# A hybrid compass mechanism combining radical pairs and magnetite crystals

## TL;DR

This paper proposes a new way that migratory birds might sense Earth's magnetic field using a combination of light-sensitive molecules and magnetic particles.

## Contribution

The novel contribution is a hybrid model combining radical pairs and magnetite to enhance magnetic field sensitivity in birds.

## Key findings

- A hybrid mechanism could amplify the Earth's magnetic field by up to 100-fold for directional sensing.
- The model suggests magnetite nanoparticles enhance radical pair sensitivity for better navigation.

## Abstract

Small night-migratory songbirds navigate thousands of kilometers assisted by an internal magnetic compass whose underlying biophysical mechanism is largely obscure. Evidence suggests that light-sensitive molecules in the birds’ eyes can respond to the Earth’s magnetic field although it is far from clear that the ensuing signal is strong enough for reliable navigation. We propose a variant of this hypothesis in which these magnetically sensitive molecules operate alongside magnetic iron oxide particles which amplify the Earth’s field. The model predicts that this partnership could increase the directional response of the sensor by up to 100-fold. By bridging biology and physics, this framework advances understanding of how migratory birds accomplish one of Nature’s most extraordinary navigational feats.

That night-migratory songbirds have a magnetic compass sense is undisputed. The nature of the sensor, however, is far from certain. The two leading hypotheses are organic radical pairs in cryptochrome flavoproteins and crystals of inorganic magnetic minerals such as magnetite. Here, we propose a magnetoreception mechanism that combines radical pair chemistry with magnetite nanoparticles. Instead of directly detecting the tiny dependence of a radical pair reaction on the direction of the Earth’s (~50 μT) magnetic field, this hybrid sensor uses a magnetic particle to amplify the Earth’s field. Directional information is thereby encoded in the response of nearby radical pairs to the intensity of a much stronger (~5 mT) magnetic field. The result is a magnetoreceptor with potentially 10 to 100-fold greater sensitivity than afforded by radical pairs alone.