Ellen Phiddian
We spend a lot of time marvelling at animals like pigeons and sharks, which can navigate with pinpoint accuracy, particularly through the sea. But new research is showing that the talent is far from universal: hawksbill turtles, for instance, use much more winding paths to get from their nesting grounds to their food.
In fact, according to a paper published in Journal of the Royal Society Interface, hawksbill turtles’ maps are less about the destination and more about the exploratory journey. GPS trackers showed that one turtle in a study even took 1306.2km to cross a distance that it could have managed in 176.4km.
“They went on quite circuitous routes to get to their foraging grounds,” says lead author Professor Graeme Hays, a researcher in marine science at Deakin University.
“Always in roughly the right direction, but very rarely perfectly orientated.”
The researchers were tracking the turtles’ paths to learn more about marine conservation.
“We’re working in in an area in the western Indian Ocean: a really isolated archipelago called the Chagos Archipelago,” says Hays.
“That archipelago lies in the heart of one of the world’s largest marine protected areas. So, the fundamental reason that we’re interested in tracking turtles is to understand what their movements are and how valuable that marine protected area is around their lives.”
The researchers tagged 22 female hawksbill turtles as they nested on Diego Garcia Island, in the archipelago.
“We put tags on nesting hawksbill turtles, and then they migrate at the end of the next nesting season back to their foraging grounds,” says Hays.
On top of their slightly messy navigational skills, the researchers were able to locate where the turtles got food.
“We found that their foraging grounds are also within the protected area,” says Hays.
But they also found that the turtles didn’t have a keen sense of direction when it came to getting back to their grounds. The average “straightness index” (the ratio between the shortest path and the actual path taken) of the turtles’ paths was 0.54. This means that the turtles travelled twice as far, on average, as they needed to.
Hays says that “crude” navigational skills are probably the reason for this – it’s unlikely that anything else is distracting the turtles from their food.
“They don’t feed when they’re away from their foraging grounds,” he says.
“That whole breeding migration might take three or four months. So those turtles that we’re tracking, they haven’t eaten for several months, and they’re really hungry.
“If I didn’t let you eat for three days, and then I put you 100 metres from a really nice restaurant and said, ‘Okay, do whatever you like’ – probably you’d make a beeline straight to that restaurant.”
The team has previously done research on green sea turtles, which can find their way over several thousand kilometres – but Hays points out that they have an easier route to follow.
“Green turtles and other species, they have foraging grounds on the coast of Africa. Some of them will swim 5000 kilometres to their foraging grounds.
“But that’s essentially quite an easy navigational task, because all they’ve got to do is swim westwards until they hit Africa – Africa’s a big target – and then they turn left or right. They’ve just got to make that correct decision, and they just follow the coastline to get to their foraging grounds.”
Hawksbill turtles, by contrast, are going under 200km – but to trickier places.
“These hawksbill turtles are going to isolated targets that are submerged banks in the open ocean. So there’s a fairly small target that they’re aiming for. The navigational challenge was a lot greater, which may explain the difficulties that they were having.”