LEVELS OF NAVIGATIONAL
ABILITY :
Many animals often travel between home and a goal, but they do not all accomplish this feat in the same manner. We group animal strategies for finding their way into three levels of ability.
1. PILOTING :
One level is piloting, the ability to find a goal by referring to familiar landmarks. The animal may search either randomly or systematically for the relevant landmarks. Although we usually think of landmarks as visual, the guidepost may be in any sensory modality. As we will see shortly, magnetic cues guide sea turtles during their oceanic travels, and olfactory cues guide salmon during their upstream migration.
2. COMPASS ORIENTATION :
A second level, called compass orientation, is the ability to head in a geographical direction without the use of landmarks. The sun, the stars, and even the earth’s magnetic field may be used as compasses by many different species. One way to demonstrate that an animal is using compass orientation is to move it to a distant location and determine whether it continues in the same direction or compensates for the displacement. If it does not compensate for the relocation, compass orientation is indicated. When immature birds of certain migratory species, such as European starlings, were displaced experimentally, they flew in the same direction as the parent group that had not been moved, and they flew for the same distance (. In other words, they migrated in a path parallel to their original migratory direction. However, because they had been experimentally displaced before beginning their migration, they did not reach their normal destination. In some cases, this meant that they ended up in ecologically unsatisfactory places.
Uses for Compass Orientation :
Compass orientation can be used in different ways—in both short-distance and long-distance navigation.
Migratory Direction of Juvenile :
Birds Most first time migrant birds reach their destination without knowing where that goal is located. They are guided by an inherited program that tells the juveniles in which direction to fly and how long to fly. This innate program is sometimes called vector navigation. What observations have supported the idea of vector navigation? Individual birds held in the laboratory f lutter in the direction in which they would be flying if they were free. When their cousins in nature have completed their migratory journey, the captive birds also cease their directional activity. Furthermore, many species, particularly those that fly from Central Europe to Africa, change compass bearing during their flight. Garden warblers (Sylvia borin) and blackcaps (S. atricapilla) held in the laboratory change the direction in which they flutter in their cages at the time that freef lying members of their population change direction. Cross-breeding studies have also shown the inheritance of migratory direction. Andreas Helbig (1991) crossbred members of two populations of blackcaps that had very different migratory directions. The orientation of the offspring was intermediate between those of the parents. Indeed, migratory direction is inherited by the additive effects of a number of genes.
Path Integration :
Besides their use in long-distance navigation, compasses can be used to improve in another type of navigation, called path integration or dead reckoning. In path integration, the animal integrates information on the sequence of direction and distance traveled during each leg of the outward journey. Then, knowing its location relative to home, the animal can head directly there, using its compass(es). A compass may also be used to determine the direction traveled on each leg of the outward journey, or the direction may be estimated from the twists and turns taken, sounds, smells, or even the earth’s magnetic field. Information from the outward journey is used to calculate the homeward direction (vector). (Thus, some authors consider path integration to be a type of vector
navigation.) The estimates of distance and direction are often adjusted for any displacement due to current or wind. Once close to home, landmarks may be used to pinpoint the exact location of home. Many types of animals use path integration to find their way around. Consider, for example, the desert ant (Cataglyphis bicolor). During its foraging forays, this insect wanders far from its nest over almost featureless terrain. After prey is located, sometimes 100 meters away from the nest, roughly the distance of a football field, the ant turns and heads directly toward home. It appears that the ant knows its position relative to its nest by taking into account each turn and the distance traveled on each leg of its outward trip. If a researcher captures an ant as it is leaving a feeding station headed for home and relocates the ant to a distant site, the ant’s path is in a direction that would have led it home if it had not been experimentally moved. How does a desert ant determine the direction and distance of its outward route? The direction is determined using the pattern of polarization of skylight. Ants determine their direction by using the pattern of skylight polarization, which is caused by the sun’s position. Desert ants determine the distance they travel using a mechanism that integrates the number of strides required to reach the goal with stride length. Matthias Wittlinger and colleagues (2007) demonstrated this internal pedometer in a very clever way. As we all know, a person with longer legs requires fewer steps to reach a goal than does a person with short legs. Therefore, the researchers predicted that manipulating the length of ant’s legs would cause the ants to misestimate the distance to the nest. The researchers collected ants at an experimental feeder and manipulated the length of the ants’ legs. They lengthened the legs of some ants by attaching pig’s bristles to the ant’s legs, creating stilts. They shortened the legs of other ants by partial amputation. The ants walking on stilts overestimated the distance to the nest, whereas the ants with stubby legs underestimated the distance. An added complication to this means of calculating the distance traveled from home is that stride length varies with rate of travel. Thus, as remarkable as this stride counting might seem, the actual mechanism of distance determination also includes an estimation of stride length. Once at home, cues from inside the nest reset the path integrator to zero, so that it can be set again by the next outward journey.
Map and Compass :
A compass may also be used with a map to calculate a homeward path. Imagine yourself abandoned in an unfamiliar place with only a compass to guide your homeward journey. Before you could head home, you would also need a map so that you could know where you were relative to home. Only then could you use your compass and orient yourself correctly.
3. TRUE NAVIGATION :
A third level of orientation, sometimes called true navigation is the ability to maintain or establish reference to a goal, regardless of its location, without the use of landmarks. Generally, this implies that the animal cannot directly sense its goal and that if it is displaced while en route, it compensates by changing direction, thereby heading once again toward the goal. Only a few species, most notably the homing pigeon (Columba livia), have been shown to have true navigational ability. Certain other groups of birds, including oceanic seabirds and swallows, are also known to home with great accuracy, as do sea turtles. Interestingly, an invertebrate, the spiny lobster (Panulirus argus), also seems to have true navigation abilities.
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