Rats are quite content going for a swim, and scientists have capitalised on that to learn about the rodent's spatial memory. The animals can learn to swim to a platform submerged a few millimetres beneath the surface of a shallow pool, surrounded by familiar objects that help them get their bearings. But can the same aquatic approach be used to find out more about how lizards navigate? Augusto Foà and colleagues from the Università di Ferrara, Italy, decided to see whether ruin lizards (Podarcis sicula)that swim occasionally are able to find a ledge submerged just below the surface of a pool using only the sun for guidance(p. 2918).
Setting up a small outdoor pool surrounded by a high screen the team gently released individual lizards into the warm shallow water and trained them to locate a platform just above the surface of the water. Once the lizards were comfortable locating the ledge, Foà submerged it, released each lizard back in the centre of the pool and waited to see if it could still locate the ledge; it did. Finally the team took away the ledge, placed each lizard in the water in a random orientation and waited to see if the animal would home in on the correct location, despite the missing ledge. Each lizard looked around for a few moments and then doggy paddled over to the edge where the ledge should have been.
Having convinced themselves that the lizards could learn to find the platform in the pool as much as seven days after training, the team tested whether the reptiles were using the sun as a compass to locate the ledge by changing the animals' body clocks. Shifting some of the lizards' clocks back by 6 h and shifting others forward by 6 h, the team then released the lizards into the pool and checked which direction the animals thought their ledges were in. The lizards whose clock had been shifted back thought that the ledge was further round to the right, while the lizards whose body clock had been shifted forward thought the ledge was further round to the left. The animals were using the sun as a compass to locate their ledges.
Finally the team set out to test how the lizards sense the sun's position. Knowing that several scientists have suggested that the reptiles sense the sun as a compass with their parietal eye, situated on the top of their heads, the team decided to test whether the lizards could locate their ledges without their parietal eyes. Having covered the `eye' with a paint blind and released the lizards in the centre of the maze, the team saw that the animals no longer headed decisively towards the ledge but headed off in random directions. And when the team repeated the experiment, but this time removed the sensory organ, the lizards also headed off in random directions, confirming that the lizards sense the sun as a compass with their parietal eye.