Obstacle Size and Trail–Clearing Activity in Leaf–Cutter Ants, Atta colombica

Tropical Ecology • February 2015 • Ecology and Evolutionary Biology
Obstacle Size and Trail–Clearing
Activity in Leaf–Cutter Ants, Atta
colombica
Maxson Jarecki
Princeton University
mjarecki@princeton.edu
Abstract
This study on Atta colombica leaf obstacle removal found that there is a power curve relationship
between leaf obstacle length and its removal time. Leaf obstacles over 5cm long were cut using the same
techniques as used in leaf harvesting. This study was conducted across 3 forests of varying rainfall levels
in Panamá. Though there were no significant differences found between forests and time periods (morning
and afternoon), a larger study may reveal differences based on humidity, elevation, temperature, sun
exposure, and microclimate variation due to weather and vegetation. Further research must be conducted
to fully examine the effect these variables have on the individual obstacle removal rates of these forests.
Keywords: Atta colombica; rainfall gradient; Panamá; trail maintenance; obstacle removal.
I. Introduction
A
tta colombica is a species of fungus grow-
ing leaf–cutter ant; its range reaches
throughout the Neotropics (Ghazoul
and Sheil 2010). It is distinguished from its
relative, Atta cephalotes, by a missing tuft of
reddish hair, and by its aboveground refuse
dumps, as cephalotes dump underground. They
are part of the myrmicine tribe Attini, along
with macrotermitine termites and some wood–
boring beetles, members of which are distin-
guished by their unique ability to cultivate and
consume fungi (Wirth et al. 2003). Atta colom-
bica colonies can contain several million ants,
and they can build large nests as deep as 6
meters underground (Ghazoul and Sheil 2010).
Their farmed fungus, Leucoagarius gongylopho-
rus, only exists in the wild within leaf–cutter
ant nests (Ghazoul and Sheil 2010).
Atta colombica antennae, mouthparts, and
legs each contribute to the obstacle removal
process that this study investigates. Dr. M.V.
Brian provides a good summary of ant phys-
iology in his book, Ants (1977). Their anten-
nae allow Atta to process the size and shapes
of objects in their environment. They can be
moved, and can be spread wide apart to com-
prehend large objects, or brought close together
to sense objects less than a millimeter in diam-
eter. The antennae also detect chemicals and
pheromones laid down by other foragers, in ad-
dition to sensing vibrations in the substratum.
Atta mouths are surrounded and enclosed by
several pairs of articulated appendages, the
most prominent of which are the mandibles
(Brian 1977). These main jaws are hollow, but
are composed of thick walls. They are solidly
attached to the ant head and can be opened
widely, and closed tightly. Finally, Atta legs
each have five main joints. The outermost joint,
however, is finely articulated by further small
joints to make a flexible “foot.” Each leg is ex-
ceptionally maneuverable, and can move over
rough and irregular terrain; even up vertical
and overhanging surfaces. The ant’s hind legs
are the longest, and are long enough to lift
their body clear off the ground (Brian 1977).
These leaf–cutting ants are the most dom-
inant herbivores in the New World tropics
1

(Wilson 1986), and as “generalist herbivores,”
Colombica ants have an enormous effect on the
ecosystems they inhabit (Wirth et al. 2003).
Their role as agricultural pests have prompted
leaf–cutting ants like Atta colombica to be
among the most studied tropical insects (We-
ber 1972; Hölldobler and Wilson 1990; Fowler
et al. 1990). Because of their role as pests, most
of the research surrounding these tropical ants
revolves around their control (Vander Meer et
al. 1990). Their yearly neotropical agricultural
damage has been valued to cost thousands of
millions of dollars (Cherrett 1986). However,
there is also much research regarding Atta be-
havior.
Specifically, their foraging strategies have
been a topic of inquiry. The distribution of Atta
foraging over a large area through trails, rather
than simply foraging around the nest, has been
a major subject of study (Cherrett 1986; Fowler
and Styles 1980; Rockwood and Hubbell 1987;
Shepherd 1982). Foraging efforts are arranged
through these trails to promote the discovery
of items in productive areas (Shepherd 1982).
Also, the recruitment pheromone laid down
on these trails can convey information about
location and quality of resources (Hangartner
1969). There is research on the performance of
workers utilizing trails (Lutz 1929; Hubbell et
al. 1980; Rudolph and Loudon 1986, Lighton
et al. 1987, Waller 1989; Shutler and Mullie
1991; Wetterer 1994; Burd 1995). There are
also many studies on the use of trail systems
(Fowler and Robinson 1979; Fowler and Stiles
1980; Shepherd 1982; Rockwood and Hubbell
1987). There is less information, though, on the
origins of these foraging trails: where they are,
how they are constructed, and how they are
maintained (Shepherd 1985; Farji Bener and
Sierra 1993; Howard 2001).
Foraging trails allow Atta ants to locate re-
sources once they have exited the nest (Höll-
dobler 1977; Shepherd 1982; Fowler and Stiles
1980). Trails have also been linked to reduc-
ing aggressive encounters between neighboring
colonies whose resource areas overlap (Vilela
and Howse 1986; Farji Bener and Sierra 1993).
Their trails are broken into two types, “trunk”
trails (like the trunk of a tree), and ephemeral
trails (Howard 2001). These trunk trails are
fairly permanent, persisting from a few months
to several years (Howard 2001). Colombica
colonies manage trail systems that average
267 meters in length, and build an estimated
2.7 kilometers of trail per year (Howard 2001).
Through his calculations, Howard concluded
that the energetic costs of trail clearing are
negligible in the context of the vast number
of available workers and their rate of harvest
(2001). It is interesting to note that these long–
lasting trails persist despite Atta’s foraging on
patchy and ephemeral leaf resources (Rock-
wood 1975; Fowler and Stiles 1980; Shepherd
1985). These trails can be easily identified in
the forest, as they are often clear of debris. This
trail clearing has been shown to facilitate more
effective locomotion (Rockwood and Hubbell
1987). In fact, Rockwood and Hubbell found
that colony investment in trail–making repaid
between four– and ten–fold in reduced travel
cost (1987). Also, the more effective applica-
tion of trail pheromones to the smoother sub-
strate offered by a cleared trail may increase
the strength and persistence of these trunk
trails, which may allow ants to relocate and
exploit resources more effectively (Wirth et al.
2003). Hölldobler and Lumsden developed a
cost–benefit equations to examine the energetic
frictional cost of trail locomotion and trail con-
struction (1982).
Cf = gNtot
Cf is frictional cost, Ntot is the number of
loads carried, and g is a cost–per–ant coeffi-
cient. G is higher in a litter–covered forest floor
and lower in a cleared area (Hölldobler and
Lumsden 1980) (Fig. 1).
Ct = aNmax + bNmaxT
Ct is the cost of clearing and maintaining
a trail. It increases with the area of the trail,
aNmax, and the amount of time the trail is used,
bNmax (Hölldobler and Lumsden 1980) (Fig. 1).
2

Figure 1: Visual representation of the costs and benefits
of trail–clearing in Atta colombica.
Atta ants in a colony are divided into dif-
ferent castes, primarily based on head size
(Wirth et al. 2003). Lugo et al. (1973) esti-
mated that up to 75% of ants on trails at a
given time do not carry leaves. They suppose
that these ants are the ones most involved in
trail–clearing. The most frequent head width
among leaf carriers is between 2.0 and 2.2mm
(Wirth et al. 2003). Howard found that larger
workers whose headwidths measure between
2.2 and 2.9mm are most likely to be involved
in trail clearing (2001). His study confirmed
that ants clearing trails are significantly larger
than those carrying leaves. Ants involved in
trail–clearing exhibit high task fidelity; clearers
tend to clear, while foragers forage (Howard
2001). Small litter items are carried off trails,
while larger items are made smaller through
the same leaf–cutting techniques Atta colombica
ants use during foraging.
This investment in altering the obstacle
greatly increases its removal energy cost, but
makes its removal possible (Howard 2001). His
study outlined the time and energy costs of
trail–clearing on a macro scale; those of a
colony. He found that the costs of removing a
kilogram of litter were approximately 3,359 ant–
hours and 4.6 kJ of collective energy (Howard
2001). He then estimated that the total cost
of trail–clearing to a colony averaged 11,000
ant–days of work (2001). The yearly energetic
cost, though, was the equivalent of just 8,000
leaf–loads (Howard 2001). There is a major
discrepancy between the importance of trail–
clearing activity and its cost. Rockwood and
Hubbell (1987) found that there can be a ten–
fold reduction in travel cost when a trail is
properly cleared. However, this instrumental
activity takes only 8,000 leaf–burdens of en-
ergy yearly. This annual cost can be recovered
in less than a day by an average–size colony
(Howard 2001)! If these findings are correct,
then trail–clearing may be one of the most ef-
fective productivity–enhancing behaviors these
social insects perform. While Howard’s study
was a colony–wide investigation into time in-
vestment, my own study involves an in–depth
look at this phenomenon on a micro scale. I in-
vestigated the relationship between leaf length
and obstacle removal time in Atta colombica
colonies during trail–clearing.
II. Methods
I conducted this study through three forests in
Panamá over a rainfall gradient:
Pipeline Road: A lowland wet evergreen for-
est with 2,131 mm rainfall per year at
27m elevation.
Parque Natural Metropolitano: A lowland
semi–deciduous forest with 1,850 mm
rainfall per year at 30m elevation.
San Lorenzo: A lowland wet evergreen forest
with 3,152 mm rainfall per year at 130m
elevation.
Over the course of 9 study days during the
month of February, I located 5 Atta colombica
colonies per forest. I used these sites to test
their leaf obstacle removal speed. I selected per-
manent “trunk” trails and excluded all trails
that were not approximately 75% cleared; trail
3

clearance level was determined visually. I laid
leaves of lengths between 1 cm and 24 cm down
in the middle of these trails and timed colonies’
obstacle removal times (ORTs henceforth).
Leaves were selected from the Swietenia
macrophylla, the big–leaf mahogany tree. Macro-
phylla is an endemic tree species that offers a
wide range of leaf lengths in a standard shape.
I gathered a large batch of macrophylla leaves
of various sizes and stored them flat in plas-
tic bags, creating a vacuum by sucking the air
from each bag, and storing them in the dark.
This was to minimize day–by–day variations in
moisture content.
I also performed an in–lab analysis on Swi-
etenia macrophylla leaf length, width, mass, and
surface area correlations (Fig. 1). This work
verified that there was a consistent linear rela-
tionship between these variables. Leaf lengths
and widths were determined with a ruler, and
leaf mass was determined with a scale. Fi-
nally, surface area calculated using computer–
analyzed photos of each leaf, through the pro-
gram ImageJ R
.
Finding complete macrophylla leaves less
than 4 cm long proved impossible, so I cre-
ated leaf fragments by cutting larger leaves
into the correct shape. I selected removal sites
at least 10 feet apart from one another to min-
imize the effect obstacles may have on each
others’ ORT. Leaves were laid down horizon-
tally across the trail to maximize blockage; each
leaf was placed upside–down to create a small
hill, increasing each obstacle’s effect on forager
movement.
I began ORT timing from the moment the
first unladen ant touched the leaf, and ended
when 95% of ants involved left the site. I also
noted the occurrence of leaf–cutting behavior
on the obstacle, as some obstacles were cut and
some were not.
Figure 2: Length and width, mass, and surface area have
linear relationships (N = 60). Linear regres-
sion in JMP 11 found significance of r2 = 0.79,
r2 = 0.91, and r2 = 0.90, respectively. Each
regression revealed a P < .0001.
I analyzed my leaf length and ORT mea-
surements using CurveExpert Professional Ver-
sion 1.2.2 (2011), a comprehensive data analysis
software by D.G. Hyams.
4

III. Results
All data gathered during the study reflects a
power curve relationship between leaf length
and obstacle removal time (n = 104, r2 = 0.619)
(Fig. 1a). The power curve equation is:
y = axb
Each location and time period indepen-
dently had power curve relationships between
leaf length and obstacle removal time (Fig. 1b–
1f).
Figure 3: Figures 1a—-1f, left to right. Each curve
follows the power equation y = axb. The
outer shaded area represents “b” standard er-
ror, while the inner represents that of “a".
The variables a and b for each data set’s
y = axb equation are located in Table 1. Their
standard errors are included in italics.
Table 1: Regression values for each data set, to be input
to the power equation y = axb.
I projected average ORTs for for 3 leaf
lengths in Table 2 using the above data. By
checking the ranges of these values according
to their standard error I discovered that there
is no significant difference between these lo-
cations and time periods. However, a larger
sample size may reveal significance.
Table 2: ORT estimations for 3 different leaf sizes. These
times were calculated from regressions of y =
axb from each site and time period.
Leaf length and presence or absence of cut-
ting were highly correlated (Kruskall–Wallis
non–parametric t–test: Z = –7.21, p<0.001) (Fig.
2). Leaves above 12 cm in length were always
cut. The two outlying data points above 12
cm in the “no” column were the only ones
placed on a severe incline, which allowed ants
to remove them quickly and without cutting.
Leaves between 12 cm and 5 cm varied, pos-
sibly due to factors outlined in the discussion.
Leaves below 5cm were never cut.
Figure 4: Leaf length and presence or absence of cutting.
5

IV. Discussion
The most significant finding of this study is the
power curve relationship found between leaf
length and ORT. One would assume that this
trail-clearing phenomena would increase lin-
early: a 5 cm object taking 5 minutes, and a 25
cm object taking 25. Instead, this exponential
increase causes a drastic difference between
objects of of different sizes; a 5 cm leaf takes 5
minutes, but a 25 cm leaf actually takes 82 (Ta-
ble 2). If there is such a significant presence of
obstacle-removing ants patrolling these trunk
trails (Lugo et al. 1973) then why is there such
a time discrepancy between the removal and
small and large leaves?
Ants can carry up to 10x their own weight
(Brian 1977); this implies that their combined
efforts should be extremely effective in moving
large objects. However, the random movement
of individual ants involved in trail-clearing
renders this strength futile, as they are often
pulling against each other. Ants communicate
by sight and smell (Shutler 1991), and have no
effective method of communication with which
to coordinate their trail-clearing efforts. Their
solution to this disharmony is to break the ob-
stacle into smaller pieces, at which point their
random movements can effectively cart off the
objects.
Atta colombica ants were consistently cutting
leaf over 12 cm long (Fig. 4). In addition to
reducing the weight of the obstacle, this action
also pared down the leaf shape to make the
load less cumbersome. Ants have difficulty
balancing carried loads properly (Wirth et al.
2003) so this cutting may help sculpt the obsta-
cle into an easier-to-balance shape. I did not
record the presence of “spine-breaking” behav-
ior in my notes, but this phenomenon was a
major component of large object removal strat-
egy. “Spine-breaking” is the cutting of leaf
obstacles all the way through the leaf midrib,
cutting the leaf in two. In addition to lessening
the weight of the load and shaping the obstacle
to be better balanced, spine-breaking also re-
duces the distance between ants involved in the
object’s removal. This increased proximity may
allow the trail-clearers to visually identify each
other, and may even allow them to coordinate
the direction they pull. This spine-breaking
was only present in extremely large leaves (18-
24cm), but happened consistently through this
obstacle group.
It is certainly possible, though, that this
spine-breaking doesn’t allow for increased ant
communication. Instead, it may facilitate large
obstacle removal simply by reducing the num-
ber of ants needed to pull on each sub-object.
Fewer ants making random removal direction
choices would increase the likelihood that they
drag in the same direction.
My sample size (N= 104) in this study did
not show any significant difference in rates
between forests, or between the morning and
afternoon time periods (Table 1). However, a
more comprehensive analysis may reveal dif-
ferences. Pipeline Road, Parque Metropolitano,
and San Lorenzo forests had differing levels
of rainfall, elevation, and temperatures. A dis-
crepancy in rates could be attributed to this
rainfall gradient. Ants are highly susceptible
to desiccation (Wirth et al. 2003) and therefore
are attuned to water loss, which can be sub-
stantial even in humid rainforest conditions
(Brian 1977).
Foraging is affected by seasonal and diur-
nal variations in sunshine, temperature, hu-
midity, wind, and rainfall, in addition to mi-
croclimate variation due to weather and lo-
cal vegetation (Brian 1977). If studied more
closely, any of these factors may play a role
in controlling the rate of obstacle removal in
Atta colombica. Though the morning and af-
ternoon data sets showed no significant differ-
ence (Table 1), further study may reveal a dis-
crepancy. Forager slackness in the afternoon
has been seen in many species of ant (Brian
1977). Air temperature also affects ant move-
ment rates; Formica aqiulonia move 2.5 times
faster for every 10C rise (Brian 1969). Finally,
the moisture content and age of of leaves may
vary between forests and time periods. It takes
approximately 4x as long for foragers to cut
and harvest older leaves (Nichols-Orians and
Schultz 1989), which would also affect obstacle
6

removal.
Further study will be necessary to test the
significance of these variables on any differ-
ence in removal rate between forests and time
periods. It would also be insightful to collect
data on spine-breaking behavior during these
future studies, as it may actually shed light on
ant communication strategies and group effort.
Trail-clearing and maintenance is an extremely
effective productivity-enhancing behavior in
Atta colombica, and may be responsible for their
success in the Neotropics. Maximizing removal
time is essential for maximizing obstacle re-
moval effort, but also may even limit predation
and parasitism on these colonies (Wirth et al.
2003).
V. Acknowledgments
I would like to thank Yves Basset, Ioanna
Chiver, and Paula Gomez for their invaluable
guidance during this amazing course, in ad-
dition to my research partners Jen Zhou and
Rachel Updike. I would also like to thank the
ants.
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Obstacle Size and Trail–Clearing Activity in Leaf–Cutter Ants, Atta colombica

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