If you were dropped out of a plane, in the middle of a desert, how would you find your way home?
“If you only have a map, you say, ‘I’m here and I want to go...north.’ But you still need to know where north is,” says Cordula Mora, a postdoctoral research associate in biology. “If you have a compass but no map, you can tell direction — where north is. However, you don’t know where you are, so you cannot determine which direction will take you home.”
The Earth is like a big bar magnet. Its magnetic field is strongest and most vertical at the poles, and is weakest and most horizontal at the equator. Many animals and even some bacteria orientate themselves based on the strength and angle of this field.
In studies carried out at the University of Auckland in New Zealand, Mora showed that homing pigeons have a magnetic sense that allows them to detect the magnetic field around the Earth. While it’s long been known that pigeons have a magnetic compass, these new results open up the possibility that pigeons may also have a magnetic map.
Mora trained four pigeons to discriminate between the presence and absence of a magnetic field. The birds jumped onto one platform if they detected a magnetic field during the trial and onto another if they did not. Correct choices were rewarded with food. They got it right between 65 percent and 70 percent of the time — significantly more often than chance alone would allow. The work was published in the November 25, 2004, issue of Nature.
Mora came to Carolina to work with Ken Lohmann, professor of biology, who has conducted many studies of loggerhead sea turtle navigation in the wild.
In her lab, eight loggerhead turtles swim up and press their noses against the glass — greeting their handler. Mora fishes out two turtles — Maria, the biggest, and Lisa.
“These guys are really well behaved because they’re used to being handled,” Mora says. “If you ask people who work with them at the coast, they’ll tell you that wild loggerheads are quite ill-tempered compared to green turtles.” But loggerheads aren’t as endangered as their counterparts, and are easier to obtain.
Lisa, like the pigeons in Mora’s earlier experiments, is being trained to respond to magnetic fields. But it’s Maria’s job to detect polarized light.
“We know that birds and insects can detect polarized light, but no one’s ever proven it with aquatic reptiles,” Mora says.
In the wild, turtles feed on transparent animals such as jellyfish or larvae. If Mora’s experiments show that turtles can detect the polarized light patterns refracted off of these animals, it might explain how turtles see their food. It will also give Mora an idea about how turtles orient themselves in the ocean when there’s not much light shining through the water.
Mora says she hopes her research will increase understanding of the sensory world of sea turtles and contribute to their continued survival. For example, her work on the turtles’ vision has applications for developing fishing gear that catches fish, but kills fewer turtles in the process.
Thomson is a senior majoring in journalism at Carolina.
