鸟类能看到地球的磁场吗?

Apart from being very useful for geologists who want to understand the planet’s past, as well as fending off the onslaught of the solar wind, and helping humans who want to navigate the old-fashioned way - with a compass - Earth’s magnetic field is essential to many migratory species: birds appear to be able to read the direction of the field and use it to guide them as they fly thousands of miles. Francesca Fazey spoke to Oxford scientist Peter Hore who suspects that proteins called cryptochromes in the birds’ eyes enable them to “see” magnetic fields...

Francesca - Close your eyes and turn around five times. Now, without opening your eyes head towards magnetic north.

Okay. So you don’t actually have to try this. And please don’t especially if you’re listening near a busy road.

But if you were able to do it, chances are you’d be a robin or any other kind of migratory bird. Scientists have worked out that birds that migrate can actually sense the direction of the magnetic field using some neat tests. Here’s Professor Peter Hore at the University of Oxford who specialises in this area…

Peter - The experiments involved testing the birds during the migratory season, so these are small migratory songbirds like robins. And during the migratory season in the spring and autumn, if you put them in a funnel shaped cage, the direction in which they hop to try and get out of the cages is the direction in which they would fly if they were released.

Francesca - Now, here comes the clever bit: using coils and currents you can change the magnetic fields that the birds are experiencing and then see if it changes the way they try and hop out of the cage and, incredibly, it did. So how do our feathered friends do this?

Peter - The leading hypothesis at the moment is that there are magnetically sensitive chemical reactions in the retinas of the bird’s eyes, and that these chemical reactions allow them to sense the direction of the field.

Francesca - A recent study has identified a specific protein with a great name - Cryptochrome 4 - that they think is the most likely candidate for a magneto receptor in these migratory species. This is because Cryptochrome 4, or Cry 4 as they like to call it, only seems to be expressed during the migratory season when they need it to guide their long journeys.

And get this: birds who don’t migrate, like chickens, don’t express Cry 4 at all. The protein is located in specific cells in the retina which are involved with vision. So does this mean the birds can actually ‘see’ the magnetic field? Peter has a classic scientists answer to this…

Peter - Maybe.

Francesca - So, no flight of fancy for us today then!

Peter - We don’t know nearly enough about how the signalling works. It is clear that the detection of the magnetic field involves the birds visual system, so the receptors are likely to be in the photoreceptor cells. We know that when the information from the retina reaches the brain it’s processed in a visual part of the brain. I think it’s too soon to say whether the birds literally see the magnetic field or whether it’s some visual impression which, of course, is a bit difficult for us to to imagine because humans don’t seem to have a magnetic sense.

Francesca - Although we have tried to find it. One study sent out blindfolded students into the woods and asked them to find their way out again. Amazingly they did without any serious injuries and the researchers suggested this was due to us having a secret sense of magnetic north much like the migratory birds. But, like many of these findings, it has never been replicated so we can’t genuinely claim to have this superpower.

Some other animals though might…

Peter - There was a high profile study a few years ago in which the scientists analysed Google Earth images of herds of wild deer and domestic cattle and found that the animal’s body axes tended to align with the Earth’s magnetic field.

Francesca - Gosh!

彼得,被证明是难以复制的,但它’s a hint at least that maybe these land animals that do migrate large distances might have some use for the Earth’s magnetic field.