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Un estudio de las interacciones sociales y dinmicas en la araa Anelosimus sp., (Theriidae) en Monteverde, Costa Rica
A study of social interactions and web dynamics in the spider Anelosimus sp., (Theriidae) in Monteverde, Costa Rica
Sociality in spiders represents serious evolutionary drawbacks, yet persists in some 20 species. Sociality of an undescribed species of spider, Anelosimus sp. (Theriidae), was studied in Monteverde, Costa Rica. The social interactions that were considered were: prey capture, kin recognition and web dynamics. I observed prey capture, whether adults or juveniles were attacking, and how long it took to first contact the prey. Adults preferentially attacked larger prey, while juveniles preferentially attacked smaller prey (p = 0.001, 2 = 10.517, df = 1, n = 23). There was also a trend showing that spiders in multiple-spider webs contacted prey faster than those in single-spider webs (p = 0.09). These results suggest that an increase in efficiency and a sharing of the prey-capture burden act as mechanisms to help make social living beneficial. A second part of the study consisted of introducing foreign spiders, both juveniles and adults, to webs and recording the reaction. The introduced adults were almost always shown aggression, while introduced juveniles were never shown aggression. This suggests the presence of kin recognition, as foreigners were recognized; smaller juveniles were largely ignored, while large foreigners were attacked and chased off. Size could be the mechanism for determining when juveniles finally disperse from the natal web. Finally, I found a positive relation between web volume and number of spiders (p = 0.001) and number of smaller webs within 50 cm (p = 0.08). This indicates that the web must increase proportionally with each additional spider in a brood. My study suggests possible mechanisms for making social living beneficial despite the serious evolutionary drawbacks.
La sociabilidad en las araas representa serias desventajas evolucionaras, encontrndose en alrededor de 20 especies. El comportamiento social no descrito en las especies de Anelosimus sp. (Theriidae), fue estudiado en Monteverde, Costa Rica. Las interacciones sociales que fueron consideradas son: la captura de presas, el reconocimiento de parentela y la dinmica de la tela.
Text in English.
Spiders--Behavior--Costa Rica--Puntarenas--Monteverde Zone
Kin recognition in animals
Araas--Comportamiento--Costa Rica--Puntarenas--Zona de Monteverde
Reconocimientos de parentela
Tropcial Ecology 2008
Ecologa Tropical 2008
t Monteverde Institute : Tropical Ecology
1A Study of Social Interactions and Web Dynamics in the Spider Anelosimus sp., (Theriidae) in Monteverde, Costa Rica. By Molly Tolins Department of Biology, University of Wisconsin Ma dison ABSTRACT Sociality in spiders represents serious evolutionar y drawbacks, yet persists in some 20 species. Sociality of an undescribed species of spider, Anelosimus sp (Theriidae), was studied in Monteverde, Costa Rica. The social interactions that were considered were: prey capture, kin recognition and web dynamic s. I observed prey capture, whether adults or juveniles were attacking, and how long it took to first conta ct the prey. Adults preferentially attacked larger prey, w hile juveniles preferentially attacked smaller prey ( p = 0.001, c 2 = 10.517, df = 1, n = 23). There was also a tren d showing that spiders in multiple-spider webs contacted prey faster than those in single-spider w ebs ( p = 0.09). These results suggest that an increase in efficiency and a sharing of the prey-capture burden act as mechanisms to help make social living benef icial. A second part of the study consisted of introducing foreign spiders, both juveniles and adults, to web s and recording the reaction. The introduced adults were almost always shown aggression, while introduced juveniles were never shown aggression. This sugges ts the presence of kin recognition, as foreigners w ere recognized; smaller juveniles were largely ignored, while large foreigners were attacked and chased of f. Size could be the mechanism for determining when ju veniles finally disperse from the natal web. Final ly, I found a positive relation between web volume and nu mber of spiders ( p = 0.001) and number of smaller webs within 50 cm ( p = 0.08). This indicates that the web must increas e proportionally with each additional spider in a brood. My study suggests po ssible mechanisms for making social living benefici al despite the serious evolutionary drawbacks. RESUMEN La socialidad araas representa serios desventajas evolucionarias, encontrndose en alrededor de 20 especies. El comportamiento social no descrito en l as especies de Anelosimus sp (Theriidae), fue estudiado en Monteverde,Costa Rica. Las interacciones socials que fueron consideradas son: captura de presas, reconocimiento de parentela y dinmica de la tela. Observ la captura de las presas, viendo si tanto adultos como juveniles realizan el ataque y cuanto tiempo t oma el primer contacto con la presa. Los adultos at acan preferiblemente a presas ms grandes, mientras que los juveniles atacan principalmente a las presas pequeas ( p = 0.001, c 2 = 10.517, df = 1, n = 23). Existe tambin una ten dencia que muestra que telas con multiples araas el tiempo de contacto con la presa es mas rapido que aquellas araas que se encuentra n solas en una sola tela (p = 0.09). Estos resultado s sugieren que un aumento en la eficiencia de captu ra y en compartir la presa pueden actuar como mecanismos pa ra el beneficio de la vida social. Una segunda par te del estudio consiste en introducir araas forneas, tanto juveniles como adultas en las telas y obse rvar la reaccin. Los adultos introducidos mostraron siemp re agresin, mientras que los juveniles nunca mostraron agresividad. Esto sugiere el reconocimien to de parentela, as, cuando los foraneos fueron reconocidos; los juveniles pequeos fueron ignorado s, mientras que los grandes fueron atacados y botad os de la tela. El tamao puede ser el mecanismo para d eterminar cuando los juveniles se dispersan finalme nte de la tela natal. Finalmente, encontr una relaci n positiva entre el volumen de la tela y el nmero de araas (p = 0.001) y el nmero de telas pequeas de ntro de los 50 cm (p = 0.08). Esto indica que las telas pueden aumentar proporcionalmente con cada araa ad icional en la camada. Mi estudio sugiere posibles mecanismos para hacer la vida social beneficiosa m s all de las desventajas evolucionarias.
2INTRODUCTION Spiders hold a formidable reputation for being sol itary and aggressive towards each other. This is generally true, but approximat ely 20 of the known 34,0000 species coexist socially (Foelix 1996). These unusual speci es practice cooperative brood care, prey capture, or both. Social spiders come from several distinct families ; one example, the Theridiids, contain a number of social species in the genus Anelosimus. These species of Anelosimus have a maternal colony, where the spiderlings remai n in the maternal web several weeks after maturation, until they are able to spin webs themselves (Jones and Parker 2002). These juveniles are more likely to survive to adult hood. Social living can also have evolutionary drawbacks such as extreme inbreeding depression (Agnarsson et al. 2006). A study on the effects of colony size on individual fitness showed that individuals benefit up to a cer tain colony size, but when the colony gets too large, individual fitness suffers (Aviles and Tufino 1998). They found a reduced chance of egg-laying females and a higher chance of infection by egg sac parasitoids (Aviles and Tufino 1998). However, sociality persi sts in these spider species, and studies have shown that the benefits can outweigh the losse s; although inbreeding has been found, resulting in smaller body sizes and longer d evelopment time, survivability and fitness do not suffer because group living and mate rnal care cushion the inbreeding effects (Aviles and Bukowski 2006). The benefits of social living differ for different forms of sociality. Some systems of sociality allow the colony to more efficiently c atch bigger prey, while others provide a higher survival rate in broods (Aviles and Tufino 1 998). The juveniles, initially smaller than their prey, benefit through communal feeding a nd sometimes rely on their mothers regurgitation (Foelix 1996). Young are less suscep tible to predators, due to their proximity to their mother, and the mother is improv ing her evolutionary fitness by ensuring the survival of her progeny (Aviles and Tu fino 1998). Interestingly, a removal study with the species Anelosimus studiosus showed that the communal living benefits the survivability of t he mother as well as her brood. Mothers in this study surrounded by their brood liv ed longer and produced a second clutch sooner than solo mothers (Jones and Parker 2 002). One possible mechanism of this increased lifespan viability is the juvenile c ontribution to prey capture. In some Anelosimus species the juveniles who grow on the natal web re ach web-weaving maturity but can remain on the web up to several additional weeks (Jones and Parker 2002). This implies that the juveniles are able to build their own webs and capture their own prey, allowing communal prey capture. This cooperation b etween mother and her brood increases the efficiency of capture prey. This kin d of cooperation is called mass-action behavior, where each individual reacts predictably to a given situation in the same way, not necessarily through social communications (Tiet jen 1986) Another benefit to social living is that more spiders mean more spiders alert for prey. The whole colony benefits by having more spider sentries on a web (Reichert 1 985). This is also seen to occur in the Anelosimus sp found in Monteverde. Furthermore, juveniles catch smaller prey, which allows them to fulfill a different feeding ni che than the mother. This also means that the mother can save her energy and focus on la rger prey items when available (Foelix 1996), which alleviates intraspecies compet ition.
3 Not only can juveniles help with more rapid and suc cessful prey capture, they may also be able to take a portion of the prey-capt ure load entirely onto themselves. Juvenile spiders are capable of catching small prey items on their own, meaning the mother can save her energy and focus on larger prey items when they are available (Foelix 1996). This could be one mechanism by whic h the juveniles improve the survivability of the mother. Most social spiders can recognize non-kin conspecif ics, and that the mother allows her brood to remain on her web suggests kin recognition. However, most social spiders do not show aggression to non-kin conspecif ics (Beavis et al 2007). Furthermore, size studies have been conducted and revealed that only juveniles below a certain size are tolerated; larger juveniles may be killed (Beav is et al 2007). Kin recognition and cooperation could be a mechanism of sociality in th is new Anelosimus species as well. Another important aspect of this species of social spider to consider is web dynamics. When a mother has a brood, the web is ex panded to provide space for the spiderlings and to increase prey-capture area. A p revious study suggested, however, that the web does not increase proportionately with the increase in spider number, meaning with more spiders there will also be an increase in spider density (Reichert 1985). This denser spider population could possibly lead to mor e rapid prey capture. Web distribution can also provide insight into the social dynamics of Anelosimus sp. As in other species of this genus, several wee ks after juveniles have reached maturity, they will leave their mothers web (Jones and Parke r 2002). However, the distance they travel before constructing their webs is unknown in this species. A clumped distribution of webs, with small webs surrounding larger central webs, could suggest the juveniles only travel short distances before settling. Webs w ithin a certain distance may house close relatives or even brood-mates. This study is investigating different aspects of t he sociality of this newly discovered, subsocial (Mitchell-Dick 2007) species of the genus Anelosimus found in Monteverde It will look at web dynamics, such as web volume t o spider number and whether the webs were oriented around a central pos sible web of origin. This study will also look at prey capture behavior as a function of social interaction in Anelosimus sp I hypothesized that the more spiders present on a web the more quickly prey items would be contacted; in addition, I predicted that adults would preferentially attack larger prey, and juveniles would attack smaller prey. A seconda ry objective of my study explored foreigner recognition. I predicted that all foreign spiders would be met with aggression when introduced to an alien web, no matter their si ze. MATERIALS AND METHODS Study Site and Web Measurements This study was performed in April and early May of 2008 in Monteverde, Costa Rica. The research was conducted at three differen t roadside sites: on the Cerro Amigo (1440m to 1655m, steep open road), near the Creativ a school (1370m to 1440m), and in Caitas (1365m to 1370m). Data was collected from 3 1 Anelosimus sp. webs total in the three sites. The following size measurements were recorded for each web: length, width,
4 and height of web, and were used to calculate web ( Fig. 1). I also recorded the number of spiders and small periphery webs within 50 cm. Fig.1: Web dimension measurements collected at the end of a branch. Length was measured parallel to the supporting branch, height was measured from the highest to the lowest points, and width was measured perpendicular to the widest part of the web. Data Collection The collected prey consisted of small (under one c entimeter) and large (over one centimeter) flies. The prey items were thrown into each web until a spider successfully made contact with a prey, and time until first cont act was recorded. Attacker and prey size were also noted. I then conducted foreigner re cognition trials, where foreign spiders, from differing distances, were introduced and aggression was noted. This was repeated ten total times, five trials with juvenile s and five trials with adults. Adults are distinguished by the two spots on their cephalothor axes. Analysis The prey and predator sizes data were analyzed usi ng a chi-squared test. Linear regressions were performed on the relationships bet ween total web volume and number of periphery webs and between total web volume and number of spiders present. Finally, the average time to first contact data for multiple spider webs and single spider webs were compared using a two-sample t-test. RESULTS Length Width Height Branch Tip
5 Prey Capture I found that adults more frequently attack big pr ey and juveniles more frequently attack small prey (adults : 7 out of 8 trials, and juveniles: 11 out of 13 trials; p = 0.001183, c2 = 10.517, df = 1; big prey: n=8, small prey: n=13; Fig. 2). I also found a trend that multiple-spider webs had a lower average time of first contact, (32.25 41.82 sec. n = 9), than single-spider webs, ( 91.9 88.3 4 sec.; p = 0.09: n = 14; Fig. 3). nrr nr r r Figure 2: The frequency of adult and juvenile spid ers attacking small and big prey on multiple-spider webs ( p = 0.001, c2 = 10.517, df = 1; Big prey: n=8, Small prey: n= 13 ).
6 rrnr rr Figure 3: A trend was shown in reduced average time to first prey contact for multiplespider webs (32.25 41.82) than single-spider webs (91.9 88.34; p = 0.09; multiple spiders: n = 9, single spiders: n = 14).
7 Foreigner Recognition Foreign adults were shown aggression 80 percent of the time, while juveniles were never shown aggression (Fig. 4). rr Figure 4. The percentages of trials that aggression was shown for adult and juvenile web transfers. (n = 5 for both adults and juveniles). Web Dynamics I found that the number of spiders in the web incre ased with web volume ( p = 0.001, n = 23; Fig. 5), and the number of periphery webs increased with web volume ( p = 0.086, n = 23; Fig. 6).
8 !"## $ % # # # & # # ###& r Figure 5. A positive regression of number of spider s present and web volume (cm3 ; p = 0.001, n=23). %'(#!" $ & & # % r Figure 6: A trend showing a positive regression of web volume (cm3) with the number of periphery webs. ( p = 0.08, n = 31).
9DISCUSSION Juvenile spiders of the Anelosimus genus often remain on the maternal web past maturity; more spiders on a web mean a higher deman d for food. My results showed that the mothers and juveniles fill different feeding ni ches, and therefore, competition and stress on the mother is reduced. This could be a me chanism for how this social living benefits both the mother and the juveniles; it esse ntially expands upwards the size range of possible prey, as the mother can focus on the la rge prey and the juveniles can focus on the small prey, and all can enjoy the feast. Juveni les may also aid the mother by keeping the web clean of little prey the mother would other wise have to remove to reduce clutter. Furthermore, the trend of faster prey contact on mu ltiple-spider webs suggests that the additional spiders aid in locating and attacking pr ey. This could lead to higher prey capture efficiency and fewer web escapes. I also found that foreign adults introduced to oth er webs were usually met with aggression, which was never the case for foreign ju veniles (Fig. 4). This could suggest that Anelosimus sp have some kin recognition or that they at least re cognize long term inhabitants. The fact that juveniles were not met w ith aggression implies that they are not seen as a threat or a burden to the adults, further ing the possibility that they actually may confer some advantages to the adult. Studies have s hown this size index, a discrimination in reaction between adults and juveniles, in other social species (Beavis et al 2007). This size index may be a mechanism for determining juven ile dispersal; once juveniles reach a size where they are in competition with the mother for larger prey, she may no longer tolerate them and act aggressively towards them, fo rcing them off the web. In addition, I found a positive trend relating web volume to number of periphery webs (Fig. 6). The larger webs were probably older and more developed; they also would have had longer to have broods, so a greater number of juveniles would have dispersed from the natal web over time. It is possi ble that juveniles do not actually disperse far from their maternal web. This may exp lain the trend seen between high web volume and an increase in periphery webs. A future study could investigate the genetic relationship of spiders living in close proximity t o one another. This limited range dispersal could have detrimental effects on populat ions as it could increase inbreeding depression. My web analysis also revealed that a mother spider with a brood, must increase her web proportionally depending on how many offspr ing she has. This means that as the number of spiders increase, the density of the spid ers does not. Multiple-spider webs in my study did not necessarily have a denser spider p opulation, yet there was a trend showing that multiple-spider webs contacted prey fa ster. This finding eliminates the idea that spiders per unit volume in a web is the determ ining factor in how quickly and successfully prey is caught, as suggested by other studies (Reichert 1985). There may be other factors, like communication or strategic dist ribution (Foelix 1996). My results help explain the evolutionary drive beh ind the social living seen in this species. Social tendencies in spiders can represen t serious evolutionary problems, such as inbreeding depression and extreme intraspecific competition. This is probably why it is seen in only about 20 spider species. However, the existence of this lifestyle implies it must have some advantages over solitary life. Comm unal living reduces the strain on the mother by sharing the burden of prey capture, reduc ing mother/young competition for
10 food and increasing prey location efficiency, thus improving the chances of successful prey capture (Jones and Parker 2002). Combined, the se benefits may be enough to help drive and perpetuate the social behavior in Anelosimus sp. This study should be the first of many on this unk nown social spider species. Future studies should continue to explore the relat ionship between sociality and prey capture. A reduction in contact time seems a likel y mechanism for helping to perpetuate social living. Further studies could also do genet ic analyses on the relatedness of colonies and adjacent webs, helping to determine th e extent of inbreeding caused by sociality in Anelosimus sp. ACKNOWLEDGEMENTS I would like to thank Tania Chavarra for helping m e locate spider webs, helping to direct my inquirie s, and advising my statistics. I could not have done it w ithout her. I would also like to thank the owners of the road to Cerro Amigo for waiting at least halfway th rough my research before clearing my study site. LITERATURE CITED AGNARSSON, I.,L. AVILS.,J.A CODDINGTON, AND W.P MADDISON. 2006. Sociality in Theridiid spiders: repeated origins of an evolu tionary dead end. Evolution, 60(11): 2342-2351. AVILES, L. AND T. C. BUKOWSKI. 2006. Group living and inbreeding depression in a subsocial spider. Proc. R. Soc. B. 273: 157-163. AVILES, L. AND P.TUFINO. 1998. Colony size and individual fitness in the social spider Anelosimus eximius. The American Naturalist. 152 : 403-418. BEAVIS, A.S., D.M. ROWELL, AND T. EVANS. 2007. Cannibalism and kin recognition in Delena cancerides (Araneae: Sparassidae), a soci al huntsman spider. Journal of Zoology, 271: 233. FOELIX, R.F. 1996. Biology of Spiders, Second Edition, pp 258-264. Oxford University Press, New York. JONES, T.C. AND P.G. PARKER. 2000. Costs and benefits of foraging associated w ith delayed dispersal in the spider Anelosimus studios us (Araneae, Theridiidae). J. Arachnol. 28: 61-69. MITCHELL-DICK, A. 2007. Evidence of subsocial behavior through a nalysis of prey capture and feeding in a new species of Anelosimus (Araneae: Theridiidae). CIEE, Fall 2007. RIECHERT, S.E. 1985. Why do some spiders cooperate? Agelana c onsociata: a case study. Behav. Ecol. Symp. Entomol. Soc. 68: 106-116. TIETJEN, W.J. 1986. Social spider webs, with special refer ence to the web of Mallos gregalis. Found in: Spiders; webs, behavior, and evolution, pp. 172-206. Edited by William A. Shear. Stanford University Press, S tanford, California.