How do turtles migrate across the ocean?
Sea turtles are renowned for their amazing orientation and navigation abilities, that allow them to perform astonishing feats of navigation such as to cross entire ocean basins or to reach isolated, remote targets like oceanic islands. Very little is known about the behavioural mechanisms permitting the turtles to do that, and investigating their navigational abilities has always been one of the main interest of the Islameta Group.
To this aim, several experiments have been done in the past, following a variety of approaches to address the issue, and taking advantage of fruitful collaborations with a number of colleagues in different parts of the world.
1. Magnetic experiments with migrating Ascension turtles
It has been proposed that the green turtles nesting at Ascension Is. rely on magnetic cues for their migration towards Brazil and back. In the area crossed by migrating turtles, some parameters of the Earth’s magnetic field indeed form a grid that may provide turtles with a position-fixing system. In particular, the lines joining points of equal geomagnetic inclination angle (isoclinics) and of equal intensity (isodynamics) intersect each other at an angle close to 90°, thus forming a sort of magnetic map by which turtles may theoretically calculate their position with respect to their targets.
To test this hypothesis, in collaboration with Graeme Hays (Univ. of Wales Swansea, UK) and Susanne Akesson (Univ. of Lund, Sweden) we tracked 7 turtles during their postnesting migration from Ascension Island to Brazil coast after they had been fitted with six powerful magnets attached to various parts of their body (see below).
This treatment produced variable artificial fields around the turtle (see below), which made reliance on a geomagnetic map impossible.
The migratory courses of magnetically-treated turtles were very similar to those of control turtles without magnets tracked along the same route, and no differences between magnetically disrupted and untreated turtles were found as regards navigational performance and course straightness.
These findings, that have published in the paper Papi et al. 2000, show that magnetic cues are not essential to turtles making their trip back to the Brazilian coast.
2. Displacement experiments with Ascension turtles
Displacement experiments involve the capture of an animal at one location and its release at a different one to test its ability to compensate for the passive dislocation. Such experimental approach is a classic method to evaluate the animal’s navigational abilities.
For the first time, we used this system on marine turtles by displacing Ascension turtle females captured during the nesting season and tracking them by satellite after releases at different distances from the island. A few, preliminary attempts of displacing turtle females had been previously done with Malaysian green turtles and South African loggerheads, with the results showing good homing abilities of displaced turtles (see Luschi et al 1996 and Papi et al. 1997).
In a first series of experiments, 18 females were displaced from Ascension to eight different points in the ocean at 60-450 km from it. Since the turtles had not completed their nesting cycle, they were therefore motivated to return to Ascension, which can be considered the target of a homing behaviour.
Turtles generally found it difficult to relocate the island soon. While four turtles moved to Brazil soon after the release, four of them made long-distance searching movements in various directions before heading to Brazil, and in total 10 reached the island.
Most the successful trips, were winding but ended with a final with a final straight segment of variable length, as if the turtles were searching for a sensory contact with the island which they obtained at various distances. The approach to Ascension mostly occurred from the Northwest, i.e. from the direction opposite to the Southeast trade winds which constantly blow in the area. This suggested a navigational role of wind-borne information originating from the island.
These findings are described in the papers Luschi et al. 2001, Åkesson et al. 2003
3. Testing the role of wind-borne cues in the island-finding behaviour of Ascension turtles
The hypotheis that trade winds have a role in the final leg of the homing process to return to Ascension, a second series of experiments was performed during the 2002 nesting season. Three satellite-tracked females were displaced to a site located 50 km north-west from the island, and three females to a simmetrical one, located 50 km southeast from it. The two sites are downwind and upwind from trade winds, respectively.
The three turtles released downwind returned to Ascension Island within 1-4 days and with rather direct routes, while turtles released upwind were not able to home quickly. Two of them eventually returned to the nesting island after 10 and 27 days, and the third headed back to Brazil after failing to find its way back to the island.
These findings strongly support the hypothesis that cues borne by SE Trade winds are used by turtles to locate Ascension Island, possibly by creating a sort of plume which makes the approach from downwind easier. The nature of the wind-borne cues used by turtles during this process is currently unknown, although the role of chemical cues and/or of sounds originating from the island, is likely.
4. Displacement experiments with migrating loggerhead turtles
Loggerhead turtles of the South African population migrate towards fixed, individually-specific residential feeding grounds mainly distributed along the African continental coast. To study the turtles’ navigational performances during migration, in years 1998-99 we captured five individuals that were about to start their postnesting migration and displaced them to distant sites, away from their usual migratory route. The experiment, which involved an huge logistical effeort to move the turtles for such a long way, was made possible only thanks to the precious help of Dr. George Hughes, which was Chief Executive Officer of the local Natal Parks Board (now Ezemvelo KwaZulu-Natal Wildlife).
A first turtle, released south of Madagascar, about 1.148 km from the capture site, moved west up to mainland Africa and then reached her feeding grounds by hugging the continental coast (see figure below, red track).
The second turtle, released further away (2.140 km) close to La Reunion Island, stopped for some time on the Madagascar east coast, then turned southward to round the island and regain the African mainland in the northwest. The precocious interruption of her tracking however did not allow us to establish the location of her residential grounds (see figure below, blue track).
Three other turtles were released off the Tanzanian coast, 2.193 km north of their nesting area, at the northern edge of the distribution of the feeding grounds along the African coast. All of them headed north, and one turtle found her residential grounds located north of the release site. The other two females started long-distance oceanic wandering in which they crossed nearly the entire Indian Ocean.
While these results are difficult to be fully interpreted given the small sample size, the behaviour of the displaced turtles hardly provide any indication of their reliance on long-distance navigational systems that would have allowed them to compensate for the displacement.
These results are reported in the papers Luschi et al. 2003; Mencacci et al. 2010.
5. Displacement experiments with Indian Ocean turtles
Thanks to a fruitful collaboration with Simon Benhamou (CEFE -CNRS Montpellier, France), in years 2003-2008 we perfomed a series of displacement experiments with green turtle females nesting in various islands of the Mozambique Channel, Western Indian Ocean. As in the Ascension experiments, turtles were captured during the nesting season and tracked after release in open sea locations distributed in different diirections and at various distances from the home island. Translocations were made by relying on different means, such as small boats, larger sailboats or even military ships.
We initially tested the methods with turtles nesting on the remote Europa Island atoll (22°20’S, 40°21’E), in the southern Mozambique channel. A total of six turtles were captured while nesting, embarked in a French Navy ship and released in two offshore distant sites with an Argos satellite trasnmitter safely attached to their carapace. Three of them were succesfully tracked during their homing journey back to Europa. Rather surprisingly, displaced turtles found some difficulties in relocating the island as they reached the home island after 13, 21 and 59 days following long, circuitous routes, hence apparently displaying poor navigational abilities. Path analysis showed that they were unable to compensate for the deflecting action of currents, which moved them away from their intended course and lowered their orientation performance, and that did not takle advantage of navigational information possibly available in water masses that had previously been in contact with their target area.
(…) TO BE COMPLETED SOON!