How migratory bats find their way in pitch black
Migratory bats can pick their seasonal flight path in total darkness several minutes after their last exposure to light, showing their magnetic compass does not need photons to work.
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The full story with proper science words explained.
Flying blind
Imagine steering through a pitch-black night with no stars or streetlamps to guide you. Migratory Nathusius' pipistrelle bats (Pipistrellus nathusii) face this challenge every autumn as they travel across Europe. Birds rely on light hitting special proteins called cryptochromes in their eyes to sense magnetic north, but mammals lack these eye proteins. This raised a puzzle: does the bat magnetic compass need light at all?
The pitch-black test
To find out, researchers caught migrating bats along a major flyway. They placed them in a circular arena to record their chosen take-off direction and their take-off latency, which is the time spent waiting before flight. Some bats took off under dim starlight, while others were tested in verified total darkness.
Remarkably, bats tested in total darkness after brief starlight exposure still pointed in their normal south-westerly migratory direction. Their average heading differed by just 11 degrees from bats under starlight. They also showed no meaningful difference in angular concentration, which measures how tightly their headings clustered together.
A lingering compass
Timing was the true test. Birds rely on a radical-pair mechanism, a magnetic chemical reaction triggered by photons that lasts mere microseconds. Yet these bats took off five to eight minutes after seeing their last photon. That delay is roughly ten million times longer than the lifespan of those light-triggered chemical pairs, ruling out a light-dependent compass. Their navigation works like a physical pocket compass needle rather than a light-powered phone screen.
What we still do not know
This work is currently a preprint that has not yet completed peer review. The tests measured initial take-off headings in a lab arena rather than full, free flights across landscapes. Furthermore, bats held in dark conditions for longer periods showed the same mean direction, but their clustering fell short of statistical significance under a Rayleigh test. Scientists still do not know the exact physical hardware inside the bat, such as magnetic mineral particles or induction sensors, that tracks Earth's field without light.
Science words
- Cryptochromes
- Light-sensitive proteins in animals that can produce magnetically sensitive chemical reactions when struck by light.
- Take-off latency
- The amount of time an animal waits between entering the testing arena and flying away.
- Angular concentration
- A measurement showing how closely multiple compass directions cluster around one shared direction.
- Radical-pair mechanism
- A chemical reaction involving paired molecules that can be guided by magnetic fields.
- Rayleigh test
- A statistical method used to check whether directional measurements point towards a specific heading or are just random.
Check it yourself
This story is based on a real research paper in Preprint by Cellarius, Beer, Sucher et al.. We write with AI help and check it against the paper, but the original is the final word.