Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 50.6 Skeletal Systems Transform Muscle Contraction into Locomotion
Animals rely on skeletal systems to convert the force of muscle contraction into movement. These systems vary widely—from hydrostatic to exoskeletons to endoskeletons—but all serve to support the body, protect internal organs, and enable locomotion in different environments.
1. How Skeletons Enable Movement
- Muscles must attach to a skeleton to generate movement through contraction
- Muscles can only pull, not push—antagonistic muscle pairs enable opposite motions
- Tendons anchor muscles to bones or exoskeletal elements
- The nervous system coordinates opposing muscle activity for smooth motion
2. Types of Skeletons
- Hydrostatic skeletons: fluid-filled chambers (e.g., worms, cnidarians)
- Muscles contract to shift internal pressure, reshaping the body (e.g., earthworm peristalsis)
- Best suited for soft-bodied, aquatic organisms
- Exoskeletons: rigid external coverings (e.g., insects, clams)
- Made of chitin or calcium carbonate
- Muscles attach internally; animals must molt to grow
- Endoskeletons: internal frameworks (e.g., vertebrates)
- Composed of cartilage, bone, or both
- Supports the body and protects vital organs like the brain and heart
3. Joint Types and Body Plan
- Ball-and-socket joints: allow rotational movement (e.g., shoulder, hip)
- Hinge joints: permit one-directional motion (e.g., elbow, knee)
- Pivot joints: enable twisting movements (e.g., neck, forearm)
- Ligaments hold bones together at joints; osteoblasts and osteoclasts remodel bone
- Larger animals require thicker bones or posture changes for structural support
4. Locomotion Types and Environments
- Swimming:
- Water supports weight but increases drag
- Streamlined shapes reduce resistance (e.g., fish, whales)
- Some animals use jet propulsion (e.g., squid), others use body waves (e.g., eels)
- Flying:
- Requires lift to overcome gravity
- Wings act as airfoils; lightweight bodies help (e.g., birds with hollow bones)
- True flight is found only in insects, birds, and bats
- Land locomotion:
- Includes walking, hopping, running, crawling
- Requires support against gravity and shock absorption
- Kangaroos use elastic tendons to store energy
- Balance and posture are essential; momentum aids faster movement
- Crawling animals use coordinated undulations to move forward
5. Energy Costs of Locomotion
- Energy use depends on locomotion type and body size
- Running is most costly per unit distance, flying is intermediate, swimming is most efficient
- Larger animals are more efficient movers than small ones (lower cost per kg)
- Efficiency is affected by shape, environment, and locomotion method
In a Nutshell
Skeletal systems transform the pull of muscles into controlled movement. From flexible hydrostatic skeletons to rigid exoskeletons and internal endoskeletons, animals adapt their structures for life in water, air, or on land. The interplay of anatomy, physics, and muscle action enables diverse and energy-efficient forms of locomotion, each shaped by evolution to meet the demands of specific environments.