The Cornwall Bat Hospital ~ nature

Nature

6th December 2006

Built in compass helps bats find their way home

Big brown bats use magnetic field to navigate long distance.

Narelle Towie

Bat mobile: bats can navigate 100 kilometres without using the sun or stars.

Alamy

Bats have a novel device for guiding them home on starless nights. In addition to their well-known sensory talents, it seems that big brown bats can tune into the Earth's magnetic field, using it as a compass to guide them to roost.

This ability comes in handy on long-distance flights, where their usual mode of navigation ? bouncing sounds waves off objects using ultrasound ? doesn't do much good.

Richard Holland from Princeton University, New Jersey, and colleagues looked at 15 North American big brown bats (Eptesicus fuscus), which travel up to 100 kilometres to find hibernation sites for the winter.

To first test the animals' natural navigational abilities, they attached small radio transmitters to the bats and transported them 20 kilometres from their roost. One by one they let them go, and tracked them from a small aircraft. All of them headed directly back to their roost.

How did they do this? Researchers have previously suggested that bats might use the direction of the sunset to set their compass. Others have found traces of magnetic materials within bats, suggesting that they might use the planet's magnetic fields to find north.

To tease these effects apart in a single experiment, the researchers put the bats inside a helmet that generates a strong magnetic field offset from the planet's true north. They let the bats watch sunset while sitting in this artificial magnetic field for about an hour and a half. "We then took them to the same release site," says Holland. And they all took off in the wrong direction. "It was a neat trick."

Animal magnetism

"This seems to be convincing experimental proof that they do have a magnetic sense," says Gareth Jones, a biologist at the University of Bristol, UK.

The disorientated bats did eventually make it home, sometimes correcting their path within just a few hours. "That could be because of the short exposure of the rotated magnetic field," says Jones ? perhaps they could correct their compass once back in the correct field. Or perhaps they switched to another method of navigation. "What this is we are just not sure. Perhaps they hit an area that is familiar to them," Holland suggests.

Many organisms, from the birds to the bees, are attuned to the world's magnetic pull. How they do it remains a bit of a mystery. Ants are thought to use tiny particles of magnetic iron minerals located throughout their bodies. When the magnetic iron reacts with the dense iron core of the earth it enables the ants body to act the same way a needle in a compass does (see 'Migratory magnetism').

Migratory birds use magnetic material along with several other environmental cues ? including the sun, stars, visual references and maybe even the polarisation of light ? to navigate.

Restless hopping

Bird navigational instincts are relatively easy to study in the lab thanks to a condition called migratory restlessness: when kept in captivity, birds will hop in the direction of their desired travel.

Bats aren't that easy to study say researchers. "Studying such small animals in the wild is labour intensive. They are too small to be tracked by current satellites," says Holland. The bats weigh just 15-25 grams, and can only carry equipment of less than 0.5 grams - too small for a standard GPS collar.

The team is trying to get funding for a satellite to specifically track such small, migrating creatures. In the meantime they will continue their research, studying further exactly what mechanisms the bats use when they get lost.