Regulation of foraging in harvester ants: minute-to-minute response
D. M. Gordon, S. Holmes and S. Nacu
Dept of Biological Sciences, Stanford University, Stanford CA 94305-5020
Dept of Statistics, Stanford University, Stanford CA 94305-4065
1
Abstract. Field experiments with the harvester ant, Pogonomyrmex barbatus,
showed how foraging activity depends on return rate. Foraging activity usually slows
down in response to a brief, 3-5 min reduction in the rate at which successful foragers
return to the nest. Foraging slows down and subsequently recovers very quickly. Colonies
are more responsive to reduced return rate on particular days. We tested whether the
returning forager or the food it carries is more important in stimulating foraging activity.
Added food was not sufficient to compensate for the decrease in the rate at which
foragers returned. Food carrying the odor of foragers, treated with extract of forager
cuticular hydrocarbons, was slightly but not significantly more effective than food alone
in stimulating foraging. These results show that an inactive forager's decision whether to
leave the nest on a foraging trip depends on its very recent encounters with returning
foragers, and suggest that both food and some behavioral cue from returning foragers are
required to stimulate foraging. Since foragers usually do not return until they find food,
each forager need make only a short-term assessment of its encounters with returning
foragers, and colony foraging activity is tightly linked to current food availability.
2
Introduction
How quickly and how much an animal responds to its environment is an essential
feature of its behavior. This is crucial to the animal's ecology, because its reactions
determine how it obtains resources in a changing environment. A tortoise and a bird
differ in their access to a berry at the end of a small branch waving in the wind, because
they differ in the speed of their reactions. Like any other behavior, foraging behavior
responds to current conditions. The foragers of a social insect colony collect material
from the external world and brings it back to the nest. The intensity of foraging depends
on the environment and on colony needs. The process that regulates foraging determines
how sensitive the system is, and how robust, in its response to changing conditions.
Ants are an enormously diverse group, both taxonomically and ecologically. Ant
species vary in the speed and magnitude of their responses to environmental conditions,
ranging in speed from quick recruitment to defense or food of some tropical plant-ants
(Janzen 1967) or some invasive species (Jaffe and Deneubourg 1992), to the timid, slow
response of some small-colony ponerine species (Jaisson, Fresneau et al. 1988). The
speed and magnitude of response create a behavioral profile, or overall "tempo" (Leonard
and Herbers 1986), which contributes greatly to determining a species' ecological role.
There has been little quantitative work on the reactivity of social insect foraging systems
and its ecological consequences. Since a social insect colony adjusts its foraging without
central control, colony response to environmental change reesults from the way that
individuals, using local information, decide whether to forage at any moment. Much is
known about how honeybees, but even in honeybees, by far the best-studied social insect,
3
few studies have considered how individual interactions lead to the short-term adjustment
of overall foraging intensity (Fernandez, Gil et al 2003).
In many social insect species, whether a forager is stimulated to leave the nest
depends on its interaction with other workers. How rapidly this works, and what
information is important, determines how foraging responds to the environment. For
example, in Polybia wasps, the probability that a wasp will leave the nest to forage is
increased when it experiences a higher rate of mandibular contact, or “biting,” from other
wasps near the nest entrance (O'Donnell 2001), so the biters regulate the intensity of
foraging. In stingless bees (Melipona), foragers are stimulated to leave the nest by the
return of other foragers (Biesmeijer, van Nieuwstadt et al 1998). Whether a honey bee
forager leaves the nest for its next foraging trip is influenced by a variety of interactions
with other foragers, including antennal contact (Fernandez, Gil et al 2003), assessment of
waggle dances (Von Frisch 1993), and food exchange.
The red harvester ant, Pogonomyrmex barbatus, which lives in the deserts of the
southwestern U. S. and Mexico, forages for randomly dispersed, long-lasting and
partially-buried seeds (Gordon 1993). Foragers, a distinct group of individuals (Gordon
1989), go back and forth between the day's foraging area and the nest, to find and retrieve
dispersed seeds and small insects (Gordon 1991). Mass recruitment using pheromone
trails is extremely rare. The intensity of foraging changes from day to day (Gordon 1986;
Gordon 1991; Gordon and Kulig 1996). For example, after a rain, foraging activity is
high, presumably because the rain exposes seeds buried in the upper layer of the soil.
Within a day, harvester ant foraging is regulated in response to the rate at which
foragers return with food. Foragers inside the nest are stimulated to leave on their next
4
foraging trip by the return of successful foragers bringing in food (Gordon 2002; Schafer
and Gordon 2006). Thus foraging activity is linked to food availability: when more food
is available, foragers return more quickly, stimulating more foragers to leave the nest.
It appears that harvester ant foragers respond to the delay between successive
encounters with other foragers. The faster successful foragers come in, the more rapidly
they will meet inactive ones waiting inside the nest, if mixing is random (Seeley and
Tovey 1994). Such assessment of the rate of encounter occurs in honeybees, whose
decision whether to leave on another foraging trip depends on the time interval they had
to wait to unload nectar (Anderson and Ratnieks 1999). Recent work shows that in
harvester ants, the initiation of foraging each day depends on the rate at which patrollers,
who scout the foraging area, return to the nest. Before the foragers leave the nest on their
first foraging trip of the day, they must experience successive encounters with returning
patrollers at intervals of about 10 sec (Greene and Gordon in review). Once foraging
begins, each forager apparently requires successive encounters within some time interval
before it leaves the nest on its next foraging trip.
Here we investigate the short-term, minute-to-minute regulation of foraging. The
timescale at which overall foraging activity is adjusted is related to the time scale at
which an inactive forager evaluates its encounters with returning ones and decides to
leave the nest. The longer it takes for inactive foragers inside the nest to evaluate the rate
at which successful foragers return, the longer it will take for the overall rate of foraging
activity to respond when the forager return rate changes.
5
We first ask how quickly overall foraging activity responds to short-term
declines, lasting only 3 min, in the rate at which successful foragers return, and how
quickly foraging recovers after the decline has ended.
Next we ask whether it is the return of a forager or its food that stimulates further
foraging activity. Foraging activity depends only on the return of successful foragers
bringing in food; foragers who return after an unsuccessful trip have no effect on foraging
activity (Schafer and Gordon 2006). To investigate whether the forager, or the food it
carries, stimulates inactive foragers to leave the nest, we diminished the flow of
successful foragers into the nest, but replaced the foragers with food. We then tested
whether food would replace foragers more effectively if the food was combined with the
odor of foragers. Previous work showed that interaction with glass beads treated with
hydrocarbon extract elicited a response similar to interaction with live workers (Greene
and Gordon 2003). Here we coated millet grains with the extract of cuticular
hydrocarbons of that colony's foragers, and used the treated millet to replace returning
foragers.
Methods
The study was performed in July –Aug 2004 and 2005 at a long-term study site
near Rodeo, New Mexico. At the site are about 300 individually-labeled colonies of P.
barbatus of known age (census methods in Gordon and Kulig 1996).
To measure response to short-term changes in the rate at which successful
foragers return to the nest, we removed returning foragers.
6
1. Foragers removed
Removals were made on 12 days in July and August 2004, with 7 mature colonies
(5 years or older), for a total of 21 trials, with each colony observed 1-4 times. Each trial
used 3 observers: one person recorded the numbers of foragers leaving the nest at the nest
entrance, one recorded the numbers of foragers returning to the nest from foraging trails,
and one removed returning successful foragers. Counts of numbers of foragers entering
and leaving the nest were made in every 10-sec interval during two periods separated by
a 3-min wait so observers could rest. The first observation period, before removals, was 5
min long; the second observation period, which began after the 3-min rest, lasted 12-14
min. During min 2-5 of the second period, as many as possible successful returning
foragers, seen to be carrying a seed, were removed from the most active foraging trail, at
least 1 m from the edge of the nest mound, and placed in a box. After the trial was over,
these foragers were returned to the nest. To estimate the time it would have taken
returning foragers to arrive at the nest entrance if they had not been removed, we used the
average of the times it took several returning foragers to reach the nest from the place
where foragers were collected. The measurements of return time were made when the
colony was undisturbed, a few minutes before removals began.
2. Foragers removed – colony and day effects
To investigate effects of colony and day, 3 mature colonies were tested
repeatedly, all 3 colonies each day, for 7 days in August 2005, for a total of 21 trials.
Methods were as in #1 above.
7
3. Foragers removed and food added
To test whether foraging activity responds to the rate at which foragers enter the
nest, or the rate at which food enters the nest, we added food while removing foragers.
Experiments were made with 15 colonies on 9 days in August 2005. Each colony was
tested only once, because harvester ant colonies store food for many months (Gordon
1993), and a colony that collects a great deal of food may subsequently decrease foraging
activity. As successful returning foragers were removed, grains of millet were dropped at
the nest entrance, a shallow, partially covered runway leading into the nest. We used
millet because the grains are within the size range of seeds the ants usually collect
(Gordon 1993), and because it seemed to be the preferred seed of several used in previous
experiments (Gordon 1991). One grain of millet was introduced to the nest for each
returning forager removed. The millet was brought into the nest by ants that came out of
the nest to retrieve it, within about 1 min of the time it was dropped atthe nest entrance.
If the millet replaced returning foragers, it would do so only after the millet had entered
the nest. Because of the delay caused by the time it took to retrieve the millet, removals
were continued for 5 minutes, not 3 min as in experiments 1 and 2 above. Returning
foragerscould be collectedmore quickly than millet could be dropped into the nest, so in
some trials it took longer than the 5-minute removal periods to add all of the millet.
Counts of numbers of foragers entering and leaving the nest were made in every 10-sec
interval for 4 min before removals began, and then continuously for 10-20 min after the
5-min period of removals ended, for a total of about 25 min.
8
4. Foragers removed and food with forager hydrocarbon extract added
To test whether food would replace foragers more effectively if the food was
combined with the odor of foragers, we coated millet grains with an extract of cuticular
hydrocarbons of that colony's foragers. In each of four colonies, about 100 foragers were
collected at least one week before the experiment and stored at -20 degrees Celsius. For
each colony, cuticular hydrocarbons were extracted from 20 foragers by soaking thawed
ants in 100% pentane for 10 min. Hydrocarbons were purified using a silica gel column
with 100% pentane as the effluent. The solution of twenty ant-equivalents of
hydrocarbon was added to a tube containing about 300 pieces of millet. The millet grains
were coated with hydrocarbon extract by allowing the pentane to evaporate with periodic
mixing of the tube. Methods were as in Experiment 3 above: foragers were removed for
5 min, while millet coated with forager hydrocarbon extract was dropped into the nest
entrance. The number of treated millet grains introduced was the same as the number of
foragers removed.
Data analysis
Adjusting for removed foragers
Foraging activity could appear to decrease in response to removals merely
because some of the foragers we removed would otherwise have gone out to forage again
during our observations. We adjusted for this by adding to the observed outflow of
foragers after removals the number of removed foragers we estimated would have left the
nest. The proportion of removed foragers estimated to be likely to leave the nest to
forage again was derived from the following data on marked individuals. In July-August
9
2004, we observed 4 colonies for 2-3 days. About 250 foragers had been uniquely
marked in each colony. The time that each marked forager entered and left the nest was
recorded. From data on 6259 foraging trips of 836 marked individuals in the 4 colonies,
we found the probability that a forager would leave the nest within 12 minutes of
returning to the nest (Fig. 1). We chose 12 min because this is the interval that
observations continued after removals. The probability ranged from 0.38 to 0.54, with a
mean of 0.47; using data only from 8-10 a.m., when removal experiments were
performed, the mean was 0.48. We used 0.47 as the probability that a forager we
removed would have gone out to forage again during our observations. Thus if 36
foragers were removed, the adjusted number added 17 foragers to the observed outflow
of foragers leaving the nest. To evaluate the effect of the adjustment factor of 0.47 on the
results, we repeated the analysis twice with arbitrarily chosen larger values of the
adjustment factor, 0.8 and then 1.0 of all removed ants replaced.
Evaluating the effect of removals
We then examined the magnitude of a colony's response to removals. To
characterize the magnitude of the response, we used the average rate of outflow of
foragers divided into two phases, before and after removals. The 'before' or undisturbed
phase began when observations began. The 'before' phase ended, and the 'after' phase
began, when removals began. The beginning of removals (T0) was the time the first
forager was removed plus the time needed for that ant to return to the nest (estimated as
the average of the times it took several ants to return from the site where ants were
collected). As a measure of the response to removals, we used the ratio of the mean
10
outflow after removals to the mean outflow before removals. The ratio A/B was
calculated as the ratio of the 10-sec counts of foragers leaving the nest averaged over
observations after T0 to the 10-sec counts of foragers leaving the nest averaged over
observations before T0 (ignoring the 3 min when observers rested). We used this ratio of
outflow after removals to outflow before removals, A/B, to ask:
1. Does foraging activity change when the rate of returning foragers decreases? We
considered outflow to decrease when A/B was less than one. We used a binomial test to
test whether outflow decreased in a significant proportion of trials, and also used a t-test
to evaluate the null hyupothesis that the mean ratio A/B was 1.
2. Does foraging activity decrease in response to a decrease in the rate of returning
foragers if the foragers are replaced by food? We used a t-test to compare the proportion
of trials in which outflow decreased in experiment 1 (removals and no food) and
experiment 3 (removals with food).
3. Does foraging activity decrease in response to a decrease in the rate of returning
foragers if the foragers are replaced by food that has the odour of foragers? We used a ttest to compare the proportion of trials in which outflow decreased in experiments 3
(removals with untreated food) and experiment 4 (removals with food treated with extract
of forager cuticular hydrocarbons). We then compared the results of experiments 1 and 2
combined (removals with no food), experiment 3 (removals with untreated food) and
experiment 4 (removals with treated food), using an ANOVA with A/B as the variate and
treatment as main effect.
4. What predicts the magnitude of the response? The magnitude of the effect of removals
varied considerably. To examine the causes of this variation, we first performed a
11
regression for the 42 trials in experiments 1 and 2 with A/B as the variate and the number
removed and the average inflow before removals (until T0), as main effects. To examine
the effect of day and colony, we performed an ANOVA for the data from experiment 2
with A/B as the variate and colony and day as main effects.
We then evaluated how long it takes a colony to respond to a decrease in the rate
at which foragers return, and estimated the duration of the response. In each trial, the
data were divided into two phases, before and after removals. The 'before' or undisturbed
phase began when observations began and ended as above when removals began at time
T0. For this analysis, the phase after removals was divided into three parts, a first part
after removals until foraging activity decreased, from time T0 to time T1, a second part
when foraging activity decreased, from T1 to T2, and a third part when foraging activity
had recovered, from T2 until the end of observations (Fig 2). In many trials the recovery
did not appear to be complete; though rates of foraging outflow increased after an initial
decrease in response to removals, the rates were never as high as they were before
removals. We used a maximum-likelihood model to choose, from all possible pairs T1
and T2 (with T1 greater than T0), the T1 and T2 that resulted in the intervals over which
outflow varies the least. In this way we estimated the mean rate at which foragers left the
nest during three intervals: 1) Undisturbed, from the beginning of observations until T1,
about 5-10 min (ignoring the observers' rest in experiments 1 and 2). 2) Response to
removals from T1 until T2. 3) Recovery from removals from T2 until the end of
observations. We estimated the time it took colonies to respond as the time elapsed
between the onset of removals and a change in foraging activity (T0 – T1), and the
duration of the response as the time until the colony recovered (T1 – T2).
12
Results
Magnitude of response
When the rate at which foragers return decreases, the rate at which foragers go out
usually decreases (Fig. 2). Of the 42 trials in which foragers were removed and no food
was added (experiments 1 and 2), outflow decreased in 31 trials, or 74% (binomial test
for no decrease, p <0.01, n = 42) (Fig. 3, Fig. 5). Both the mean and median ratio of
outflow after to before removals (A/B) was 0.84, with a range from .24 to 1.93. The mean
ratio A/B was significantly less than 1 (t-test, t = -2.59, n = 42, one-sided p = 0.01). This
trend persisted even with unrealistically large adjustment factors for the proportion of
removed foragers replaced (0.8, decrease in 29 of 42 trials, and 1, decrease in 26 of 42
trials). However, in the other 11 trials, outflow increased, usually only slightly.
The magnitude of response to forager removal did not depend on the number of
foragers removed or on the rate at which foragers were returning before removals began
(multiple regression for 42 trials in experiments 1 and 2, Number removed, coefficient 0.05, < 0.5, Inflow when undisturbed coefficient –0.0, < 0.9). The number of foragers
removed ranged from 18 to 76; the average rate of returning foragers per 10 sec before
removals ranged from 0.42 to 5.52.
There were significant differences among days in response to forager removal,
and no significant differences among colonies (ANOVA for 21 trials in experiment 2, day
SS 2.3, df 6, F 3.2, p < 0.04; colony SS 0.4, df 2, F 1.6, p < 0.2).
13
Foragers vs food
Food did not effectively compensate for a decrease in the rate at which foragers
return. When foragers were removed and a corresponding number of millet grains were
dropped into the nest, foraging decreased in 11 of 15 or 73% of trials (Fig. 4). The mean
ratio of forager outflow after to before removals (A/B) was 0.89; the median was 0.80,
with a range from 0.1 to 1.99. These results are similar to those in which no food was
offered, indicating that the arrival of abundant food had little effect on the response to a
decrease in forager return rate.
Food treated with forager hydrocarbon extract appeared to compensate more
effectively for foragers than food alone (Fig 5). In 4 trials, the ratio of forager outflow
after to before removals (A/B) showed a decrease in foraging activity in 2 of 4 trials
(50%), and in the other 2, it increased, giving a mean A/B of 1.16 and median of 1.09.
The difference in number of trials when foraging decreased between the two treatments,
millet and millet treated with hydrocarbons, was not significant (1-sided t-test, t = 0.67, p
= 0.3), perhaps due to the small number of trials with treated millet.
There was no significant difference in the ratio of forager outflow after to before
removals among the three treatments: removals with no food (experiments 1 and 2),
removals with food (experiment 3) and removals with treated food (experiment 4)
(ANOVA, SS 0.386, F 0.829, df 2, p = 0.4).
Timing of response
When a colony responds to a decrease in the rate of returning foragers, by
decreasing the rate at which more foragers leave the nest, this happens very quickly (Fig.
14
2). In the 31 of 42 trials in experiments 1 and 2, in which foraging decreased in response
to removals, the decrease occurred with an average of, 2-3 minutes (mean interval
between removals at T0 and onset of decreased foraging rate at T1, 188 ± 172 sec,
median 110 sec).
The reduction in foraging activity in response to removals lasted about as long as
the removals did. In the 31 trials in which foraging decreased in response to removals
lasting 3 min, the response lasted about 5 minutes (mean interval between onset of
decreased foraging at T1 and recovery at T2, 320 ± 225 sec, median 320 sec).
The timing of response to forager removals was similar when food was offered to
replace the removed foragers. In these experiments, forager removals lasted 5 min, but it
sometimes took an additional 2-3 min to add all of the food. In the 11 trials in which
forager outflow decreased, the decrease occurred within an average of 3-4 min (mean
interval between removals at T0 and onset of decreased foraging rate at T1, 228 ± 139
sec, median 270 sec). Again, the response lasted about as long as removals did, about 8
min (mean interval between onset of decreased foraging at T2 and recovery at T3, 534 ±
207 sec, median 440 sec).
Discussion
Foraging activity changes in response to very short-term declines in the return of
successful foragers, on the scale of 3 minutes. This means there is a constant, momentto-moment adjustment of foraging activity. These results indicate that whether a forager
leaves the nest depends on its very recent interactions with incoming, successful foragers.
15
Food does not compensate for a decline in the rate at which foragers return to the
nest (Fig 5). This supports the conclusion that behavioral interactions between ants, as
well as the food itself, stimulate foraging activity. Whether a returning forager carries
food is clearly important. When returning foragers are deprived of their food and allowed
to return, foraging activity decreases (Gordon 1991). When returning foragers without
food are removed, there is no effect on foraging activity (Schafer et al. 2006). But our
results here show that food without a forager is not enough to stimulate foraging. Food
treated with extract of forager cuticular hydrocarbons was slightly, but not significantly
more effective in stimulating foraging activity than food alone. Hydrocarbon profiles
differ among task groups in harvester ants, and ants distinguish the task of the ants they
meet using these task-specific differences in hydrocarbon profile (Wagner et al 1998;
Greene and Gordon 2003) It appears that a grain of millet treated with hydrocarbon
extract is perceived more as a forager than is millet alone. Further work is needed to
determine what information is conveyed by foragers when they return to the nest with
food.
Foraging activity is tuned with remarkable speed to the rate at which foragers
return (Fig 2). Within about 2-3 minutes, colonies responded to a decrease in the rate at
which successful foragers return. When foraging ceased, it stopped for about 5 minutes,
about as long as the diminished rate of foraging had lasted. Since the decline in overall
foraging rate, which reflects the aggregate response of many individual foragers,
occurred only 2-3 minutes after removals began, the interval over which an individual
inactive forager evaluates the rate of returning foragers must be 3 minutes or less.
Similarly, since foraging activity recovers only a few minutes after the decline in return
16
rate has ended, foragers must evaluate the end of the decline quite rapidly. These results
indicate that inactive foragers respond to their very recent experience of encounters.
It is surprising that the ants can assess a change in return rate so quickly, because
the interval between the return of successive foragers is quite variable on the scale of
minutes. Suppose the probability that a forager leaves the nest depended on its
calculation of a moving average rate of forager return, with an average calculated on the
scale of minutes. When the rate of forager return changes, often the variation in intervals
between encounters would exceed the shift in the mean rate, so the ant would fail to
detect the change. However, in our experiments, colonies usually detected very rapidly a
change in the rate of forager return. It appears that inactive foragers respond somehow to
their immediate experience of encounters, or if they calculate an average rate it is over a
very short interval. One possibility is that a threshold number of encounters within a very
short time is required to stimulate foraging. Other work showed that at the onset of
foraging, encounters between patrollers and foragers must occur at intervals of about 10
sec to stimulate forager activity (Greene and Gordon in review). A short interval over
which encounters are evaluated, or a very rapid decay in the ant's memory of encounters,
would produce the rapid response to a change in the rate of forager return.
Colonies usually respond to a decline in the rate at which foragers return by
slowing down the rate at which foragers leave the nest. This occurred in about 75% of
trials. However it is puzzling that in 25% of trials, foraging activity did not slow down
when the rate of return is diminished. Whether a colony slowed foraging activity did not
depend on the number of ants removed, or on the intensity of foraging before removals
began. The only variable to predict whether colonies responded to removals was the day.
17
Previous work has repeatedly shown day-to-day variation in foraging activity (Gordon
1984; Gordon 1986; Gordon 1991; Gordon and Kulig 1996). Variation in foraging
activity is presumably linked to variation in food supply (Adler and Gordon 2003): most
foragers continue to search until they find food, and successful foragers stimulate more
foraging activity. Thus on days when foraging activity is high, food availability is
presumably high as well. It may be that ants respond to food quality as well as quantity.
There is week-to-week variation in which seeds are collected (Gordon 1993), and so on
the days when different trials were conducted, it is likely that different seeds were
available. Perhaps on days when highly preferred or unusual seeds arecoming into the
nest, foragers are more likely to go out even if they receive less stimulation from
returning foragers, and on such days, removals have little effect.
Our results here add to the growing number of examples of coordination in social
insect colonies based on encounter rate (Gordon 1993). The role of encounter rate in
forager activation, including encounters between returning and inactive foragers in honey
bees (Fernandez, Gil et al 2003), or between biters and foragers in paper wasps
(O'Donnell 2001), is reviewed above. The ant Temnothorax albipennis uses encounter
rate to assess nestmate density and thus the suitability of new nest sites (Pratt 2005). A
harvester ant worker is more likely to change tasks to do midden work when its rate of
encounter with midden workers is high (Gordon 1999). Intervals between loads of water
influence nest construction behavior in wasps (Jeanne and Nordheim 1996). Rates of
interaction may be fundamental in many aspects of task partitioning and task allocation in
social insects (Gordon 1999; Anderson et al 2002).
18
An intriguing evolutionary question is whether there is any advantage, for ants
foraging for seeds, in a colony's ability to respond within minutes to fluctuations in the
rate at which successful foragers return. Predation on harvester ants is rare, so the main
cost of foraging is related to desiccation: since harvester ants obtain most of their water
from metabolizing the fats in seeds, foraging ants are using water in order to obtain more
(MacKay 1985). Seeds are stored inside the nest for months and possibly years (Gordon
1993). When little food is available, the cost of water loss by foragers may be too high to
offset the benefit of the food that foragers collect. By adjusting foraging effort to food
availability, harvester ants may minimize water loss. However, the rapidity of response
is remarkable in ants that forage for very long-lasting resources, seeds that stay on the
ground, or buried near the surface of the soil, until removed by ants, rodents or the wind.
Since harvester ants use resources that can be retrieved by a single ant and do not require
opportunistic recruitment, perhaps their rapid response to changes in foraging rate is
merely a consequence of a system that limits the interval over which an ant must track
encounters. Each individual ant need make only a short-term assessment of its
encounters with others, and colony foraging activity is linked to food availability.
Acknowledgements
Many thanks to our field assistants, Blair Beverly, Nathan Clement, LeAnn
Howard, and Laura McDonald for their hard work and good humor. Helen McLendon,
Miko Tsukimoto, and Jonson Yee did the work on marked ants used to estimate forager
departure times. We are very grateful to Michael Greene for the extractions of forager
hydrocarbons and preparation of the treated millet. The invaluable contributions of our
19
undergraduate assistants were made possible by the Stanford Office of the Vice Provost
for Undergraduate Education. SN was supported by a grant from the Stanford Bio-X
Initiative.
20
Figure legends
Fig 1 Probability that a forager leaves the nest for another trip within the indicated
interval. Data from 6259 trips by 836 uniquely marked foragers.
Fig 2. Mean outflow in response to removals. Each point shows the average outflow for
42 trials in which foragers were removed beginning at 420 sec. Removals ended at 600
sec. Error bars show the standard error of the mean? Foraging activity fluctuates before
removals, when colonies were undisturbed, and then decreases after removals.
Fig 3. A. Cumulative number of foragers leaving the nest (above) and outflow (below) in
one trial. Removals began at 420 sec and ended at 600 sec.
T0, time the first forager was removed plus the average time for foragers to return to the
nest from the site of collection; T1, onset of decreased foraging, estimated by maximumlikelihood model; T2, onset of recovery from decreased foraging. In the upper graph, the
solid line shows cumulative number adjusted for the removed foragers; dashed line shows
the cumulative number observed. In the lower graph, the solid line shows adjusted
outflow per 10 sec, and the dashed line shows the observed outflow. The solid horizontal
lines show the means for 3 intervals: 1) from beginning of observations until response to
removals begins at T1, 2) response to removals T1-T2, 3) foraging recovered T2 until end
of observations.
B. Same as in A, for a trial in which there was no decrease in foraging in response to
removals. In the lower graph, there are no means shown because no response to removals
was detected.
21
Fig 4. Same as Fig 3, for one trial in which food was supplied to replace removed
foragers. Foraging decreased despite the food.
Fig 5 Comparison among treatments. Each box plot shows the ratio of average outflow
after to average outflow before removals (A/B) in three treatments: NF no food,
experiments 1 and 2; M millet, experiment 3; MHC millet treated with hydrocarbons,
experiment 4.
22
Fig 1
23
Fig 2
24
Fig 3A
25
Fig 3B
26
Fig 4
27
Fig 5
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