Biomechanics of Animal Movement

Biomechanics

• Study of animal movement
• Often referred to as "the mechanics of life"
• Examines how animals navigate and adapt to their environment

Biomechanics of Extinct Animals

• Provides insights into animal forms not seen today.
• Helps understand the physical limitations of extinct animals.
• Studies functional adaptations like terrestrial movement and flight.
• Examples: Quetzalcoatlus (pterosaur), Patagotitan (sauropod)

Skeletal Adaptations to Size

• Small animals do not scale proportionally.
• Small animals have thinner bones and crouched postures.
• Larger animals have thicker, more robust bones and a straighter, column-like posture.
• Examples: Wood mouse, Elephant

Posture

• Sprawling posture was ancestral to tetrapods.
• Erect posture, seen in dinosaurs and mammals, requires special adaptations.
• Open hip socket in dinosaurs supports erect posture.
• Examples: Early tetrapod trackway, Early tetrapod Seymouria

Quadrupedalism

• Movement on all four limbs.
• Ancestral locomotion in most tetrapods, providing stability.
• Movement involves rotation of the chest and hips.

Bipedalism

• Movement on two hind limbs.
• Obligate bipedalism is seen in birds and humans.
• Obligate bipeds typically have erect postures.
• Ancestral to dinosaurs, enabling flight and increased manual dexterity in humans.
• Occasional or partial bipedalism observed in other mammals.
• Examples: Ostriches, dinosaurs like Tyrannosaurus

Bipedalism—Further Innovations

• Enabled new innovations like flight (in birds).
• Increased manual dexterity and tool use in humans.

Bipedalism—Other Forms

• Occasional bipedalism observed in other mammals and reptiles.
• Kangaroos adopt tripodal stance using three limbs.

Postural Enigma

• Spinosaurus (Late Cretaceous theropod).
• Inhabiting Egypt & Morocco
• Unlike other theropods, its wrists can't pronate for hand support
• Fossils of its hands or forearms are missing

Speed

• Scientists measure running speeds from live animals.
• Analyze stride length with statistical analysis.
• Historical biomechanicist, Dr. Robert McNeill Alexander, derived relationships between speed, stride length, and body size in dinosaurs.

Alexander's Dinosaur Speed Calculator

• Predicted running speeds based on stride length and body size.
• Formulas to calculate speed were developed using empirical observations on modern animals, and applied to dinosaur fossils.

Speed—Dinosaurs (How Fast?)

• Small dinosaurs could run fast (>35 km/hr).
• Large dinosaurs like Tyrannosaurus were slower (<10 km/hr).
• Some results yielded high speeds for large dinosaurs.

Adaptations to Speed and Size

• Cursorial animals have longer distal limbs than proximal.
• This increased running efficiency and speed.
• Crural index (tibia-to-femur ratio) is an important factor.
• Graviportal animals have longer proximal limbs for stability and robustness.
• This helps in supporting large body sizes. Different types of foot postures; Plantigrade (entire foot touches ground), Digitigrade (only part of the foot), Unguligrade (only toes or tips of toes touch ground).

Adaptations to Speed and Size—Further Considerations

• Erect digitigrade and unguligrade postures reduce locomotion costs.
• Speed increases with size, but the rate of increase decreases (quadratic scaling).
• This limits speed in larger animals.

Evolution of the Horse

• Shows the evolutionary history and anatomical changes through time, from ancestors of the horse to the modern-day horse
• Detailed representations of fossil horses

Increasing Stride

• Multiple strategies to increase stride and speed without changing limb length or posture.
• Strategies include lateral and sagittal spine movements, and pelvic rotation during movement. These movements aid in more efficient stride and faster speeds.
• The adaptations are inherited, dictating movement.

Conserving Energy

• Pendular movements (gravity) help conserve energy.
• Flexibility (springs) in joints help conserve energy.
• Tendons and ligaments act as natural springs for conserving energy.
• Fusion and digit reduction increase organism support and stability.

Physics and Convergence

• Trends in posture, stance, and locomotion repeat through evolutionary history.
• Repeated forms adapted for similar functions through convergent evolution.

Conclusions

• A general trend exists in evolving toward straighter postures and more robust, graviportal limbs with larger animals.
• Cursorial animals typically have longer distal legs, and are usually digitigrade or unguligrade.
• Speed can be increased in larger animals through stride length enhancements.
• How bodyplans (plans for constructing a body) are inherited dictates evolution.
• Vertebrate animals have adaptations for conserving energy during movement.