Nature’s Depths

Over much of Planet Earth’s Northern Hemisphere, change is in progress. The Autumnal Equinox is behind us and the days are continuing to get shorter. Temperatures are dropping—a few weeks ago they were often in the eighties, now they are dipping down even into the thirties.

Magical colors are all around us. The green forests vibrate with red and gold.

The prairies that once were flower-filled now glow with the stems of mature grasses. Butterflies are nowhere to be seen—most are hiding in their overwintering stages and the monarchs have set off on their journey to the mountains of Mexico, a journey that can be as long as 2,500 miles.

Not long ago the wetlands teemed with tree swallows and barn swallows and red-winged blackbirds. Now many of them are still and the branches of the trees are naked of birds. There is a small lake near us where I used to see around a dozen great blue herons every time I went to visit. Now there are none. The ducks are moving about; every day there is a new group, or perhaps none. Two days ago there was one remaining pair of double crested cormorants where once they had been abundant. Now even that pair is gone. Only the ring-billed gulls remain. In other areas, sandhill cranes are present in greater numbers than before, in some protected areas numbering in the thousands, stopping on their way from marshy areas further north to the rich opportunities available in the south.

And the geese . . . One of the most evocative classics in the American literature about the natural world is Aldo Leopold’s A Sand County Almanac—”Sand County” because that is where he lived in Wisconsin; “almanac” because the chapters go month by month, always creating in words a vivid sense of what is happening all around us. The chapter for March is called “The Geese Return.” This is how it starts:

     “One swallow does not make a summer, but one skein of geese, cleaving the murk of a March thaw, is the spring.
      . . .
     They weave low over the marshes and meadows, greeting each newly melted puddle and pool. Finally, after a few pro forma circlings of our marsh, they set wing and glide silently to the pond, back landing-gear lowered and rumps of white against the far hill. Once touching water, our newly arrived guests set up a honking and splashing that shakes the last thought of winter out of the brittle cattails. Our geese are home again!”

And this is how it closes:

     “Every March since the Pleistocene, the geese have honked unity from China Sea to Siberian Steppe, from Euphrates to Volga, from Nile to Murmansk, from Lincolnshire to Spitzbergen. Every March since the Pleistocene, the geese have honked unity from Currituck to Labrador, Matamuskeet to Ungave, Horseshoe Lake to Hudson’s Bay, Avery Island to Baffin Land, Panhandle to Mackenzie, Sacramento to Yukon.

     By this international commerce of geese, the waste corn of Illinois is carried through the clouds to the Arctic tundras, there to combine with the waste sunlight of a nightless June to grow goslings for all the lands between. And in this annual barter of food for light, and winter warmth for summer solitude, the whole continent receives as net profit a wild poem dropped from the murky skies upon the muds of March.”

Here are two words from that poem arriving on a nearby lake in the spring, wings set and gliding silently, just as Aldo Leopold described.

Remember the golden goslings of the early summer that we saw in the previous post? They are now grown up and hard to distinguish from their parents, but they still mostly stay together as a family. Seven, including the parents, is a typical family size. Two or three families often get together to form a flock, whether for feeding or for flying.

And here, photographed a few fall days ago, is a flock of geese taking off from my favorite heron-watching lake. Increasing activity in the form of rearing up and flapping of wings, often several birds at the same time (upper picture), often precedes the flock’s takeoff (lower picture).

In this way, signs of migration are all around us. Given the profound environmental changes that stem from dense human settlement—among them urban sprawl and vast agricultural fields replacing forest and prairie—as well as increasing global warming and other changes in the climate, the exact migration patterns of birds, mammals, insects, sea turtles, and other organisms are also changing. Seeing Canada geese throughout the winter in places which they once abandoned for warmer areas is a conspicuous example. But the presence of resident populations of migratory species does not mean that migrations have stopped. It means only that some members of the species stay and take advantage of new resources that are available due to the human presence and/or the warmer winters due to increasing levels of greenhouse gases, while other members of the same species continue to undertake the demanding journey south.

HEADING SOUTH

Our migratory kin travel south in the fall and back north in the spring. They do this over distances of hundreds or thousands, even tens of thousands of miles. The longest known migration is that of Arctic terns, modest-sized sea birds who fly from the Arctic (where they breed) to the Antarctic (where they winter) and then back, a total distance of some 25,000 miles. They make this round trip annually throughout a lifetime of two or three decades.

How is this possible? Let us side-step the question of how different species manage to obtain enough energy to fly, walk, or swim for such distances and ask the other big question: How do they know in what direction they should be traveling? A good overview of this basic question is given here.

We humans have learned how to navigate reliably over great distances. Remembering how we obtain the necessary directional information may help us uncover how other organisms do this. Here are some well-established sources of information used in human navigation:

     • prior knowledge of landmarks, including oceanic currents
     • maps (paper or electronic)
     • the sun
     • the stars
     • the Earth’s magnetic field

Which of these information sources do migrating animals use? All of them, except for GPS! We do not have a comprehensive account of the mechanisms guiding navigation for any species, but we do know important components. For example, there is evidence that both birds and bees use landmarks—they learn and remember the terrain in which they are living. But to use learned landmarks to get to a desired destination also requires them, just as it does us, to know their momentary location in relation to the desired location, in other words, to know those two locations on a map. Thus, birds and bees must build an internal map of some sort.

For other cues to be useful requires other supporting mechanisms. For example, using the sun or the stars requires knowing the time of day (or night), information provided by a biological clock. Not only that, to use the stars for navigation requires knowing the season because the pattern of stars in the sky at any given clock hour shifts nightly and goes through a complete annual cycle. And there’s yet more. The pattern of stars in the sky at a particular time on a particular night of the year depends on the latitude of the observer, their location between the pole and the equator. This changes substantially as a long-distance migration proceeds.

Employing stellar navigation is a highly sophisticated process! We know that birds can use it while flying at night—if we set them up in a planetarium and change the pattern of stars we display to them, they adjust their flight direction accordingly. How they do this remains a mystery.

MAGNETIC NAVIGATION

Using the Earth’s magnetic field for navigation is just as challenging. It requires having a sensitive biological compass, a mechanism for determining the direction of that field, weak as it is. The field is effectively constant, so clock time, season, and latitude are not concerns, but having a biological mechanism for detecting the field is essential. We humans use manufactured compasses we purchase in a store for this purpose. What could a bird or other organism possibly use? Three hypotheses that are very different but not mutually exclusive have been proposed in recent years and are being actively investigated:

     1. Intracellular magnets—Magnetic structures within single cells, especially in birds’ beaks, that have properties resembling those of compass needles.
     2. Magnetically responsive visual pigments—Molecules in the eyes that behave differently depending on their orientation relative to a magnetic field and thus could generate a signal useful in navigation.
     3. Inner ear currents—Electrical currents generated within the inner ear as a bird moves through the Earth’s magnetic field.

Let’s look at these one at a time.

     1. Intracellular magnets—In the 1970s, it was observed that certain bacteria swim in a direction that is regulated by the magnetic field to which they are exposed. Curious investigators found that such bacteria build within themselves special microscopic structures which are now called magnetosomes (magnetic bodies). These behave much like the compass needles we are familiar with—the Earth’s magnetic field physically aligns them so that one end points toward the pole (in the northern hemisphere north, in the southern hemisphere south) and the bacterium, which has no separate steering mechanism, swims accordingly. Magnetosomes get their magnetic properties from an iron-containing protein called magnetite.

This surprising finding prompted a search for magnetite and magnetosomes in birds and other animals, especially ones that migrate for long distances. Magnetite has indeed been found in birds, most consistently in their beaks, but there are two big problems. The first is basically a gap in our understanding: it is not at all clear how a cell would generate the electrical signals that the nervous system requires to guide flight just because it contains magnetite.

The second, more challenging problem, is a peculiarity in the response of birds to the magnetic field around them. We know that Northern Hemisphere birds migrate south in the fall and north in the spring. However, if we place such a bird in a laboratory setting in which we can control its environment, keep everything constant except the direction of the magnetic field, and suddenly precisely reverse the direction of that field (technically fairly easy to do), the bird pays no attention. It keeps flying in its original direction! A magnetically sensitive bacterium, in contrast, is immediately forced to reverse directions.

Clearly, something unusual and counterintuitive is going on here. When these results were first published, scientists fascinated by magnetic navigation immediately started seeking alternative mechanisms that would give birds information about the alignment of the Earth’s magnetic field but not about its polarity.

     2. Magnetically responsive visual pigments—Over the last thirty years or so, there has been an accumulation of circumstantial evidence that specific pigments (called cryptochromes) in the eyes of birds play an important role not in a bird’s vision but in its magnetic navigation. These truly are pigments—they absorb light energy and they are most responsive to just a portion of the spectrum of visible light, the short wavelength (blue and green) portion of that spectrum. However, they have an additional property. Absorbing a quantum of light supplies any pigment molecule with energy. That is what happens in chlorophyll and is the start of the long chain of steps whose outcome is the synthesis of the sugars that are the foundation of all the food we eat. In the case of cryptochromes, the absorbed energy results in the formation of pairs of molecules whose properties are either matched or different in one specific respect at the sub-atomic, electron level. They are referred to as a radical pair. The proportion of the matched pairs and different pairs is governed by the orientation of the cryptochrome molecules in the magnetic field. Hence, cryptochromes generate an orientation-dependent molecular signal. What’s more, they are the only proteins in the animal world that are known to do this.

As with magnetosomes, the way this initial cryptochrome signal is converted into the electrical signals required by the nervous system has not yet been worked out. However, the system does account for the peculiar non-response of migrating birds to a reversal in the orientation of the magnetic field in which they find themselves—like birds, radical pairs reflect the alignment of the magnetic field but not its polarity. This is one piece of evidence among many making the cryptochrome hypothesis highly attractive.

     3. Inner ear currents—If you move a magnet within a loop of wire or a loop of wire along a magnet, you will generate an electrical current within the wire. This is called induction. On a large scale, this is how electrical power generators work: some mechanical source of energy—whether wind or steam or water spilling out of a dam—is used to rotate a massive coil of wire around a magnet or a magnet around the coil and this rotation induces current within the wire that is ultimately brought to our houses for us to use.

Many years ago, it was proposed that the semicircular canals of mammalian inner ears, crucial for the sense of balance, might resemble coils of wire because they are filled with a salty fluid that is an electrical conductor. When these canals are moved within the Earth’s magnetic field, they might generate induced electrical currents, and these might ultimately cause nerve impulses to be transmitted to the brain to guide navigation. Recent work is showing that this is a plausible hypothesis for the magnetically-guided navigation of birds. It has an interesting advantage—it would work equally well during the day and during the night, whereas the functioning of the cryptochrome pigments fails in the dark.

These three hypotheses invoke very different mechanisms, but there is no reason why they might not all function within a single organism and, indeed, complement each other. For example, it has been proposed that the main role of magnetite is not to guide flight along a north-south track but to give a bird information about where it is located on a magnetic map of its route. This would be of great help in keeping it on track over huge distances. The role of cryptochromes might be to enable very precise alignment with the desired flight trajectory. And the role of inner ear currents might be to provide magnetic information during night flight, especially on nights when the stars are not visible.

All the scientists working in this field agree that we are far from having the full story. This is science in process!

MAGNETO-SENSITIVE HUMANS?

Birds do it; whales, bats, and many other mammals do it; turtles do it; fish do it; insects do it; even bacteria do it. So, can we humans orient ourselves in the Earth’s magnetic field?

This has been a controversial area of investigation. In the 1980s a Scottish researcher by the name of Robin Baker assembled a raft of evidence indicating that yes, we can. His evidence was criticized for not being sufficiently rigorous. However, scientists are curious and are forever seeking new ways to approach intriguing questions. Here are two recent lines of evidence that were not available to Baker in the 1980s but that now suggest that he may well have been right.

     1. Human brain waves are affected by the magnetic field around us. Experiments at Caltech and at the University of Tokyo published in 2019 indicate that steady magnetic fields (not the magnetic pulses that are used to stimulate brains by inducing electrical currents within them) modify our brainwaves. The subjects were not aware of anything special when the orientation of these fields was altered, but the changes in their brainwaves (reductions in alpha-waves, for those in the know) are generally indicative of sensory input reaching the brain and of the brain paying attention. Here is a fine review of this work.

This finding supports the possibility that we may respond to the Earth’s magnetic field, possibly in a useful way, even though we are not aware of it. A lack of awareness of significant sensory input would not be a new finding—by no means all of the information the brain uses to keep us functioning enters our conscious awareness. Among many other examples, it has been known for a long time that we respond to specific chemicals (pheromones), especially those released by the opposite sex, without being aware of it.

     2. Humans, too, have cryptochromes. Not only that, our cryptochromes are sensitive to the Earth’s magnetic field. How did we find this out? Fruitflies (Drosophila), among the favorite organisms for molecular genetics, orient in a magnetic field. They have cryptochromes. If you deprive them of their cryptochromes by disabling specific genes, the flies’ ability to respond to magnetic fields is lost. If you replace those non-functional genes with human genes that produce human cryptochromes, the flies’ magnetic sense is restored. Human cryptochromes enable fruitflies to sense the Earth’s magnetic field!

Of course, this experiment does not prove that we ourselves use our cryptochromes for this purpose. For example, the same cryptochromes play a role in our biological clocks. Nevertheless, we now have a demonstration that our bodies have a mechanism for sensing the Earth’s magnetic field. A thoroughgoing skeptic (as many great scientists are!) will have to show that we have such a mechanism but that we don’t use it.

THE BROAD PERSPECTIVE

Over and over again in Nature’s Depths, we have focused on stories that illustrate the degree to which we humans are a part of the natural world, of life on Earth. Yes, we have special attributes—no other creatures have such a highly developed language or such amazing intellectual powers, and none have generated an agricultural revolution and later an industrial revolution. Nevertheless, other creatures do communicate, they do solve problems, and they do build structures. There is not an unbridgeable gap between us.

When we look at other organisms, we recognize how interrelated these organisms are in the Earth’s biosphere and how finely tuned they are to their overall environment, including its non-living components. We need to remember that this interrelatedness and fine tuning applies to human beings as well, often in ways that do not come prominently to our attention. The ability to respond to the Earth’s magnetic field is an example of an ability that we may have without being aware of it. How many other such abilities might there be? How might we be entangled in the whole Earth system in ways that remain to be discovered? These are questions well worth pondering.