How does an animal having toes create more friction than not?

How does an animal having toes create more friction than not?

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I was looking at the morphology of different animals when I found that apparently part of the reason vertebrate mammals have toes is that they grip the ground. But, why is that better than having a solid foot with no toes if friction increases with the surface area in contact with the ground?

I would say that the premise that "the surface area of contact with the ground increases in case of solid foot" is incorrect.

In practice, the ground is very far from being even and flat, and the animals need to navigate themselves throughout different terrains that include very uneven ground, stones, growing & lying plants, etc. In that case, having toes, in general, offer an advantage of gripping to the ground (and increasing contact area in multiple directions) in a large set of terrains. Then, different animals will develop their toes in different shape/form tailored to the dominating usage (including, but not limited to gripping the ground).

Biology chapter 33 quizzes

B) All cnidarians except corals are in the medusa stage.

C) This phylum has more species than any other phylum.

D) The bodies of its members are organized around a gastrovascular cavity.

A) the chitinous exoskeleton cannot grow

B) the environment degrades the exoskeleton, which therefore must be shed and replaced

C) arthropod appendages generally increase in number as the animal ages

D) the exoskeleton is progressively reabsorbed by body tissue

A) coral and starfish . Echinodermata

B) snail and coral . Annelida

C) bass and giant squid . Mollusca

D) giant squid and snail . Mollusca

A) All possess a water vascular system, which permits movements via tube feet.

B) Larval forms show evidence of bilateral symmetry, which is mostly lost in the adult form.

C) One of the classes in this phylum is Asteroidea, the sea stars.

D) Many species possess an internal calcareous skeleton and spiny dermal projections.

B) that have a mantle that produces a shell

C) that are hermaphrodites

D) with a closed circulatory system

A) absorbing nutrients from the guts of their hosts

B) paralyzing small crustaceans with stinging cells

C) scraping bacteria and algae from hard substrates

D) performing photosynthesis

A) use their pseudocoeloms as hydrostatic skeletons

B) have characteristically long bodies with external, but not internal, segmentation


Bipedal walking and running are the normal human gaits. Apes and a population of Japanese macacques sometimes walk bipedally (Napier & Napier, 1967). Kangaroos and a few rodents hop bipedally. Birds on the ground walk, run or hop. Some lizards run bipedally, and cockroaches have been filmed running bipedally at their highest speeds (Full & Tu, 1991).

In bipedal walking and running, the feet move alternately, half a cycle out of phase with each other. Such gaits are generally classed as walking if the duty factor (the fraction of the time for which each foot is on the ground) is greater than 0.5, and running if it is less than 0.5. In hopping, the feet generally move more or less simultaneously. Jerboas and crows, however, use a peculiar out-of-phase hopping gait, in which the phase difference between the feet is neither zero nor half a cycle (Hayes & Alexander, 1983). Gaits like this are sometimes described as skipping (Minetti, 1998).

In the remainder of this paper I consider only walking and running, the gaits normally used by humans. Apes on the ground usually travel quadrupedally. They make only occasional use of bipedalism, often in the context of display. Bipedal walking is the normal slow gait of birds, and running is the fast terrestrial gait of many of them. There seems to be a tendency for birds that spend a lot of their time in trees to use hopping as their fast gait, and for other birds to run. Lizards vary in the use they make of bipedalism. Many species are exclusively quadrupedal. Some such as Uma are bipedal for only a small proportion of their strides, but others such as Callisaurus are frequently bipedal for many successive strides (Irschick & Jayne, 1998, 1999a). Basiliscus is well known for its ability to run bipedally for short distances over the surface of water (Glasheen & McMahon, 1996). The cockroach Periplaneta runs on all six legs at low speeds, but at high speeds (1.0𠄱.5 m s 𢄡 ) it makes about half its runs on four legs (the middle and hind legs) and half on the hind legs only (Full & Tu, 1991).

Dynamic similarity

I will refer frequently to the concept of dynamic similarity (see, for example, Alexander, 2003). Two bodies are geometrically similar if one could be made identical to the other by multiplying all its linear dimensions by the same factor λ. By an extension of the same idea, two motions are dynamically similar if they could be made identical by multiplying all linear dimensions by a factor λ,all times by a factor τ, and all forces by a factor φ. For example,the motions of two pendulums of different lengths, swinging through the same angle, are dynamically similar. Strict dynamic similarity requires geometric similarity.

Two systems can only have dynamically similar motion in particular circumstances. If gravitational forces are important, ratios of (gravitational force/inertial force) must be the same for the two systems, at corresponding stages of their motions. For this to be possible, the systems must be moving with equal Froude numbers [speed 2 /(gravitational acceleration× length)]. A fuller explanation of this point can be found in Alexander(2003). In calculating a Froude number, any length characteristic of the systems may be used for example, leg length is generally used in discussions of running. If viscous forces are important, dynamic similarity is conditional on equality of ratios of viscous forces to inertial forces, which requires equal values of the Reynolds number(speed × length × fluid density/viscosity). For dynamic similarity of vibrating systems, the Strouhal numbers (frequency × length/speed)must be equal. Froude, Reynolds and Strouhal numbers are dimensionless. Other dimensionless numbers define conditions for dynamic similarity, in systems for which other kinds of forces are important.

The Function of Fingerprints

For those of you who understand natural selection, there is clearly no evolutionary advantage to fingerprints helping put criminals in jail, so there must be some other reason for their development. As it turns out, fingerprints also go by another name &ndash dermal ridges &ndash and when looked at under a microscope, those tiny patterns of swirls, loops and whorls look like a topographic map, complete with valleys and ridges. This textured nature of the fingerprints mean that they significantly increase friction on the surfaces they touch &ndash everything from tree branches to footballs. By increasing the contact area between the fingers and the other surface, it is easier to hang on to and grip safely.

Roughly five million years ago, before primates came out of the trees and began to move upright throughout the world, the need for that extra grip was crucial. Leaping from tree to tree without falling, precisely grasping rocks and other makeshift tools, and climbing rapidly to avoid predators required a confident grip that wouldn&rsquot slip. Wherever fingerprints have been found in the natural world, it has been determined that they serve a very similar purpose.

That being said, there has been some research on the frictional function of fingerprints that has found this accepted argument lacking. The opposing theory states that fingerprints, due to their rather rubbery nature, allow the skin to deform and avoid the development of blisters, unlike our palms, fingerpads and soles (of our feet), which can often blister. This argument doesn&rsquot full hold up, however, because human palms are also covered in dermal ridges.

Furthermore, some researchers have also proposed that fingerprints actually reduce contact area, which would lower the frictional coefficient. Others even argue that fingerprints are designed to help maintain a grip on wet surfaces, as the small channels between the ridges would be able to shift water away from the contact point. Whatever they&rsquore true purpose, they are important, and very difficult to get rid of, unless you&rsquore joining a secret government agency, of course.

Research is limited on this subject, but it is ongoing, both in humans and in the animal kingdom. Speaking of fingerprints in other animals, the form that their fingerprints take, as well as the species where fingerprints have independently developed, may surprise you!

38.3 Joints and Skeletal Movement

By the end of this section, you will be able to do the following:

  • Classify the different types of joints on the basis of structure
  • Explain the role of joints in skeletal movement

The point at which two or more bones meet is called a joint , or articulation . Joints are responsible for movement, such as the movement of limbs, and stability, such as the stability found in the bones of the skull.

Classification of Joints on the Basis of Structure

There are two ways to classify joints: on the basis of their structure or on the basis of their function. The structural classification divides joints into bony, fibrous, cartilaginous, and synovial joints depending on the material composing the joint and the presence or absence of a cavity in the joint.

Fibrous Joints

The bones of fibrous joints are held together by fibrous connective tissue. There is no cavity, or space, present between the bones and so most fibrous joints do not move at all, or are only capable of minor movements. There are three types of fibrous joints: sutures, syndesmoses, and gomphoses. Sutures are found only in the skull and possess short fibers of connective tissue that hold the skull bones tightly in place (Figure 38.23).

Syndesmoses are joints in which the bones are connected by a band of connective tissue, allowing for more movement than in a suture. An example of a syndesmosis is the joint of the tibia and fibula in the ankle. The amount of movement in these types of joints is determined by the length of the connective tissue fibers. Gomphoses occur between teeth and their sockets the term refers to the way the tooth fits into the socket like a peg (Figure 38.24). The tooth is connected to the socket by a connective tissue referred to as the periodontal ligament.

Cartilaginous Joints

Cartilaginous joints are joints in which the bones are connected by cartilage. There are two types of cartilaginous joints: synchondroses and symphyses. In a synchondrosis , the bones are joined by hyaline cartilage. Synchondroses are found in the epiphyseal plates of growing bones in children. In symphyses , hyaline cartilage covers the end of the bone but the connection between bones occurs through fibrocartilage. Symphyses are found at the joints between vertebrae. Either type of cartilaginous joint allows for very little movement.

Synovial Joints

Synovial joints are the only joints that have a space between the adjoining bones (Figure 38.25). This space is referred to as the synovial (or joint) cavity and is filled with synovial fluid. Synovial fluid lubricates the joint, reducing friction between the bones and allowing for greater movement. The ends of the bones are covered with articular cartilage, a hyaline cartilage, and the entire joint is surrounded by an articular capsule composed of connective tissue that allows movement of the joint while resisting dislocation. Articular capsules may also possess ligaments that hold the bones together. Synovial joints are capable of the greatest movement of the three structural joint types however, the more mobile a joint, the weaker the joint. Knees, elbows, and shoulders are examples of synovial joints.

Classification of Joints on the Basis of Function

The functional classification divides joints into three categories: synarthroses, amphiarthroses, and diarthroses. A synarthrosis is a joint that is immovable. This includes sutures, gomphoses, and synchondroses. Amphiarthroses are joints that allow slight movement, including syndesmoses and symphyses. Diarthroses are joints that allow for free movement of the joint, as in synovial joints.

Movement at Synovial Joints

The wide range of movement allowed by synovial joints produces different types of movements. The movement of synovial joints can be classified as one of four different types: gliding, angular, rotational, or special movement.

Gliding Movement

Gliding movements occur as relatively flat bone surfaces move past each other. Gliding movements produce very little rotation or angular movement of the bones. The joints of the carpal and tarsal bones are examples of joints that produce gliding movements.

Angular Movement

Angular movements are produced when the angle between the bones of a joint changes. There are several different types of angular movements, including flexion, extension, hyperextension, abduction, adduction, and circumduction. Flexion , or bending, occurs when the angle between the bones decreases. Moving the forearm upward at the elbow or moving the wrist to move the hand toward the forearm are examples of flexion. Extension is the opposite of flexion in that the angle between the bones of a joint increases. Straightening a limb after flexion is an example of extension. Extension past the regular anatomical position is referred to as hyperextension . This includes moving the neck back to look upward, or bending the wrist so that the hand moves away from the forearm.

Abduction occurs when a bone moves away from the midline of the body. Examples of abduction are moving the arms or legs laterally to lift them straight out to the side. Adduction is the movement of a bone toward the midline of the body. Movement of the limbs inward after abduction is an example of adduction. Circumduction is the movement of a limb in a circular motion, as in moving the arm in a circular motion.

Rotational Movement

Rotational movement is the movement of a bone as it rotates around its longitudinal axis. Rotation can be toward the midline of the body, which is referred to as medial rotation , or away from the midline of the body, which is referred to as lateral rotation . Movement of the head from side to side is an example of rotation.

Special Movements

Some movements that cannot be classified as gliding, angular, or rotational are called special movements. Inversion involves the soles of the feet moving inward, toward the midline of the body. Eversion is the opposite of inversion, movement of the sole of the foot outward, away from the midline of the body. Protraction is the anterior movement of a bone in the horizontal plane. Retraction occurs as a joint moves back into position after protraction. Protraction and retraction can be seen in the movement of the mandible as the jaw is thrust outwards and then back inwards. Elevation is the movement of a bone upward, such as when the shoulders are shrugged, lifting the scapulae. Depression is the opposite of elevation—movement downward of a bone, such as after the shoulders are shrugged and the scapulae return to their normal position from an elevated position. Dorsiflexion is a bending at the ankle such that the toes are lifted toward the knee. Plantar flexion is a bending at the ankle when the heel is lifted, such as when standing on the toes. Supination is the movement of the radius and ulna bones of the forearm so that the palm faces forward. Pronation is the opposite movement, in which the palm faces backward. Opposition is the movement of the thumb toward the fingers of the same hand, making it possible to grasp and hold objects.

Types of Synovial Joints

Synovial joints are further classified into six different categories on the basis of the shape and structure of the joint. The shape of the joint affects the type of movement permitted by the joint (Figure 38.26). These joints can be described as planar, hinge, pivot, condyloid, saddle, or ball-and-socket joints.

Planar Joints

Planar joints have bones with articulating surfaces that are flat or slightly curved faces. These joints allow for gliding movements, and so the joints are sometimes referred to as gliding joints. The range of motion is limited in these joints and does not involve rotation. Planar joints are found in the carpal bones in the hand and the tarsal bones of the foot, as well as between vertebrae (Figure 38.27).

Hinge Joints

In hinge joints , the slightly rounded end of one bone fits into the slightly hollow end of the other bone. In this way, one bone moves while the other remains stationary, like the hinge of a door. The elbow is an example of a hinge joint. The knee is sometimes classified as a modified hinge joint (Figure 38.28).

Pivot Joints

Pivot joints consist of the rounded end of one bone fitting into a ring formed by the other bone. This structure allows rotational movement, as the rounded bone moves around its own axis. An example of a pivot joint is the joint of the first and second vertebrae of the neck that allows the head to move back and forth (Figure 38.29). The joint of the wrist that allows the palm of the hand to be turned up and down is also a pivot joint.

Condyloid Joints

Condyloid joints consist of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone (Figure 38.30). This is also sometimes called an ellipsoidal joint. This type of joint allows angular movement along two axes, as seen in the joints of the wrist and fingers, which can move both side to side and up and down.

Saddle Joints

Saddle joints are so named because the ends of each bone resemble a saddle, with concave and convex portions that fit together. Saddle joints allow angular movements similar to condyloid joints but with a greater range of motion. An example of a saddle joint is the thumb joint, which can move back and forth and up and down, but more freely than the wrist or fingers (Figure 38.31).

Ball-and-Socket Joints

Ball-and-socket joints possess a rounded, ball-like end of one bone fitting into a cuplike socket of another bone. This organization allows the greatest range of motion, as all movement types are possible in all directions. Examples of ball-and-socket joints are the shoulder and hip joints (Figure 38.32).

Link to Learning

Watch this animation showing the six types of synovial joints.

Career Connection


Rheumatologists are medical doctors who specialize in the diagnosis and treatment of disorders of the joints, muscles, and bones. They diagnose and treat diseases such as arthritis, musculoskeletal disorders, osteoporosis, and autoimmune diseases such as ankylosing spondylitis and rheumatoid arthritis.

Rheumatoid arthritis (RA) is an inflammatory disorder that primarily affects the synovial joints of the hands, feet, and cervical spine. Affected joints become swollen, stiff, and painful. Although it is known that RA is an autoimmune disease in which the body’s immune system mistakenly attacks healthy tissue, the cause of RA remains unknown. Immune cells from the blood enter joints and the synovium causing cartilage breakdown, swelling, and inflammation of the joint lining. Breakdown of cartilage causes bones to rub against each other causing pain. RA is more common in women than men and the age of onset is usually 40–50 years of age.

Rheumatologists can diagnose RA on the basis of symptoms such as joint inflammation and pain, X-ray and MRI imaging, and blood tests. Arthrography is a type of medical imaging of joints that uses a contrast agent, such as a dye, that is opaque to X-rays. This allows the soft tissue structures of joints—such as cartilage, tendons, and ligaments—to be visualized. An arthrogram differs from a regular X-ray by showing the surface of soft tissues lining the joint in addition to joint bones. An arthrogram allows early degenerative changes in joint cartilage to be detected before bones become affected.

There is currently no cure for RA however, rheumatologists have a number of treatment options available. Early stages can be treated with rest of the affected joints by using a cane or by using joint splints that minimize inflammation. When inflammation has decreased, exercise can be used to strengthen the muscles that surround the joint and to maintain joint flexibility. If joint damage is more extensive, medications can be used to relieve pain and decrease inflammation. Anti-inflammatory drugs such as aspirin, topical pain relievers, and corticosteroid injections may be used. Surgery may be required in cases in which joint damage is severe.


The pelvis or pelvic girdle is made from three fused bones: the ilium, ischium and pubis bones. It is the connection point for the leg and sacrum of the spinal column. In humans the pelvis is much shorter and wider than other mammals because it holds organs such as our stomachs. In four legged mammals the pelvis is longer to give more area for muscles to attach to. The pelvis is also a useful bone in determining the sex of a human skeleton. The female pelvis in wider to accommodate childbirth and the shape of the pubis is much flatter.

38.1 Types of Skeletal Systems

A skeletal system is necessary to support the body, protect internal organs, and allow for the movement of an organism. There are three different skeleton designs that fulfill these functions: hydrostatic skeleton, exoskeleton, and endoskeleton.

Hydrostatic Skeleton

A hydrostatic skeleton is a skeleton formed by a fluid-filled compartment within the body, called the coelom. The organs of the coelom are supported by the aqueous fluid, which also resists external compression. This compartment is under hydrostatic pressure because of the fluid and supports the other organs of the organism. This type of skeletal system is found in soft-bodied animals such as sea anemones, earthworms, Cnidaria, and other invertebrates (Figure 38.2).

Movement in a hydrostatic skeleton is provided by muscles that surround the coelom. The muscles in a hydrostatic skeleton contract to change the shape of the coelom the pressure of the fluid in the coelom produces movement. For example, earthworms move by waves of muscular contractions of the skeletal muscle of the body wall hydrostatic skeleton, called peristalsis, which alternately shorten and lengthen the body. Lengthening the body extends the anterior end of the organism. Most organisms have a mechanism to fix themselves in the substrate. Shortening the muscles then draws the posterior portion of the body forward. Although a hydrostatic skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an efficient skeleton for terrestrial animals.


An exoskeleton is an external skeleton that consists of a hard encasement on the surface of an organism. For example, the shells of crabs and insects are exoskeletons (Figure 38.3). This skeleton type provides defence against predators, supports the body, and allows for movement through the contraction of attached muscles. As with vertebrates, muscles must cross a joint inside the exoskeleton. Shortening of the muscle changes the relationship of the two segments of the exoskeleton. Arthropods such as crabs and lobsters have exoskeletons that consist of 30–50 percent chitin, a polysaccharide derivative of glucose that is a strong but flexible material. Chitin is secreted by the epidermal cells. The exoskeleton is further strengthened by the addition of calcium carbonate in organisms such as the lobster. Because the exoskeleton is acellular, arthropods must periodically shed their exoskeletons because the exoskeleton does not grow as the organism grows.


An endoskeleton is a skeleton that consists of hard, mineralized structures located within the soft tissue of organisms. An example of a primitive endoskeletal structure is the spicules of sponges. The bones of vertebrates are composed of tissues, whereas sponges have no true tissues (Figure 38.4). Endoskeletons provide support for the body, protect internal organs, and allow for movement through contraction of muscles attached to the skeleton.

The human skeleton is an endoskeleton that consists of 206 bones in the adult. It has five main functions: providing support to the body, storing minerals and lipids, producing blood cells, protecting internal organs, and allowing for movement. The skeletal system in vertebrates is divided into the axial skeleton (which consists of the skull, vertebral column, and rib cage), and the appendicular skeleton (which consists of the shoulders, limb bones, the pectoral girdle, and the pelvic girdle).

Link to Learning

Visit the interactive body site to build a virtual skeleton: select "skeleton" and click through the activity to place each bone.

Human Axial Skeleton

The axial skeleton forms the central axis of the body and includes the bones of the skull, ossicles of the middle ear, hyoid bone of the throat, vertebral column, and the thoracic cage (ribcage) (Figure 38.5). The function of the axial skeleton is to provide support and protection for the brain, the spinal cord, and the organs in the ventral body cavity. It provides a surface for the attachment of muscles that move the head, neck, and trunk, performs respiratory movements, and stabilizes parts of the appendicular skeleton.

The Skull

The bones of the skull support the structures of the face and protect the brain. The skull consists of 22 bones, which are divided into two categories: cranial bones and facial bones. The cranial bones are eight bones that form the cranial cavity, which encloses the brain and serves as an attachment site for the muscles of the head and neck. The eight cranial bones are the frontal bone, two parietal bones, two temporal bones, occipital bone, sphenoid bone, and the ethmoid bone. Although the bones developed separately in the embryo and fetus, in the adult, they are tightly fused with connective tissue and adjoining bones do not move (Figure 38.6).

The auditory ossicles of the middle ear transmit sounds from the air as vibrations to the fluid-filled cochlea. The auditory ossicles consist of three bones each: the malleus, incus, and stapes. These are the smallest bones in the body and are unique to mammals.

Fourteen facial bones form the face, provide cavities for the sense organs (eyes, mouth, and nose), protect the entrances to the digestive and respiratory tracts, and serve as attachment points for facial muscles. The 14 facial bones are the nasal bones, the maxillary bones, zygomatic bones, palatine, vomer, lacrimal bones, the inferior nasal conchae, and the mandible. All of these bones occur in pairs except for the mandible and the vomer (Figure 38.7).

Although it is not found in the skull, the hyoid bone is considered a component of the axial skeleton. The hyoid bone lies below the mandible in the front of the neck. It acts as a movable base for the tongue and is connected to muscles of the jaw, larynx, and tongue. The mandible articulates with the base of the skull. The mandible controls the opening to the airway and gut. In animals with teeth, the mandible brings the surfaces of the teeth in contact with the maxillary teeth.

The Vertebral Column

The vertebral column , or spinal column, surrounds and protects the spinal cord, supports the head, and acts as an attachment point for the ribs and muscles of the back and neck. The adult vertebral column comprises 26 bones: the 24 vertebrae, the sacrum, and the coccyx bones. In the adult, the sacrum is typically composed of five vertebrae that fuse into one. The coccyx is typically 3–4 vertebrae that fuse into one. Around the age of 70, the sacrum and the coccyx may fuse together. We begin life with approximately 33 vertebrae, but as we grow, several vertebrae fuse together. The adult vertebrae are further divided into the 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae (Figure 38.8).

Each vertebral body has a large hole in the center through which the nerves of the spinal cord pass. There is also a notch on each side through which the spinal nerves, which serve the body at that level, can exit from the spinal cord. The vertebral column is approximately 71 cm (28 inches) in adult male humans and is curved, which can be seen from a side view. The names of the spinal curves correspond to the region of the spine in which they occur. The thoracic and sacral curves are concave (curve inwards relative to the front of the body) and the cervical and lumbar curves are convex (curve outwards relative to the front of the body). The arched curvature of the vertebral column increases its strength and flexibility, allowing it to absorb shocks like a spring (Figure 38.8).

Intervertebral discs composed of fibrous cartilage lie between adjacent vertebral bodies from the second cervical vertebra to the sacrum. Each disc is part of a joint that allows for some movement of the spine and acts as a cushion to absorb shocks from movements such as walking and running. Intervertebral discs also act as ligaments to bind vertebrae together. The inner part of discs, the nucleus pulposus, hardens as people age and becomes less elastic. This loss of elasticity diminishes its ability to absorb shocks.

The Thoracic Cage

The thoracic cage , also known as the ribcage, is the skeleton of the chest, and consists of the ribs, sternum, thoracic vertebrae, and costal cartilages (Figure 38.9). The thoracic cage encloses and protects the organs of the thoracic cavity, including the heart and lungs. It also provides support for the shoulder girdles and upper limbs, and serves as the attachment point for the diaphragm, muscles of the back, chest, neck, and shoulders. Changes in the volume of the thorax enable breathing.

The sternum , or breastbone, is a long, flat bone located at the anterior of the chest. It is formed from three bones that fuse in the adult. The ribs are 12 pairs of long, curved bones that attach to the thoracic vertebrae and curve toward the front of the body, forming the ribcage. Costal cartilages connect the anterior ends of the ribs to the sternum, with the exception of rib pairs 11 and 12, which are free-floating ribs.

Human Appendicular Skeleton

The appendicular skeleton is composed of the bones of the upper limbs (which function to grasp and manipulate objects) and the lower limbs (which permit locomotion). It also includes the pectoral girdle, or shoulder girdle, that attaches the upper limbs to the body, and the pelvic girdle that attaches the lower limbs to the body (Figure 38.10).

The Pectoral Girdle

The pectoral girdle bones provide the points of attachment of the upper limbs to the axial skeleton. The human pectoral girdle consists of the clavicle (or collarbone) in the anterior, and the scapula (or shoulder blades) in the posterior (Figure 38.11).

The clavicles are S-shaped bones that position the arms on the body. The clavicles lie horizontally across the front of the thorax (chest) just above the first rib. These bones are fairly fragile and are susceptible to fractures. For example, a fall with the arms outstretched causes the force to be transmitted to the clavicles, which can break if the force is excessive. The clavicle articulates with the sternum and the scapula.

The scapulae are flat, triangular bones that are located at the back of the pectoral girdle. They support the muscles crossing the shoulder joint. A ridge, called the spine, runs across the back of the scapula and can easily be felt through the skin (Figure 38.11). The spine of the scapula is a good example of a bony protrusion that facilitates a broad area of attachment for muscles to bone.

The Upper Limb

The upper limb contains 30 bones in three regions: the arm (shoulder to elbow), the forearm (ulna and radius), and the wrist and hand (Figure 38.12).

An articulation is any place at which two bones are joined. The humerus is the largest and longest bone of the upper limb and the only bone of the arm. It articulates with the scapula at the shoulder and with the forearm at the elbow. The forearm extends from the elbow to the wrist and consists of two bones: the ulna and the radius. The radius is located along the lateral (thumb) side of the forearm and articulates with the humerus at the elbow. The ulna is located on the medial aspect (pinky-finger side) of the forearm. It is longer than the radius. The ulna articulates with the humerus at the elbow. The radius and ulna also articulate with the carpal bones and with each other, which in vertebrates enables a variable degree of rotation of the carpus with respect to the long axis of the limb. The hand includes the eight bones of the carpus (wrist), the five bones of the metacarpus (palm), and the 14 bones of the phalanges (digits). Each digit consists of three phalanges, except for the thumb, when present, which has only two.

The Pelvic Girdle

The pelvic girdle attaches to the lower limbs of the axial skeleton. Because it is responsible for bearing the weight of the body and for locomotion, the pelvic girdle is securely attached to the axial skeleton by strong ligaments. It also has deep sockets with robust ligaments to securely attach the femur to the body. The pelvic girdle is further strengthened by two large hip bones. In adults, the hip bones, or coxal bones , are formed by the fusion of three pairs of bones: the ilium, ischium, and pubis. The pelvis joins together in the anterior of the body at a joint called the pubic symphysis and with the bones of the sacrum at the posterior of the body.

The female pelvis is slightly different from the male pelvis. Over generations of evolution, females with a wider pubic angle and larger diameter pelvic canal reproduced more successfully. Therefore, their offspring also had pelvic anatomy that enabled successful childbirth (Figure 38.13).

The Lower Limb

The lower limb consists of the thigh, the leg, and the foot. The bones of the lower limb are the femur (thigh bone), patella (kneecap), tibia and fibula (bones of the leg), tarsals (bones of the ankle), and metatarsals and phalanges (bones of the foot) (Figure 38.14). The bones of the lower limbs are thicker and stronger than the bones of the upper limbs because of the need to support the entire weight of the body and the resulting forces from locomotion. In addition to evolutionary fitness, the bones of an individual will respond to forces exerted upon them.

The femur , or thighbone, is the longest, heaviest, and strongest bone in the body. The femur and pelvis form the hip joint at the proximal end. At the distal end, the femur, tibia, and patella form the knee joint. The patella , or kneecap, is a triangular bone that lies anterior to the knee joint. The patella is embedded in the tendon of the femoral extensors (quadriceps). It improves knee extension by reducing friction. The tibia , or shinbone, is a large bone of the leg that is located directly below the knee. The tibia articulates with the femur at its proximal end, with the fibula and the tarsal bones at its distal end. It is the second largest bone in the human body and is responsible for transmitting the weight of the body from the femur to the foot. The fibula , or calf bone, parallels and articulates with the tibia. It does not articulate with the femur and does not bear weight. The fibula acts as a site for muscle attachment and forms the lateral part of the ankle joint.

The tarsals are the seven bones of the ankle. The ankle transmits the weight of the body from the tibia and the fibula to the foot. The metatarsals are the five bones of the foot. The phalanges are the 14 bones of the toes. Each toe consists of three phalanges, except for the big toe that has only two (Figure 38.15). Variations exist in other species for example, the horse’s metacarpals and metatarsals are oriented vertically and do not make contact with the substrate.

Evolution Connection

Evolution of Body Design for Locomotion on Land

The transition of vertebrates onto land required a number of changes in body design, as movement on land presents a number of challenges for animals that are adapted to movement in water. The buoyancy of water provides a certain amount of lift, and a common form of movement by fish is lateral undulations of the entire body. This back and forth movement pushes the body against the water, creating forward movement. In most fish, the muscles of paired fins attach to girdles within the body, allowing for some control of locomotion. As certain fish began moving onto land, they retained their lateral undulation form of locomotion (anguilliform). However, instead of pushing against water, their fins or flippers became points of contact with the ground, around which they rotated their bodies.

The effect of gravity and the lack of buoyancy on land meant that body weight was suspended on the limbs, leading to increased strengthening and ossification of the limbs. The effect of gravity also required changes to the axial skeleton. Lateral undulations of land animal vertebral columns cause torsional strain. A firmer, more ossified vertebral column became common in terrestrial tetrapods because it reduces strain while providing the strength needed to support the body’s weight. In later tetrapods, the vertebrae began allowing for vertical motion rather than lateral flexion. Another change in the axial skeleton was the loss of a direct attachment between the pectoral girdle and the head. This reduced the jarring to the head caused by the impact of the limbs on the ground. The vertebrae of the neck also evolved to allow movement of the head independently of the body.

The appendicular skeleton of land animals is also different from aquatic animals. The shoulders attach to the pectoral girdle through muscles and connective tissue, thus reducing the jarring of the skull. Because of a lateral undulating vertebral column, in early tetrapods, the limbs were splayed out to the side and movement occurred by performing “push-ups.” The vertebrae of these animals had to move side-to-side in a similar manner to fish and reptiles. This type of motion requires large muscles to move the limbs toward the midline it was almost like walking while doing push-ups, and it is not an efficient use of energy. Later tetrapods have their limbs placed under their bodies, so that each stride requires less force to move forward. This resulted in decreased adductor muscle size and an increased range of motion of the scapulae. This also restricts movement primarily to one plane, creating forward motion rather than moving the limbs upward as well as forward. The femur and humerus were also rotated, so that the ends of the limbs and digits were pointed forward, in the direction of motion, rather than out to the side. By placement underneath the body, limbs can swing forward like a pendulum to produce a stride that is more efficient for moving over land.

How Big Can a Land Animal Get?

Imagine taking a helicopter to an uncharted island, only to be ambushed by a massive ape-like creature standing more than 100 feet tall and weighing 158 tons. With shocking strength, this simian foe sends a tree trunk right through your chopper, before going on to crush, stomp and bellow his way through your friends for the next two hours. This is the plot of the movie Kong: Skull Island, a new take on the 80-year-old franchise based around the infamous King Kong.

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Now, Skull Island never claims to hew to scientific accuracy. But we had to ask: Could a creature as large as this living skyscraper ever exist on our planet? Would it have the strength to crush helicopters in its hands, or would it merely collapse under its own weight?

To start, let's be clear that there's no way King Kong or any other gigantic ape is living somewhere undetected on Earth. "King Kong as shown in the movie probably isn't a physically viable organism," says Jonathan Payne, a paleobiologist at Stanford University who has done extensive research on how body size has evolved over the history of life. The main reasons: Gravity and biomechanics.

If you took an animal and blew it up in size, mathematics dictates that the creature's mass would increase cubically, or by a power of three. However, by the same ratio of size increase, the width of the creature's body, and thus its bones and muscles, would increase only by a power of two, says Payne. "As you get bigger you need to dedicate more and more of your body mass to your bones to support yourself," he says. 

That’s why you don't see creatures like daddy longlegs—those spider-like arachnids that appear in your bathroom and are usually no bigger than an inch long—clocking in at much larger sizes. "Their legs would shatter under their bodyweight," Payne says. (Shudder.)

Because of these laws, taking your typical 350-pound Western gorilla and simply scaling it up by a factor of 20 would be physically impossible the resulting creature's skeleton and muscles wouldn't be able to support its mass. Larger animals need bigger and thicker limbs to hold themselves up, says University of New Mexico paleoecologist Felisa Smith, which makes it unlikely that any creature on land has ever exceeded 100 tons.

"Poor King Kong couldn't even roll over," says Smith—much less attack people and helicopters.

So it's no surprise that Earth's biggest terrestrial animals—elephants—today fall far short of King Kong size. African elephants, for instance, can reach about 13 feet tall and weigh up to 7.5 tons. In the past, however, life got far larger: Dinosaurs like the Titanosaur weighed in at nearly 80 tons󈟚 times larger than the African elephants of today, but still nowhere near as big as the fictional King Kong.

The reason has to do with the fact that dinosaurs were reptiles, and today we live in an age dominated by mammals. To maintain their higher body temperatures, warm-blooded mammals spend about 10 times more energy than cold-blooded reptiles do on their metabolisms. This is energy that a mammal can't devote to increasing its body size. So it makes sense that the largest mammals we know of are roughly one-tenth as large as the largest reptiles ever found, Smith says.

What about the blue whale, which is believed to be the largest animal to ever exist on Earth, weighing in at more than 200 tons? In water, the rules are different. Water's buoyancy helps support the bodies of sea creatures, taking some of the strain off their muscles and skeletons. Smith says blue whales could theoretically get even bigger than they are presently, but biologists believe that the relatively short gestation period of blue whales for their body size—just 11 months—limits their size.

(Similarly, it’s possible that on a planet with lower gravity than Earth’s, such as Mars, terrestrial creatures less encumbered by their loads could grow much larger.) 

But there's another major factor that limits an animal's size: food. A 158-ton ape is going to need a lot of food to support itself, and it is not likely to find that amount of food on Skull Island, unless helicopters full of tasty humans crash there regularly.

Usually, getting one’s hands on more food means having access to proportionally more territory, Smith says. Blue whales swim across ranges of thousands of miles to find krill to eat, and African elephants can cover up to 80 miles in a day looking for vegetation. Large animals tend to get smaller on islands to compensate for the fact that there are usually fewer potential food sources, Payne says, such as the extinct dwarf elephant species that once lived on islands in the Mediterranean Sea. So if anything, King Kong would more likely be a dwarf gorilla than a massive one.

What evolutionary pressures would make it more appealing to be a larger animal, given the obvious drawbacks? "There has to be a selective advantage for being bigger," Smith says. For example: not getting eaten. Since smaller animals are more easily picked off by predators, natural selection can drive a species to get bigger to help defend itself better. This can be a tradeoff, however, since larger animals tend to move a lot slower than smaller ones (see the above lesson on biomechanics).

Being a lot bigger also means you can get a lot more food, Payne says. The classic example is the giraffe, whose massive height allows it to reach vegetation that no other animal can. Similarly, blue whales can filter large amounts of water with their baleen teeth, which allows them to capture up toو,000 pounds of finger-sized krill per day.

Let's face it: Scientifically speaking, King Kong may be as much a leap of imagination as Hollywood itself. But Payne isn't willing to fully rule out the possibility of life ever getting that large. "I don't like to ever say never on these things," he says. "Every time you think that life can't do something, it often figures out ways to do it … Life surprises us in all kinds of ways."

Editor's Note, March 22, 2017: This article initially misstated that increasing a creature's mass cubically would increase it by a factor of three. It has been corrected.

Strange animal movements

Some animals don't move in the traditional sense. Others move in unique ways which might never have expected.

The Portuguese man o' war is a jellyfish like creature which doesn't actually swim. It has a bladder which inflates and deflates depending on whether it wants to be closer or further away from the sea's surface. Some parasites and even types of fish can move, but choose not to. Instead, they latch on to other animals and let them do the moving for them.

Some creatures, like the golden wheel spider can make themselves cartwheel down sand banks. Armadillos can move in a special way, similar to a hedgehog, where they turn into a ball for protection. They do not do this so they can move, although if they where on a hill, it is likely they would roll down it.

Some animals move in a very special way. They are able to stick to certain surfaces so they can travel vertically as well as horizontally. These include snails which use a mucus like slime to climb up surfaces. Others, like geckos, use hundreds of microscopic hairs called setae to climb up vertical surfaces.

If you want to read similar articles to How do Animals Move, we recommend you visit our The Animal Kingdom category.