The Dung Beetle and the Stars
Dung beetles have long surprised scientists with their remarkable navigation abilities,
despite the relatively small sizes of their brains.
Scientists have also known that after dung beetles collect the dung, they do a little
dance on top of it before moving it in a straight line to their destination.
The dancing was once written off as a “happy dance,” but now we know better—it actually
helps the dung beetle get its bearings as it dances around in circles, looking at the
stars.
This behaviour, as well as the fact that dung beetles have specialized eyes which analyse
the direction of light polarization, first led researchers to hypothesize that the beetles
might be using the sky to navigate.
During the day, it was easy to see that they were using a pattern of polarized light around
the sun in order to navigate.
But no one knew exactly how they continued to navigate at night, which led to the hypothesis
that they might be using the moon.
To test the hypothesis, researchers led by Marie Dacke conducted a variety of different
experiments.
They set up a circular arena with high walls in South Africa, blocking out views of everything
but the sky.
They then timed dung beetles—in this case, Scarabaeus satyrus—on how long they took
to cross the arena.
The beetles were tested in different conditions: with the moon out, with no moon, and with
an overcast sky.
Some of the beetles were equipped with cardboard caps which forced their eyes downward, so
they weren’t able to see the sky at all under any conditions.
What the researchers found was that dung beetles had a tough time staying in a straight line
when there were clouds obscuring the sky and when they were wearing the caps.
However, when there was a clear sky with no moon, the beetles were still able to navigate
correctly.
This led to the idea that they were using stars to navigate.
That led to a potential problem.
As Dacke said, “We thought that they could be using the stars [for orientation], but
dung beetles have such small eyes that they don’t have the resolution, or sensitivity,
to see individual stars.”
To investigate this further, the researchers repeated the experiments under slightly different
conditions.
They moved the arena to a planetarium where they were able to control which stars the
dung beetles were able to see.
Once again, some of the beetles wore caps.
This time, they showed the beetles the brightest stars, showed only the Milky Way, and then
showed the entire sky.
What they found was that the beetles were slow to cross the arena when the brightest
stars were visible, but they crossed at normal speed when only the Milky Way was visible
and when the entire sky was visible.
The beetles with caps had difficulty crossing no matter what.
The researchers concluded that the beetles must be using the Milky Way in order to navigate.
This falls in line with the findings of a similar experiment that used a different type
of dung beetle, the Scarabaeus zambesianus, which was unable to move in a straight line
when the Milky Way wasn’t visible.
Of course, in the Northern Hemisphere, the Milky Way is less distinct than in the Southern
Hemisphere, and it’s possible that dung beetles in the Northern Hemisphere are using
a different gradient of light than the southern beetles.
But the revelation that southern beetles are able to use the Milky Way to navigate is big
news from the animal kingdom.
They are the first insects known to use the stars for navigation (outside of our sun,
which insects like honey bees do use as one of many reference points for navigation; this
is particularly fascinating because these bees will adjust for the change in the sun’s
location at a given time of day when communicating locations of things to their fellow bees.
Even if their communication takes many hours, they’ll adjust accordingly just fine for
the change in position of the sun.)
In any event, the ability to roll a ball of dung in a straight line is crucial for the
survival of dung beetles.
After they create a ball of dung, they have to roll it away from the pile as quickly as
possible to avoid having their prize stolen by one of their peers.
Once the pile of dung is a safe distance away, the dung beetle will bury it and it becomes
food for the beetle’s offspring.
Bonus Facts: • Non-insects that are known to use stars
for navigation, include many different types of birds, a few types of seals, and, of course,
humans.
It’s thought that the ability to read the stars might be even more widespread among
other members of the animal kingdom than we once thought, but a significant amount of
research still needs to be done to confirm or deny this.
• Arguably among the most skilled animal navigators is the homing pigeon, which uses
magnetic cues, patterns of sunlight, and physical landmarks to locate their homes.
After an initial trip, pigeons can’t be stopped by magnets, blindfolds, or even getting
shipped off to a different time zone in a sealed box—they can always find their way
back home.
They’re a bit like salmon, who likely use magnetic signatures in order to find their
way back to the streams in which they hatched.
Another animal that uses magnetic fields to navigate is the sea turtle.
In addition, Robins can actually see magnetic fields, but only with one of their eyes.