THE EFFECTS OF WIND ON FORAGING SRATEGIES OF ATTA CEPHALOTES LEAF- CUTTER ANTS By MICHAEL JOHN RODRIGUEZ B.S., Southeastern Louisiana University, 2011 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Master of Science Department of Ecology and Evolutionary Biology 2014 This thesis entitled: The effects of wind on foraging strategies of Atta cephalotes leaf-cutter ants written by Michael John Rodriguez has been approved for the Department Ecology and Evolutionary Biology Dr. Michael Breed _____ Dr. Rebecca Safran Date The final copy of this thesis has been examined by the signatories, and we Find that both the content and the form meet acceptable presentation standards Of scholarly work in the above mentioned discipline. iii Rodriguez, Michael John (M.S., Department of Ecology and Evolutionary Biology) The effects of wind on foraging strategies of Atta cephalotes leaf-cutter ants Thesis directed by Professor Dr. Michael Breed Abstract: Successful foragers alter their behavior in response to variation in local conditions, resulting in reduction of foraging costs and maximization of resource gain. In eusocial colonies, individuals may adjust their own efforts to maximize the productivity of the colony as a whole. Maximization of colony productivity can be achieved through sub-maximal individual performance. Attine leaf-cutter ant foragers often cut leaf fragments shorter than the hind legs could allow, suggesting other factors contribute to load size determination. Several studies have shown reasons why leaf-cutter ants cut smaller loads than they could maximally carry. The effects of wind speed on leaf-cutter load size selection were examined in this study and showed conditions in which leaf-cutter foragers change their behavior to cut larger loads than normal. In response to wind treatments, foragers cut larger leaf loads and fewer minima workers hitchhiked on those leaves. This study highlights behavioral plasticity of leaf-cutter foragers in response to local conditions on the foraging trail, and it adds to our knowledge of resource allocation in central-place foraging systems. Key words: behavioral flexibility, ethology, foraging, load size selection, parasitism, social insects iv Acknowledgements Special thanks for funding from the Organization for Tropical Studies, the University of Colorado, Boulder Department of Ecology and Evolutionary Biology, Colorado Diversity Initiative, and Petridish.org contributors including: Sandra Blair, Michael Bonner, Kevin Westmoreland, Jasmine Hegman, Nicole Gorden, James Waters, Jason Steiner, Samuel J. Ciurca, Jr., James Jorasch, Gaurav Vaidya, Jaime Pawelek, Daniel Ewald, Kwame Hagan, Nicholas Barbieri, Bob Peake, Kenneth Trease, Kelly Stewart, Mustafa, Susan Fay, and Alan F. Rodriguez. Thank you to the staff and researchers of La Selva and the participants and instructors of OTS Tropical Biology course 2012-1. Thanks to my adviser, Dr. Michael Breed, and lab mates for guidance and reviews. Thank you to Dr. Catherine and Mac Macgregor. v CONTENTS Introduction...............................................................................................1 Leaf-cutter ant background.................................................................2 Foraging efficiency ......................................................................... 3-7 Hitchhiker ants ..............................................................................7 Purpose of the study ............................................................................8 Methods ............................................................................................... 9-13 Results.. ............................................................................................. 13-19 Discussion ......................................................................................... 20-24 Conclusion .................................................................................. 24-25 References ......................................................................................... 26-30 vi FIGURES Figure 1. Leaf fragment area between wind treatments ...............................................14 2. Leaf fragment area and forager head width................................................. 15 3. Forager head width between wind treatments ..............................................15 4. Interaction of forager head width and leaf fragment area between wind treatments................................................................................................16 5. Hitchhiker numbers between wind treatments ..............................................17 6. Foraging trail traffic density and leaf fragment mass ..................................18 7. Leaf fragment area between leaf type ...........................................................19 8. Forager head width between leaf type ..........................................................19 1 Introduction Successful foragers alter their behavior in response to a shifting external environment (Charnov 1976). This results in reduction of foraging costs and maximization of resource gain if local conditions vary. In eusocial colonies, individuals specializing in certain tasks may adjust their own efforts to maximize the productivity of the colony as a whole (Burd 1996a; Constant et al. 2012). Theory predicts that individuals adjust their foraging efforts to achieve optimal resource input into the nest (Charnov 1976; Wetterer 1989; Burd 1996a; Burd and Howard 2008). Interactions among nestmates and external cues allow individuals to alter their behavioral effort, especially as different biotic (parasitism, predation, nest-mate interactions) and abiotic conditions (rain, wind, temperature) vary (Hölldobler and Wilson 1990; Feener and Moss 1990; Gordon 2002; Angilletta et al. 2008; Elizalde and Folgarait 2012). Behavioral variability within a colony and behavioral flexibility displayed by individuals may contribute to the evolutionary success of colony phenotypes and to the ecological success of the eusocial Hymenoptera (Wilson 1971; Constant et al. 2012). My results from studies of foraging in a leaf-cutter ant, Atta cephalotes, add to our understanding of the complex interplay between costs and benefits in central place foraging systems and how individual behavior and recruitment behavior of leaf- cutter ants is plastic in response to abiotic and biotic external conditions, such as wind (Rudolph and Loudon1986), traffic on the foraging trail (Farji-Brener et al. 2011), or the presence of very small workers, called minima, riding on a carried load (Feener and Brown 1992; Orr et al. 1995; Porter et al. 1995). Leaf-cutting ants 2 A small percentage of ant species have polymorphic workers (Wetterer 1994), and within the fungus growing ants, family Formicidae, subfamily Myrmicinae, tribe Attini, only some species have workers with different size castes. Workers of the monomorphic Attini forage for insect carcasses, waste, and other detritus that do not require further processing. The polymorphic Attini are specialist foragers on vegetable matter (seeds, flowers, and leaves). This plant material needs special treatment to convert to a suitable substrate for fungal gardens grown by the ants and used as food. Handling of plant material in preparation for use in fungal gardening includes fragmentation, chewing, and regurgitation onto the fungal garden (Wilson 1980a, b). Leaf-cutting ants in the genera Atta and Acromyrmex have highly polymorphic workers, resulting in a high division of labor with workers specializing on tasks appropriate for their size (morphological polyethism) (Wilson 1980a, b). In Atta cephalotes leaf-cutter ants, the species addressed in this study, the largest workers can be 200 times the size of the smallest workers (Weber 1972). The large, major, workers and intermediate, media, workers act as soldiers and foragers, made effective by their large bodies and mandibular muscles. The smaller, or minima, workers take care of the brood and garden and maintain the trails (Hughes and Goulson 2001). Inside the nest, minima gardeners fragment the forage into a pulp that they feed to basiodiomycetes fungi which are cultured as the primary food source for their developing larvae. To a lesser extent adults also feed on the fungi (Weber 1972; Quinlan & Cherrett 1979; Benckiser 2011). The fungal gardens are analogous to human agriculture (Benckiser 2011). Minima workers are primarily fungal caretakers but have a secondary role as defenders of adult ants against parasitic phorid flies (Phoridae: Pseudoacteon sp.). Minima workers hitchhike rides aboard leaf fragments carried back by larger workers (Wilson 1980a, b; Feener and Moss 3 1990; Vieira-Neto et al. 2006) and fend off these flies. Foragers can travel for hours over long distances on dedicated and manicured trails, recorded to span 300 m from the nest entrance, with a mean distance of 50 m (Lewis et al. 1974; Wetterer 1990). Foragers travelling back to the nest carrying loads are more vulnerable to parasitoid attack than unladen workers, and hitchhiking ants offer defense, rearing up on hind legs to grab the fly or to block oviposition (Feener and Moss 1990; Vieira-Neto et al. 2006). Considering the long travel times of foragers and their vulnerability while carrying loads, hitchhikers offer a valuable defense from parasitoid enemies. In addition to defense, hitchhikers sometimes work to remove trichomes on