Prof Dr Henrik Mouritsen has been researching the magnetic sense of birds for more than 15 years. He has now summarised the current state of research in the journal Nature. In this interview, the neurobiologist talks about his fascination with birds and why basic research is so important.
QUESTION: Mr Mouritsen, you have ringed more than 6000 birds in your life. What was your most memorable experience as a bird researcher?
ANSWER: As a student, I was in the Philippines, where we had to make our way through the rainforest with a machete. We caught a small flycatcher that had a bright orange tail with an upside-down black T in it. This bird had never been described in a bird book before! We felt like explorers. A fantastic experience for a young person interested in birds.
QUESTION: How did you become interested in birds?
ANSWER: I had a teacher at primary school who was very interested in birds. He was very inspiring and sparked my interest. I kept that interest. As a pupil and student, I was a so-called twitcher: every time a rare bird species was spotted somewhere in Denmark, we went out - no matter what else we had to do. At the time, I began to ask myself: why would a bird from Mongolia suddenly turn up in Denmark? Since it was mainly young birds, it was clear that something must have gone wrong genetically or mechanistically. I started to read the literature on bird migration research. To be honest, I wasn't very impressed.
Knowing the details
QUESTION: What was known at the time about how birds find their way over thousands of kilometres?
ANSWER: It was clear that birds could use the magnetic field, the sun and the stars to navigate. But opinions differed widely as to which of these was most important. And, of course, scientists knew that birds needed a mechanism to sense the magnetic field. However, most studies focused on the behaviour of birds. Virtually nothing was known about the molecular biology of the magnetic sense and the corresponding brain functions. During my doctoral thesis, I realised that in order to really understand animal behaviour, you have to start knowing the details. That's why I went to Canada in 1999 to learn about neurophysiology.
QUESTION: You have been researching at the University of Oldenburg since 2002. What are the most important milestones in your work?
ANSWER: In 2000, scientists theorised that a certain class of molecules, cryptochromes, enable migratory birds to perceive the magnetic field as a visual pattern. We therefore began to investigate this molecule and the bird's brain in more detail. In 2004, my colleague, the neurobiologist Reto Weiler, and I, together with our research groups, actually found cryptochrome in the retina of migratory birds. My colleague Eric Jarvis from Duke University and I and our teams then discovered a region in the brain known as cluster-N in 2005. We were able to show that this region is light-dependent and, very likely, processes information from the magnetic compass. A little later, in 2009, we showed that this region is light-dependent: If we switch off the Cluster-N region, the birds' solar and stellar compasses still work, but the magnetic compass does not. We know this from behavioural experiments. In the beginning, these were just assumptions. Today, I am 99 percent sure that the cluster N region is a centre of the magnetic compass in the bird's brain. The cryptochrome we found back then was probably not the right molecule. We recently identified another chryptochrome that is more likely to be involved in the process. However, we were close at the time. These were our most important early milestones.
Light as a lever
QUESTION: You have been working closely with your British colleague Peter Hore for about ten years. What are you currently working on?
ANSWER: We have started to investigate the biophysical mechanism of the magnetic sense in detail. However, we cannot describe what happens in a bird's eye using classical mechanics. We have to look at it quantum mechanically. However, I am not a quantum chemist. That's why I'm working with Peter Hore and his colleagues at Oxford University. Peter has come up with an analogy to explain the complicated processes in a bird's eye: The cryptochrome molecule is like a boulder standing on its flat side. This means that it is in equilibrium. A small fly could never knock it over. However, we have a lever - that is the light. This lever places the stone on its corner. This is the state of the molecule in which a so-called radical pair is present. This is very far from equilibrium and only exists for about a microsecond. But the molecule is very sensitive to disturbances. Now it doesn't just matter whether the little fly lands, but also where it lands.
QUESTION: And in this analogy, the small fly corresponds to the Earth's magnetic field...
ANSWER: Yes. Depending on how the molecule is orientated relative to the magnetic field, the bird sees more or less light. In other words, birds can see the magnetic field. The exciting thing is that this mechanism responds very easily to the interactions between the weak geomagnetic field and the radical pair. The energy of these interactions is about a million times lower than what was originally thought to be the limit for biological senses. What if other biological sensory systems also have similar intermediates? There could also be other senses that respond to very weak stimuli.
The value of basic research
QUESTION: You originally wanted to understand how birds find their way by sensing the Earth's weak magnetic field. Now you have stumbled across something that could also occur elsewhere...
ANSWER: Yes. And that could fundamentally change our understanding of biological sensory systems. I would never have thought that when I started. And as soon as we found experimental evidence from biology and physics for this mechanism, a lot of people became interested in it. This is because similar processes take place in quantum computers, for example. The problem is that the electrons in a quantum computer cannot be kept stable for very long, and certainly not at room temperature. However, it seems that birds have solved at least part of this problem in their magnetic compass. As a result, governments and companies have suddenly become interested in bird migration research. This is a good example of the value of real basic research: if you want to understand systems that are still completely unexplored, then you might get completely new solutions. That's the good thing about the German science system - it still supports basic research very well.
Fabulous workshops
QUESTION: In 2014, your working group published another important result in Nature. Namely, that noise from electronic devices interferes with the magnetic compass of birds. Do you now know how this happens?
ANSWER: We are investigating this question together with our partners in Oxford. What is clear is that time-dependent magnetic fields interfere with the interaction between the electrons and the Earth's magnetic field. But we don't yet have a complete picture of why the magnetic compass of birds reacts so sensitively to the very weak anthropogenic electronic noise.
QUESTION: You obviously feel very comfortable as a researcher in Germany. Why did you originally come to Oldenburg?
ANSWER: I selected Oldenburg for two very specific reasons: When I started, we didn't yet know what happens in the eye and brain of birds. It was clear that if I wanted to understand the visual system, I had to work with someone who understood the sense of sight better than I did. That was Reto Weiler - and his working group. I also wanted to work with Franz Bairlein from the Institute for Bird Research in Wilhelmshaven. He knows a lot about birds and how to keep them. Since then, the University of Oldenburg has always supported my research. We also have marvellous electronic and mechanical workshops here: I could buy my equipment off the shelf. But then I would have to replace all the magnetic parts. Because I can't carry out magnetic measurements on the brain if there are iron parts nearby. That's why the workshops have to modify my equipment. They do that really well. There are only a few universities that have so many good workshops.
Quantum mechanics and ecology
QUESTION: In your review, you point out some unanswered questions for research. What are your most important goals for the future?
ANSWER: If we want to understand how birds navigate, we need to understand the biophysics and quantum mechanical aspects of magnetic sense. Where exactly are the sensors? What is their structure and with whom do they interact? We need to understand the physiology, the electrical signals and the signalling pathways. And we want to understand how the brain brings all this together with other information relevant to navigation. We don't know where this happens. That's what we want to find out. We also want to find out how birds store spatial information in the brain. Are there place and compass neurones? We want to look at these questions at the molecular, neuroanatomical and biophysical level, but also at the behavioural level - both in the laboratory and in the wild. This is the reason why we here in Oldenburg will be applying to the German Research Foundation (DFG) for a Collaborative Research Centre (SFB) that will bring together researchers from quantum mechanics to ecology.
QUESTION: Who are your partners in this project?
ANSWER: A good two thirds of the leading scientists come from Oldenburg. We are cooperating with Franz Bairlein, Onur Güntürkün from the University of Bochum, Miriam Liedvogel from the Max Planck Institute for Evolutionary Biology in Plön, Elmar Behrmann from the University of Cologne, Nachum Ulanovsky from the Weizmann Institute in Israel and Peter Hore from Oxford. These experts cover fields of research that are not represented in Oldenburg. We hope to be able to answer about half of the 20 questions I posed at the end of my Nature review with this proposed SFB.
Interview: Constanze Böttcher
