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1 Effect of minima on the velocity of the workers in Atta cephalotes Nicholas J. Fabeck Department of Biology University of Minnesota-Twin Cities Abstract: The leaf-cutting ant, Atta cephalotes, creates trails of foragers which carry leaf fragments to the nest. A division of labor is created based on ant body size (Stevens 1983). The smallest members of the caste system, the minima, hitchhike on the leaves of workers returning to the nest. My study was performed on a large colony in Monteverde, Costa Rica to research the cost of effi ciency to the colony in relation to velocity with minima travelers. I hypoth esized that load size affects velocity of the worker and predicted that an increased load size will negatively influence velocity. I collected Atta workers (n = 240) with leaf fragments and their mini ma and measured their velocity and ant, minima, and fragment mass. Though it was discovered that more minima travel on smaller leaves ( 2 = 1990.13 p = < 0.001, df = 2), there was no significant relationship between increased load mass and velocity. The heavier load by the minima may be a trade off by foragers for the prot ection from parasitic flies that prey on workers. Resumen: La hormiga zompopa, Atta cephalotes crea senderos de forraj eo por los cuales cargan segmentos de hojas a sus nidos. Existe una divisin de trabajo basada en el tamao de la hormiga (Stevens 1983). Los miembros ms pequeos de este sistema de castas, las mnimas, viajan en las hojas de las obreras que regresan a sus nidos. Realice este estudio con una colonia grande en Monteverde, Costa Rica, con el propsito de investigar el costo en eficiencia a la colonia de las mnimas. Mi hiptesis era que el tamao de la carga afectara la velocidad de las obre ras y predije que un increm ento en el tamao de la carga influenciara negativamente la velocidad. Yo recolecte obreras de Atta con sus fragmentos de hojas y sus mnima y med su velocidad y el peso de la hormiga, mnima y hoja. Aunque descubr que ms mnimas viajan en hojas pequeas, no hubo una relaci n significativa entre peso y velocidad. Una carga ms pesada por causa de la mnima pueda ser un in tercambio por los forrajeador es por proteccin de las moscas parsitas que atacan obreras. Introduction: In neotropical forests, Atta cephalotes a leaf-cutting ant, create highways of individuals carrying various leaf fragments back to th eir underground nest. The colony consists of approximately five million ants set in a caste system based on size differences starting with minima ants approximately 2 mm in length, media workers 10 mm, and soldiers 20 mm (Stevens 1983). Together they work to make these organized highways, constantly cutting, carrying, and depositing leaves in the nest. These pieces are cut and chewed, then placed into a large fungus garden (Holldobler and Wilson 1990). Larger colonies will forage 20-80 m away from the nest harvesting from trees, whereas smaller colonies have physically smaller workers a nd take from ferns and grasses (Wetterer 1994). However, questions arise as to how the hitchhiking minima riding on leaf fragments aid leaf-carrying workers. A 1995 experiment in Costa Rica, found that vibrations produced by foragers while cutting leaves was extremely effective in recruitment of minima. Howeve r, minima need to be in close proximity of the noise to have an effect on recruitment (Roces and Holldobler 1995). A study in Panama discovered a beneficial rela tionship between minima and Atta workers with its predator (family Phoridae). The investigation showed that the presence of hitchhikers significantly reduced the parasites time spent on the leaves as well as the frequency of
2 attack (Feener Jr. and Moss 1990). Hitchhikers were also observ ed to be preparing leaves prior to being deposited as they were traveling (Linksvayer et al. 2002). The purpose of this study was to investigate if the minima have some effects on the velocity of leaf carrying ants in the colony. My hypothesi s is that load size effects velocity of the worker. I predicted that a) with the presence of minima it will increase the load size and have a negative influence in work ers velocity and b) wo rker ants of larger masses will carry increased load sizes as well as more minima and c) larger leaves will have more minima. Methods: The study was performed in Bajo del Tigr e, in Monteverde, Costa Rica. A large nest was used to collect observations. Two s ticks were placed vertically into the ground 70 cm apart from one another along the Atta highway. A stopwatch was used to record the time it took for the ant to travel the complete distance. Time began the moment an ant crossed the starting line (first stick) and ended when it reached the second stick. The first 50 ants whose velocities were recorded were collected with their leaf fragment and minima and placed into numbered vials. Ants were collected for five days, however only 40 ants were collected on day 3 due to poor weather conditions. Vial s were brought to the Monteverde Biological Station to measure an t mass, fragment mass, and minima mass on an electronic balance. Regressions were used to determine relationships for load sizes, minima mass, ant mass, and velocity. A ch i square test was also performed to study frequency of minima as it varied between l eaf size and velocity. Leaf categories were created based on mass: small (0.003-0.039 g), medium (0.040-0.079 g), and large (0.0800.130 g). Velocity categories are: slow (.0034-.0099 m/s), medium (.0100-.0199 m/s), fast (.0200-.0273 m/s). Large minima were .003 g or higher. Results: Leaf fragments mass ranged from 0. 003-0.126 grams. Ant mass ranged from 0.001-0.045 grams. Three regressions (Total load size Vs. velocity, Number of minima Vs. velocity, Leaf fragment size Vs. Veloc ity) were run and found to not be significant (Table 1). A positive relationship was seen between total load mass and ant size (F = 162.92, p = < 0.001, df = 1, figure 1a) and between minima mass and ant mass (F = 11.5, p = 0.0008, df = 1 figure 1b). The data also s howed a positive rela tionship between both mass of minima and leaf fragment size (F = 34.96, p = < 0. 001. df = 1, figure 2a) and number of minima versus leaf fragme nt size (F =12.58, p= 0.0005 df=1, figure 2b). These two regressions (Fig. 2a and 2b) dem onstrated relationships however both had low R2 values (0.128 and 0.050 respectively), whic h indicates a weak relationship. More minima travel on smaller leaves ( 2 = 1990.13 p = < .001, df = 2. figure3). A higher frequency of larger minima travel on leaves of slower workers (2 = 153.69 p = < 0.0001 df = 2, Figure 4). Discussion: The original hypothesis, that load size is related to velocity, was not supported by the data. An increased load size has no si gnificant relationship with velocity. Colony efficiency in terms of velocity is maintain ed with hitchhikers traveling on leaves. As predicted, larger ants carried heavier load sizes. This is corroborated by a study in 1994, which found that larger ants on average cut denser leaves (Wetter 1994). Carrying a
3 heavier load (which includes minima) may be more beneficial to the ant because of the protection from parasitic flies. Parasitic flies te nd to prefer larger ants (Tonhasca Jr. and Braganca 2000) and therefore ants that are mo re susceptible may choose to carry minima. The higher frequency of larger minima trav eling on slower individuals could also be explained by this benefit. The presence of minima on leaves decreases the frequency of harassment of predators (Feener Jr. and Moss 19 90). It is also possi ble that the slower workers may be more prone to attack. The discovery by Freener and Moss (1990) leads me to believe this decreased harassment is the reason why slower travelers have more minima in their velocity category. We can s ee that as velocity categories increase, the amount of total minima being car ried is decreasing (Fig. 3). It is important to note that there is a higher total amount of minima traveling on smaller leaves. Since load mass and speed both have a positive relationship with ant size smaller ants most likely were taking smaller leaf fragments with minima (Rezania 2004). Since this study was performed in the morning, smaller ants may have been more common. A study found that susceptible foragers are protected against parasitism by a shift in the worker size distribution toward smaller, unsusceptible sizes during the day when parasitoids are active and larger ants at night when parasitoids are inactive (Feener Jr and Brown 1993). Larger ants may be more likely to be seen at night carrying minima on their larger leaf fragments. As stated above the slower moving ants may necessitate minima for defensive purposes. Its also po ssible this behavior developed as a defense mechanism against parasitic f lies. Minima may appear larger to predators on smaller leaves as opposed to larger fragments and therefore reduce the amount of attack by flies. It would be interesting to see how minima body size plays a role in protecting the worker. Larger minima may cover more leaf area. Also a study on leaf size preferences of Phoridae flies attacking Atta cephalotes may shed light on this behavior. Future studies would benefit by using a diffe rent method of measuring the ants. Measuring body sizes, e.g. femur length, may lead to a more signifi cant relationships because of more precise measurements of minima. Acknowledgements: I would like to thank my project advisor, Tania Chavarria, for assisting me with data analysis and supporting me when initial experiment designs failed. The other members of the faculty I thank: Karen Masters for picking me up after my original site was bulldozed by construction workers, Pablo Allen for helping me with excel analysis and Spanish translation, Camryn Pennington for listening to me talk out my possible explanations and helping edit my paper as well as Hannah Frank for editing. Also, Jess and Maria for helping me weigh ants. Frank Joyce for allowing me to work on his property and the Monteverde Biological Station for giving me supplies to carry out this project. Literature Cited: Feener, D.H. Jr. and K. A.G. Moss. 1990. Defense against parasites by hitchhikers in leaf-cutting ants: a quantitative assessment. Behavioral Ecology and Sociobiology 26: 17-29. Feener, D. H. Jr. and B.V. Brown 1993. Ovip osition Behavior of an Ant-Parasitizing Fly, Neodohrniphora curvinervis (Diptera: Phoridae), and Defense Behavior by Its LeafCutting Ant Host Atta cephalotes (Hymenoptera: Formicidae). Journal of insect Behavior 6: 675-688. Holldobler, B. and E.O. Wilson. 1990. The Ants The Belknap Press of Harvard University press, Cambridge, Mass. pp. 596-600.
4 Linksvayer, T. and A.C. McCal1, R.M. Jensen, C.M. Marshall, J.W. Miner and M. J. McKone. The Function of Hitchhiking Behavior in the Leaf-cutting Ant Atta cephalotes Biotropica 34: 93-100. Rezania, Z. 2004. Worker Size, Load Ma ss, Speed and Foraging Efficiency in Atta cephalotes CIEE Tropical Ecology and Conservation Fall: 127-137. Roces, F. and B. Holldobler. 1995. Vibrational communication between hitchhikers and foragers in leaf-cutting ants (Atta cephalotes). Behavioral Ecology and Sociobiology 37: 297-302. Stevens, G. 1983 Atta cephalotes (Zompopas, leaf-cutting ants) In: Costa Rican Natural History, D.H. Janzen ed. University of Chicago Press, Chicago, pp. 688-691. Tonasca, A. Jr. and M.A.L Braganca 2000. Forager size of leaf-cutting ant Atta sexdens (Hymenoptera: Formicidae) in mature eucalyptus forest in Brazil. Revista de Biologia Tropical 48. Wetterer, J.K. 1994. Forager polymorphism, size-matching, and load delivery in the leaf-cutting ant, Atta cephalotes. Ecological Entomology 19: 57-64. Wetterer, J.K. 1994. Ontogenetic Changes in Forager Polymorphism and Foraging Ecology in Leaf-cutting Ant Atta cephalotes Oecologia 98 : 235-238.
5 Table 1: Regressions preformed for Atta comparing velocity and an independent factor found to be insignificant. The data included 240 individual Atta cephalotes workers carrying minima on leaves and was collected over 5 days in Monteverde, Costa Rica. Regression F value p value df R2 Total load size Vs. Velocity 0.363 0.547 1 0.001 Number of minima Vs. Velocity 3.14 0.079 1 0.012 Leaf fragment size Vs. Velocity 0.366 0.545 1 0.001 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 00.0050.010.0150.020.0250.030.0350.040.0450.05 Ant mass g A
6 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 00.0050.010.0150.020.0250.030.0350.040.0450.05 ant mass g Figure 1: A) Ant mass versus total lo ad mass. The minima mass and leaf fragment mass were added together a nd compared with work mass. The data included 240 Atta workers with minima th at were weighed at the Monteverde Biological Station in Costa Rica. A positive regression was seen with linear equation: to tal load size y = 0.011 + 2.08X (R2 = .406). B) Ant mass versus Mass of Minima (R2 =.046, linear equation: mass minima y = 0.001 + 0.056 X). Minima mass only re gistered if the vial was not zeroed on the scale; consequently only whole values were seen. B
7 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 leaf mass g 0 1 2 3 4 5 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 leaf frag size g Figure 2: A) Minima mass vs. Leaf mass. A positive relationship was shown in Atta workers collected in Costa Rica. Linear Relationship: mass minima y = 0.001 + 0.026X. B) Number of Minima vs. Leaf mass shows a positive relationship. Linear equation: # minima y = 1.10 + 8.22X A B
8 0 50 100 150 200 250 300 small medium big leaf size Figure 3: Total number of minima in l eaf size category. Leaves carried by Atta were subdivided by size: small (0.003-0.039 g), medium (0.040-0.079 g), and large (0.080-0.130 g). More minima were found on smaller leaves. 0 10 20 30 40 50 60 70 80 90 100 Slow mediumfast velocity category small big Figure 4: Total number of minima sepa rated by mass and velocity. Velocity categories are: slow (.0034-.0099 m/s), medium (.0100-.0199 m/s), fast (.0200-.0273 m/s). A large minima was .003 g or higher. There is a higher frequency of larger minima mass on i ndividuals in the slower category.
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Fabeck, Nicholas J.
Efecto del minima en la velocidad de los trabajadores en Atta cephalotes
Effect of minima on the velocity of the workers in Atta cephalotes
The leaf-cutting ant, Atta cephalotes, creates trails of foragers which carry leaf fragments to the nest. A division of labor is created based on ant body size (Stevens 1983). The smallest members of the caste system, the minima, hitchhike on the leaves of workers returning to the nest. My study was performed on a large colony in Monteverde, Costa Rica to research the cost of efficiency to the colony in relation to velocity with minima travelers. I hypothesized that load size affects velocity of the worker and predicted that an increased load size will negatively influence velocity. I collected Atta workers (n = 240) with leaf fragments and their minima and measured their velocity and ant, minima, and fragment mass. Though it was discovered that more minima travel on smaller leaves (2 = 1990.13 p = < 0.001, df = 2), there was no significant relationship between increased load mass and velocity. The heavier load by the minima may be a trade off by foragers for the protection from parasitic flies that prey on workers.
Las hormigas zompopas, Atta cephalotes, crean senderos de forrajeos por las cuales cargan segmentos de hojas a sus nidos. Existe una divisin de trabajo basada en el tamao de la hormiga (Stevens 1983). Los miembros ms pequeos de este sistema de castas, las minimas, viajan en las hojas de las obreras que regresan a sus nidos. Mi estudio se realiz en una colonia grande en Monteverde, Costa Rica con el propsito de investigar el costo de eficiencia de la colonia en relacin a la velocidad de los viajeros minimas.
Text in English.
Tropical Ecology 2007
Ecologa Tropical 2007
t Monteverde Institute : Tropical Ecology