Kinesis is a fundamental biological concept describing how an organism responds to changes in its environment through movement. It is a behavioral response where an organism adjusts its activity level, such as speed or turning rate, when exposed to a stimulus like light, temperature, or humidity. This orientation behavior helps the organism aggregate in areas that are more favorable for its survival.
Defining Kinesis
Kinesis is formally defined as a non-directional movement response to the intensity of a stimulus, rather than its direction. The movement remains random; the organism is not moving toward or away from the stimulus source. Instead, the organism regulates its movement to spend more time in beneficial environments and less time in unfavorable ones. The core mechanism involves a change in the animal’s rate of movement proportionate to the strength of the external factor. For instance, an unpleasant stimulus, like dryness, prompts increased random movement, raising the probability of the organism finding a moist area. This simple behavioral mechanism is highly effective for organisms that lack the sensory capabilities for true directional navigation.
The Two Forms of Kinesis
Orthokinesis
Orthokinesis is a change in the speed or frequency of the organism’s movement in relation to the intensity of the stimulus. An organism exhibiting orthokinesis moves quickly when it encounters an unfavorable condition, but slows down significantly, or even stops, once it enters a favorable zone. This change in speed directly controls the amount of time the organism spends in a particular area.
Klinokinesis
Klinokinesis involves a change in the rate of turning or the frequency of direction changes. The organism maintains a relatively constant speed but turns more frequently when the stimulus intensity is high, indicating poor conditions. Conversely, it turns much less often when the stimulus intensity is low, moving in straighter paths and increasing the chances of staying within the desirable area.
Kinesis vs. Taxis
Kinesis requires a clear distinction from taxis, which is a more complex and directional movement. Taxis is movement where the organism moves directly toward or away from the source of the stimulus. For example, positive phototaxis describes movement toward a light source, while negative chemotaxis describes movement away from a specific chemical concentration. This requires the organism to sense the stimulus gradient and orient its body axis relative to the source.
Kinesis, by contrast, is a simpler, non-directional response that does not require the organism to sense the stimulus gradient or orient itself. The organism simply changes its activity level based on whether the stimulus intensity is high or low. The resulting movement is random, yet the behavioral strategy leads to adaptive aggregation in the preferred environment.
Real-World Biological Examples
A classic example of kinesis is the behavior of woodlice, also known as pillbugs, in response to humidity, a behavior termed hygrokinesis. Woodlice require high humidity to prevent desiccation. When a woodlouse moves from a damp area into a dry one, it responds to the decrease in humidity by increasing its movement speed, a display of orthokinesis. This faster movement increases the likelihood that it will quickly leave the dry, unfavorable area.
The woodlouse also increases its rate of turning when it detects dryness, demonstrating klinokinesis, which helps prevent it from traveling far in a straight line. Once the animal re-enters a moist environment, its speed and turning rate decrease, causing it to stay put in the favorable microhabitat. Another example involves the flatworm Dendrocoelum lacteum, which exhibits klinokinesis by turning more frequently when exposed to increasing light intensity, ensuring it remains aggregated in darker areas.