Geometry explains the benefits of division of labour in a leafcutter ant

Many ant species have morphologically distinct worker sub-castes. This presumably increases colony efficiency and is thought to be optimized by natural selection. Optimality arguments are, however, often lacking in detail. In ants, the benefits of having workers in a range of sizes have rarely been explained mechanistically. In Atta leafcutter ants, large workers specialize in defence and also cut fruit. Fruit is soft and can be cut by smaller workers. Why, therefore, are large workers involved? According to the geometry hypothesis, cutting large pieces from three-dimensional objects like fruit is enhanced by longer mandibles. By contrast, long mandibles are not needed to cut leaves that are effectively two-dimensional. Our results from Atta laevigata support three predictions from the geometry hypothesis. First, larger workers cut larger fruit pieces. Second, the effect of large size is greater in cutting fruit than leaves. Third, the size of fruit pieces cut increases approximately in proportion to the cube of mandible length. Our results are a novel mechanistic example of how size variation among worker ants enhances division of labour.     

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.     

Polymorphism: a weak influence on worker aggregation level in ants

Abstract.  1. Aggregation of individuals, a basic behaviour in social species, plays an essential role in many aspects of animal life (reproduction, defence, and alimentation). Understanding how this phenomenon is modulated is important to comprehend the social organisation of the group.
2. In social insects, aggregation is influenced by environmental (e.g. the light level) and social (e.g. polyethism in monomorphic ants) factors. Ants display a great variation of biological characteristics (e.g. queen number, polymorphism, division of labour, etc.) that are likely to influence the level of inter-attraction and so the aggregation.
3. The present research focused on one biological characteristic: the morphological castes (minors, majors), testing the hypothesis that minors will aggregate more than majors due to their greater need to fight against the loss of heat and to increase their self-protection.
4. Aggregation experiments were conducted on two highly polymorphic species, Atta sexdens rubropilosa and Solenopsis interrupta , using the two extreme morphological castes (majors and minors).
5. All castes exhibited a low level of aggregation: 40–50% of workers assembled for both species, the biggest cluster involving 20% of the total population. The lack of difference between morphological castes in the aggregation shows the weak influence of polymorphism on the interactions between ants.
6. It is concluded that the main factor modulating the aggregation behaviour is polyethism, i.e. the division of labour associated with the presence of an outside-the-nest experience: workers that only take care of the brood, without outside world experience (brood-tenders) assembling more than foraging workers (foragers).     

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.     

Foraging by Atta sexdens (Formicidae: Attini): seasonal patterns, caste and efficiency

Abstract. 1. Atta sexdens changes diel periods of foraging, the size of its foraging territory, the numbers and lengths of foraging trails, and its rate of foraging with respect to seasonality in subtropical Paraguay. Leaf loads are significantly larger in warmer months than loads carried in cooler months.
2. Foragers segregate into three labour groups in the field: a small subset climb trees, cut large quantities of vegetation, and drop them to the ground; the second subset of foragers searches out these leaf caches, cut diem into smaller pieces, and carry and deposit the leaf fragments on the foraging trail; the third subset of workers retrieves leaf fragments on the trail and carry them to the nest.
3. Pitfall trapping shows a large degree of patchiness in activity, with media workers dominating the foraging population, more so closer to vegetation which is being harvested.
4. The recovery efficiency of the multi-staged foraging behaviour is estimated to be only 49%, with the recovery of leaf caches near 50%. The impact of A. sexdens may, thus, be twice as great as previous estimates on their herbivory.     

The ontogeny of foragwehaviour in desert ants, Cataglyphis bicolor

Abstract. 1. Individually foraging desert ants, Cataglyphis bicolor , exhibit short foraging lives (half lifetime, i.e. half-time of the exponential decay function: 4.5 days), in which they perform 3.7 ± 1.9 foraging runs per day.
2. During their short lifetime foraging period the ants increase the duration of their foraging round trips (up to 40.0 ± 24.6 min per run), the maximal distance of individual foraging runs (up to 28.2 ± 4.1 m), and their foraging success, i.e. the ratio of successful runs to the total number of runs (up to 0.70).
3. The parameter that increases most dramatically during a forager's lifetime is direction fidelity, i.e. the tendency to remain faithful to a particular foraging direction.
4. A model based on some simple behavioural rules is used to describe the experimental findings that within an isotropic food environment individual ants develop spatial foraging idiosyncrasies, and do so at a rate that increases with the food densities they encounter.
5. Finally, it is argued that in functional terms direction fidelity is related to the navigational benefits resulting from exploiting familiar (route-based) landmark information, and hence reduces round-trip time and by this physiological stress and predatory risk.     

Hitchhiking and the removal of microbial contaminants by the leaf‐cutting ant Atta colombica

1. The ecologically dominant leaf‐cutting ants exhibit one of the most complex forms of morphological caste‐based division of labour in order to efficiently conduct tasks, ranging from harvesting fresh leaf material to caring for the vulnerable fungal crop they farm as food. While much of their division of labour is well known, the role of the smallest workers on foraging trails is puzzling. Frequently these minim workers hitchhike on leaf fragments and it has been suggested that they may act to reduce the microbial contamination of leaf material before they enter the nest. Here we investigated this potentially important role of minims with field colonies of Atta colombica. 2. We experimentally increased the microbial load of leaf fragments and found that this resulted in minims hitchhiking on leaf fragments for longer. Furthermore, we show that leaves naturally have a significant microbial load and that the presence of hitchhikers reduces the microbial load of both experimentally manipulated and natural leaf fragments. 3. Intriguingly, the microbial load of leaves high in the canopy where ants were foraging was much lower than closer to the ground where the ants avoided cutting leaves. This suggests that the often perplexing foraging patterns of leaf‐cutting ants may in part be explained by the ants avoiding leaves that are more heavily contaminated with microbes. 4. The removal of microbial contaminants is therefore an important role of hitchhiking minim workers in natural colonies of Atta leaf‐cutting ants, although other tasks such as trail maintenance and defence also explain their occurrence on trails.     

Effects of aphids on foliar foraging by Argentine ants and the resulting effects on other arthropods

Abstract.  1. Although interactions between ants and honeydew-producing insects have received considerable study, relatively little is known about how these interactions alter the behaviour of ants in ways that affect other arthropods. In this study, field and greenhouse experiments were performed that examined how the presence of aphids ( Aphis fabae solanella ) on Solanum nigrum influenced the foraging behaviour of Argentine ants ( Linepithema humile ) and, in turn, modified the extent to which ants deter larval lacewings ( Chrysoperla rufilabris ), which are known aphid predators.
2. A field experiment demonstrated that the level of foliar foraging by ants increased linearly with aphid abundance, whereas no relationship existed between the level of ground foraging by ants and aphid abundance.
3. In the greenhouse, as in the field, foliar foraging by ants greatly increased when aphids were present. Higher levels of foliar foraging led to a twofold increase in the likelihood that ants contacted aphid predators. As a result of these increased encounters with ants, lacewing larvae were twice as likely to be removed from plants with aphids compared with plants without aphids. Once contact was made, however, the behaviour of ants towards lacewing larvae appeared similar between the two experimental groups.
4. Argentine ants drive away or prey upon a diversity of arthropod predators and parasitoids, but they also exhibit aggression towards certain herbivores. Future work should attempt to quantify how the ecological effects that result from interactions between honeydew-producing insects and invasive ants, such as L. humile , differ from those that result from interactions between honeydew-producing insects and native ants.     

Limiting the success of stem borers (Prodoxus quinquepunctellus) in yuccas: indirect effects of ants, aphids, and fruit position

Abstract.  1. Although the moth–yucca mutualism is often studied as a pairwise interaction, yucca plants are also the sole host for a variety of other visitors. One of these additional visitors is a stem-boring moth, Prodoxus quinquepunctellus.
2. In this study, it is shown how the reproductive success of Prodoxus indirectly depends on the interactions between yuccas and their pollinators ( Tegeticula , Prodoxidae) as well as the indirect effects of ants and aphids.
3. Aggressive wood ants foraging on yuccas will attack adult Prodoxus moths while attempting to oviposit. This reduces the number of eggs laid in yucca stalks, leading to fewer larvae feeding in the stalks.
4. Once in the stalk, the survival of Prodoxus eggs/larvae depends upon the rate at which the flowering stalks dry out during fruit maturation. Portions of the stalk above the highest fruit dry out quickly and survivorship approaches zero in these dry sections, while larvae in green sections of the flowering stalk have significantly higher survival rates. The presence of aphids feeding on the stalk slows down the rate of stalk drying and could lead to increased survival of Prodoxus larvae.
5. Overall, ants have strong indirect effects on P. quinquepunctellus by controlling how many eggs are laid in the stalk and by influencing the distribution of aphids. However, it is primarily the presence and position of the fruit that can affect larval survivorship, and fruit position is a function of pollinator visits and resource limitation. These complex interactions illustrate the importance of studying the yucca–moth mutualism in a community context.     

Ontogenetic changes in forager polymorphism and foraging ecology in the leaf-cutting ant Atta cephalotes

In the leaf-cutting ant Atta cephalotes (L.) small colonies produce a relatively narrow size-range of small workers, whereas large colonies produce a much wider size-range of workers. In this study, I compared the foraging of four small A. cephalotes colonies (fewer than 5000 workers) with published data on foraging of large colonies to examine how colony size and worker size-range may be related to foraging ecology in leaf-cutting ants. I found that the foraging ecology of small A. cephalotes colonies is very different from that of large colonies. In small colonies, a relatively narrow size-range of foragers (1.4–6.7 mg, mean 3.3 mg) cut primarily herbs (ferns, grasses, and other small herbaceous plants) located within 7 m of the nest. In contrast, in large colonies, a broader size-range of workers (1.4–30 mg, mean 7.3 mg) participate in foraging, generally harvesting from trees 20–80 m from the nest, with larger workers cutting on trees with thicker and tougher leaves. Small colonies' dependence on small herbaceous plants near the nest may have a profound impact on distribution of A. cephalotes. A. cephalotes colonies are rarely found in primary forest, where the low occurrence of small herbaceous plants in the understory may preclude the establishment of young colonies.     

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.     

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.     

Shear adhesive performance of leaf‐cutting ant workers (Atta cephalotes)

Wingless arboreal ants must resist the force of gravity while traversing substrates in their environment. For leaf‐cutting ants like Atta cephalotes, foraging may also include a ca. 30 m vertical descent while carrying a load 1–6 times their body mass. We hypothesized that heavier and larger ants would carry heavier and larger loads and that adhesive performance would positively correlate with load mass. We found no relationship between ant mass, body length, head width, or adhesive performance, and the load size an ant carried. In addition to workers carrying vegetative loads (most often leaves), workers in an active foraging trail also include smaller workers riding on the leaves carried by larger workers, and large major workers, providing protection from aerial and ground attacks (Soldiers), respectively. Despite varying functional roles, all foraging ants require secure attachment to the substrate. We measured shear adhesive performance of each foraging role and found that Soldiers produced the highest shear adhesive forces. However, when controlling for tarsal pad area, we found that ants carrying loads have higher shear adhesive performance per unit area than those riding on leaves, and that Soldiers have the lowest shear adhesive performance per unit area. This suggests that while leaf choice does not appear to be dictated by size, mass, or shear adhesive performance of individual ants, overall, ants who carry leaves adhere more strongly given their pad size than those who do not. Abstract in Spanish is available with online material.     

The Leaf-Cutting ant <Emphasis Type="Italic">Atta Sexdens rubropilosa</Emphasis>, FOREL, 1908 Prefers Drier Chambers for Garbage Disposal

Leave cutting ants rely on a fungus garden as their main food supply. This garden produces debris that must be disposed by workers, as it may favor the contamination of the fungus. We assumed that the growth of undesired microorganisms on garbage would increase with humidity, therefore drier areas should be more suitable for garbage disposal. Accordingly, we tested the hypothesis that leave-cutting ants Atta sexdens rubropilosa choose drier chambers for garbage disposal. We found that 30 out of 30 sub-colonies tested for hygropreference chose drier chambers for garbage disposal when offered a choice between dry (RH=25±5% SD) and humid (RH=95±5% SD) chambers.     

Unpaved roads alter foraging patterns of the leafcutter ant Atta colombica

Leaf cutting ants are dominant herbivores and influential ecosystem engineers in the Neotropics. It has been suggested that habitat disturbances alter the architecture of foraging trail systems for colonies in their vicinity; however, the evidence remains scarce. In this study we investigated the effect of unpaved roads dissecting tropical lowland forest habitat on the structure of leafcutter foraging trail systems and foraging effort. We mapped trail systems for 16 mature Atta colombica colonies located at different distances from unpaved roads. Our results suggest exploitation of unpaved roads by leafcutters provides favorable foraging conditions, causing significant differences in foraging trail structure.     

Parallel foraging cycles for different resources in leaf‐cutting ants: a clue to the mechanisms of rhythmic activity

1. Leaf‐cutting ants display regular diel cycles of foraging, but the regulatory mechanisms underlying these cycles are not well known. There are, however, some indications in the literature that accumulation of leaf tissue inside a nest dampens recruitment of foragers, thereby providing a negative feedback that can lead to periodic foraging. We investigated two foraging cycles occurring simultaneously in an Atta colombica colony, one involving leaf harvesting and the other exploiting an ephemeral crop of ripe fruit. 2. Leaf harvesting followed a typical diel pattern of a 10–12 h foraging bout followed by a period of inactivity, while fruit harvesting occurred continuously, but with a regular pre‐dawn dip in activity that marked a 24 h cycle. 3. Although the results of the present study are drawn from a single field colony, the difference found is consistent with a mechanism of negative feedback regulation acting in parallel on two resources that differ in their rates of distribution and processing, creating cycles of formation and depletion of material caches. 4. This hypothesis should provoke further interest from students of ant behaviour and some simple manipulative experiments that would begin to test it are outlined. Any role of resource caches in regulating foraging by Atta colonies may have similarities to the logistics of warehouse inventories in human economic activity.     

Human disturbance promotes herbivory by leaf‐cutting ants in the Caatinga dry forest

Anthropogenic disturbances are known to modify plant–animal interactions such as those involving the leaf‐cutting ants, the most voracious and proliferating herbivore across human‐modified landscapes in the Neotropics. Here, we evaluate the effect of chronic anthropogenic disturbance (e.g., firewood collection, livestock grazing) and vegetation seasonality on foraging area, foliage availability in the foraging area, leaf consumption and herbivory rate of the leaf‐cutting ant Atta opaciceps in the semiarid Caatinga, a mosaic of dry forest and scrub vegetation in northeast Brazil. Contrary to our initial expectation, the foraging area was not affected by either disturbance intensity or the interaction between season and disturbance intensity. However, leaf consumption and herbivory rate were higher in more disturbed areas. We also found a strong effect of seasonality, with higher leaf consumption and herbivory rate in the dry season. Our results suggest that the foraging ecology of leaf‐cutting ants is modulated by human disturbance and seasonality as these two drivers affect the spectrum and the amount of resources available for these ants in the Caatinga. Despite the low productivity of Caatinga vegetation, the annual rates of biomass consumption by A. opaciceps are similar to those reported from other leaf‐cutting ants in rain forests and savannas. This is made possible by maintaining high foraging activity even in the peak of the dry season and taking benefit from any resource available, including low‐quality items. Such compensation highlights the adaptive capacity of LCA to persist or even proliferate in human‐modified landscapes from dry to rain forests.     

The effect of wilting on the selection of leaves by the leaf-cutting ant Atta laevigata

Using field assays of leaf preference, we tested the hypothesis that wilting affects the selection of leaves by the leaf-cutting ant Atta laevigata (Fr. Smith). Detached leaves were left to air-dry until noticeably wilted. The area removed by the ants from wilted leaves was significantly greater than the area removed from fresh leaves, this effect being observed in several plant species, in leaves of different age, and in assays with different ant colonies. Leaves collected from water-stressed plants were also preferred to leaves from non-stressed plants. A. laevigata was found to employ a two-stage, size-related, strategy when cutting plants. Larger workers climbed the plant stem and dropped whole leaves to the ground by severing their petioles; smaller workers cut the lamina of the dropped leaves. The ants frequently left dropped leaves on the ground, until the next foraging day or even later, when they were harvested in a wilted condition in preference to newly-dropped leaves.It is possible that during wilting some repellent substances evaporate or become less effective, thus enhancing leaf palatability. Alternatively or in addition, changes in nutrient and water content may have rendered wilted leaves more palatable to leaf-cutting ants.     

Temporal and spatial patterns in the emigrations of the army ant Dorylus (Anomma) molestus in the montane forest of Mt Kenya

Abstract.  1. The emigration behaviour of the army ant Dorylus ( Anomma ) molestus was studied in the montane forest of Mt Kenya. This species forages by massive swarm raids (mean width 10.3 m ± 4.6 m SD), which are assumed to have a strong negative impact on the densities of prey populations.
2. For non-reproductive colonies the stay duration in a nest is highly variable (median 17, range 3–111 days). This suggests that the frequency of emigrations is not dictated by a brood cycle as an underlying endogenous pattern generator.
3. Colony density is high (mean nearest neighbour's distance 82.1 m ± 29.4 m SD) and mean foraging range is 75.0 m, so encounters with neighbouring colonies occur frequently.
4. The straight line emigration distance is on average 92.7 m (± 29.7 SD). The emigration direction is random with respect to absolute bearing and also relative to the direction of the previous emigration. However, the emigration direction is influenced by the location of the nearest neighbour. Colonies typically emigrate directly away from their nearest neighbour.
5. Local food depletion is likely to be the ultimate cause for emigrations in this species, because emigration distance is larger than foraging range and colonies move away from their nearest neighbour. A small percentage of emigrations may be triggered by pangolin attacks on nests.
6. Contrary to the prediction of a recently developed mathematical model for epigaeic swarm-raiding Dorylus ( Anomma ) species, D.  ( A .)  molestus colonies do not engage in intraspecific battles. Possible reasons for the absence of fights between colonies despite apparently fierce intraspecific competition are discussed.     

Leaf nutrient content and host plant selection by leaf-cutter ants, Atta laevigata, in a Neotropical savanna

The selection of nutrient-rich leaves by leaf-cutter ants ( Atta spp.) is thought to be of indirect benefit to these ants by promoting the growth of their symbiotic fungus. However, relatively few studies have analyzed the influence of leaf nutrient content on host plant selection by leaf-cutter ants, and conflicting results have been found. We compared the content of eight nutritional elements plus the non-nutrient aluminum between leaves harvested by colonies of Atta laevigata (Smith) (Hymenoptera: Formicidae: Attini) and leaves collected randomly within their foraging areas. In addition, we evaluated whether leaf nutrient content explained the frequency with which these ants attacked and defoliated some of the tree species found in the study area. For 2 years, we monitored 17–26 trees from 15 species and determined the number of times each plant was attacked and the amount and type of foliage removed. Leaves harvested by A. laevigata presented significantly higher concentrations of N, P, K, Zn, and Cu than those collected randomly. This result is likely to reflect the foraging pattern presented by these ants, which were selective both in terms of the plant species and age of leaves most commonly attacked. Young leaves were the only or the main leaf type exploited in many species, and in comparison to mature leaves these presented significantly higher concentrations of P and K. Large differences in the mean number of ant attacks on the tree species studied were also observed, and those presenting more leaf N tended to be the most frequently attacked.