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Travel routes to remote
ocean targets reveal the
map sense resolution for a
marine migrant
Hays, G.C., Atchison-Balmond, N., Cerritelli, G., Laloë,
J.O., Luschi, P., Mortimer, J.A., Rattray, A., and Esteban,
N., ‘Travel routes to remote ocean targets reveal the map
sense resolution for a marine migrant’ – Journal of the
Royal Society Interface
https://doi.org/10.1098/rsif.2021.0859
In 1873, Charles Darwin marvelled at the ability of sea
turtles to find isolated island breeding sites, but the
details of how sea turtles and other animals navigate
during these migrations remains perplexing. To help solve
this long-standing enigma, this study tracked 22 critically
endangered hawksbill turtles (Eretmochelys imbricata)
after they had finished nesting in the Chagos Archipelago
all the way to their foraging grounds in other parts of the
Indian Ocean. The team considered the likely resolution
of any mapping sense used in migration, based on the
navigational performance across different scales (tens to
thousands of kilometres).
Bertarelli Foundation
The studies found that these turtles often followed
circuitous paths when migrating short distances. For
example, one turtle travelled 1,306.2 km, seven times the
beeline distance to the target foraging ground of only
176.4 km. When off the beeline to their target, turtles
sometimes corrected their course both in the open ocean
and when encountering shallow water. The results provide
compelling evidence that hawksbill turtles only have a
relatively crude map sense in the open ocean, almost
certainly using a geomagnetic map that only corrects
them when they are a long way off route. The existence
of widespread foraging and breeding areas on isolated
oceanic sites points to target searching being common in
sea turtles in the final stages of migration.
Hawksbill turtles typically migrate much shorter distances
than green turtles, around 150 km compared to up to
5,000 km. An earlier paper about tagged green turtles in
2020 by the authors provided some of the best evidence
to date for the ability of turtles to reorient in the open
ocean, but similarly only at a crude level. Sea turtles
therefore locate isolated targets through a roughly targetoriented ocean crossing, open ocean course corrections
and then localised search closer to the target. Sea turtle
navigational abilities are therefore far from perfect but
may simply be as good as possible within the constraints
of their sensory ability.