The human body is able to obtain information about our environment in unique ways. Our eyes convert photons of reflected light into electrical and chemical signals that our brain processes into images of the world around us. Our noses are full of specialized cells that detect millions (or even trillions!) of different scents floating around in our environment: aromatic flowers, the putrescent stench of skunk, and tart citrus fruits all activate neural signaling. Our ears are specialized for detecting the compression of molecules in the air, from the deep, low frequency rumbles of 20 Hz to the piercing screams of 20,000 Hz.
The human body is able to detect photons of light, airborne chemicals, and sound waves. But there is one ever-present force to which our bodies are seemingly agnostic: magnetism.
Like ancient mariners relying on a compass to guide them home, the honeybee (Apis mellifera) uses the Earth’s magnetic forces to help them chart their path back to the hive. Unlike sailors, however, honeybees don’t need a tool to tell them which way is north: they have a compass built directly into their bodies.
Being able to sense their surroundings provides a huge survival advantage for ancient, pre-modern bees. Naturally, it helps with the detection and avoidance of predators, but it also helps them find food. Coming across a delicious pollen-dusted flower is good; remembering how to return to this food source with a swarm of fellow bees is even better. They must depend on all their senses to successfully navigate the highly complex environment, and collectively, all the inputs from the range of sensory modalities help point them to the goal.
But let’s back up a bit: Why does the Earth emit a magnetic field to begin with? Deep underneath the rocky crust of our planet, molten iron alloys are constantly flowing back and forth. The movement of these metals creates a magnetic field that extends millions of miles into space. Surprisingly, despite the tremendous volume of space engulfed by this magnetosphere, the strength of the field is several times weaker than a simple refrigerator magnet. But don’t conflate this weakness with uselessness: without the magnetosphere, the surface of the planet would be bombarded by a steady stream of harmful solar radiation.
More about BEEhavior
Although our human stomachs might figuratively guide us to the fridge, the honeybee abdomen literally points them north. They possess this intrinsic navigational ability because of a population of cells in their abdomen called trophocytes. These cells provide nutrition to nearby cells, but this support function isn’t what makes them unique: honeybee trophocytes have tiny, half-micron wide iron granules within the cell. These granules are so dense in trophocytes: there are about 1.25 grams of iron per cubic centimeter of cytoplasm. Discovering the iron within these trophocytes was one of the earliest clues indicating that the honeybee might be able to sense magnetic fields. Upon exposing these trophocytes to a magnet, the iron granules interact with intracellular structural proteins,physically distorting the cell and aligning it with the North Pole.
For Homo sapiens,iron is the most prevalent heavy metal in our body. The major proteinous component of red blood cells, hemoglobin, relies on iron to carry out its primary function of oxygen delivery. Iron-containing proteins contribute to generating cellular power in mitochondria. Iron is used as a cofactor by nuclear proteins for such essential functions as DNA replication and cell cycle progression. All this iron – yet we can’t detect magnetic north.
So, why did honeybees develop this remarkable ability to detect magnetic fields? Odds are good that a random evolutionary mutation left some bees with this capability, and this new ability proved helpful for the survival of the species. Honeybees work together in a shared societal structure. Each bee within a hive acts to fulfill whatever task improves the viability of the hive, thus improving the individual odds of survival. When the communal food supply gets low, brood pheromone (BP) is produced, which triggers the bees to leave the hive in search of pollen, the carbohydrate-filled and nutrient-rich product of flowering plants. BP levels also increase when the queen is ready to lay eggs, causing the bees to stockpile food in anticipation of the hungry new mouths that need to be fed.
Often times, worker bees travel great distances and navigate complex environments to find food (sometimes as far as seven miles!). Of course, one single bee could never carry enough to feed the whole hive. After a fruitful journey, the scout has to return home to recruit the help of their worker bee sisters (All worker bees are female!). Worker bees can then be mobilized to leave the hive and find the food source on their own. To navigate this expansive, open-world, they must rely on their senses – including their ability to detect the Earth’s magnetic field.
Not only do these magnetosensitive trophocytes help the bees navigate, they are also able to help encode a memory of the sensation of a unique magnetic field. In one study, honeybees were given the choice of drinking from one of two dishes of sugar water. While one dish was sweetened with 20% sugar, the other was even more nutritious at 50% sugar. One or the other feeding conditions was paired with an experimenter-produced magnetic field, with the Earth’s natural magnetic field serving as the control condition. After learning the association between the preferred sucrose drink and a specific magnetic field condition, both sucrose dishes were switched out for pure water. The bees returned to the dish of water associated with the magnetic field that was observed at the 50% sucrose dish. Furthermore, attaching tiny magnets to the abdomen of bees impaired their ability to accurately identify the difference between two identical-looking food sources.
How do humans affect honeybees, and how can I help?
Honeybees truly have remarkable navigational talents that help them bring food home. But this same evolutionary survival skill might be bringing death to the species. Honeybees are facing a crisis due to the increased toxicity of man-made pesticides. This colony collapse disorder might be due to worker bees who carry pesticides back to the hive, which then wipe out the whole colony from within. We have to do our part to be kind to bees – learn more about what you can do here.
Austin Lim is a dancer, artist, lover of all things brain-related, and a Postdoctoral Fellow with the Surmeier lab at Northwestern University. He holds a Ph.D. in Neurobiology from The University of Chicago. You can find him on Twitter and on his own website.