- Tropical leafcutter ants don’t need to stop and ask for directions
- They have internal magnetic compasses
Leafcutter ants are famous for traveling far from their home nests in search of leaves; the ants don’t eat the leaves themselves, but rather use them as fodder for a fungus that they grow in
their colonies as food.
A few years ago, biologists Robert Srygley and Andre Riveros
accidentally discovered that the ants were also using an internal compass to
navigate. When the researchers used a powerful magnetic pulse to disrupt the
local magnetic field, foraging leafcutters lost their way. When the magnetic
field was reversed, some of the ants marched off in the opposite
direction—making them the only insect known to use such a built-in compass.
If that weren’t remarkable enough, the ants are also known
to use landmarks and even stars to help keep track of their positions while
they’re out and about. (See “Dung Beetles Navigate Via the Milky Way, First
Known in Animal Kingdom.”)
Herding Ants
In the new experiments, the scientists tested the responses
of two groups of leafcutters to a change in the local magnetic field.
One group consisted of wild leafcutters taken from a natural
ant colony. Using flakes of barley, the scientists lured these ants to an
outdoor feeder that they set up. After two days, the ants had forged a new
trail to the feeder. (Also see “How Leafcutter Ants Evolved From Farmers Into
Cows.”)
The other group consisted of ants reared in a lab, in
plastic boxes that lacked soil, where they stayed for about a year, explained
Srygley, who is with the USDA’s Agricultural Research Service.
The scientists then set up an outdoor, circular arena, which
they filled with sand and surrounded with a coil made of copper wire that they
could use to manipulate the local magnetic field.
Next, the team took ants from both groups and plunked them
down separately inside the arena. They then shifted the local magnetic field by
90 degrees in the horizontal direction and watched what happened, according to
the study, published in the August issue of the journal Behavioral Ecology and
Sociobiology. (Watch a video of leafcutter ants.)
The ants raised in a soil-free environment managed to find
their way home, but the soil-exposed ants seemed confused. They marched in a
direction that faced neither home nor the direction of the shifted magnetic
field.
Magnetic Mineral
What was going on? The scientists think that the indoor ants
navigated using a less precise sense, called proprioception, that allows them
to keep track of how far they go and in which direction they turn.
This “proprioception compass” appears to work well for short
distances, but over longer distances small errors can add up, making this
compass less reliable. (Also see “New Theory on How Homing Pigeons Find Home.”)
For longer distances, the scientists think leafcutters rely
on their internal magnetic compasses. Other studies have linked this magnetic
sense to specks of magnetite (a magnetic mineral found in soil) in their
antennae. This explains why the wild ants seemed to lose their way when the
magnetic field was shifted.
The indoor ants were raised in a soil-free environment, and
thus lacked access to magnetite, so they didn’t have magnetic compasses that
could be confused by the magnetic field change (the sand in the arena lacked
magnetite).
Getting Magnetized
While the new findings strengthen the link between magnetite
found in soil and the leafcutter’s magnetic compass, questions remain about how
the particles are incorporated into the ants’ bodies.
Leafcutters aren’t known to ingest soil directly, and they
primarily feed on the fungus they raise in their colonies.
One idea, Srygley said, is that the leafcutters are
inadvertently feeding magnetite-packed soil to their fungus, in addition to
leaves.
“The [magnetite] would then be fed to the larvae, and the
larvae would incorporate it into their antennae when they get close to
adulthood,” Srygley said.
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