Leaf Area, Worker Size and Number of Hitchhiking Minima in Atta cephalotes Daniel Shimek Department of Biology, Gustavus Adolphus College ABSTRACT Several species of leaf cutter ants, including Atta cephalotes exhibit an interesting behavior of hitc hhiking minima. Minima are the smallest of the polymorphic castes of leaf cutter ants and the function and origin of this hitchhiking behavior is not completely understood. There are several hypotheses for this behavior, and the most popular include defense against parasites and cleaning the leaf fragment of contaminants. Studies have investigated some patterns of hitchhiking behavior including different frequencies during day and night and distance from colony. This study examined the rel ationship between head width, leaf area and the number of hitchhiking minima in A cephalotes Ants were collected from a single colony in Bajo del Tigre, Monte verde, Costa Rica. The number of minima, head width, and lea f fragment area were recorded. Ants with mimina present had significantly higher mean head width and carried leaf fragments with greater mean area than those without. This implies that larger ants could require more defenses from parasites or larger leaf fragments contain more contaminants. RESUMEN Varias especies de zompopas, incluyendo Atta cephalotes exhiben un comportamiento interesante de las mnimas que viajan sobre las hojas. Mnima es la ms pequea de las castas de las zompopas y la funcin y el origen de este comportamiento no es completamente entendido an. Existen varias hiptesis para est e comportamiento, y la ms popular incluye la defensa contra parsitos y limpiar los pedazos de hoja de contaminantes. Estudios han investigado algunos patrones de este comportamiento incluyendo diferentes frecuencias durante el da y la noche y distancia al nido. Este estudio examina la relacin entre el ancho de la cabeza, rea de la hoja y el nmero de mnimas en A. cephalotes Las hormigas fueron recolectadas de un nico nido en Bajo del Tigre, Monteverde, Costa Rica. El nmero de mnimas, ancho de la cabeza y rea del fragmento cargado se anotaron. Hormigas con mnimas presentan cabezas significativamente ms grandes y adems tambin cargan trozos ms grandes que aquellos sin mnimas. Esto implica que hormigas ms grandes pueden necesitar ms defe nsas contra parsitos o que los trozos ms grandes contienen ms contaminantes. INTRODUCTION Atta cephalotes is a species of leaf cutter ant that harvests leaf fragments in order to grow a fungus for consumption. These leaf cutter ants are important and d ominant he rbivores in tropical forests and can consume 12 17% of total leaf production per year (Hlldobler & Wilson 1990). Leaf cutter ants have polymorphic castes There are three worker castes consisting of soldiers who are th e largest and defend other ants. Soldiers are found on the trails and within the nest. Media ar e the foragers and generalists. Media are the most common caste found on trails and perform the function of cutting and carrying leaf fragments to the nest. M inima are the smallest cast e and are most commonly found within the nest caring for the fungus and larvae However, minima are unusually and often found outside the nest along trails and often times hitchhiking on leaves carried b y media foragers This hitchhiking behavior is not ve ry common on the trails (Feener & Moss 1989).
There have been many suggested hypotheses for the p resence of minima hitchhikers. The most popular and prevalent hypothesis is that minima defended the for agers from parasitoids such as p horid flies (Phoridae) (Vieira Neto et al 2006). The flies lay eggs on the head of the ant and the larvae hatch and burrow into the head of the ant, eventually killing it. Leaf carriers can attempt to defend themselves but are rarely successful (Feener & Moss 1989). Minima ha ve been observed in an erect head up posture apparently ready to defend the media (Linksvayer et al 2002) However, phorid flies are largely diurnal in natural systems and hitchhiking behavior has been observed during nocturnal foraging (Vieira Neto et al 2006). Most minima found on trails are not hitchhiking so other explanat ions of hitchhiking behavior have been hypothesized. It is possible that hitchhiking may be done to conserve energy, feed on sap, clean leaf fragments or are marooned on the leaf fr agment ( Linksvayer et al 2002) The energy conservation hypothesis states that minima that hitchhike do so to conserve energy on their way back to the nest. However the support for this hypothesis is weak at best (Feener & Moss 1989). Worker ants of A. c ephalotes have been shown to consume a large amount of leave sap to fulfill metabolic energy requirements (Griffith &Hughes 2010). Some scientists have suggested that hitchhiking ants simply get marooned on leaf fragments and are unable to leave the fragme nt (Viera Neto et al 2006). This suggestion has largely been refuted in recent studies. Minima are known to remove contaminants in the fungus garden and could perform this function on leaf fragments before they enter the colony. Minima have been observed in a head down posture licking the surface leaf fragment (Linksvayer et al 2002) S tudies have shown increased occurrence of hitchhiking on contaminated leaf fragments and corn flakes (Vieira Neto et al 2006, Griffiths & Hughes 2010). As the functions of hitchhiking are becoming better understood, studies have investigated further patterns of hitchhiking. One pattern that has been found is that h itchhiking occurs more commonly in the forest canopy where foraging occurs rather than close to the nest ( Zi sook 2006, Griffiths & Hughes 2010 ). However there is little information on the factors that cou ld increase the likelihood of a minima hitchhiking su ch as leaf area and head size of the laden ant Parasitic flies tend to prefer larger ants (Tonasca & Braga nca 2000). Several phorid flies land on the leaf fragment in order to lay eggs in the head of the ant (Vieira Neto et al 2006). One species of phorid fly ( Apocephalus attophilus ) has be en shown to not parasitize ants without leaves (Feener & Moss 1989). This suggests ants with larger bodies and heads that carry larger leaf fragments would be at a g reater risk from phorid flies. This is based on the assumption that a larger head correlates to a bility to carry a larger leaf. These ants would therefore benef it from frequently having hitchhikers. Also, a larger leaf are could possibly contain more contaminants making it a high priority for cleaning. These predictions formed th e purpose for this experiment. The objective of this study was to examine whether a wider head or a larger leaf head would predict a higher occurrence of m inima hitchhiking.
METHODS Study Site This study was conducted on the property of Frank Joyce near Bajo del Tigre Rese rve in Monteverde, Costa Rica. The life zone of the study site is considered premontane wet forest with an elevation of ~1400 meters and mean annual rainfall of 2000 4000mm (Harber 2000) Experimental Procedure Five hundred and twenty eight ants were collected during the week of November 11 th 18 th 2010, between 10am 2pm. I observed 264 laden ants whose leaves had zerp minima, 240 with one minima and 23 with two minima. Ants were collected from a single col ony and a single trail. The specific trail was chosen because it was observed to conta i n the highest density of ants. I collected all ants from the same location on the trail approximately 15m from the colony. A marker was placed a long the trail and the first media which crossed the marker without minima was collected. Head width was calcula ted as the distance between the ey es and measured with a caliper. I measured the leaf area using a transparent grid featuring 25 mm 2 squares. I rejected ants carrying leaf stems or other leaf fragments which could not be measured with the grid. In order to collect medias with minima, the vicinity around the marker was observed and the first ant with a hitchhiker encountered was collected. I measured head width and leaf area in the same manner as ants without hitchhikers. Statistical Analyses I tested the mean head size by category of number of minima for statistically significant differences using a one way ANOVA A one way ANOVA was used to deter mine differences in leaf area. A Tukey Kramer HSD was used for post hoc comparisons between the thr ee categories for both head size and leaf area. The correlation between head size and leaf area was tested by a bivariate linear fit. RESULTS Head Size The mean head size for all ants was 0.2284915 cm ( 0.0431126 ). There was a significant difference in the mean head length between ants with zero, one, and two minima ( One Way ANOVA, F=3.4482, df=2, p=0.0325, R 2 =0.01299 Fig. 1). Post hoc tests revealed mean head width of ants with zero minima ( 0.223617 cm 0.00264) was lower than ants with one minima (0.233604 cm 0.00277 ; Tukey Kramer HSD ). Ants with two minima had a mean head width of 0.231087 cm 0.00895 Leaf Area The mean leaf area of all ant fragments was 85.94047619 mm 2 ( 31.314012 ). The maximu m and minimum leaf areas were 250 and 18.75 mm 2 respectively. The mean leaf area between laden ants carrying leaves with zero minima, one minima, and two minima differed significantly (One Way ANOVA, F= 7.9001 df=2, p= 0.0004 R 2 = 0.02927 Fig. 2). Mean leaf area in laden ants carrying leaves with one and two minima was greater than laden ants carrying leaves with zero minima (Tukey Kramer HSD) There was a
20% increase in mean leaf area from zero ( 81.274 1.9024 ) to two minima ( 102.717 6.4454 ). Ants with one minima showed a 10% increase in mean lea f area compared to those with zero minima ( 89.401 1.9953 and 81.274 1.9024 respectively). Similar to head width, the R 2 value for leaf area and number of minima is quite low ( 0.02927 ) suggesting other factors might be a str onger indicator. Head Width and Leaf Area Correlation The re was a positive correlation between head width and leaf area ( F= 150.7042 df=1, p<0.0001, R 2 = 0.223033 Fig. 3). The ant with the largest head width of 0.52c m carried a leaf fragment of 137.5 mm 2 An ant with a head width of 0.42cm carried the largest l eaf fragment, with an area of 250mm 2 The smallest ant, 0.145cm head width, was found wit h a leaf fragment area of 37.5mm 2 The smallest leaf fragment was 18.75mm 2 and was transported by an ant wit h a head width of 0.16cm. Head size and leaf area were positively correlated (R 2 = 0.223033 ). This suggests ants with larger heads are capable or more likely carry larger leaf fragments. Also it can be inferred that ants with large heads and larger leafs tog ether have the greatest frequency of minima hitchhikers. DISCUSSION A plausible explanation for why hitchhikers are likely to be found on ants with larger heads is the ant protection hypothesis that states hitchhikers defend the leaf carriers from ovipositor attacks (Feener & Moss 1989) P arasites prefer larger ants since they are more visible targets (Tonasca & Braganca 2000 ) Minima have been shown to suc cessfully def end against phorid flies (Linksvayer et al 2002). It has been hypothesized that the presence of hitchhikers can reduce parasitism by up to 75% (Feener & Moss 1989). Media ants can attract minima through short ranged communications including pheromones (Vie ira Neto et al 2006). Perhaps most hitchhiking minima only do so when summoned by a forager. This evidence strongly suggests that minima play an important role in parasitoid defense. Hitchhikers may be found more often on larger leaf fragments for seve ral reasons. Some species of phorid flies land on leaf fragments prior to ovipositing eggs so larger leaf fragments might increase the chance of being parasitized (Griffiths & Hughes 2010). A larger leaf fragment might be beneficial to the fly to avoid de tection or act as a landing pad or bulls eye. This puts ants carrying a large leaf fragment at a higher risk. It is also possible that larger leaf fragments might contain more fungal contaminants beca use of increased area. Minima c ould respond by hitchhiking and cleaning these larger leaf fragments before they enter the colony and contaminate the fungus garden. Another possibility is that larger leave s could contain more leaf sap. Many of Atta spp. workers obtain most of t heir metabolic energy from sap. Perhaps larger leaf fragments contain more sap and th is entices minima to hitchhike. There are potential future studies that could addre ss previously raised questions. To determine whether larger ants are at a higher risk, one could examine the rate s of phorid parasitism and compare the mean head size of ants that become pa rasitized to those that do not. This could clarify whether the increased frequency of hitchhiking on large ants is a response to phorid flies. Another interesting study could focus on the contaminants on leaf fragments immediately after cutting to determine whether amount
of contaminant cover dif fers by size of leaf fragments. This could confirm or refute one of my speculations as to why minima are frequently found on larger leaf fr agments. Hitchhiking on leaves in A. cephalotes appears to be a behavioral adaptation to several selective pressures including parasite defense and fungal contaminants. This behavior seems to have evolved multiple times in different species of ants ind icating a similar pattern of ovipositor behavior in their parasites. ACKNOWLEDGEMENTS I would like to thank my advisor Anjali Kumar for help and guidance in fin ding and completing this study. Moncho Caldern receives thanks for his patience and help. Ra quel Martinez deserves thanks for her helpful information and insight into this project. I would finally like to thanks Pablo Allen for assistance with the statistical analyses and Frank Joyce for allowing me to use the Atta cephalotes colony located on his property. LITERATURE CITED Feener D.H. and K.A.G. Moss. 1989. Defense against parasites by hitchhikers in leaf cutting ants: a quantitative study. Behavioral Ecology and Sociobiology 26: 17 29. Grif fiths, H.M., and W.O.H. Hughes. 2010. Hitchhiking and the removal of microbial contaminants by the leaf cutting ant Atta colombica Ecological Entomology 35: 529 537. Harber, W. A. 2000. Plants and Vegetation. In: Monteverde: Ecology and Conservation of a Tropical Cloud Forest N. M. N adka rni and N. T. Wheelwright, eds. Oxford University Press, New York, NY Hlldobler, B. and E.O. Wilson. 1990. The Ants Harvard University Press, Cambridge, MA. Linksvayer, T.A., A.C. McCall, R.M. Jensen, C.M. Marshal l, J.W. Miner, and M.J. McKone. 2002. The function of hitchhiking behavior in the leaf cutting ant Atta cephalotes Biotropica 34(1): 93 100. Tonasca, A. Jr. and M.A.L. Braganca 2000. Forager size of leaf cutting an Atta sexdens (Hymenoptera: Formicidae) in matu re eucalyptus forest i n Brazil. Revista de Biologia Tropical 48. Vieira Neto, E.H.M., F.M. Mundim, and H.L. Vasconcelos. 2006. Hitchhiking behavior in leaf cutter ants: An experimental evaluation of three hypoth eses. Insectes Sociaux 53: 326 332. Zisook, R. 2006. Variation of hitch hiking incidence and density in space and time in two Atta cephalotes colonies: effects of the presence of phorid flies. University of California, Educati on Abroad Program, Spring 2006.
Figure 1: The mean head length of Atta cephalotes ants featuring three different amoun ts of hitchhiking minima. Similar letters indicate statistically significant difference in means ( One Way ANOVA, F=3.4482, df=2, p=0.0325, R 2 =0.01299 ). The object of this study was to determine what individual characteristics of A. cephalotes workers could be used to predict frequency of hitchhikers. A single colony was studied in Monteverde, Costa Rica. 0.21 0.215 0.22 0.225 0.23 0.235 0.24 0.245 Zero One Two Head Width (cm) Number of Minima A A
Figure 2: The mean area of leaf fragments carried by Atta cephalotes workers and the corresponding number of minima hitchhikers. Similar letters indicate statistically significant differences ( One Way ANOVA, F=7.9001, df=2, p=0.0004, R 2 =0.02927 ). It was hypothesized that mimina would be more frequently found on leaves with larger areas. 0 20 40 60 80 100 120 Zero One Two Leaf Area (mm2) Number of Minima A B B A
Figure 3: Correlation of head width (cm) and leaf area (mm 2 ) of 528 Atta cephalotes media workers ( Bivariate linear fit, F=150.7042, df=1, p<0.0001, R 2 =0.223033 ) It was predicted that ants with larger bodies could possibly carry larger leaf fragments or larger leaf fragments might require larger workers to carry. 0 50 100 150 200 250 0.1 0.2 0.3 0.4 0.5 Leaf Area (mm2) Head Width (cm)
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rea foliar, tamao de los trabajadores, y el nmero de mnima en Atta cephalotes
Leaf area, worker size, and number of hitchhiking minima in Atta cephalotes
Several species of leaf-cutter ants, including Atta cephalotes, exhibit an interesting behavior of hitchhiking minima. Minima are the smallest of the polymorphic castes of leaf-cutter ants and the function and origin of this hitchhiking behavior is not completely understood. There are several hypotheses for this behavior, and the most popular include defense against parasites and cleaning the leaf fragment of contaminants. Studies have investigated some patterns of hitchhiking behavior including different frequencies during day
and night and distance from colony. This study examined the relationship between head width, leaf area and the number of hitchhiking minima in A. cephalotes. Ants were collected from a single colony in Bajo
del Tigre, Monteverde, Costa Rica. The number of minima, head width, and leaf fragment area were recorded. Ants with mimina present had significantly higher mean head width and carried leaf fragments with greater mean area than those without. This implies that larger ants could require more defenses from parasites or larger leaf fragments contain more contaminants.
Varias especies de zompopas, incluyendo Atta cephalotes, exhiben un comportamiento interesante de las mnimas que viajan sobre las hojas. Mnima es la ms pequea de las castas de las zompopas y la funcin y el origen de este comportamiento no es completamente entendido an. Existen varios hiptesis para este comportamiento, y la ms popular incluye la defensa contra los parsitos y limpiar los pedazos de hoja de los contaminantes. Los estudios han investigado algunos patrones de este comportamiento incluyendo las diferentes frecuencias durante el da y la noche y la distancia al nido. Este estudio examina la relacin entre el ancho de la cabeza, el rea de la hoja y el nmero de mnimas en A. cephalotes. Las hormigas fueron recolectadas de un nico nido en Bajo del Tigre, Monteverde, Costa Rica. Se anotaron el nmero de mnimas, el ancho de la cabeza y el rea del fragmento cargado. Las hormigas con mnimas presentan cabezas significativamente ms grandes y adems tambin cargan trozos ms grandes que aquellos sin mnimas. Esto implica que las hormigas ms grandes pueden necesitar ms defensas contra los parsitos o que los trozos ms grandes contienen ms contaminantes.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone
Alimentos de animales
Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology Fall 2010
Ecologa Tropical Otoo 2010
t Monteverde Institute : Tropical Ecology