In an attempt to find clues, biologists have now installed transmitters on the shells of three giant sea turtles and booked air time on a Nasa satellite to track their movements. From an orbit 600 miles in space, the satellite is monitoring the daily progress of the three turtles as they head south from Ascension to warmer waters.
But the turtles are not the only wildlife now being tracked by Nasa satellites. More than 1,000 animals, birds and fish have been fitted with transmitters and are being monitored with the kind of satellite technology originally designed for tracking the activities of potential enemies. The movements of nearly 100 different species, including African elephants and Indian tigers, north American moose and reindeer, albatrosses and eagles, whales and basking sharks, polar bears and rhinos, are lighting up the screens at the world's only animal tracking satellite centre in Toulouse. From Alaska to Australia, biologists have been queuing up to put transmitters on wildlife to collect data which is, both literally and metaphorically, manna from heaven for studying migratory patterns. It is the availability of reasonably priced satellite time and the arrival of small, purpose-designed transmitters that has led to a big increase in the numbers of projects.
"When all this started the only bird that could be used was the albatross because it was big enough to carry the transmitter which weighed around 200g," explains Philippe Grof, business development manager of Argos, an affiliate of the French Space Agency, which runs the animal tracking service. "Now they can be as small as 15g which means that almost anything above a weight of 1.5kg can carry one."
Biologists who want to take part, buy and install their own transmitter at around pounds 2,000 a time. How the units are installed varies among animals - on the turtle it is glued to the shell, penguins have a sling holding it in place, birds carry it strapped to their body, and with larger animals such as polar bears, the transmitter is carried on a fixed collar - but once there, they they have a working life of around three to four months before power begins to fade.
As signals from the transmitters will not travel through water they come with a salt water switch that turns the device off when it is submerged. There is little point in having the transmitter working and wasting battery power underwater.
Once the transmitters are in place, Argos track the animals at a cost of around pounds 8 a day. For that fee, the company monitors the position and daily progress of the animal. The centre takes in the raw data using antennae in Brittany, Alaska and Washington, and then deciphers the information before passing it on to the subscriber.
Direction, speed and the final destination is the usual set of data that is looked for. The daily position checks are particularly crucial because they can help to give clues as to how the animal is navigating and adjusting its position. Some animals, for instance, are thought to find their way by using the positions of the sun, moon or stars. The earth's magnetic field may also be a factor, and the electric field created by ocean currents could also act as a pointer.
"For small distances, biologists can use VHF transmitters and follow the animals with a hand-held direction antenna. But where large distances are involved, that it not possible," says Grof. "If you want to follow the three-month migration of the stork from Germany to the Sudan, for instance, or swans migrating from Europe to southern Africa, the only way is to use the satellite. We give global coverage of the earth and can follow anything [that spends time on land] almost anywhere as long as it has a working transmitter.
"The information is given in the form of digital graphics. From that they can study the behaviour of the animals, how fast they are travelling, where they stop to eat, the frequency of directional changes, and so on," says Grof.
Dr Walter Reiss, a researcher at the University of California, says satellite monitoring has proved invaluable. "It has certainly helped with behavioural studies, and is crucial in conservation and environmental projects too. We can, for instance, use it to identify any detours in traditional routes which may be due to climatic changes, pollution, or development. We will be able to keep a check over time on how migratory habits are influenced by agricultural changes or deforestation."
For Dr Graeme Hay, a marine biologist with the University of Wales at Swansea, the satellite system offered the opportunity to try and solve the riddle of the Ascension turtle, and this summer he loaded a dozen transmitters into his luggage, flew out to the island and looked for volunteers among the 10,000 reptiles. The turtles, whose shells measure up to 5ft in diameter and who have a potential lifespan longer than the average human, are thought to make the journey to Ascension every three or four years. Dr Hay, now back home, is analysing the data he has so far received from Argos and says the first turtle has been tracked for 1,500 miles and is now off the coast of Brazil. "It only took 35 days and that is pretty good going, an average of around 40 miles a day, and it appears to have gone very much in a straight line. A second has left this week and seems to be going in the same direction."
He and his team, working with the University of Pisa, hope the data will eventually help them to come up with a hypothesis about the navigational techniques the turtles use. It is conceivable that memory may play some part in the travelling abilities of the older turtles, but that does not explain how first timers make the journey.
For Dr Hay and his team the satellite technology brought an unexpected finding too, when more than half the original transmitters were destroyed during the vigorous lovemaking of the turtles.
"What we have demonstrated clearly is that as soon as two turtles mate transmitters however hi-tech or securely protected, don't have a chance," he says ruefully. But, as he admits, it was not really sur- prising, given that an average male weighs around 250kg and that the love-making usually lasts for a transmitter-grinding 10 hours. !
Arctic Tern: The number one migrator which spends the summer in the Arctic where it rears its young, and then flies south around 9,000 miles to the Antarctic for a second cold summer. At the end of the southern summer they head another 9,000 miles back to the North Pole.
Salmon: After they are hatched, young salmon swim down river to the sea where they spend the next few years fattening up. As adult fish, they migrate thousands of miles back to the river where they were hatched. It is suspected that they home in on the chemical signature of the river.
Eels: Adult European eels migrate south to the Sargasso Sea to lay their eggs. The larvae spend the next three years making the return journey and arrive as elvers. The elvers swim up rivers such as the Severn. Those that evade fishermen evolve into yellow eels. When they mature they head south to repeat the cycle.
Wildebeest: Gnus have been known to migrate up to 1,000 miles to try and find water and food.
Lemmings: These small, short-tailed rodents are not mainstream migrators in that they only move out in large numbers when there is overpopulation or when the food runs out. Despite popular myth that they are suicidal and throw themselves off cliffs, those that die do so either through starvation or by drowning as they try to cross rivers.