WHY ANIMALS DON’T READ MAPS
If birds drove cars, they wouldn’t argue about the GPS directions it seems. Migrating birds rarely get lost, but how they know which way to fly has remained a mystery. Until now…
When you hear the term ‘quantum physics’ there are a few things that might come to mind: the Large Hadron Collider, unintelligible equations and Star Trek gobbledygook. Those of us who are more au fait with the realm of teeny-tiny science will know about particles tunneling through a wall, the uncertainty principle and an ill-fated cat in a box. Animals almost certainly aren’t on anyone’s list. But that’s all about to change. Kyle Pastor explains that science’s most poorly understood specialism may have solved a one hundred year old biological mystery. Quantum science is about things that are extremely small, with quantum mechanics exploring objects at the scale of protons and electrons. Because we are talking about things at such a small scale, it is difficult to appreciate the effects of quantum mechanics (such as ‘ tunneling’) in the everyday world. But this could be about to change. In the past, scientists have only been able to see quantum effects when things are very cold. But we live in a warm environment, so all the molecules that make up the world around us jiggle around so much that they wash out the details that quantum mechanics give us. However, new research has revealed that biological systems like plants and birds use these hidden quantum effects to their advantage. Birds are much bigger than an atom (and are warm and squishy), so they would seem to be among the least likely places to see quantum-ness. But this is where the European robin comes in… As many of you will know, birds are able to navigate the globe with amazing accuracy. In general, this is done by sensing the Earth’s magnetic field. For a long time it was believed that there was a small piece of magnetic material in the beak of birds that gave them the direction of the magnetic field’s ‘polarity’ – much like how a normal compass works. But it is now believed that this is not the whole story and, in fact, the small amounts of metal in the beak may often have nothing to do with navigation. In a report published in a recent issue of
Nature, Nature, researchers examined how birds in the laboratory react to changes in the magnetic field, and found these birds to in fact be blind to the magnetic polarity (which way is North or South). They determined this by using magnetic fields set up at different strengths and different angles, which showed that birds sensed the ‘inclination’ – or angle – of the magnetic field compared to the horizontal instead of sensing the polarity. If you take a compass and hold it flat, its needle will tell you the direction of magnetic North. The compass is showing you the polarity of the magnetic field. Take that same compass and flip it 90 degrees sideways. The direction the needle now shows is called the angle dip or inclination of the magnetic field. In the Northern Hemisphere it points down, and in the south it points up (see image on the next page). This, of course, is very useful for navigation. Now, back to our robin. With the passing of the seasons, the robin uses the Earth’s magnetic field to find its way between Africa and Northern Europe in a search for the best weather. The robin’s ‘inclination compass’, which it uses to navigate these journeys, has been discovered not in the beak but in the eye. The scientists found that the bird lost its magnetic sense when flying in the dark – meaning that the magnetic sense must need light to activate it, and so must
be somewhere in the eye. This finding makes perfect sense: it is probably giving robins the ability to actually see the Earth’s magnetic field. Now that’s an incredible ability to have. Located at the back of the eye, chemical compounds called cryptochromes respond to magnetic force by performing a quantum reaction called a ‘pair-radical reaction’. This reaction ‘measures’ the inclination of the magnetic field from any given location. But how? Here comes the tricky stuff… Imagine you want to compare a bunch of weights on a balance beam by placing them on each side of the balance. If you do this, the balance beam will tip in the direction of the heaviest weights. By looking at how much it is tilting – by measuring the angle of the tilt – you can figure out how unbalanced it is. When light excites the cryptochrome compounds in the eye, they can exist in two states, the singlet state (S) and triplet state (T); these states are a bit like weights on opposite sides of the balance beam. If the S and T states are equal in number, we have the equivalent of a balanced beam. This is the situation if there is no magnetic field. But when the bird experiences a magnetic field with an inclination, the balance is tipped and the states ‘pile up’ on one side more than the other. The robin’s brain interprets this imbalance in S and T states as a change in the magnetic field – just like we would measure the angle of an unbalanced beam. Voilà, we have a biological quantum compass! At the moment scientists still need some more evidence to confirm that this is a quantum effect and not anything else. They plan to do this by using ‘Quantum Control’, a set of techniques that can rule out anything that may not be quantum. Put simply, they will see how the birds react in a mix of fluctuating magnetic fields. Careful analysis of what the birds do will confirm whether the quantum reactions truly are in control. It’s all pretty deep stuff, and there are some links below about quantum biology to give a bit more of an in-depth explanation. And even if it is tricky to understand, ‘Quantum Control’ is a fantastic name. It should forever be used as the name for all super-villain death machines. Or comic book heroes’ superpowers. Perhaps the title for the next Bond movie? Now who said physics was boring?