Leafcutter ants are known to reject previously accepted plants if they prove to be unsuitable for their symbiotic fungus: a phenomenon that involves avoidance learning. Workers need to associate the ...detrimental effects of the incorporated plant with its characteristics, chemical and/or physical features, thus allowing plant recognition at the foraging site on subsequent days, and its avoidance. We addressed the question of to what extent olfactory memory underlies delayed avoidance of plants unsuitable for the symbiotic fungus. Odour is an important plant characteristic used as a recognition cue outside the nest during foraging.
Acromyrmex ambiguus foragers were able to learn about a plant’s unsuitability for their symbiotic fungus, rejecting this substrate in a binary choice experiment in the laboratory. Presented with leaf discs of two plant species simultaneously, one known to be unsuitable and one suitable, individual foragers significantly preferred the suitable plant species. When presented only with the plant odour, foragers steered away from the odour of the known unsuitable plant species, moving towards the simultaneously presented odour of a suitable plant species. Foragers were therefore able to identify plant species and recall information about substrate suitability for the fungus through the plant’s odour alone. None the less, foragers showed a significantly stronger avoidance of known unsuitable substrate when they could make contact with the leaf discs, suggesting the use of additional cues for plant identification. It is argued that not only appetitive, but also aversive learned responses are involved in the process of plant recognition by foraging leafcutter ants.
The size of underground ant nests positively correlates with their worker number, suggesting that during colony growth, workers respond to increased space demands by enlarging the existing nest. In ...leaf-cutting ants, the presence of brood and a growing symbiotic fungus are expected to generate spatial demands workers should respond to. We investigated to what extent the presence of in-nest stores, such as brood and fungus, and actual space availability influence the digging behavior of leaf-cutting ants and lead to the adjustment of nest enlargement. In the laboratory, we offered worker groups of Acromyrmex lundi a nest site consisting of either a tunnel, representing reduced available space, or a tunnel and a small chamber, representing ample available space as a starting point for nest enlargement. Workers could move and store offered brood and fungus into the nest structure, and enlarge it as needed. The presence of brood and fungus influenced digging activity as well as the architecture of the nest. In the presence of relocated brood, digging activity was higher in reduced space than in ample space, suggesting that high worker density stimulated excavation. When workers relocated fungus and brood into ample space, digging activity was similar as in their absence, yet a larger chamber and fewer tunnels were excavated. During the process of nest enlargement, workers were observed to initially excavate space in excess, which was refilled with part of the removed soil pellets, thus leading to a reduction of available space. Pellet deposition seemed to be opportunistic, with workers refilling unused space. Results indicate that the size of a leaf-cutting ant nest does not simply depend on the number of inhabiting workers. Rather, workers adjust the enlargement of their nest space depending on the current space available and the presence of in-nest stores.
Nest ventilation should be particularly relevant for the huge colonies of leaf-cutting ants, genus Atta. Considerable amounts of O2 are consumed and CO2 produced by both the fungus gardens and the ...ants inside nest chambers, which are located at deep soil layers characterized by high CO2 and low O2 concentrations. In this work, passive nest ventilation was investigated in field Atta capiguara and Atta laevigata nests, first, by evaluating air movements through the nest using propane as tracer gas as well as the CO2 and O2 concentrations of the circulating air, and second, by exposing the internal nest morphology with the use of cement casts and excavations. Results showed that even though outflow of CO2-rich air and inflow of O2-rich air occurred at high-placed and low-placed openings, respectively, supporting a wind-induced interpretation of air movements through the nest, circulating air was never detected inside fungus chambers. The CO2 and O2 levels inside the fungus chambers increased and decreased with increasing soil depth, respectively, and were in the range observed in the soil phase. Based on the underground nest architecture, it is concluded that although the external shape of the nest induces underground air circulation, the inflowing air is unable to directly reach the fungus chambers. It is argued that colony respiration completely depends on diffusive flows between the chamber air and the adjacent nest and soil atmospheres. Circulating air, although not directly renewing the air inside the nest chambers, may contribute to colony respiration by increasing the capacity of the nest and soil airs to act as an O2-source and a CO2-sink, because of the decrease in the CO2 and the increase in the O2 levels in the underground air phase. Possible adaptations of both ants and fungus to the high CO2 and low O2 concentrations usually found in soils are discussed.
Like many ants inhabiting temperate regions, leaf-cutting ant colonies of a number of Acromyrmex species build thatched nests that achieve more stable temperatures than those of the environment. ...Workers are expected to counteract short-term variations in nest microclimate by modifying the thatch structure, for instance by reducing thatch thickness or by permeating the thatch with openings so as to favor heat exchanges with the outside. Such thermoregulatory responses may compromise the control of nest humidity because of the concomitant water losses resulting from the escape of humid air to the environment. We asked whether water losses through the thatch trigger deposition of nest material as a regulatory response for humidity control. In a laboratory colony of A. heyeri, we defined four equidistant spots on the nest thatch through which losses of air with different humidities were experimentally simulated. The deposition of building material on the spot was recorded as indicative building response. Results showed that material deposition occurred significantly more frequently with higher humidity content of the air leaving the spot. Dry air leaving the nest hardly triggered material deposition. The observation of a continuous turnover of thatch material indicates that the building of a nest thatch is a permanent process, and that short-term responses for the control of nest climate, as the deposition of building material to suppress an outflow of humid air, are guided by specific cues such as the humidity content of the air stream.
Flexible division of labour is a key feature of social insects. Our understanding of individual response behaviour and how it is integrated into a functioning colony is still patchy. Most theoretical ...studies addressing this question are based on the assumption that workers have intrinsic and often fixed response thresholds for task-related stimuli. Here, we investigated the plasticity in worker fanning response behaviour in a bumblebee, Bombus terrestris. Using a temperature-controlled brood dummy, we first asked whether the fanning response is modulated by rate of temperature change. Second, we examined to what extent the fanning response is influenced by recent fanning experience. Our results show that the individual fanning response is modulated by both the rate of temperature increase and recent experience. Workers responded at lower temperatures and with a higher probability when the temperature of the brood dummy increased slowly compared with more rapid temperature increases. Workers that repeatedly responded to an increase in brood dummy temperature with fanning showed a significant decrease in their response thresholds, whereas the response thresholds of control workers that experienced the same treatment but did not gather fanning experience remained unchanged. The decrease in response threshold was pronounced when the time interval between two successive fanning events was less than 6h. When 16h or more separated fanning events, individual fanning responses returned to higher threshold levels. We suggest that experience-dependent modulation of response thresholds plays an important role in the behavioural differentiation of workers and the flexibility of insect colonies.
► Bumblebee workers control the temperature of their brood by incubating and fanning. ► Using a brood dummy, we investigated the plasticity in the individual fanning response. ► Fanning response behaviour is modulated by rate of temperature increase. ► Fanning response thresholds are modulated by previous experience in task performance. ► The effect of experience depends on the time interval between task performances.
Summary
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Leaf‐cutting ants (LCA) are highly polyphagous but at the same time highly selective dominant herbivores of neotropical rainforests. While a whole range of leaf properties has been ...identified to influence interspecific host choice, the mechanisms underlying intraspecific and intra‐individual herbivory patterns remain obscure. Here, we evaluate the plant‐stress hypothesis by analysing whether and how drought stress of food plants positively affects LCA food plant selection.
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In bioassays with whole plants of Piper marginatum, Atta colombica workers harvested more than twice the leaf area of stressed than of vigorous control plants. Within individual plants, the attractiveness of a given leaf increased with its stress level.
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Drought stress induced an accumulation of proline and non‐structural carbohydrates in the plant tissue by a factor of 3·5 and 2, respectively, accompanied by a decrease in the leaf water content of about 35%. Moreover, samples taken from preferred leaves within a plant contained more osmolytes than those representing the whole plant.
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Ants were shown to detect and prefer these osmolytes in bioassays conducted with leaf discs that had been experimentally enriched with proline and/or sucrose. We therefore propose a mechanism that links the preference of LCA to drought stress via the osmolyte concentrations within the leaves.
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Our results support the plant‐stress hypothesis, confirming that stressed plants and plant parts are more attractive to LCA. This may account for intraspecific and intra‐individual herbivory patterns of LCA, thus influencing populations of host species through the discrimination of drought‐sensitive individuals.
Nest foundation in the leaf-cutting ant Atta sexdens is claustral, and the single queen completely relies on its body reserves throughout, approximately, 9 weeks until the first workers emerge and ...initiate foraging. Nest digging is much time- and energy-consuming, and it is an open question how queens decide on the length of the tunnel they dig and therefore the depth of the initial chamber. Shallow founding nests may be energetically cheaper to dig, but queens may be more exposed to changing environmental variables. Deeper nests, on the other hand, may be climatically more stable and suitable, but more expensive to dig. We hypothesized that the maximal nest depth excavated by Atta founding queens may represent the outcome of an evolutionary trade-off between maximizing nest depth and minimizing energy expenditure during digging, so as to save energy for the long claustral phase. We tested this hypothesis by comparing the fitness consequences of increased digging effort in queens that were experimentally stimulated to excavate a complete founding nest either once, twice or three times consecutively compared to control queens that did not dig. Fitness was quantified as mortality rates, rates of egg-laying and offspring production, and size of the fungus garden until the emergence of the first workers. Results showed that, in contrast with the initial expectations, fungus growth, egg-laying rates and offspring production were not affected by the increased digging effort in the experimentally induced successive excavations. However, a significant higher mortality was observed in queens with increased digging effort, i.e., those that dug two or three nests consecutively. It is argued that in queens a behavioral mechanism for the control of nest depth has evolutionary been selected for as a trade-off between maximizing nest depth, to favor protection of the queen against unsuitable environmental variables, and minimizing energy expenditure during digging, which significantly affects survival.
This study investigates the techniques of nectar feeding in 11 different ant species, and quantitatively compares fluid intake rates over a wide range of nectar concentrations in four species that ...largely differ in their feeding habits. Ants were observed to employ two different techniques for liquid food intake, in which the glossa works either as a passive duct-like structure (sucking), or as an up- and downwards moving shovel (licking). The technique employed for collecting fluids at ad libitum food sources was observed to be species-specific and to correlate with the presence or absence of a well-developed crop in the species under scrutiny. Workers of ponerine ants licked fluid food during foraging and transported it as a droplet between their mandibles, whereas workers of species belonging to phylogenetically more advanced subfamilies, with a crop capable of storing liquids, sucked the fluid food, such as formicine ants of the genus
Camponotus. In order to evaluate the performance of fluid collection during foraging, intake rates for sucrose solutions of different concentrations were measured in four ant species that differ in their foraging ecology. Scaling functions between fluid intake rates and ant size were first established for the polymorphic species, so as to compare ants of different size across species. Results showed that fluid intake rate depended, as expected and previously reported in the literature, on sugar concentration and the associated fluid viscosity. It also depended on both the species-specific feeding technique and the extent of specialization on foraging on liquid food. For similarly-sized ants, workers of two nectar-feeding ant species,
Camponotus rufipes (Formicinae) and
Pachycondyla villosa (Ponerinae), collected fluids with the highest intake rates, while workers of the leaf-cutting ant
Atta sexdens (Myrmicinae) and a predatory ant from the
Rhytidoponera impressa-complex (Ponerinae) did so with the lowest rate. Calculating the energy intake rates in mg sucrose per unit time, licking was shown to be a more advantageous technique at higher sugar concentrations than sucking, whereas sucking provided a higher energy intake rate at lower sugar concentrations.
Thermal preferences for location of brood and fungus by workers of the thatching grass-cutting ant Acromyrmex heyeri were investigated in the laboratory, using a temperature gradient between 10 ...degree C and 37 degree C. Workers from a large laboratory colony were motivated to pick up exposed brood and fungus by removing a small part of the protective nest thatching. Single workers carrying pieces of fungus or brood items were then gently removed from the colony and placed at one end of the temperature gradient, either at 10 or 37 degree C. They were allowed to relocate the fungus or brood following their temperature preference. Both the probability of relocating an item and the temperatures selected were recorded after 45 minutes. The probability of relocation largely depended on the starting temperature, being higher for workers starting at 37 degree C than at 10 degree C, for both fungus and brood-carrying workers. For those relocated items, the selected temperatures averaged 25 degree C and 24 degree C for fungus and brood, respectively, when workers started from the hot end of the gradient. When workers started from the cold end of the gradient, the selected values were slightly lower, 22 degree C and 21 degree C for fungus and brood, respectively. These values match closely the temperature range in which maximal growth of the isolated attine ant fungus is achieved, between 20 and 25 degree C, as reported in the literature, indicating that the thermoregulatory behavioral responses of workers maximize fungal growth. Thermal preferences and the construction of thatch mounds are discussed as regulatory responses for the control of fungus growth in leaf-cutting ants inhabiting temperate regions.
Microclimatic conditions in the giant nests of the leaf-cutting ant Atta vollenweideri were experimentally examined to address the question whether increasing concentrations of the respiratory gases ...affect colony respiration. Measurements of CO sub(2) concentrations of less than 2.8% at different depths inside mature field nests indicated good ventilation, even at 2 m depth. Passive ventilation was driven by wind and promoted by the shape of the nest. It did not influence nest temperature nor relative humidity. During rain and flooding, small colonies closed all nest openings to protect the nest from excessive water influx and damage. Measurements in a small nest indicated that, as a consequence of closure, the CO sub(2) concentration increased rapidly. This situation was simulated in the laboratory, using a small colony of Atta sexdens as a model system. Colony respiration rates were shown to be reduced as a function of increasing CO sub(2) concentration. Based on literature data on ant respiration, it is suggested that the respiration of the symbiotic fungus was reduced, and not that of the ants. Since the brood of leaf-cutting ants feeds exclusively on the fungus, reduced nest ventilation and its effects on respiration rates may compromise colony growth. While mature nests provide the colony with good microclimate under all weather conditions during summer, developing colonies are confronted with a tradeoff between minimizing the risk of inundation and assuring adequate gas exchange inside their nests.
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