Adaptive Value

Route 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 Salmon

Homing 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.  

   Caribbean spiny lobster Panulirus argus [3]
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].