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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
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, Cost a 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 worke r and predicted that an increased load size will negatively influence velocity. I collected A tta 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 Ã°c 2 = 1990.13 p = < 0.001, df = 2, there was no significant rela tionship 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. Resumen : La hormiga zompopa, Atta cephalotes , crea senderos de forrajeo por lo s cuales cargan segmentos de hojas a sus nidos. Existe una divisiÃ³n de trabajo basada en el tamaÃ±o de la hormiga Stevens 1983. Los miembros mÃ¡s pequeÃ±os de este sistema de castas, las mÃnimas , viajan en las hojas de las obreras que regresan a sus nid os. Realice este estudio con una colonia grande en Monteverde, Costa Rica, con el propÃ³sito de investigar el costo en eficiencia a la colonia de las mÃnimas. Mi hipÃ³tesis era que el tamaÃ±o de la carga afectarÃa la velocidad de las obreras y predije que u n incremento en el tamaÃ±o de la carga influenciarÃa negativamente la velocidad. Yo recolecte obreras de Atta con sus fragmentos de hojas y sus mÃnima s y medÃ su velocidad y el peso de la hormiga, mÃnima y hoja. Aunque descubrÃ que mÃ¡s mÃnimas viajan en hoj as pequeÃ±as , no hubo una relaciÃ³n significativa entre peso y velocidad. Una carga mÃ¡s pesada por causa de la mÃnima pueda ser un intercambio por los forrajeadores por protecciÃ³n de las moscas parÃ¡sitas que atacan obreras. Introduction: In neotropical fo rests, Atta cephalotes , a leaf cutting ant, create highways of individuals carrying various leaf fragments back to their 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 and take from ferns and grasses Wetterer 1994. Howeve r, 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. Howe ver, 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 relationship between minima and Atta workers with its predator
2 family Phoridae. The investiga tion showed that the presence of hitchhikers significantly reduced the parasite s time spent on the leaves as well as the frequency of attack Feener Jr. and Moss 1990. Hitchhikers were also observed to be preparing leaves prior to being deposited as the y 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 hypothesis 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 worker s velocity and b worker ants of larger masses will carry increased load sizes as well as more minima and c larger leaves will have more minima. Method s: The study was performed in Bajo del Tigre, in Monteverde, Costa Rica. A large nest was used to collect observations. Two sticks 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. Vials were brought to the Monteverde Biological Station to measure ant 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 chi square test was also performed to study frequency of minima as it varied between leaf size and velocity. Leaf categories were created based on mass: small 0.003 0.039 g, medium 0.040 0.079 g, and large 0.080 0.130 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. Velocity were run and found to not be significan t 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 showed a positive relationship bet ween both mass of minima and leaf fragment size F = 34.96, p = < 0. 001. df = 1, figure 2a and number of minima versus leaf fragment size F =12.58, p= 0.0005 df=1, figure 2b. These two regressions Fig. 2a and 2b demonstrated relationships, however b oth had low R 2 values 0.128 and 0.050 respectively, which indicates a weak relationship. More minima travel on smaller leaves Ã°c 2 = 1990.13 p = < .001, df = 2. figure3. A higher frequency of larger minima travel on leaves of slower workers Ã°c 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 significant relationship with velocity. Colony
3 efficiency in terms of velocity is maintained 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 heavier load which includ es minima may be more beneficial to the ant because of the protection from parasitic flies. Parasitic flies tend to prefer larger ants Tonhasca Jr. and Braganca 2000 and therefore ants that are more susceptible may choose to carry minima. The higher fr equency of larger minima traveling 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 1990. It is also possible that the slower workers ma y 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 see that as velocity categories increase, the amount of total minima being carried 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 distrib ution 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. It s also possible this behavior developed as a defense mechanism against parasitic flies. 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 f lies attacking Atta cephalotes may shed light on this behavior. Future studies would benefit by using a different method of measuring the ants. Measuring body sizes, e.g. femur length, may lead to a more significant relationships because of more precise me asurements 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. an d 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. Oviposition Behavior of an Ant Parasitizing Fly, Neodohr niphora curvinervis Diptera: Phoridae, and Defense Behavior by Its Leaf Cutting Ant Host Atta cephalotes Hymenoptera: Formicidae. Journal of insect Behavior 6: 675 688.
4 Holldobler, B. and E.O. Wilson. 1990. The Ants The Belknap Press of Harvard Univer sity press, Cambridge, Mass. pp. 596 600. 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. Wor ker Size, Load Mass, 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 cephalote s. 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 200 0. 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 c ephalotes. 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 f or 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 R 2 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
6 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 Ant mass g Total Mass g 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 ant mass g leaf frag size Figure 1: A Ant ma ss versus total load mass. The minima mass and leaf fragment mass were added together and compared with work mass. The data included 240 Atta workers with minima that were weighed at the Monteverde Biological Station in Costa Rica. A positive regression was seen with linear equation: total load size y = 0.011 + 2.08X R 2 = .406. B Ant mass versus Mass of Minima R 2 =.046, linear equation: mass minima y = 0.001 + 0.056 X. Minima mass only registered if the vial was not zeroed on the scale; consequently only whole values were seen. A 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 Minima mass 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 number minima 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 + 0.0 26X. 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 number of minima Figure 3: Total number of minima in leaf size category. Leaves carried by Atta were subdivided by size: sma ll 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 medium fast velocity category Number of minima small big Figure 4: Total number of minima separated 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 individuals in the slower category.