Sea turtles' impressive navigation feats rely on surprisingly crude
'map'

Date:
July 16, 2020
Source:
Cell Press
Summary:
Since the time of Charles Darwin, scientists have marvelled at
sea turtles' impressive ability to make their way -- often over
thousands of kilometers -- through the open ocean and back to the
very places where they themselves hatched years before. Now,
researchers have evidence that the turtles pull off these
impressive feats of navigation with only a crude map to guide them
on their way.



FULL STORY ========================================================================== Since the time of Charles Darwin, scientists have marveled at sea
turtles' impressive ability to make their way -- often over thousands of kilometers - - through the open ocean and back to the very places where
they themselves hatched years before. Now, researchers reporting in the
journal Current Biology on July 16 have evidence that the turtles pull
off these impressive feats of navigation with only a crude map to guide
them on their way, sometimes going far off course before correcting
their direction.


==========================================================================
"By satellite tracking turtles travelling to small, isolated oceanic
islands, we show that turtles do not arrive at their targets with pinpoint accuracy," says Graeme Hays of Australia's Deakin University. "While
their navigation is not perfect, we showed that turtles can make course corrections in the open ocean when they are heading off-route. These
findings support the suggestion, from previous laboratory work, that
turtles use a crude true navigation system in the open ocean, possibly
using the earth's geomagnetic field." Despite much study of sea turtle navigation, many details were lacking. Hays' team realized that was in
part because most sea turtles return to spots along the mainland coast,
which are also the easiest places to find.

For the new study, his team had attached satellite tags to nesting green turtles (Chelonia mydas) out of an interest in learning about the extent
of the turtles' movements and to identify key areas for conservation. In
the process, they realized that, by serendipity, many of the tracked
turtles travelled to foraging sites on isolated islands or submerged
banks. It allowed them to explore in more detail how turtles make their
way to such small and harder-to- find islands.

In total, the researchers recorded the tracks of 33 green sea turtles
migrating across the open ocean from their nesting beaches on the island
of Diego Garcia (Indian Ocean) to their foraging grounds across the
western Indian Ocean, many of which were isolated island targets. Using individual-based models that incorporated ocean currents, they then
compared actual migration tracks against candidate navigational models
to show that 28 of the 33 turtles didn't re- orient themselves daily or
at fine-scales.

As a result, the turtles sometimes travelled well out of their way
-- several hundred kilometers off the direct routes to their goal --
before correcting their direction, often in the open ocean. Frequently,
they report, turtles did not reach their small island destinations
with pinpoint accuracy. Instead, they often overshot and or spent time searching for the target in the final stages of migration.

"We were surprised that turtles had such difficulties in finding their
way to small targets," Hays says. "Often they swam well off course and sometimes they spent many weeks searching for isolated islands.

"We were also surprised at the distance that some turtles migrated. Six
tracked turtles travelled more than 4,000 kilometers to the east African
coast, from Mozambique in the south, to as far north as Somalia. So,
these turtles complete round-trip migrations of more than 8,000
kilometers to and from their nesting beaches in the Chagos Archipelago."
The findings lend support to the notion that migrating sea turtles use
a true navigation system in the open ocean. They also provide some of
the best evidence to date that migrating sea turtles have an ability to re-orient themselves in deep waters in the open ocean, the researchers
say. This implies that they have and rely on a map sense. But the results
also show that their map lacks fine details, allowing them to operate
only at a crude level.

As a result of this imperfect navigation system, the turtles reach their destination only imperfectly. In the process, the turtles spend extra
energy and time searching for small islands.

The findings also have implications for the turtles' conservation,
Hays says.

Turtles travel broadly across the open ocean once nesting season has
finished.

As a result, he says, "conservation measures need to apply across these
spatial scales and across many countries." The researchers say that
they hope the next generation of tag technology will allow them to
directly measure the compass heading of migrating turtles as well as
their location. "Then we can directly assess how ocean currents carry
turtles off-course and gain further insight into the mechanisms that
allow turtles to complete such prodigious feats of navigation," Hays says.


========================================================================== Story Source: Materials provided by Cell_Press. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Graeme C. Hays, Giulia Cerritelli, Nicole Esteban, Alex Rattray,
Paolo
Luschi. Open Ocean Reorientation and Challenges of Island Finding
by Sea Turtles during Long-Distance Migration. Current Biology,
2020; DOI: 10.1016/j.cub.2020.05.086 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200716111631.htm

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