Locomotion size

Limb Postures and Locomotion

The session focuses on how animals deal with the constraints of size, particularly the challenges faced by larger animals due to increased limb force. It also addresses the energetic implications of different limb postures and constraints to legged locomotion.

The core reading material is a review article by Diplin Clemente, which synthesizes existing literature on how animals cope with the challenges of being large, examining various hypotheses and explanations.

Advantages of Being Big

Larger animals can travel further distances due to a lower cost of transport. This is because they have a longer stride length, requiring fewer steps to cover the same distance, thus minimizing their cost of transport.

Disadvantages of Being Big

As an animal's mass increases, the force its skeleton needs to support also increases. While animals could theoretically counteract these forces by developing much larger legs, this would lead to a trade-off with speed, making them very slow.

Strategies for Animals to Grow Larger Within Safety Limits

Animals can potentially grow larger while staying within their safety limits by:

Modifying the material properties of their bones and muscles.
Changing the geometric shape of their bones.
Altering the way their bones are loaded (related to limb posture).
Modifying the architecture of their skeletal muscle.
Accepting decreases in locomotive performance by slowing down.

The paper explores the evidence for and against each of these potential strategies.

Bone and Muscle Strength

Early studies on bone failure stresses did not find significant differences across animals of different weights. Similarly, skeletal muscle properties are highly conserved across species, with consistent muscle proteins (actin, myosin).

Studies on the mechanical properties of skeletal muscle across different animal sizes found little difference in fiber diameter and force generation capability. This suggests that muscle properties are relatively constant across different animal sizes.

While one modeling study suggested potential changes in bone trabecular architecture in mammals and birds with size, the general consensus is that bone and muscle properties are relatively consistent, particularly in vertebrates, making this an unlikely primary explanation for dealing with size constraints.

Bone Thickness

Some studies suggest that bone dimensions scale isometrically. However, there is no overarching scaling theory that applies to all animals. There is no definitive relationship that easily fits for bone thickness.

Reducing Musculoskeletal Stress Through Locomotion

Larger animals reduce musculoskeletal stress through locomotion by employing a more upright limb posture, which enhances their effective mechanical advantage (EMA).

Crouched Posture in Feathered Species

Feathered species maintain a crouched posture even at larger sizes possibly to aid in generating horizontal ground reaction forces, which are important for maneuvering. The effective mechanical advantage diagrams typically focus on vertical ground reaction forces, potentially overlooking the significance of horizontal forces.

Komodo Dragons

Komodo dragons defy typical animal posture and sizing rules. They can reach masses over 100 kg but maintain an extremely crouched sprawling limb posture. Geometric scaling would suggest that the stresses on their bones and muscles should be at failure point during normal locomotor activity. However, they can move quite fast, making this even more puzzling.

Altering Muscle Mechanical Properties

Some animal groups alter the mechanical properties of their muscles by:

Increasing the force-generating and withstanding capability of their muscles.
Increasing their muscle mass or cross-sectional area.
Decreasing their muscle fiber length.

While the fundamental properties of muscle (actin, myosin) remain consistent, changes in fiber length and muscle cross-sectional area can enhance force generation and withstanding capabilities.

Reducing Peak Stress by Slowing Down

Slowing down reduces peak stress by increasing stance time and duty factor. This spreads the force over a longer period, thus reducing the actual force experienced by the limb.

Extant Mammals Above Optimum Mass for Speed

Only a small percentage (around 3-4%) of extant (still living) mammals are above the optimum mass for speed.

Giant Extinct Species

Possible explanations for how extinct species, like dinosaurs, were able to grow so large include:

Different Environmental Conditions: The environment and ecology may have been different, providing a significant advantage to larger animals.
Different Mechanisms: Extinct species may have had unique mechanisms for withstanding the additional stresses of being large that are not seen in extant animals.

It may be that current methods of observing extant animals may not apply for extinct ones due to too many differences; therefore skewing data.

Size vs Speed

There is a trade-off between size and speed. Large size typically means reduced speed unless other adaptations optimize locomotion.

Big animals have a more upright limb and a larger effective mechanical advantage, compared to smaller animals that have a more crouched limb. There are exceptions, such as the barony lizards and some feathered species, that do not follow this pattern.

The mechanisms by which massive extinct species achieved high speeds are not fully understood.

It is now likely maladaptive to be very large and slow, given changes in the environment and ecology.

Limb Postures and Locomotion

The session focuses on how animals deal with the constraints of size. Larger animals face increased limb force, and the session addresses the energetic implications of different limb postures. Constraints to legged locomotion are also discussed.

Advantages of Being Big

Larger animals can travel further distances due to a lower cost of transport. This advantage arises because they have a longer stride length, reducing the number of steps needed to cover the same distance, thus minimizing their cost of transport.

Disadvantages of Being Big

As an animal's mass increases, so does the force its skeleton needs to support. If animals were to counteract these forces by developing much larger legs, it would lead to a trade-off with speed, making them very slow and less agile.

Strategies for Animals to Grow Larger Within Safety Limits

Animals can potentially grow larger while staying within their safety limits by:

Modifying the material properties of their bones and muscles.
Changing the geometric shape of their bones.
Altering the way their bones are loaded (related to limb posture).
Modifying the architecture of their skeletal muscle.
Accepting decreases in locomotive performance by slowing down.

The paper explores the evidence for and against each of these potential strategies.

Bone and Muscle Strength

While one modeling study suggested potential changes in bone trabecular architecture in mammals and birds with size, the general consensus is that bone and muscle properties are relatively consistent, particularly in vertebrates. Thus, modifications in bone and muscle strength aren't a primary explanation for dealing with size constraints.

Bone Thickness

Some studies suggest that bone dimensions scale isometrically. However, there is no overarching scaling theory that applies to all animals. So far, there is no definitive relationship that easily fits bone thickness.

Reducing Musculoskeletal Stress Through Locomotion

Larger animals reduce musculoskeletal stress through locomotion by employing a more upright limb posture. This posture enhances their effective mechanical advantage (EMA), reducing the forces experienced by the limbs.

Crouched Posture in Feathered Species

Feathered species maintain a crouched posture even at larger sizes, possibly to aid in generating horizontal ground reaction forces, which are important for maneuvering. The effective mechanical advantage diagrams typically focus on vertical ground reaction forces, potentially overlooking the significance of horizontal forces. Also, the crouched position may be related to flight requirements and stability

Komodo Dragons

Altering Muscle Mechanical Properties

Some animal groups alter the mechanical properties of their muscles by:

Increasing the force-generating and withstanding capability of their muscles.
Increasing their muscle mass or cross-sectional area.
Decreasing their muscle fiber length.

Functional Significance of Muscle Adaptations

While the fundamental properties of muscle (actin, myosin) remain consistent, changes in fiber length and muscle cross-sectional area can enhance force generation and withstanding capabilities. Shorter fibers can produce more force but over a shorter range of motion, beneficial for high-force, low-speed activities.

Reducing Peak Stress by Slowing Down

Slowing down reduces peak stress by increasing stance time and duty factor. This spreads the force over a longer period, thus reducing the actual force experienced by the limb, and minimizing risk of injury.

Extant Mammals Above Optimum Mass for Speed

Only a small percentage (around 3-4%) of extant (still living) mammals are above the optimum mass for speed, indicating a potential evolutionary trade-off or constraint.

Giant Extinct Species

Possible explanations for how extinct species, like dinosaurs, were able to grow so large include:

Different Environmental Conditions: The environment and ecology may have been different, providing a significant advantage to larger animals. For example, higher oxygen levels or different predator-prey dynamics.
Different Mechanisms: Extinct species may have had unique mechanisms for withstanding the additional stresses of being large that are not seen in extant animals. This could involve novel bone structures, muscle arrangements, or physiological adaptations.

It may be that current methods of observing extant animals may not apply for extinct ones due to too many differences; therefore skewing data. The fossil record provides limited insight into soft tissues and physiological processes, making it challenging to fully understand these adaptations.

Size vs Speed

There is a trade-off between size and speed. Large size typically means reduced speed unless other adaptations optimize locomotion. These adaptations can include changes in muscle architecture, limb posture, or skeletal structure.

The mechanisms by which massive extinct species achieved high speeds are not fully understood. Further research and biomechanical modeling are needed to explore these adaptations.

It is now likely maladaptive to be very large and slow, given changes in the environment and ecology. Modern environments pose different challenges, such as human activity and habitat fragmentation, that may disadvantage slow-moving, large animals.