Eciton burchell i i : Polymorphism of Submajor Caste and Foraging E fficiency Samantha Alger Department of Biology , University of Rhode Island ABSTRACT Due to the link between efficiency and fitness, there should be selective pressure for morphology and behavior that promotes foraging efficiency. Among social insects, selective pressures act on individuals , shaping the physical castes of a colony. The highly polymorphic army ant, Eciton burchelli i , has castes with morphological adaptations to allow task specialization . This study investigates what selective pressures are acting to shape the polymorphism of the submajor caste in E. burchell i i . Ants were collected to find a relation ship between ant caste and the preyÂ€s biomass, width, and length . Results show that prey width is the greatest pressure e ffecting transportation efficiency , and in turn, shaping the evolution of the specialized porter caste i.e., submajor . Results also indicate that of all the castes, submajors are found to be the most morphologically different and the y carry the most different sized prey. Possible explanations to Eciton Â€s physically exaggerated submajor caste include the ir outstanding need of transport efficiency and unique prey preferences. RESUMEN Debido a la uniÃ³n entre eficiencia y Ã©xito reproductivo , debe haber una presiÃ³n selectiva por comportamiento y morfologÃa que promueve la eficiencia en el forrjeo. Entre insectos sociales, las presiones selectivas actÃºan en individuos dando origen a difer entes castas en la colonia. La altamente polimÃ³rfica hormiga arriera, Eciton burchelli , posee castas con adaptaciones morfolÃ³gicas que permiten estas especializaciones. Este estudio investiga que presiones selectivas actÃºan moldeando el polimorfismo de l a casta submayor. Hormigas fueron colectadas para encontrar una relaciÃ³n entre la casta y la biomasa de la presa, ancho y largo. Los resultados muestran que el ancho de la presa es el mayor factor que ejerce una presiÃ³n selectiva en la eficiencia de tran sporte, por lo tanto, moldeando la evoluciÃ³n de la casta. Los resultados tambiÃ©n indican que todas las castas, submayores parecen ser las mÃ¡s diferentes morfolÃ³gicamente y estas cargan presas de diversos tamaÃ±os. Posibles explicaciones para las castas ex ageradas de Eciton incluyen una necesidad sobresaliente de transporte y preferencia Ãºnica de presas. INTRODUCTION Fitness is increased when an animal maximizes foraging efficiency, which generally correlates to a faster handling time Orians & Pearson 1979 ; Krebs & Davies 1993. For social insects , like army ants, g reater foraging efficiency indicates an increase in colony growth rate, and a greater fitness of all members of a colony Franks 1985 a . Due to this relationship between efficiency a nd fitness, there should be selective pressure for morphology and behavior th at promotes foraging efficiency. Socialization among insects, and especially ants, has allowed for their incredible ecological success Gotwald 1995 ; Powell & Franks 2005 . Thei r success is expressed in the fact that while ants and termites make up only 2% of the earth s 900,000 known insect species, they constitute for more than half of the insect biomass Wilson & Ho ll dobler 2005. An aspect
of insect sociality allowing for thi s success is caste systems. The creation of worker castes is driven by the increasing efficiency of division of labor in a colony Oster & Wilson 1978. This efficiency comes with specialized work forces with task dependent adaptations. These adaptations can be expressed as morphological differences among castes . A rmy ants , a highly specialized group, have castes that are behaviorally as well as morphologically separated Harvell 1994 . While it is clear that these morphological and beh avioral differences create efficiency among castes, the ecological pressures causing these differences are not yet clear. The circumstances needed for the evolution of specialized castes presents a key issue in the understanding of social species. Eciton bu r chellii , a well studied army ant species, has behavioral characteristics that may have great selective pressure on maximizing transportation efficiency. The ir aboveground life style puts them more at risk to kleptoparasites during prey retrieval than any other species of New World army ants , which typically spend life partly or exclusively subterranean Rettenmeyer 1963; Powell & Franks 2005. Th is vulnerability may play a role in the fast tempo of E . burchellii Rettenmeyer 1963 . Lower transportation costs benefit E . burchellii by helping them to avoid kleptoparasitism. In addition, their strict raid migration schedule , dependent on larval development , brings further time limitations on foraging time Franks et al. 1999 . E citon burchellii caste evolution may be partly driven by this need to maximize foraging and transportation efficiency. Morphology varies greatly among the workers of E. burchellii ; corresponding to different behavioral role s, or castes, within the colony Powell & Franks 2005, Franks 2005, Franks 1985b. A morphologically exaggerated submajor caste exists in E. burchellii . This caste is more distinct and exaggerated in E. burchellii than in any other army ant species known to have this caste Powell & Franks 2005. Submajors are the second largest caste next to majors i.e., soldiers and have disproportionately large legs, head, and grasping mandibles Franks 1985 b , Powell & Franks 2005 . These characteristics make submajors better porters of prey Franks 1985b; Gotwald 1995; Franks et al. 1999; Powell & Franks 2005 . Because their legs are the longest in proportion to body size of all E. burchellii workers , they can run the fast est when not burdened with prey Franks 1985 a . This characteristic allows them to improve transport efficiency by returning to the raid quickly after depositing food within the bivouac Franks 1985 . Due to the large size of submajors, and because unit trans portation costs decrease with increasing worker size, they are able to carry heavier and larger prey items in relation to their body weight than any other caste Franks 1985 a . Based on this speciesÂ€ overpowering need for efficient transport, the morpholog ical characteristics of E. burchellii submajors , and their resulting specialty in carrying prey, it would appear that food transport is the ecological pressure causing the evolution of this caste. FIGURE 1 . Arrows represent head width measurement.
In this study , I explore prey transportation efficiency as an e volutionary pressure on E. burchellii submajor evolution . I plan to invest igate the roles and importance of 1 prey biomass 2 prey width and 3 prey length on E. burchellii . I predict that prey biomass is driving the selection for the production of the submajor caste. Therefore, prey biomass will be the best indicator of caste differentiation in E. burchellii. METHODS T he stu dy was conducted in San Luis, Costa Rica on the trails of the University of Georgia Ecolodge . All specimens were collected along the Camino Real trail and closely surrounding forest . Data collection occur red during afternoons from 13 April 2009 to 6 May 2009 . Because all colonies studied were in the nomadic phase, where they were moving their colony every night to a new locati on, it is difficult to determine the exact number studied. A probable estimation is 3 6 different colonies. For each data collection period I spent 2 3 hours collecting both individual s and groups of ants traveling with prey items along the column back to the bivouac. I concentrated my efforts on a 1 2 meter portion of the column and collected ants as they passed. Every 30 minutes, I did a 5 minute collection of individuals not carrying ant prey items along a one meter portion of t he column to determine caste distribution in E. burchellii colonies . I placed all specimens into 70% alcohol vials or bags . Before all calculations, specimens were briefly dried with a paper towel. I measured each individualÂ€s head width with a cali per and placed it into castes N = 664. Head width was measur ed between the eyes Fig. 1 and cas te was determined using FranksÂ€ definition of caste distinctions by head width Franks 1985a . I weighed individuals and groups of porters with a scale to the nearest 0.001 mg N =174. The prey the porters had in tow was also weighed in the same m anner N =174. The maximum length and width of each prey item was measured with a caliper N =236. Relationships between prey measurements and caste w ere analyzed using one way ANOVA . Differences between pairs of means were determined using a Tukey post hoc test. RESULTS A trend of all head widths of all ants measured shows a greater morphological difference in submajors when compared to the rest of the castes minors and medias Fig 2 . Submajors carried the most different prey based on width ANOVA, F ratio= 11.3661, P < 0 .001, df = 235 ; Fig. 3. Submajors carried the most different prey based on prey length ANOVA, F = 6.8147, P F IGURE 2 . Head width distribution . Distinctions of caste d individuals are bracketed 1 minor 2 m edia and 3 subm ajor.
= 0 .0013, df = 235 ; Fig. 4 and on prey biomass ANOVA, F ratio= 3.2813, P =0.04, df = 173 ; Fig. 5. Overall , submajors carried the widest, longest, and heaviest prey items Figs. 3, 4, and 5. DISC USSION In comparing the head width distributions of all ants measured, I found that submajors appear to be the most morphologically different from other worker castes Fig. 2 . A clear distinction is apparent between submajor headwidth and the headwidths of the other two castes . The distribution of the headwidths for media an d minor are less distinguishable from each other and a smooth gradient exists. This difference in morphology shows submajors may be specialized for a specific task within the colony, a behaivoral role which medias and minors do not need to accomplish. In constrast to my pr ediction , I found that prey biomass does not play the greatest role in predicting caste d etermination . In fact, I found that prey width best deter mines the caste of its carriers while p rey length is the second best determinant and p rey biomass is the last. For all three analysis, I found tha t submajors carried more varied prey than media and minor castes . Among the three castes observed and measured carrying prey, submajors are the most morphologically different and carry the most different sized prey. T he morphological differences of submajo rs makes them better suited for wider prey . In comparison to minors and medias, submajors have the longest legs, largest head and largest mandibles. FIGURE 3. The mean prey w idth carried by each caste. Letters designate different means castes. FIGURE 4. The mean prey length carried by each caste. Letters designate different caste means FIGURE 5. The mean prey biomass carried by each caste. Letters designate different caste means.
Like all other army ants, E. burchellii individuals carry prey items slung bene ath their bodies Franks 1985b . T his load transport method is mechically efficient and adheres to E . burchellii Â€s time limited lifestyle Powell & Franks 2005. The submajor caste of E. burchellii Â€s i s adapted to transport awkward prey loads : their longer legs increase ground cle arance, while their large head and mandibles allow for a more powerful grip to hold the load well above the substrate Paul 2001. The inefficiencies of under body prey transport is typically noted with wider prey, not necessarily longer prey. Long legs ma y make carrying wider prey more efficient, where this morphological variation may not change the efficiency of carrying long prey items. Long prey can be tackled by groups of ants forming a line and carrying the item in tandum. Accordingly, m y results indi cate that prey width is more of a pressure on caste differentiation. If all army ants carry prey slung below their bodies, what ecological pressures are at work that make E. burchellii Â€s submajor caste more exaggerated than other army ant species ? Possibly, their diet preference appear s to be a factor. E citon burchellii Â€s diet is composed of approxi mat e ly 50% social insect larvae and 50% large arthr opods Franks 1983. These large arthropod prey sources are captured and then dismembered into a multi tude of different sized portions . E citon burchellii is the only Eciton that is not a specialist on social insect larvae as prey Powell & Franks 2005. T he submajor caste has evolved as a specialized caste for awkward prey loads which typically differ greatly in width. Contrasting c aste composition and diet of other species such as E. mexicanum proves this assertion. This species lacks a submajor caste and specializes on monomorphic poneroid ant larvae as prey Rettenmeyer et al. 1983 . I f E. burchellii had maintained a stri ct larvae only diet, transport ation efficiency would be greater and submajors may not have evolved Powell & Franks 2005. Well kn own theories of evolution explain how morphological characteristics often develop through ecological pressures and niche partitioning. However, little is known about the pressures that act to develop castes within social species. My results strongly sugges t that prey size, in particular width, is the driving force behind the evolution of E. burchellii Â€s submajor caste. The special ization of the submajor caste in porting awkward loads may explain these results. In investigating the pressures that act on caste evolution, my results attribute to the overall understanding of social insects and their evolutionary history. Future studies could delve deeper into the selective forces acting on the submajor and other c astes. Due to submajorÂ€s extreme efficiency, one may expect to find greater numbers in caste distribution analysis. Future studies may want to focus on other selective forces for the submajor caste and attempt to find an answer to their relatively small pr oportion s . In addition, measuring the velocity of carrying ants with prey in tote would offer a better explanation as to the efficiencies of different castes and may offer insight to the specialties of other castes.
ACKNOWLEDGMENTS I wish to most gratefully t hank Anjali Kumar for her expertise and support through discussion, careful guidance , and answering my endless questions . I also wish to thank Yi men Ar aya for his helpful discussions on this project and generous statistical assistance. The pho tos of ant specimen and procedure would not have be en possible without the help of Luke Manlove. In addition, I wou ld also like to thank Scott Kos iba for his field and lab support. This project would not have been possible without Fabricio a nd the Universi ty of GeorgiaÂ€s E colodge facilities. I would like to thank the EstaciÃ³n BiolÃ³gica de Monteverde for much needed supplies. Lastly, I most importantly would like to thank my parents, Edward and Jacqueline Alger. If not for their funding, I would not have had the opportunity to pursue this project. LITERATURE CITED F RANKS , N. R. 1983. Ecology and Population regulation in the army ant Eciton burchelli. In E.G. Leigh, A. S. Rand , and D. M. Windsor Ed s .. The ecology of a tropical forest: seasonal rhythms and long term changes , pp. 389 395. Oxford University Press . F RANKS , N.R. 1985 a . Reproduction, foraging efficiency and worker polymorphism in army ants. In B. Holldobler , and M. Lindauer Ed s .. Experime ntal Behavioral Ecology and Sociobiology , pp. 9 107. G. Fischer Verlag, Stuttgart. F RANKS , N.R. 1985 b . Teams in social insects: groups retrieval of prey by army ants Echiton burchelli , Hymenoptera: Formicidae . Behavioral Ecology and Sociobiology . 18: 425 426. F RANKS , N. R., A.B. S ENDOVA F RANKS , J. S IMMONS , AND M. M OGIE . 1999 . Convergent e volution, s uperefficient t eams and t empo in o ld and n ew w orld a rmy a nts. Proceedings: Biological Sciences . 266: 1697 1701. G O T WALD , W. H. 1995. Army Ants: The Biology of Social Predation. Cornell University Press, Ithaca, NY. H ARVELL , D. C. 1994. The e volution of p olymorphism in c olonial i nvertebrates and s ocial i nsects. The Quarterly Review of Biology . 69: 155 185. K REBS , J. R. , AND N. B. D AVIE S . 1993. An Introduction to Behavioral Ecology. Bla ckwell Science Ltd, Osney Mead, Oxford. O RIANS , G. H., AND N. E. P EARSON . 1979. On the theory of central place foraging. In D. J . Horn, G. R. Stairs, and R. D. Mitchell Ed s .. Analysis of Ecological Systems, Ohio State University Press, Columbus. O STER , G. F. , AND E. O.W ILSON . 1978. Caste and ecology in the social insects. Princeton University Press. P OWELL , S., AND N. R. F RANKS . 2005. Caste e volution and e cology: a s pecial w orker for n ovel p rey . Proceeding s: Biological Sciences . 272: 2173 2180. P AUL , J. 2001. Mandible movements in ants. Comparative Biochemistry and Physiology, A: Comparative Physiology. 131: 7 20. R ETTENMEYER , C. W. 1963. Behavioral studies of army ants. University of Kansas. Sci . Bull . 44: 281 465. R ETTENMEYER , C. W., R. C HADAB C REPET , M. G. N AUMANN , AND L. M ORALES . 1983. Comparative foraging by neotropical army ants. In P. Jaisson Ed.. Social insects in the tropics , Vol. 2: Proceedings of the 1 st International Symposium of I. U. S. S. I. and the Society of Entomology of Mexico, Coyoyoc, Morelos, Mexico, November 1980, pp. 59 73. UniversitÃ© Paris Nord , Paris, France. W ILSON , E.O. AND B. H OLLDOBLER 2005. Eusociality: Origin and Consequences. Proceedings of the National Academy of Sciences of the United States of America. 102: 13367 13371.
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Eciton burchelli: polimorfismo de las principales sub-castas y la eficiencia de forrajeos
Eciton burchellii: polymorphism of submajor caste and foraging efficiency
Due to the link between efficiency and fitness, there should be selective pressure for morphology and behavior that promotes foraging efficiency. Among social insects, selective pressures act on individuals, shaping the physical castes of a colony. The highly polymorphic army ant, Eciton burchellii, has castes with morphological adaptations to allow task specialization. This study investigates what selective pressures are acting to shape the polymorphism of the submajor caste in E. burchellii. Ants were collected to find a relationship between ant caste and the preys biomass, width, and length. Results show that prey width is the greatest pressure effecting transportation efficiency, and in turn, shaping the evolution of the specialized porter caste (i.e., submajor). Results also indicate that of all the castes, submajors are found to be the most morphologically different and they carry the most different sized prey.
Possible explanations to Ecitons physically exaggerated submajor caste include their outstanding need of transport efficiency and unique prey preferences.
Este estudio investiga que presiones selectivas actan moldeando el polimorfismo de la casta submayor en E. burchelli. Las hormigas fueron colectadas para encontrar una relacin entre la casta, la biomasa, la anchura y la longitud de la presa.
Text in English.
Tropical Ecology 2009
Ecologa Tropical 2009
Castas de hormigas
t Monteverde Institute : Tropical Ecology