Calf muscle Eye muscle (lateral rectus FIGURE 50.18 Skeletal muscles have different proportions of fast-twitch and slow twitch fibers. The muscles that move the 5c0 eve contain tly fast. whereas the deep muscle of the leg(the soleus)contains mostly slow-twitch fibers Time(msec) The calf muscle(gastrocnemius)is intermediate in its composition Muscle Metabolism during Rest and Exercise tigue are not entirely understood. In most cases, however, Skeletal muscles at rest obtain most of their energy from muscle fatigue is correlated with the production of lactic the aerobic respiration of fatty acids. During exercise, mus- acid by the exercising muscles. Lactic acid is produced by he anaerobic respiration of glucose, and glucose is ob- cle glycogen and blood glucose are also used as energy tained from muscle glycogen and from the blood.Lactate make ATP, which is needed for(1) the movement of the production and muscle fatigue are therefore also related sources. The energy obtained by cell respiration is used to cross-bridges during muscle contraction and (2) the pump- cogen ing of Cat** into the sarcoplasmic reticulum for muscle re- Because the depletion of muscle glycogen plac laxation. ATP can be obtained by skeletal muscles quickly on exercise, any adaptation that spares muscle glycogen will by combining ADP with phosphate derived from creati improve physical endurance. Trained athletes have an in- phosphate. This compound was produced previously in the creased proportion of energy derived from the aerobic res- resting muscle by combining creatine with phosphate de- their muscle glycogen reserve. The greater the level of on o atty acids, resulting in a slower depletion of rived from the aTP generated in cell respiration physical training, the higher the proportion of energy de- Skeletal muscles respire anaerobically for the first 45 to rived from the aerobic respiration of fatty acids. Because 90 seconds of moderate-to-heavy exercise, because the car- the aerobic capacity of endurance-trained athletes is higher diopulmonary system requires this amount of time to suffi- than that of untrained people, athletes can perform more ciently increase the oxygen supply to the exercising mus- cles. If exercise is moderate, aerobic respiration contributes exercise before lactic acid production and glycogen deple the major portion of the skeletal muscle energy require- ments following the first 2 minutes of exercise Endurance training does not increase muscle size. Whether exercise is light, moderate, or intense for a Muscle enlargement is produced only by frequent periods given person depends upon that person's maximal of high-intensity exercise in which muscles work against for aerobic exercise. The maximum rate of oxygen con high resistance, as in weight lifting. As a result of resis- sumption in the body(by aerobic respiration) is called the ance training, type II(fast-twitch)muscle fibers become thicker as a result of the increased siz ze and number of maximal oxygen uptake, or the aerobic capacity. The inten- their myofibrils. Weight training, therefore, causes skele- sity of exercise can also be defined by the lactate threshold. tal muscles to grow by hypertrophy(increased cell size) This is the percentage of the maximal oxygen uptake at rather than by cell division and an increased number of which a saprobic respiration. For average, healthy people,cells result of an for example ficant amount of blood lactate appears when exercise is performed at about 50 to 70% of the maxi mal oxygen uptake Muscles contract through summation of the contractions of their fibers, producing tension that may result in shortening of the muscle. Slow-twitch skeletal Muscle Fatigue and Physical Training muscle fibers are adapted for aerobic respiration and Muscle fatigue refers to the use-dependant decrease in the are slower to fatigue than fast-twitch fibers, which are ability of a muscle to generate force. The reasons for fa more adapted for the rapid generation of power. ter 0 Locomotion 1011
Muscle Metabolism during Rest and Exercise Skeletal muscles at rest obtain most of their energy from the aerobic respiration of fatty acids. During exercise, muscle glycogen and blood glucose are also used as energy sources. The energy obtained by cell respiration is used to make ATP, which is needed for (1) the movement of the cross-bridges during muscle contraction and (2) the pumping of Ca++ into the sarcoplasmic reticulum for muscle relaxation. ATP can be obtained by skeletal muscles quickly by combining ADP with phosphate derived from creatine phosphate. This compound was produced previously in the resting muscle by combining creatine with phosphate derived from the ATP generated in cell respiration. Skeletal muscles respire anaerobically for the first 45 to 90 seconds of moderate-to-heavy exercise, because the cardiopulmonary system requires this amount of time to sufficiently increase the oxygen supply to the exercising muscles. If exercise is moderate, aerobic respiration contributes the major portion of the skeletal muscle energy requirements following the first 2 minutes of exercise. Whether exercise is light, moderate, or intense for a given person depends upon that person’s maximal capacity for aerobic exercise. The maximum rate of oxygen consumption in the body (by aerobic respiration) is called the maximal oxygen uptake, or the aerobic capacity. The intensity of exercise can also be defined by the lactate threshold. This is the percentage of the maximal oxygen uptake at which a significant rise in blood lactate levels occurs as a result of anaerobic respiration. For average, healthy people, for example, a significant amount of blood lactate appears when exercise is performed at about 50 to 70% of the maximal oxygen uptake. Muscle Fatigue and Physical Training Muscle fatigue refers to the use-dependant decrease in the ability of a muscle to generate force. The reasons for fatigue are not entirely understood. In most cases, however, muscle fatigue is correlated with the production of lactic acid by the exercising muscles. Lactic acid is produced by the anaerobic respiration of glucose, and glucose is obtained from muscle glycogen and from the blood. Lactate production and muscle fatigue are therefore also related to the depletion of muscle glycogen. Because the depletion of muscle glycogen places a limit on exercise, any adaptation that spares muscle glycogen will improve physical endurance. Trained athletes have an increased proportion of energy derived from the aerobic respiration of fatty acids, resulting in a slower depletion of their muscle glycogen reserve. The greater the level of physical training, the higher the proportion of energy derived from the aerobic respiration of fatty acids. Because the aerobic capacity of endurance-trained athletes is higher than that of untrained people, athletes can perform more exercise before lactic acid production and glycogen depletion cause muscle fatigue. Endurance training does not increase muscle size. Muscle enlargement is produced only by frequent periods of high-intensity exercise in which muscles work against high resistance, as in weight lifting. As a result of resistance training, type II (fast-twitch) muscle fibers become thicker as a result of the increased size and number of their myofibrils. Weight training, therefore, causes skeletal muscles to grow by hypertrophy (increased cell size) rather than by cell division and an increased number of cells. Muscles contract through summation of the contractions of their fibers, producing tension that may result in shortening of the muscle. Slow-twitch skeletal muscle fibers are adapted for aerobic respiration and are slower to fatigue than fast-twitch fibers, which are more adapted for the rapid generation of power. Chapter 50 Locomotion 1011 Time (msec) Contraction strength Eye muscle (lateral rectus) Calf muscle (gastrocnemius) Deep muscle of leg (soleus) FIGURE 50.18 Skeletal muscles have different proportions of fast-twitch and slowtwitch fibers. The muscles that move the eye contain mostly fast-twitch fibers, whereas the deep muscle of the leg (the soleus) contains mostly slow-twitch fibers. The calf muscle (gastrocnemius) is intermediate in its composition
Comparing Cardiac and from one side of the cell to the other Smooth muscles Intercalated The thick filaments are attached either disks to structures called dense bodies. the Cardiac and smooth muscle are similar functional equivalents of Z lines, or to in that both are found within internal the plasma membrane. Most smooth organs and both are generally not muscle cells have 10 to 15 thin fila under conscious control. Cardiac mus ments hick filament, are cle. however, is like skeletal muscle in 3 per thick filament in striated muscle that it is striated and contracts by fibers means of a sliding filament mecha- nism. Smooth muscle(as its name im- sarcoplasmic reticulum; during a con plies) is not striated. Smooth muscle traction Ca++ enters from the extracel does contain actin and myosin fila- FIGURE 50.19 lular fluid. In the cytoplasm, Ca++ ments,but they are arranged less reg- Cardiac muscle. Cells are organized into binds to calmodulin, a protein that is ularly within the cell long branching chains that interconnect, structurally similar to troponin. The forming a lattice; neighboring cells are Ca++-calmodulin compl Cardiac muscle linked by structures called intercalated enzyme that phosphorylates(adds a osIn Cardiac muscle in the vertebrate Unlike the case with striated muscles heart is composed of striated muscle this phosphorylation is required for the cells that are arranged differently from the fibers in a myosin heads to form cross-bridges with actin. skeletal muscle. Instead of the long, multinucleate cells This mechanism allows gradations in the strength of that form skeletal muscle, cardiac muscle is composed of contraction in a smooth muscle cell, increasing contraction shorter, branched cells, each with its own nucleus, that strength as more Ca++ enters the cytoplasm. Heart patients interconnect with one another at intercalated discs(fig- sometimes take drugs that block Ca** entry into smooth ure 50.19). Intercalated discs are regions where the mem- muscle cells, reducing the cells ability to contract. This branes of two cells fuse together, and the fused mem- treatment causes vascular smooth muscle to relax, dilating junctions permit the diffusion of ions, and thus the heart must do to pump blood through them t of work the spread of electric excitation, from one cell to the next. In some smooth muscle tissues, the cells contract only The mass of interconnected cardiac muscle cells forms a when they are stimulated by the nervous system. These single, functioning unit called a myocardium. Electric im- muscles line the walls of many blood vessels and make up es Deg ontaneou Isly in a specific region of the my- the iris of the eye. Other smooth muscle tissues, like those cardium known as the pacemaker. These impulses are not in the wall of the gut, contains cells that produce electric initiated by impulses in motor neurons, as they are in impulses spontaneously. These impulses spread to adjoin- skeletal muscle, but rather are produced by the cardiac ing cells through gap junctions, leading to a slow, steady muscle cells themselves. From the pacemaker, the im contraction of the tissue pulses spread throughout the myocardium via gap junc Neither skeletal nor cardiac muscle can be greatly tons, causing contraction. stretched because if the thick and thin filaments no longer The heart has two myocardia, one that receives blood overlay in the sarcomere, cross-bridges cannot form.Un- from the body and one that ejects blood into the body. Be like these striated muscles. smooth muscle can contract cause all of the cells in a myocardium are stimulated as a even when it is greatly stretched. If one considers the de unit,cardiac muscle cannot produce summated contrac- gree to which some internal organs may be stretched-a tions or tetanus. This would interfere with the alternation uterus during pregnancy, for example-it is no wonder that between contraction and relaxation that is necessary for these organs contain smooth muscle instead of striated Pumping. Smooth muscle Cardiac muscle cells interconnect physically and Smooth muscle surrounds hollow internal organs, includ- electrically to form a single, functioning unit called a ing the stomach, intestines, bladder, and uterus, as well as myocardium, which produces its own impulses at a all blood vessels except capillaries. Smooth muscle cells are pacemaker region. Smooth muscles lack the long and spindle-sh and each contains a single organization of myofilaments into sarcomeres and lac cleus. They also contain actin and myosin, but these con- sarcoplasmic reticulum but contraction still occurs as ractile proteins are not organized into sarcomeres. Parallel myofilaments slide past one another by use of cross- arrangements of thick and thin filaments cross diagonally 1012 Part XIlI Animal Form and Function
Comparing Cardiac and Smooth Muscles Cardiac and smooth muscle are similar in that both are found within internal organs and both are generally not under conscious control. Cardiac muscle, however, is like skeletal muscle in that it is striated and contracts by means of a sliding filament mechanism. Smooth muscle (as its name implies) is not striated. Smooth muscle does contain actin and myosin filaments, but they are arranged less regularly within the cell. Cardiac Muscle Cardiac muscle in the vertebrate heart is composed of striated muscle cells that are arranged differently from the fibers in a skeletal muscle. Instead of the long, multinucleate cells that form skeletal muscle, cardiac muscle is composed of shorter, branched cells, each with its own nucleus, that interconnect with one another at intercalated discs (figure 50.19). Intercalated discs are regions where the membranes of two cells fuse together, and the fused membranes are pierced by gap junctions (chapter 7). The gap junctions permit the diffusion of ions, and thus the spread of electric excitation, from one cell to the next. The mass of interconnected cardiac muscle cells forms a single, functioning unit called a myocardium. Electric impulses begin spontaneously in a specific region of the myocardium known as the pacemaker. These impulses are not initiated by impulses in motor neurons, as they are in skeletal muscle, but rather are produced by the cardiac muscle cells themselves. From the pacemaker, the impulses spread throughout the myocardium via gap junctions, causing contraction. The heart has two myocardia, one that receives blood from the body and one that ejects blood into the body. Because all of the cells in a myocardium are stimulated as a unit, cardiac muscle cannot produce summated contractions or tetanus. This would interfere with the alternation between contraction and relaxation that is necessary for pumping. Smooth Muscle Smooth muscle surrounds hollow internal organs, including the stomach, intestines, bladder, and uterus, as well as all blood vessels except capillaries. Smooth muscle cells are long and spindle-shaped, and each contains a single nucleus. They also contain actin and myosin, but these contractile proteins are not organized into sarcomeres. Parallel arrangements of thick and thin filaments cross diagonally from one side of the cell to the other. The thick filaments are attached either to structures called dense bodies, the functional equivalents of Z lines, or to the plasma membrane. Most smooth muscle cells have 10 to 15 thin filaments per thick filament, compared to 3 per thick filament in striated muscle fibers. Smooth muscle cells do not have a sarcoplasmic reticulum; during a contraction, Ca++ enters from the extracellular fluid. In the cytoplasm, Ca++ binds to calmodulin, a protein that is structurally similar to troponin. The Ca++-calmodulin complex activates an enzyme that phosphorylates (adds a phosphate group to) the myosin heads. Unlike the case with striated muscles, this phosphorylation is required for the myosin heads to form cross-bridges with actin. This mechanism allows gradations in the strength of contraction in a smooth muscle cell, increasing contraction strength as more Ca++ enters the cytoplasm. Heart patients sometimes take drugs that block Ca++ entry into smooth muscle cells, reducing the cells’ ability to contract. This treatment causes vascular smooth muscle to relax, dilating the blood vessels and reducing the amount of work the heart must do to pump blood through them. In some smooth muscle tissues, the cells contract only when they are stimulated by the nervous system. These muscles line the walls of many blood vessels and make up the iris of the eye. Other smooth muscle tissues, like those in the wall of the gut, contains cells that produce electric impulses spontaneously. These impulses spread to adjoining cells through gap junctions, leading to a slow, steady contraction of the tissue. Neither skeletal nor cardiac muscle can be greatly stretched because if the thick and thin filaments no longer overlay in the sarcomere, cross-bridges cannot form. Unlike these striated muscles, smooth muscle can contract even when it is greatly stretched. If one considers the degree to which some internal organs may be stretched—a uterus during pregnancy, for example—it is no wonder that these organs contain smooth muscle instead of striated muscle. Cardiac muscle cells interconnect physically and electrically to form a single, functioning unit called a myocardium, which produces its own impulses at a pacemaker region. Smooth muscles lack the organization of myofilaments into sarcomeres and lack sarcoplasmic reticulum but contraction still occurs as myofilaments slide past one another by use of crossbridges. 1012 Part XIII Animal Form and Function Intercalated disks FIGURE 50.19 Cardiac muscle. Cells are organized into long branching chains that interconnect, forming a lattice; neighboring cells are linked by structures called intercalated discs
Modes of animal locomotion Animals are unique among multicellular organisms in their ability to actively move fror om one place to another. Loco- motion requires both a propulsive mechanism and a control mechanism. Animals employ a wide variety of propulsive ate the necessary force. The quantity, quality, and position Thrust of contractions are initiated and coordinated by the ner Reactive vous system. In large animals, active locomotion is almost always produced by appendages that oscillate--appendicule locomotion-or by bodies that undulate, pulse, or undergo Trout peristaltic waves--axial locomotion Lateral While animal locomotion occurs in many different forms, the general principles remain much the same in all groups. The physical restraints to movement---gravity and frictional drag-are the same in every environment, differ- Push ing only in degree. You can conveniently divide the envi- ronments through which animals move into three types, each involving its own forms of locomotion: water, land Locomotion in water Many aquatic and marine invertebrates move along the Reactive bottom using the same form of locomotion employed by orce terrestrial animals moving over the land surface Flatworms employ ciliary activity to brush themselves along, round- force worms a peristaltic slither, leeches a contract-anchor extend creeping. Crabs walk using limbs to pull themselves along; mollusks use a muscular foot, while starfish use unique tube feet to do the same thing Moving directly through the water, or s FIGURE 50.20 sents quite a different challenge. Water's buoyancy reduces Movements of swimming fishes. (a) An eel pushes against the the influence of gravity. The primary force retarding for water with its whole body, (b) a trout only with its posterior half ward movement is frictional drag, so body shape is impor Whales also swim using undulating body waves, but un- tant in reducing the friction and turbulence produced by like any of the fishes, the waves pass from top to bottom swimming through the water Some marine invertebrates swim using hydraulic pro and not from side to side. The body musculature of eels sion. Scallops clap their shells together forcefully, while and fish is highly segmental; that is, a muscle segment al squids and octopuses squirt water like a marine jet. All ternate with each vertebra. This arrangement permits the aquatic and marine vertebrates, however, swim. smooth passage of undulatory waves along the body Swimming involves using the body or its appendages Whales are unable to produce lateral undulations because to push against the water. An eel swims by sinuous undu- mammals do not have this arrangement. lations of its whole body(figure 5020a). The undulating Many tetrapod vertebrates swim, usually with appendic body waves of eel-like swimming are created by waves of ular locomotion. Most birds that swim, like ducks and muscle contraction alternating between the left and right geese, propel themselves through the water by pushing axial musculature. As each body segment in turn pushes against it with their hind legs, which typically have against the water, the moving wave forces the eel feet. Frogs, turtles, and most marine mammals als forward th their hind legs and have webbed feet. Te Fish, reptiles, and aquatic amphibians swim in a way brates that swim with their forelegs usually have these similar to eels, but only undulate the posterior(back) por- limbs modified as flippers, and pull themselves through the tion of the body(figure 50.206)and sometimes only the water. These include sea turtles, penguins, and fur seals. A caudal (rear)fin. This allows considerable specialization of few principally terrestrial tetrapod vertebrates, like polar the front end of the body, while sacrificing little propulsive bears and platypuses, swim with walking forelimbs not Chapter 50 Locomotion 1013
Modes of Animal Locomotion Animals are unique among multicellular organisms in their ability to actively move from one place to another. Locomotion requires both a propulsive mechanism and a control mechanism. Animals employ a wide variety of propulsive mechanisms, most involving contracting muscles to generate the necessary force. The quantity, quality, and position of contractions are initiated and coordinated by the nervous system. In large animals, active locomotion is almost always produced by appendages that oscillate—appendicular locomotion—or by bodies that undulate, pulse, or undergo peristaltic waves—axial locomotion. While animal locomotion occurs in many different forms, the general principles remain much the same in all groups. The physical restraints to movement—gravity and frictional drag—are the same in every environment, differing only in degree. You can conveniently divide the environments through which animals move into three types, each involving its own forms of locomotion: water, land, and air. Locomotion in Water Many aquatic and marine invertebrates move along the bottom using the same form of locomotion employed by terrestrial animals moving over the land surface. Flatworms employ ciliary activity to brush themselves along, roundworms a peristaltic slither, leeches a contract-anchorextend creeping. Crabs walk using limbs to pull themselves along; mollusks use a muscular foot, while starfish use unique tube feet to do the same thing. Moving directly through the water, or swimming, presents quite a different challenge. Water’s buoyancy reduces the influence of gravity. The primary force retarding forward movement is frictional drag, so body shape is important in reducing the friction and turbulence produced by swimming through the water. Some marine invertebrates swim using hydraulic propulsion. Scallops clap their shells together forcefully, while squids and octopuses squirt water like a marine jet. All aquatic and marine vertebrates, however, swim. Swimming involves using the body or its appendages to push against the water. An eel swims by sinuous undulations of its whole body (figure 50.20a). The undulating body waves of eel-like swimming are created by waves of muscle contraction alternating between the left and right axial musculature. As each body segment in turn pushes against the water, the moving wave forces the eel forward. Fish, reptiles, and aquatic amphibians swim in a way similar to eels, but only undulate the posterior (back) portion of the body (figure 50.20b) and sometimes only the caudal (rear) fin. This allows considerable specialization of the front end of the body, while sacrificing little propulsive force. Whales also swim using undulating body waves, but unlike any of the fishes, the waves pass from top to bottom and not from side to side. The body musculature of eels and fish is highly segmental; that is, a muscle segment alternates with each vertebra. This arrangement permits the smooth passage of undulatory waves along the body. Whales are unable to produce lateral undulations because mammals do not have this arrangement. Many tetrapod vertebrates swim, usually with appendicular locomotion. Most birds that swim, like ducks and geese, propel themselves through the water by pushing against it with their hind legs, which typically have webbed feet. Frogs, turtles, and most marine mammals also swim with their hind legs and have webbed feet. Tetrapod vertebrates that swim with their forelegs usually have these limbs modified as flippers, and pull themselves through the water. These include sea turtles, penguins, and fur seals. A few principally terrestrial tetrapod vertebrates, like polar bears and platypuses, swim with walking forelimbs not modified for swimming. Chapter 50 Locomotion 1013 Eel Trout Thrust Reactive force Lateral force Push 90˚ Trout Reactive force Push 90˚ Lateral force Thrust FIGURE 50.20 Movements of swimming fishes. (a) An eel pushes against the water with its whole body, (b) a trout only with its posterior half. (a) (b)
FIGURE 50.21 Animals that hop or leap use their rear legs to propel themselves through the air. The powerful leg muscles of this frog allow it to explode from a crouched position to a takeoff in about 100 milliseconds Locomotion on land arthropods and vertebrates achieve faster gaits by overlap- ng the leg movements of the left and right sides. For ex- The three great groups of terrestrial animals--mollusks, ample, a horse can convert a walk to a trot, by moving di arthropods, and vertebrates--each move over land in dif ferent ways agonally opposite legs simultaneously Mollusk locomotion is far less efficient than that of the The highest running speeds of tetrapod vertebrates, secrete a path of mucus that they glide along, pushing with galts Whe gallop of a horse, are obtained with asymmetric other groups. Snails, slugs, and other terrestrial mollusks n galloping, a horse is never supported by more a muscular foot than two legs, and occasionally is supported by none. This Only vertebrates and arthropods(insects, spiders, and reduces friction against the ground to an absolute mini ing speed. With their larger number of legs, crustaceans)have developed a means of rapid surface loco- arthropods cannot have these speedy asymmetric gaits, be motion. In both groups, the body is raised above the round and moved forward by pushing against the ground cause the movements of the legs would interfere with each othe vith a series of jointed appendages, the legs Because legs must provide support as well as propulsion, Not all animals walk or run on land. Many insects, like grasshoppers, leap using strong rear legs to propel them it is important that the sequence of their movements not selves through the air. Vertebrates such as kangaroos, rab- shove the body s center of gravity outside of the legs'zone of support. If they do, the animal loses its balance and falls bits, and frogs are also effective leapers(figure 50.21) It is the necessity to maintain stability that determines the Many invertebrates use peristaltic motion to slide over sequence of leg movements, which are similar in verte- exhibited by snakes and caecilians(legless amphibians) brates and arthropods Most snakes employ serpentine locomotion, in which the The apparent differences in the walking gaits of these body is thrown into a series of sinuous curves. The move- two groups reflects the differences in leg number. Verte- ments superficially resemble those of eel-like swimming, brates are tetrapods(four limbs), while all arthropods have but the similarity is more apparent than real. Propulsion six or more limbs. Although having many legs increases sta ar to reduce the not by a wave of contraction undulating the body but by a bility during locomotion, they also appear to reduce the simultaneous lateral thrust in all segments of the body in maximum speed that can be attained The basic walking pattern of all tetrapod vertebrates is contact with the ground. To go forward, it is necessary that left hind leg(Lh), left foreleg(LF, right hindleg(RH), the strongest muscular thrust push against the ground op- posite the direction of movement. Because of this, thrust right foreleg(RF, and then the same sequence again and tends to occur at the anterior(outside)end of the inward- again. Unlike insects, vertebra begin to walk with any of the four legs, and not just the posterior pair. Both curving side of the loop of the snake's body es can 101 Part XIlI Animal Form and Function
Locomotion on Land The three great groups of terrestrial animals—mollusks, arthropods, and vertebrates—each move over land in different ways. Mollusk locomotion is far less efficient than that of the other groups. Snails, slugs, and other terrestrial mollusks secrete a path of mucus that they glide along, pushing with a muscular foot. Only vertebrates and arthropods (insects, spiders, and crustaceans) have developed a means of rapid surface locomotion. In both groups, the body is raised above the ground and moved forward by pushing against the ground with a series of jointed appendages, the legs. Because legs must provide support as well as propulsion, it is important that the sequence of their movements not shove the body’s center of gravity outside of the legs’ zone of support. If they do, the animal loses its balance and falls. It is the necessity to maintain stability that determines the sequence of leg movements, which are similar in vertebrates and arthropods. The apparent differences in the walking gaits of these two groups reflects the differences in leg number. Vertebrates are tetrapods (four limbs), while all arthropods have six or more limbs. Although having many legs increases stability during locomotion, they also appear to reduce the maximum speed that can be attained. The basic walking pattern of all tetrapod vertebrates is left hind leg (LH), left foreleg (LF), right hindleg (RH), right foreleg (RF), and then the same sequence again and again. Unlike insects, vertebrates can begin to walk with any of the four legs, and not just the posterior pair. Both arthropods and vertebrates achieve faster gaits by overlapping the leg movements of the left and right sides. For example, a horse can convert a walk to a trot, by moving diagonally opposite legs simultaneously. The highest running speeds of tetrapod vertebrates, such as the gallop of a horse, are obtained with asymmetric gaits. When galloping, a horse is never supported by more than two legs, and occasionally is supported by none. This reduces friction against the ground to an absolute minimum, increasing speed. With their larger number of legs, arthropods cannot have these speedy asymmetric gaits, because the movements of the legs would interfere with each other. Not all animals walk or run on land. Many insects, like grasshoppers, leap using strong rear legs to propel themselves through the air. Vertebrates such as kangaroos, rabbits, and frogs are also effective leapers (figure 50.21). Many invertebrates use peristaltic motion to slide over the surface. Among vertebrates, this form of locomotion is exhibited by snakes and caecilians (legless amphibians). Most snakes employ serpentine locomotion, in which the body is thrown into a series of sinuous curves. The movements superficially resemble those of eel-like swimming, but the similarity is more apparent than real. Propulsion is not by a wave of contraction undulating the body, but by a simultaneous lateral thrust in all segments of the body in contact with the ground. To go forward, it is necessary that the strongest muscular thrust push against the ground opposite the direction of movement. Because of this, thrust tends to occur at the anterior (outside) end of the inwardcurving side of the loop of the snake’s body. 1014 Part XIII Animal Form and Function FIGURE 50.21 Animals that hop or leap use their rear legs to propel themselves through the air. The powerful leg muscles of this frog allow it to explode from a crouched position to a takeoff in about 100 milliseconds
Samoan flying fox (fruitbat) FIGURE 50.22 Flight has evolved three times among the vertebrates. These three very different vertebrates all have lightened bones and forelimbs transformed into wings Among vertebrates(figure 50.22), flight first evolved Flight has evolved among the animals four times: insects, some 200 million years ago among flying reptiles called pterosaurs(extinct flying reptiles), birds, and bats. In all pterosaurs. a very successful and diverse group, pterosaurs four groups, active flying takes place in much the same way nged in size from individuals no bigger than sparrows to Propulsion is achieved by pushing down against the air pterodons the size of a fighter plane. For much of this time, hey shared the skies with birds, which most paleontologists with wings. This provides enough lift to keep insects in the believe evolved from feathered dinosaurs about 150 million air. Vertebrates, being larger, need greater lift, obtaining it years ago. How did they share their ecological world for 100 with wings that are convex in cross section. Because air million years without competition driving one or the other must travel farther over the top surface, it moves faster, from the skies? No one knows for sure. Perhaps these early a in birds and most insects, the raising and lowering of the birds were night fliers, while pterosaurs flew by day creating lift over the wing vings is achieved by the alternate contraction of exte Such an arrangement for sharing resources is not as un ensor likely as it might at first appear. Bats, flying mammals which muscles(elevators) and flexor muscles(depressors). Four evolved after the pterosaurs disappeared with the dinosaurs, nsect orders(containing flies, mosquitoes, wasps, bees, and are night fliers. By flying at night bats are able to shop in a beetles), however, beat their wings at frequencies from 100 store with few other customers and a wealth of food: night- to more than 1000 times per second, faster than nerves can flying insects. It has proven to be a very successful approach carry successive impulses! In these insects, the flight mus- cles are not attached to the wings at all but rather to the One-quarter of all mammal species are bats stiff wall of the thorax, which is distorted in and out by their contraction. The reason that these muscles can beat Locomotion in larger animals is almost always produced so fast is that the contraction of one set stretches the other by appendages that push against the surroundings in triggering its contraction in turn without waiting for the some fashion, or by shoving the entire body forward by arrival of a nerve im an undulation Chapter 50 Locomotion 1015
Locomotion in Air Flight has evolved among the animals four times: insects, pterosaurs (extinct flying reptiles), birds, and bats. In all four groups, active flying takes place in much the same way. Propulsion is achieved by pushing down against the air with wings. This provides enough lift to keep insects in the air. Vertebrates, being larger, need greater lift, obtaining it with wings that are convex in cross section. Because air must travel farther over the top surface, it moves faster, creating lift over the wing. In birds and most insects, the raising and lowering of the wings is achieved by the alternate contraction of extensor muscles (elevators) and flexor muscles (depressors). Four insect orders (containing flies, mosquitoes, wasps, bees, and beetles), however, beat their wings at frequencies from 100 to more than 1000 times per second, faster than nerves can carry successive impulses! In these insects, the flight muscles are not attached to the wings at all but rather to the stiff wall of the thorax, which is distorted in and out by their contraction. The reason that these muscles can beat so fast is that the contraction of one set stretches the other, triggering its contraction in turn without waiting for the arrival of a nerve impulse. Among vertebrates (figure 50.22), flight first evolved some 200 million years ago among flying reptiles called pterosaurs. A very successful and diverse group, pterosaurs ranged in size from individuals no bigger than sparrows to pterodons the size of a fighter plane. For much of this time, they shared the skies with birds, which most paleontologists believe evolved from feathered dinosaurs about 150 million years ago. How did they share their ecological world for 100 million years without competition driving one or the other from the skies? No one knows for sure. Perhaps these early birds were night fliers, while pterosaurs flew by day. Such an arrangement for sharing resources is not as unlikely as it might at first appear. Bats, flying mammals which evolved after the pterosaurs disappeared with the dinosaurs, are night fliers. By flying at night bats are able to shop in a store with few other customers and a wealth of food: nightflying insects. It has proven to be a very successful approach. One-quarter of all mammal species are bats. Locomotion in larger animals is almost always produced by appendages that push against the surroundings in some fashion, or by shoving the entire body forward by an undulation. Chapter 50 Locomotion 1015 Eastern bluebird Pterosaur (extinct) Samoan flying fox (fruitbat) FIGURE 50.22 Flight has evolved three times among the vertebrates. These three very different vertebrates all have lightened bones and forelimbs transformed into wings