First published online January 30, 2009
Journal of Experimental Biology 212, 499-505 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.022988
Priority rules govern the organization of traffic on foraging trails under crowding conditions in the leaf-cutting ant Atta colombica
A. Dussutour1,2,3,*,
,
S. Beshers2,
J. L. Deneubourg3 and
V. Fourcassié1
1 Centre de Recherches sur la Cognition animale, UMR CNRS 5169,
Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex
4, France
2 Unit of Social Ecology, Université Libre de Bruxelles, Boulevard du
Triomphe, B-1050 Bruxelles, Belgium
3 Department of Entomology, 320 Morrill Hall, University of Illinois at
Urbana-Champaign, 505 S. Goodwin Avenue, Urbana, IL 61801, USA
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Fig. 1. Head-on collisions between (A) outbound and inbound unladen ants and (B)
outbound and inbound laden ants. After a collision, either the inbound or the
outbound ant moves to the bridge side to allow the passage of the oncoming
ants. The follower ants might benefit from the passage of the leading ant (the
ant that was given way) before the ant that gave way returns to the top of the
bridge.
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Fig. 2. Distribution frequency of the size of the groups of successive ants
traveling in the same direction for the two types of bridges used in the
experiment. The distributions for the two directions have been pooled on the
graph. The distribution frequency of the group size obtained with a random
sequence of ants generated on the basis of an equal probability of occurrence
of nestbound and outbound ants is also represented. N=5 replicates
for each bridge. The inset shows the distribution frequency as a function of
group size in natural log scale.
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Fig. 3. Proportion of laden ants for each size of the groups of inbound ants. The
number of laden and unladen ants are indicated in bold and regular font style,
respectively. The results of five replicates have been pooled. The dashed line
shows the expected proportion of laden ants for each group size, i.e. the mean
proportion of laden ants in the inbound flow of the five replicates (0.24). We
used a  2 goodness-of-fit test to test whether the proportion
of laden ants for each group size differed from the expected value. A
significant result means either that laden ants are underrepresented (for
group size <5) or overrepresented (for group size >5) in the groups.
*P<0.05, **P<0.01,
***P<0.001.
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Fig. 5. Expected and observed proportions of inbound groups led by a laden ant as a
function of group size. The expected proportion corresponds to the proportion
of laden ants observed for each group size (see
Fig. 3). If the laden ants were
equally distributed within the group, we would expect a 0.24 chance to observe
them in the first position. We used a 2 goodness-of-fit test
to test whether the proportion of laden ants for each group size differed from
the expected value. The significant result for group size >3 means that
laden ants are significantly more likely to occupy the first position than any
other position within the groups. The results of five replicates have been
pooled. *P<0.05, **P<0.01,
***P<0.001.
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Fig. 6. Probability of a follower ant of benefiting from the passage of the leading
ant according to its position in a group of inbound or outbound ants. The
leading ants are the ants that are given way to after a head-on encounter
(N=99, N=301 and N=294 for inbound unladen ants,
outbound ants and laden ants, respectively). The ant occupying the first
position is the one immediately behind the leading ant. The probability for
each ith position is computed by dividing the number of ants in the
ith position that benefited from the passage of the leading ant by
the total number of groups whose size is  (i+1). For example, an
ant immediately behind (1st position) a laden ant has a 0.99 probability of
benefiting from the passage of the leading ant. The probability value becomes
0.59 if the leading ant is unladen. The inset shows the distribution of the
number of ants that benefited from the passage of a leading ant. The dotted
line within the boxplots represents the median, and the lower and upper
boundaries of the boxes represent, respectively, the 25th and 75th
percentiles, while the whiskers extend to the smallest and largest values
within 1.5 box lengths. The asterisks represent the outliers. The bars bearing
different letters are significantly different at the 1% level of significance
(one-way ANOVA followed by a Bonferroni post-hoc test).
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© The Company of Biologists Ltd 2009
