page_title: Role of skin in locomotion
section_title: Bats
hierarchical_section_title: Role of skin in locomotion / Specialized function in vertebrates / Aerial locomotion / Bats
context_page_description: 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.
context_section_description: Bats rely on skin on their wings to generate lift and thrust used in flight. Therefore, the structure of the bat wing skin is different from the skin of the bat body. Bat wing skin consists of two thin layers of epidermis with a thin layer of dermis/hypodermis located between the epidermal layers whereas  the skin of the bat body consists of a single layer of epidermis with a thicker layer of dermis internal to the epidermis. 
  Within the dermal and epidermal layer of bat wing skin, the connective tissue and muscle fibers provide the structural support. The connective tissue fibers within bat wing skin consists of collagen and elastin fiber bundles arranged in a "regular mesh like scaffolding", which the nerves, skeletal muscle fibers and blood vessels embed themselves into. Of the muscles that insert themselves into the mesh scaffolding, larger muscles anchor the skin to the bone and control the membrane tension and camber of the bat wing during flight, whereas smaller muscles, which originate from within the mesh scaffolding, attach to collagen fibers within the fiber network and modulate bone loading and allow for precise control of wing shape and tension. 
  As seen in snakes, local structural differences within the arrangement of the fibers change the mechanical properties of local area, but there are general characteristics that describe the mechanical behavior of bat wing skin. Within the mesh scaffolding of bat wing skin, collagen fibers cross bones perpendicular to the long axes of the bones, therefore mechanical properties of bat wing  skin oriented perpendicular to the long axes of the bones exhibit a lower stiffness than the skin that is oriented parallel to the long axes of the bodies. 
  Stiffer skin is necessary for bat wing skin oriented in the direction parallel to the long axes of the bones to prevent too much deformation of bat wing skin during flight(with respect to the bone), resulting in the shearing of the bat wing skin off of the bone. Flexible skin is necessary for the direction perpendicular to the long axes of the bones for facilitating the shape changes needed for movement and control during flight.  This anisotropy of bat wing skin is also useful as method of storing and releasing elastic energy, particularly during the downstroke. During the downstroke, the bat extends its wing  and the wing skin experiences an aerodynamic force. The wing skin expands and  counteracts the aerodynamic force. After the wing is fully extended, the orientation of the wing is nearly vertical and the aerodynamic forces on the wing are reduced. As the aerodynamic force is reduced, the wing recoils, drawing the digits of the wings together in preparation for the upstroke.
caption: Drawing of the bat wing skin showing the fibers of "mesh like scaffolding"
exif: {}