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Foraging efficiency and polymorphism of the submajor caste in E. burchellii Nathan A. Sellers Department of Environmental Sciences, Allegheny College A BSTRACT Army ants are superefficient in their retrieval of prey. There are four distinct worker cas tes in Eciton burchellii each important to the high efficiency of foraging. When prey is too large for an individual it is segmented by workers into manageable pieces then conveyed to the bivouac. Workers carrying prey were collected to determine the rel ationship between prey siz e and the worker(s) carrying prey as well as examine prey as an evolutionary pressure for selection of the submajor caste Results show length and width are poor determinants of which caste is performing the retrieval task, as al l castes are equally as likely to carry prey of any size. Prey b iomass was shown to be a significant factor in determining which caste carried which prey with the submajor caste carry ing the heaviest of prey items. RESUMEN Las hormigas arrieras son muy eficientes en la recuperacin de presas. Existen cuatro castas diferentes en Eciton burchellii cada una importante para la alta eficiencia en el forrajeo. Cuando una presa es muy grande para un solo individuo, las hormigas la dividen en pedazos manejable s y as transportados al bivouac. Las obreras que cargan presas que ellos colectan para determinar la relacin entre el tamao de las presas que las obreras cargan como una fuerza evolucionaria a la seleccin de la casta submayor. Los resultados muestran que mientras el largo y el ancho son factores no determinantes en cual casta realiza la recoleccin, todas las castas son igualmente capaces de cargar presas de cualquier tamao. La biomasa de las presas resulto ser el factor significativo en la determin acin de cual casta carga cual presa, con la casta submayor cargando las presas ms pesadas. I NTRODUCTION E.O Wilson and Holldobler (2005) likened superefficient insect societies to a factory inside a fortr ess. The factory being the queen and nurse work ers, surrounded by the fortress, or nest and the workers who build, repair and guar d it. Ants and termites are excellent example s of these superefficient societies because though they together are only 2% of approximately 900,000 globally known insect spe ci es they form over half of the total insect biomass (Wilson & Holldobler 2005). The key to success for these superefficient eusocial societies is the specialization that occurs in the ir workforce ( Oster & Wilson 1978, Powell & Franks 2006 ) Eusocial soci eties are composed of many workers living together in cooperation, usually around a central female or queen and the specialization which occurs in these societies allows for workers to be performing diff erent tasks at the same time making these societies highly efficient (Holldobler & Wilson 1990 ) The army ant sub family Ecitoninae includes about 150 species o f army ants (Rettenmeyer et al. 1983). New world army ants are abundant in humid lowlands from northern Peru to southern Mexico (Holldobler & Wilson 1990) and many of these ants specialize on feeding on individuals and larvae of other eusocial insect colonies, though some also feed on other
arthropods (Franks 1985a). Division of labor among the workers of these species is highly import ant to their e fficiency (Franks 1985 a ) and is characterized by accomplishment of different tasks by sp ecialized individuals (Robinson 1992). Eciton spp. are n otably different from other new world army ants in that they retrieve prey and migrate above ground ( Rettenmey er 1963) A well studied species of Eciton in the new world, Eciton burchellii form groups of workers, or teams, to retrie ve large r prey items (Franks 1985b). E. burchellii workers are polymorphic and divisible into four worker castes (Franks 1985a, Go twald 1995, Franks et al 1999). From largest to smallest individuals they are majors who are defensive workers with large sickle shaped mandibles submajors who are dominant porters of prey items media who are a generalist caste and are abundant in all behavioral role s and minums who are found d isproportionally in the bivouac but also are bridge builders and assist in prey transport (Fran ks 1985a, Franks et al. 1999). When raiding the colony forms a 10 15m wide swarm that will taper off into a tru nk column in the rear closer to the bivouac (Holldobler & Wilson 1990, Gotwald 1995). The swarm can travel over 200 meters in one day looking for prey, and af ter find ing prey they efficiently pin it down, kill and section it (if large enough) into pieces for travel back to the bivouac (Gotwald 1995). Submajors are fundamental components to E. burchellii prey transport (Franks 1985b Powell & Franks 2005 ). These highly adapted workers have disproportionately long legs in comparison to their body which allows them to more efficiently carry wider, longer and heavier prey (Franks 1985a) While this caste makes up only 3% of the colony it is present in 26 % of the events in which prey is transported back to the bivouac which suggests a specialist porter rol e within the colony ( Franks 1985a Franks et al. 1999) This superefficient prey transport caste wa s likely selected for due to mixed diet constraints of transport efficiency ( Jurgan 2001 Powell & Franks 2005) It is well known that all retrievers of pr ey, whether individuals or members of a team, carry prey slung beneath their bodies (Franks 1985a, Gotwald 1995) and that submajors are best suited for this task as they have disproportionally long legs (Powell & Franks 2005). As E. burchellii feed on oth er nonhymenopteran arthropods as 50% of their diet submajor specialization is highly important to the transport ation this more awkwardly shaped prey group (Powell & Franks 2005 Powell & Franks 2006 ). Eciton burchellii colonies coordinated efforts in swa rming as we ll as bivouac movement shows they work to optimize time. As previously stated much evidence shows that as prey biomass increases so does the biomass of the individual or teams which are carrying the prey item. Here I look E. burchellii prey and see how they could be an evolutionary pressure in the natural selection for the submajor caste. MATERIALS AND METHODS Study Site This study was conducted in the premonta n e forest of San Luis, Costa Rica on the trails of the University of Georgia Eco lodge as well as the trails of La Finca Bella (11 00m elevation). Data was collected colonies of E. burchellii were used in this study. Data Collection Sampling periods consi sted of presenting a raiding colony with a cockroach After the ants killed and segmented the cockroach and a segment was being moved towards the bivouac it was
picked up and placed into bags of 70% ethanol. O ther prey items being carried by workers were also p icked up and placed in ethanol All ants of a carrying team were collected and placed together in ethanol bags. Measurement of prey and Eciton burchellii workers S pecimens of prey and E. burchellii workers were left to dry overnight. The intereye width of each individual worker was measured using a caliper and each ant was placed into a caste based on efinition of caste distinctions (Franks 1985a). Prey items and individual ants were weighed using a scale to the nearest 0.0001 g. The len gth and width of each prey item was also measured to the nearest mm with a caliper. Relationships between prey length, width and biomass and caste were analyzed using one way ANOVA s A Tukey Kramer HSD test was used for pairwise comparisons of av erage pre y biomass and caste. RESULTS In this study 37 minums, 168 medias and 64 submajors were collected, totaling in 268 E. burchellii workers. As prey biomass increased so did the total biomass of workers carrying the prey item (Fig. 1). No difference was s een in the prey items carried by the castes (minum, media and submajor) based on length (One way ANOVA, F errordf, totaldf =0.9096, P= 0 .4039; Fig. 2 ) and width (One way ANOVA, F errordf, totaldf = 1.2061, P= 0 .3010 ; Fig. 3 ) of prey item Submajors carried he avier prey than any other caste (One way ANOVA, F ratio=5.8797, P= 0 0032; Fig. 4 ). Submajors carried the heaviest prey items while the media and minums were shown to carry prey items of similar biomass (Tukey Kramer HSD= 0 5) Average biomass of worker caste was from heaviest to lightest, submajors (0.0126 g ), medias ( 0 .0061 g ) and minums (0.0035 g ; Fig. 5). Figure 1. Comparing prey item to caste carrying the prey shows a s the biomass of the prey item increases so does the biomass of the worker(s).
Figure 2. Mean length carried by each caste ( standard error of the mean ). Each caste is equally as likely to carry prey of any length. Sample size was 168, 37, 64 for media, minum and submajor respectively. Figure 3 Mean prey width carried by each caste ( standard error of the mean ). Each caste was shown to be equally as likely to c arry prey of any width. W idth was determined to be a more significant factor than prey length. Sample siz es for media, minum, submajor was 168, 37, 64 respectfully. Figure 4 Mean biomass carried by each caste ( standard error) Tukey Kramer HSD test showed that submajors carried overall heavier eight than the other castes. Ot her castes (media, minum) showed no difference in weight they would carry. Sample sizes for med ia, minum, submajor was 168, 37, 64 respectfully.
Fig ure 5 Average weight of the castes was calculated to determine that each c aste average weight was found to correlate with the general caste size assumptions (minums lightest, media middle weight and submajors the heaviest). Heavier weight in submajor s could also be a factor in their carrying of larger prey. DISCUSSION E. bu rchellii are evolved to be highly efficient via their highly polymorphic workforce (Oster & Wilson 1978). Gener al foraging theory assumes foraging workers increase their fitness by maximizing the rate of ener gy delivery to a central place, in this case th e bivouac (Gotwald 1995) and the morphology of the submajors and carrying of heavier prey items found here supports this. Division of labor in E. burche llii workers increases foraging efficiency and therefor e increases the growth rate of the colony. Inc reasing the growth rate decreases generation time and contributes back to the fitness of all members of the colony (Franks 1985a). As new colonies formed through division of ol d ones fitness of all members determines rate of colony division, which depends on growth rate (Franks 1985a). All w orkers of E. burchellii are highly efficient. In this study b iomass was found to be the best determinant of which caste was carrying prey items back to the bivouac. Bo th length and width were not seen to be an import ant factor as any of the worker s (other than maj ors) would individually, or as a team, carry prey of any length and/or width. These data conflict with almost all other literature found concerning caste a nd prey retrieval patterns which show clear patterns in prey sizes and porters ( Rettenmeyer 1963, Franks 1985a Franks 1985b, Gotwald 1995, Franks et al. 1999, Anderson & Franks 2001, Powell & Franks 2005, Powell & Franks 2006 ). These discrepancies in the data could well be due to a deficit in sample size. Prey biomass however was shown to be a significant factor in determining which caste did the work (Fig. 4,5) which does match previous data The challenge of a wider diet for E. burchellii, encompassing a wide variety of hyme nopteran and other arthropo d prey (Holldobler & Wilson 1990) resulted in the evolution of submajors for transport of more awkwardly shaped arthropod prey (Powell & Franks 2006). Most n ew world army ants specialize on the larvae of other eu social hymenopt eran s and therefore do not have the specialized submajor caste for prey transport (Franks et al, 1999) B y encompas sing arthropods into their diet E. burchellii overall foraging efficiency increases (Gotwald 1995 ), as they do not have to forage as far for their wider range of prey The submajor s are especially important because they transport more prey items back to the bivouac at lower energy costs than the other castes due to their disproportionally long legs (Gotwald 1995) also adding to increase colony fitness
Hundreds of th ousands of workers compose E. burchellii colonies (Holldobler & Wilson 1990, Franks et al. 1999) and all are able to perform ing tasks to increase the efficiency of the colony (Oster & Wilson 1978). Worker morphology is clearly an important factor to the su ccess of E. burchellii colonies. Without the specialization of each caste the colony could not be as efficient as they clearly are. A CKNOWLEDGMENTS I offer m any thanks to Anjali Kumar without whose help support and guidance I could not have done this project. I would also like to thank my host family, Hugo, Lila and Jason for providing much needed assistance in prey capture as well as the staff of the University of Georgia Ecolodge for letting me use their land and for their constant vigilance for arm y ants My project could not have been completed without the help of Dan Brunelle, whose eyes and company kept me on the search or without Brendan Boyer and Valarie Milici who provided much needed support and distraction throughout. Also thanks to Christ in e Isab ella and Anjali Kumar for revie wing my paper and helping make it the final product it is now. LITURATURE CITED Anderson, Carl & N.R. Franks. 2001. Teams in animal societies. Behavioral Ecology 12(5):534 540. Franks, Nigel R. 1985a. Reproduction, foraging efficiency and work polymorphism in army ants. B. Holldobler and M. Linauer (Eds.) Experimental Behavioral Ecology and Sociobiology, pp. 9 107. G. Fischer Verlag, Stuttgart. Franks, Nigel R. 1985b. Teams in Social Insects: groups retrieval of pre y by army ants (Eciton burchellii, Hymenoptera, Formicidae). Behavioral Ecology and Sociobiology. 18:425 426. Franks, Nigel R., A.B. Sendova Franks, J. Simmons and M. Mogie. 1999. Convergent evolution, superefficient teams and tempo in old and new world ar my ants. The Royal Society. 266:1697 1701. Gotwald, William H., Jr. 1995. Army Ants: The biology of social predation. The Cornell Series In Arthropod Biology. Cornell University Press. Pp. 41 161. Holldobler, Bert and Edward O. Wilson. 1990. the Ants. Bal knap Press of Harvard University Press. Cambridge, Massachussets. Pp. 573 595. Jurgan Paul 2001. Mandible movements in ants. Comparative Biochemistry and Physiology, A: Comparative Physiology. 131(1):7 20. Oster, George F. and Edward O. Wilson. 1978. Caste and ecology in social insects. Princeton University Press. Powell, Scott and Nigel R. Franks. 2005. Caste evolution and ecology: a special worker for novel prey. Biological Sciences. 272:2173 2180. Powell, Scott and Nigel R. Franks. 2006. Ecology a nd evolution of worker morphological diversity: a comparative analysis with Eciton army ants. Functional Ecology. 20:1105 1114. Rettenmeyer, C.W. 1963. Behavioral studies of army ants. University of Kansas. Sci. Bull. 44(9):281 465. Rettenmeyer, C.W., R. C hadab Crepet, M.G. Nauman and L. Morales. 1983. Comparative foraging by neotropical army ants. In P. Jaisson (Ed.). Social inse cts in the tropics (Vol. 2): Proceedings of the 1 st International Symposium o f I.U.S.S.I. and the Society of Entomol ogy of Mexico C oyo yoc Morelos, Mexico. November 1980, pp. 59 73. Universite Paris Nord, Paris, France. Robinson, Gene E. 1992. Regulation of Division of Labor in Social Insects. Annual Review of Entomology. 37: 637 655. Wilson, Edward O. and Bert Holldobler. 2005. E usociality: origin and consiquences. Proceedings of the National Acadamy of Sciences of the United States of America. 102(38):13367 13371.
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Sellers, Nathan, A.
Eficiencia de forrajeo y el polimorfismo de la casta submayor en E. burchellii
Foraging efficiency and polymorphism of the submajor caste in E. burchellii
Army ants are superefficient in their retrieval of prey. There are four distinct worker castes in Eciton burchellii each
important to the high efficiency of foraging. When prey is too large for an individual it is segmented by workers into manageable pieces then conveyed to the bivouac. Workers carrying prey were collected to determine the
relationship between prey size and the worker(s) carrying prey as well as examine prey as an evolutionary pressure for selection of the submajor caste. Results show length and width are poor determinants of which caste is performing the retrieval task, as all castes are equally as likely to carry prey of any size. Prey biomass was shown to be a significant factor in determining which caste carried which prey, with the submajor caste carrying the heaviest
of prey items.
Las hormigas arrieras son muy eficientes en la recuperacin de presas. Existen cuatro castas diferentes en Eciton burchellii cada una importante para la alta eficiencia en el forrajeo. Cuando una presa es muy grande para un solo individuo, las hormigas la dividen en pedazos manejables y as transportados al bivouac. Las obreras que cargan presas que ellos colectan para determinar la relacin entre el tamao de las presas que las obreras cargan como una fuerza evolucionaria a la seleccin de la casta submayor. Los resultados muestran que mientras el largo y el ancho son factores no determinantes en cual casta realiza la recoleccin, todas las castas son igualmente capaces de cargar presas de cualquier tamao. La biomasa de las presas resulto ser el factor significativo en la determinacin de cual casta carga cual presa, con la casta submayor cargando las presas ms pesadas.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone--San Luis
Costa Rica--Puntarenas--Zona de Monteverde--San Luis
Tropical Ecology Spring 2011
Ecologa Tropical Primavera 2011
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