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Efecto del tamao de la colonia en la estructura por edades y el comportamiento de Metabus gravidus (Araneidae) en el Bosque Nuboso de Monteverde
Effect of colony size on age structure and behavior of Metabus gravidus (Araneidae) in the Monteverde Cloud Forest
Metabus gravidus are semi-social orb-weaving spiders living in colonies of up to 70 individuals over slow moving water. Buskirk (1975a) found that the time budget of spider behaviors differs within colonies of various sizes. I recorded the frequency and time spent on orb maintenance, aggressive behavior, spider
displacement, and prey capture for spiders of varying sizes. Spider and web sizes were measured and then I observed 21 spiders of each size for an hour to record the time and frequency of the four behaviors. In the
twenty-four colonies studied, small spiders were more abundant. Small colonies had a greater number of small spiders and large colonies contained more large spiders. Length of behavior did not correspond with
spider size in small, medium, or large colonies, but the frequencies show behavioral trends. A higher frequency of aggressive behavior was found in large spiders and a higher count for maintenance was found in small spiders. It is possible that small spiders are deterred from dispersing to large colonies with a greater number of large spiders because an increase of spider size correlates with an increase in aggressive behavior (Buskirk 1975a).
Metabus gravidus es una araa tejedora de orbe que posee un comportamiento semi-social que vive en colonias de hasta 70 individuos sobre el agua de movimiento lento. Buskirk (1975a) encontr que los comportamientos de las araas difieren en las colonias de diferentes tamaos. Registr la frecuencia y el tiempo dedicado al mantenimiento de la telaraa, el comportamiento agresivo, el desplazamiento de la araa, y la captura de presas para las araas de diferentes tamaos.
Text in English.
Spiders--Behavior--Costa Rica--Puntarenas--Monteverde Zone
Araas--Comportamiento--Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology 2007
Ecologa Tropical 2007
Araas tejedoras de orbe
t Monteverde Institute : Tropical Ecology
1 Effect of colony size on age structure and behavior of Metabus gravidus (Araneidae) in the Monteverde Cloud Forest Laura Peterson Department of Geography, Northern Michigan University ABSTRACT Metabus gravidus are semi social orb weaving spiders livin g in colonies of up to 70 individuals over slow moving water. Buskirk (1975a) found that the time budget of spider behaviors differs within colonies of various sizes. I recorded the frequency and time spent on orb maintenance, aggressive behavior, spider displacement, and prey capture for spiders of varying sizes. Spider and web sizes were measured and then I observed 21 spiders of each size for an hour to record the time and frequency of the four behaviors. In the twenty four colonies studied, small spi ders were more abundant. Small colonies had a greater number of small spiders and large colonies contained more large spiders. Length of behavior did not correspond with spider size in small, medium, or large colonies, but the frequencies show behavioral trends. A higher frequency of aggressive behavior was found in large spiders and a higher count for maintenance was found in small spiders. It is possible that small spiders are deterred from dispersing to large colonies with a greater number of large spiders because an increase of spider size correlates with an increase in aggressive behavior (Buskirk 1975a). RESUMEN Metabus graviduses es una araa de tela orbicular que posee comportamiento semi social vive en colonias de hasta setenta individuos s obre agua de movimiento lento. Buskirk (1975a) encontr los comportamientos de araas difiere en colonias de varios tamaos. Registr la frecuencia y el tiempo dedicado al mantenimiento de la tela, el comportamiento agresivo, el desplazamiento de la araa, y captura de la presa para araas de varios tamaos. La araa y los tamaos de los teleraas fueron medidos y luego observe 21 araas de cada tamao durante una hora y registr el tiempo y la frecuencia de los cuatro comportamientos. En las veinticuatro c olonias las araas, pequeas fueron ms abundantes. En pequeas colonias hay un mayor nmero de pequeos araas y las colonias grandes contuvieron araas ms grandes. La longitud del comportamiento no correspondi al tamao de araa en colonias pequeas, m edias, o grandes, pero las frequencies muestran algunas tendencies en los comportamientos. Una frecuencia ms alta del comportamiento agresivo fue encontrada en araas grandes y ms alta fue la frequencia del mantenimiento en pequeas araas. Es possible q ue pequeas araas no se dispersen a colonias grandes con un mayor nmero de araas grandes porque un aumento del tamao de araas guarda relacion con un aumento en el comportamiento agresivo (Buskirk 1975a). INTRODUCTION Metabus gravidus is a parasocia l spider living in colonies of 2 to 70 individuals over slow moving streams (Buskirk 1975a). They are communal and territorial but not cooperative in web building or prey capture. Colonial behavior for orb weaving spiders can be beneficial for the use of support lines to increase spinning efficiency, web takeover, defense from predators, and a closer mating proximity. Site preference of M.
2 gravidus is not randomly distributed along streams but influenced by water turbulence, the slope of the river, and s tream width (Buskirk 1975a). Colony size depends on the space available and the number of adult spiders at the site. Only 5 12% of spiderlings that hatch from an egg case in the colony eventually build a web in the same community (Buskirk 1975a). Colon y selection and web location preference of hatchlings and adults is not well studied. Juvenile spiders tend to build webs near the bank and have a greater web spinning time because of threats from larger spiders. Spinning is delayed because small spiders retreat or move to a different location when competing with adult spiders (Rayor and Wetz 2000). Juvenile and adult spiders in larger colonies benefit from increased prey capture (Spiller 1992). Metabus gravidus is able to increase fitness by catching m ore prey, conserving energy and preventing web loss. Aggressive behaviors and web maintenance are two costly and energy expending behaviors (Craig 1989). Studies show that large spiders spend more time building webs that are closer to the water (Melchoir s 2005) and small spiders expend more time and energy defending their webs (Trussel 1999). We can assume that site selection of orb weaving spiders is influenced by existing colonies not on habitat alone (Buskirk 1975a), but the costs and benefits of join ing certain colonies are unknown. Studying spiders of various ages in different sized colonies may increase the understanding of M. gravidus location, behavior, and benefits of social communities. The structure of many social groups can be shaped by envi ronmental factors as well as intraspecific interactions and behaviors (Cody 1973; Connell 1975; Schoener 1974). The purpose of this study is to explain the effect of colony size on behavior and energy expenditure of M. gravidus. I predicted to find more aggression in large spiders and increased web maintenance of small spiders in large colonies than medium and small colonies because studies by Trussel (1999) show that juvenile spiders spend more time defending their webs. Large spiders (12 mm in length) benefit by expending less energy in any colony size because they build their webs over the stream and catch the most prey (Buskirk 1975a). MATERIALS AND METHODS Study site I studied M. gravidus colonies along the Quebrada Maquina (elev. 1500 m, 10¡20'N, 84¡41W) at the Estacin Biolgica de Monteverde located in Monteverde, Puntarenas Province, Costa Rica. The study area is premontane moist forest dominated by evergreens. The morning is the peak time for M. gravidus activity (Buskirk 1975a) and I typica lly observed anywhere from 7 am to 12 pm. I randomly selected twenty four colonies with a wide range of individuals per colony along the Quebrada Maquina. I collected data for three weeks from late April into May 2007. Data Collection I first determin ed the frequency of distribution of colony size, spider size, and web size frequency within colonies. To measure web and spider size for each individual in a colony I used a 31cm ruler and a 1 m stick with a margin of error of 0.10 mm. I measured the spi ders from the tip of the abdomen to the head while they were
3 undisturbed on the web. Male spiders and those without a web were included in the spider census. I made web measurements as a vertical and horizontal line through the center of the hub. To ma ke support lines and webs visible I used baby powder for more accuracy. From observations of size trends throughout the colonies I created three size ranges of 1 3 mm for small (juvenile), 4 7 mm for intermediate, and 8 12 mm for large (adult) spiders. Colony size ranges were small (0 10 spiders in the colony), medium (11 20 spiders), and large (21 40 spiders). I observed one spider of each size range, if present in the colony, for one hour. Data included timing the behaviors and recording the frequen cy type of each behavior. Observations of individuals included a) orb maintenance, b) aggressive behavior, c) spider displacement, and d) prey capture. Orb maintenance consisted of clearing the web of debris or spinning silk and repairing the web. I cou nted spider displacement if an intruder successfully took over the web. Aggressive behavior was characterized by web jerking in the presence of an intruder, chasing, fleeing, or fighting. I noted prey capture if the spider moved towards the prey, bit it, wrapped it, and carried it back to the hub (Buskirk 1975b). Colonies that disappeared due to dry streambeds after the spider census were not able to be included in the behavioral part of the study. I used Chi square goodness of fit tests to compare spi der size in small, medium, and large colonies and to find size trends throughout the twenty four colonies combined. I also used a Chi square test to compare the frequency of observed behaviors for maintenance, aggression, displacement, and prey capture. I then used ANOVA tests to compare the time for each behavior between spiders in small, medium, and large colonies. RESULTS Of the 184 M. gravidus spiders measured in all twenty four colonies, there was a significantly higher number of small spiders (Fi g 1. Chi squared goodness of fit test; 2 = 11.43, df = 2, p = .003). 0 20 40 60 80 100 120 140 Large Medium Small Spider size Number of spiders FIGURE 1. Small M. gravidus spiders (1 3 mm) were more abundant in the twenty four colonies studied, followed by large (8 13 mm), and medium (4 7 mm) in the Monteverde cloud forest (N =184).
4 The data shows that large colonies had a greater number of large spiders and small colonies had more small spiders (Fig 2. Chi square goodness of fit test; 2 = 39.58, df = 4, p = 5.29E 08). 0 10 20 30 40 50 60 70 Small Medium Large Colony size Number of spiders Small spiders Medium spiders Large spiders _______________________________________________________________________ FIGURE 2. A significant trend between the number of spiders and colony sizes of M. gravidus spiders was found in Monteverde, Costa Rica. Small spiders were more abundant in smalle r colonies and larger spiders were higher in number in medium and large colonies. Behavioral observations showed statistically significantly higher frequency of aggression from large spiders, more maintenance from small spiders, and increased prey capture among medium sized spiders (Fig. 3. Chi squared goodness of fit test; 2 = 29.24, df = 6, p = 5.47E 05). 0 5 10 15 20 25 30 35 40 45 50 Maintenance Aggression Displacement Prey Capture Observed behavior Frequency Small spider Medium spider Large spider _______________________________________________________________________
5 FIGURE 3. The frequency of each behavior recorde d during the hour of spider observation. Small M. gravidus spiders spent more times maintaining their webs where as large spiders had increased aggressive movements. The difference between length of time for each type of behavior in small, medium, and l arge spiders was not significant. There was no significant trend between spider size and maintenance time (ANOVA test: F = 2.07, df effect = 2, df error = 61, p = .135), aggressive movements (ANOVA test: F = 1.12, df effect = 2, df error = 61, p = .334), or prey capture (ANOVA test: F = 1.12, df effect = 2, df error = 61, p = .334). I dismissed observations of displacement from the ANOVA testing because of low numbers. DISCUSSION Age structure and behavioral trends in M. gravidus colonies may show how spiders incr ease benefits while minimizing costs in certain size colonies. In the twenty four colonies studied, small spiders were more abundant compared to medium and large spiders (Figure 1). M. gravidus breeds year round but the number of spiderlings increase at the beginning of the tropical wet season in March, April and May. The spiders mature in five months, their eggs hatch, and the adult usually survives until the end of the year (Buskirk 1975a). The high reproduction rate of M. gravidus may explain the ove rall high frequency of small spiders. Patterns of spider size distribution show that a greater number of small spiders inhabit smaller colonies where as more adult (large) spiders are found in larger colonies (Figure 2). A possible explanation for this i s that aggression tends to increase with spider size (Trussell 1999) and with more adults in large communities, small spiders may prefer smaller colonies. When building webs, large spiders often tear support lines and webs of small spiders that interfere or connect to their webs (Buskirk 1975a). It is possible that smaller colonies with less large spiders provide spiderlings and juveniles with a less aggressive environment. Increased numbers of small spiders in small colonies and large spiders in large c olonies may occur from colony fidelity of spiders growing from spiderling to adult in the same colony. Most spiderlings disperse from their natal colony, allowing for colonies to remain populated year round (Uetz 1986). Colonies show a higher ratio of fe males with high colony fidelity creating the permanent colonies (Shelton 1992). Once the spiderlings have established themselves in a colony they may eventually create the larger colonies with large spiders over time. It is possible that large spiders in crease fitness by living in larger colonies through web support from other spiders, which would explain the increase of large spiders in large colonies (Figure 2). In large colonies small spiders are the support webs for large spiders building webs across the stream (Buskirk 1975a). Other tropical communal spiders such as Anelosimus eximius have shown to have significantly higher fitness in colonies with over 23 individuals compared to smaller colonies (Avils and Tufio 1998). The large spiders can bene fit from increased prey capture, shorter web building time, and less vulnerability to predation (Buskirk 1975a). I found a significant difference between the frequency of the observed behaviors and spider size (Figure 3). The results showed that aggres sion was highest among large
6 spiders, followed by medium sized spiders. Maintenance was notably higher for small, and then medium sized spiders, where as displacement and prey capture were not similar in frequency between spider sizes. Higher maintenanc e in smaller spiders may be a result of the aggression from larger individuals. Studies by Buskirk (1975a) show that larger spiders are more likely to tear the webs of smaller spiders, and that large spiders further from the bank are more defensive. Larg e spiders are more vulnerable to takeovers because of their large webs, causing increased aggression between large spiders (Buskirk 1975b). Another interesting observation was the increased number of males on the support lines in larger colonies that main ly disturbed the large spiders rather than smaller spiders. Males are often responsible for orb takeovers from adult females (Buskirk 1975b), which may also explain increased aggression in large spiders. Higher frequency of aggressive behaviors in large spiders could also be a product of limited resources in colonies at the time of observation (Figure 3). When food is scarce, defense costs increase to defend webs from intruders (Provencher and Vickery 1988). Increased intraspecific competition and terri toriality of large spiders in times of limited resources may cause the lack of small spiders in large colonies. With more aggression, small spiders may not prefer to live in colonies with larger spiders. The relationship between spider size and length o f the observed behaviors (maintenance, aggression, and prey capture) was found to be statistically insignificant. These behavioral results may not have accurately reflected trends because of physical and environmental variables. Morning feeding of M. gra vidus periods are influenced by prey captures from the previous night (Buskirk 1975a), which may have occurred for some observed spiders. Measurements of aggression times may have been skewed because while one aggressive move from a large spider may deter an intruder, it could take many movements for a smaller spider defend its web. Many times maintenance is based on abiotic factors such as wind and rain that could tear a web depending on web site (Craig 1989). Personal observations included web destruct ion by dragonflies and falling fruits, leading to increased maintenance time because of chance events. Further work on behavioral trends between spider age (size) in different sized colonies in the field could be significant if compared to those in an env ironment to control more variables. It is possible that patterns between spider size and behavior could be significant if observations during evening hours were included. Prey capture throughout the day and limited resource studies may explain the freque ncy of behaviors as well. Nearest neighbor data in large and small colonies may also influence behaviors in different sized colonies. Dispersal of spiderlings could help identify which colonies small spiders prefer and if they continue to live in the sa me colony over time. Continuing data collection on M. gravidus would allow for a better understanding of costs and benefits of certain size colonies for all parasocial spiders. ACKNOWLEDGEMENTS Thanks to Karen and Alan Masters for introducing me to the species Metabus gravidus Thank you Tania Chavarria, Camryn Pennington, Tom McFarland, and Eric Lawyer for their advice and revisions.
7 LITERATURE CITED Avils, L. and P. Tufio. 1998. Colony size and individual fitness in the social spider Anelos imus eximius The American Naturalist. 153(3): 403 418. Buskirk, R.E. 1975a. Coloniality, activity patterns and feeding in a tropical orb weaving spider. Ecology 56:1314 1328. Buskirk, R.E. 1975b. Aggressive display and orb defence in a colonial spider, M etabus gravidus Animal Behavior. 23: 560 567. Cody, M.L. 1973. Competition and the structure of bird communities. Princeton University Press. Princeton, New Jersey, USA. Connell, R.K. and D.J. Futuyma. 1975. On the measurement of niche breadth and overl ap. Ecology. 52: 567 576. Craig, C.L. 1989. Alternative foraging modes of orb weaving spiders. Biotropica. 21(3): 257 264. Melchoirs, K. 2005. Colonial placement behaviors of Metabus gravidus (Araneidae). CIEE. Summer. Provencher, L. and W. Vickery. 1988. Territoriality, vegetation complexity, an biological control: the case for spiders. The American Naturalist. 132:257 266. Rayor, L.S and G.W. Uetz. 2000. Age related sequential web building in the colonial spider Metepeira incrassata an adaptive spacing strategy. Animal Behavior 57: 1251 1259. Schoener, T.W. 1974. Resource partitioning in ecological communities. Science. 185:27 39. Shelton, A. 1992. Web site tenacity and behavior of the social spiders Metabus gravidus. CIEE. Summer. Spiller, D.A. 19 92. Relationship between prey consumption and colony size in an orb spider. Oecology 90: 457 466. Trussel, B.N. 1999. Motivations behind aggressive encounters in colonies of Metabus gravidus spiders of different sizes. CIEE. Fall. Uetz, G.W. 1986. Web bui lding and prey capture in communal orb weavers. In W.A. Shear (Ed.). Spiders: Web, behavior, and evolution, pp. 207 231. Stanford University Press.