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Moving around in the environment is critical to finding new sources of food, mates, and shelter. The simplest movements, kineses, simply involve speeding up if conditions are unsuitable and slowing down if conditions are suitable. Kinetic movements are not directed with respect to environmental stimuli, and consequently they require very simple sensory and neural systems.
The next level of orientation in local navigation comes with taxes. A taxis is directed with regards to a specific stimulus. An moth which flies towards a light is positively phototactic. A cockroach hiding from light is negatively phototactic. Any environmental stimulus whose source is directional can be used to orient using a taxis (light=phototaxis, gravity=geotaxis, heat=thermotaxis, etc.). Directional information for taxes can come from sequential comparisons, as when a worm first samples by moving its head to one side and then samples again by moving its head to the other side. Simultaneous comparisons can also be used; these require two sensory organs which are spatially separated so that triangulation can be used to determine the source of the stimulus (for example, paired eyes or ears).
Often straight movements are thwarted by obstacles (if the animal is an ant, these could be small pebbles, for larger animals, trees and boulders are effective barriers). How can an animal maintain movement in a relatively fixed direction when its path is frequently deflected? An interesting example of a simple mechanism used by many animals to maintain their course of movement is counterturning; each change of direction is balanced by a change in the opposite (counter) direction when it becomes possible to do so.
More complex movement patterns are derived by moving at an angle relative to the stimulus, rather than directly towards or away from the stimulus. Angled movements are useful because they can be used in homing. An animal which leaves its nest and moves at an angle with respect to a landmark or the sun needs to only reverse that angle to return to its nest; this is called compass orientation.
Bell, W. J. 1991. Search behaviour. Chapman and Hall:London.
Animals "home" when they return to a central place, such as their nest or their territory. Homing is a frequent activity, occuring after foraging bouts or other relatively local movements. This distinguishes homing from migration, which is a long distance movement between two distinct habitats. The key element to understanding homing behavior is determining which cues provide the directional information that allows animals to move between their home and other locations. Also, animals may use environmental information hiearchically, so that one type of homing information is important when they are distant from their home, and others become more important as they come closer to their goal. These examples show how different animals accomplish the task of homing.
Migrations allow animals to take advantage of habitats which are favorable in different seasons, or which are favorable for different stages in an animal's lifecycle. I use three examples to illustrate the complexities of migration behavior:
Animals disperse by leaving their natal area and finding new territories or home ranges. Dispersal is a topic in both behavior and ecology; many of the ecological principles concerning dispersal apply to both plants and animals, but of course here I only talk about animal dispersal. There are five key causes of dispersal: competition, inbreeding avoidance, kin-competition avoidance, breeding dispersal, and colonization dispersal.
Dobson, F. S. 1982. Competition for mates and predominantly juvenile male dispersal in mammals. Anim. Behav. 30:1183-1192.
copyright ©2001 Michael D. Breed, all rights reserved