May 2019, ScientificAmerican.com 79FROM
“THE REMARK ABLE VISUAL CAPACITIES OF NOCTURNAL INSECTS: VISION AT THE LIMITS WITH SMALL EYES
AND TINY BRAINS,”BY ERIC J. WARRANT, INPHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B,VOL. 372; APRIL 5, 2017zoologists started the tests under dusklike illumination, then
turned down the light to levels as low as dim starlight. As dark as
the surroundings became, the moths could still tell yellow from
blue. Since that study, Kelber’s team has found nocturnal color
vision in carpenter bees and geckos. She hopes to test for color
vision in fruit bats and in owls, whose nocturnal hunting prowess
has usually been ascribed to keen hearing or big eyes.
Frogs can see color in the dark as well, distinguishing blue
from green. Animal physiologist Kristian Donner of the Univer-
sity of Helsinki in Finland and his colleagues tested European
common frogs for phototaxis, a behavior by which the frogs typ-
ically hop toward light. Donner wondered if they would be
choosy about the color of the light. Decades ago lab tests on
frogs’ rod cells had shown that some specifically reacted to blue
light, whereas others responded to green. To find out what the
cell differences meant for frog behavior, Donner’s group placed
17 amphibians, one at a time, in a bucket with two windows on
opposite sides. The scientists shined blue light in one side and
green light in the other. Then they measured the frequency and
direction of the frog hops at different light levels.
When the bucket was completely dark, the hops were ran-
dom. But as soon as the researchers let in the least possible
amount of light, the frogs showed a clear preference for green.
“At the very limit for vision, they can still differentiate between
blue and green,” Donner says. For human comparison, his stu-
dents stuck their heads in the bucket and could not see any
light, much less tell green from blue.
It’s not certain why the amphibians jumped toward green
light. Perhaps, Donner speculates, frogs get clues from the stars.
Starlight is made up of relatively long wavelengths, and green
light wavelengths are longer than blue, so green coming into
the bucket might hint at starlit open spaces and a route to
escape from the container.STARLIT PATHS
if frogs indeed follow the stars, they would not be the only
animals that do so. Dung beetles travel in a perfectly straight
line on moonless nights, when the only light comes from stars.The movement is a good strategy for a beetle with a nice fresh
bit of dung, says James Foster, a sensory biologist at Lund. It
wants to leave the scrum of other beetles at the dung pat and
find a quiet patch of ground to dig in with its prize. Going
straight, rather than weaving or turning about, will get the bee-
tle away from the pat as quickly as possible.
How do the beetles do it? Foster’s Lund adviser Marie Dacke,
Warrant and other researchers had already discovered that the
insects use what they can see above them to find their way
around. The scientists put cardboard visors on the critters so
they could not see the sky. Then they let the insects loose in a
circular arena and tracked the way each traveled to the edge.
When capped, the beetles took much more circuitous routes,
indicating that something in the sky was important to them.
The researchers suspected the beetles might use the pattern
of stars for orientation, like six-legged sailors navigating with
constellations. To test this idea, Dacke and her colleagues
brought the beetles, with their dung balls, to a planetarium
where skylight patterns could be easily controlled. Under either
a simulation of a full starry sky or just the bright streak of the
Milky Way, the beetles sped straight to the circle’s edge in under
a minute. They took longer if the galaxy was absent. It was the
first time any animal was shown to orient itself using this band
of stars. (After publication in the journal Current Biology in
2013, the work earned a tongue-in-cheek Ig Nobel Prize in Biol-
ogy and Astronomy.)
More recently, Foster investigated how dung beetles might
use the Milky Way to go in one particular direction. Seen from
our planet, the galaxy’s thick band of stars is a fairly symmetri-
cal line. From the beetles’ perspective, the line would look just
the same when they are moving forward or backward. Yet the
insects do not get turned around.
Foster suspected that the beetles kept track of subtle dif-
ferences in light intensity between one end of the Milky Way
and the other. When he analyzed photographs of the galaxy
taken from the beetles’ South African habitat, he found that
the intensity of light from the northern and southern ends of
the Milky Way indeed differed by at least 13 percent and some-Trouble at Night
When it is dark, light-detecting cells in the eye have little chance
of catching the few photons (units of light) that might reveal an
object. This diagram shows 400 photoreceptor cells trying to
discern a circle. With only six photons coming in (left), the circleremains identical to its dark surroundings. As photon numbers
increase, so does the contrast between the circle and environ-
ment. But only after levels are boosted 1,000 times (right) does
the object become clear.© 2019 Scientific American