1 Effect of Ground Temperature and Nest Differences on Productivity and Worker Size in Atta cephalotes (Hymenoptera: Formicidae) Hannah Fried Petersen Departments of Environmental Studies and Biology, The University of Pittsburgh ABSTRACT Atta cephalot es is one of the few species that does better in grassy pastures and secondary growth and t heir abundance, persistent foraging, and that they collect from a lot of different tree species has earned them the reputation of huge agricultural pests A. cephalotes are p oikilotherms meaning their internal body temperature fluctuates greatly with temperature. For this reason, it has been purported that these leaf cutter ants will only collect leaf fragments at certain temperatures and that different sized workers have dif ferent temperature tolerances. The objective of this study was to look for relationships between temperature, productivity, and worker size in nests in Monteverde, Costa Rica. I productivity was more responsive to changes in temperature and that nest and temperature interact to predict worker size during foraging Because o f A large impact on the e c osystem in Monteverde (whether viewed as good or bad), it is important and interesting to know more about what regulates the speed of their foraging. RESUMEN Atta cephalotes es una especie que se dessrrolla mejor en pastos y bosques de crecimiento secundario. Su abundancia, forrajeo persistente, y el hecho que recolectan muchos tipos diferentes de rboles les da la reputacin de grandes pestes agrcolas. A. cephalotes es una especie poiquilotrmica lo que significa que la tem peratura interna de su cuerpo flucta con la temperatura ambiental. Por esta razn, se piensa que las hormigas que cortan hojas solamente recogen fragmentos de hojas durante temperaturas especificas y que trabajadores de tamaos diferentes tienen toleranci as de temperature diferentes. El objetivo de este estudio fue entender las relaciones entre temperatura, productividad, y tamao de trabajadores de esta especie en Monteverde, Costa Rica. Descubr que la productividad de algunos nidos fue mas afectada por la temperatura que otros.Tambin que temperatura e identidad del nido interactuan para predecir el tamao de los trabajadores durante el forrajeo. Dado el impacto grande que tiene Atta cephalotes en los ecosistemas de Monteverde (si bueno o malo), es imp ortante e interesante saber ms de lo que limita y acelera el forrajeo. INTRODUCTION For social and poikilothermic insects such as the leaf cutting ant Atta cephalotes temperature affects development, phenology and for aging activity (Crist & Williams 1999) It has been suggested that there is a range of temperatures under which the A. cephalotes worker caste will forage A. cephalotes is found throughout Costa Rica mainly in pastures or secondary growth (Hlldobler & Wilson 1990). These leaf cutters are broadly polymorphic ants the workers range in size from 2mm in length to 20 mm in length (with workers within a single colony possibly ranging 250 fold in mass) (Stevens 1983) In another species of thermophilic ants ( Cataglyphis velox ), it was found that thermal tolerance was size related with la rger workers being able to withstand higher
2 temperatures (Cerda & Retana 1997). It has been suggested that colony fitness is related to fungus production (which is related to foraging productivity) Additionally, another study on the army ant Eciton bu rchellii found that the ants retreated from warm edges, steered around hot patches in the forest, and appear ed in many ways to be living near the upper limit of their temperature tolerance. This study also found that A. cephalotes was more tolerant of high temperatures than E. burchellii but still showe d a n egative response to temperature (Meisel 2006). Atta cephalotes nests are very common in Monteverde, Costa Rica. The majority of the A. cephalotes nests in Monteverde are in the sun (in cleare d pastures or young secondary growth). However, some nests are completely shaded. Also, the topographic locations of the nests are very varied. There is not much known about how the interaction between temperature and foraging is affected by topographic po sitioning but it is clear that there is an effect. Because A. cephalotes has this optimum temperature range that influences the number of foraging hours they have in a day, correlations between the productivity and thus fitness of A. cephalotes in the M onteverde area should be related to temperature. The goal of this study was to investigate the relationship between temperature and productivity as well as temperature and worker size in Atta cephalotes. The results were expected to show a peak and trough in foraging activity along a temperature range (productivity declining when it is too hot or too cold). MATERIALS AND METHODS S TUDY AREA The study was conducted on three different Atta cephalotes nests within Monteverde, Costa Rica. Monteverde is classified as Tropical Lower Montane Wet Forest. Nest one is in an open pasture owned by Don Juan coffee company. The grass growing in the pasture is about one foot high and the nest is in full sun. Also, the nest itself is barely visible partly because of the grass but also maybe because a large part of it is underground. Ne st two nest seems has a very large obvious pile where the ants are discarding. Nest three is on private property in Caitas. This nest is also in an open pasture b ut the grass here is much shorter. This nest appears to be significantly younger than the other two judging from the size of the nest. Nest t wo in Bajo del Tigre is at 1300m elevation and the other two nests are slightly higher in elevation. D ATA COLLECTION All three nests had multiple established collecting paths. For all three nests I chose one path based on which had the most activity. Measurements and collecting were taken one meter from the entrance to the ne st on workers returning to the nest Data were collected every half hour while observing. Every 30 minutes, I took ground and air temperature measurements. I took ground temperature measurements by placing the thermometer on the ground (in the sun if prese nt) along the collecting path. I allowed temperature to stabilize for three minutes before recording. For air temperature I held the thermometer in the sun and allowed it to stabilize for three minutes. At a I collected the first 20 workers carrying leaf fragments to pass. Additionally at this time, I counted how long it took 50 carrying
3 workers to pass that same point (as a measure of productivity). Data were collected early in the morning and later in the afternoon (when temperatures drastically rise and fall) in an attempt to collect specimen and record productivity for the full range of temperatures experienced by the nest. Finally head measurements for the collected ants were taken using a caliper. Therefore each 30 minute temperature and productivity measurement had 20 measured worker sizes to go along with it. These 20 measurements were averaged to give an average worker size associated with each temperature measurement. RESULTS G R OUND TEMPERATURE EFF ECT ON PRODUCTIVITY Regression analyses were r u n for all three nests. Ground temperature and air temperature were highly correlated; ground temperature was used a s it i s the most influential for ant activity Nest o ne (Fig. 1) showed a negative trend though it was not statistically significant (F= 2.621, P=0.1191, df=24), there being a slight increase in productivity with an increase in temperature. Nest t wo (Fig. 2) shows no relationship between ground temperature and productivity (F=0.3306, P=0.5811, df=9). The re lationship between ground temper ature and productivity in nest t hree is marginally significant (F=4.7072, P=0.0582, df=10 ; Fig. 3 ) meaning that productivity increases as temperature increases. FIGURE 1. Re lation between ground temperature a nd productivity (as measured by the time it took 50 Atta cephalotes ants to pass a designated point along collecting path) in nest o ne (Don Juan) (N=24). Regression was not significant (p > 0.05).
4 FIGURE 2. Relation between ground temperature and productivity (as measured by the time it took 50 Atta cephalotes ant s to pass a designated point along collecting path) in n est t wo (Bajo del Tigre). (N=10). Regression was not significant (p > 0.05). FIGURE 3. Relation between ground temperature and productivity (as measured by the time it took 50 Atta cephalotes ants to pass a designated point along collecting path) in n est t hree (open field in Caitas ). (N=11). Regression was marginally significant (p = 0.058 )
5 W ORKER SIZE DIFFERENC ES BETWEEN NESTS It was found that there is a significant difference in average worker size between th e nests (F = 17.8716, df = 41, p < 0.0001 ; Fig 4 ) n est o ne and n est t wo have significantly bigger ants than n est t hree T here are no significant differences between n est o ne and n est t wo FIGU RE 4. A verage carrying worker size for each of the three A. cephalotes nests. An ANOVA was used to determine differences between nests, same letter above columns mean similar size of ants (Tukey test, p > 0.05). E FFECT OF TEMPERATURE AND NEST ON WORKER S IZE Figure 5 plots the relationship between temperature and size of workers for all three nests on the same graph. I performed an analysis of covariance (ANCOVA) to evaluate how worker size related to ground temperature and nest. The ANCOVA model expl ained 68% of the variance in workers size ( F = 15.84, df = 42, p < .0001 ) There is a significant effect of nest (F = 18.19 df = 2, p < .0001) and of ground temperature (F = 12.12 df = 1, p = 0.0013 ), including an intera c tion between the 2 variables (F = 4.47, df = 2, p = 0.018 ) This means that there were different re s ponses towards increasing ground temperature between nests, as can be seen in Fig. 5. There was a b igger effect in ne st two than in nests one and t hree.
6 FIGURE 5 Ground t e ffect on the size of carrying workers (measured by head size s of collected workers ) (N=43). DISCUSSION The inconsistent relationship between first 50 (product ivity) and ground temperature in nests one, two, and three is probably due to differences between the nests. Differences in colony size, foraging rhythms or microclimate location could be causing the nests to respond differently to variable thermal enviro nments (Crist & Williams 1999). None of the nests were found to have a definitive minimum and maximum temperature when they either start or stop working, respectively (although for nest one there is a point when ground temperature reached 36 o C and the colony stopped working) For nest t wo, this can probably be explained by the fact that the colony only experiences a small range of temperatures because it is in the shade. Had there been larger fluctuations in temperature, the colony may have showed a r esponse in productivity. Nest t hree is the youngest of the nests so the workers are the smallest. In 2007, Azc rate et al. found that worker speed (and thus productivity) had a higher positive correlation with temperature for smaller ants. This could expl ain the statistically significant relationship between temperature and productivity in nest t hree. Nests one and t wo had on average bigger worker s size than nest t hree. This can be explained by the relationship between colony age and worker size. Older colonies have larger workers (Wilson 1983).
7 The trend that arises Figure 5 is particularly interestin g. Plo tted independently, only nest t wo shows a significant decrease of size wit h increasing temperature though nest one shows the same trend. Nest t hree simply does not have the same size range of workers that the other two nests have. So, sending out larger workers is not even an option small workers have to be working all the time because there are only small workers in the nest. It does seem that the smaller workers in n est t hree start working at warmer temperatures. Nests one and two have lar ger work while nest t The overall trend is only appreciated when all data is considered together it may be that the full range of worker size (which can only be modeled if all three nests are together) is needed to see the interaction. This model show s us again that there are significant differences between nests that explain muc h of the variation. Crist and Williams ( 1999 ) found that ant response to soil temperature may be affected by topographic variability, the amount of plant cover on or around the nests, or natural shifts in foraging rhythms. All of these factors are certainly different between the three nests. All three nests are act ing differently This implies that sweeping hypotheses and predictions about Atta cephalotes must consider differences between every nest. But the study did also show th at ground temperature affects productivity and size at least for some nests. This conclusion is interesting and implies some limits and activation factors for a keystone species in tropical ecosystems. AC KNOWLEDGMENTS This project was made possible by all of the professors of the CIEE Tropical Ecology and Conservation program. I would especially like to thank Pablo Allen for advising my research and always being available, and the teaching assistants Yime n Araya and Moncho Caldern for their patience and resourcefulness. I would like to thank Frank Joyce for letting me observe a nest on his property. I would also like to thank Katie Graziano and Gracy Huntley for showing me nests. LITER ATURE CITED Azc rate, F. M., E Kovacs, and B. Peco. 2007. Microclimatic Conditions Regulate Surface Activity in Harvester Ants Messor barbarus. Journal of Insect Behavior, Vol. 20, No. 3. Cerda, X., and J. Retana. 1997. Links between Worker Polymorphis m and Thermal Biology in a Thermophilic Ant Species. Oikos, Vol. 78, No. 3: 467 474. Crist, T.O. and J.A. Williams. 1999. Simulation of Topographic and Daily Variation in Colony Activity of Pogonomyrmex occidentalis (Hymenoptera: Formicidae) Using a Soil Temperature Model. Environmental Entomology 28: 659 668. Hlldobler, B., and E.O. Wilson. 1990. The Fungus Growers The Ants. Harvard University Press. pp 596 608. Meisel, J.E. 2006. Thermal Ecology of the Neotropical Army Ant Eciton burchelli Ec ological Applications, 16(3): pp. 913 922.
8 Stevens, G. C. 1983. Atta cephalotes. Costa Rican Natural History D.H. Janzen, ed. The University of Chicago Press. pp 688 690. Stradling, D.J. 1978. The Influence of Size on Foraging in the Ant, Atta cephalote s and the Effect of Some Plant Defence Mechanisms. Journal of Animal Ecology 47: 173 188 Whitford, W. G., and G. Ettershank. 1975. Factors affecting foraging activity in Chihuahuan desert harvester ants. Environ. Entomol. 4: 689 696. Wilson, E. O. 1983. Caste and Division of Labor in leaf cutter ants (Hymenoptera: Formicidae: Atta). Behavioral Ecology and Sociobiology 14: 55 60.
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Efecto de la temperatura del suelo y las diferencias de jerarqua en la productividad y el tamao de los trabajadores en Atta cephalotes (Hymenoptera Formicidae)
Effect of ground temperature and nest differences on productivity and worker size in Atta cephalotes (Hymenoptera Formicidae)
Atta cephalotes is one of the few species that does better in grassy pastures and secondary growth and their abundance, persistent foraging, and that they collect from a lot of different tree species has earned them the reputation of huge agricultural pests. A. cephalotes are poikilotherms meaning their internal body
temperature fluctuates greatly with temperature. For this reason, it has been purported that these leaf-cutter ants will only collect leaf fragments at certain temperatures and that different sized workers have different temperature tolerances. The objective of this study was to look for relationships between temperature, productivity, and worker size in nests in Monteverde, Costa Rica. It was found that some nests productivity was more responsive to changes in temperature and that nest and temperature interact to predict worker size during foraging. Because of A. cephalotes large impact on the ecosystem in Monteverde (whether viewed as good or bad), it is important and interesting to know more about what regulates the speed of their foraging.
Atta cephalotes es una especie que se desarrolla mejor en los pastos y en los bosques de crecimiento secundario y su abundancia, forrajeo persistente, y el hecho que recolectan muchos tipos de rboles diferentes les da la reputacin de grandes pestes agrcolas. A. cephalotes es una especie poiquilotermia lo que significa que la temperatura interna de su cuerpo flucta con la temperatura ambiental. Por esta razn, se piensa que las hormigas que cortan-hojas solamente recogen los fragmentos de las hojas durante las temperaturas especficas y que los trabajadores de los diferentes tamaos tienen tolerancias de temperatura diferentes. El objetivo de este estudio fue entender las relaciones entre la temperatura, la productividad, y el tamao de trabajadores de esta especie en Monteverde, Costa Rica. Descubr que la productividad de algunos nidos fue ms afectada por la temperatura que otros. Tambin que la temperatura e identidad del nido interactan para predecir el tamao de los trabajadores durante el forrajeo. Dado el impacto grande que tiene Atta cephalotes en los ecosistemas de Monteverde (ya sea visto como bueno o malo), es importante e interesante saber ms de lo que limita y acelera el forrajeo.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone
Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology Spring 2010
Ecologa Tropical Primavera 2010
t Monteverde Institute : Tropical Ecology