Natal homing

Natal homing is the homing process by which some adult animals return to their birthplace to reproduce.

Sea Turtles
There are several different kinds of marine animals that demonstrate natal homing. The most commonly known is the sea turtle. These Loggerhead sea turtles are thought to show two different types of homing. The first of which comes in the early stages of life. When first heading out to sea, the animals are carried out by tides and currents with little swimming involved. Recent studies now show that the animals demonstrate homing to feeding grounds near their natal birth place.

Turtles of a specific natal beach show differences in their mitochondrial DNA haplotypes that distinguish them from turtles of other nesting areas. 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.

Salmon
The migration of North Pacific Salmon from the Ocean to their freshwater spawning habitat is one of the most extreme migrations in the animal kingdom. The life cycle of a salmon begins in a freshwater stream or river that dumps into the ocean. After spending four or five years in the ocean and reaching sexual maturity, many salmon return to the same streams they were born in to spawn. There are several theories on how salmon are able to do this.

One theory 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.

Other theories rely on the fact that salmon have an extremely strong sense of smell. One theory 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 theory 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.

Blue-fin Tuna
Atlantic bluefin tuna spawn on both the east and west shores of the Atlantic Ocean. When a Blue-fin Tuna hatches, there is a chemical imprint in the animal's otoliths based on the water's chemical properties. Fish born in different regions will show clear differences here. Studies of the commercial fishing industry in the United States show that the population of Blue-fin Tuna in the North Atlantic is made up fish hailing from both coasts. While the fish may live in close proximity out in the Atlantic, they return to their natal region to spawn. Electronic tagging done over several years showed that 95.8% of the yearlings tagged in the Mediterranean Sea returned there to spawn. Results for the Gulf of Mexico were 99.3%. With the overfishing of this species, scientists have much to learn about their spawning habits in order to sustain the population for both a reliable food source and a healthy ecosystem.

Geomagnetic Imprints
Natal homing is, in theory, made possible by something called geomagnetic imprinting. The theory is that the animal has a geomagnetic map imprinted in their brain guiding them. Animals that stray are believed to do so because of a poor imprint. This poor imprint, and straying, could be what sustains these species for so long. There are still uncertainties how these animals can migrate hundreds of miles back to where they were born for them to lay their offspring. Scientists believe they have a couple different navigational tools, but they primarily find their way from using the Earth’s magnetic field which is geomagnetic imprinting. Although they use the Earth's magnetic field, scientists argue it due to abrupt and somewhat unpredictable changes in the Earth’s field, these animals’ ecological processes and ultimately the population may be altered. The Earth’s magnetic field strength is very inconsistent and the polarity even flips every 200,000 years on average and would affect the natal homing characteristics of many marine animals.

Geomagnetic imprinting is believed to be performed by sea turtles and salmon by imprinting on either the intensity or the inclination angle of the Earth’s magnetic field at their birthplace. Although these animals have the ability to imprint on the Earth’s magnetic field, there are still some navigational errors that occur due to either the poor imprinting or from occasional changes in the magnetic field’s strength or angle. The inclination angle of the Earth's magnetic field is recorded by the imaginary lines the field creates, where the lines are parallel to the Earth at the equator and are perpendicular to the landscape of the north and south poles. Between the inclination angle and the intensity of the magnetic field, every location on the Earth has a unique geomagnetic imprint.

Chemical Cues
Chemical cues have been proven to be a reliable navigational tool for marine animals by scientists experimenting on young salmon. The 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.

Effect of Thermal Pollution on Natal Homing (Chum Salmon)
Thermal pollution, which refers to the degradation of water quality by changing the ambient water temperature, has a serious effect on natal homing of Chum salmon. Chum salmon is a typical cold water fish that prefer water around 10 oC. When water temperature is raised due to thermal pollution, Chum salmon tends to dive into deep water for thermoregulation. This reduces the time Chum salmon spent in surface water column and reduce the chance for Chum salmon to approach natal river since chemical cue for natal homing is concentrated on surface water.

Evolution
It has been studied and recorded by scientists that at a beach in eastern Mexico, where Kemp’s ridley turtles nest, a navigational error from the inclination angle over a period of one decade would lead the turtles only within an average of 23 kilometers from their natal region. Other locations resulted in navigational errors of over one hundred kilometers in the same period of time. Results from this study show that the navigational tool of geomagnetic imprinting is believed to only navigate the marine animals close to where they were born and then the animals rely on chemical cues of the tributaries and rivers to direct them to back to their birthplace. 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 repoduce 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.

Forward
Although scientists have been studying marine animals that perform natal homing for years, they are still not positive that geomagnetic imprinting and chemical cues are the only navigational tools they use for their incredible migrations. There is still much more research to be done until scientists can fully understand how these animals can travel such great distances to reproduce. Fortunately as technology has progressed there are several tools available to scientists now such as data loggers equipped with magnetometers that can easily be attached to the animals. Not only do they give data showing the animal relative to the Earth's magnetic field but some also give latitude based on this, longitude based on light levels, temperature, depth, etc. Pop-up satellite archival tag are used to gather data anhave the ability to transfer this data via Argos System satellites to the scientist.