Magnetic Orientation Biology 342 Fall 06 |
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Adaptive ValueRoute adjustment: Sea Turtles
Migratory path of hatchling
loggerhead turtles [3]
Hatchling
sea turtles are often cited as performing one of the most spectacular
and arduous long-distance migration in the animal kingdom. The
young turtles must hatch from their eggs on the beach, travel across
the sand to reach the water, then fight the waves to make it to the
open sea. After reaching the open sea these hatchlings follow the
North Atlantic gyre in the Sargasso Sea before they return to the
southeastern US where their feeding grounds are located.
Hatchlings stay in the gyre for at least several years, often
making the circular loop several times. The route that the
hatchlings follow in the gyre is perfect for the needs of the
vulnerable turtles. The migratory route is situated in a very
resource-rich region of the sea, and straying from the route can be
fatal. There are points along the northern and southern legs
where the current branches off from the route and could lead hatchlings
into waters that are far too cold for their survival. The fact
that young turtles have a navigation system which allows them to
re-orient themselves to their correct route ensures that they will
survive through the years of circling the gyre. Without the ability to
readjust if swept astray, it is likely that such a vast migration of
hatchling sea turtles would not be possible.
Hatchling logger head turtles [3] Homing: Pigeons, Spiny Lobsters, and SalmonHoming is the ability to
orient toward a "home destination" or, more generally, return to an
original location. Homing pigeons are well known for their
ability to use homing to return to their lofts even after being
displaced over a 1000 km. While today this behavior is utilized by
humans for sport, it is thought that homing pigeons are a descendant of
Mediterranean rock doves which use homing most likely to return to
their nest full of young after a long period of feeding [13]. Homing
pigeons are thought to use a magnetic sense at least in part to find
their way back home. This ability to return home even after large
displacements is valuable especially if home is place of great safety
or if this is the location of offspring.
In the case of spiny lobsters, the first invertebrate discovered to use magnetic orientation, homing
is used to return to the rock crevices and holes that they call home.
The Caribbean spiny lobster is a nocturnal forager and will often
travel significant distances to feed. At the end of a feeding
session the lobsters must return in the dark to their protective homes.
Living in a good crevice significantly decreases the mortality
rate of spiny lobsters so being able to return to a known and
functional crevice ensures greater protection for predators once day
light comes [2].
Sockeye salmon Oncorhynchus nerka [15] A final example of an organism that uses a magnetic sense for homing is the sockeye salmon. Like all salmid fish the sockeye returns their natal streams in order to breed. Although this migration that can be thousands of kilometers is very energetically taxing, the protected spawning grounds are ideal for reproductive success. Further, fidelity to natal breeding sites has created genetic isolation and increased specialized adaptations to specific natal habitats [5,14]. |