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Natal homing, or natal philopatry, is the homing process by which some adult that have animal migration away from their juvenile return to their birthplace to reproduce. This process is primarily used by such as and salmon, although some and also practice similar reproductive behaviors. Scientists believe that the main cues used by the animals are geomagnetic imprinting and olfactory cues. The benefits of returning to the precise location of an animal's birth may be largely associated with its safety and suitability as a breeding ground. When like the Atlantic puffin return to their natal breeding colony, which are mostly on islands, they are assured of a suitable climate and a sufficient lack of land-based predators.
Sea turtles born in any one area differ genetically from turtles born in other areas. The newly hatched young head out to sea and soon find suitable feeding grounds, and it has been shown that it is to these feeding areas that they return rather than to the actual beach on which they started life. Salmon start their lives in freshwater streams and eventually travel down-river and are washed out to sea. Their ability to travel back, several years later, to the river system in which they were spawned is thought to be linked to olfactory cues, the "taste" of the water. Atlantic bluefin tuna spawn on both the east and west shores of the Atlantic Ocean but intermingle as they feed in mid-ocean. Juvenile tuna that have been tagged have clearly shown that they almost invariably return to the side of the Atlantic on which they were spawned.
Various theories have been put forward as to how the animals find their way home. The geomagnetic imprinting hypothesis holds that they are imprinted with the unique magnetic field that exists in their natal area. This is a plausible theory but has not been proven to occur. Pacific salmon are known to be imprinted on the water chemistry of their home river, a fact that has been confirmed experimentally. They may use geomagnetic information to get close to the coast and then pick up the olfactory cues. Some animals may make navigational errors and end up in the wrong location. If they successfully breed in these new sites, the animal will have widened its breeding base which may ultimately increase the species' chances of survival. Other, unknown means of navigation may be involved, and further research is needed.
Turtles of a specific natal beach show differences in their mitochondrial DNA haplotypes that distinguish them from turtles of other nesting areas.(Bowen, 2004) Many turtles from the same beaches show up at the same feeding areas. Once reaching sexual maturity in the Atlantic Oceans, the female Loggerhead makes the long trip back to her natal beach to lay her eggs. The Loggerhead sea turtle in the North Atlantic cover more than 9,000 miles round trip to lay eggs on the North American shore.
One hypothesis is that they use both chemical and geomagnetic cues that allow them to return to their birthplace. The Earth's magnetic field may help the fish navigate the ocean to find the spawning region. From there, the animal locates where the river dumps into the sea with the chemical cues unique to the fish's natal stream.(Lohmann, 2008)
Other hypotheses rely on the fact that salmon have an extremely strong sense of smell. One hypothesis states that salmon retain an imprint of the odor of their natal stream as they are migrating downstream. Using this memory of the odor, they are able to return to the same stream years later. Another smell-related hypothesis states that the young salmon release a pheromone as they migrate downstream, and are able to return the same stream years later by smelling the pheromone they released.
In animal behavior, the term "imprinting" refers to a special type of learning. Exact definitions of imprinting vary, but important aspects of the process include the following: (1) the learning occurs during a particular, critical period, usually early in the life of the animal; (2) the effects last a long time; and (3) the effects cannot be easily modified. For natal homing, the concept is that animals like sea turtles and salmon imprint on the magnetic field of their home area when young, and then use this information to return years later.
Geomagnetic imprinting has not been proven to occur, but it appears to be plausible for several reasons. The earth's magnetic field varies across the globe in such a way that different geographic areas have different magnetic fields associated with them. Also, sea turtles have a well-developed magnetic sense and can detect both the intensity (strength) of the Earth's field as well as the inclination angle (angle at which the field lines intersect the earth's surface). Thus, it is plausible that sea turtles, and maybe salmon also, can recognize their home areas using the distinctive magnetic fields that exist there.
Many of the classical studies demonstrating olfactory imprinting in salmon were carried out by Arthur Hasler and his colleagues. In one particularly famous experiment, young salmon were imprinted with artificial chemicals and were released into the wild to perform their normal migrations. Almost all of the young fish returned to the same stream that had also been artificially imprinted with the same chemicals, proving that the fish do use chemical cues to return to their natal region.
These navigational errors have actually strengthened the evolutionary trait of natal homing for marine animals by resulting in some animals straying from their birthplace. Most animals return to their natal region because they know it is a safe place to lay their eggs. These regions will usually have few predators, the correct temperature and climate, and will have the right type of sand for turtles because they cannot lay eggs in wet and muddy environments.
The few animals that do not return to their natal region and stray to other places to reproduce will provide the species with a variety of different locations of reproduction, so if the original natal locations have changed, the species will have expanded to more places and will ultimately increase the species' survival chances.
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