Module 3

Magnetic Compass

Pigeons sense the Earth’s magnetic field for compass and possibly map use. Two candidate mechanisms compete: magnetite-based in the upper beak (Fleissner 2003) and radical-pair quantum-spin chemistry in retinal CRY4 (Hore & Mouritsen 2016). The field remains contested; this module covers the evidence on both sides.

1. Magnetite Hypothesis

Fleissner 2003 reported dendritic iron-rich structures in the upper beak, near trigeminal-nerve terminals, consistent with single-domain magnetite crystals acting as biological compass needles. Anaesthetising the ophthalmic branch of the trigeminal disrupts pigeon magnetic-orientation behaviour (Mora 2004). However, Treiber 2012 re-examined the beak structures with confocal/Prussian blue staining and concluded the iron-rich cells are macrophages, not neurons — casting the magnetite hypothesis into doubt. Eder 2014 partially confirmed Treiber. The debate is ongoing.

2. Radical-Pair Mechanism

Schulten & Ritz 2000 proposed that blue-light-excited cryptochromes (CRYs) in the retina form a spin-correlated radical pair whose singlet/triplet interconversion is modulated by the Earth’s magnetic field. Zapka 2009 demonstrated light dependence of the compass; Xu 2021 Nature showed isolated avian CRY4 exhibits magnetic-field-dependent radical-pair chemistry in vitro.

\[ [\text{FAD}^{\bullet-}\cdots\text{Trp}^{\bullet+}]\quad\xrightleftharpoons[\ \text{spin-selective products}\ ]{\vec B\text{-dependent mixing}} \]

The radical-pair mechanism requires coherence times ~10–100 μs at body temperature — a remarkable biological quantum-coherence observation if confirmed.

Simulation: Radical-Pair Response & Field Inclination

Python

script.py39 lines

import numpy as np, matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

# Radical-pair singlet yield vs magnetic field angle
# Approx: S yield = 0.5 + 0.5*cos(2*theta) (spin-selective product formation)
theta = np.linspace(0, 2*np.pi, 200)
field_strengths = [10, 25, 50, 75, 100]

fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(14, 5), facecolor='#0a0a1a')
for ax in (ax1, ax2):
    ax.set_facecolor('#111827'); ax.tick_params(colors='#cbd5e1')
    for s in ax.spines.values(): s.set_color('#334155')
    ax.grid(True, color='#334155', alpha=0.3)

for B in field_strengths:
    S_yield = 0.5 + 0.3*np.cos(2*theta) * (B/50)**0.5
    ax1.plot(np.degrees(theta), S_yield, lw=2, label=f'B = {B} uT')
ax1.set_xlabel('Angle to field (deg)', color='#cbd5e1')
ax1.set_ylabel('Singlet product yield', color='#cbd5e1')
ax1.set_title('Radical-pair compass: CRY4 anisotropic response',
              color='#c7d2fe', fontweight='bold')
ax1.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')

# Geomagnetic field: inclination vs latitude
lat = np.linspace(-90, 90, 200)
incl = np.degrees(np.arctan(2*np.tan(np.radians(lat))))
ax2.plot(lat, incl, color='#818cf8', lw=2.5)
ax2.axhline(0, color='#fbbf24', ls='--', label='Equator (horizontal field)')
ax2.set_xlabel('Geographic latitude (deg)', color='#cbd5e1')
ax2.set_ylabel('Field inclination (deg)', color='#cbd5e1')
ax2.set_title('Dipole field inclination',
              color='#c7d2fe', fontweight='bold')
ax2.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')

plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0a1a')
print('Field strength ~25-65 uT; birds use inclination as latitude cue')
print('Radical-pair mechanism (Ritz 2000): FAD* + Trp+ pair in CRY4')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

3. Compass vs. Map

The Earth’s field provides direction (compass) and two potential positional cues: field intensity and inclination (latitude). Wiltschko 2005 cleanly demonstrated a compass role in pigeons; a magnetic map role (for positional computation) is more controversial and likely restricted to long-distance displacements outside the learned area.

Key References

• Fleissner, G. et al. (2003). “Ultrastructural analysis of a putative magnetoreceptor in the beak of homing pigeons.” J. Comp. Neurol., 458, 350–360.

• Treiber, C. D. et al. (2012). “Clusters of iron-rich cells in the upper beak of pigeons are macrophages, not magnetosensitive neurons.” Nature, 484, 367–370.

• Hore, P. J. & Mouritsen, H. (2016). “The radical-pair mechanism of magnetoreception.” Annu. Rev. Biophys., 45, 299–344.

• Xu, J. et al. (2021). “Magnetic sensitivity of cryptochrome 4 from a migratory songbird.” Nature, 594, 535–540.

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