Why in the News?

Scientists at Macquarie University, Sydney, have discovered that two species of nocturnal bull ants (Myrmecia pyriformis and Myrmecia midas) rely on polarised moonlight for navigation.

What is Polarised Moonlight?  

• Polarised moonlight refers to moonlight that has undergone scattering in Earth’s atmosphere, causing its waves to oscillate in a specific direction.
• Unlike direct moonlight, which is unpolarised, the light that scatters in the sky becomes linearly polarised, meaning its electric field aligns in a fixed plane.
• The moon emits unpolarised light, but when it interacts with air molecules and dust particles in the atmosphere, it scatters and becomes polarised.
• The intensity of polarised moonlight is much lower than polarised sunlight, making it harder for most animals to detect.
• The pattern of polarisation in moonlight remains stable, allowing nocturnal animals to use it as a reliable navigation tool.
• Why is it Important for Navigation?
  • Many nocturnal animals, including bull ants (Myrmecia pyriformis and Myrmecia midas), rely on celestial cues to orient themselves.
  • Unlike the moon’s direct position, which changes with phases and cloud cover, the polarisation pattern remains detectable throughout the night.
  • This enables ants to navigate effectively even under crescent or waning moons, where light intensity is significantly lower.

E-Vector Pattern and Ant Navigation

• Polarised moonlight forms a distinct pattern in the sky, known as the E-vector pattern.
• This pattern shifts based on the moon’s position, but its orientation remains stable, allowing insects like ants to use it as a natural compass.
• The E-vector pattern aligns at 90° to the moon’s direct light, creating a predictable navigation reference.
• How do Bull Ants use it?
  • Ants detect the E-vector pattern in the night sky using their specialised compound eyes, which are sensitive to polarised light.
  • Even in dim conditions, they adjust their movements according to the orientation of polarised moonlight.
  • Researchers found that when the E-vector was artificially rotated, the ants changed their paths accordingly, confirming that they rely on this pattern.
  • When the moonlight disappeared (during a new moon phase), the ants struggled to navigate, further proving their dependence on polarised lunar light.

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