Load size selection by foraging leaf-cutter ants (Atta cephalotes)

ABSTRACT.

• 1 Velocity of load-carrying Atta cephalotes (L.) foragers increases with increasing ant size and decreasing load size.
• 2 Foragers are selective in the sizes of loads they carry, but heavier loads would apparently increase their rate of leaf transport to the nest (mg of leaf m s^?1).
• 3 Even for very thin leaves, leaf diameter is not correlated with ant body size despite the method of cutting (rotating around a fixed point on the leaf edge).
• 4 When cutting leaves of different densities, load mass is more closely matched to ant size than is load surface area. This implies that ants choose loads based on mass rather than surface area, and thus the several possible disadvantages associated with carrying loads of large surface area (e.g. increased disturbance by wind or rain) are unlikely explanations of why ants do not select larger loads.
• 5 The relationship beween forager size and load size is made more complex by further selectivity at the level of colony recruitment: larger ants recruit to higher-density (thicker) leaf types.
• 6 Gross leaf transport rate is not maximized by foraging A.cephalotes, but net rate of energy intake cannot be assumed to follow the same pattern. If costs/time (not measured) are constant with changing load size, then the net rate of energy intake is not maximized. An alternative hypothesis is that costs/time increase with larger loads, thereby decreasing net rate of gain for larger loads.

     

Diel changes in forager size, activity, and load selectivity in a tropical leaf-cutting ant, Atta cephalotes

Abstract. 1. The leaf-cutting ant Atta cephalotes (L.) in a Costa Rican tropical moist forest showed diel changes in foraging activity. In most colonies studied, foraging was primarily nocturnal, although in a few colonies it was primarily diurnal.
2. In all colonies studied, mean forager mass was larger at night than during the day.
3. At night, most foragers carried freshly cut leaf fragments, whereas during the day a large proportion carried dried fragments and other vegetable matter collected from along their trail.
4. Along one trail, where foraging was primarily nocturnal, the match between ant mass and load mass was compared for laden ants at night and during the day. Laden ants at night were larger, carried relatively heavier loads, and showed a higher degree of matching between their mass and load mass than those foraging during the day.
5. A comparison of load masses of ants coming down a local tree and of ants picking up marked fragments from along their trail suggested that the diel difference in load mass and in the match between ant mass and load mass were related to the greater proportion of ants carrying freshly cut leaf fragments at night. Fresh fragments weighed more due to higher water content, and the match between ant mass and load mass was greater for ants cutting fresh fragments than for ants picking up abandoned fragments from along their trail.
6. Possible explanations for the diel changes in forager size and activity are discussed.     

Forager size and ecology of Acromyrmex coronatus and other leaf-cutting ants in Costa Rica

I compare forager size and foraging ecology of the leaf-cutting ant Acromyrmex coronatus (Fabricius) with published data on three other leaf-cutter species in Costa Rica, Atta cephalotes (L.), Acromyrmex octospinosus (Reich), and Acromyrmex volcanus Wheeler. Intra-and interspecific differences in forager size in these leaf-cutting ants appear to reflect the economics of harvesting different preferred resources. Ac. coronatus colonies have relatively small foragers (mean mass=3.4±1.4 mg) that cut almost exclusively the thin, soft leaves and other parts of small herbaceous plants. Similarly, small A. cephalotes colonies have small foragers (3.3±1.0 mg) that attack the leaves of small herbaceous plants. In contrast, mature A. cephalotes colonies have a wider sizerange of foragers (7.3±4.1 mg) that primarily attack the leaves of trees, with larger foragers cutting thicker, tougher leaves. In A. cephalotes, the match of forager size to leaf type (both ontogenetically and behaviorally) increases foraging efficiency. Extreme forager polymorphism in mature A. cephalotes colonies appears to broaden the diversity of tree species that they can exploit efficiently. Ac. octospinosus and Ac. volcanus both have large, relatively monomorphic foragers (13.3±4.2 mg and 30.6±4.3 mg, respectively) that typically scavenge for pieces of fallen vegetation, such as dead leaves, fruit, and flowers, in addition to cutting herbs. The large foragers of Ac. octospinosus and Ac. volcanus appear to be well suited as generalist foragers, able to cut or collect any desirable vegetation encountered. Ac. coronatus is similar to A. cephalotes in other ways. Both Ac. coronatus and A. cephalotes establish and maintain cleared trunk trails for foraging, and both have minima workers that hitchhike on the loads carried by foragers, apparently serving to protect the larger foragers from attack by phorid flies. Trunk trails and hitchhikers are not known for Ac. octospinosus and Ac. volcanus. That A. coronatus and A. cephalotes show little overlap in geographic distribution within Costa Rica may relate both to differences in habitat requirements and to interspecific competition.     

Temporal variation in foraging activity and efficiency and the role of hitchhiking behaviour in the leaf-cutting ant, Atta cephalotes

Two phenomena are integral to the foraging behaviour of leaf‐cutting ants in the genus Atta: hitchhiking (where small ants ride on leaf fragments carried by larger workers) and rhythmic foraging (where foraging activity shows marked fluctuations over time). While parasitism by phorid flies has been implicated in eliciting both behaviours, recent research suggests fungal contaminants and the need to procure sap also play a key role in eliciting hitchhiking. For wild colonies of Atta cephalotes L. (Hymenoptera: Formicidae), we investigated the extent to which hitchhiking frequency varied in space and time and the foraging performance of day‐time and night‐time workers. Day‐time foragers were considerably smaller than nocturnal foragers, a trend previously described as a response to diurnal phorids. Despite their smaller size, day‐time foragers had higher foraging performance, perhaps as a consequence of decreased trail congestion. Larger leaf‐carriers were more likely to carry hitchhikers and hitchhiking frequency was higher at night, an observation that conflicts with the parasitoid defence hypothesis, but not with the leaf sap and fungal defence hypotheses. Hitchhikers constitute a major proportion (typically 12%) of the loads carried by workers, and have three times the effect of leaf fragment mass on forager velocity. However, they reduced energetic efficiency by only 2.6% and provisioning rate by 5.9%. Our results provide partial support for the parasitoid defence hypothesis, but suggest that both the risks of parasitism and the opportunity‐cost to foraging associated with carrying hitchhikers may be low.     

Load-size determination in the leaf-cutting ant, Atta cephalotes

I examined load-size determination by a highly polymorphic leaf-cuttingant, Atta cephalotes, cutting leaves of artificial trees (branchesplaced in the top of plastic tubes). I compared load size forants cutting thin leaves (starfruit, Averrhoa carambola) andthick leaves (grapefruit, Citrus parodist). At each source,larger ants cut larger fragments. Distance from the nest hadno effect on load size. The mass of fragments cut by an antof a given size was significantly greater when cutting grapefruitleaves. The leaf area cut, however, showed no significant differencebetween the two leaf types. Leaf area increased approximatelyin proportion to ant body mass to the 0.6 power. As a resultof their method of load-size determination, ants of a givensize cut heavier loads when cutting the thicker leaves. Thisdifference, however, was counteracted at the colony level byrecruitment of larger ants, which cut smaller area fragmentsrelative to their body mass, to cut at thicker leaf sources.     

Mechanisms affecting load size determination inAtta cephalotes L. (Hymenoptera,Formicidae)

Summary The size of leaf-cutting ant foragers correlates more closely with the weight than the area of the leaf fragments they cut. This implies that the mechanism of load size determination is not a simple function of body geometry. Ants were found not to adjust the radius of cut to compensate for experimental changes in leaf weight during the process of cutting. However, ants changed their cutting behaviour according to the thickness of the region of the fragment being cut. Ants decreased their cut radius when cutting through leaf veins. Similarly, when cutting artificial laminae of varying thicknesses, ants reduced the cut arc radius when traversing thicker regions.Cut radius was not principally controlled by the position of the rear legs nor the overall body length but by mechanisms associated with the head and thorax of the ant.     

Field study on the foraging characteristics of a ponerine ant,Hagensia havilandi Forel

Summary A field study of the foraging strategy used by the ponerine ant,Hagensia havilandi is reported. They have permanent nests in the leaf litter of coastal forests.H. havilandi is a diurnal forager and collects a variety of live and dead arthropods. These predatory ants exhibit individual foraging with no cooperation in the search for or retrieval of food items. Three colonies were observed and showed similar temporal and spatial foraging patterns. The paths of individual ants were followed and the results showed that the foragers exhibit area fidelity, and return to the nest via a direct route on finding on prey item. Several foragers did not return to the nest at dusk but returned the following morning. Occasionally a limited amount of tandem recruitment was displayed.     

The ‘truck‐driver’ effect in leaf‐cutting ants: how individual load influences the walking speed of nest‐mates

The foraging behaviour of social insects is highly flexible because it depends on the interplay between individual and collective decisions. In ants that use foraging trails, high ant flow may entail traffic problems if different workers vary widely in their walking speed. Slow ants carrying extra‐large loads in the leaf‐cutting ant Atta cephalotes L. (Hymenoptera: Formicidae) are characterized as ‘highly‐laden’ ants, and their effect on delaying other laden ants is analyzed. Highly‐laden ants carry loads that are 100% larger and show a 50% greater load‐carrying capacity (i.e. load size/body size) than ‘ordinary‐laden’ ants. Field manipulations reveal that these slow ants carrying extra‐large loads can reduce the walking speed of the laden ants behind them by up to 50%. Moreover, the percentage of highly‐laden ants decreases at high ant flow. Because the delaying effect of highly‐laden ants on nest‐mates is enhanced at high traffic levels, these results suggest that load size might be adjusted to reduce the negative effect on the rate of foraging input to the colony. Several causes have been proposed to explain why leaf‐cutting ants cut and carry leaf fragments of sizes below their individual capacities. The avoidance of delay in laden nest‐mates is suggested as another novel factor related to traffic flow that also might affect load size selection The results of the presennt study illustrate how leaf‐cutting ants are able to reduce their individual carrying performance to maximize the overall colony performance.     

Leaf density and a trade-off between load-size selection and recruitment behavior in the ant Atta cephalotes

This study considers the interplay between individual load-size selection and recruitment behavior in the leaf-cutting ant Atta cephalotes. Foraging workers anchor themselves on the leaf edge by their hind legs and pivot around them while cutting arcs from leaves. Since workers not only cut leaves but also lay chemical trails to recruit nestmates, we investigated whether there is conflict of motivation affecting the workers' decision either to quickly inform nestmates about a newly discovered food source, or instead to cut full-load leaf fragments, which could delay recruitment. Workers were presented with leaves of privet of three different grades of toughness (measured as leaf density=mass/area) as sources of different quality, and load-size selection and recruitment behavior by harvesting-satiated and harvesting-deprived workers were measured. The following results were obtained. (1) Leaf density affected individual load-size selection: both harvesting-satiated and harvesting-deprived workers were found to cut smaller leaf fragments from the denser leaves. (2) Harvesting-deprived workers cut smaller fragments than harvesting-satiated workers, and therefore saved cutting time. The fragments cut were smaller only during the initial phases of the recruitment process, when information about the discovery needed to be transferred. (3) Harvesting-deprived workers showed higher recruitment rates than harvesting-satiated workers. A considerable number of ants were observed to return to the nest unladen. During the initial phases, the ratio of laden/unladen workers was lower than that for harvesting-satiated workers, and increased with the development of the tograging process. (4) Scout workers confronted with familiar leaves ran back to the nest laying chemical trails without even contacting the leaves. They relied on olfactory cues to start recruiting nestmates, and leaf density played no role in their decisions. (5) When confronted with unfamiliar leaves, on the other hand, they assessed leaf quality by probing bites at the leaf edge, although no actual cuts occurred. In this situation, the resulting recruitment rates depended on physical leaf traits, being higher for the tenderer leaves. (6) Workers foraging on unfamiliar leaves cut smaller fragments than workers cutting familiar leaves, and most of them displayed trail-laying behavior when returning to the nest. The results support the hypothesis of a trade-off between time spent collecting and that invested to recruit nestmates. During the initial phases of exploitation of a newly discovered food source, workers reduced their individual carrying performance in order to return earlier to the colony for further recruitment.     

Head-on encounter rates and walking speed of foragers in leaf-cutting ant traffic

Summary. Trail traffic of the leaf-cutting ant Atta cephalotes involves intermingled flows of outbound and returning foragers. Head-on encounters between workers from the opposite flows are a common occurrence in this traffic. Each encounter momentarily delays the two ants involved, and these small delays might pose a significant cost to the colony's foraging performance when summed over thousands of workers along many metres of trail. We videotaped outbound and returning foragers over a 1 m course, and measured the encounter rates they experienced and their velocity. Our analysis indicates that locomotion speed is diminished by increasing encounter rate, but that the effect is small relative to the effects of ant body size and load mass. Head-on encounters allow exchange of information and leaf fragments between workers, and we consider how the benefits of such encounters may make this form of traffic organization superior to segregated outbound and returning lanes, despite the measurable c ost of encounters in mixed traffic.     

Leaf cutter ants (Atta cephalotes) harvest baits offering sodium chloride rewards

Recent studies of attraction to sodium chloride baits suggest that diverse ant species forage for salt. We used experimental presentations of salt baits to test whether leaf cutter ants (Atta cephalotes) are attracted to and harvest salt-treated paper baits that offer no other resources. Atta foragers were most attracted to sucrose baits (positive control), but more foragers touched and cut salt-treated baits than water-treated baits (negative control). Furthermore, the ants removed more paper from the salt baits than from water-treated baits. We conclude that leaf cutter ants expend time and energy to harvest salt in the absence of other rewards. Salt could be harvested for the workers’ consumption, or it could be fed to the fungus gardens in the ants’ nest.     

Crowding increases foraging efficiency in the leaf-cutting ant <Emphasis Type="Italic">Atta colombica</Emphasis>

Many animals, including humans, organize their foraging activity along well-defined trails. Because trails are cleared of obstacles, they minimize energy expenditure and allow fast travel. In social insects such as ants, trails might also promote social contacts and allow the exchange of information between workers about the characteristics of the food. When the trail traffic is heavy, however, traffic congestion occurs and the benefits of increased social contacts for the colony can be offset by a decrease of the locomotory rate of individuals. Using a small laboratory colony of the leaf-cutting ant Atta colombica cutting a mix of leaves and Parafilm, we compared how foraging changed when the width of the bridge between the nest and their foraging area changed. We found that the rate of ants crossing a 5 cm wide bridge was more than twice as great as the rate crossing a 0.5 cm bridge, but the rate of foragers returning with loads was less than half as great. Thus, with the wide bridge, the ants had about six times lower efficiency (loads returned per forager crossing the bridge). We conclude that crowding actually increased foraging efficiency, possibly because of increased communication between laden foragers returning to the nest and out-going ants. Received 15 December 2006; revised 16 February 2007; accepted 19 February 2007.     

Interactions Increase Forager Availability and Activity in Harvester Ants

Social insect colonies use interactions among workers to regulate collective behavior. Harvester ant foragers interact in a chamber just inside the nest entrance, here called the ''entrance chamber''. Previous studies of the activation of foragers in red harvester ants show that an outgoing forager inside the nest experiences an increase in brief antennal contacts before it leaves the nest to forage. Here we compare the interaction rate experienced by foragers that left the nest and ants that did not. We found that ants in the entrance chamber that leave the nest to forage experienced more interactions than ants that descend to the deeper nest without foraging. Additionally, we found that the availability of foragers in the entrance chamber is associated with the rate of forager return. An increase in the rate of forager return leads to an increase in the rate at which ants descend to the deeper nest, which then stimulates more ants to ascend into the entrance chamber. Thus a higher rate of forager return leads to more available foragers in the entrance chamber. The highest density of interactions occurs near the nest entrance and the entrances of the tunnels from the entrance chamber to the deeper nest. Local interactions with returning foragers regulate both the activation of waiting foragers and the number of foragers available to be activated.     

Is foraging in the desert ant,Messor pergandei (Hymenoptera: Formiceidae), limited by water?

Abstract.

• 1 The seed-harvesting ant Messor (Veromessor) prrgandei (Mayr) is a common inhabitant of southwestern deserts of the U.S.A. Foragers vary in size from less than 1 mg to more than 10 mg in body mass and may travel over 80 m on a single foraging trip. Their small size, long foraging range, and hot, arid habitat suggest that water stress may limit foraging activity. We examined intercolony and interindividual variation in water loss of M.pergandei foragers under several different situations in the field.
• 2 Colonies differed significantly in minimum critical water content (W_c) of individual foragers (water content below which foragers are incapable of normal locomotion). In one colony small workers had disproportionately higher W_c than larger workers; in the other colony W_c was isometric with body size.
• 3 Groups of workers confined in the field approached W_c only after normal foraging stopped and substrate temperatures exceeded 45°C, while water content of individual foragers did not approach the W_c during normal foraging periods. Moreover, seed load and distance travelled did not negatively affect forager water content, as measured on return to the nest: indeed, our results suggest that forager hydration level may influence load selection and/or foraging distance. We conclude that, under normal circumstances, foraging in M.pergandei is not water-limited.

     

The effect of load length,width and mass on transport rate in the grass-cutting ant <Emphasis Type="Italic">Atta vollenweideri</Emphasis>

In the present study we investigated the economics of load transport in the grass-cutting ant Atta vollenweideri by focusing on the effects of load mass, width and length on individual transport rates. Both running speed of foragers and the amount of material transported a given distance per unit time, i.e. gross material transport rate, were evaluated in both field and laboratory colonies. In order to separate the effects of load mass, load length and width on transport rate, workers were presented with paper fragments which differed twofold either in length, width or mass, but not in the other parameters. When controlling for fragment mass, both running speed of foragers and gross material transport rate were observed to be higher when they carried short fragments: A twofold increase in fragment length had a marked negative effect on manoeuvrability during transport and, as a consequence, on material transport rate. In contrast, if fragment mass was doubled and length maintained, running speed differed according to the mass of the loads, with heavier fragments being transported at the slower pace. For the sizes tested, heavy fragments yielded a higher transport rate in spite of the slower speed of transport, as they did not slow down foragers so much that it counterbalanced the positive effects of fragment mass on material transport rate. Doubling the width of the fragments without changing their mass had no influence on running speed and transport rate. When presented with a choice of dropped fragments differing in the size variables mentioned above, workers discriminated among fragments of different size and preferred shorter fragments, thus rejecting loads that are associated with higher travel times and lower material transport rates. It is argued that, based on the energetics of cutting, workers might maximize their individual harvesting rate by cutting long grass fragments, since the longer a grass fragment, the larger the amount of material harvested per unit cutting effort. Our results indicate, however, that larger loads negatively affect transport rates. The sizes of the fragments cut by grass-cutting ants under natural conditions may represent the outcome of an evolutionary trade-off between maximizing harvesting rate at the cutting site and minimizing the effects of fragment size on material transport rates.     

Polydomy and the organization of foraging in a colony of the Malaysian giant ant Camponotus gigas (Hym. / Form.)

In Kinabalu National Park, Borneo we observed four colonies of the Malaysian giant ant Camponotus gigas in a primary forest. These predominantly nocturnal ants have underground nests, but forage in huge three-dimensional territories in the rain forest canopies. The colony on which our study was mainly focused had 17 nests with about 7000 foragers and occupied a territory of 0.8 ha. To improve observation and manipulation possibilities, these nests were linked at ground level by 430 m of artificial bamboo trail. A group of specialist transport worker ants carried food from `source' nests at the periphery to the central `sink' nest of the queen. Transport of food between nests started immediately after the evening exodus of the foragers. Transporter ants formed a physical subcaste among the minors and behaved according to predictions of the central-place foraging theory. Their load size was about five times that of the average forager and grew proportionally with head width. Longer distances were run by ants with greater head width and larger gross weight. Transporter ants that ran more often took heavier loads. Experiments with extra-large baits revealed that C. gigas used long-range recruitment to bring foragers from different nests to “bonanzas” at far distant places. The foraging strategy of C. gigas is based on a polydomous colony structure in combination with efficient communication, ergonomic optimization, polyethism and an effective recruitment system. Received: 16 March 1998 / Accepted: 24 August 1998     

The short-term regulation of foraging in harvester ants

In the seed-eating ant Pogonomyrmex barbatus, the return ofsuccessful foragers stimulates inactive foragers to leave thenest. The rate at which successful foragers return to the nestdepends on food availability; the more food available, the morequickly foragers will find it and bring it back. Field experimentsexamined how quickly a colony can adjust to a decline in therate of forager return, and thus to a decline in food availability,by slowing down foraging activity. In response to a brief, 3-to 5-min reduction in the forager return rate, foraging activityusually decreased within 2–3 min and then recovered within5 min. This indicates that whether an inactive forager leavesthe nest on its next trip depends on its very recent experienceof the rate of forager return. On some days, colonies respondedmore to a change in forager return rate. The rapid colony responseto fluctuations in forager return rate, enabling colonies toact as risk-averse foragers, may arise from the limited intervalover which an ant can track its encounters with returning foragers.     

Dynamic matching of forager size to resources in the continuously polymorphic leaf-cutter ant, Atta colombica (Hymenoptera, Formicidae)

Abstract.  1. Ergonomic optimisation theory proposes that by increasing variation in worker morphology, social insect colonies may increase their dietary breadth; however, little is known about how this relationship operates at the colony level. This study examines the colony-level pattern of forager size allocation to resource sites in a natural setting.
2. Using a biologically relevant measure of toughness, it is shown that leaf-cutter ant colonies exploit a variety of plant resources that vary significantly in toughness at any given time.
3. Forager size is shown to be matched to the toughness of plant material, with larger ants harvesting tougher material.
4. Furthermore, outbound foragers travelling to a harvest site are matched in size to the toughness of plant material contained within the site and are not a random selection of available foragers. The match between forager size and plant toughness may reduce the number of wasted trips and ill-matched foragers.
5. The observed colony-level pattern of forager allocation could be the result of learning by individual foragers, or the result of information shared at the colony level.     

Cutters,carriers and transport chains: Distance-dependent foraging strategies in the grass-cutting ant <Emphasis Type="Italic">Atta vollenweideri</Emphasis>

Summary Most studies on leaf-cutting ant foraging examined forest species that harvest dicot leaves. We investigated division of labor and task partitioning during foraging in the grass-cutting ant Atta vollenweideri. Workers of this species harvest grass fragments and transport them to the nest for distances up to 150 m along well-established trunk trails. We recorded the behavior of foraging ants while cutting and monitored the transport of individually-marked fragments from the cutting site until they reached the nest. A. vollenweideri foragers showed division of labor between cutting and carrying, with larger workers cutting the fragments, and smaller ones transporting them. This division was less marked when plants were located very close to the nest and no physical trail was present, i.e., the cutter often transported its own fragment back to the nest. On long foraging trails, the transport of fragments was a partitioned task, i.e., workers formed transport chains composed of 2 to 5 carriers. This sequential load transport occurred more often on long than on short trails. The first carriers in a transport chain covered only short distances before dropping their fragments, and they were observed to turn back and revisit the patch. The last carriers covered the longest distance. The probability of dropping the carried fragment on the trail was independent of both worker and fragment size, and there was no particular location on the trail for dropping, i.e., fragments were not cached. Transport time of fragments transported by a chain was longer than for those transported by single workers all the way to the nest, i.e., sequential transport did not save foraging time. Two hypotheses concerning the possible adaptive value of transport chains are discussed. The first one argues that sequential transport may lead to an increased material transport rate compared to individual transport. The second one considers sequential transport as a way to enhance the information flow among foragers, thus leading to a quicker build-up of workers at particular harvesting places. It is suggested that rather than increasing the gross transport rate of material, transport via chains may favor the transfer of information about the kind of resource being actually harvested.Received 19 December 2002; revised 14 March 2003; accepted 19 March 2003     

The size–distance relationship in the wood ant Formica rufa

1. The size–distance relationship among honeydew‐collecting foragers of the red wood ant Formica rufa was investigated. Within the colony territory, the size (as measured by head width) and fresh weight of samples of foragers were determined for ants ascending and descending trees near, and farther from, the central nest mound. 2. The mean size of the ants was significantly higher at far trees than at near trees in six out of the seven colonies investigated, confirming the general presence of the size–distance relationship. 3. In three colonies, a load–distance relationship was also found. For a given head width, honeydew‐carrying ants descending far trees were significantly heavier than those descending near trees (i.e. they were carrying heavier loads from trees farther away from the central nest mound). 4. This is the first time that both load–distance and size–distance relationships have been reported in foraging workers from the same ant colony. 5. The combined effects of these characteristics suggest that colony foraging efficiency is enhanced by far trees being visited by the larger workers that then return with heavier loads of honeydew.