Dokument-ID Dokumenttyp Verfasser/Autoren Herausgeber Haupttitel Abstract Auflage Verlagsort Verlag Erscheinungsjahr Seitenzahl Schriftenreihe Titel Schriftenreihe Bandzahl ISBN Quelle der Hochschulschrift Konferenzname Quelle:Titel Quelle:Jahrgang Quelle:Heftnummer Quelle:Erste Seite Quelle:Letzte Seite URN DOI Abteilungen OPUS4-6730 Dissertation Pielström, Steffen On the Role of Local Information in the Spatial Organisation of Collective Nest Digging in the Leaf-Cutting Ant Atta vollenweideri (Forel, 1893) 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. 2013 urn:nbn:de:bvb:20-opus-79118 Theodor-Boveri-Institut für Biowissenschaften