A new paper in the Proceedings of the Royal Society B by Mansfield et al. reports the first long-term tracks of newly hatched loggerhead sea turtles after they enter the ocean.
Here is the abstract:
Abstract
Few at-sea behavioural data exist for oceanic-stage neonate sea turtles, a life-stage commonly referred to as the sea turtle ‘lost years’. Historically, the long-term tracking of small, fast-growing organisms in the open ocean was logistically or technologically impossible. Here, we provide the first long-term satellite tracks of neonate sea turtles. Loggerheads (Caretta caretta) were remotely tracked in the Atlantic Ocean using small solar-powered satellite transmitters. We show that oceanic-stage turtles (i) rarely travel in Continental Shelf waters, (ii) frequently depart the currents associated with the North Atlantic Subtropical Gyre, (iii) travel quickly when in Gyre currents, and (iv) select sea surface habitats that are likely to provide a thermal benefit or refuge to young sea turtles, supporting growth, foraging and survival. Our satellite tracks help define Atlantic loggerhead nursery grounds and early loggerhead habitat use, allowing us to re-examine sea turtle ‘lost years’ paradigms.
This is a fascinating example of how informative simple observation can be. Consider the mystery of the sea turtle life cycle. The babies hatch from nests on the sandy beaches of the eastern seaboard of the U.S. and various Caribbean islands. Upon hatching they make their way to the sea, swim far offshore, and are lost from view. Bigger turtles can be observed at sea, caught in fishing nets. Some can be caught and sampled, and their DNA compared with turtle populations from around the world. This suggest that turtles from the eastern U.S. somehow make their way to the other side of the Atlantic , even as far as the Azores, as they mature. Then years after hatching, adults return to the same beach where they were born. Mothers haul themselves ashore, dig a nest in the sand, and lay dozens of leathery eggs.
What exactly happens in the years between first entry into the ocean and the return to the beach where hatching occurred. Do they drift passively? Swim in particular direction? Some combination? If only we could just ride along and watch?
That, in effect, is what this recent paper has tried to accomplish, at least for the first several months in a hatchling’s life. The study used solar-powered satellite transmitters which are small enough to ride along on the carapace of the turtle, and powerful enough to send a signal that reports where the turtle is, if not let us actually see the world from the turtle’s perspective. Still, knowing where the turtle is tells us a lot.
The study reports tracks from 17 turtles that hatched on a beach in northeastern Florida and then were tracked for up to 220 days, for distances ranging from 200 to 4300 kilometers. See the tracks superimposed on maps of the North Atlantic color coded to show variation in ocean depth (top) and sea surface temperature (bottom):
Notice that the tracks (the white filaments in the upper map, the dark filaments in the lower) initially follow the course of the gulf stream, which may suggest that the turtles simply drift with prevailing currents.
Over the longer term the tracks show that turtles depart from the Gulf Stream, and not in random directions. Instead they move east and south rather than north or west, which takes them into the “North Atlantic Subtropical Gyre,” the great swirling pool of water in the western North Atlantic. That they move in this direction when departing the Gulf Stream is an essential step in their life cycle, because it moves them toward the vast Sargassum seaweed beds in the Gyre that will be their feeding and sheltering grounds as they grow. Let’s have a closer look of one of the turtles as it veers away from the Gulf Stream:
The paths relative to ocean currents suggest that the turtles are not only passively drifting (though they may do a lot of that), but also making directed movements.
Other evidence about the turtle and their Umwelt comes from the tags: the batteries stayed fully charged. Since the power for the charging came from solar cells, this suggests that the turtles spent a lot of time near the surface rather than diving down to swim or feed. This may have been to aid swimming (because of the drag from the tags on their back) or, more likely, to bask in the sun’s warmth.
How do they do this? How do the turtles guide themselves on a course that would have defeated many human mariners during our history of oceanic exploration? What is it like to be a loggerhead turtle during its life cycle?
More specifically, what features of the environment can tell a turtle, bobbing in the vastness of the ocean, far out of sight of land and carried an unknown distance by the mighty Gulf Stream, which direction to turn to head toward the Sargasso Sea (which more or less coincides with the Gyre), let alone how to steer back home?
We don’t yet know the answer to this, but the leading hypothesis is that the turtles can take a reading from local features of the earth’s magnetic field that vary in a systematic way over the whole of the ocean. Reading the magnetic coordinates specifies a heading that is appropriate for their goal. Lohmann and colleagues (2001) published experimental evidence for this idea, by giving neonate sea turtles (in a laboratory) the magnetic coordinates for different parts of the North Atlantic around the Gyre, and observing that they adopted magnetic headings that would be appropriate for each of those regions.
That study by Lohmann et al. suggested two amazing things about the Umwelt of the loggerhead turtle. First, that like von Uexküll’s tick, they do not need to have a very sophisticated understanding of their task to solve a complicated problem. In the turtle’s case, it is as if they come into the world programmed with a list of magnetic headings corresponding to different magnetic map coordinates. Nothing that requires them to have experienced it before, and certainly nothing in the way of a comprehensive understanding of ocean currents. The other amazing thing is that the turtles’ knowledge base is specific to the part of the globe where they live and their species has evolved. They do not have a generic or abstract knowledge of any part of the ocean or the earth’s magnetic field except the North Atlantic. They are creatures of their particular place, and their brain and behavior are tuned to that part of the world.
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Lohmann KJ, Cain SD, Dodge SA, Lohmann CMF 2001 Regional magnetic fields as navigational markers for sea turtles. Science 294, 364–366. (doi:10.1126/science.1064557)
Mansfield KL, Wynekan J, Porter WP, Luo J. 2014 First satellite tracks of neonate sea turtles redefine the “lost years” oceanic niche. Proc. Roy. Soc. Lond. B. 281 (1781) 20133039. (doi:10.1098/rspb.2013.3039)