Locomotor Behavior: How Animals Move and Adapt

Locomotor behavior is the active, self-propelled movement of an organism from one location to another. This capacity for movement is fundamental to most animal species, serving survival-related functions including finding food, seeking mates, and escaping predators. The expression of this behavior is diverse, shaped by an animal’s physical form and the environment it inhabits, representing a direct interaction with its surroundings.

Classifications of Locomotion

Movement on land, or terrestrial locomotion, contends with gravity, requiring robust skeletal and muscular systems. Walking is a common form, while running is a faster gait exemplified by the cheetah, which utilizes elastic energy in its tendons. Hopping is used by kangaroos, characterized by powerful hind limbs and a long tail for balance. Crawling and slithering, seen in snakes, involves using the body surface to generate friction for propulsion.

Aquatic locomotion involves moving through water, a dense medium that presents drag. Fish use body and caudal fin (BCF) propulsion, creating undulations to generate thrust, from an eel’s whole-body wave to a salmon’s tail-focused movement. While fish undulate laterally, marine mammals like dolphins flex their spines vertically due to their terrestrial ancestry. Other animals, like sea turtles, use flippers as hydrofoils to generate lift-based thrust.

Aerial locomotion is divided into powered flight and gliding. Powered flight, seen in birds and bats, uses flapping wings to generate lift and thrust, requiring specializations like lightweight bones and powerful flight muscles. Gliding is an unpowered descent where an animal like a flying squirrel uses a membrane or flattened body to travel horizontally while falling. This is an energy-efficient method for moving between trees.

Neurological Control of Movement

The generation of rhythmic movements like walking or swimming originates from specialized neural circuits known as Central Pattern Generators (CPGs). These networks of neurons are located within the spinal cord and brainstem. CPGs can produce coordinated, rhythmic output signals without requiring continuous sensory input or commands from the brain for every movement. They function like an embedded motor program that automatically generates the basic sequence of muscle contractions, such as the alternating movement of legs during walking.

These CPGs are responsible for the fundamental rhythm and pattern of locomotion, coordinating the timing of muscle groups. In quadrupedal animals, for instance, CPGs in the cervical and lumbar regions of the spinal cord control the forelimbs and hindlimbs, respectively. Neural feedback between the circuits controlling the left and right limbs ensures stable and efficient gaits.

While CPGs create the foundational rhythm, higher brain centers provide voluntary control. The motor cortex initiates, stops, and modifies movements by sending signals to the CPGs to adjust speed or change direction. Neurons in the motor cortex also encode parameters like force and direction, allowing for precise, goal-oriented actions.

The Role of Sensory Feedback

Although CPGs can generate movement patterns without sensory input, locomotion in a variable world depends on the continuous refinement of these patterns through sensory feedback. This process allows an animal to adapt its movements in real-time to navigate complex and unpredictable environments. The interaction between pre-programmed motor outputs and incoming sensory information ensures that movement is both stable and responsive.

A primary source of this feedback is proprioception, the body’s internal sense of its own position and movement in space. Sensory receptors in muscles, tendons, and joints provide constant information to the central nervous system about limb position and the forces being exerted. This feedback allows an animal to instantly adjust its footing on an uneven surface or to modify the force of its leg extension on different inclines.

The vestibular system, located in the inner ear, is another source of sensory information. This system detects head movements and accelerations, providing the brain with information about balance and spatial orientation. As an animal moves, the vestibular system sends signals that trigger reflexive adjustments in posture to maintain stability. Complementing these internal senses is vision, which allows an animal to identify obstacles, plan a path, and make adjustments to its trajectory.

Developmental Stages of Locomotion

The ability to move is not fully formed at birth but develops over an organism’s lifespan in a process known as ontogeny. This progression results from both the physical maturation of the nervous and musculoskeletal systems and learning through experience. The development of locomotion in human infants provides a clear example of this sequential acquisition of motor skills. Early movements include involuntary reflexes, such as the stepping reflex observed in newborns.

As the infant’s nervous system and muscle strength develop, these early reflexes give way to more controlled, voluntary actions. The infant learns to support its own weight, leading to the milestone of crawling. This stage involves mastering the complex coordination of moving all four limbs in a reciprocal pattern. Crawling allows the infant to explore its environment independently for the first time, a significant step in cognitive and social development.

The final major stage in this sequence for humans is the transition to bipedal walking, which requires greater balance, coordination, and strength than crawling. Learning to walk is a gradual process of trial and error, where the infant progressively refines its balance and gait. This developmental sequence illustrates how complex locomotor behaviors are built upon simpler foundational abilities through biological maturation and practice. Similar progressions, adapted to different body plans and environments, are seen across the animal kingdom.