Gliding: A common adaptation among mammals


Guest Lecture by Brian Stafford

  1. Gliding is an energy efficient means of locomotion
    1. Gliding is an example of convergence: it has evolved independently in 3 separate groups of mammals: Order Rodentia, Order Dermoptera, and in some marsupials


  2. Although they are not closely related, gliding mammals have morphological features in common:
    1. Patagium - the membrane of skin stretching from forelimb to hindlimb (also to head and tail in some species) - the points of attachment vary by species (wrist to ankle; elbow to knee)
    2. An extension of cartilage from the forelimb (see Figure 17.8 in text)
      1. serves as an attachment point for the patagium
      2. allows an increase in the size of the patagium
      3. may serve to stabilize the "wing"
      4. may allow for control of direction while gliding (maneuverability)


  3. Gliding mammals must solve similar problems when in the air
    1. There are 4 forces acting on an animal in the air:
      1. weight - pulls downward
      2. lift - pulls upward
      3. drag - decreases forward motion
      4. thrust - increases forward motion


    2. For an airborne object to remain airborne:
      lift > weight
      thrust > drag

    3. For gliding mammals, thrust comes from:
      1. initial launch (pushing off of a substrate)
      2. potential energy (of falling) transformed into velocity


    4. In order to travel further while gliding, an animal must increase lift or reduce drag

    5. One way to increase lift is to increase the surface area of the patagium; long fingers and extensions of cartilage from the forelimb are ways of increasing surface area (SA)
      1. Wing loading = (weight of animal/SA): animals with a lower wing loading (high SA) can glide at slower speeds without stalling. Higher wing loading animals must glide faster to remain in the air - there are several drawbacks to this:
        1. gliding faster means less maneuverability
        2. harder to stop or avoid obstacles
        3. harder to avoid predators (such as owls)


    6. One way to reduce drag is to have a relatively large, rectangular wingspan instead of a shorter, square-shaped wingspan:
      1. Aspect ratio = (wingspan)^2/SA
        High aspect ratio: low drag, more agile, stalls at low angle of attack
        Low aspect ratio: high drag, less agile, stalls at higher angle of attack (more vertical)


    7. The cartilage on the forelimbs may be another way to reduce drag: the wingtip can be bent up to reduce the amount of induced drag:
      1. Induced drag results when high pressure air slips out from underneath the wing and moves to the top of the wing - this is most pronounced at the wingtips
      2. Induced drag will pull the wingtips down and retard forward motion
      3. If the wingtips are bent upwards, the high pressure air that moves to the top of the wing pushes out to the sides instead of pulling the wingtips down - this may also help stabilize the wing and allow the animal to stay on course if knocked to the side by a gust of wind
      4. Reduction in induced drag increases gliding distances and helps maintain lift at high angles of attack


    8. Elliptical wings may also reduce induced drag - tapered wingtips help minimize the loss of high pressure air under the wing


  4. Flying lemurs (colugos)
    1. They have the most complete patagium of any gliding mammal - it extends from the neck to the digits of the forelimbs, along the sides of the body and hind limbs, and encloses the tail
      1. May represent an evolutionary step toward powered flight - probably share a common ancestor with bats


    2. Two genera (text only indicates one): Cynocephalus and Galeopterus
      1. Why should these be considered separate genera instead of two species under a single genus?
        1. differences in teeth/jaw:

          Cynocephalus Galeopterus
          blade-like teeth serrated teeth
          more robust teeth teeth not as robust
          larger area on lower jaw for muscle attachment


        2. The differences in their teeth indicate differences in ecology such as food quality or structure, as well as differences in foraging strategies and possibly mating strategies