Tuesdays with Turtles – Navigation

If you did not know already, sea turtles tend to nest on (or close to) the beaches they were born at. It is known that they use the Earth’s magnetic fields to navigate. Here’s an article on the navigational natations (I strain to alliterate at times, but natation means swimming!) of the Green Turtle.

ScienceDaily: How Do Marine Turtles Return To The Same Beach To Lay Their Eggs?

I flagged this article not because it tells us much new, but just adds some nuances to sea turtle navigation.

The study has shown that the marine turtles’ navigation system allows them to maintain their course towards the egg-laying site wherever they find themselves. It is almost as if they were equipped with a compass pointing towards the beach in question. So they can correct any deflection they are subject to: transport by boat, ocean currents… But, unlike human navigators, they are not able to correct for ocean drift in plotting their course. So the movements recorded by the satellite are a combination of deliberate action by the turtles and the effect of currents. So it appears that the turtles’ navigation system is relatively simple and may cause them to be wander at sea for long periods during adverse ocean conditions. One turtle released 250 km from its egg-laying site on Europa traveled more than 3 500 km in two months before returning there!

Well, that’s interesting, if not surprising. It’s one thing to have a magnetic bookmark of a destination in your head (pretty wonderful thing, I wish I had it!) and swim continually towards this destination. It’s quite another thing to keep track of complex parameters like ocean drift and keep correcting continuously.

In essence, what they’re saying is that turtles home in on their destinations, but don’t always take the shortest way in because they tend to drift and not correct for this drift dynamically. It is not evolutionarily necessary because turtles probably do not get thrown way of course often enough that evolving even more sophisticated navigational systems (Garmin?) would provide a significant enough survival advantage.

In the Mozambique Channel, between the east coast of Africa and Madagascar, on the beaches of the French Islands of Europa and Mayotte, they caught turtles at the beginning of their egg-laying cycle, so that the animals were strongly impelled to return to this area to complete their cycle. After having Argos transmitters fitted to their shells in order to satellite track their return journey to the beach, the animals were released in open sea, several hundred kilometers from the egg-laying site.

Now that’s just mean! Imagine being kidnapped, tagged and released many miles away from home, and having to find your way back. Apparently, these turtles did it. Man, that would be such a cool navigational aid to have, quite a party trick!

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