Is Traffic Organized on Trails of the Leaf cutting Ant Atta cephalotes? Angela C. Rao Department of Zoology, University of Washington ___________________________________________________________________ ABSTRACT The leaf cutting tribe Attini Formicidae: Myrmicinae shows numerous adaptations that increase foraging efficiency. Such adaptations are seen in their highly polymorphic caste systems, trails branching from many different exit holes, clearing and maintenance of foraging trails, and pheromonal signaling and recruitment. It is reasonable to assume that mechani sms exist that would reduce incidence of collisions, traffic jams, etc., on foraging trails. This hypothesis was tested by recording trail density and time, linear distance, and direction of ant travel on the trails of Atta Cephalotes and looking for certa in trends. Ants traveling towards the nest were not found to vary much from those traveling away from the nest. Because of this and the lack of distance and speed dependence on ant density or direction, it was concluded that some form of organization may e xist on foraging trails. The most likely explanation for this is the presence of some organization method such as pheromone trails with a maximum trail width and yielding of ants traveling towards the foraging source to ants carrying leaves. RESU MEN La tri bu de cortadoras de hojas Attini Formicidae: Myrmicinae muestran muchas adaptaciones para aumentar la eficiencia de forrajeo. Esas adaptaciones se pueden ver en su alto polimorfismo de castas, el uso de senderos en aberturas diferentes de los nidos, el m antenimiento y limpieza de senderos y el uso de feromonas para marcar senderos. Es razonable asumir que existen mecanismos que podran reducir la incidencia de colisiones en los senderos. Esta hiptesis se probo al tomar dato s de densidad, tiempo, distanci a linear y la direcci n a la que la hormiga viaja en el sendero. Las hormigas que viajaron hacia el nido no mostraron diferencias con las que lo hicieron lejos del sendero. Porque no se encontraron una dependencia en la distancia y la velocidad con la dens idad o la direcci n, se concluy que algn tipo de organizacin debe existi r en los senderos. La explicaci n ms aceptable es que la organizaci n se da por el uso de feromonas con un ancho mximo de sendero donde las hormigas viajan acarreando las hojas. INTRODUCTION Leaf cutting ants of the tribe Attini Formicidae: Myrmicinae are critical herbivores of the Neotropics, cutting 12 17% of tropical leaf matter. Leaf tissue is then cultivated to grow a Basidiomycete fungus for larvae Holldobler and Wilson 1990, Wilson 1971. Atta cephalotes colonies are abundant in disturbed areas and secondary forest with nests
s ometimes containing over a million workers and a thousand fungus gardens Holldobler and Wilson 1990, Howard 1991, Hoyt 1996. Several castes contribute to the functioning of the colony. Attine ants have developed many adaptations that increase their foraging efficiency. There is an extremely polymorphic caste system that functions simila r to an assembly line Holldobler and Wilson 1990. Major workers, with their disproportionately enlarged, heart shaped heads, cut and carry the leaves. Minima occasionally ride on top of leaves carried by workers, possibly protecting the major workers fro m parasitic flies. Soldiers guard the nest, and different worker castes are involved in areas of cultivation of the fungus Leucocoprinm gonylophora Holldobler and Wilson 1990. The use of trunk trails up to 100 meters long, branching from various exit holes of the nest are also key in maintaining high efficiency levels as is the clearing and maintenance of those trails Holldobler and Wilson 1990, Howard 1991. Recruitment behavior as well as trail systems are very important adaptations for social insects Shepherd 1985. Leaf cutting ants mark their trails with pheromones that are stored in the poison gland and released through the sting Hogue 1993, Holldobler and Wilson 1990. Use of pheromones for recruitment is a highly evolved trait of some in sects. These behaviors combined with many other adaptations have caused leaf cutter societies to be regarded as extremely complex and resourceful. Foraging trails vary in activity with time, season, weather, colony size, etc. As ant density increases on tr ails, is foraging efficiency maintained? It is possible that efficiency declines. However, optimal efficiency theories would suggest in order for the colony's fitness can be maximized Alcock 1984, Howard 1991, Shepherd 1985. There may be
mechanisms, like "traffic patterns", to maintain trail efficiency. Such mechanisms have never been previously explored. This study attempted to explore the possibility that leaf cutters have developed organized traffic, such lane division, to further increase efficiency. The distance and speed of workers as they walk along trails was measured to study the effects of A. cephalotes density on trail efficiency. If the linear distance traversed by an ant in a given length of trail increases with ant density, efficiency has dec line. Likewise, if the speed at which ants traverse a given length of trail declines with ant density, efficiency would have decreased. If efficiency is maintained as ant density increases, distance traversed and speed should not change. In this event, tra ffic would require organization, perhaps along well defined "sub trails" or with traffic organized as on human highways, where one side is moving in only one direction. MATERIALS AND METHODS Research was conducted in the premontane moist life zone of Cai tas, Costa Rica Holdridge 1967 as well as on the premises of the Monteverde Ecofarm in Monteverde, Costa Rica. The ants were observed in disturbed areas that support several colonies of A. cephalotes, as well as during the daytime which is when one is mo st likely to find them foraging during the wet season Rockwood 1975. When a trail suitable for observation was located, the width was measured and a digital photograph taken of a 10 20 cm portion of the trail to later calculate ant density with. After a photograph was taken, two pieces of wood were placed on each side of the trail to prop up a meter long, 30 cm wide piece of observation glass. Using a dry erase marker, an estimate of the midline of the trail was marked on the glass. A single ant was the n observed as it traveled on the trail
from one end of the glass to the other. The ant's direction was recorded, alternating between away from and towards the nest, as was the time that the ant took to travel the meter long portion of trail. The ant's path of travel was traced on the glass with a dry erase marker and men transferred onto a piece of wax paper or tissue paper with the relative location of the nest indicated. The photographs taken that day were transf erred on a computer to be examined. The number of ants appearing in the photograph was counted and their direction of travel recorded, so long as they were within the 10 20 cm portion indicated by measuring tape la id n ext to the trail. The ant counts, trai l width, and photographed portion length were used to calculate the density of ants on the trails in ants/cm 2 String was laid down on the traced paths to determine the linear distance each ant traveled overall. This was repeated for the linear distance tr aveled on each side of the midline with respect to the nest, to determine if there was a preference for either side which would test the possibility of trail organization that is like human highways. RESULTS A correlation matrix revealed a significant relationship between trail width and the distance that returning ants those with leaves traveled see Table 1. In this case, wider trails resulted in longer, more circuitous routes for ants. All other significant correlations were autocorrelations of sp eed with completion time, and different measures of trail densities and trail distances. The average ant walked a distance of 108.4 cm 5.12 SD along the meter length of trails studied. Distance did not differ significantly between ants leaving the nest without leaves and those returning with leaves t=0.856, df =73, n=38 away, 37
towards, p=0.3947. Ants spent nearly equal time on left and right sides of the trail 56.2 32.1 left, t=1.475; 52.3 31.8 right, t= 1.389 and this was not different for ants coming or going 61.5 32.9 left away; 50.7 30.8 left towards; 47.4 33.3 right away; 57.5 29.7. The speed at which the ants walked the meter of trails studies is indicated by both completion time and speed. Completion time was, on average, 58.4 seconds 23.1. In this case, ants carrying leaves toward the nest were statistically slower, 61.5 s 18.9, than ants without away from the nest, 55.3 26.4 t= 1.164, df=73, n=38 away 37 towards, p=0.2482. Likewise, speed, which averaged 2.1 cm/s 0.74, was slower for ants with loads, some 1.9 cm/s 0.59, compared to those without, 2.3 0.82 t=2.318, df=73 n=38 away, 37 towards, p=0.0233. Trail width and ant density varied. Ave rage trail width was 11.8 cm 4.3 and ant density ranged from 0.01 ants/cm 2 to 2.4. Perhaps most important was the lack of correlation found between ant density and both distance traversed and speed. Figure 1 shows distance traveled with ant density. It i s clear that distance varied widely, even at low ant density 100 cm to 118 cm. At higher ant density ants tended to walk 105 110 cm, which is close to the mean of 108 cm. No significant relationship was found R 2 =0.007, p=0.4615. Similarly, Figure 2 shows speed with ant density. Again no significant relationship was found R 2 =0.010, p=0.4026 as speed varied widely both at low and high ant densities 0.75 cm/s to 4.3 cm/s. At high densities ants tended to walk w ith a speed of 1.5 2.0 cm/s, close to the mean of 2.0, and at low densities ants walked with a speed of 0.75 4.0 cm/s. Figure 3, showing speed with trail width, also shows no significant relationship R 2 =0.028, p=0.1535. Again, speed varied widely, both a t small and large trail widths 0.75 4.3 cm/s.
DISCUSSION Previously described attinian adaptations and optimal foraging theories Alcock 1984, Howard 1991, Shepherd 1985 suggest that a system of traffic on leaf cutter ant foraging trails would increase foraging efficiency and, hence, fitness. Such organization would explain the apparent lack of major collisions and gridlock on the trails of A. cephalotes. This question was explored by recording the travel of 75 individual ants. Ther e was virtually no difference found between the travel of ants heading towards the foraging site and ants carrying leaves back to the nest Figure 1. For both groups, the speed and distance traveled showed no dependence on the trail width or ant density. This suggests that there is some way that the ants can control for more ants on the trails and narrower trail widths, which would, if there was no trail organization, cause slower traffic and more confusion. The best explanation for the relationships found is that there is some form of organization on the foraging trails but not as highly organized as human highways. Ants do not organize themselves right and left, as ants tended to spend equal amounts of time on both sides of the trail. Whatever organizing principles are acting upon the trails were not measured directly in this study. A possible method for organization would have to have a maximum trail width to keep the confusion down, or else more ants, espec ially those carrying leaves, would be found to wander off. This reasoning comes from the finding that increasing trail width decreased efficiency in ants with leaves. There also seems to be a yielding rule seen in casual observation. The ants carrying leav es seem to have the right of way. It was also observed that when ants yielded to others, they seemed to try to return to their original path of travel suggesting a possible pheromonal cue.
Future studies could explore the possibility of these organized tra ils with maximum widths and yielding rules. Recording the number of collisions and who yields to who, in addition to the data taken during this experiment, would be a good beginning. It would also be good to record whether yielding ants return to their ori ginal paths after yielding to other ants and how long this takes. A. cephalotes ants apparently do not travel on their foraging trails in a random matter, but the methods of organization needs further investigation. ACKNOWLEDGEMENTS I would like to thank A lan Masters for his guidance and problem solving. I am also very grateful for the assist ance and support from the Santamar a family, Andrew Rodstrom, Will Wieder, Mauricio Garc a, and Karen Masters. The cooperation of the Monteverde Ecofarm was also essential to the success of this study, and I thank the people there. I am forever in debt to the ants who were kind enough to let me observe their lives and especially to the soldier ants who, in their better judgement, did not attack m e. I extend an apology as well, to the two soldier ants that I killed, one maliciously and the other by accident. LITERATURE CITED Alcock, John. 1984. Animal Behavior: An Evolutionary Approach. Third Edition. Sinauer Associates, Inc., Sunderla nd, Massachusetts. Hogue, C.L. 1993. Latin American Insects and Entomology. University of California Press, Berkeley, California. Holdridge, L.R. 1967. Life Zone Ecology. Revised Edition. Tropical Science Center, San Jos
Costa Rica. Holldobler, B. and E.O Wilson. 1990. The Ants. The Belknap Press of Harvard University Press, Cambridge, Massachusetts. Howard, J.J. 1991. Resource quality and cost in the foraging of leaf cutter ants. In: Ant Pla nt Interactions. Huxley, C.R. and D.F. Cutler eds.. Oxford Science Publications, Oxford, England, pp.42 50. Hoyt, E. 1996 the Earth Dwellers: Adventures in the Land of Ants. Touchstone, New York, New York. Longino, John T. and Paul E. Hanson. 19 95. The ants Formicidae. In: The Hymenoptera of Costa Rica. P. E. Hanson and I. D. Gauld, eds. Oxford University Press, Oxford, England, pp. 609 612. Rockwood, Larry L. 1975. The effects of seasonality on foraging in two species of Leaf cutting ants Att a in Guanacaste Province, Costa Rica. Biotropica 7: 3, pp. 176 193. Shepherd, John D. 1985. Adjusting foraging effort to resources in adjacent colonies of the leaf cutter ant, Atta colombica. Biotropica 17: 3, pp. 245 252. Wilson, E.O. 1971. The Insect Societies. The Belknap Press of Harvard University Press, Cambridge, Massachusetts.