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1Differential response to vibration stimulus in Metabus gravidus (Araneidae) communities John McLaughlin Department of Earth and Environmental Sciences, Nor theastern University ABSTRACT This study is an investigation on the effect of dis tance between neighboring spiders on the reaction t o an artificially generated vibration stimulus. The stud y was conducted using a tuning fork at 320 Hz in or der to generate a vibration intended to stimulate the spid er web. Metabus gravidus individuals that were closer to the source of vibration (mean 9.5 2.75 cm) tended to react by fleeing, and those farther from the so urce (mean 16.5 4.8 cm) typically reacted by approachi ng. The spider closest to the source of vibration r eacted by fleeing and approaching, while the neighboring s pider exhibited no response more often than expecte d by chance (X2 = 28.4). Thus individuals who maintain a greater di stance to near neighbors benefit by minimizing interfering vibrations from neighboring webs. RESUMEN Este estudio es una investigacin sobre el efecto de la distancia entre araas vecinas y la reaccin ante una vibracin artificial estimulada. El estudio fue he cho usando un diapason a 320 Hz para generar una vibracin en la tela de araa. Individuos de Metabus gravidus que se encontraban cerca de la fuente de vibracin (promedio 9.5 2.75 cm) tienden a escapa r, y los que se encuentran lejos da la fuente (prom edio 16.5 4.8 cm) tpicamente tienden a acercarse. La s arenas cercanas a la fuente de vibracin reaccion an alejandose y acercandose, mientras que las arenas v ecinas no exhiben ninguna respuesta mayor a la esperada (X2 = 28.4). As individuos que mantienen una gran dist ancia con los vecinos se ven beneficiados por una menor interferencia con las vibraciones en las telas vecinas. INTRODUCTION The strategy of communal living was developed in or der to make life easier, and many organisms on earth have come to exploit the benefit s of social living. Sociality can provide a variety of benefits including increased r eproductive success, increased defense against predation, and more efficient feeding. (Bus kirk 1975a) Communal living in spiders is rather rare; of the 3 4,000 species of arachnids there are only 20 species known to live in colonies. Amon g these 20 species there is a lot of variability in the community style. Some orb weaver s only live in a colony during the juvenile stage of life, under maternal care. Other species share a single common sheet web that is built and maintained communally (Uetz 1 986). In the case of Metabus gravidus the spiders share a common structural scaffolding of non-viscid silk strung over streams. Individuals build and maintain their own o rbs (Buskirk 1975a). By sharing structural support, the spiders are able to exploit a unique niche that would be otherwise inaccessible to solitary spiders (Buskirk 1975a). Some members of the M. gravidus community benefit more from the cooperation of others. Past studies indicate trends between spi der size/age and their location within the colony. This information was supplemented by an investigation of insect traffic
2 within different microhabitats of a stream. It was shown that there is more insect traffic closer to the water surface over the center of the stream, as opposed to areas that are farther from the water surface and closer to the st ream bank (Buskirk 1975a; Potosek 2000). Not coincidentally, older/larger spiders wer e shown to dominate the locations of high insect traffic (Buskirk 1975a). Metabus gravidus individuals are quite active in defending orb orie ntation. Dominance of web location is asserted through aggre ssive interactions usually initiated by larger spiders. The spiders use vibratory signal s to assert aggression, such as bouncing in place in the hub of the web, plucking radii, and charging other spiders that have come too close. The majority of aggressive interactions occur during the web building process; when a spider builds a web too close another. Usual ly vibratory displays are sufficient in communicating aggression, but it can get to the poi nt of destroying parts of a neighboring web or physical confrontation (Buskirk 1975b). The energy spent in aggressive encounters can be an important cost to communal liv ing. They not only have energy costs, but also distract the spider from being able to attack potential prey items (Uetz 1986). It is understandable that a spider is so territoria l over its web; not only does it rely on its web as a source of food, but it is essential ly an extension of its sensory structure (Foelix 1996). Vibrations are crucial in the commun ication behavior of Aranids, and are used in aggressive interactions, courtship behavior s and prey location (Uetz 1986). Orbweavers tend to orient themselves in the hub of the ir webs, allowing the spiders to read and respond to vibrations from all directions. When an object (prey, debris, or perhaps an intruder) enters the web, the impact creates oscill ations that are sent down the radii to the center of the web. After detecting the vibration, t he spider uses a technique of plucking the silk, sending oscillations to the source of vib ration in order to read the oscillations that rebound from the ensnared object. This behavio r allows the spider to characterize the nature of the ensnared object before committing to a more energetically costly response, such as attacking or fleeing (Foelix 1996). The communal spider web can serve a variety of bene fits. In colonies of orbweavers, the aggregation of webs can serve as a gau ntlet to insects. If an insect enters one web and bounces free, there is a high likelihood th at it will fly into another web within the colony (Uetz 1986). The aggregation of orbs als o extends the sensory perception of the spider beyond its own orb, so that it is able t o sense vibrations of neighboring webs. This can act as an important early warning system w hen a predator is in the colony (Tietjen 1986). Vibrations from the neighbors web can also be costly; responding to vibrations of a neighbors web could be wasted ener gy, especially considering that there is minimal prey theft among M. gravidus (Buskirk 1975b). The aggression of the older/larger spiders makes the distance between orb s relative to the age of the spider, in that the older/larger spiders exhibit a greater dis tance to the nearest neighbor than do the younger spiders in the colony (Tietjen 1986). The m ost-fit spiders maximize the distance from their neighbors which could mean that there is some benefit. I propose that the greater distance to neighbor allows older/larger sp iders to focus on their own web, in that they will respond to less vibration generated in ne ighboring webs and thus waste less energy. Thus I hypothesize that spiders with a grea ter distance to the nearest neighbor will respond to vibrations generated in a neighbor s web less often.
3MATERIALS AND METHODS Site Location The study was conducted along Rio Alondra in San Lu is, Costa Rica. I found several colonies upstream of where Rio Alondra crosses the road that leads to Catarata San Luis, in between the University of Georgia Ecolodge and t he entrance to Catarata San Luis. Experimental Methods Ten days of data collection occurred between April 2 and April 15 2008, between 1:00pm and 6:00p.m. To begin data collection, two neighbor ing spiders were selected from a colony. I clouded the webs with baking flour in ord er to better see the intricacies of the webs. At times this agitated the spiders, so no dat a were collected until the spiders had returned to the hub for at least one minute. I meas ured the shortest distance by web between the two spiders. A tuning fork tuned to 320 Hz (E) was used to generate a vibration that would simulate a prey item. This val ue is in the middle portion of frequencies that are transmitted through the web of a spider, as well as into the web of another spider (Masters et al1986). I feel it is al so important to state that M. gravidus individuals to not show selection of prey based on previous experience (Buskirk 1975). Even though the tuning fork may generate an unfamil iar oscillation, I was able to assume that the spiders would react to it. A vibration was generated and applied directly on a non-viscid support line on the periphery of one of the two orbs, and the distance from the source of vibration to the spider closest to it was recorded. The vibration was always applied on a side of the orb opposite to the neares t neighbor, so that the vibration would have to travel through the first web before reachin g the web of the neighbor (for experimental clarity, the spider closest to the sou rce of vibration was referred to as Spider 1 and the spider farthest from the source of vibration was referred to as neighbor) The tuning fork was left on the silk fo r five seconds. The reactions of spiders were characterized into the following categories: a pproach, fleeing, plucking, or no reaction. An approach is defined as any movement aw ay from the hub toward the source of vibration. Fleeing is defined as any movement fr om the hub away from the source of vibration. Plucking is defined by the jerking of th e web radii by the spider while remaining in the hub. No reaction is defined by the lack of any noticeable response to the vibration.
4 FIGURE 1. Image depicting the application of vibrat ion. Spider 1 was always the spider closest to the vibration. Data Analysis A Kruskall-Wallis test was used to analyze the rela tionship between the reaction of Spider 1 and the distance from the source, the dist ance to nearest neighbor and the reaction of Spider 1, and the distance to near neig hbor and the reaction of the neighboring spider. A chi-squared test was used to compare the frequency of each response of the two neighboring spiders. RESULTS I was able to find six colonies that were consisten tly active, ranging from two to 17 active individuals. In comparing the distance from the source of vibrat ion to the reaction of the nearest spider, it was shown that spiders closer to the sou rce of vibration tended to flee more often (mean 9.5 cm +/2.75), those farther away ha d a greater tendency to approach (mean 16.5 cm +/4.8). Plucking (mean 13.5 cm +/4.9) (Kruskall-Wallis test, H = 17.6, p < 0.001; Figure 2).
5 nr n FIGURE 2. Mean distance from the source of vibratio n, for each reaction behavior type. The spider closest to the source of vibration is an alyzed. Spiders closer to the source of vibration were more likely to flee, spiders farther from the source of vibration were more likely to approach. Plucking was exhibited over the middle range of distances (N = 57, H = 17.6, p < 0.001). As the distance between spiders increased, the neig hbor was more likely to show no response(mean 27.52 cm +/15.7; Kruskall-Wallis te st, H = 6.96, p = 0.07; Fig. 3). nrr n FIGURE 3. Mean near neighbor distance is shown for each of the four reaction behaviors of the neighbor spider. No response was most common at greater distances (N = 57, H = 6.96, p = 0.07).
6 The spider closest to the source of vibration appr oached and fled more often than expected, while the spider farther from the source exhibited no response more often than expected. Plucking reactions for both spiders were close to the expected values (chisquared test, X = 28.4; Fig. 4). nr r rn FIGURE 4. The observed number of behaviors for both spider 1 (the spider closest to the source) and the neighboring spider (spider farthest from the source) are compared. Spider 1 approached and fled more times, and never showed no response (chi-squared = 28.4). Comparing the distance to the nearest neighbor to t he reaction of the spider nearest the source of vibration did not show any si gnificant results (Kruskall-Wallis test, H = 1.2, p = 0.54) On three occasions, spiders left the colony retrea t after the application of a vibration and proceeded to occupy empty web hubs. T he distance from the source of the vibration to the retreat was greater than 30 cm on two of the occasions, and was not measured on the third. One time a spider that was o n the shared support line responded to a vibration by occupying a web, but no distance dat a were recorded. DISCUSSION
7 The amount of silk a vibration travels through has an effect on the distance the vibration travels, as well as the strength of that vibration. There were some interesting data when the distance between neighbors was related to the r eaction of the neighboring spider. The neighboring spider had a higher incidence of no-rea ction as the distance to the near neighbor was increased. This illustrates that the v ibration is indeed diluted as it travels through one web and continues through to the next. This agrees with my idea that a spider presumably benefits from maintaining a great er distance to the near neighbor in that it is not as influenced by vibrations generate d in a neighbors web. The significance of this result was supplemented by a chi-squared te st comparing the frequency of the four different reaction types between Spider 1 and the n eighbor spider. The spider whose web was closest to the source of vibration always exhib ited a reaction, whereas no-reaction was the most frequent behavior of the neighboring s pider, especially as the distance between neighbors was increased (Figure 3). This di fference in behavior illustrates the effect of distance on the travel of vibration throu gh the spider colony. The fact that the neighboring spider does not respo nd to weaker vibrations does not necessarily mean that the vibration goes unfelt ; it could be that it is too subtle to be of interest to the spider. I say this because of the f ew occasions where spiders came out from the colony retreat, in order to occupy empty webs a fter the application of a vibration on an occupied web. The estimated distances from the r etreat to the source of vibration were greater than 30 cm, yet the spiders were still inte rested in the vibration. Peak insect activity is the late afternoon, and a corresponding trend of orb-building occurs just prior to it (Buskirk 1975). Perhaps vibrations from prey capture of other webs functions as an alarm clock to spiders in retreat, signaling the ri se of insect activity. A significant trend was shown when comparing the di stance of the source of vibration to the reaction of the spider closest to it (Figure 1). Vibrations generated close to the spider resulted in fleeing reactions. This c ould perhaps be due to the fact that the close distance produced a stronger vibration that m ight simulate a large/dangerous prey item. The most common reaction was plucking which a lso occurred over the greatest range of distances, overlapping both the lower rang e of the distances, which elicited approach and the upper range of distances, which el icited flight. That plucking is the most common reaction, occupying the median range of distances, coincides with the use of this behavior by orb-weavers to read the source of web vibrations (Foelix 1996). Before approaching a prey item it is beneficial to characterize the size and intensity of the prey item to avoid danger and/or wasted energy, esp ecially considering that the tuning fork likely creates unfamiliar frequencies. It is a lso important to consider that a plucking reaction requires less energy than either flight or approach. The tendency to approach vibrations generated farther away from the hub coul d imply that the gentler vibration simulates a more attractive prey item, or at least a less intense one. It could also simulate the approach of an intruding conspecific spider, wh ich is generally not tolerated and often results in aggressive encounters as the spider trie s to defend its web (Buskirk 1975). Yet another possibility is that the spider approaches a vibration generated farther away out of curiosity: being able to sense a vibration that is not quite intimidating, but not being able to characterize it. I was unable to find a trend between the distance t o the nearest neighbor and the reaction of the spider closest to the source of vib ration. This indicates that the spiders do not react to signals based on the presence of a clo se neighbor. This can be accredited to
8 the minimal levels of prey theft within the colony, something that is much more common in other communal spider species (Buskirk 1975). Th ese spiders live communally, which must mean that the benefits of the community outwei gh the costs (Tietjen 1986). Though the amount of prey theft is low in this species, or b take-over is quite common (Buskirk 1975). This increases the benefit of having a great er distance to the nearest neighbor; a longer support line to the web means that the spide r occupying the hub has more time to react to the intruding spider. There have been many studies done on M. gravidus that document the patterns of arrangement of spiders and orbs within the communit y. This study is an attempt to investigate some of the reasons the colony is consi stently arranged the way it is, and it is evident that effect of vibration differs with the a mount of silk it travels through. It would be interesting to conduct a study that compared the reactions of M. gravidus to other orbweaving species, both solitary and colonial. It wou ld also be of interest to broaden the scope of the data I have collected, with emphasis o n finding larger colonies and testing a wider range of frequencies. This could provide a mo re encompassing depiction of the preference of frequency as well as an examination o f the effect of colony size on behavior. ACKNOWLEDGEMENTS Much thanks to Rio Alondra and the Univerity of Geo rgia Ecolodge that allowed me to study in the forest surrounding it. I would also li ke to thank Tania for her wealth of constructive criticism, Pablo for his statistical k nowledge, Kateyln for her paper probing, and Karen for her contagious excitement. LITERATURE CITED Buskirk, R. E. 1975. Coloniality, activity patterns and feeding in a tropical orb-weaving spider. Ecology 56: 1314-1328. Buskirk, R. E. 1975. Aggressive display and orb def ense in a colonial spider, Metabus gravidus Animal Behavior 23: 560-567. Masters, M.M, Hubert, S.M., Moffat, A.J.M. 1986. Tr ansmission of vibration in a spiders web. in Shear, W.A. (Ed.) Spiders: Webs, Behavior, and Evolution pp 172-206. Stanford University Press. Potosek, J. Spider size, web location, and prey cap ture in the colonial orb-weaver Metabus gravidus. 2000 CIEE: Tropical biology and conservation program. Fa ll 2000. 154-165. Foelix, F. 1996. Biology of Spiders 2nd Edition. Oxford University Press, New York. 256264. Tietjen, W.J. 1986. Social spider webs, with specia l reference to the web of Mallos gregalis In Shear, W.A. (Ed.) Spiders: Webs, Behavior, and Evolution pp 172206. Stanford University Press. Uetz, G.W. 1986. Web building and prey capture in c ommunal orb weavers. In Shear, W.A. (Ed.) Spiders: Webs, Behavior, and Evolution pp 172-206. Stanford University Press.
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Diferencia de respuesta al estimulo de vibracin en las comunidades de Metabus gravidus (Araneidae)
Differential response to vibration stimulus in Metabus gravidus (Araneidae) communities
This study is an investigation on the effect of distance between neighboring spiders on the reaction to an artificially generated vibration stimulus. The study was conducted using a tuning fork at 320 Hz in order to generate a vibration intended to stimulate the spider web. Metabus gravidus individuals that were closer to the source of vibration (mean 9.5 2.75 cm) tended to react by fleeing, and those farther from the source (mean 16.5 4.8 cm) typically reacted by approaching. The spider closest to the source of vibration reacted by fleeing and approaching, while the neighboring spider exhibited no response more often than expected by chance (X2 = 28.4). Thus individuals who maintain a greater distance to near neighbors benefit by minimizing interfering vibrations from neighboring webs.
Este estudio es una investigacin sobre el efecto de la distancia entre las araas vecinas y la reaccin ante una vibracin artificial estimulada.
Text in English.
Spiders--Behavior--Costa Rica--Puntarenas--San Luis
Araas--comportamiento--Costa Rica--Puntarenas--San Luis
Tropical Ecology 2008
Ecologa Tropical 2008
t Monteverde Institute : Tropical Ecology