Organisms must constantly monitor their surroundings to survive and reproduce. Behavioral ecology examines how innate behaviors are shaped by natural selection to increase fitness. A fundamental way organisms interact with their environment is through movement responses to external factors like light, temperature, or chemicals. These automatic movements allow a creature to place itself in a more favorable location or escape from a harmful one. These responses are classified into two broad categories based on whether the movement is directed by the stimulus’s location or simply by its intensity.
Taxis: Directed Movement
Taxis is a movement that is always directional. The organism moves either directly toward or directly away from the source of a stimulus. The orientation of the organism’s body is precisely guided by the stimulus. Movement toward the stimulus is termed positive taxis, while movement away is negative taxis.
A common example is phototaxis, which is movement in response to light. Single-celled organisms like Euglena exhibit positive phototaxis, swimming toward the light source to maximize photosynthesis. Chemotaxis involves movement dictated by a chemical gradient, such as when a bacterium moves toward a higher concentration of a nutrient. Conversely, a mosquito flying away from a chemical repellent demonstrates negative chemotaxis.
Kinesis: Non-Directed Movement
Kinesis is a non-directional movement where the rate or frequency of activity changes in response to the overall intensity of a stimulus, not its location. The organism does not orient itself toward or away from the stimulus source. It simply changes its movement pattern when conditions become unfavorable, increasing the probability of eventually leaving a poor environment.
Kinesis can be divided into two main subtypes based on the change in movement.
Orthokinesis
Orthokinesis is a change in the speed of movement; the organism moves faster in unfavorable conditions and slows down when it finds a preferred environment.
Klinokinesis
Klinokinesis involves a change in the frequency of turning or directional changes. For instance, a small flatworm may increase its turning rate when exposed to bright light, moving randomly until it finds a dark, shaded area.
Key Distinctions and Survival Examples
The fundamental difference between taxis and kinesis rests on the role of directionality. Taxis is an orientation-based response, directed by the spatial location of the stimulus. Kinesis is a rate-based response, where the organism’s speed or turning frequency is modulated by the intensity of the stimulus, without reference to its source.
These distinct behaviors serve different survival strategies. A moth displaying positive phototaxis moves in a straight line toward a distant light source. This directed movement ensures it reaches a specific point, often for navigation or mating. In contrast, a woodlouse exhibits orthokinesis in response to humidity, moving quickly on a dry surface but slowing down significantly in damp areas. This non-directional change in speed effectively keeps the woodlouse in the moist environment it requires.