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Busby, Mary Kathryn
Reconocimiento familiar en las araas sociales Anelosimus sp. en Monteverde, Costa Rica
Kin recognition in the social spiders Anelosimus sp. in Monteverde, Costa Rica
Anelosimus spiders are social and use pheromonal and vibrational cues to distinguish conspecifics from
predators and prey. Juveniles live in their natal web with adults until maturity, at which time they disperse.
Given that juveniles are more likely to disperse to locations near to their natal web and that Anelosimus has
developed mechanisms for kin recognition, these spiders would be expected to exhibit aggressive behavior
less frequently in the presence of intruding spiders from nearby webs than towards intruders from distant
webs. This hypothesis was tested by introducing spiders to new webs from distant locations as well as from
nearby locations. There was no significant relationship found between web distance and level of aggression
(C2 = 5.47, df = 2, p = 0.06) nor between colony size and level of aggression (C2 = 2.43, df = 2, p = 0.30).
A linear regression comparing colony size to level of aggression yielded no significant results for ignoring
behavior (F = 0.94, df =1 and 18, p = 0.34), approaching behavior (F = 3.36, df = 1 and 18, p = 0.08), nor
aggressive behavior (F = 0.29, df = 1, p = 0.59). According to this data there is little indication that spiders
change their level of aggression due to colony size or distance between webs. This study provides data that
can be used to explain the purpose and function of kin-recognition behaviors
Las araas Anelosimus son individuos sociales que utilizan pistas como vibraciones y feromonas para distinguir conspecficos entre los depredadores y las presas. Los juveniles viven en sus telas natales con los adultos hasta que maduran, tiempo al cual se dispersan. Con esto, los juveniles tienden mas a dispersarse a telas ubicadas cerca de la tela natal y estas especies tienen un mecanismo de reconocimiento de parentela, se espera en estas araas un comportamiento agresivo menos frecuente con la presencia de araas cercanas a la tela que con las araas intrusas de telas lejanas.
Text in English.
Kin recognition in animals
Reconocimiento de parientes en los animales
Tropical Ecology 2008
Ecologa Tropical 2008
t Monteverde Institute : Tropical Ecology
Kin recognition in the social spiders Anelosimus sp. in Monteverde, Costa Rica Mary Kathryn Busby Department of Integrative Biology, University of Texas at Austin ABSTRACT Anelosimus spiders are social and use pheromonal and vibrational cues to distin guish conspecifics from predators and prey. Juveniles live in their natal web with adults until maturity, at which time they disperse. Given that juveniles are more likely to disperse to locations near to their natal web and that Anelosimus has developed m echanisms for kin recognition, these spiders would be expected to exhibit aggressive behavior less frequently in the presence of intruding spiders from nearby webs than towards intruders from distant webs. This hypothesis was tested by introducing spiders to new webs from distant locations as well as from nearby locations. There was no significant relationship found between web distance and level of aggression Ã°C Ã°2 = 5.47, df = 2, p = 0.06 nor between colony size and level of aggression Ã°C Ã°2 = 2.43, df = 2, p = 0.30. A linear regression comparing colony size to level of aggression yielded no significant results for ignoring behavior F = 0.94, df =1 and 18, p = 0.34, approaching behavior F = 3.36, df = 1 and 18, p = 0.08, nor aggressive behavior F = 0.29 , df = 1, p = 0.59. According to this data there is little indication that spiders change their level of aggression due to colony size or distance between webs. This study provides data that can be used to explain the purpose and function of kin recogniti on behaviors. RESUMEN Las araÃ±as Anelosimus son individuos sociales que utilizan pistas como vibraciones y feromonas para distinguir entre conespecÃficos entre depredadores y presas. Los juveniles viven en sus telas natales hasta que maduran, tiempo al cual se dispersan. Con esto, los juveniles tienden mÃ¡s a dispersarse a telas ubicadas cerca de la tela natal y estas especies tienen un mecanismo de reconoci miento de parentela, se espera en estas arenas un comportamiento agresivo menos frecuente con la presencia de araÃ±as cercanas a la tela que con araÃ±as intrusas de telas lejanas. Esta hipÃ³tesis se probÃ³ colocando araÃ±as en nuevas telas desde distintas distancias asÃ como araÃ±as de telas cercanas. No existe una relaciÃ³n entre la distancia y el nivel de agresiÃ³n Ã°C Ã°2 = 5.47, df = 2, p = 0.06 tampoco entre el tamaÃ±o de la colonia y el nivel de agresiÃ³n Ã°C Ã°2 = 2.43, df = 2, p = 0.30. Una regresiÃ³n linear para comparar el tamaÃ±o de la colonia y el nivel de agresiÃ³n no mostrÃ³ ningÃºn resultado como comporta miento evasivo F=0.94, df=1 and 18, p=0.34, comportamiento de aproximaciÃ³n F=3.36, df=1 and 18, p=0.08 , y comportamiento agresivo F=0.29, df=1, p=0.59. De acuerdo a estos datos no hay un indicio de que las araÃ±as cambien su nivel de agresiÃ³n debido al tamaÃ±o de la colonia o a la distancia entre telas. Este estudio provee datos que expliquen los comportamientos de reconocimiento de parentela. INTRODUCTION Sociality has evolved independently several times in spiders and to differing degrees. Periodic social species are those that have associations with relatives that disintegrate prior to the mating season. In permanent social species the juveniles often remain at their natal web year round. It is thought that permanent social traits evolve by suppress ing this phase of dispersal of juveniles at a certain age Seibt et al. 1988.
Anelosimus spiders are among the few spiders that practice true sociality Foelix 1996. While they are less social than the eusocial insects in that all members are capable of reproducing and there is no real hierarchy or system of rank, they form communal webs in which all members share duties Foelix 1996. Anelosimus contains four permanently social species AvilÃ©s et al. 1998. Anelosimus web colonies are built in disturbed areas such as roadsides or forest edges and clearings. They build these webs as a collective effort and the webs increase in size as the number of members increases Tietjen 1986. There may be up to thousands of members per colony or there may be colonies consisting of only one female Vollrath 1988. A colony may exist in one location for years Seibt et al. 1998. Anelosimus spiders share web building, community maintenance, and prey capture tasks among members Foelix 1996. Juveniles remain at their na tal web until adulthood, at which point most of them disperse. Many, however, stay in their natal webs as adults and sometimes forgo their own reproduction to aid in the rearing of relatives offspring Jones 2001. This kin selection behavior increases in dividual fitness when spiders are highly related. Parental care which exists in Anelosimus and tolerance of juveniles result in fitness trade offs. Competition for food resources during periods of rapid growth is a drawback that in many species has cause d juveniles to disperse during their high growth period Powers et al. 2003. Another drawback is that dispersing juveniles frequently disperse to locations near to their natal web. While some juveniles disperse beyond local areas, many stay close enough t o result in inbreeding depression AvilÃ©s et al. 1998. The benefits of communal living must outweigh the costs for this trait to have evolved. Ultimately, kin selection increases the chances of an individual s own genes being perpetuated by close relative s. Proximately, parents and juveniles fill different feeding niches: juveniles may capture a wider variety of prey sizes and help keep the web clean of smaller prey Tollins 2008. Other fitness benefits to having a communal lifestyle in spiders include ha ving less demand on individual silk production and better defense against predation Jones 2001. Because Anelosimus spiders live communally, there must be some system in place by which spiders can recognize their kin. Evolutionarily this trait should have evolved to prevent spiders from killing conspecifics carrying their own genes, as an individual s decision to attack or tolerate a stranger would be governed by a cost/benefit ratio Seibt et al. 1988. These recognition mechanisms have been shown to be both vibrational and pheromonal Foelix 1996. In the species Anelosimus jucundus , spiders frequently disperse to locations within five meters of their natal web Powers et al. 2003. Due to this factor, one would expect nearby webs to be more likely than distant webs to contain related spiders. One would also expect spiders to be more likely to show aggressive behavior towards spiders from distant webs than towards spiders from nearby webs. This study attempts to determine if there is a relationship betw een web distance and aggressive behavior. If a spider from a distant web is introduced to a new web, it should be more likely to cause an aggressive reaction than the introduction of a spider from a nearby web. One would also expect webs with a higher colo ny population to show higher levels of aggression towards intruders because there are more individuals capable of defending the web.
METHODS Twenty two webs of Anelosimus spp. Theridiidae were located down the length of the road leading to the Cerro Am igos tower in Monteverde, Costa Rica from July 15 until August 2, 2008. Twenty of these were manipulated and used for data collection. The other two were used to supply additional spiders. Data was recorded for both of the two extra webs, but no experiment s were performed on these webs. Webs were marked with numbered flagging tape, designating each web with a number from one to 22. The size of each web was recorded as length by width in centimeters, with length being the length down the supporting branch on which the web was built and width being the horizontal dimension perpendicular to the length. Distance between each web was measured in paces, which were later converted to meters. The number of individual spiders inhabiting each web was also counted and recorded. Two foreign spiders were introduced to each of the 20 experimental webs. Of these two, one originated from a web near the web to which it was introduced and the other originated from a web distant to the web to which it was introduced. A nearby w eb almost always meant a next door neighbor, ten meters away or fewer. For instance, a spider introduced to web number nine from web number eight would be considered a next door neighbor and may be used as a near web. Distant webs were chosen in such a w ay as to maximize the distance between the two webs and to minimize damage to the web population size by removing spiders from small web colonies. Observations were recorded in terms of the level of aggression displayed by the defending spider defensive spider is defined as the spider who is in its own web when the intruder is introduced. Aggression levels were grouped into three categories: ignoring, approaching, and aggressive. Ignoring means the defensive spider made no movement or indicati on of awareness that an intruder had been introduced. Approaching means the defensive spider moved from its place and may have climbed to a location very close to the intruder but made no attempt to chase, push, or bite the intruder. Chasing, pushing, or biting behaviors were dubbed aggressive. Spiders were transferred from web to web using the simple method of forceps and vial. The spiders were taken from their home web using forceps, transferred to a glass vial marked with their web number of origin, and their length was measured in centimeters. Since these spiders stay in an outstretched position, their lengths were measured from the tip of the front leg to the tip of the back leg. They were then carried to one of the other 20 webs, either nearby or far from their original web, and placed in the new web using forceps. Efforts were made to prevent anything other than the intruding spider from touching the new web. That is, forceps were held near enough for the spider to climb onto the new web but not n ear enough for the forceps to cause warning vibrations in the web. The relationship between aggression behavior and distance between webs was analyzed using a Chi squared test. The relationship between colony size and aggressive behavior was analyzed using regression analysis with the statistical software STATISTICA. By dividing colony sizes into big and small, signifying colonies with populations greater than nine and less than nine respectively, relationship between colony size and level of aggression was tested using another Chi squared test.
RESULTS There was no significant difference found between web distance and aggression level Ã°C Ã°2 = 5.47, df = 2, p = 0.06. Based on another Chi squared test there was also no significant difference between colo ny size and aggression level Ã°C Ã°2 = 2.43, df = 2, p = 0.30. Linear regressions for colony size and ignoring behavior F = 0.94, df = 1 and 18, p = 0.34, approaching behavior F = 3.36, df = 1 and 18, p = 0.08, and aggressive behavior F = 0.29, df = 1, p = 0.59 were not significant. DISCUSSION This study indicates that there is little relationship between the effects of colony size or distance of webs and level of aggression against intruders. This requires more studies to ascertain why the relationshi p between these variables is minimal. Perhaps with a larger sample size of webs the data may prove significant. Similarly, the relationship between web distance and aggression behavior is close to being significant and may prove to be so if more data is co llected Fig. 1. Another hypothesis to explain the lack of significance is that there is either a lack of or overabundance of relatedness between nearby webs. If juveniles regularly disperse to distances further than the ones used in this study, then the likelihood that spiders from the distant webs are related is unknown. There may be a need for repeating this experiment with a wider distance between webs to find out if this relationship is significant. Personal observation suggested that there might be a relationship between aggression of spiders and the presence of an egg sac. Adult spiders clutching egg sacs appeared more active and more likely to display aggressive behaviors. This factor would most likely depend on the time of year in which the study i s conducted. Other experimenters have noticed dramatic changes in web composition and behavior of members across different seasons. For instance during the dry season spiders tend to be less active, since activity causes them to desiccate more quickly. The number of adults, juveniles, and eggs is highly variable throughout the year. Therefore if this experiment is repeated in a different time of year it may yield different results Vollrath 1988. Along those lines, different weather conditions on data coll ection days may have played a role in spider behavior. Several days were misty or raining, while others were sunny. This study could have been improved with more information regarding the meaning of different spider behaviors. Primarily, it is uncertain wh ether approaching behaviors should be considered aggressive or benign. Also, based on personal observation, there is a wide variation in the behavior of individual spiders. This is especially evident when capturing spiders. There were many different resp onses on the part of Anelosimus spiders to being pursued by a pair of forceps, including hiding, dropping to the ground, and playing dead. Further investigation may clarify the connection between morphology and behavioral response to threat.
ACKNOWLEDGM ENTS I would like to thank Tania ChavarrÃa for the invaluable expertise, advice, and time she contributed to this project. I would also like to thank Karen Masters, CIEE, and the EstaciÃ³n BiolÃ³gica de Monteverde for providing insight and resources.
LITE RATURE CITED AvilÃ©s, L. and G. Gelsey. 1998. Natal dispersal and demography of a subsocial Anelosimus species and its implications for the evolution of sociality in spiders. Zoology 76: 2137 2147. Foelix, R.F. 1996. Biology of Spiders . pp 258 264. Oxford University Press. New York, New York. Jones, T. 2001. Delayed juvenile dispersal benefits both mother and offspring in the cooperative spider Anelosimus studiosis Araneae: Theridiidae. Behavioral Ecology 13:142 148. Powers, K. and L. AvilÃ©s. 2003. Natal dispersal patterns of a subsocial spider Anelosimus cf. Jucundus Theridiidae. Ethology 109: 725 737. Seibt, U. and W. Wickler. 1988. Interspecific tolerance in social Stegodyphus spiders Eresidae, Araneae. Arachnol. 16: 35 39. Tietjen, W. 1986. Social Spider Webs, with Special Reference to the Web of Mallos gregalis . Shear W.Red. Spiders: Webs, Behavior, and Evolution . pp 172 206. Stanford University Press. Stanford, California. Tollins, M. 2008. A Study of Social Interactions and Web Dynamics in the Spider Anelosimus sp ., Theridiidae in Monteverde, Costa Rica. CIEE. Spring 2008. Vollrath, F. 1986. Environment, reproduction, and the sex ratio of the social spider Anelosimus eximius Araneae, Theridiiae. Arachnol.14: 267 281.
0 2 4 6 8 10 12 14 16 18 ignored approached aggresive Behavior Frequency near far Figure 1 . Black bars show number of instances of each behavior per nearby spider that was introduced. White bars show number of instances of each behavior per far away spider that was introduced Ã°C Ã°2 = 5.47, df = 2, p = 0.06. 0 2 4 6 8 10 12 14 ignored approached aggresive Behavior Frequency small big Figure 2 . Black bars show number of instances of each behavior observed per small colony. White bars show number of instances of each behavior observed per big colony Ã°C Ã°2 = 2.43, df = 2, p = 0.30.
-0.5 0 0.5 1 1.5 2 2.5 0 5 10 15 20 25 30 35 40 # Approached Figure 3 . Regression line shows relationship between number of times approaching behavior was observed and population of web colony F=3.36, df=1 and 18, p=0.08. Table 1 . Linear re gression results show relationship between number of times each behavior was observed and the population of web colony. Regression F Degrees Freedom p Colony size vs. ignored 0.94 1 and 18 0.34 Colony size vs. approached 3.36 1 and 18 0.08 Colony size v s. aggressive 0.29 1 and 18 0.59