February 5, 2019, Lund University
Of all nocturnal animals, only dung beetles can hold their course
using polarized moonlight. Researchers at Lund University in Sweden have now
shown that beetles can use polarized light when its signal strength is weak,
which may allow them to find their bearings when artificial light from cities
swamps natural moonlight.
"Our investigation reveals that these beetles would be
sufficiently sensitive to orient themselves underneath a light-polluted city
sky during a full moon. But they
would be lost during a quarter or crescent moon,"
explains James Foster, researcher at the Department of Biology at Lund
University.
Moonlight becomes polarized when it strikes particles in the upper
atmosphere on its journey to Earth. In the study, the researchers show that the
strength of this polarized light signal changes depending on the moon's phase.
Polarized light from a quarter moon is only one-third as strong as at full
moon, and just one-fifth as strong for a crescent moon.
"This is the very first evidence that the polarization of
skylight changes over the lunar cycle," says James Foster.
Together with colleagues in Lund, Germany, South Africa and the
U.S., he tested how dung beetles cope
with this weak signal at
night when using polarization as their compass to roll a dung ball in a
straight line across the African savannah. On clear nights, they perceive this
polarized light in a blue sky like humans observe on a sunny day—but thousands
of times darker.
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