“Building behaviour and the control of nest climate in Acromyrmex leaf-cutting ants” Dissertation zur Erlangung des naturwissenschaftlichen Doktorgrades der Bayerischen Julius-Maximilians-Universität Würzburg vorgelegt von Leonardo Martin Bollazzi Sosa Aus Las Piedras, Uruguay. Würzburg, März 2008 2 Eingereicht am: 19 März 2008 Mitglieder der Promotionskommission: Vorsitzender: Prof. Dr. M. J. Müller Gutachter: Prof. Dr. Flavio Roces Gutachter: Prof. Dr. Judith Korb Tag des Promotionskolloquiums: 28 Mai 2008 Doktorurkunde ausgehändigt am: …………………………………………………… 3 Contents Summary..................................................................................................................................6 Zusammenfassung ...............................................................................................................9 1. Introduction and general aim...................................................................................12 1.1. Specific aims and experimental approach.........................................................14 2. Thermal preference for fungus culturing and brood location by workers of the thatching grass-cutting ant Acromyrmex heyeri.................................................16 2.1. Introduction............................................................................................................16 2.2. Methods..................................................................................................................18 2.3. Results....................................................................................................................19 2.4. Discussion..............................................................................................................22 3. Soil temperature, digging behaviour, and the determination of nest depth in South American species of Acromyrmex leaf-cutting ants.....................................25 3.1. Introduction............................................................................................................25 3.2. Methods..................................................................................................................27 3.2.1. Effect of soil temperature on worker’s digging performance ..........28 3.2.2. Soil temperature selected by workers to start digging.....................28 3.2.3. Workers responses to increasing and decreasing soil temperatures while digging .............................................................................30 3.2.4. Soil temperature and observed nest depths in South American species of the genus Acromyrmex.................................................................31 3.3. Results....................................................................................................................33 3.4. Discussion..............................................................................................................41 3.4.1. Behavioural responses to soil temperature.......................................41 3.4.2. The determination of nest depth in Acromyrmex .............................44 3.4.3 Nest depth as an adaptation for the maintenance of proper nest temperatures .....................................................................................................46 4. Adapting to temperate climate through building behaviour: comparative thermal biology in thatched and subterranean grass-cutting ant nests (Acromyrmex heyeri)..........................................................................................................48 4.1. Introduction............................................................................................................49 4.2. Methods..................................................................................................................50 4 4.2.1. Fungus garden temperature in both thatched and subterranean nests ...................................................................................................................51 4.2.2. The effect of incoming solar radiation and colony presence on fungus temperature of thatched nests...........................................................52 4.2.3. Thermal properties of thatch and soil.................................................53 4.3. Results....................................................................................................................54 4.3.1. Fungus garden temperature: seasonal comparisons between thatched and subterranean nests ..................................................................54 4.3.2. The effect of incoming solar radiation on daily changes of fungus garden temperature in both nest types .........................................................56 4.3.3. The effect of colony presence on the temperature of thatched nests ...................................................................................................................61 4.3.4. Thermal properties of the thatch material and soil...........................62 4.4. Discussion..............................................................................................................63 4.4.1. Temperature in thatched and subterranean nests: the thatch limits heat exchanges with the environment...........................................................63 4.4.2. The maintenance of proper temperature in thatched nests............66 4.4.3. Thermoregulatory benefits of thatched nests ...................................67 4.A. Appendix................................................................................................................69 4.A.1. Fungus garden temperature in both thatched and subterranean nests ...................................................................................................................69 4.A.2. The effect of incoming solar radiation and colony presence on fungus temperature of thatched nests...........................................................69 4.A.3. Thermal properties of thatch and soil ................................................70 4.A.4. Nest morphology of thatched and subterranean nests...................71 5. Building behaviour for climate control in leaf-cutting ants: Acromyrmex heyeri workers trade off thermoregulation for humidity control............................74 5.1. Introduction............................................................................................................74 5.2. Methods..................................................................................................................77 5.3. Results....................................................................................................................79 5.4. Discussion..............................................................................................................82 6. To build or not to build: circulating dry air organizes collective building for climate control in the leaf-cutting ant Acromyrmex ambiguus ..............................86 6.1. Introduction............................................................................................................86 5 6.2. Methods..................................................................................................................89 6.2.1. Experimental setup ...............................................................................89 6.2.2. Experimental series...............................................................................91 6.2.3. Experimental procedure .......................................................................93 6.3. Results....................................................................................................................93 6.4. Discussion..............................................................................................................96 7. General discussion .......................................................................................................99 7.1. Climatic variables as cues for building behaviour: the maintenance of a proper nest climate .....................................................................................................100 7.2. Use of climatic factors as cues for building: its adaptive value ...................101 References..........................................................................................................................105 Erklärung.............................................................................................................................119 Lebenslauf...........................................................................................................................120 Publikationsliste ................................................................................................................121 Danksagung........................................................................................................................123 6 Summary The present 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