October 27, 2010 § 1 Comment
October 18, 2010 § Leave a comment
Solvay Public Conference, Sunday 17th of October 2010
Wolfgang Wiltschko: ‘The magnetic compass of birds’
Since the 1960s, Professor Wolfgang Wiltschko and colleagues of the Goethe-Universität Frankfurt am Main in Germany have been conducting experiments that have provided proof that birds use the Earth’s magnetic field to migrate.
Migratory birds are highly mobile organisms. In order to navigate successfully over long distances, they need to be able to do two things:
1) determine the course of the goal, e.g. 180° degrees South,and
2) use a compass mechanism.
Arctic Terns (Sterna paradisaea), whose yearly migration takes them from their northern breeding grounds all the way to the Antarctic and back (a 44,300 mile round-trip), spend the long summers in the Arctic and migrate southwards at the beginning of autumn. Pied flycatchers (ficedula hypoleuca), who also make use of long northern summers to breed, migrate southeastward to Central Europe and then move again southwestward; thus bypassing the harsh natural barriers of the Alps and the Sahara desert to reach western Africa. This SE-SW (or NE-NW on the way back) pattern is one often observed in migratory birds’ flightpaths.
According to Wiltschko, these birds all share an innate migratory compass. They use several such compasses: solar, stellar and the Earth’s magnetic field. It is the last one of these that Wiltschko homed in on in his conference yesterday at the Solvay Institute in Brussels.
At about the time the word heimweh, a word of Swiss origin meaning homesickness or longing for home, was introduced to mainstream European vocabulary in the mid 1700s, it was realised that the Earth is a big magnet. This magnet has two magnetic poles, the North Magnetic Pole and the South Magnetic Pole, close to but not aligned exactly to the tilt of the Earth’s axis.
The Earth’s magnetic field extends thousands of kilometres into space. Wiltschko and colleagues have been studying birds’ behaviour relative to this magnetic field for the last forty-odd years and have determined that migratory birds display a reliable directional preference while migrating.
By catching the birds on their migratory paths and placing them in an enclosed space where the magnetic field has been artificially adjusted – say to 120° off Magnetic North – they have been able to deduce that these birds are using an innate compass to follow the Earth’s magnetic field in their seasonal migrations.
The European Robin (Erithacus rubicula), for example, which breeds in most parts of Europe, is a partial migrant (i.e. not all individuals migrate). The magnetic compass of these birds has been tested at an experimental station in Frankfurt-am-Main where robins are caught and tested en route from Scandinavia to northern Africa. By placing the robin in an enclosed funnel cage and sending an artificially adjusted magnetic current through the cage with a similar intensity to the Earth’s magnetic field at a given latitude, it has been found that the bird will move relative to the North-South vector (an imaginary line drawn along a magnetic field) of the adjusted field on being set free.
By analysing the magnetic compass of robins, it was discovered that it is primarily this vertical component of the mechanism (the North-South vector) that determines the preferred migratory paths of the bird; i.e. even if the direction of this vector is adjusted, the robin will still go ‘poleward’.
In another similar experiment carried out on domestic chickens (Gallus gallus domesticus) in Australia, a social stimulus was added to the directional one. By placing a newly hatched chick in a glass box containing a red ball and four screens – one at each of the four cardinal points (North, South, East and West) – after two days the chick will gravitate toward the ball as though it were its mother. The ‘mother’ (red ball) is then removed from the chick’s view and placed behind a screen in the northern corner of the box. The chick searches the box for the missing ball, and finds it behind the northern screen. When the ball is once again removed and this time the magnetic current is switched 90°, the chick will spontaneously move toward the adjusted north (in this case really the West or East) and look behind the screen there.
In both these experiments it can be deduced that the bird trusts the ‘vertical component’ of the magnetic field. In migratory birds, however, one important anomaly results from this reliance on the vertical component; at the equator, the inclination of the Earth’s magnetic field is neutral (i.e. the field lines are approximately parallel to the Earth’s surface) and so migratory birds cannot use the magnetic field at the equator to navigate. It is only when the birds move north or south of the equator, where the field lines are set at a steeper angle to the Earth’s surface, that they can perceive their latitude and direction again.
“E come li stornei ne portan l’ali/ nel freddo tempo, a schiera larga e piena …”
“And as in cold weather, circling in serried groups, / starlings are lifted skyward on their wings …”
Dante, Inferno, V.
It has been shown that starlings introduced to Australia from the northern hemisphere have been able to adjust their compasses and fly southward (towards the pole) instead of northward (towards the equator) in their seasonal migrations.
It has also been discovered that birds can adapt to much weaker magnetic fields. As we have seen, the magnetic field at equatorial latitudes is very weak; therefore, birds need to get tuned to a field before they can make use of it for navigation.
In other words, birds need to make a mental map of a region before they can navigate their way around it. In the same way that a child has an instinct to spontaneously learn language in order to be able to ‘navigate’ the immediate world around it, the birds integrate the ‘lay of the land’ across the target region. This innate compass (or ‘navigational language’) is progressively fine-tuned by adding to it features of the landscape, e.g. sounds, smells and geological features seen from above when the bird is in flight. The magnetic compass is thus the backbone of the birds’ navigational aptitude.
The effectiveness of the magnetic compass is also increased by addition of other compasses; stellar and solar. Similar mechanisms have been found and studied in bats, mole rats, butterflies and sea turtles.
As often in experimental science, hunches give rise to unexpected discoveries. Wiltschko and his team discovered that the navigational abilities of robins were linked to the right eye (and thus to the left hemisphere of the brain). By placing a hood over the robin’s right eye – the left one was also tried, with little or no navigational impairment – it was discovered that the magnetic compass of these birds is lateralized in favour of the right eye.
From all of the above experimental results, which have also been carried out on numerous other species of birds, two main hypotheses present themselves concerning the magnetic compass of migrating birds:
1. Transitory magnetoreception*: a light-dependent process in photopigment in the birds’ eyes.
In this hypothesis, it is primarily light waves (or photons) that incline the bird to fly in a particular direction. The strongest argument for this hypothesis has come from experiments where the direction and intensity of light waves hitting the retina of the bird have been fiddled with. In these experiments, light at longer wavelengths (light at the red end of the spectrum) has been shown to disrupt the birds’ compass completely, whereas white and blue light have little or no adverse effect on the compass.
The exact point on the spectrum at which this disruption occurs has also been determined. It seems that in most cases, it is on the border between green and yellow light where the compass suddenly breaks down. It is therefore a tiny shift (a few nanometres) in the spectrum of the light entering the birds’ eyes that ends up disorienting the birds’ compass very significantly.
2. In the second, or permanent magnetoreception, hypothesis it is believed that the navigational aptitude in birds can be traced to the presence of magnetite (iron oxide) in their beaks. Wiltschko and his colleagues are convinced, however, that the magnetite found in birds’ beak serves only as a kind of rudder the bird uses to position itself correctly in flight; rather than functioning as an inbuilt magnetic needle.
Finally, as a zoologist, Wiltschko is keen to point out that this ‘phylogenetic’, or commonly inherited, characteristic in modern birds can be traced back as far as the Late Cretaceous era; 70 to 90 million years ago. The inbuilt navigational devices of the early orders of Galloanserea (fowl) and Neoaves – the two main orders from which modern birds have evolved – can be seen in action today in our chickens, pigeons and Passerines (perching birds). All modern birds have magnetic compasses inherited from this time. It’s how they find their way home.
* Magnetoreception is the ability to detect a magnetic field.
October 13, 2010 § Leave a comment
October 11, 2010 § 1 Comment
October 11, 2010 § 3 Comments
Mr Rhinoceros was hot and bothered. He trudged lazily across the savannah under the hot afternoon sun. He was fed up with hot weather and nasty flies nibbling his ears and getting in his eyes. He was on his way to the watering hole where all the animals of the reserve went to drink and wallow in the cool muddy waters. “I’m going to stick all day long in the mud”, he thought to himself, “and if anybody talks to me I’ll pretend I’m asleep”. With that, he gave a tremendous sigh that ended with a grunt. At last, in the haze, he saw the pool ahead and began to feel better already at the thought of wading into the oozing mud and covering himself up to the nostrils. But when he got to the edge of the pool, he saw that it was almost dried up and that if he were lucky he might be able to get mud up to his knees. “This must be why no-one is around”, he mused, “they’ve all gone to the delta”. He was in no mood to follow them such a distance, so he began slowly treading towards the centre of the pool. “Well, at least no-one is here to bother me,” he exclaimed out loud. Just then, he realized that three of his stumpy feet were stuck in the mud and that he couldn’t move. His fourth (back, right) foot was left dangling in the air. He knew that if he put it in he had no chance of getting out and that night would come and bring crocodiles and other sneaky predators who would wait for him to get weak and then gobble him up alive! He began to struggle but this only made his three feet sink faster into the thick black mud. It was then he started to panic, and he almost lost his footing altogether.
He began to sob, and wish that he had never left his shady spot under the trees that morning. Just as he was beginning to give up hope of ever getting free, he saw what he thought had been a stone in the middle of the pool moving in his direction. At first, he was afraid it was a crocodile coming to get him but then he remembered that crocodiles don’t have backs shaped like stones. He then saw that this “stone” had four legs, and that it even had a head! It was moving very slowly and seemed not to have a care in the world. At last, it arrived alongside the rhinoceros and poked out its head. “I’m looking for some water” it remarked, yawning widely when it said the word “WATER”. “I have been here a long time and I can’t find a drop; I’m beginning to get thirsty”. Mr. Rhinoceros looked at this strange creature and said, “I didn’t know stones drank water?”. “They don’t”, replied the “stone” distractedly, “but I do, and I’m asking you if you’ve seen any hereabouts”. All this was very strange to the rhinoceros; but just then it struck him that the stone could help him to get unstuck. “If you help me get out of this mess, I’ll bring you to find some water. There is a river not far from here where I’m sure you can get some”, he offered. The “stone” looked at him for what seemed like a very long time and then said to him nonchalantly, “Alright, it’s a deal.” Though, to tell the truth, he didn’t look in the least bit convinced.
Mr. Rhinoceros had once seen his mother use stones as steps to cross a running river; and so, he had an idea: “If you stand still over here I can place my one free foot on top of you and pull my other feet free. Then, I’ll be able to roll over to the edge of the pool, put you on my back, and we can go in search of water”. The “stone” made no reply for a few moments, as if it were trying to take in what it had just been told. Then, ever so slowly turning, it began to walk over to where the rhinoceros had indicated nodding with his horns. It placed itself bravely beneath the one free foot and, to the astonishment of Mr. Rhinoceros, drew in its legs and neck and became an immobile stone again.
The rhinoceros placed its foot on the curved “back” and, applying its weight, tugged with all its might to get its other feet free. For a few moments, nothing budged, then, all of a sudden, one of the hind feet came unstuck making a loud suction noise: “SHGLOOP”. The rhinoceros could now fall over on one side (its two right feet being free) and, in doing so, yank out the other two on the left. It lay half-submerged and exhausted in the mud, with its clodded toes sticking in the air, and looking rather like an enormous beetle toppled helplessly on its back.
For a moment, Mr. Rhinoceros was so exhausted and happy that he forgot everything else. When he came to his senses again, the “stone” was nowhere to be seen. He looked left and right, arching his strong neck this way and that. “Well”, he said, “that’s a pity. It must have changed its mind.” Presently, he squirmed his way backwards over towards the edge of the pool. When he was on his feet again, he set off lazily in the direction of the delta. He had sauntered about ten yards when he heard a strange but familiar voice in his ear: “AWFULLY kind of you, sir”.
He almost jumped with fright (he was really a very sensitive rhinoceros). It was the “stone”. It was sitting on Mr. Rhinoceros’s back, nestling between his big hump and his shoulder blades. When he recovered from this shock, he thanked the “stone” for his assistance, said he hoped he hadn’t been too much of a burden, and promised there would be plenty of water to drink very soon. The “stone” said laconically, “Yes, that’s good”, and then added, “It has been at least a month since I had some water. My poor throat is dry as a bone.” And he emphasised the word ‘bone’. So, the pair set off for the delta; the rhinoceros walking at a slow steady pace and talking about the weather, the flies, and the general state of affairs on the savannah. All three were “a bother and a nuisance”, in his estimation. The “stone” merely said “Mmmm” and “Ahhh” from time to time and the rhinoceros, thinking that he must be parched (the poor fellow) reassured him constantly saying: “Not long now,” or, “Hold on there,” and went on complaining about everything.
And so, with the gentle bumping and swaying of the rhinoceros’s gait and the monotonous drone of his voice, the “stone” (who was really a turtle) was soon soundly asleep. He was dreaming that he was little again and that he was at the seaside with his mother and father. They were teaching him how to swim. Each time he ventured into the water, a big wave would come crashing down and toss him upside down or back-to-front. He was terrified, and paddled his little legs with all his might, he tiny eyes closed. His mother nudged him back in the right direction again and his father swam in front of him to protect him from the pounding surf. At last, he made it past the big waves and was soon paddling gleefully around in the water. He was free for the first time and yet he felt the sea powerfully surging around him. When he surfaced for air, he saw that the land was further away and that his mother and father were still there, no bigger than two shiny pebbles on the shore. He ducked his head and tried to swim back towards them. It was no use, however; his limbs were too weak to carry him against the strong current. Soon, the land was out of sight. The water seemed colder now, and so he began to paddle slowly, letting himself be carried by the tide. By nightfall, he had travelled a long way. The sea was inky black. And yet, before the little turtle’s eyes swam shoals of creatures infinitely smaller and brighter than the stars above him.
When he awoke, he found himself on land again. This time, however, it was not the sandy shore where he had left his parents. Instead, it was what seemed to be a slate grey rock half-submerged in the water. Nothing grew there, not even one shrub. His throat was very dry. He stretched out his neck and tasted the water. It was fresh and sweet! He drank his fill and moved towards the edge of the rock, letting his body slide out into the cool current….