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Polyethism in Eciton burchellii army ants : a look at task specializations of worker castes Colleen Nell Department of Biology, Drake University ABSTRACT Insect societies, specifically Eciton burchellii army ants have had high ecological success due to polyethism within the colony. This division of labor within the colony increases efficiency and therefore fitness of the colony, which has selected for morphologically and behaviorally defined castes; majors, submajors, media, and minims. Samples of the se castes show strong division of labor and roles throughout the colony. Majors are shown to restrict themselves to the column and bivouac where defense is of highest necessity. Submajors have morphologically evolved to carry large prey and maintain colu mn flow. Media are found to be generalists, reinforcing essential functions of the colony, and the smallest caste, minims play key roles within the structure of the bivouac and bridging for prey transport. Collectively these roles increase the efficiency of the colony and contribute to the ecological success of E.burchellii. RESUMEN Las sociedades de insectos, especficamente las hormigas arrieras Eciton burchellii tienen un alto xito ecolgico debido al politesmo entre colonias. La divisin de labor es entre las mismas colonias aumenta la eficiencia y xito reproductivo de la colonia, la cual ha sido seleccionada por castas definidas morfolgicamente y por comportamiento, mayores, submayores, medias y mnimas. Muestras de estas castas muestran una fu erte divisin de labores entre la colonia. Las mayores estn restringidas a las columnas y bivaques donde la defensa es la mayor necesidad. Las submayores estn morfolgicamente adaptadas para cargar presas grandes y mantener el movimiento de la colonia. Las medias son generalistas, reforzando funciones esenciales de la colonia, y las castas ms pequeas, mnimas, juegan un papel clave en la estructura del bivaque y formando puentes para el transporte de presas. Colectivamente estas funciones aumentan l a eficiencia de la colonia y contribuyen al xito ecolgico de E. burchellii. Key words: army ants; Eciton burchellii; polyethism; task specialization; division of labor; castes INTRODUCTION Polyethism in a broad sense can be defined as a division of labor in accordance with size (Wilson 1971). Division of labor is characterized by groups of specialized individuals performing tasks, simultaneously or sequentially, to increase efficiency (Robinson 1992). The functionality of social insect colonies is h ighly dependent on the variation in behaviors performed by different colony members. The high ecological success of social insects, such as Hymenoptera and Isoptera, across ecosystems, may be attributed to this decentralization of colony behaviors. For exa mple, termites and ants, which exhibit strict around two percent of all insect species (Wilson & Holldobler 2005). Through evolutionary time, castes have become morpholo gically specialized for specific tasks or behaviors within eusocial species (Powell & Franks 2005). Task specialization and castes have evolved independently many times in several species, indicating selection pressure for specialization is strong and col onies may be more efficient with caste specialization for specific behaviors (Franks, et al 2001). Castes may specialize on specific tasks like reproduction, foraging, and/or colony defense. Specialization in such ways means increased efficiency, which allows for increased
colony growth rates and hence greater colony fitness (Ratnieks & Anderson, 1998; Franks 1985). Army ants (Ecitoninae) are an example of an organism with well defined morphological divisions that are thought to have discrete task specia lization. The tasks in army ant colonies are extremely partitioned, with a continuum from one physogastric reproductive queen to tiny nurse workers. Army ants are central place foragers, requiring a high energy output for most hours of the day, feeding o n prey only once returned to the bivouac. This puts a high emphasis on the flow of the column and successful prey return. Specialization among tasks therefore is very beneficial to the maintenance and growth of the colony. Instead of all groups expendin g large amounts of energy foraging on a daily basis, task partitioning and specialization of those that are foraging can maximize the ratio of input to output (Orians and Pearson 1979). Eciton burchellii is one of the most well studied species of army an t because of its diurnal, aboveground foraging habit (Powell & Franks 2005). Within the workers of the E. burchellii the castes are highly polymorphic, with a size differential of 8.1 mm between the smallest and largest members (Gotwald and Kupiec 1975). The castes are colony size, and generalist diet mean castes must be highly behaviorally specialized. There are many tasks to be performed, from foraging, transportin g prey, and protecting from kleptoparasites. These task related divisions have selected for the evolution of the five recognized castes within the workers; the majors (soldiers), submajors, media, minim, and the reproductives (Franks 1985). The largest workers, the majors have hook like mandibles that have advanced so far defensively they appear incapable of assisting in foraging or grasping prey, limiting them to a specific protective role in the colony (Topoff 1971). The submajors are the most heavily studied caste. Consisting of only three percent of colony workers this caste carries 26 % of prey, and are morphologically specialized to do so by disproportionately long legs to carry heavier and wider pieces. Although all castes run at the same speed with prey, this caste is able to run much faster without prey and returns more frequently to the swarm front, therefore playing a large role in prey transport (Franks 1985). The great majority of research done on task specialization in army ants has conce ntrated on these two larger castes, with not much known about the media and minim castes. The minims are the smallest caste, and most abundant in the bivouac where they are thought to act as brood nurses. Minims are believed to also play a role in egg ha ndling and feeding larvae, although few studies have been conducted (Topoff 1971). Remaining is the most abundant caste, the media, with a body size falling between the minim and submajors (Franks 1985). Because of their moderate size and abundance they are thought to potentially be generalists in terms of task performance, performing all within colony tasks with the same frequency (Franks 1985). However, with selective pressures causing distinct castes in E. burchellii it is difficult to believe the med ia do not have some specialized role within the colony. This study is aimed to take a closer look at these potentially specialized roles within the colony of the worker castes in E. burchellii in terms of task performance. It will also explain the benefits of task specialization in terms of evolutionary significance. MATERIALS AND METHODS Study site This study was conducted around San Luis, Costa Rica, on trails at the University of Georgia Ecolodge and in the surrounding forest at 1000 meters above sea level. Data was collected in early afternoon from April 8 th 2010 to April 25 th 2010. Samples were taken
from between 4 9 colonies, however an exact number is hard to determine because all colonies studied were in their nomadic phase, moving bivouac sites each night. Sampling Samples were taken of the ants in the bivouac, column, and swarm regions of the colony when accessible. In addition samples were taken of the individuals seen killing prey, carrying prey to the bivouac, forming bridges in the colum n, and hanging to form the bivouac. All the regions, ants carrying prey, and ants forming bridges were sampled for 2 minute collection periods, with breaks between areas to prevent disturbing the natural composition of the workers. To collect the ants kil ling prey, prey of moderate sizes (10 mm in length to 40 mm length) were thrown into the swarm. Once the prey had ceased movement it was removed along with the ants on it. Ants hanging in the bivouac were collected by pulling a sample of the linked worke rs, then waiting a 30 minute period for the bivouac to settle from the disturbance before repeating. All samples collected were put in 70 percent ethanol bags. Each region and task was sampled 6 times except for those killing the prey, which were sampled 14 times. Measurements The head widths of all specimens (n= 2228) were then measured with caliper at between the eyes of the ants. These measurements along with defining morphological features of the submajor and major castes were used to caste the ant definition of caste distinctions by head width (Franks 1985). RESULTS FIGURE 1: Head width distribution of all E. burchellii ants sampled (n=2228) in San Luis valley of Costa Rica.
Head width measurements showed notable morphological differences between the four worker castes, and the media caste were the most numerous of those sampled (Fig 1). There were significant differences between the worker sizes and task performance, showing that specific castes performed specific castes (F 6,2 258 = 34.7063; p < 0.0001; Fig. 2). The majors were more frequently found in the bivouac and column than the swarm ( 2 (2, N = 68) = 18.02, p < 0.001; Fig. 3a). Respectively, the majors were distributed with 24 percent in the bivouac and 29 percent in the column (Fig 3a). Thirty seven percent of submajors sampled were carrying prey, which was significantly greater than expected ( 2; (3, N = 108) = 85.26, p < 0.001; Fig. 3b). Submajors were also found more often in the column than any other region of the c olony (Fig. 3b). The submajors were responsible for 24.5 percent of all carrying done within the colony. The media allocated themselves fairly evenly across all tasks and regions (Fig 3c). They did 77.45 percent of the killing in the swarm, which is mor e than any other task, while major and submajor castes were reduced in this task in comparison to their normal colony composition. The media were also found to carry 81.25 percent of larvae within the carrying task, which was significantly more than expec ted in the composition of the carrying task ( 2 (1, N = 64) = 6.53, p < 0.001; Fig. 3c). The minims composed hanging and bridging tasks significantly more than expected in their respective regions ( 2 (1, N = 321) = 72.75, p < 0.001; (1, N = 411) = 521.12 p < 0.001; Fig 3d). Fifty one percent of the bridging comprised of the minims, and 38.25 percent of the hanging. These were both much higher than the normal occurrence of this caste in the regions where these tasks take place (12.22% in the column and 18.68% in the bivouac). FIGURE 2: Mean head widths ( standard error) of Eciton burchellii as distributed by tasks and region. ANOVA shows that mean head widths are statistically different between tasks and regions (F 6,2258 =34.7063; p<0.0001). Tukey test shows that tasks compared to respective regions of occurrence are significantly different in mean head width between the bivouac and hanging ants, the column and carrying ants, and the column and bridges.
FIGURE 3: E. burchellii membership allocat ions to tasks sampled, by castes; Majors (a), submajors (b), larvae. DISCUSSION Majors Statistical tests showed that the majors were most freq uently found in the column and bivouac. This coincides with other research that has determined their role to be largely protective (Topoff 1971). As the largest caste they are the most expensive to produce and are therefore the least abundant in the colo ny with only 5.25 percent of ants sampled. As larger body size to be most effective in colony protection. Therefore, the majors are found in the bivouac, specialized to protect the queen and respond to predation (Rettenmeyer 1963). Optimality theory states that energy expended is most beneficial if concentrated closer to locations of higher importance, as shown with the majors (Parker & Smith 1990). In the column, the majors, although doing little in terms of carrying and bridging are still a key component to the functionality. As fitness is maximized by the rate of delivery of energy to the bivouac from the swarm, the colony has divided labor to maintain maximal flow; the majors are on standby to protect food sources returning and to intervene in the case of disturbance (Orians & Pearson 1979). This is optimal because waste to the colony. Submajors The data on the submajors reaffirmed previous studies by Franks (1985), in that they are specialized in prey transport, allocating the highest percentage of their time to this activity. They were also found in higher densities in the column than other regions,
supporting that they are a porter caste, focusing most energy expenditure on traveling back and forth from the swarm and bivouac. The entire colony is dependent on maintenance of the flow of the column, as the consistenc y and rate of flow are determinant in the energy gain (Orians & Pearson 1979). Specialization of the submajor caste morphologically through leg length and larger body size minimizes transport costs. The submajors can run at an equal pace to the other cas tes while carrying a disproportionate weight, maximizing speed and prey size transported (Franks 1985). Although only 37% of the carrying was done by the submajors, this can still be considered a specialization because they make up such a small portion of the colony with 8%. Groups are often assembled to cooperatively retrieve prey and the submajor is the key component in the process, the first to pick up the prey, with additional smaller workers added as needed to maintain standard retrieval speed (Franks 1986). Therefore, polyethism seems to differentiate roles in team completion of this task, with emphasis on the larger submajor size for weight transport and potentially different team roles of other castes participating in carry (Anderson & Frank 2001; Franks 1986). As E .burchellii have a generalist diet of varying prey sizes this may constrain transport efficiency if submajors were to carry all prey, leading to the assistance of other castes as well (Powell & Franks 2005). The submajors did not part icipate in carrying the larvae of E. burchellii during emigration. This may further indicate specialization in carrying larger items or team carry roles, because as package size decreases there is less of a necessity to employ larger worker sizes in carry ing as efficiency will decrease. This may represent passing of a task threshold associated with carrying, to be taken over by the next size range, media workers (Theraulaz, et al. 1998). Media The media caste appeared very generalized across all zo nes and activities. This generalist approach is reflected in the morphology of the media, with hind leg length to body length ratios ranging between that of the smallest and largest workers, and having a moderate body size (Franks 1985). Making up the lar gest portion of the total colony, the media balances many of the implications of extreme specialization as seen in caste polyethism. With a caste moderating between the morphological extremes of the others, moderate efficiency can be maintained via complet ing tasks others have been specialized to do, literally filling in the pieces. This generalized behavior increases the functionality of the colony because the media have fewer restrictions on their actions (Tienderen 1991). Although generalists have capabi lities applied to many tasks, within the media caste there is potential for specializations or team components within the caste that act in reducing generalist costs, however further investigated is needed. Extreme specializations of the major, submajor, a nd minim castes may jeopardize the colony in disturbance. Without a generalized caste to fill in the spaces, the loss of individuals from a certain caste could have expansive results to the functionality of the colony. As a morphological median between th e other castes, as generalists the media cut efficiency losses that may occur when these specialized castes perform across tasks. In addition if there are insufficient members of a specialized caste, the media are able to aid in all tasks with limited dec rease in efficiency and to fluctuate to areas that require higher demand throughout the day This flexibility is an essential feature to the success of division of labor in social insects (Theraulaz, et al. 1998). Looking at caste distribution within ea ch task, the media can be seen at highest proportionality in the killing of prey (74.5%). When killing, E. burchellii surround the prey and anchor it to the ground with their hooked tarsal claws, while other members climb up the prey, killing it and break ing it to pieces for transport to the bivouac (Schneirla 1971). The media may be the optimal caste for killing prey because they have
a moderate body size that can balance the potential trade off of enough strength to hold the prey in fewer numbers, witho ut having as high a developmental cost to the colony as the major and submajors do. Although not necessarily a specialization of the media caste, this is an example of the way that having a generalized caste is benefiting the colony. With the media doing most of the killing, this allows the other castes to concentrate on the tasks they have evolved to complete most efficiently, putting energy expended developmentally to optimal use (Parker & Smith 1990; Ratnieks & Anderson 1998). As noted with the major workers, the swarm and therefore the killing ants are the least protected throughout all regions of the colony. Having a higher frequency of the media in this region also provides a safety net for any deaths or predation that may occur. Losing the media will have the least impact on the total colony as they act in the least specialized role, mainly for redundancy of the specialized castes. Minims The minim workers, previously assumed to be brood nurses because of their high frequency in the bivouac (Franks 1985), appear to have importance in other tasks as well. The minims appear to be large component of the formation of the bivouac itself. This is beneficial to the colony because as the minims have the smallest body size and therefore leg proportio ns the hanging formation they create will be a more closely knit wall. The E. burchellii bivouac is known to be maintained at temperatures close to 28.5C in the core, which promotes higher brood growth rates (Franks 1989). A tighter hanging wall seems op timal in protecting the larvae brood and queen from external exposure. This could provide smaller gaps between individuals, which may potentially allow for greater manipulation of temperatures and internal conditions of the bivouac. Also, as the smallest members the minims are the least expensive developmentally to produce, therefore losses of workers may be reduced if the bivouac is attacked. The minims are not specialized defensively, but the majors can be found mainly in the bivouac patrolling along t his wall of minims to supplement any defensive response needed (C. Nell, pers. obs). The minims also constituted the majority of bridging that was found along the column. As previously mentioned, the column is extremely important in the total function and success of the colony. A higher rate of flow maintained in the column along generation time, and therefore an overall increase in the collective fitness of the colony (Franks 1985). This puts a large emphasis on maintenance of the flow of the column returning prey. To maintain the standard rate of flow in the column, bridges are formed by individuals, latching together in order to create a more fluid path for prey tr ansport (Powell & Franks 2006). As found by Powell and Franks (2006), utilizing bridges shows a clear increase in the daily prey intake of the colony. As the submajors and media have a larger body size they are more essential components of the column be cause they can carry disproportionately heavier and larger prey items than the minims. Also as previously noted the submajors can run the fastest without prey to return faster for more prey. Therefore it is most optimal to the maintenance and flow of the column for the minims to perform the bridging task (Parker & Smith 1990). Polyethism within the E. burchellii has clear implications relating to task specialization of the worker castes. Over evolutionary worker castes have developed as a response to spe cific demands within the colony such as more efficient prey transport and defensive roles. Even the generalist approach of the media caste is notably selected for in stabilization and redundancy of necessary colony tasks, which is an important factor in e cological success. The roles of the castes are key components of the evolutionary
success of E. burchellii which have contributed to the maximal efficiency and functionality for the colony. ACKNOWLEDGEMENTS Special thanks to the University of Georgia E colodge, whose administration so graciously allowed me to use their trails and facilities to make this project happen and to David Gardner for his assistance in the thrillingly long hours of ant measuring. Additional thanks to Pablo Allen for his high fas hion white rubber boots and Dr. Anjali Kumar for teaching me how to smell out army ants. Without such useful skills and knowledge passed down from Anjali, this project would not have been so successful. LITERATURE CITED A NDERSON C., AND N. R. F RANKS 2001. Teams in Animal Societies. Behavioral Ecology. 5: 534 540. F RANKS N. R 1985. Reproduction, foraging efficiency and worker polymorphism in army ants. In B. Holldobler, and M. Lindauer (eds.) Experimental Behavioral Ecology and Sociobiology, pp. 9 107. G. Fischer Verlag, Stuttgart. F RANKS N. R. 1986 Teams in social insects: group retrieval of prey by army ants ( Eciton burchelli, Hymenoptera: Formicidae). Behavioral Ecology and Sociobiology. 18:425 429. F RANKS N. R. 1989. Thermoregula tion in army ant bivouacs. Physiological Entomology. 14:397 404. F RANKS N. R., A. B. S ENDOVA F RANKS J. S IMMONS AND M. M OGIE 2001. Convergent evolution, superefficient teams and tempo in old and new world army ants. Proceedings: Biological Sciences. 266:1697 1701. G OTWALD W. H. J R ., AND B. M. K UPIEC 1975. Taxonomic implications of doryline worker ant morphology: Cheliomyrmex morosus (Hymenoptera: Formicidae). Annals of the Entomological Society of America. 68:961 971. O RAINS 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 (Eds.). Analysis of Ecological Systems, Ohio State University Press, Columbus. P ARKER G.A. AND J. M. S MITH 1990. Optimality theory in evolutionary biology. Nature. 348: 27 33. P OWELL S., AND N. R. F RANKS 2005. Caste evolution and ecology: a special worker for novel prey. Proceedings: Biological Sciences. 272: 2173 2180. P OWELL S. AND N. R. F RANKS 2006. How a few help all: living pothole plugs speed pry delivery in the army ant Eciton burchellii Animal Behaviour. 73: 1067 1076. R ATNIEKS F. L. W. and C. Anderson. 1998. Task Partitioning in insect socieites. Insect Societies. 46: 95 108. R ETTENMEYER C. W 1963. Behavioral studi es of army ants. University of Kansas Science Bulletin. 44:281 465. R OBINSON G. E. 1992. Regulation of division of labor in insect societies. Annual Review of Entomology. 37:637 65. S CHNEIRLA T. C. 1971. Army Ants: A study in social organization. W. H. Freedman and Company, San Francisco. S EELY T. D. 2002 When is self organization used in biological systems? Biological Bulletins. 202: 314 318. T HERAULAZ G., E. B ONABEAU AND J. D ENEUBOURG 1998. Response threshold reinforcement and divi sion of labour in insect societies. Proceedings of the Royal Society of London. 265: 327 332.
T OPOFF H 1971. Polymorphism in army ants related to division of labor and colony cyclic behavior. The American Naturalist. 105:529 548. V AN T IENDERE N P. H. 1991. Evolution of generalists and specialists in spatially heterogeneous environments. Evolution. 45:1317 1331. W ILSON E. O. 1971. The Insect Societies Pg 156 160. Harvard College. Harvard University Press. W ILSON E. O., AND B. H OLLDOBL ER 2005. Eusociality: Origins and consequences. Proceedings of the National Academy of Sciences of the United States of America. 110:13367 13371.
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Polietismo en las hormigas arrieras Eciton burchellii: Un vistazo a las especialidades de trabajo de los obreros de las castas
Polyethism in Eciton burchellii army ants: a look at task specializations of worker castes
Insect societies, specifically Eciton burchellii army ants have had high ecological success due to polyethism within the colony. This division of labor within the colony increases efficiency and therefore fitness of the colony, which has selected four morphologically and behaviorally defined castes; majors, submajors, media, and minims. Samples of these castes show strong division of labor and roles throughout the colony. Majors are shown to restrict themselves to the column and bivouac where defense is of highest necessity. Submajors have morphologically evolved to carry large prey and maintain column flow. Media are found to be generalists, reinforcing essential functions of the colony, and the smallest caste, minims play key roles within the structure of the bivouac and bridging for prey transport. Collectively these roles
increase the efficiency of the colony and contribute to the ecological success of E.burchellii.
Las sociedades de insectos, especficamente las hormigas arrieras Eciton burchellii tienen un alto xito ecolgico debido al politesmo entre las colonias. La divisin de labores entre las mismas colonias aumenta la eficiencia y el xito reproductivo de la colonia, la cual ha sido seleccionada por las castas definidas morfolgicamente y por el comportamiento en, mayores, submayores, medias y mnimas. Las muestras de estas castas muestran una fuerte divisin de labores entre la colonia. Las mayores estn restringidas a las columnas y vivaques donde la defensa es la mayor necesidad. Las submayores estn morfolgicamente adaptadas para cargar presas grandes y mantener el movimiento de la colonia. Las medias son generalistas, reforzando funciones esenciales de la colonia, y las castas ms pequeas, mnimas, juegan un papel clave en la estructura del vivaque y forman puentes para el transporte de las presas. Colectivamente estas funciones aumentan la eficiencia de la colonia y contribuyen al xito ecolgico de E. burchellii.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone--San Luis
Costa Rica--Puntarenas--Zona de Monteverde--San Luis
Tropical Ecology Spring 2010
Division of labor
Ecologa Tropical Primavera 2010
Tareas de especializacin
Divisin del trabajo
t Monteverde Institute : Tropical Ecology