45.6B: Movement and Migration
- Distinguish between kinesis, taxis, and migration in response to stimuli
Innate behaviors: movement and migration
Innate or instinctual behaviors rely on response to stimuli. The simplest example of this is a reflex action: an involuntary and rapid response to stimulus. To test the “knee-jerk” reflex, a doctor taps the patellar tendon below the kneecap with a rubber hammer. The stimulation of the nerves there leads to the reflex of extending the leg at the knee. This is similar to the reaction of someone who touches a hot stove and instinctually pulls his or her hand away. Even humans, with our great capacity to learn, still exhibit a variety of innate behaviors.
Kinesis and taxis
Another activity or movement of innate behavior is kinesis: undirected movement in response to a stimulus. Orthokinesis is the increased or decreased speed of movement of an organism in response to a stimulus. Woodlice, for example, increase their speed of movement when exposed to high or low temperatures. This movement, although random, increases the probability that the insect spends less time in the unfavorable environment. Another example is klinokinesis, an increase in turning behaviors. It is exhibited by bacteria such as E. coli which, in association with orthokinesis, helps the organisms randomly find a more hospitable environment.
A similar, but more-directed version of kinesis is taxis: the directed movement towards or away from a stimulus. This movement can be in response to light (phototaxis), chemical signals (chemotaxis), or gravity (geotaxis). It can be directed toward (positive) or away (negative) from the source of the stimulus. An example of a positive chemotaxis is exhibited by the unicellular protozoan Tetrahymena thermophila . This organism swims using its cilia, at times moving in a straight line and at other times making turns. The attracting chemotactic agent alters the frequency of turning as the organism moves directly toward the source, following the increasing concentration gradient.
Migration as innate behavior
Migration is the long-range seasonal movement of animals. An evolved, adapted response to variation in resource availability, it is a common phenomenon found in all major groups of animals. Birds fly south for the winter to get to warmer climates with sufficient food, while salmon migrate to their spawning grounds. The popular 2005 documentary March of the Penguins followed the 62-mile migration of emperor penguins through Antarctica to bring food back to their breeding site and to their young. Wildebeests migrate over 1800 miles each year in search of new grasslands.
Although migration is thought of as an innate behavior, only some migrating species always migrate (obligate migration). Animals that exhibit facultative migration can choose to migrate or not. Additionally, in some animals, only a portion of the population migrates, whereas the rest does not migrate (incomplete migration). For example, owls that live in the tundra may migrate in years when their food source, small rodents, is relatively scarce, but not migrate during the years when rodents are plentiful.
Key Points
- Innate behaviors are instinctual, relying on responses to stimuli.
- Kinesis is the undirected movement in response to a stimulus, which can include orthokinesis (related to speed) or klinokinesis (related to turning).
- Taxis is the directed movement towards or away from a stimulus, which can be in response to light (phototaxis), chemical signals ( chemotaxis ), or gravity (geotaxis).
- Migration is an innate behavior characterized as the long-range seasonal movement of animals; it is an evolved, adapted response to variation in resource availability.
- Migration is a variable innate behavior as some migrating species always migrate (obligate migration) while in other animals, only a portion of the population migrates (incomplete migration).
Key Terms
- orthokinesis : the speed of movement of the individual is dependent upon the intensity of the stimulus
- taxis : the movement of an organism in response to a stimulus; similar to kinesis, but more direct
- kinesis : the undirected movement of an organism in response to an external stimulus