Role of skin in locomotion describes how the integumentary system is involved in locomotion. Typically the integumentary system can be thought of as skin, however the integumentary system also includes the segmented exoskeleton in arthropods and feathers of birds. The primary role of the integumentary system is to provide protection for the body. However, the structure of the skin has evolved to aid animals in their different modes of locomotion. Soft bodied animals such as starfish rely on the arrangement of the fibers in their tube feet for movement. Eels, snakes, and fish use their skin like an external tendon to generate the propulsive forces need for undulatory locomotion. Vertebrates that fly, glide, and parachute also have a characteristic fiber arrangements of their flight membranes that allows for the skin to maintain its structural integrity during the stress and strain experienced during flight.
Soft bodied locomotion in invertebrates
The term "Soft Bodied" refers to animals which lack typical systems of skeletal support - included in these are most insect larvae and true worms. Animals that are soft bodied are constrained by the geometry and form of their bodies. However it is the geometry and form of their bodies that generate the forces they need to move. The structure of soft bodied skin can be characterized by a patterned fiber arrangement, which provides the shape and structure for a soft bodied animals. Internal to the patterned fiber layer is typically a liquid filled cavity, which is used to generate hydrostatic pressures for movement.Robert E. Shadwick. Foundations of animal hydraulics: Geodesic fibres control the shape of soft bodied animals. J Exp Biol, 211(3):289–291, February 2008. Some animals that exhibit soft bodied locomotion include starfish, octopus, and flatworms.
Hydrostatic skeleton A hydrostatic skeleton uses hydrostatic pressure generated from muscle contraction against a liquid filled cavity. The liquid filled cavity is commonly referred to as the hydrostatic body. The liquid within the hydrostatic body acts as an incompressible fluid and the body wall of the hydrostatic body provides a passive elastic antagonist to muscle contraction, which in turn generates a force, which in turn creates movement.R. B. Clark and J. B. Cowey. Factors controlling the change of shape of certain nemertean and turbellarian worms. J Exp Biol, 35(4):731–748, December 1958. This structure plays a role in invertebrate support and locomotor systems and is used for the tube feet in starfish and body of worms. A specialized version of the hydrostatic skeleton is a called a muscular hydrostat, which consists of a tightly packed array of three-dimensional muscle fibers surrounding a hydrostatic body.William M. Kier and Kathleen K. Smith. Biomechanics(Structures and Systems): A Practical Approach. (1992). A. A. Biewener, ed. New York: IRL Press at Oxford Univ. Press. Examples of muscular hydrostats include the arms of octopus and elephant trunks.
Fiber arrangement
The arrangement of the connective tissue fibers and muscle fibers create the skeletal support of a soft bodied animal. The arrangement of the fibers around a hydrostatic body limits the range of movement of the hydrostatic body (the "body" of a soft bodied animal) and defines the way the hydrostatic body moves.
Muscle fibers Typically muscle fibers surround the hydrostatic body. There are two main types of muscle fibers orientations that are responsible for the movement: the circular orientations and longitudinal orientations.William M. Kier and Kathleen K. Smith. Tongues, tentacles and trunks: the biomechanics of movement in muscular-hydrostats.Zoological Journal of the Linnean Society, 83(4):307–324, 1985. Circular muscles decrease the diameter of a hydrostatic body, resulting in an increase in the length of the body, whereas longitudinal muscles shortens the length of a hydrostatic body, resulting in an increase in the diameter of the body. There are four categories of movements of a hydrostatic skeleton: elongation, shortening, bending and torsion. Elongation, which involves an increase in the length of a hydrostatic body requires either circular muscles, a transverse muscle arrangement, or radial muscle arrangement. For a transverse muscle arrangement, parallel sheets of muscle fibers that extend along the length of a hydrostatic body. For a radial muscle arrangement, radial muscles radiate from a central axis along the axis perpendicular to the long axis. Shortening involves the contraction of the longitudinal muscle. Both shortening and bending involve the contraction of longitudinal muscle, but for bending motion some of the antagonistic muscles work synergistically with longitudinal muscles. The amplitude of movements are based upon the antagonistic muscles forces and the amount of leverage the antagonistic muscle provides for movement. For the torsion motion, muscles are arranged in helical layers around a hydrostatic body. The fiber angle (the angle the fiber makes with the long axis of the body) plays a critical role in torsion, if the angle is greater than 54°44', during muscle contraction, torsion and elongation will occur. If the fiber angle is less than 54°44', torsion and shortening will occur.
Connective tissue fibers The arrangement of connective tissue fibers determines the range of motion of a body, and serves as an antagonist against muscle contraction. The most commonly observed connective tissue arrangement for soft bodied animals consists of layers of alternating right and left-handed helices of connective tissue fibers which surround the hydraulic body. This cross helical arrangement is seen in the tube feet starfish, different types of worms and suckers in octopus. This cross helical arrangement allows for the connective tissue layers to evenly distribute force throughout the hydrostatic body. Another commonly observed connective tissue fiber range is when the connective tissue fibers are embedded within a muscle layer. This arrangement of connective tissue fibers creates a stiffer body wall and more muscle antagonism, which allows for more elastic force to be generated and released during movement. This fiber arrangement is seen in the mantle of squid and the fins in sharks.
Specialized function in vertebrates
Swimming and undulatory locomotion The skin of these animal that use undulatory motion to locomote have several distinct characteristics. The skin of these animals consists of cross-helical arrangement of collagen and elastin fibers embedded in the dermal layer of skin,D.M. Pearson.Functional aspects of the skin in polypterus fishes. Zool. J. Linn. Soc. 72, 93–106. 1981 a two-dimensional stiffness. which permits bending at small curvatures and resists bending at high curvatures and skin is attached directly to the underlying muscles. Fish, shark, and snakes are all examples of animals that locomote using undulatory locomotion.
Eel
The cross helical fiber arrangement of the two dermal fibers types collagen and elastin, are responsible for the mechanical properties of the skinL. Yang. Mechanical properties of native and cross-linked type i collagen fibrils. Biophysical Journal, 94(6):2204–2211, March 2008. such as the two dimensional stiffness seen in the eel skin. In the longitudinal direction, eel skin behaves like a pure fiber system, with a lessertensile strength than skin in the hoop direction. The skin in the hoop direction exhibits a higher elastic modulus than the skin in the longitudinal direction.M R. Hebrank. Mechanical properties and locomotor functions of eel skin. Biol Bull, 158(1):58–68, February 1980. The two dimensional stiffness allows for the body of the eel to be modeled a pressurized cylinder with the fiber angle of the cross helical arrangement dictating the method by which the eel moves. Eel skin behaves like skin having a fiber angle greater than 45°. In an eel with the cross helical fiber arrangement, muscle contraction in the anterior region bends the fish, and so the skin on the convex side is extended in the longitudinal direction. The extension in the longitudinal direction produces contraction in the hoop direction as the fiber angle decreases until these dimensional changes are resisted by the body of the eel. The skin becomes skin, and additional longitudinal force (applied by skin) results in force being transmitted along the tail. Therefore, changes in fiber angle of the cross helical arrangement in eel skin allows for the transmission of force through the skin during swimming. The skin act like an external tendon allowing for an eel to generate a greater propulsive force per muscle contraction. In addition to the eel skin acting as an external tendon, the skin attaches directly to the underlying muscle, which allow for the eel to generate an even greater force per muscle contraction.
Longnose gar Due to the heavily scaled skin of the Longnose gar, some of the mechanical properties differ from model of describing how eel skin adds movement. The scale row resists longitudinal forces, which unlike eel skin, makes the skin stiffer in the longitudinal direction, providing myomeres with leverage and anchorage for pulling tendons. At low curvatures, it appears that the dermis is slack on both the concave and convex sides of the body. When the dermis is placed in tension, and resistance to bending is developed, which is referred to as flexural stiffness of the fish skin. The flexural stiffness is a result of the two dimensional stiffness of fish with heavily scaled skin, such as the longnose gar. This mechanical property of fish skin is important to the way a fish swims, because this mechanical property passively stiffens the body, which would otherwise would have been done muscularly.Long, J.H. Jr., Hale, M.E., McHenry, M.J.* and M.W. Westneat. (1996). Functions of fish skin: flexural stiffness and steady swimming of longnose gar, Lepisosteus osseus. Journal of Experimental Biology 199, 2139-2151. The flexural stiffness of fish skin act in a manner similar to the mechanism by which eel skin acts as an external tendon, however in the case of fish skin, the flexural stiffness acts as a mechanism to decelerate body movement rather than to generate a propulsive force.
Snake
Snakes are one of the few vertebrates in which the skin alone is sufficient for locomotion. During Rectilinear locomotion, the skeleton remains fixed, while the skin is alternately lifted and pulled forward, and then allowed to contact the ground and pulled backwards, propelling the body forward.
One of the interesting aspects of snakeskin are folds of intersquamous skin between longitudinally oriented scale rows.Savitzky, A. H., Townsend, V. R., Jr, Hutchinson, D. A. and Mori, A. (2004). Dermal characteristics, scale row organization, and the origin of macrostomy in snakes. J. Morphol. 260,325 The function of these folds is to permit the circumference of the snake to increase, allowing prey to pass into the stomach during feeling., S. J. Mullin. Adaptations facilitating facultative oophagy in the gray rat snake, Elaphe obsolete spiloides. Amphibia-Reptilia 17, 387-394. 1996 Snakes differ from eels in the direction in which the skin is stiffer, the dorsal scale rows are more flexible in snake than in eels because the dorsal scale row associated with stretching. Differences in the local dermal structures, such as variations in the diameters and orientation of collagen fibers within the intersquamous skin create local differences in the mechanical properties of the snake skin, thus allowing it to adapt to the stresses and strains during the feeding process.Gabriel Rivera, Alan H. Savitzky, and Jeffrey A. Hinkley. Mechanical properties of the integument of the common gartersnake, thamnophis sirtalis (serpentes: Colubridae). J Exp Biol, 208(15):2913–2922, August 2005.
Aerial locomotion