Odor Trail through Ant Country
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Odor Trail through Ant Country The desert ant’s use of its own built-in GPS – consisting of a sun-compass-based path integration system and visual landmarks – in locating its nest is a known phenomenon. Researchers recently ascertained, however, that this system also includes a sense of smell. Even more surprising is the discovery that these animals learn to distinguish between different odors in the nest environment, and use these like a map. Markus Knaden and his team at the Max Planck Institute for Chemical Ecology in Jena set out to search for clues in ant country.
FOCUS_Orientation
A desert ant runs through the test channel to the feeder.
The walls of the channel prevent the ant from seeing its
environment. Under these test conditions, the ant can only
navigate using its sense of smell.
TEXT MARCUS ANHÄUSER
K
athrin Steck looks a bit tired. contained in the nest odor of desert they meet one another or arrive at the
That’s not surprising – she has ants. Does every nest have its own nest, they touch one another extensive-
been up all night. The biolo- odor? Or do all nests smell the same? ly with their antennae,” says Knaden.
gist has been traveling since What odor emanates from the en- Once a foraging ant finds a food source,
2:00 a.m. Her trip started with trance, a roughly two-centimeter hole it informs its companions in the nest by
a two-hour taxi ride through the dark- in the encrusted ground of a desiccated means of an attractant, a pheromone
ness from the Tunisian town of Maha- salt lake? This is the typical habitat of trail on the ground. The ants follow this
rès to Monastir. From there, she flew the subject of their research – the feisty, odor trail to the food source and then
back to Germany, took a train from shiny black ant Cataglyphis fortis. carry the food back to the nest. The more
Leipzig airport to Jena and, instead of animals use the trail, the more reinforced
going home, went straight to the labo- A TRAIL OF ODORS FOR it becomes. The result is the familiar ant
ratory at the Max Planck Institute for THEIR CONSPECIFICS trails, teeming with insects.
Chemical Ecology. However, the Cataglyphis genus does
The reason for the urgency is the For two weeks, Steck stood all day, every not deploy these types of ant trails at all.
small glass of clear liquid in her back- day, in the blazing sunshine in temper- “They are lone warriors when it comes
pack. It contains invisible odors ex- atures of up to 50 degrees Celsius. Bühl- to food,” says the behavioral ecologist.
tracted from the nest entrance of a mann extracted the odors at the nest In the salt lake, only individual ants are
desert ant. “These must be cooled as entrance while Steck pursued the long- seen flitting around, sometimes zigzag-
Photo: MPI for Chemical Ecology
much as possible and then quickly legged ants on their journeys across the ging around while foraging, sometimes
processed,” says the 34-year-old post salty desert floor. hot-footing it toward the entrance hole
doc, who works with Bill Hansson, an The olfactory orientation of ants has of the nest. “A pheromone trail would
olfaction expert. long been established – even if it is not quickly disappear in the salt pan, where
Kathrin Steck and her colleagues obvious to everyone. The ants pick up temperatures reach almost 60 degrees,”
Cornelia Bühlmann and Markus Knaden the scent of their food and identify says Knaden. There are ant species that
want to discover the substances that are friends and foes in ant country. “When do leave odor trails in such territories,
4 | 10 MaxPlanckResearch 17
FOCUS_Orientation
» Cataglyphis occupies a very special place in the world
of ant research. It is known as a “navigational genius.”
“but these are mostly species that are ac- Over the decades, researchers discov-
tive in the evenings or at night.” This ered that the insect actually uses sever-
means that they engage in foraging al techniques to cross the salt pans in
when temperatures cool down. search of rich pickings and return di-
rectly to its nest. “They use a path in-
DESERT ANTS WITH BUILT-IN GPS tegration system – based on a sun com-
pass for measuring directions and a
In addition, laying an odor trail is ben- step counter for measuring distances
eficial only if there are plentiful food covered – and landmarks, to find the
sources to exploit, that is, for ants who nest entrance,” says Knaden, summa-
are carrying in those seeds that are ly- rizing the ants’ repertoire. Their facet
ing on the ground behind a bush. eyes are thus bigger than those of
However, Cataglyphis fortis ants have many other ants. They also have very
a particular liking for dead insects. large, mobile antennae.
And they carry these back to the nest Yet, until now, hardly anybody had
in one piece. bargained on their sense of smell for
Markus Knaden has spent 14 years guiding them across the hot, hostile
researching Cataglyphis, a genus with salt flats. Not even Knaden. It was only
approximately 60 species, of which 6 when he arrived in Bill Hansson’s de-
can be found in the Sahara, its main partment in 2006 that the use of the
habitat. Cataglyphis occupies a very Formicidae nose as a homing device
special position in the world of ant re- came to his attention. “I had hoped methyl salicylate, decanal, nonanal
search. It is known as a “navigational that the ants had learned not only vis- and indole. “We took environmental
genius.” This is probably a slight exag- ual landmarks, but also possibly odor odors that are not connected with
geration; many other animal species cues,” says the Max Planck scientist. food, but rather are produced, for ex-
have comparable orientation skills, but Knaden and Steck first began system- ample, by plants and are highly vola-
only a few of them are as well re- atically researching the substances that tile at high temperatures,” says Knaden.
searched as the Cataglyphis. ants actually smell – or don’t smell. In a Food odors would have been too at-
Since the 1960s, scientists have special laboratory such as the one oper- tractive. And the ants are not supposed
been investigating how ants get their ated by Bill Hansson they have all the to be attracted by the odor – they are
bearings in the desert. Back then, the equipment they need to systematically supposed to be able only to identify it
father of Cataglyphis research, neuro- investigate odors. Steck tested a wide va- and remember it.
physiologist Rüdiger Wehner from the riety of substances from the odor librar-
University of Zurich, pitched his tents ies and, using ‘electroantennograms,’ RESEARCH ON DRY LAND
in the Tunisian town of Maharès and demonstrated how the olfactory nerves
began to examine the navigational abil- in the antennae respond to odors. The odor researchers then wanted to
ity of these long-legged, elegant desert The two researchers then concen- find out whether the animals are capa-
runners, which are roughly the same trated on four odors on which they ble of using these odors for orientation
size as the common wood ants. trained the ants for their experiments: in the area around the nest entrance,
18 MaxPlanckResearch 4 | 10
above: Different strokes: The entrances to
many of these nests consist of only an
inconspicuous, thumbnail-size hole in the
ground; other ants of the same species create
striking nest mounds. One possible reason
could be that returning ants use the mound as
a landmark and therefore find their way back
faster to the safety of the nest.
right: Kathrin Steck marks individual ants.
Using checklists, the researchers can see how
often the individual animals have already
Photos: MPI for Chemical Ecology (2)
performed the relevant training.
for example, in the same way that they
use visual landmarks, such as a bush or
a stone, to pinpoint the small entrance
hole. The area studied is a desiccated
salt lake in Tunisia, six kilometers long,
three kilometers wide, about one hour
4 | 10 MaxPlanckResearch 19
above: The test site shows the salt pan in the
background. Several such salt pans are dot-
ted throughout Tunisia. This one measures
approximately three by six kilometers.
below: In temperatures of up to 50 degrees
Celsius, Kathrin Steck stands in the blazing
sunshine and tracks the ants’ search runs in
the channel
from the coast and the town of Maha-
rès, where the scientists were staying.
The mecca of Cataglyphis research. Rü-
diger Wehner discovered the coastal
area in 1968 and set up his base there.
Markus Knaden, who completed his
doctorate with Wehner and worked
with him as a post doc, also began
Photos: MPI for Chemical Ecology (2)
working with desert ants in this area.
When he moved to Bill Hansson’s
department in Jena in 2006, he initial-
ly had some bad luck: his ant popula-
tion in Maharès was almost decimated.
“We assume that it had something to
do with the major use of pesticides in
the nearby olive tree plantations,” says
20 MaxPlanckResearch 4 | 10
FOCUS_Orientation
Large test fields painted on the ground allow
the researchers to record the ants’ extensive
search runs. This method was established
by Rüdiger Wehner, the father of Cataglyphis
research in the late 1960s, and remains a
proven technique to this day.
Knaden. He and Steck were thus forced ing ants. “We are the crazy people who el. The ants then have no clues as
to search for a new population – and come every year with new equipment to where the hole is. Nevertheless,
they found one at the said salt lake, 75 and a jeep,” says Knaden, smiling. they find it – because the nest has a
kilometers away. Knaden saw the ad- The scientists set up their experi- particular smell. The researchers know
vantage of their salt flat immediately ment in the salt lake, which contains this because each time the ants on the
on their first visit: “The nests there were water only during the winter. For their panel reach the wind vane, which in-
literally exploding with ants.” The first experiments, they needed only a dicates the wind direction in front
nests were much bigger than those in round, five-millimeter-thick plastic and of the entrance, they turn and scurry
Maharès, which is beneficial for carry- aluminum panel with a one-centimeter to the hole. On the leeward side, how-
ing out spot checks and performing sta- hole in the middle, which was placed ever, they wander around, searching,
tistical evaluations. over a nest entrance. Using this meth- even if they are only a few centimeters
od, Bühlmann and Knaden were able to from their own home. A first impor-
SWITCHING TO LANDMARK demonstrate that odor is actually a fac- tant finding.
ORIENTATION tor in helping the ants find the en-
trance when they return to the nest: FORAGING TRIP IN
Even though research conditions are once the sun compass and path integra- THE ALUMINUM CHANNEL
better at the salt pan, the scientists tor have brought the ant to within a
from Jena don’t want to move there meter of the nest, the animal then For the subsequent, more complex
Photo: MPI for Chemical Ecology
lock, stock and barrel; their base camp switches to landmark orientation. “The tests, the field researchers needed alu-
continues to be in Maharès. “The coast ant cannot actually see the hole itself, minum channels measuring two me-
is simply nicer,” says Knaden. Better so it remembers a nearby stone or ters long and seven centimeters wide,
climate, a fishing port, a few restau- bush,” says Knaden. U-shaped and open at the top, which
rants. And the locals know the re- In the experiment, the researchers are placed on the ground. These chan-
searchers, as they spend entire days can exclude such information by cov- nels are connected via a tube with an
standing in the scorching heat, watch- ering the nest entrance with the pan- upturned bucket positioned over the
4 | 10 MaxPlanckResearch 21
top: The researchers paint several spots entrance hole to the Cataglyphis nest. ple, on the wall opposite the hole.
of color on the abdomen of each ant The scientists use this test channel to They evidently use the spot as a land-
so that they can distinguish between
the ants on the training runs.
hide all visual cues and the horizon mark and link this to the location of
from the ants’ field of vision: “The an- the entrance. Would the same thing
bottom: Markus Knaden and Kathrin
Steck look at the experimental data on imals scuttle out of their nest under work with odors?
the computer. the bucket. They go through the tube Because Knaden and Steck were not
and arrive at the side wall of the alu- sure whether even one (and if so which)
minum channel through a tiny hole. of the four odors identified in the lab-
There they look for the food that we oratory – methyl salicylate, decanal,
have laid out and come back to this nonanal and indole – could ultimately
hole,” says Markus Knaden, describing be important, they drizzled the four
the experiment. substances separately to form an odor
Would the ants also be guided by square in the training channel in front
an odor that does not emanate from of the entrance hole. The ants learned
the hole itself, but originates from its very quickly that their entrance could
environment? When Knaden and be found in this odor field. When they
Steck examined this issue in their returned after their foraging trip in the
channel experiment, they made an ex- aluminum channel, they just searched
citing discovery that once again illus- inside the square for the entrance and
trates the complex sensory abilities of found it immediately.
these small insects. But something first
had to go wrong before they realized A MAP OF VARIOUS ODORS
the ants’ abilities.
Photos: MPI for Chemical Ecology (2)
The hole in the side wall of the alu- Unfortunately, the control experiments
minum channel is so small that it is yielded no conclusive results: the re-
very difficult for the insects to find un- searchers offered an odor field again,
less it is marked. “It takes them forev- but this time without an entrance. The
er to find it,” says Markus Knaden. ants did remember the odor and initial-
They keep running by the hole. How- ly systematically frequented the odor
ever, they find it immediately if the re- field, “but each time they hurried out
searchers paint a black spot, for exam- again over the lines of the square,” says
22 MaxPlanckResearch 4 | 10
FOCUS_Orientation
The Jena-based odor specialists set up camp
in the Tunisian coastal town of Maharès. It
has been the mecca for Cataglyphis research
since 1968, when Swiss zoologist Rüdiger
Wehner discovered the town and decided to
use it as a base for his research.
stink – is: carbon dioxide, or stale air.
“The animals breathe in the nest, food
is consumed there – all of this causes
the level of CO2 in the air to rise,” ex-
plains Knaden. That could account for
the nest odor, but he isn’t sure.
To find out exactly, Cornelia Bühl-
mann and Kathrin Steck collected the
nest odors mentioned at the start of
this article using an aspirator. Here in
Jena, Steck puts the odors back into
the refrigerator, and now there is just
one more thing to do: go home and
go to bed.
Knaden, describing the ants’ behavior. arate sensory inputs – as they do for see-
And then the researcher realized they ing – which means that they rely on
had made a serious error: “We had both of their antennae. According to GLOSSARY
mixed up the odors. In the control Knaden, “Ants that only had one an-
channel, they were in the wrong loca- tenna could no longer find their way Electroantennograms (EAG)
tions in relation to where the entrance around.” This type of stereo smelling Electrophysiological discharge of so-
hole was supposed to be.” among animals has been known for a called summating potential (super-
imposed action potential of several
The ants were visibly confused long time; researchers are sure that rats odor receptors) over the entire
by this. When Kathrin Steck arranged and even humans use such a method. antenna when an odor is applied.
the odors in the same order in the odor What they did not realize is that ants Path integrator
square as they were in the test channel also have this facility. Desert ants use a path integration
in front of the entrance hole, everything Yet a number of questions remain system. This consists of a compass,
was back to normal again: the ants unanswered: Which landmarks are to determine where they are heading,
and an odometer to measure the
looked for the nest entrance only with- more important for the ants – visual distances they have traveled. The
in the odor field as they had learned to or olfactory? And does the distribution compass uses a light phenomenon in
do previously. “We had not expected the of odors at the nest entrance in the the sky that is invisible to the human
odor array to be important for orienta- natural environment really play a role? eye and stretches across the entire
celestial hemisphere as a large-scale
tion purposes,” says Knaden. After all, the experiment with the
polarization pattern. Cataglyphis
The animals clearly navigate not aluminum channels is an artificial measures the distance it has traveled
only on the basis of one or two odors in arrangement. It is possible that the by integrating the steps it has taken
the mix and link these in their memory ants may use such an odor map less at on the journey. Rüdiger Wehner and
with the entrance hole, but they actual- the nest entrance but more when for- his colleagues discovered this by arti-
ficially lengthening or shortening the
ly remember the individual locations of aging. A typical smell of home could length of the desert ants’ legs before
the odor drops. Or, in a nutshell: Cat- also be enough for an ant to find its
Photo: MPI for Chemical Ecology
they returned to the nest from a feed-
aglyphis smells in 3-D – a completely sur- own nest. er. The result was that animals manip-
prising and conclusive result that nobody Following his latest field trip, ulated in this way over- or underesti-
mated the return distance by exactly
had reckoned with after 40 years of re- Markus Knaden is somewhat skeptical
the amount by which the animals had
search on Cataglyphis’ navigational skills. about this: “It looks like there’s one modified their step length due to the
In order to be able to orientate general nest smell,” he says. Steck, change in the length of their legs.
themselves spatially in such an olfacto- Bühlmann and he already suspect what
ry landscape, the animals need two sep- the smell – or perhaps one should say
4 | 10 MaxPlanckResearch 23
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