1. Why be a gliding mammal?
    1. Gliding is an energy efficient means of locomotion

    2. Gliding gives rapid access to food sources

  2. Gliding is an example of convergence.
    1. There have been at least 7 independent evolutions of gliding in Mammalia (plus 2 extinct taxa).

      1. Order Rodentia

      2. Order Marsupialia

      3. Order Dermoptera

        1. Recent evidence suggests that Dermoptera should have two genera [Cynocephalus volans (Phillipine flying lemur) and Galeopterus variegatus (Sundaic flying lemur)] instead of two species under one genus (Cynocephalus volans and Cynocephalus variegatus). The distinction between the two is based on tooth and cranial morphology (see photos, C. variegatus; C. volans):

        C. volans

        G. variegatus

        narrow rostrum/cranium

        broad rostrum/cranium

        blade-like incisor

        serrated incisor

        robust mandible

        more slender mandible

        The differences may reflect changes in food quality or structure (C. volans is folivorous; G. variegatus is frugivorous), foraging, or mating strategies.

  3. Although they are not closely related, gliding mammals have morphological features in common.

    1. Patagium (most complete in Dermoptera) - the membrane of skin stretching from forelimb to hindlimb (also head to 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 that is usually flexed dorsally

      1. usually extends from the elbow or wrist

      2. serves as an attachment point for the patagium

      3. may reduce drag and serve to stabilize the "wing"

      4. may allow for control of direction while gliding (maneuverability)

    3. Long, slender finger bones and forelimbs (ex: flying lemurs) - this trait is shared by Chiroptera.

    4. When the patagium does not enclose the tail, the tail, is often "feathery" and the hair is dorsoventrally flattened

  4. Gliding mammals must solve similar problems when in the air
    1. There are 4 forces acting on an animal in the air (these forces also apply to any other airborne object such as an airplane):
      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 substrate)
      2. potential energy (of falling) transformed into velocity


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

      1. The cartilage on the forelimb may be a way to reduce induced drag - it allows the wingtip to be bent up (flexed dorsally):
        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 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

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

        1. Wind tunnel testing of this theory showed that elliptical wings don't appear to reduce induced drag (except slightly at low angles of attack) but they do provide wing stability

      3. Another way to reduce drag is to have a relatively large, rectangular wingspan instead of a shorter, square-shaped wingspan:

      4. One way to increase lift is to increase the surface area of the patagium; long slender fingers and extensions of cartilage from the forelimb are ways of increasing surface area (SA).

        1. Wing loading = Weight of animal/Surface Aarea of wing
          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:

          • gliding faster means less maneuverability

          • harder to avoid obstacles

          • harder to avoid predators