Database Reference
In-Depth Information
reproduction, at a certain time of their seasonal activity (e.g., bird migrations),
life cycle (i.e., eel, or atlantic salmon,
Salmo salar
migrations) or life history
across generations (i.e., monarch butterfly,
Danaus plexippus
). When individuals
have reached the final destination of their long displacements, they range over
shorter distances to select and use local resources.
At the other end of the movement behavior continuum, and very commonly,
animal species show a
sedentary
behavior, that is, they occupy a
home range
.
Use of space, and therefore movement, is tightly linked to the use of resources:
the former ecological concept cannot be understood without taking into account
the latter. Many animals use stable refuge for egg laying, rearing the juveniles,
accumulating reserves, overriding unfavorable climatic conditions, and so forth.
These organisms alternate the use of the refuge with excursions in the external
environment where they search for resources (e.g., food and mates). Clearly
for these organisms, it is vital to recover quickly and safely their refuge. The
homing pigeon (
Columba livia
) represents the paradigmatic example of this
behavior. For homing successfully an animal needs two tools: a map to know
its own position (with respect to home) and a mechanism (usually a taxis) to
move in the right direction. Many orienting mechanisms based on different cues
(sun, moon, stars, magnetic field, light polarization, and so forth) have been
demonstrated, although the maps used in animal navigation are more elusive.
The earth's magnetic field can be used as a global map. At shorter distances it
is indeed possible to use olfactory maps, and in the area usually explored by
animals, a memory-based landmark map can be effective. Landmarks can be of
different kinds, and usually these are naturally present in the environment, but
sometimes these are pheromones purposely laid by the animals themselves, such
as in trail-following of ants, snails and butterflies.
Navigation
, the ability to use
“compasses and maps,” has been demonstrated for several species of animals in
different taxonomic groups.
Another broad line of research on animal movement is represented by the
use of space outside the refuge, if any, by the animal. When an organism
faces contradictory requirements while exploiting an environment, a trade-off
between the needs of minimizing risks and maximizing resource acquisition
exist. Thus, the available space is not used at random; some areas are preferred
and others avoided. The ranging movements of the animal are therefore led
by the optimal use of available space and resources and are constrained by
both physical (e.g., presence of natural obstacles) and biological (presence of
competitors and predators) factors. The resulting area is defined as home range,
or as territory, depending on whether it is defended or not against intruders.
Many methods for computing home range size have been proposed. Now
there is a general agreement that kernel density distribution methods represent
an appropriate approach for describing the structure of home ranges. Quite
recently, an innovative approach has been proposed based on the formulation
