Host-plant selection,diet diversity,and optimal foraging in a tropical leafcutting ant

Summary A month-long study was conducted on the comparative foraging behavior of 20 colonies of the leafcutting ant, Atta cephalotes L. in Santa Rosa National Park, Guanacaste Province, Costa Rica. The study was conducted during the middle of the wet season, when trees had mature foliage and the ants were maximally selective among species of potential host plants. The colonies always gathered leaves from more than a single tree species but on average one species constituted almost half the diet with the remaining species being of geometrically decreasing importance. Colonies exhibited greater diversity in their choice of leaves and lower constancy of foraging when the average quality of resource trees was lower, as predicted by elementary optimal foraging theory. Furthermore, the ants were more selective of the species they attacked at greater distances from the nest. However, the ants sometimes did not attack apparently palatable species, and often did not attack nearby individuals of species they were exploiting at greater distances.A classical explanation for why leafcutting ants exploit distant host trees when apparently equally good trees are nearer, is that the ants are pursuing a strategy of conserving resources to avoid long-term overgrazing pressure on nearby trees. We prefer a simpler hypothesis: (1) Trees of exploited species exhibit individual variation in the acceptability of their leaves to the ants. (2) The abundance of a species will generally increase with area and radial distance from the nest, so the probability that at least one tree of the species will be acceptable to the ants also increases with distance. (3) The ants forage using a system of trunk-trails cleared of leaf litter, which significantly reduces their travel time to previously discovered, high-quality resource trees (by a factor of 4- to 10-fold). (4) Foragers are unware of the total pool of resources available to the colony. Therefore once scouts have chanced upon a tree which is acceptable, the colony will concentrate on harvesting from that tree rather than searching for additional sources of leaves distant from the established trail.     

Leaf caching in Atta leafcutting ants: discrete cache formation through positive feedback

We examined the occurrence, mechanism and costs and benefits of leaf caching in laboratory colonies of two species of leafcutting ants, Atta cephalotes and A. colombica. If foragers returning to the nest are unable to enter because of a temporary bottleneck caused by leaves building up they may deposit their leaf pieces outside the nest entrance, forming a leaf cache. Similar leaf caches occur in the field at foraging trail junctions, obstacles on the trail and within nest entrance tunnels. Foraging ants carrying leaves were presented with different-sized leaf caches and the number dropping their leaves on the cache was recorded. The probability of a forager dropping her leaf was positively correlated with the size of the cache that she encountered. Therefore, positive feedback played a role in the formation of nest entrance caches. Cached pieces were more likely to be retrieved than noncached pieces but the time taken to retrieve leaf pieces from a cache was greater than from scattered groups of leaves. We suggest that the strategy of flexible nest entrance caching is an adaptive response to fluctuating food availability and collection. Copyright 2000 The Association for the Study of Animal Behaviour.     

New role for majors in Atta leafcutter ants

Abstract.  1.  Atta (Hymenoptera: Formicidae) leafcutter ants display the most polymorphic worker caste system in ants, with different sizes specialising in different tasks. The largest workers (majors) have large, powerful mandibles and are mainly associated with colony defence.
2. Majors were observed cutting fallen fruit and this phenomenon was investigated in the field by placing mango fruit near natural Atta laevigata and Atta sexdens colonies in São Paulo State, Brazil.
3. Ants cutting the fruit were significantly heavier (mean = 49.1 mg, SD = 11.1 mg, n = 90) than the ants carrying the fruit back to the nest (mean = 20.9 mg, SD = 9.2 mg, n = 90).
4. Fruit pieces cut by majors were small (mean = 15.9 mg), approximately half the weight of leaf pieces (mean = 28.5 mg) cut and carried by media foragers. It is hypothesised that it is more difficult to cut large pieces from three-dimensional objects, like fruit, compared to two-dimensional objects, like leaves, and that majors, with their longer mandibles, can cut fruit into larger pieces than medias.
5. The study shows both a new role for Atta majors in foraging and a new example of task partitioning in the organisation of foraging.     

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.     

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.     

Performance and fate of tree seedlings on and around nests of the leaf‐cutting ant Atta cephalotes: Ecological filters in a fragmented forest

Habitat fragmentation is currently the most pervasive anthropogenic disturbance in tropical forests and some species of leaf‐cutting ants of the genus Atta (dominant herbivores in the neotropics) have become hyper‐abundant in forest edges where their nests directly impact up to 6% of the forest area. Yet, their impacts on the regeneration dynamics of fragmented forests remain poorly investigated. Here we examine the potential of Atta cephalotes nests to function as ecological filters impacting tree recruitment. Growth, survival and biomass partitioning of experimentally planted seedlings (six tree species) were examined at eight spatially independent A. cephalotes colonies in a large Atlantic Forest fragment. Seedling performance and fate (leaf numbers and damage) were monitored up to 27 months across three habitats (nest centre, nest edge and forest understorey). Plants at illuminated nest centres showed twice the gross leaf gain as understorey individuals. Simultaneously, seedlings of all species lost many more leaves at nests than in the forest understorey, causing a negative net leaf gain. Net leaf gain in the shaded understorey ranged from zero (Licania and Thyrsodium species) to substantial growth for Copaifera and Virola, and intermediate levels little above zero for Protium and Pouteria. Also seedling survival differed across habitats and species, being typically low in the centre and at the edge of nests where seedlings were often completely defoliated by the ants. Lastly, seedling survival increased strongly with seed size at nest edges while there was no such correlation in the forest. Our results suggest that Atta nests operate as ecological filters by creating a specific disturbance regime that differs from other disturbances in tropical forests. Apparently, Atta nests favour large‐seeded tree species with resprouting abilities and the potential to profit from a moderate, nest‐mediated increase in light availability.     

Infidelity of leafcutting ants to host plants: resource heterogeneity or defense induction?

Summary Leafcutting ants have strong among- and within-plant preferences, and generally abandon plants long before they are completely defoliated. Two tropical deciduous forest tree species preferred by the leafcutting ant Atta colombica were studied to determine how variation in resource quality affects ant selectivity and partial defoliation of plants. Significant differences in palatability and leaf characteristics of Spondias mombin and Bursera simaruba were found among trees and among leaf types within trees, but not among branches within trees. No short-term responses to experimental defoliation of up to 50% of total canopy were found in either species. Leaf nutrient and poisture content were positively correlated, and phenolic content negatively correlated, with the palatability of Spondias mombin, a species containing hydrolyzable tannins. Leaf moisture and phenolic content were both positively correlated with the palatability of Bursera simaruba, which contains predominantly condensed tannins. The results suggest that variation in leaf quality among and within plants is at least a partial explanation for ant selectivity and partial defoliation of preferred species. There is no evidence that rapidly induced changes in plant chemistry affect ant decisions to abandon these plants. Instead, it appears likely that ants abandon plants once high-quality leaf patches are exhausted. Quantitative variation in leaf nutrients, moisture, and secondary chemicals all appear to contribute to ant preferences for individuals and tissues of highly palatable plants.     

Phenotypic integration in an extended phenotype: among‐individual variation in nest‐building traits of the alfalfa leafcutting bee (Megachile rotundata)

Structures such as nests and burrows are an essential component of many organisms’ life‐cycle and require a complex sequence of behaviours. Because behaviours can vary consistently among individuals and be correlated with one another, we hypothesized that these structures would (1) show evidence of among‐individual variation, (2) be organized into distinct functional modules and (3) show evidence of trade‐offs among functional modules due to limits on energy budgets. We tested these hypotheses using the alfalfa leafcutting bee, Megachile rotundata, a solitary bee and important crop pollinator. Megachile rotundata constructs complex nests by gathering leaf materials to form a linear series of cells in pre‐existing cavities. In this study, we examined variation in the following nest construction traits: reproduction (number of cells per nest and nest length), nest protection (cap length and number of leaves per cap), cell construction (cell size and number of leaves per cell) and cell provisioning (cell mass) from 60 nests. We found a general decline in investment in cell construction and provisioning with each new cell built. In addition, we found evidence for both repeatability and plasticity in cell provisioning with little evidence for trade‐offs among traits. Instead, most traits were positively, albeit weakly, correlated (r ~ 0.15), and traits were loosely organized into covarying modules. Our results show that individual differences in nest construction are detectable at a level similar to that of other behavioural traits and that these traits are only weakly integrated. This suggests that nest components are capable of independent evolutionary trajectories.     

An odd activity of an ant (Cataglyphis bicolor); ≪visiting≫

Summary An unexplained form of behaviour connecting nests of an antCataglyphis bicolor F. is described. Individuals from one nest visit another wandering around the nest entrance, for periods of up to half an hour or more. Their behaviour is quite distinct from any seen in normal foraging. These individuals come from the upper end of the range of size; more frequently but not always from a larger to a smaller nest. Nests are normally antagonistic. These ants back away rapidly from an ant of the nest visited.     

Morphological differences between extranidal and intranidal workers in the ant Temnothorax rugatulus,but no effect of body size on foraging distance

Most ant genera are thought to have monomorphic workers, indicating perhaps a high degree of flexibility in task allocation, and the well-studied genus Temnothorax is an example of this. However, considerable size variation may exist between individuals. In addition, though workers can show flexible behavior, it has been shown that individuals may consistently differ in their task profiles. Here we test whether body size variation among workers affects foraging behavior. Two main hypotheses were tested: first, whether larger ants forage at greater distance from the nest, and second, whether larger individuals show a higher propensity to work outside of the nest. Our results showed that ant body size does not significantly affect foraging distance. However, larger ants were more likely to be found outside the nest. Though Temnothorax ants are a common model system, this is the first study demonstrating task allocation based on body size, which is fixed in adults. Our study suggests that particularly small species may have to be examined carefully for body size variation before concluding that body size is uniform and therefore irrelevant for task allocation.     

Evidence that the fungus cultured by leaf-cutting ants does not metabolize cellulose

Leaf‐cutting ants are a very specialized group of ants that cultivate fungus gardens in their nests, from which they obtain food. The current opinion is that the fungus cultivated by leaf‐cutting ants digests cellulose. Here we reassess the cellulose‐degrading capability of the fungus by using two complementary approaches tested in four Attini species (genera Atta and Acromyrmex): (1) ability of fungus to grow in cellulose; and (2) lignin/cellulose ratio in the refuse material dumped outside the nest, as an indicator of cellulose consumption. We found that (1) the fungus did not grow in cellulose, and (2) the lignin/cellulose ratio was much lower in the ants' refuse than in material digested by cellulose‐digesting organisms, such as brown‐rot fungus, termites, and ruminant mammals. This evidence strongly suggests the inability of the fungus to degrade cellulose. Therefore, the fungus–ant symbiosis and the ecological role of leaf‐cutting ants need to be reconsidered.     

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     

Ants on the Move: Resource Limitation of a Litter‐nesting Ant Community in Costa Rica1

The leaf litter of tropical wet forests is replete with itinerant ant nests. Nest movement may help ants evade the constraints of stress and disturbance and increase access to resources. I studied how nest relocation and environmental factors may explain the density, size, and growth of leaf litter ant nests. I decoupled the relationships among litter depth, food abundance, and nest availability in a 4‐mo manipulation of food and leaf litter in a community of litter‐nesting ants in a lowland wet forest in Costa Rica. Over 4 mo, 290 1 m² treatment and control plots were sampled without replacement. Nest densities doubled in response to food supplementation, but did not decrease in response to litter removal or stress (from litter trampling). The supplementation of food increased the utilization of less favored nesting materials. In response to food supplementation and litter trampling, arboreal ants established nests in the litter, and growth rates of the most common ants (Pheidole spp.) increased. Colony growth was independent of colony size and growth rates of the most abundant ants. In general, I conclude that litter‐nesting ant density is driven primarily by food limitation, that nest relocation behavior significantly affects access to resource and the demographic structure of this community, and that nest fission may be a method to break the growth–reproduction trade‐off.     

Comportement desFormica devant un obstacle «infranchissable»

Summary Formica polyctena can send information along their path about an obstacle. A simple mechanism (abnormal workers agitation) help the ants to find another way behind the obstacle.Formica rufa can dig a tunnel under a large circular wall that blocks all accesses to the nest. They can also open a hole in the wall, passing building materials through. These materials can be carried also underground, to repair the central part of the nest whose access is blocked. That is another proof of the extreme plasticity of the building activity among red ants.     

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.     

Impact of plant cover on the cavity‐nesting ant Temnothorax crassispinus

1. Plant communities influence the availability of important resources for ants, such as nest sites and food, as well as environmental conditions. Thus, plants affect the abundance and distribution of ants. 2. In a field experiment, the influence of plant cover on the settlement of nest sites and per‐capita productivity of sexual individuals by the ant Temnothorax crassispinus was analysed. In July 2014, in five areas with patches of alien balsam Impatiens parviflora, and another five of native balsam I. noli‐tangere, transects composed of artificial nests were established; the nest sites were situated inside patches of balsams, and outside of them. Four hundred and forty artificial nests were used. One year later, the nests were collected. 3. Colonies of the ants three times more often inhabited nest sites outside the patches of both balsams. Besides, colonies with queens were more frequently found in nest sites located away from balsams. The per‐capita productivity of sexual individuals was higher in nests collected in patches of balsam, and the colonies from patches of alien balsam produced a more female‐biased sex ratio. 4. In terms of the impact on the ant, no clear differences were found between the alien balsam and the native one. The most important factor affecting the fitness of ants in areas dominated by balsams is the presence of herbaceous plant cover rather than whether the plant is alien or native.     

Novel fungal disease in complex leaf‐cutting ant societies

Abstract 1. The leaf‐cutting ants practise an advanced system of mycophagy where they grow a fungus as a food source. As a consequence of parasite threats to their crops, they have evolved a system of morphological, behavioural, and chemical defences, particularly against fungal pathogens (mycopathogens). 2. Specific fungal diseases of the leaf‐cutting ants themselves have not been described, possibly because broad spectrum anti‐fungal defences against mycopathogens have reduced their susceptibility to entomopathogens. 3. Using morphological and molecular tools, the present study documents three rare infection events of Acromyrmex and Atta leaf‐cutting ants by Ophiocordyceps fungi, agenus of entomopathogens that is normally highly specific in its host choice. 4. As leaf‐cutting ants have been intensively studied, the absence of prior records of Ophiocordyceps suggests that these infections may be a novel event and that switching from one host to another is possible. To test the likelihood of this hypothesis, host switching was experimentally induced, and successfully achieved, among five distinct genera of ants, one of which was in a different sub‐family than the leaf‐cutter ants. 5. Given the substantial differences among the five host ants, the ability of Ophiocordyceps to shift between such distant hosts is remarkable; the results are discussed in the context of ant ecological immunology and fungal invasion strategies.     

Endophytic fungi increase the processing rate of leaves by leaf‐cutting ants (Atta)

1. Fungal endophytes are microfungi that reside asymptomatically inside of leaf tissues, increasing in density and diversity through time after leaves flush. Previous studies have suggested that the presence of fungal endophytes in the harvest material of leaf‐cutting ants (Atta colombica, Guérin‐Méneville) may negatively affect the ants and their fungal cultivar. 2. In the present study, it was tested whether the presence and diversity of fungal endophytes affected the amount of time necessary for leaf‐cutter ants to cut, process, and plant leaf material in their fungal garden. It was found that ants took 30–43% longer to cut, carry, clean, and plant leaf tissue with high relative to low endophyte abundance, and that the ants responded similarly to leaf tissue with high or low endophyte diversity. 3. It was further investigated whether the fungal cultivars' colonisation rate was greater on leaf material without fungal endophytes. No difference in the ants' cultivar colonisation rate on leaf tissue with high or low endophyte abundance was observed.     

Speed versus accuracy in decision-making ants: expediting politics and policy implementation

Compromises between speed and accuracy are seemingly inevitable in decision-making when accuracy depends on time-consuming information gathering. In collective decision-making, such compromises are especially likely because information is shared to determine corporate policy. This political process will also take time. Speed–accuracy trade-offs occur among house-hunting rock ants, Temnothorax albipennis. A key aspect of their decision-making is quorum sensing in a potential new nest. Finding a sufficient number of nest-mates, i.e. a quorum threshold (QT), in a potential nest site indicates that many ants find it suitable. Quorum sensing collates information. However, the QT is also used as a switch, from recruitment of nest-mates to their new home by slow tandem running, to recruitment by carrying, which is three times faster. Although tandem running is slow, it effectively enables one successful ant to lead and teach another the route between the nests. Tandem running creates positive feedback; more and more ants are shown the way, as tandem followers become, in turn, tandem leaders. The resulting corps of trained ants can then quickly carry their nest-mates; but carried ants do not learn the route. Therefore, the QT seems to set both the amount of information gathered and the speed of the emigration. Low QTs might cause more errors and a slower emigration—the worst possible outcome. This possible paradox of quick decisions leading to slow implementation might be resolved if the ants could deploy another positive-feedback recruitment process when they have used a low QT. Reverse tandem runs occur after carrying has begun and lead ants back from the new nest to the old one. Here we show experimentally that reverse tandem runs can bring lost scouts into an active role in emigrations and can help to maintain high-speed emigrations. Thus, in rock ants, although quick decision-making and rapid implementation of choices are initially in opposition, a third recruitment method can restore rapid implementation after a snap decision. This work reveals a principle of widespread importance: the dynamics of collective decision-making (i.e. the politics) and the dynamics of policy implementation are sometimes intertwined, and only by analysing the mechanisms of both can we understand certain forms of adaptive organization.