TY - JOUR A1 - Roces, Flavio A1 - Pielström, Steffen T1 - Soil Moisture and Excavation Behaviour in the Chaco Leaf-Cutting Ant (Atta vollenweideri): Digging Performance and Prevention of Water Inflow into the Nest N2 - The Chaco leaf-cutting ant Atta vollenweideri is native to the clay-heavy soils of the Gran Chaco region in South America. Because of seasonal floods, colonies are regularly exposed to varying moisture across the soil profile, a factor that not only strongly influences workers' digging performance during nest building, but also determines the suitability of the soil for the rearing of the colony's symbiotic fungus. In this study, we investigated the effects of varying soil moisture on behaviours associated with underground nest building in A. vollenweideri. This was done in a series of laboratory experiments using standardised, plastic clay-water mixtures with gravimetric water contents ranging from relatively brittle material to mixtures close to the liquid limit. Our experiments showed that preference and group-level digging rate increased with increasing water content, but then dropped considerably for extremely moist materials. The production of vibrational recruitment signals during digging showed, on the contrary, a slightly negative linear correlation with soil moisture. Workers formed and carried clay pellets at higher rates in moist clay, even at the highest water content tested. Hence, their weak preference and low group-level excavation rate observed for that mixture cannot be explained by any inability to work with the material. More likely, extremely high moistures may indicate locations unsuitable for nest building. To test this hypothesis, we simulated a situation in which workers excavated an upward tunnel below accumulated surface water. The ants stopped digging about 12 mm below the interface soil/water, a behaviour representing a possible adaptation to the threat of water inflow field colonies are exposed to while digging under seasonally flooded soils. Possible roles of soil water in the temporal and spatial pattern of nest growth are discussed. KW - ants KW - fungi KW - surface water KW - vibration KW - acoustic signals KW - physical properties KW - analysis of variance KW - fungal structure Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-111298 ER - TY - THES A1 - Pielström, Steffen T1 - On the Role of Local Information in the Spatial Organisation of Collective Nest Digging in the Leaf-Cutting Ant Atta vollenweideri (Forel, 1893) T1 - Über die Rolle lokaler Information in der räumlichen Organisation des kollektiven Nestgrabeverhaltens bei der Blattschneiderameise Atta vollenweideri (Forel, 1893) N2 - 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. N2 - Viele Ameisenarten graben unterirdische Nester. Eines der wohl eindrucksvollsten Beispiele ist die Chaco-Blattschneiderameise Atta vollenweideri aus der Gran Chaco Region in Südamerika. Die Nester dieser Art gehören zu den größten von Insekten gebauten Strukturen auf unserem Planeten, und an ihrer Konstruktion sind vermutlich Millionen von Individuen über mehrere Jahre beteiligt. Die Mechanismen, die der Organisation des kollektiven Nestgrabeverhaltens zu Grunde liegen, sind weitgehend unbekannt. Berücksichtigt man allerdings allein die Dimensionen des gebauten Nestes im Vergleich zur Größe, und den vermutlich begrenzten Sinnes- und Kognitionsleistungen der einzelnen Arbeiterin, so liegt die Vermutung nahe, daß den Organisationsmechanismen überwiegend Reaktionen auf lokale Reize innerhalb der Wahrnehmungsreichweite zu Grunde liegen. Zu diesen lokalen Reizen gehören vermutlich Umweltvariablen, Reize, die mit den Bedürfnissen der Kolonie zusammenhängen, und Reize, die der räumlichen Koordination gemeinsamer Bemühungen dienen. Die vorliegende Dissertation untersucht die Rolle lokaler Reize in der Organisation des kollektiven Grabeverhaltens bei Atta vollenweideri auf diesen drei Ebenen. Sie beschreibt Experimente (1) zur Reaktion grabender Arbeiterinnen auf Unterschiede in der Bodenfeuchte, die in diesem Zusammenhang eine wichtige Umweltvariable darstellt, (2) zum Einfluß der verfügbaren Nestgröße auf die Nesterweiterung und (3) zur räumlichen Koordination grabender Arbeiterinnen durch Reize, die von anderen grabenden Arbeiterinnen ausgehen. Die Versuche zur Bodenfeuchte zeigen eine Präferenz für feuchte Materialen, die hohe Grabe- und Transportraten ermöglichen. Demzufolge kann die Verteilung des Wassers im Boden um ein Nest erheblichen Einfluß auf die Entwicklung der Nestform haben. Andererseits zeigen die Ergebnisse aber auch, daß Arbeiterinnen vermeiden, in extrem nassen Materialen zu graben. In Anbetracht der regelmäßigen Überflutungen in der Gran Chaco Region lässt sich diese Ablehnung als eine Adaptation interpretieren, die hilft, Wassereinbrüche ins Nest zu vermeiden. In den Experimenten zum Einfluß der Nestgröße gruben die Ameisen weniger, wenn ihnen ein größeres Nest zur Verfügung stand. Wenn das Nest im Laufe des Grabeprozesses künstlich schlagartig vergrößert wurde, passte sich die Graberate dem neuen Volumen an. Diese Beobachtungen bestätigen die Hypothese, daß die Grabeaktivität abhängig vom Platzbedarf reguliert wird, vermutlich, weil die Individuendichte im Nest das Grabeverhalten beeinflusst. Andererseits zeigen die Beobachtungen aber auch eine zeitabhängige, intrinsische Abnahme der Motivation zu graben. Zudem korreliert die Graberate nur dann mit der Nestgröße, wenn Nester von vergleichbarer Form in Betracht gezogen werden. Die Verteilung eines ähnlichen Nestvolumens auf drei kleinere Kammern statt einer größeren führte zu deutlich niedrigeren Graberaten. Eine mögliche Erklärung für diese Beobachtung liegt in der Verteilung der Arbeiterinnen im Nestinnern, die möglicherweise mit der Nestgeometrie variiert. Unterschiede in der Verteilung der Individuen können bei gleichem Nestvolumen unterschiedliche lokale Individuendichten zur Folge haben. Des weiteren werden zwei verschiedene Stimuli beschrieben, die zur räumlichen Koordination des kollektiven Grabeverhaltens genutzt werden. Zum einen erhöht sich die Wahrscheinlichkeit, daß eine ankommende Arbeiterin sich den Grabearbeiten an einer bestimmten Stelle anschließt, wenn auf dem Weg dorthin, nahe der betreffenden Stelle, frisch ausgegrabene Bodenpartikel, sogenannte pellets, abgelegt wurden. Das passiert in Folge der sequentiellen Arbeitsteilung beim Transportieren der Erde, denn der Transport der pellets erfolgt in Arbeitsketten, ähnlich wie der Transport von Blattfragmenten an der Oberfläche. Zum anderen stridulieren die Arbeiterinnen beim Graben und erzeugen dadurch Vibrationssignale von kurzer Reichweite, die ebenfalls Nestgenossinnen zum Graben an derselben Stelle motivieren. Es werden also zwei sich gegenseitig ergänzende Mechanismen beschrieben, die es erlauben, grabende Arbeiterinnen an einer bestimmten Stelle zu konzentrieren. In beiden Fällen erhöht ein Reiz, der durch aktuelle Grabeaktivität in unmittelbarer Nähe generiert wird, die Wahrscheinlichkeit, daß der Reizempfänger in der Nähe anderer grabender Arbeiterinnen zu Graben beginnt. Besonders in einem ansonsten, in Bezug auf das Grabeverhalten reizarmen Umfeld können derartige Mechanismen wichtig sein, um den kollektiven Grabebemühungen eine gemeinsame Richtung zu geben. Dementsprechend lässt sich argumentieren, daß die räumliche Organisation des Grabeverhaltens auf der Nachahmung von Entscheidungen basiert. Individuen kopieren die Entscheidungen ihrer Nestgenossinnen, wo gegraben wird, indem sie auf Reize reagieren, die von der Grabeaktivität anderer Arbeiterinnen ausgehen. KW - Blattschneiderameisen KW - Nestbau KW - Verhalten KW - Organisation KW - Stridulation KW - Transport KW - Boden KW - leaf-cutting ants KW - nest building KW - organisation KW - stridulation KW - transport Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-79118 ER - TY - JOUR A1 - Pielström, Steffen A1 - Roces, Flavio T1 - Sequential Soil Transport and Its Influence on the Spatial Organisation of Collective Digging in Leaf-Cutting Ants JF - PLoS ONE N2 - The Chaco leaf-cutting ant Atta vollenweideri (Forel) inhabits large and deep subterranean nests composed of a large number of fungus and refuse chambers. The ants dispose of the excavated soil by forming small pellets that are carried to the surface. For ants in general, the organisation of underground soil transport during nest building remains completely unknown. In the laboratory, we investigated how soil pellets are formed and transported, and whether their occurrence influences the spatial organisation of collective digging. Similar to leaf transport, we discovered size matching between soil pellet mass and carrier mass. Workers observed while digging excavated pellets at a rate of 26 per hour. Each excavator deposited its pellets in an individual cluster, independently of the preferred deposition sites of other excavators. Soil pellets were transported sequentially over 2 m, and the transport involved up to 12 workers belonging to three functionally distinct groups: excavators, several short-distance carriers that dropped the collected pellets after a few centimetres, and long-distance, last carriers that reached the final deposition site. When initiating a new excavation, the proportion of long-distance carriers increased from 18% to 45% within the first five hours, and remained unchanged over more than 20 hours. Accumulated, freshly-excavated pellets significantly influenced the workers' decision where to start digging in a choice experiment. Thus, pellets temporarily accumulated as a result of their sequential transport provide cues that spatially organise collective nest excavation. KW - animal behavior KW - ants KW - confidence interval KW - decision making KW - foraging KW - fungal structure KW - fungi KW - hormone transport Y1 - 2013 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-96275 ER -