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First published online May 8, 2007
Journal of Experimental Biology 210, 1804-1812 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02769
Homing strategies of the Australian desert ant Melophorus bagoti II. Interaction of the path integrator with visual cue information
Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia
Present address: ARC Centre of Excellence in Vision Science and Centre for Visual Sciences, Research School of Biological Sciences, Australian National University, PO Box 475, Biology Place, Canberra, ACT 2601, Australia (e-mail: ajay.narendra{at}anu.edu.au)
Accepted 5 March 2007
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Summary |
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 TOP Summary IntroductionMaterials and methodsResultsDiscussionReferences |
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Key words: distant cues, route cues, path integrator, context, interaction, ants, Melophorus bagoti
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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). Hence
visual cues in the foraging territory are vital for homing ants
(Collett et al., 1992;
Collett et al., 2003b;
Seidl and Wehner, 2006).
Non-trail forming ants that inhabit landmark-rich rain forests return to the
nest by either matching the contrast in the canopies
(Hölldobler, 1980) or
using landmark and celestial information
(Beugnon et al., 2005).
Individually foraging desert ants that inhabit featureless habitats are guided
primarily by the path integrator to steer themselves towards the goal
(Wehner et al., 1996). During
path-integration, ants continuously monitor the direction and distance
travelled on the foraging journey, to update a vector and compute the shortest
distance to return to the nest (Wehner and
Wehner, 1990; Collett and
Collett, 2000; Wehner and
Srinivasan, 2003).
Path-integration is a self-centred system of navigation that accumulates
errors (Müller and Wehner,
1988), and because of this ants use the cues along the route, when
available, to fine-tune their home vector
(Collett, 1992).
Path-integration is often the initial navigational strategy of
non-trail-forming ants in unfamiliar terrain. With repeated foraging, an area
becomes familiar and the information derived from the landmarks along the
route is also used. The visual landmark information is thus utilised as a
supplementary strategy (Wehner et al.,
1996) for homing (Burkhalter,
1972; Wehner and Räber,
1979; Bisch-Knaden and Wehner,
2001; Collett et al.,
2001). In some cases, the action associated with the visual
landmarks suppresses the performance of the path integrator
(Collett et al., 1998;
Kohler and Wehner, 2005).
However, the interaction between the path integrator and the visual cues
present in the foraging territory remains obscure. This paper reports the
findings of an investigation into this interaction in the Australian desert
ant, Melophorus bagoti, which primarily navigates by establishing and
adhering to individualistic routes (Kohler
and Wehner, 2005).
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Materials and methods |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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;
Muser et al., 2005). M.
bagoti ants are scavengers, and feed primarily on insects roasted in the
desert heat.
Experimental set up
Ants were trained to forage within a corridor of landmarks flanking the
route from the nest to a food source. Homebound trajectories of trained ants
were recorded in the presence and absence of vector information and
route-cues, in both the familiar training field and an unfamiliar test
field.
The training field (15 mx30 m) was set up around a nest where no distinct landmarks such as trees were present. All tussocks in the foraging area were cleared using a lawn mower. Here, the ants were trained to reach a feeder by travelling north through a 20 m long route-mark corridor (20 m length and 1 m width; Fig. 1A). The route-mark corridor was constructed by erecting two rows of cylinders, each cylinder measuring 60 cm in height and 16 cm in diameter. Cylinders were placed on each side of the nest and the feeder, 0.5 m away, and along the path, where they were placed alternately 0.5 m to the left and right of the nest–feeder line, at 2 m intervals. A feeder with watermelon and cookie crumbs as food, was sunk into the ground at the end of the corridor. During training, the interior walls of the feeder was intentionally kept grainy to help ants climb out of the feeder. During testing, however, a thin coating of fluon was applied on the interior walls to prevent ants from escaping. The training field was divided into a grid of 0.5 m squares using strings and tent pegs. Paths of homing ants were recorded on squared paper.
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A test field (15 mx30 m) was established in an unfamiliar area,
250 m away from the nest site. A route-mark corridor similar to the one
in the training field was set up. The test field also was cleared of tussocks
and divided into a grid of 0.5 m squares wherein homebound trajectories were
recorded.
Training regime
Ants that reached the feeder for the first time were marked with distinct
colours for identification and allowed to return to the nest. Following
marking, each ant revisited the feeder at least 20 times in a given day and
were trained to reach the feeder and return to the nest for 14 consecutive
days. Ants that arrived to the feeder on the 15th day and picked up a cookie
crumb were captured in a plastic vial and released either in the training
field or in the test field where their paths were recorded in different test
conditions. Each ant was tested only once.
Testing
Based on their vector information, ants were divided to two groups: (1)
full-vector ants: ants caught at the feeder before they had begun their
homebound run, and (2) zero-vector ants: ants caught close (within 50 cm) to
the nest entrance while returning to the nest. Zero-vector ants are so called
as they have nearly run off their entire home vector.
Ant paths within the route-mark corridor
The homebound paths of ants that arrived at the feeder, after travelling
through the route-mark corridor, were recorded under three conditions,
full-vector ants in the presence of route-marks (FvRm+), full-vector ants in
the absence of route-marks (FvRm–) and zero-vector ants in the presence
of route-marks (ZvRm+), in both the unfamiliar test field and familiar
training area, giving a total of six conditions. In the training field, the
paths taken by the ants were recorded until they reached the nest; all the
tested ants reached the nest, taking on average less than 40 s. In the test
field, the paths of full-vector ants were recorded until the ants began to
search, while the paths of zero-vectors ants were recorded for a 5-min period.
Search was identified by a turn 
50° from the homing trajectory and
extending for at least 0.5 m. The start of search was identified with a 100%
reliability between two experimenters in an earlier sample.
Ant paths following a sideways displacement
Ants of this group were also trained within the route-mark corridor for 14
successive days. Ants that arrived at the feeder on the 15th day and picked up
a cookie crumb, were captured and displaced sideways, 1.5 m, 3 m, 6 m and 10 m
west of the nest-feeder line. Paths of homing ants were recorded on squared
paper until they reached the nest. The route-marks were not moved during the
tests. As a control group, zero-vector ants were captured and were displaced
to a point 10 m west of the nest-feeder line.
Analysis
Trajectories of homebound paths were digitised. Data evaluation was carried
out using statistical methods prescribed for circular distributed data by
Batschelet (Batschelet, 1981).
Circles of 0.5 and 5 m radii were drawn around the release point and angular
positions were recorded when the trajectories crossed each circle for the
first time. The mean direction 
and length of the mean vector
r were computed for each condition. The mean angles of circular
observations was compared by Watson–Williams test using the circular
statistics program Oriana (Kovach,
2004). Among ants displaced sideways, the distribution of
orientations at both 0.5 m and 5 m were analysed using the V test to
determine whether ants from each group were significantly oriented towards
their true nest position.
Distance travelled before engaging in a search was determined for ants recorded in the test field and among ants displaced sideways. For ants whose paths were recorded in the training field, the absolute deviation of an ant's path from the nest–feeder line was measured at every 1 m interval (Fig. 1B). The overall average deviation was computed for each ant, following which the average deviation for each condition was determined. Less deviation would suggest ants adhering to a nest–feeder path, whereas greater deviation would suggest otherwise. For ants displaced sideways, the point of entry of each ant into the route-mark corridor was determined and was compared among the five laterally displaced groups. Data were checked for normality and non-parametric analyses were conducted when required. Search behaviour is described, but not analysed in detail. Another paper in preparation is devoted to the analysing the search patterns.
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50°; (b) one turn: paths of ants characterised by a single turn
50°, these are usually 90° turns directed towards the route-mark
corridor; (c) zero loops: paths of ants with more than one turn
s–1 50°, but the paths never form loops; and (d)
one loop: paths of ants that turn 50° and form a loop/s. |
Results |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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Orientation of ants in the presence of route marks (FvRm+) was directed towards the fictive nest and differed significantly from the orientation of ants in the absence of route marks (FvRm–) at both 0.5 m (P<0.05, Watson–Williams test) and 5 m (P<0.05, Watson–Williams test) after release (Fig. 3A). The initial orientation of FvRm+ ants at 0.5 m after release was significantly different from the orientation after the ants had travelled a distance of 5 m (P<0.05, Watson–Williams test; Fig. 3A). However, the initial orientation of FvRm– ants at 0.5 m after release did not differ from the orientation after the ants had travelled a distance of 5 m (P>0.1, Watson–Williams test; Fig. 3A). The initial orientation of ZvRm+ ants at 0.5 m was directed towards the fictive nest, but thereafter they switched to a search around the release point, and continued searching for the entire recording duration of 5 min (Fig. 2A). None of the ZvRm+ ants crossed the 5 m circle during the search and hence their orientation at 5 m is not shown.
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Homing in familiar training field
(a) Ants released within the route-mark corridor
All ants from the three groups, FvRm+, FvRm– and ZvRm+, reached the
nest located 20 m away from the release point
(Fig. 2B). The homing paths of
the three groups deviated by different extents from the nest–feeder line
(P<0.001, ANOVA), the deviation being maximum in FvRm–
(0.603±0.31 m) and minimum in FvRm+ ants (0.125±0.06 m; mean
± s.d.) The paths of FvRm+ and ZvRm– were indistinguishable
except for an initial search exhibited by zero-vector ants. The deviation of
the paths of ZvRm+ ants (0.178±0.07 m) from the nest-feeder line was
similar to the deviation exhibited by FvRm+ ants (P>0.05,
Bonferroni's test) and was less than the deviation of FvRm– ants
(P<0.01, Bonferroni's test).
The mean orientations of FvRm+ ants at 0.5 m and 5 m after release did not differ (P>0.1, Watson–Williams test; Fig. 3B) and both were directed towards the nest (Ps<0.001, V test). In the FvRm– ants too, the mean orientations at 0.5 m and 5 m after release did not differ (P>0.1, Watson–Williams test; Fig. 3B) and both were directed towards the nest (Ps<0.001, V test). Zero-vector ants initially exhibited a search, but then travelled within the route-mark corridor (Fig. 2B). Their mean orientations at 0.5 m and 5 m after release did not differ (P>0.1, Watson–Williams test; Fig. 3B) and both were oriented towards the nest (Ps<0.001, V test).
(b) Ants displaced sideways
Ants displaced sideways from the nest–feeder line, 1.5 m, 3 m, 6 m,
10 m and Zv10 m, reached the nest successfully (Figs
4,
5,
6). Ants that entered the
route-mark corridor travelled within the corridor until they reached the nest.
The point at which they entered the route-mark corridor varied in all the five
groups (P<0.001, ANOVA). Ants from the 1.5 m, 3 m, 6 m and 10 m
group entered the corridor 12.09±4.72 m (N=21),
7.66±3.86 m (N=20), 9.87±4.65 m (N=18) and
1.56±0.85 m (N=21; means ± s.d.) respectively, away
from the nest. Zero-vector ants, Zv10 m, displaced laterally entered the
corridor 12.08±4.61 m (N=15; means ± s.d.) away from
the nest.
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)] strongly suggest that the ants are predominantly guided by
the path integrator in this section. The distance travelled by relying on the
path integrator was nearly half the distance of the outbound journey of 20 m
in all four groups (9.15±3.77 m, N=10, 1.5 m group;
11.05±2.88 m, N=12, 3 m group; 9.50±3.28 m,
N=18, 6 m group; 9.23±2.35 m, N=21, 10 m group; means
± s.d.; P=0.35, ANOVA; Fig.
4). As shown in Table
1, only few ants from the 1.5 m and 3 m groups initially relied on
the path integrator (i.e. ants pooled from `one turn', `no loop' and `one
loop' categories), and by contrast, all ants from the 6 m and 10 m group
relied on the path integrator. The direction of the second segment implies
strongly that ants are being guided by an aspect of the visual scene.
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The majority of ants from the 1.5 m and 3 m group entered the corridor either without a turn or with a single sharp turn that identifies a switch from relying on path-integration to route following. Few ants from these two groups searched in loops before entering the route-mark corridor (Table 1). In contrast, all the ants from the 6 m and 10 m group searched in loops, following which they reached the nest, by heading towards a direction intermediate between that dictated by the nest and the route-mark corridor. Zero-vector ants too searched in loops, following which they travelled in oblique paths to reach the route-mark corridor (Table 1). In all groups, once the ant hit the route-mark corridor, it travelled within the corridor to reach the nest.
Displaced ants to which vector information was available were initially oriented towards the true nest (Ps<0.001, V test; Fig. 5). At a distance of 5 m from the release point, their orientation changed significantly (Ps<0.05, Watson–Williams test), towards a direction intermediate between that dictated by the path integrator and the nest (Fig. 5). Ants without vector information, Zv10 m, both initially and after 5 m were oriented towards the true nest direction (Ps<0.001, V test; Fig. 6). However, it is important to note that these ants initially exhibited a large directional scatter (r=0.38), which reduced after the ants had travelled a distance of 5 m (r=0.92).
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Discussion |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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),
wherein they adhere to a particular route while returning to the nest
(Kohler and Wehner, 2005).
However, the ants are often exposed to severe dust storms that displace them
from the familiar route. In such a scenario, relying on route cues alone would
be perilous for a homing ant, and hence alternative or complementary
strategies to reach the nest are required.
Homing without vector information
The initial orientation of ZvRm+ ants in the training field was towards the
fictive nest position (Fig. 2B,
Fig. 3B) and this was the
result of a residual vector (zero-vector ants were captured close to the
nest), or the direction dictated by route-marks or the familiar context
consisting of micro-landmarks and distant cues. But even in an unfamiliar
context, as was the case in a test field, the initial orientation of ZvRm+
ants was towards the nest (fictive) position
(Fig. 2A,
Fig. 3A). This was certainly
because of a residual vector, or the direction dictated by the route-marks.
The presence of familiar route-marks in an unfamiliar context of the test
field, did not elicit homing, and the ants engaged in a search behaviour
immediately after a short homeward run. Comparing the homing abilities of
these ants with those that have no vector or route-mark information,
ZvRm–, would be interesting. This is a condition not tested in this
experiment. From the results here, I would predict that ZvRm– ants too
would reach the nest, but only if in a familiar context.
Homing with vector information and the importance of contextual cues
FvRm+ ants deviated the least from the nest–feeder line in the
training field, whereas in a unfamiliar test field they travelled nearly
half-way towards the nest, interweaving from one route-mark to the other
(compare Fig. 2A with 2B). The
interweaving, which appears as if the ants are querying the presence of
familiar route-marks in an unfamiliar context, was not exhibited by ants
homing in a familiar context of the training field. Ants that travelled in the
absence of route-marks (FvRm–) in the training field
(Fig. 2B) deviated by a
significantly greater distance from the nest–feeder line, whereas in the
test field the lack of route-marks altered the orientation of the homing ant
(Fig. 2A). In the training
field the maximum deviation of the paths of FvRm– ants from the
nest-feeder line was a result of the absence of familiar route-marks amidst
familiar contextual cues. The deviation is quite possibly due to an
interweaving search carried out by homing ants for the route-marks that were
absent. The route-marks themselves could provide directional guidance when
present amidst familiar contextual cues. In an unfamiliar test field, since
the context was entirely different, FvRm– ants did not search for the
route cues. Thus in the test field the paths of FvRm– ants were straight
compared with the paths of FvRm+ ants whose paths interweaved from one
route-mark to another (Fig.
2A). Thus, route cues are not essential, but when present in a
familiar context, `tighten' the ant's path. However, in the absence of
familiar contextual cues, route cues alone cannot elicit homing and their role
is perhaps to provide directional cues to an homing ant.
Ants with vector information, irrespective of the presence or absence of
route-marks, travelled nearly half the distance of the outbound journey before
engaging in a search. This partial distance travelled by relying on the path
integrator was similar to the proportion travelled by ants in an unfamiliar
test field after reaching a food source in an open field
(Narendra, 2007). The path
integrator is thus relied upon when visual cues that form a familiar context
are absent. Isolated landmarks can often be misidentified hence relying on
contextual cues can aid in disambiguating individual cues
(Collett et al., 2003a).
Hence, context plays an important role in insect navigation.
Homing by laterally displaced ants
Ants displaced laterally encounter a navigation problem similar to ants
that are blown off by dust storms, i.e., a homing ant is presented with three
conflicting directional choices, (1) the path integrator, (2) distant cues,
and (3) the familiar route (Fig.
7). All ants with vector information displaced 6 m and 10 m away
showed a distinct switch from initially relying on the path integrator to
following visual cues (Fig. 4).
In contrast, most ants displaced close to the route-mark corridor at 1.5 m and
3 m distance, reached the nest primarily by adhering alongside the route-mark
corridor or travelling within the corridor and did not demonstrate any
distinct switch from relying on path-integrating to route-following. Ants with
vector information that were displaced close to the route at 1.5 m hit the
route close to the release point, whereas those displaced 10 m away hit the
route close to the nest. Thus the position where the ants hit the route did
not bear any relevance to their route-following ability, and upon hitting the
familiar route ants travelled along the route to reach the nest.
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Information from the path integrator was, however, unavailable for zero-vector ants, Zv10 m, and hence these ants relied on distant cues for homing (Fig. 6). Upon release, the ants directed their search towards the distant cues. The search led them closer towards the familiar route-mark corridor. But rather than travelling laterally to reach the familiar path, they travelled in strikingly oblique paths, heading in a direction intermediate to that dictated by the distant cues and the route cues. Similar to the behaviour of ants in other conditions, once the zero-vector ants hit the familiar path in a familiar context, they adhered to their path until they reached the nest (Fig. 6).
The ability to adhere to a familiar route upon reaching it, corroborates
with previous findings (Wehner et al.,
2006), in which both full-vector and zero-vector ants were
captured and displaced locally, a few meters away from the homing path. This
behaviour of adhering to a familiar route is exhibited by homing pigeons as
well (Biro et al., 2004;
Lipp et al., 2004).
Experienced birds that had been displaced nearly 3 km from the route
approached the route obliquely, and once they hit the route they travelled
along their familiar path to reach their goal
(Biro et al., 2006). These
paths were similar to their earlier flight paths before being displaced.
Nature of distant cues
In the absence of route cues and vector information, distant cues guide
ants towards the nest (Fig. 7).
Distant cues take precedence over vector information among ants displaced
sideways, causing the ants to orient towards the nest immediately upon
release. What might these distant cues be? In the ant's semi-arid desert
habitat these cues could either be a distant skyline or a single conspicuous
landmark such as a distant tree whose view changes little as the insect moves
around the nest. The use of distant cues while homing has been demonstrated in
wood ants, Formica japonica
(Fukushi, 2001), where the
ants used the distant skyline for homing. Hence the paths of laterally
displaced wood ants did not remain parallel to one another but converged at
the nest. Fukushi (Fukushi,
2001) suggested that the distant cues used by wood ants are
panoramic cues. Further research is needed to identify the nature of these
distant cues.
In summary, M. bagoti ants displaced from their familiar route rely on distant landmarks for homing. The ants follow routes in familiar foraging terrain, but the route marks are not essential and are utilised only when present in a familiar context. Locally displaced ants return to the nest by compromising between three navigational strategies. Ants far away from the familiar route, compromise between the directional dictates of the path integrator and the distant cues and ants nearby to the route compromise between the directional dictates of the distant cues and the familiar route cues. If the ants encounter the familiar path in a familiar context, they adhere to the familiar path until they reach the nest.
This research has demonstrated the interaction between the different navigational strategies and has highlighted the importance of distant cues and contextual information for homing desert ants.
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Acknowledgments |
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