“All zoology is really ecology. We cannot fully understand the lives of animals without understanding our microbes and our symbioses with them.”
― Ed Yong, in I Contain Multitudes
There and back again
The honey bee’s waggle dance. Bats’ echolocation. Salmon’s riverine olfaction. Penguins’ magnetoreception. London taxi-drivers’ spatial-cognitive mapping.
Over the course of evolution, we animals have devised a number of innovative ways to get from here to there, and especially, to find our way home.
In Hawai‘i, green sea turtles navigate hundreds of miles from the main Hawaiian Islands every few years to nest in the French Frigate Shoals, the largest atoll in the Northwest Hawaiian Islands. After returning to their home beaches, female sea turtles make a nest with a clutch of up to 180 eggs.
Once hatched, baby sea turtles scramble for the water, hopefully dodging hungry predators, and dispersing into the open ocean for several years. Scientists call this 5–10 years of open-ocean existence for young turtles ‘the lost years,’ as there is still much we don’t know about this vulnerable period of juvenile sea turtles’ life cycle.
After these lost years, floating around in the open ocean, Hawaiian green sea turtles are ‘recruited’ –– or put down roots — in particular foraging grounds across the Hawaiian islands (usually places with tasty seaweed). They then proceed to munch their way through life over the next several years, foraging, sleeping, basking, and foraging some more.
Eventually, once young female sea turtles reach 20–30 years of age, they miraculously trace back their 500-mile journey across the middle of the Pacific to reach their home beach of origin in the French Frigate Shoals. Once there, they continue their ancient tradition of making a nest to initiate a new sea turtle life cycle, as they’ve done for millions of years.
The ability of sea turtles – not to mention birds, lobsters, and whales – to detect the earth’s magnetic field to guide their way home is widely known but surprisingly little studied. “Despite the prevalence of natal homing,” write sensory biologists Catherine Lohmann and Kenneth Lohmman in their recent overview of the growing research on animals’ magnetic sensing, “the mechanisms underlying it have received little attention.”
“For some animals, there’s something it feels like to be such an animal. There is a self, of some kind, that experiences what goes on.”
— Peter Godfrey-Smith, in Other Minds
The emerging science of magnetotactic bacteria (MTB)
In addition to sea turtles, many other animals, from penguins to salmon, navigate confidently across vast distances of what seems to be featureless ocean-scapes to return to their place of birth.
For example, salmon appear to traverse large expanses of ocean to return to their home rivers by detecting unique magnetic signatures or ‘cues’ of specific coastal areas, imprinted on their brains at birth. Once magnetically pulled to the entrance of their spawning grounds, they switch to a chemical system of navigation to complete the journey up the river, what scientists call a “biphasic navigational strategy.”
Growing evidence suggests sea turtles use a biphasic strategy as well: following magnetic cues for finding home, and chemical cues for getting around once there.
In addition to biphasic navigation, sea turtles actually have two kinds of magnetic senses: 1) a ‘compass sense’ and 2) a ‘map sense.’ Where the magnetic compass sense helps them go in the ‘right’ direction, their magnetic map sense allows them to intuit their geographic position on the earth.
However, despite all that we have learned about magnetic navigation in animals like sea turtles over the past fifty years, how it actually works –what mechanisms underlie this capability – remains a deep scientific mystery. As a new study puts it:
“the identity of the magnetic-sensing organ and its structure and/or apparatus within such animals remains elusive — a sense without a receptor.”
While there is still much we don’t know, one tantalizing hypothesis is gaining traction from a study with new findings on ‘MTB’ or symbiotic ‘magnetotactic bacteria.’
The crazy idea is this: it may not be a special sensory organ in animals’ bodies or brains that enables them to use magnetism to navigate. Instead, it appears some animals have created a symbiotic partnership with a special kind of ‘magneto’ bacteria that serves as the underlying mechanism for this magnetic superpower in certain animals.
As Robert Fitak, Assistant Professor of Biology at the University of Central Florida, says, “The search for a mechanism has been proposed as one of the last major frontiers in sensory biology and described as if we are ‘searching for a needle in a haystack.”
Indeed, the needle has been well buried, and the potential that symbiotic magnetic bacteria are the mechanism behind magnetic sensing in animals has remained a fun hypothesis to think about, but without much in the way of evidence.
Unraveling the mystery of magnetoreception
There hasn’t been much evidence until recently that is. Two recent studies on host-bacteria symbiosis have provided the first empirical evidence that there might just be something to this MTB hypothesis.
Magnetotactic bacteria (MTB) are bacteria that contain magnetosomes. Megnetosomes are magnetic mineral crystals enveloped by a membrane. At the bottom of the sea, researchers found ocean-dwelling eukaryotic protists covered in MTB. Their research showed that eukaryotes acquired a magnetic sense from their MTB clothing, enabling them to direct the movement of the larger host organism they formed a part of.
Another study found that when reed warblers ––a migrating bird known to use magnetic detection to navigate––were given an antibiotic that targeted their MTB, the birds lost their sense of direction.
Building on this new evidence, Fitak and his research team compiled a database of all known bacteria containing magnetosomes, revealing 92-MTB species discovered over the past 39 years.
The results showed that MTB is everywhere. Humans are chock full of them! But it seems that only some animals have evolved the symbiotic partnership with these magnetic bacteria needed to harness the amazing powers of navigation they can bestow on their host.
As Fitak points out, we still don’t really know where magnetotactic bacteria would live in an animal. They suspect it is likely in nervous tissue, like the brain, or the eye. But how these bacteria actually communicate with their host organism remains an enduring mystery, but a mystery new research is beginning to unravel. As Fitak’s team writes,
“Even if a symbiotic magnetic sense is indeed widespread, a further challenge lies in unraveling the mechanism by which the host and MTB communicate, i.e.how does a host, such as birds sense the bacterial reaction to changes in the ambient magnetic field? All in all, the symbiotic magnetic-sensing hypothesis is a hypothesis worth considering.”
I wonder if humans too contain these latent magnetotactic powers within our microbiome? Potential symbiotic relationships that lie in waiting, perhaps to be activated someday through a new spark of microbial collaboration?
“Our darkest fiction is full of Orwellian dystopias, shadowy cabals, and mind-controlling supervillains. But it turns out that the brainless, microscopic, single-celled organisms that live inside us have been pulling on our strings all along.”
― Ed Yong, in I Contain Multitudes: The Microbes Within Us and a Grander View of Life