Magnetic Entanglement

It’s all around us, all the time, and as humans we have come to rely on Earth’s magnetic field for navigation. Is it the same for bees, a creature for which navigation is so vitally important?

Fig 1. Diagrammatic representation of the Earth’s geomagnetic field detailing the main components thought to be available to animals for magnetoreception. Lines of magnetic flux (shown on right), emanate from the Earth’s iron core at the south magnetic pole (Sm) in the Southern Hemisphere (S) and travel to the north magnetic pole (Nm) in the Northern Hemisphere (N).

It’s taken as common knowledge that honey bees, like ants, migratory birds and sea creatures, are sensitive to the planet’s magnetic field. That they should be was first suggested at the end on the 19th century, but the first convincing evidence wasn’t produced until 1965. A research group in Frankfurt that included Wolfgang Wiltschko demonstrated that migratory European Robins could detect the geomagnetic field as a directional compass. But just exactly what did being ‘magnetosensitive’ mean?

Since then, despite not knowing exactly what animals were sensing, or how, or indeed when they paid attention to the sense itself, it’s now accepted that many animals are known to be magnetosensitive, including bacteria, honey bees, cockroaches, newts, turtles, rodents, lobsters, fish, bats, and many birds. While ‘affected by’ or ‘sensitive to’ do not automatically mean ‘use’, detecting and orienting yourself with respect to the Earth’s magnetic field could potentially be done equally well in the atmosphere, in the deep oceans, and on the ground. The magnetic field appears to be more universal and constant than any other navigational cue, so using it to get around seemed an obvious use.

What’s in a field?

Finding evidence for any organs or structures that might be involved in detecting the properties of the field and explaining how it provides information that is acted upon still proves to be very difficult. It was not a simple job either to anticipate just what kind of information could be obtained or how useful it could be. If you imagine the earth as a giant magnet then the most obvious bit of information is polarity, in our terms, a ‘North’ or ‘South’ direction. Direction though, is not location. [See Fig 1]

Two other features of the field might help. As the field curves around a globe, parallel to the surface at the magnetic equator but perpendicular to the poles, the other piece of ‘positional’ information is the angle the field makes relative to the Earth’s surface. We call that the ‘inclination’. The intensity or strength of the field also varies, but not by much, and not in any systematic way. So, while there might be some local ‘character’ that might allow you to learn to identify a unique place it’s difficult to image how field strength might be used to get you to that place. [See Fig 2]

Fig 2. In this merged aeromagnetic anomaly map of the state of Virginia, USA, the colour scale shows deviations in the strength of the Earth’s dipole field. Although some long-distance migrants evidently extract positional information from the general dipole field, minerals in the Earth’s crust often generate local field anomalies (such as this map shows). Thus, it is unlikely that bees would use dipole strength as a navigational aid.

So that’s a long way of suggesting that animals (who do not share the same needs), may be sensitive to different features (or combination of features) of magnetic fields. They may use the Earth’s magnetic field in different ways AND therefore use different ‘apparatus’ to do it. For instance, Robins apparently, sense the field inclination, and not polarity.

Bee-ing sensitive

In 1972 Martin Lindauer published a paper1 describing how the orientation of honey bees’ combs and their foraging dances could be altered by manipulating the magnetic field they could sense with a device called a helmholz-coil2. Subsequent research (based on evidence from bacteria) suggested that an iron oxide (magnetite), which could be found in all sorts of animals, would react to a magnetic field and could therefore be the basis of some sensory structure. Theoretically, if the iron could be found, somewhere close by there ought to be some link to the nervous system that would enable perception3.

The problem is that iron is an essential omnipresent component everywhere in biological cells and, since the body is transparent to magnetic fields the candidate cells could be anywhere. Worse, experimenter's labs were awash with magnetic material, contaminating their work and confounding their results4.

So, do honey bees possess this iron in their makeup?

In honey bees two possible sites lead the field as potential locations for their magnetic ‘sense’, the anterior dorsal region of the abdomen, where evidence suggests magnetite particles are present, and in the ventral abdomen where there are iron granules in cells in the subcuticular fat layer. Few studies, or none if you want to be critical, have produced robust evidence that the presence of iron is functional in a navigational sense.

It has been suggested that exposure of live bees to an intense magnetic field alters the magnetization of the ferromagnetic particles in their abdomens so that, unlike comparable control bees, they failed to respond to an anomaly in a controlled magnetic field that signalled a sugar reward. These studies, ten or fifteen years later, are still debated and lack support; do the iron particles just represent stored dietary iron? Are the associative learning choices they faced ecologically relevant, and can the results be reproduced? At the moment magnetite-based reception is hypothesis, not theory.

Irrelevant noise?

The other research problem, especially but not just for honey bees, is that so often the ability to sense a magnetic field appears somewhat irrelevant. Some animals that do appear to respond to changes in a field do so very slowly. Despite its ubiquity, very few animals appear to rely on our magnetic field completely, if at all. If they do refer to it, its status appears relegated to ‘backup’ or ‘supporting info’, while celestial data, way-points, path-integration, image-matching and so on take centre-stage. Testing the ability in a laboratory, with all sorts of creatures, is frequently far from conclusive and needs scale and repetition.

Maybe that shouldn’t be surprising. The Earth’s magnetic field is, as far as energy goes, a pretty weak phenomenon. Thermal and electrical energy are orders of magnitude larger, and there is no obvious way in which the ‘signal’ could be amplified above the energetic ‘noise’ by something analogous to eyes or ears5. As long as they are there, there are plenty of alternative clues to your whereabouts that are faster and easier to use than a tiny magnetic field. If you are a night-migrating song bird, or an ocean-going sea turtle the world is very different and there’s no alternative, but we have to ask if a magnetic sense in honey bees, if it is employed at all, is just an obsolete vestigial ability with very little contemporary ecological relevance?

Dave Black is a commercial-beekeeper-turned-hobbyist, now retired. He is a regular science writer providing commentary on “what the books don't tell you”, via his Substack Beyond Bee Books, to which you can subscribe here.

References

1. Martin Lindauer, 1972. Magnetic Effect on Dancing Bees. Anim Orientat Navig 559–567.

2. Helmholtz coils are a special form of electromagnet in which a there is a pair of parallel, coaxial, wire coils separated by a distance equal to their radius. They generate a uniform magnetic field between the coils when an identical electric current is passed through each coil. The system can be ‘tuned’ to negate the effect of the Earth’s magnetic field or produce a uniform field with a known strength and direction. https://www.electricity-magnetism.org/helmholtz-coils/

3. Kuterbach, D.A., Walcott, B., Reeder, R.J., and Frankel, R.B. (1982). Iron-Containing cells in the honey bee (Apis mellifera). Science 218, 695–697. https://doi.org/10.1126/science.218.4573.695.

4. Kobayashi, A.K., Kirschvink, J.L., and Nesson, M.H. (1995). Ferromagnetism and EMFs. Nature 374, 123. https://doi.org/10.1038/374123a0.

5.  Johnsen, S., Lohmann, K.J., Warrant, Eric.J., 2020. Animal navigation: a noisy magnetic sense? Journal of Experimental Biology 223, jeb164921. https://doi.org/10.1242/jeb.164921