@article{RoemerCosarinskyRoces2020,
  author    = {R{\"o}mer, Daniela and Cosarinsky, Marcela I. and Roces, Flavio},
  title     = {Selection and spatial arrangement of building materials during the construction of nest turrets by grass-cutting ants},
  series = {Royal Society Open Science},
  volume    = {7},
  journal   = {Royal Society Open Science},
  doi       = {10.1098/rsos.201312},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-230458},
  year      = {2020},
  abstract  = {Ants build complex nest structures by reacting to simple, local stimuli. While underground nests result from the space generated by digging, some leaf- and grass-cutting ants also construct conspicuous aboveground turrets around nest openings. We investigated whether the selection of specific building materials occurs during turret construction in Acromyrmex fracticornis grass-cutting ants, and asked whether single building decisions at the beginning can modify the final turret architecture. To quantify workers' material selection, the original nest turret was removed and a choice between two artificial building materials, thin and thick sticks, was offered for rebuilding. Workers preferred thick sticks at the very beginning of turret construction, showed varying preferences thereafter, and changed to prefer thin sticks for the upper, final part of the turret, indicating that they selected different building materials over time to create a stable structure. The impact of a single building choice on turret architecture was evaluated by placing artificial beams that divided a colony's nest entrance at the beginning of turret rebuilding. Splitting the nest entrance led to the self-organized construction of turrets with branched galleries ending in multiple openings, showing that the spatial location of a single building material can strongly influence turret morphology.},
  language  = {en}
}
@article{CosarinskyRoemerRoces2020,
  author    = {Cosarinsky, Marcela I. and R{\"o}mer, Daniela and Roces, Flavio},
  title     = {Nest Turrets of Acromyrmex Grass-Cutting Ants: Micromorphology Reveals Building Techniques and Construction Dynamics},
  series = {Insects},
  volume    = {11},
  journal   = {Insects},
  number    = {2},
  issn      = {2075-4450},
  doi       = {10.3390/insects11020140},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-200680},
  year      = {2020},
  abstract  = {Acromyrmex fracticornis grass-cutting ants construct conspicuous chimney-shaped nest turrets made of intermeshed grass fragments. We asked whether turrets are constructed by merely piling up nearby materials around the entrance, or whether ants incorporate different materials as the turret develops. By removing the original nest turrets and following their rebuilding process over three consecutive days, age-dependent changes in wall morphology and inner lining fabrics were characterized. Micromorphological descriptions based on thin sections of turret walls revealed the building behaviors involved. Ants started by collecting nearby twigs and dry grass fragments that are piled up around the nest entrance. Several large fragments held the structure like beams. As a net-like structure grew, soil pellets were placed in between the intermeshed plant fragments from the turret base to the top, reinforcing the structure. Concomitantly, the turret inner wall was lined with soil pellets, starting from the base. Therefore, the consolidation of the turret occurred both over time and from its base upwards. It is argued that nest turrets do not simply arise by the arbitrary deposition of nearby materials, and that workers selectively incorporate large materials at the beginning, and respond to the developing structure by reinforcing the intermeshed plant fragments over time.},
  language  = {en}
}
@article{FalibeneRocesRoessler2015,
  author    = {Falibene, Augustina and Roces, Flavio and R{\"o}ssler, Wolfgang},
  title     = {Long-term avoidance memory formation is associated with a transient increase in mushroom body synaptic complexes in leaf-cutting ants},
  series = {Frontiers in Behavioural Neuroscience},
  volume    = {9},
  journal   = {Frontiers in Behavioural Neuroscience},
  number    = {84},
  doi       = {10.3389/fnbeh.2015.00084},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-148763},
  year      = {2015},
  abstract  = {Long-term behavioral changes related to learning and experience have been shown to be associated with structural remodeling in the brain. Leaf-cutting ants learn to avoid previously preferred plants after they have proved harmful for their symbiotic fungus, a process that involves long-term olfactory memory. We studied the dynamics of brain microarchitectural changes after long-term olfactory memory formation following avoidance learning in Acromyrmex ambiguus. After performing experiments to control for possible neuronal changes related to age and body size, we quantified synaptic complexes (microglomeruli, MG) in olfactory regions of the mushroom bodies (MB) at different times after learning. Long-term avoidance memory formation was associated with a transient change in MG densities. Two days after learning, MG density was higher than before learning. At days 4 and 15 after learning when ants still showed plant avoidance MG densities had decreased to the initial state. The structural reorganization of MG triggered by long-term avoidance memory formation clearly differed from changes promoted by pure exposure to and collection of novel plants with distinct odors. Sensory exposure by the simultaneous collection of several, instead of one, non-harmful plant species resulted in a decrease in MG densities in the olfactory lip. We hypothesize that while sensory exposure leads to MG pruning in the MB olfactory lip, the formation of long-term avoidance memory involves an initial growth of new MG followed by subsequent pruning.},
  language  = {en}
}
@phdthesis{Halboth2018,
  author    = {Halboth, Florian},
  title     = {Building behavior and nest climate control in leaf-cutting ants: How environmental cues affect the building responses of workers of \(Atta\) \(vollenweideri\)},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-161701},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2018},
  abstract  = {The present work investigates the influence of environmental stimuli on the building behavior of workers of the leaf-cutting ant Atta vollenweideri. It focuses on cues related to the airflow-driven ventilation of their giant underground nests, i.e., air movements and their direction, carbon dioxide concentrations and humidity levels of the nest air. First, it is shown that workers are able to use airflow and its direction as learned orientation cue by performing learning experiments with individual foragers using a classical conditioning paradigm. This ability is expected to allow workers to also navigate inside the nest tunnels using the prevailing airflow directions for orientation, for example during tasks related to nest construction and climate control. Furthermore, the influence of carbon dioxide on the digging behavior of workers is investigated. While elevated CO2 levels hardly affect the digging rate of the ants, workers prefer to excavate at locations with lower concentrations and avoid higher CO2 levels when given a choice. Under natural conditions, shifting their digging activity to soil layers containing lower carbon dioxide levels might help colonies to excavate new or to broaden existing nest openings, if the CO2 concentration in the underground rises. It is also shown that workers preferably transport excavated soil along tunnels containing high CO2 concentrations, when carbon dioxide levels in the underground are elevated as well. In addition, workers prefer to carry soil pellets along outflow tunnels instead of inflow tunnels, at least for high humidity levels of the air. The material transported along tunnels providing outflow of CO2-rich air might be used by workers for the construction of ventilation turrets on top of the nest mound, which is expected to promote the wind-induced ventilation and the removal of carbon dioxide from the underground. The climatic conditions inside the nest tunnels also influence the structural features of the turrets constructed by workers on top the nest. While airflow and humidity have no effect on turret structure, outflow of CO2-rich air from the nest causes workers to construct turrets with additional openings and increased aperture, potentially enhancing the airflow-driven gas exchanges within the nest. Finally, the effect of airflow and ventilation turrets on the gas exchanges in Atta vollenweideri nests is tested experimentally on a physical model of a small nest consisting of a single chamber and two nest tunnels. The carbon dioxide clearance rate from the underground was measured depending on both the presence of airflow in the nest and the structural features of the built turrets. Carbon dioxide is removed faster from the physical nest model when air moves through the nest, confirming the contribution of wind-induced flow inside the nest tunnels to the ventilation of Atta vollenweideri nests. In addition, turrets placed on top of one of the tunnel openings of the nest further enhance the CO2 clearance rate and the effect is positively correlated with turret aperture. Taken together, climatic variables like airflow, carbon dioxide and humidity levels strongly affect the building responses of Atta vollenweideri leaf-cutting ants. Workers use these environmental stimuli as orientation cue in the nest during tasks related to excavation, soil transport and turret construction. Although the effects of these building responses on the microclimatic conditions inside the nest remain elusive so far, the described behaviors are expected to allow ant colonies to restore and maintain a proper nest climate in the underground.},
  subject      = {Verhalten},
  language  = {en}
}
@article{FalibeneRocesRoessler2015,
  author    = {Falibene, Agustina and Roces, Flavio and R{\"o}ssler, Wolfgang},
  title     = {Long-term avoidance memory formation is associated with a transient increase in mushroom body synaptic complexes in leaf-cutting ants},
  series = {Frontiers in Behavioral Neuroscience},
  volume    = {9},
  journal   = {Frontiers in Behavioral Neuroscience},
  number    = {84},
  doi       = {10.3389/fnbeh.2015.00084},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-125522},
  year      = {2015},
  abstract  = {Long-term behavioral changes related to learning and experience have been shown to be associated with structural remodeling in the brain. Leaf-cutting ants learn to avoid previously preferred plants after they have proved harmful for their symbiotic fungus, a process that involves long-term olfactory memory. We studied the dynamics of brain microarchitectural changes after long-term olfactory memory formation following avoidance learning in Acromyrmex ambiguus. After performing experiments to control for possible neuronal changes related to age and body size, we quantified synaptic complexes (microglomeruli, MG) in olfactory regions of the mushroom bodies (MBs) at different times after learning. Long-term avoidance memory formation was associated with a transient change in MG densities. Two days after learning, MG density was higher than before learning. At days 4 and 15 after learning—when ants still showed plant avoidance—MG densities had decreased to the initial state. The structural reorganization of MG triggered by long-term avoidance memory formation clearly differed from changes promoted by pure exposure to and collection of novel plants with distinct odors. Sensory exposure by the simultaneous collection of several, instead of one, non-harmful plant species resulted in a decrease in MG densities in the olfactory lip. We hypothesize that while sensory exposure leads to MG pruning in the MB olfactory lip, the formation of long-term avoidance memory involves an initial growth of new MG followed by subsequent pruning.},
  language  = {en}
}
@phdthesis{Pielstroem2013,
  author    = {Pielstr{\"o}m, Steffen},
  title     = {On the Role of Local Information in the Spatial Organisation of Collective Nest Digging in the Leaf-Cutting Ant Atta vollenweideri (Forel, 1893)},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-79118},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {Many ant species excavate underground nests. One of the most impressive examples is the Chaco leaf-cutting ant Atta vollenweideri from the Gran Chaco region in South America. The nests excavated by the workers of that species are among the largest insect-built structures on the planet. They are ecavated over years possibly involving millions of working individuals. However, the mechanisms underlying the organisation of collective nest digging in ants remain largely unknown. Considering the sheer dimensions of the nest in comparison to the size and presumably limited perceptual and cognitive abilities of the single worker, the assumption can be made that organising mechanisms are mostly based on responses of individuals to local stimuli within their perceptual range. Among these local stimuli that guide nest digging we can expect environmental variables, stimuli that relate to the requirements of the colony, and stimuli related to the spatial coordination of collective effort. The present thesis investigates the role of local stimuli from these three categories in the organisation of collective digging behaviour in the Chaco leaf-cutting ant. It describes experiments on (1) how workers respond in the context of digging to differences in soil moisture, which comprises an important environmental variable; (2) how available nest space influences nest enlargement; (3) and how the spatial coordination of excavating workers is implemented by responding to stimuli arising from nest mates while engaged in digging behaviour. The experiments on soil water content show that workers prefer to dig in moist materials that allow for fast excavation and transport rates. Accordingly, an unequal distribution of water in the soil around a nest can influence how the nest shape develops. On the other hand, results also indicate that workers strongly avoid excavating in extremely moist materials. Regarding the abundant occurrence of flooding events in the Gran Chaco region, the latter can be interpreted as an adaptation to avoid water inflow into the nest. In the experiments on the effect of nest space, the ants excavated less when presented with larger nests. When a large amount of space was suddenly added to the nest during the digging process, excavation rates decreased according to the new volume. These observations confirm the hypothesis that digging activity is regulated according to space requirements, possibly because crowding conditions inside the nest influence excavation behaviour. However, observations also indicate an intrinsic decrease of digging motivation with time. Moreover, excavation rates correlate with nest size only when comparing nests of similar shape. Distributing a similar nest volume to three smaller chambers, instead of one, resulted in drastically decreased digging rates. A possible explanation for that observation lies in the distribution of workers inside the nest that may vary according to nest geometry: a different distribution of individuals can lead to in different local crowding conditions in similar nest volumes. Furthermore, two different stimuli are described that are used in the spatial coordination of collective digging effort. First, fresh soil pellets deposited close to the digging site on their way from the surface increase the probability that arriving workers join excavation efforts at the same site. The deposition of pellets on the way is a consequence of sequential task partitioning during soil transport. The pellets are carried in transport chains that closely resemble the modalities of leaf transport observed at the surface. Second, workers stridulate while digging. The short-ranged vibrational signals produced thereby also attract nest mates to excavate at the same location. Accordingly, two mutually complementing mechanisms are described that allow to concentrate excavators at one location. In both cases, a local stimulus that is generated by current close-by excavation activity increases the probability of the stimulus receiver to dig close to other excavators. In an environment otherwise poor in digging stimuli, these mechanisms can be especially important to give collective digging efforts a common direction. As a consequence it can be argued that the spatial organisation of collective digging is based on choice copying. Individuals copy nest mate decisions on where to excavate by responding to local stimuli provided by nest mate digging activity. Taken together, responses to local stimuli can determine the direction of nest growth, aid in preventing the inflow of surface water into the nest, guide the adjustment of nest size to colony requirements and spatially coordinate collective digging efforts. Even though it cannot be ruled out that digging responses based e.g. on spatial memory or long-term experience exist, the results presented here clearly demonstrate that responses to local information account for many important aspects of nest development.},
  subject      = {Blattschneiderameisen},
  language  = {en}
}
@article{BollazziRoces2010,
  author    = {Bollazzi, Martin and Roces, Flavio},
  title     = {The thermoregulatory function of thatched nests in the South American grass-cutting ant, Acromyrmex heyeri},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-68225},
  year      = {2010},
  abstract  = {The construction of mound-shaped nests by ants is considered as a behavioral adaptation to low environmental temperatures, i.e., colonies achieve higher and more stables temperatures than those of the environment. Besides the well-known nests of boreal Formica wood-ants, several species of South American leaf-cutting ants of the genus Acromyrmex construct thatched nests. Acromyrmex workers import plant fragments as building material, and arrange them so as to form a thatch covering a central chamber, where the fungus garden is located. Thus, the degree of thermoregulation attained by the fungus garden inside the thatched nest largely depends on how the thatch affects the thermal relations between the fungus and the environment. This work was aimed at studying the thermoregulatory function of the thatched nests built by the grass-cutting ant Acromyrmex heyeri Forel (Hymenoptera: Formicidae: Myrmicinae). Nest and environmental temperatures were measured as a function of solar radiation on the long-term. The thermal diffusivity of the nest thatch was measured and compared to that of the surrounding soil, in order to assess the influence of the building material on the nest's thermoregulatory ability. The results showed that the average core temperature of thatched nests was higher than that of the environment, but remained below values harmful for the fungus. This thermoregulation was brought about by the low thermal diffusivity of the nest thatch built by workers with plant fragments, instead of the readily-available soil particles that have a higher thermal diffusivity. The thatch prevented diurnal nest overheating by the incoming solar radiation, and avoided losses of the accumulated daily heat into the cold air during the night. The adaptive value of thatching behavior in Acromyrmex leaf-cutting ants occurring in the southernmost distribution range is discussed.},
  subject      = {Acromyrmex heyeri},
  language  = {en}
}
@phdthesis{Saverschek2010,
  author    = {Saverschek, Nicole},
  title     = {The influence of the symbiotic fungus on foraging decisions in leaf-cutting ants - Individual behavior and collective patterns},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-52087},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {Foraging behavior is a particularly fascinating topic within the studies of social insects. Decisions made by individuals have effects not only on the individual level, but on the colony level as well. Social information available through foraging in a group modulates individual preferences and shapes the foraging pattern of a colony. Identifying parameters influencing foraging behavior in leaf-cutting ants is especially intriguing because they do not harvest for themselves, but for their symbiotic fungus which in turn influences their plant preferences after the incorporation of the substrate. To learn about the substrates' unsuitability for the fungus, ants need to be able to identify the incorporated substrate and associate it with detrimental effects on the fungus. Odor is an important plant characteristic known to be used as recognition key outside the nest in the context of foraging. Chapter 1 shows that foragers are able to recall information about the unsuitability of a substrate through odor alone and consequently reject the substrate, which leads to the conclusion that inside the nest, odor might be enough to indentify incorporated substrate. Identification of plant species is a key factor in the foraging success of leaf-cutting ants as they harvest a multitude of different plant species in a diverse environment and host plant availability and suitability changes throughout the year. Fixed plant preferences of individuals through innate tendencies are therefore only one factor influencing foraging decisions. On the individual as well as the colony level, foraging patterns are flexible and a result of an intricate interplay between the different members involved in the harvesting process: foragers, gardeners and the symbiotic fungus. In chapter 2 I identified several conditions necessary for na{\"i}ve foragers to learn about the unsuitability of substrate inside the nest. In order to exchange of information about the unsuitability of a substrate, the plant in question must be present in the fungus garden. Foragers can learn without own foraging experience and even without experiencing the effects of the substrate on the fungus, solely through the presence of experienced gardeners. The presence of experienced foragers alone on the other hand is not enough to lower the acceptance of substrate by na{\"i}ve foragers in the presence of na{\"i}ve gardeners, even if experienced foragers make up the majority of the workforce inside the nest. Experienced foragers are also able to reverse their previous negative experience and start accepting the substrate again. The individual behavior of foragers and gardeners with different experiential backgrounds in the presence of suitable or unsuitable substrate inside the fungus chamber was investigated in chapter 3 to shed some light on possible mechanisms involved in the flow of information about substrate suitability from the fungus to the ants. Gardeners as well as foragers are involved in the leaf processing and treatment of the applied leaf patches on the fungus. If the plant material is unsuitable, significantly more ants treat the plant patches, but foragers are less active overall. Contacts between workers initiated by either gardeners or foragers occur significantly more frequent and last longer if the substrate is unsuitable. Even though experienced gardeners increase na{\"i}ve foragers' contact rates and duration with other workers in the presence of suitable plant patches, na{\"i}ve foragers show no differences in the handling of the plant patches. This suggests that foragers gain information about plant suitability not only indirectly through the gardening workers, but might also be able to directly evaluate the effects of the substrate on the fungus themselves. Outside the nest, foragers influence each other the trail (chapter 4). Foraging in a group and the presence of social information is a decisive factor in the substrate choice of the individual and leads to a distinct and consentaneous colony response when encountering unfamiliar or unsuitable substrates. As leaf-cutting ants harvest different plant species simultaneously on several trails, foragers gain individual experiences concerning potential host plants. Preferences might vary among individuals of the same colony to the degree that foragers on the same trail perceive a certain substrate as either suitable or unsuitable. If the majority of foragers on the trail perceives one of the currently harvested substrates as unsuitable, na{\"i}ve foragers lower their acceptance within 4 hours. In the absence of a cue in the fungus, na{\"i}ve foragers harvesting by themselves still eventually (within 6 hours) reject the substrate as they encounter experienced gardeners during visits to the nest within foraging bouts. As foraging trails can be up to 100 m long and foragers spend a considerable amount of time away from the nest, learning indirectly from experienced foragers on the trail accelerates the distribution of information about substrate suitability. The level of rejection of a formerly unsuitable substrate after eight hours of foraging by na{\"i}ve foragers correlates with the average percentage of unladen experienced foragers active on the trail. This suggests that unladen experienced foragers might actively contact laden na{\"i}ve workers transmitting information about the unsuitability of the load they carry. Results from experiments were I observed individual laden foragers on their way back to the nest backed up this assumption as individuals were antennated and received bites into the leaf disk they carried. Individuals were contacted significantly more often by nestmates that perceived the carried leaf disk as unsuitable due to previous experience than by nestmates without this experience (chapter 6). Leaf-cutting ants constantly evaluate, learn and re-evaluate the suitability of harvested substrate and adjust their foraging activity accordingly. The importance of the different sources of information within the colony and their effect on the foraging pattern of the colony depend on the presence or absence of each of them as e.g. experienced foragers have a bigger influence on the plant preferences of na{\"i}ve foragers in the absence of a cue in the fungus garden.},
  subject      = {Blattschneiderameisen},
  language  = {en}
}
@phdthesis{Froehle2009,
  author    = {Fr{\"o}hle, Kerstin},
  title     = {Mechanismen zur Regulierung der Nestgr{\"o}ße w{\"a}hrend des Koloniewachstums bei Blattschneiderameisen},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-46311},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2009},
  abstract  = {Die Strukturen der Ameisennester, so wird seit einiger Zeit vermutet, entstehen aufgrund eines selbstorganisierten Prozesses, bei dem die einzelne Ameise nur {\"u}ber lokale Informationen verf{\"u}gt, ohne eine {\"U}bersicht {\"u}ber das globale Muster zu haben. Die Gesamtstruktur resultiert demnach viel eher durch multiple Interaktionen, die entweder direkt zwischen den Individuen oder zwischen den Individuen und ihrer Umgebung stattfinden. Ziel dieser Arbeit war es, die Kriterien zu untersuchen, nach denen sich die Blattschneiderameisen w{\"a}hrend des Nestbaus richten, um so die Frage zu beantworten, ob es f{\"u}r die Entstehung der Strukturen nur der Interaktion mit der Umgebung bedarf oder ob direkte soziale Interaktionen auch einen Einfluss darauf haben. Betrachtet wurde dazu die Kontrolle der Nestgr{\"o}ße w{\"a}hrend verschiedener Stadien der Kolonieentwicklung: in der Gr{\"u}ndungsphase, in der die K{\"o}nigin die Entscheidungen alleine und ohne soziale Interaktionen f{\"a}llt; in der darauf folgenden Etablierungsphase, in der Arbeiterinnen entweder alleine oder in kleinen Gruppen die bereits existierenden Strukturen ver{\"a}ndern; sowie im adulten Stadium, in der die Baut{\"a}tigkeit von mehreren Tausend Arbeiterinnen ausgef{\"u}hrt werden kann. K{\"o}niginnen graben unverz{\"u}glich nach dem Hochzeitsflug ein Gr{\"u}ndungsnest, das aus einem vertikalen Tunnel und einer horizontalen Kammer besteht, in welcher die erste Brut und der Pilz gez{\"u}chtet werden. Um ein Gr{\"u}ndungsnest zu graben, muss die K{\"o}nigin zuerst mit ihren Mandibeln kopf{\"u}ber am Boden graben. Hierbei legt sie einen Tunnel an, der einen etwas gr{\"o}ßeren Durchmesser als sie selbst besitzt. Ist dann die gew{\"u}nschte Tunnell{\"a}nge erreicht, so wechselt sie vom vertikalen Tunnel zum horizontalen Kammergraben, worauf anschließend der Tunnel verschlossen wird. Die Frage, die sich nun stellt, ist, wie Atta vollenweideri K{\"o}niginnen die L{\"a}nge des Tunnels bewerten, um den Wechsel zum Kammergraben einzuleiten. Aufgrund der Ergebnisse wird angenommen, dass die K{\"o}niginnen sowohl die L{\"a}nge des Tunnels, wahrscheinlich {\"u}ber Propriozeption, als auch die Grabezeit absch{\"a}tzen und mit einer internen Referenz vergleichen. Wurde demnach weder die erwartete L{\"a}nge noch die maximal schon investierte Zeit erreicht, so fuhren die K{\"o}niginnen fort den Tunnel zu verl{\"a}ngern. Der Wechsel vom Tunnel zum Kammergraben wurde dann eingeleitet, wenn die K{\"o}niginnen, in Abh{\"a}ngigkeit von den jeweiligen Bodenbedingungen, entweder zuerst die erwartete L{\"a}nge oder die zu investierende Zeit erreicht hatten. Daraufhin fingen sie an die Kammer zu bauen, wobei sie die nun ausgegrabenen Lehmpartikel dazu benutzten, den Tunnel zu verschließen. Diese wurden von oben bis unten komplett verschlossen, womit die Kammergr{\"o}ßen von den Tunnell{\"a}ngen abh{\"a}ngig waren. Wurden die K{\"o}niginnen jedoch mit Tunneln konfrontiert, die experimentell {\"u}ber die erwartete L{\"a}nge hinaus verl{\"a}ngert wurden, so wurden diese nicht mehr {\"u}ber die komplette Strecke, sondern in mehreren Teilabschnitten verschlossen. Dies deutet darauf hin, dass bei der Regulierung der Kammergr{\"o}ße ein weiterer Mechanismus involviert ist. Nach 2-3 Monaten schl{\"u}pfen in der Regel die ersten Arbeiterinnen, womit die Kolonie in die Wachstumsphase eintritt. Mit dem Wachsen der Kolonie wird das Gr{\"u}ndungsnest ver{\"a}ndert, wobei die Arbeiterinnen die bereits existierende Pilzkammer vergr{\"o}ßern und neue Tunnel anlegen. Nach welchen Kriterien sie sich dabei richten, war allerdings nicht bekannt. Gezeigt werden konnte, dass Acromyrmex lundi Arbeiterinnen anfangen ein Nest zu vergr{\"o}ßern, wenn sich der frei f{\"u}r die Ameisen zur Verf{\"u}gung stehende Platz innerhalb des Nestes reduziert und dass sie aufh{\"o}ren, wenn wiederum gen{\"u}gend Platz vorhanden ist. Eine Zunahme in der Gruppengr{\"o}ße (1, 2, 6 und 12 Tiere) bewirkte somit, einen proportionalen Anstieg des ausgegrabenen Volumens und damit der Arbeitsleistung der Kolonie. Ob beim Graben aber eher die schon vorhandenen Pilzkammer vergr{\"o}ßert oder neue Tunnel angelegt werden, hing von der Stimuluskombination ab. So bewirkte ein Platzmangel, ausgel{\"o}st durch eine, relativ zur Nestgr{\"o}ße, große Zahl an Arbeiterinnen, das bereits existierende Tunnel verl{\"a}ngert oder neue angelegt wurden. Eine Kammervergr{\"o}ßerung konnte dagegen nur beobachtet werden wenn Pilz vorhanden und der Platz in der Kammer reduziert war. Die Arbeiterinnen reagierten dabei, auf dieselben Stimuli mit denselben Verhaltensmustern, unabh{\"a}ngig davon ob sie alleine oder in einer Gruppe gruben. Je mehr Ameisen sich aber in der Gruppe befanden desto mehr wurden die Kammern zun{\"a}chst vergr{\"o}ßert, wobei sich jedoch keine Korrelation mit der Gruppengr{\"o}ße zeigte. Dies l{\"a}sst darauf schließen, dass die Vergr{\"o}ßerung von den sich gleichzeitig am Graben beteiligenden Ameisen abh{\"a}ngt, die die Kammern so lange vergr{\"o}ßern bis gen{\"u}gend Platz vorhanden ist. Die Zahl der Ameisen die sich jedoch am Graben beteiligen nimmt mit steigender Gruppengr{\"o}ße zu, weswegen die Kammern bei großen Ameisenzahlen gr{\"o}ßer wurden. Gleichzeitig mit dem Vergr{\"o}ßern fingen die Ameisen jedoch an ausgegrabene Lehmpartikel in der Kammer zu deponieren. Dies bewirkte, dass vor allem gr{\"o}ßere Kammern im Nachhinein verkleinert wurden, bis ein bestimmter Abstand zum Pilz erreicht war, bei dem eventuell zwei Ameisen aneinander vorbeilaufen konnten. Somit hatte die Einlagerung der Lehmpartikel in der Kammer zur Folge, dass die Kammergr{\"o}ße im Nachhinein besser dem Pilzvolumen angepasst wurde. {\"A}hnlich wie bei der Kammervergr{\"o}ßerung verhielt es sich beim Anlegen der Tunnel. Auch diese wurden umso breiter je mehr Tiere sich gleichzeitig am Graben beteiligten und wurden dann im Nachhinein durch Einlagerung von Lehmpartikeln auf eine bestimmte Breite reduziert. Zus{\"a}tzlich wurden die Tunnel aber auch umso l{\"a}nger je mehr Ameisen sich in der Gruppe befanden, weshalb die Nestgr{\"o}ße {\"u}ber die Gr{\"o}ße der Gruppe reguliert wurde. Acromyrmex lundi Nester bestehen in der Regel aus einer großen zentralen Pilzkammer und aus mehreren Tunneln, die diese mit der Erdoberfl{\"a}che verbinden. Wie die Ameisen in dem adulten Stadium die Gr{\"o}ße der Pilzkammer regulieren, wurde bisher noch nicht untersucht. Als m{\"o}gliche Kriterien, nach denen sich die Ameisen richten k{\"o}nnten, wurde sowohl das vorhandene Pilzvolumen als auch die Anzahl an Arbeiterinnen in Betracht gezogen. Gezeigt werden konnte, dass die Kammern umso gr{\"o}ßer werden, je mehr Pilzvolumen vorhanden ist. Aufgrund dessen wird angenommen, dass der Pilz beim Bau der Pilzkammer als Vorlage dient und somit das Grabeverhalten r{\"a}umlich organisiert. Eine Erh{\"o}hung der Ameisenzahlen bewirkte dagegen eine Vergr{\"o}ßerung des Nestvolumens durch das Anlegen von Tunneln. Dadurch nahm das insgesamt ausgegrabene Volumen und damit die Grabeaktivit{\"a}t mit der Gr{\"o}ße der Kolonie zu. Allerdings stieg es nicht, wie bei den kleinen Gruppen beobachtet werden konnte, proportional zur Koloniegr{\"o}ße an. Vermutet wird, dass sowohl die Kammer- als auch die Nestvergr{\"o}ßerung {\"u}ber die Individuendichte reguliert wird. Demnach w{\"u}rden die Tiere anfangen zu graben, wenn die Individuendichte {\"u}ber einen Schwellenwert ansteigt und aufh{\"o}ren, wenn die Dichte wiederum unter diesen Schwellenwert f{\"a}llt. Allerdings gibt es Hinweise darauf, dass die Grabeaktivit{\"a}t nicht nur {\"u}ber die Individuendichte, sondern zus{\"a}tzlich noch durch ein rhythmisches Graben in der Nacht geregelt zu sein scheint. Zusammengenommen konnte also gezeigt werden, dass K{\"o}niginnen auf Stimuli in ihrer Umgebung reagieren, indem sie die Tiefe des Gr{\"u}ndungsnestes durch das Absch{\"a}tzen der schon gegrabenen Tunnell{\"a}nge bestimmen. Das Nestgraben erfolgt allerdings nicht nach einem einfachen Stimulus-Antwort-Mechanismus, sondern die K{\"o}niginnen richten sich zus{\"a}tzlich noch nach der Zeit, was einen internen Messfaktor darstellt. Ebenfalls scheint die Kammergr{\"o}ße durch mindestens zwei Mechanismen reguliert zu werden. Somit fließen sowohl bei der Bestimmung der Tunnell{\"a}nge als auch bei der Regulation der Kammergr{\"o}ße mehrere Kriterien in die Entscheidung mit ein. Ebenso wie die K{\"o}niginnen reagieren einzelne Individuen auf unterschiedliche Stimuli in ihrer Umgebung, wodurch unterschiedliche Neststrukturen entstehen k{\"o}nnen. So fangen Ameisen an ein Nest zu vergr{\"o}ßern, wenn sich der zur Verf{\"u}gung stehende Platz innerhalb des Nestes reduziert. W{\"a}chst der Pilz so reduziert sich der Abstand zwischen Pilz und Kammerwand, was f{\"u}r die Tiere ein Signal ist, die Kammer zu vergr{\"o}ßern. Dabei wird der Pilz als Vorlage verwendet, der das Graben r{\"a}umlich organisiert. Ist der Platz innerhalb des Nestes dagegen aufgrund des Koloniewachstums reduziert, so fangen die Arbeiterinnen an Tunnel auszugraben, so dass die Nestgr{\"o}ße der Koloniegr{\"o}ße angepasst wird. Allerdings, so wird vermutet, h{\"a}ngt die Anzahl der sich am Graben beteiligenden Ameisen sowie auch deren Arbeitsleistung von der Gr{\"o}ße der Gruppe ab. Demnach sind die Individuen nicht nur sensitiv auf die Stimuli, die aus ihrer Umgebung kommen, sondern {\"a}ndern ihr Verhalten auch in Abh{\"a}ngigkeit von dem sozialen Umfeld, in dem sie sich befinden.},
  subject      = {Nestgr{\"o}ße},
  language  = {de}
}
@phdthesis{BollazziSosa2008,
  author    = {Bollazzi Sosa, Leonardo Martin},
  title     = {Building behaviour and the control of nest climate in Acromyrmex leaf-cutting ants},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-27610},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {This work was aimed at experimentally studying whether climatic variables act as environmental cues for workers' building behaviour in leaf-cutting ants of the genus Acromyrmex, and to what extent building responses account for the maintenance of nest climate in a proper range for the inhabiting colony. Specifically, this work presents independent analysis in different Acromyrmex species with disparate ecology and nesting habits, aimed at understanding to what extent: i) temperature and humidity act as cues for workers' building behaviour, ii) inter- and intraspecific differences in the nesting habits observed in South American Acromyrmex are based on distinct building behaviours and on the variation in regional climate across continent, iii) differences in nest architecture account for the maintenance of nest climate in a proper range for colony members and, iv) climatic variables trigger building responses aimed at controlling short-term changes in nest climate. It is first experimentally shown that soil temperature acts as a cue for workers' digging behaviour. Acromyrmex lundi workers were observed to respond to both soil temperature as well as its changes, and to decide accordingly where to start or whether to stop digging. The soil temperature range preferred by workers to dig, between 20°C and maximally 30.6°C, matches the range at which colony growth is expected to be maximized. Temperature-sensitive digging might therefore lead to the establishment of the fungus chambers in soil layers with a proper range of temperatures for colony growth. Based on that, it was hypothesized that nest depth in Acromyrmex largely depends on the depth at which this temperature range is located across the soil profile, i.e., the higher the temperature in the superficial soil layers, the deeper the nest location, since soil temperature decreases with increasing depth. A bibliographic survey on nesting habits of 21 South American Acromyrmex species confirmed that the warmer the soil temperature at 50 cm depth throughout the South American continent, the higher the number of species presenting subterranean nests, compared with those inhabiting superficial nests. Temperature-sensitive digging in Acromyrmex would therefore explain the geographical distribution of nesting habits observed for this genus in the South American continent, i.e., subterranean in the northern tropical regions, and superficial in the southern temperate ones. In addition, results showed that Acromyrmex colonies from temperate regions indeed achieve thermoregulatory benefits through the determination of nest depth based on thermoregulatory needs. In sympatrically-occurring colonies of the grass-cutting ant A. heyeri, temperature inside superficial thatched nests was higher, and more suitable for colony growth, than that inside subterranean nests. This temperature surplus was even higher in spring, at the time of production of sexual brood, than in winter or summer. It was demonstrated that such temperature surplus was brought about by the low thermal diffusivity of the nest thatch, which prevents diurnal nest overheating by the incoming solar radiation, and avoids losses of the accumulated daily heat into the cold air during night, thus leading to high average nest temperatures. Although highly advantageous for colonies in terms of nest temperature, the determination of nest depth based on thermoregulatory needs may differentially affect nest ventilation and humidity depending on how nest exposition influences the exchange of nest air with the outside air. For instance, colonies with a superficial nesting habit might benefit from improved nest ventilation, but be at risk of desiccation due to their exposition and the consequent humidity losses into the dry outside air. Results demonstrated that in two Acromyrmex species, short-term regulatory building responses triggered and spatially organized by climatic variables occur, and may counteract undesired changes in internal nest humidity. Workers of the thatching grass-cutting ant A. heyeri, for instance, closed a number of nest-thatch openings as a response to desiccation of the outside air, even at a nest temperature that otherwise triggered the response of opening them so as to reduce nest temperature. In the leaf-cutting ant A. ambiguus, the direction of the airflow inside nest tunnels was shown to act as a cue for spatially guiding the building behaviour of plugging nest entrances. However, workers only responded if the humidity content of the circulating air was low, trading therefore nest ventilation for humidity maintenance.},
  subject      = {Verhaltens{\"o}kologie},
  language  = {en}
}