Mammalian Structure and Function: Locomotion

  1. Characteristics of living mammals (*diagnostic)

    1. Skeletal features
      1. Two occipital condyles*
      2. Long bones have epiphyses*
      3. Tendency towards fusion of bones (e.g., pelvis, cranium)

    2. Skeletal structure

      1. Axial skeleton
        1. Five well-differentiated types of vertebrae
          Significance of vertebral specialization and relationship to homeothermy

          1. Cervical (including atlas=1, axis=2)
          2. Thoracic
          3. Lumbar
          4. Sacral
          5. Caudal

        2. Ribs

      2. Appendicular skeleton
        1. Basic parts
          1. Pectoral girdle + forelimbs + manus + pollex + phalanges
          2. Pelvic girdle + hindlimbs + pes + hallux + phalanges

    3. Ancestral pattern
      1. Five digits
      2. Phalangeal formula: 2-3-3-3-3

  2. Locomotion

    1. Types
      1. Walking, running, cursorial (terrestrial, usually quadrupedal)
        video: dogs
        1. Many specialized for moving quickly in a terrestrial habitat
        2. Pattern of footfalls depends on species and speed of travel
        3. Humans are the only truly bipedal species
        4. Modes of terrestrial locomotion
          1. Plantigrade
          2. Digitigrade
          3. Unguligrade

      2. Saltatorial (jumping, springing, bounding)
        1. quadrupedal of rabbit hopping
        2. bipedal (ricochetal)
          video of kangaroo hopping (best footage at about 1:30)
          • enlarged feet on propulsive limbs
          • center of gravity shifted toward rear of body
          • big hindquarters
          • long tail (acts as counterbalance)

      3. Swimming
        1. amphibious (semiaquatic)
          video: otter swimming
          • thick fur
          • webbed feet (used for propulsion)
          • tail may be flattened, provides stability and propulsion
        2. aquatic
          video: sea lion swimming
          • fore and hind limbs modified into flippers for propulsion
          • thick layer of subcutaneous fat (blubber)
          • basically no externally visible tail
        3. marine (fully aquatic)
          video: dolphin swimming
          • fusion of cervical vertebrae
          • forelimbs modified into flippers (provides stability)
          • elimination of hind limbs and most of pelvic girdle (not visible externally, but remnant bones may remain)
          • tail flattened dorso-ventrally into a fluke (provides propulsion)
          • thick layer of subcutaneous fat (blubber)

      4. Gliding and Flying (volant)
        video:fruit bat flying
        video: sugar glider gliding
        • Characterized by a patagium (membrane)
        • Gliding has evolved independently at least 5 times in marsupials, rodents and dermopterans

      5. Climbing (arboreal, scansorial)
        video: squirrel climbing
        1. Claws
        2. Prehensility
          • Usually associated with larger body size relative to substrate
          • Occurs in hands, feet, tail
          • Associated with loss of claws (nails instead)
          • Associated with friction pads
        3. Suspensory (hanging)
          • Associated with long, curved digits
          • Associated with fusion of cervical vertebrae

      6. Digging and burrowing (fossorial)
        video: mole burrowing
        • Usually dig with forelimbs, but sometimes with incisor teeth (e.g., mole rats)
        • Large claws
        • Short, broad forelimbs (if forelimbs are used for digging)
        • Forelimbs splayed to the sides (if forelimbs are used for digging)
        • More vibrissae than terrestrial mammals, in more locations on the body
        • Small eyes
        • Often lack pinnae

  1. Case Studies

    1. Ungulates: Perissodactyla and Artiodactyla
      1. Not a formal taxonomic division

      2. Shared features

        1. Herbivory
        2. Extreme cursorial adaptations, including syndactyly and hooves

      3. Evolution of cursoriality
      4. Biomechanics of cursoriality (demonstration)

        1. Increasing stride length

          1) Lengthen limbs
          2) Loss or reduction of clavicle
          3) Dorso-ventral flexion of spine
          video: cheetah showing dorso-ventral flexion of spine while running
          Dorso-ventral flexion improves respiratory efficiency (locomotor and respiratory systems are mechanically coupled)

        1. Increasing stride rate

          1) More joints in limb
          2) Reduction in weight of distal limb elements
          • loss of digits
          • loss of structures associated with lateral movement
          • concentration of musculature close to body core and minimization of musculature at ends of limbs

        1. Other connective tissue specializations related to cursoriality and herbivory

          1) Nuchal ligament
          2) Springing ligament

    2. Cetaceans

      Think-Pair-Share: How do you turn a cow into a whale?

      1. Morphological characteristics related to aquatic lifestyle

        1. Fusiform-shaped body
        2. Large body size
        3. Virtually hairless
        4. Blubber
        5. No sebaceous glands
        6. Tail flattened dorso-ventrally into a fluke
        7. Forelimbs are modified into flippers
        8. Vestigial hindlimbs
        9. Dorsal fin in most species
        10. Skull

        1. Specialized muscles to control blow hole
        2. Neck is short and cervical vertebrae are often fused

      1. Diving and swimming

        1. Very fast swimmers

          • Drag, turbulent flow, laminar flow
          • Adaptations that promote laminar flow

            1) Lack of hair except for a few vibrissae
            2) Few external appendages/protuberances
            3) Parabolic body form

        1. Respiratory/circulatory adaptations permitting prolonged submersion

          • Normal vertebrate diving adaptations (AKA the diving reflex)
            1) Bradycardia - In cetaceans, heart rate drops to half its normal rate during long dives
            2) Peripheral vasoconstriction - Capillaries in the extremities narrow, causing blood to be redistributed to vital organs
          • Extra capillaries in the lungs
          • Use 3 times as much of the oxygen from a breath of air as do terrestrial mammals
          • Can force almost all air out of the lungs when exhaling
          • Twice as high a concentration of red blood cells
          • 2-9 times as much myoglobin in muscle tissue
          • High tolerance to lactic acid
          • High tolerance to carbon dioxide

        1. Can tolerate very high pressures associated with deep dives

          • Usual mammalian response to increased pressures
          • Cetacean adaptations to deep diving

    1. Primates

      1. General characteristics (adaptations for arboreality?)

        1. Locomotion

      2. Quadrupedal
        1. Terrestrial

          terrestrial quadruped skeleton
          • Narrow thorax
          • Restricted shoulder joint
          • Forelimbs and hind limbs of similar lengths
          • Short digits
          • Tail reduced

        2. Arboreal

          arboreal quadruped skeletal diagram
          • Narrow thorax
          • Forelimbs and hindlimbs of similar lengths
          • Shoulder joint allows some lateral motion
          • Long, prehensile digits
          • Long tail

      3. Vertical clinging and leaping
        photo series

        leaper skeletal diagram
        1. Hindlimbs longer than forelimbs
        2. Long, prehensile digits
        3. Long lumbar region of vertebral column

      4. Brachiation (and semibrachiation)

        brachiator skeletal diagram
        1. Broad thorax
        2. Forelimbs longer than hindlimbs
        3. Reduced or absent pollex
        4. Mobile shoulder joint
        5. Short lumbar region
        6. Mobile hip joint
        7. No tail (OR long and prehensile tail in semibrachiators)

          video: gibbon (brachiation; start at 1:40)
          video: spider monkey (semibrachiation)