Lesson 15: How little turtles know where to go Flashcards
migration in the north Atlantic gyre
- evidence that turtles use positional (map) information (not just compass information) from the Earth’s magnetic field
where does map information and earth’s magnetic field come from
the magnetic inclination angle and the magnetic intensity
where does compass information come from
the directionality (polarity) of the field lines
- inclination angle gets steeper as you move toward the poles
- intensity gets stronger as you move toward the poles
approaching the north pole
large inclination angle
near the equator
small inclination angle
isoline
is a line that we draw connecting all the points that are the same
- ex: everywhere along this (in the picture) inclination angle is the same: 60 degrees
- the line is called an isoline of inclination or an isocline
magnetic coil system
- with this coil system, we were able to control the INTENSITY and INCLINATION at the same time
the north atlantic gyre test
testing the idea that turtles can sense inclination angles and use them to figure out location
— 60 degree isoclinic in red at northern edge of gyre
– 30 degree isoclinic in red at the southern edge of gyre
response to the 60 degree inclination angle
- most went south
response to 30 degree inclination angle
- most went east to northeast
– east didn’t seem right – we only changed inclination – maybe they needed the field to be more realistic – not the full reaction
main take away from the inclination experiments
hatchling loggerheads can distinguish between different magnetic inclination angles and respond to them in ways that make sense
north Atlantic gyre testing: intensity
testing the idea that turtles can sense magnetic intensity to figure out their location
isolines of equal magnetic intensity
- intensity also increases as you go north and forms isolines
- everywhere along the line indicated – the intensity is the same: 45000 nt>
^^ this line is called isodynamic or isoline of intensity
isodynamic
isoline of intensity
tilt of lines
intensity lines are tilted relative to inclination lines so individual places in the ocean have unique combinations of intensity and inclination - each place has its own magnetic address
experiment
either varied just intensity or just inclination while keeping other parameters constant
- this produced combinations of inclination and intensity that don’t actually exist in nature
RESULTS
- high intensity more on west side, turtles swam east
- but if given high intensity on the other side of the gyre, ten turtles swam west
- can sense intensity – go in the directions that make sense
- hatchlings responded to different magnetic addresses in ways that kept them n the gyre system
innate?
- the hatchlings had never been in the water so the gyre migration responses are clearly innate
BUT - we also know that turtles are capable of doing more than just responding to innate signals about position
experimetn: can turtles use the magnetic field to find specific places along the US coast
- the north-south coastline intersects with the patterns of isolines so every place along the coast is marked by its own inclination angle and its own intensity
- SO THEORETICALLY a turtle could just swim north or south along the coast until it hit the magnetic field of the specific location
magnetic field experiment set up
to test if turtles can find specific locations using magnetic fields, we used ocastal juveniles (teens) that had a feeding area in Melbourne Beach, FL. Then we made a really big magnetic coil system
- captured the teens from the reef in melbourne beach where they feed
- tehtered them up in a tank in melbourne beach (near capture site)
- tehter was connected to electronics that recorded the direction the turtle was swimming
- the elcetronics and coil were controlled from computers inside
- used computers to change magnetic field and monitor swimming direction
- and then we gave them the magntic field (inclination and intensity off Jekyll Island off the Georgia coast)
OR - gave them the magneticfield (inclination and intensity) of Tavernier Key in south flordia
CONCLUSION
- juvenile green turtles have a geomagnetic positioning system that assists them in navigating to specific locations
—- in other words — they have the basics of a magnetic map!!!!!!!
- up near jekyll island, went south
- down near tavernier key, went north
what were the 2 islands
jekjyll island (north) and tavernier key (south)
what about adult turtles
- female sea turtles show Natal Homing in which they come back to the nest on or near the same beaches where they themselves hatched
Geomagnetic imprinting hypothesis
suggests that little turtles learn the magnetic address of their home beach and come back to it years later as adults
why is testing the geomagnetic imprinting hypothesis a tough problem
- life cycle takes a minimum of 20 years so no good way to tag them and wait for them to come back
over time what can happen to the magnetic field lines
they can wobble and move a little
converging isolines
increased nest density
diverging isolines
decreased nest density
converging isoclinics
increased nest density relative to average
diverging isoclinics
decreased net density relative to average
20 years later
calculated the nesting density for each country in Flordia over the last 20 years and matched it to whether or not the field lines and converged
- there would be no affect of secular variation on nest density if turtles were not using magnetic field to find their favorite beach
