Pitched against the mighty North Atlantic, it would seem that newly hatched loggerhead turtles would stand little chance of influencing their destiny. Perceived wisdom held that after swimming from their home beach the youngsters got picked up by the powerful currents circulating in the North Atlantic, where they could spend as much as the next 15 years being carried around the ocean. However, Nathan Putman and Ken Lohmann from the University of North Carolina, USA, were less sure. Together with other colleagues, they had already shown that simulations of the magnetic field from various locations in the North Atlantic affected the tiny animals’ choice of direction, with the minute migrants selecting the best orientation to keep them in the circulating current. However, many ecologists did not believe that the hatchlings’ efforts could influence their North Atlantic odyssey. Intrigued, Putman and Lohmann decided to take a computational approach; they set millions of cyber-turtles loose in a simulated North Atlantic to find out how much they affect their own course (p. 1863).
‘We wanted an ocean model that would depict weather events that were forced using wind, so getting a model that was very fine scale was important for those fine scale little animals’, says Putman. So, the duo teamed up with oceanographer Thomas Shay from the University of North Carolina, USA, to simulate 5 years of ocean circulation. Then, having successfully constructed their cyber-ocean, they had to figure out how to simulate the turtle voyagers. ‘We came across Philippe Verley, who devised this wonderful software that was called ICTHYOP that was designed to study the movement of fish larvae’, explains Putman. Visiting Verley at the Laboratoire de Physique des Océans in France, Putman and Verely discussed how they could convert the fish simulations into turtles migrating through the ocean. After a number of modifications to the software, Putman was ready to set hundreds of thousands of simulated turtle hatchlings loose in the cyber-ocean.
Setting the youngsters to swim for 1, 2 or 3 h per day at 0.2 m s–1, the program allowed the turtles to drift with the current for the remainder of the day. Then, when the turtles entered specified regions of the ocean, the program allocated them a swimming direction in roughly the same orientation that the young turtles had selected in the laboratory. ‘We were interested in modelling realistic navigation behaviour so we wanted a wide spread of around 80 deg around the directions that the hatchlings had picked’, explains Putman.
After months of computation and analysis, the duo realised that even the most minimal amount of swimming had affected the simulated hatchlings’ trajectories. Comparing the swimming turtles with simulations of turtles that drifted exclusively, the duo could see that the courses of the swimming turtles were less dispersed than those of the turtles that had been cast purely to the sea. ‘The swimming behaviour seemed to be compensating for the tendency of dispersion. It’s like the turtles were swimming to make the currents approximate what we see in the textbooks’, says Putman.
Having found that swimming for even a tiny fraction of a day could help the simulated turtles stay on course, the duo is keen to track the progress of recently embarked hatchlings to find out how much of an impact swimming makes in practice. ‘The plan is to put tiny little tags on turtles. We’ll know where the turtle goes, but it won’t tell us if they are swimming or drifting, so we’ll look at the ocean circulation models along the turtles’ paths and subtract the current velocity to determine when they are swimming’, explains Putman.