The world as we
see it is full of a bewildering number of hues, enhancing life with a
spectacular array of shades. What if there’s another layer of information and
detail we can’t sense, though? Humans only see a small portion of the
electromagnetic spectrum. Unlike us, some animals see polarized light, which
enhances their world even more, analogous to adding color to a black and white
image. In a study at the University of Bristol published in Current Biology February 2012, Dr.
Shelby Temple, Professor Justin Temple, and collaborators have discovered that
cuttlefish have the most acute polarized vision of any animal observed so far.
What
makes polarized light different? Polarized light rays vibrate in one plane as
opposed to multiple planes like other light rays. For humans, polarized light
usually means reducing glare with sunglasses or watching a movie in 3D, some
organisms have greater purposes for it. In other animals, polarized light is
used for navigation and orientation. In 2006, Savannah sparrows were found to
use polarized light to determine where north and south is more accurately. The
researchers believe that animals may use polarized light in predator/prey
detection and in signaling and communication, especially in cephalopods like
octopus, squid, and cuttlefish. The cuttlefish species studied here cannot see
what humans consider to be visible color wavelengths, so polarized may
substitute as a method of seeing the details of its world.
So,
how do you give a cuttlefish an eye exam? To examine the sensitivity of
cuttlefish vision, the researchers took LCD computer monitors and removed the
polarizing filter. A video was played for the cuttlefish in which an expanding
circle mimicked the appearance of a looming predator. These circles were shown at different levels of grayscale
and different angles of light so they were only visible to creatures with
polarized sensitivity. When they detected the movement, cuttlefish responded by
changing skin patterns. Cuttlefish responded to differences as small as one
degree of polarization, significantly more precise than previous species like
octopus and crayfish examined which responded to differences of 10 to 20
degrees.
The researchers
also analyzed low- and high-resolution sight by examining images of aquatic
life. Because we can’t see differences in polarization, false colors were
substituted, and the team found that while animals disappeared into the
background in the low-resolution images, they were visible in the
high-resolution analysis. This leads Professor Temple to note that camouflage
in the polarization dimension has not been investigated and could be the reason
why cuttlefish and other predators may have developed high sensitivity to
polarized light. The mesmerizingly detailed patterns cuttlefish flash on their
skin may also use polarization for detailed signaling and communication, but
more research is needed. This study is a great beginning to investigation of
alternative ways to sense the world and could be applicable for finding out how
animals and plants signal each other- imagine if there were giant ‘DO NOT EAT
ME; I’M POISONOUS’ signs we’re not even aware of in everyday life. Perhaps
eventually humans too can tap into this other level of vision for navigation
and communication purposes.
http://www.sciencedirect.com/science/article/pii/S0960982212000115
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