Animal movements are direct responses to environmental changes, enabling them to find food, avoid predators, locate mates, and seek suitable habitats. Detecting a stimulus and reacting by moving is fundamental to an animal’s survival and reproduction.
Taxis
Taxis refers to a directed movement or orientation of an organism in response to an external stimulus. If an animal moves towards the stimulus, it is called positive taxis; if it moves away, it is negative taxis. This directional response is a precise way for organisms to navigate their environment.
Phototaxis is movement in response to light; moths exhibit positive phototaxis towards light, while cockroaches demonstrate negative phototaxis away from it. Chemotaxis involves responding to chemical signals, such as bacteria moving towards nutrients or away from toxins to locate resources and avoid danger.
Geotaxis involves movement in response to gravity. Some aquatic microorganisms display positive geotaxis by swimming downwards, while others show negative geotaxis by moving upwards in the water column. Thigmotaxis is the response to physical contact. Many insects, like cockroaches, exhibit positive thigmotaxis by seeking out tight spaces and maintaining contact with surfaces, providing a sense of security.
Rheotaxis describes directed movement in response to a current of water or air. Fish often exhibit positive rheotaxis, swimming against the current to maintain position and conserve energy. This behavior also aids migration in species like salmon.
Kinesis
Kinesis is a non-directed or random movement of an organism in response to the intensity of an external stimulus. Unlike taxis, the organism does not move towards or away from the stimulus in a specific direction. Instead, it changes its speed of movement or frequency of turning, which helps it remain in favorable conditions or leave unfavorable ones. This response is about adjusting activity levels rather than directional navigation.
There are two main types of kinesis. Orthokinesis involves a change in the speed of an organism’s movement based on the stimulus intensity. For instance, woodlice, which prefer humid environments, will move faster in dry conditions and slow down or become less active when they encounter higher humidity. This increased movement in unfavorable conditions raises the probability of the woodlouse finding a more suitable, moist area.
Klinokinesis refers to a change in the frequency or rate of turning in response to stimulus intensity. An example is the behavior of flatworms, which turn more frequently when exposed to increasing light. This higher turning rate in bright areas means they spend more time in darker, preferred environments, as each turn allows them to sample a new direction. Bacteria like E. coli also exhibit klinokinesis by increasing their tumbling frequency in less favorable chemical gradients, allowing them to randomly explore and eventually find more hospitable environments. This undirected exploration helps organisms aggregate in optimal zones without needing to sense a specific direction.