side and
legs straight and together, the body is a square; with
arms extended above the head and legs spread, the
body is a circle.
The Proportionality of the
Human Body
 Leonardo da Vinci also expanded further on
the fractional proportionality of the human
body, as many subsequent artists have done,
including Michelangelo for his sculpture of
David.
 The human body is essentially symmetrical
and proportional.
 The body can be divided in half by the hip
bones, meaning that the length of the legs is
equal to the length of the torso (from the top of
the head to the hip bones).
Comparing Proportions
 The same observation cannot be made
for frogs, who seem to have legs that
account for 2/3 of the body length.
 If this were to be true for humans, the
average 12 year old, with a height of
150cm would have legs that were a
meter long!
 Do you think you would be able to see
the difference?
Humans vs. Frogs
Why the Difference?
 Why do you think that frogs have such
long back legs?
 What purpose could their legs serve that
humans don’t need?
 What benefit would it be to have hind legs
that were much larger than our upper
body?
Movin’ and Shakin’
 Frogs do not walk, they jump; therefore,
they must have enough force to propel their
body forward and up
 Humans and other walkers need only the
strength to balance their weight on one leg
(if biped, and two legs if quadruped) while
lifting and extending the other.
 This very different requirement for motion is
what causes the frog’s anatomy to be so
different in the hind legs.
Walking and Jumping
 Note the differences between the frog and other
quadruped, the dog.
 The dog walks, typically, and though it can
jump, walking is the most common means of
motion.
 Frogs generally jump. Some frogs can “walk,”
but the movement is still a shorter jumping
motion.
 Their modes of action can be seen in their
bones and muscles.
 Examine!
Dog vs. Frog: The Hind Legs
 Frogs have relatively long leg bones in comparison to other
quadrupeds.
 Frogs have a hip structure that allows for rotation of the leg
during jumping that dogs and other quadrupeds do not
need.
 Frogs have only one bone in the calf instead of two like
other animals (quadrupeds and bipeds) that gives more
strength and leverage but less detailed motion, like walking.
 Frogs have an extra joint in their lower leg which gives
more stability and power for jumping.
Os Coxae (Ilium and Ischium)
Femur
Ilium
Tibiofibula
Femur
Ischium
Tibia and Fibula
Tarsal
Bones
Tarsal Bones
Astragalus
Calcaneum
Movement of Frog—Jumping
 Jumping requires a great deal of energy, so the
muscles of the back leg of a frog must be relatively
large.
 Jumping also requires a wide angle of rotation
around the hip joint for take-off and landing.
 Rotation also occurs at take-off around the knee
joint.
 Scientists study the axis of rotation of a bone
mathematically to determine the bone strength
and other clues that help us to better understand
anatomy.
As a Frog Jumps
As you can see, the frog
uses its long legs and
many joints to rotate the
legs to get a great deal of
power while jumping.
Internal rotation of the tibiofibula at the starting jump
position enhances
Joint Rotation Axes
Kargo, W. J. et al. J Exp Biol 2002;205:1683-1702
Kargo, W. J. et al. J Exp Biol 2002;205:1683-1702
Movement of a Frog—
Swimming
 In swimming, frogs use their back legs
like fish use their flippers.
 The extra rotation that frogs have
around their hip and knee joints allows
for strong kicking that propels them
forward in the water.
 The conformation of a frog’s leg is
also better suited for swimming with
the knee joint of a relaxed leg located
around mid-body, giving the frog a lot
of power to kick.
Alert! MUTATIONS AHEAD!
 In 1995, a group of Minnesota students
discovered something that shook the
ecological world—the gross malformations of
frogs.
 The students found very few frogs in areas
that were once heavily populated with
amphibians, and those frogs they did find
had mutations like extra or missing limbs,
improperly placed eyes and mouths or
malformed spines.
 What had happened?
Searching for Answers
 Ecologists and other scientists set out to
find out the extent of the problem.
 They found that mutation among
amphibians was worldwide, as was the
decline in amphibian populations.
 What exactly was happening to these
frogs?
 What effects did these mutations have on
the individual frog? On the ecosystem?
 The scientists set out to find answers.
What happens when
something goes wrong
 Frogs and other
amphibians, due to
their development
cycle, are often
found with mutations
as adults.
 A mutation may be
as simple as extra
toes, or as serious
as having an extra
pelvis and two extra
legs.
 What causes these
mutations?
Frog Life Cycle
 Frogs and other amphibians
have a life cycle that makes
them very vulnerable to
mutations.
 Since they are undergoing
constant anatomical
changes throughout the first
part of their life, they are
very susceptible to factors
that could cause mutational
shifts in their anatomy.
Causes of Mutations
 Much speculation surrounds the exact causes of
the mutations in amphibians.
 One cause that’s been identified is the trematode
parasite Ribeiroia ondatra. Because of
environmental factors like increased
temperatures and contaminated water, the
trematode population has grown.
 Other mutations are caused without the presence
of the parasite, and are probably chemically
driven, meaning they are caused by pollutants.
 Another hypothesis is that the mutations are
caused by overexposure to UV radiation due to a
depleted ozone layer.
 What do you think?
Other Kinds of Mutations
 The word “mutation” is used most often when describing
genetics and DNA.
 While the kinds of anatomical mutations found in frogs are
the result of these genetic mutations, they are not the only
results of mutations in DNA.
 Mutations that we view as anatomical, such as a frog
having an immobile leg, might also be the result of
something else other than altered DNA. It could be the
result of an injury or a trauma.
 Not all mutations are bad!
 Scientists now believe that the genetic variation found in
all terrestrial life forms originally came from DNA
mutations.
 Genetic variations in a population allow natural selection
to occur.
How a Mutation Affects
the Skeleton Chemicals that are absorbed during the
development stage will cause a significant
change in the frog’s DNA that will create a
mutation.
Parasites infect tadpoles and alter DNA during
limb formation, causing missing limbs, extra
limbs or other problems.
Most mutations deal with the skeletal system.
Examine the frogs to the left.
•The top frog has a malformed hip and
only one leg.
•The middle frog has an underdeveloped
leg. Note the fused and bent bone of the
right leg.
•The final frog has multiple bones in the
left leg. It has two femurs and five bones
in the lower leg. The joints are improperly
formed and therefore the limb is immobile.
Why isn’t having
more legs
beneficial?
Amphibian Malformation Pictures
 You would think that having multiple legs would help
the frogs jump better or that having more bones would
provide better leg strength, but this unfortunately is not
the case.
 The extra bones disrupt the proper leverage that the
normal bones would provide.
 Extra limbs offset the natural balance and movement
of the frog and are generally nonfunctional.
 Also, the malformations are often just the tip of the
iceberg when it comes to anatomical problems in the
frog.
The Future of Frogs
 Many scientists have said that frogs and other
amphibians are indicator or sentinel species for
environmental changes—they are the
proverbial canary in the mineshaft.
 Since frogs live the first half of their lives in the
water they are extra-sensitive to changes in the
water’s quality and overall changes in the
environment.
 Many ecologists suggest that the same
environmental changes that are causing gross
frog mutations may eventually cause harmful
health effects in the human population.