Blog Author: Professor Adrian Thomas, CSO, Animal Dynamics
True navigation is tough. You have to work out where you are, where you want to go, and the direction to head in to get there. The Duke of Edinburgh award hunters charging around the local countryside at this time of year might use a map and compass. Not all of them get lost. Some animals are expert at true navigation, being able to return home after long foraging trips, after migrating or after being displaced long distances to places they have never before visited. Until recently it was not clear how they did this –what do they use for a map and how do they detect compass directions?
Recent work by Kishkinev and colleagues has shown that at least for Eurasian Reed Warblers from Austria, magnetic cues alone are sufficient to provide both map and compass, but only if the full 3D magnetic field is provided. Kishkinev and colleagues caught Eurasian Reed Warblers at the Biological Station at Lake Neusiedl in Illmitz, south-eastern Austria, and tested the birds for preferred migratory direction. When the birds had access to all the usual natural cues (vision of the sky and horizon, access to open air, natural magnetic field) they preferred to head in the normal migratory direction – South East. When they used coils to shift the magnetic field to what would be expected near the city of Neftekamsk, Russia, the birds instead corrected their preferred migratory direction to South West, which would have been the correct direction from Neftekamsk but not from Illmitz. This experiment suggests Reed Warblers can use the magnetic field not just for compass direction, but also as a map: as the authors put it “On this basis, the evidence is now very strong that adult night-migratory reed warblers have a magnetic map and that they can use it to compensate for large geographical displacements”. That may work particularly well over Eurasia because there is a well-marked magnetic gradient running both North-South and East-West, forming a grid-like pattern ideal for localisation.
How birds detect magnetic fields is less clear. Recent research from Oxford University Chemistry Professor Peter Hore and colleagues suggests that Chryptochrome 4, may be the magnetoreceptor. Chryptochromes are light-sensitive, and are found in the retinas of birds (and other animals). When exposed to light they form a pair of radicals with unpaired electrons that have correlated spins. Theoretical work, and now lab-work in vitro shows that the states of these electrons can be affected by magnetic fields in ways that are consistent with what would be required for a magnetoreceptor. Exactly how it would work is unclear and controversial – and bird navigation is a famously and spectacularly combative field, so other researchers are already disputing not the findings (which are solid) but whether they are relevant, and if they are (in the classic approach to negate impact) whether they are new….
However animals detect magnetic fields, their ability to navigate without GPS is something that would be extremely valuable given that several states have already demonstrated the ability to turn off, spoof or otherwise interfere with GPS signals, and that GPS is only available with a clear view of the GPS-satellites; difficult in cities, under tree-cover or even under heavy cloud-cover, impossible underwater. GPS-denied navigation is something Animal Dynamics has been working on in various ways, and our DASA funded bio-inspired MagNav project (based on the techniques Tuna use to navigate) demonstrated in tests off Falmouth that conventional off the shelf magnetic sensors alone can provide North-South localisation better than 15km without any other information. Not exactly pinpoint, but good enough for trans-oceanic navigation, and enough to bring a navigator close enough for sensor-fusion with other systems (visual, chemosensory, infrasound) to get them home.
Kishkinev et al., 2021, Navigation by extrapolation of geomagnetic cues in a migratory songbird. Current Biology 31, 1563–1569 April 12, 2021 https://doi.org/10.1016/j.cub.2021.01.0
Xu, J., Jarocha, L.E., Zollitsch, T. et al. Magnetic sensitivity of cryptochrome 4 from a migratory songbird. Nature 594, 535–540 (2021). https://doi.org/10.1038/s41586-021-03618-9