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La evidencia de la conducta subsocial mediante el anlisis de la captura de presas y la alimentacin en una nueva especie de Anelosimus (Araneae: Theridiidae)
Evidence of subsocial behavior through analysis of prey capture and feeding in a new species of Anelosimus (Araneae : Theridiidae)
Sociality is a little studied, little occurring, and unique trait in spider biology. Recently, in the genus Anelosimus, where sociality has originated eight or nine times, research has shown that more quasisocial species exist at low elevations and almost all subsocial species at higher elevations. The purpose of this study was to determine the sociality of a new species of Anelosimus, and add to the knowledge base of social Anelosimus spiders. Analysis of prey capture and feeding has provided evidence for a subsocial classification. In 12 webs of a female and her offspring, juveniles coordinated efforts to capture prey. Almost all of the spiders in the web fed communally (R=0.97; p=.0001). Juveniles more adeptly captured prey smaller than their body size prey, and spent longer capturing prey larger than their body. Other natural history aspects of these spiders were described, including guarding of egg sacks, web heights of 0.2m-2.3m, and tendency of webs to be found on tips of branches. With as much knowledge as to the extent of sociality in Anelosimus, we may be able to accurately analyze trends, and provide insight into the mechanisms of evolution and their origins.
El comportamiento social en las araas es poco frecuente y estudiado, as mismo parece estar ligado a un nico rasgo en la biologa de las araas. Recientemente, en el genero Anelosimus donde el comportamiento social se ha originado ocho o nueve veces, las investigaciones han demostrado que mas especies cuasi sociales se encuentran a elevaciones bajas y casi todas las especies subsociales a mayores elevaciones. El propsito de este estudio fue determinar el grado de sociabilidad en una nueva especie de Anelosimus y aadir mas conocimiento a la base de sociabilidad en este genero de araa.
Text in English.
Predator & prey
Depredador y presa
Tropical Ecology 2007
Ecologa Tropical 2007
t Monteverde Institute : Tropical Ecology
1 Evidence of subsocial behavior through analysis of prey capture and feeding in a new species of Anelosimus Araneae: Theridiidae Aaron Mitchell Dick Department of Biology, Indiana University Bloomington ABSTRACT Sociality is a little studied, littl e occurring, and unique trait in spider biology. Recently, in the genus Anelosimus , where sociality has originated eight or nine times, research has shown that more quasisocial species exist at low elevations and almost all subsocial species at higher ele vations. The purpose of this study was to determine the sociality of a new species of Anelosimus, and add to the knowledge base of social Anelosimus spiders . Analysis of prey capture and feeding has provided evidence for a subsocial classification . In 1 2 webs of a female and her offspring, juveniles coordinated efforts to capture prey. Almost all of the spiders in the web fed communally R=0.97; p=.0001. Juveniles more adeptly captured prey smaller than their body size prey, and spent longer capturing prey larger than their body. Other natural history aspects of these spiders were described, including guarding of egg sacks, web heights of 0.2m 2.3m, and tendency of webs to be found on tips of branches. With as much knowledge as to the extent of social ity in Anelosimus , we may be able to accurately analyze trends, and provide insight into the mechanisms of evolution and their origins . RESUMEN El comportamiento social en las araÃ±as es poco frecuente y estudiado, asÃ mismo parece ser ligado a un Ãºnico rasgo en la biologÃa de las araÃ±as. Recientemente, en el gÃ©nero Anelosimus donde el comportamiento social se ha originado ocho o nueve veces, las investigaciones han demostrado que mÃ¡s especies cuasisociales se encuentran a elevaciones bajas y mÃ¡s especies subsociales a mayores elevaciones. El propÃ³sito de este estudio fue d eterminar el grado de sociali dad en una nueva especie de Anelosimus y aÃ±adir mÃ¡s conocimiento a la base de socialidad en este gÃ©nero de araÃ±a. El anÃ¡lisis en la captura de presas y el co mportamiento de alimentaciÃ³n provee evidencia de comportamiento subsocial en esta especie. En las 12 telas analizadas una hembra y sus crÃas, los juveniles coordinan esfuerzos p ara capturar la presa. Casi tod o s los individuos en una tela se alimentan comun almenteR=0.97; p=.0001. Lo s juveniles captura n mayormente l a presa mÃ¡s pequeÃ±as que ellos y el tiempo de captura es mucho mayor con las presas de mayor tamaÃ±o que ellos. Otros aspectos de la historia natural que se describieron en este estudio incluyen el cuido de las bolsas de huevos, la altura de las telas que fue entre 0.2 y 2.3 m y la tendencia de las telas a encontrase al final de las ramas. Esto aÃ±ade conocimiento a los aspectos sociales en Anelosimus y con ellos tal vez se provee informaciÃ³n a los me canismos de evoluciÃ³n y sus orÃgenes.
2 INTRODUCTION Social spiders represent a remarkable evolutionary phenomenon in that it has originated multiple times in only a few species. In the family Theridiidae, sociality has originated independently 1 8 or 19 times among only 23 species Agnarsson et al . 2006. Repeated origins allow clear observation of how sociality evolved, and how it explains the evolution of some confounding traits, i.e. those selected at the group level Agnarsson et al . 2006; A viles and Tufino 1998. Spider sociality is a rare phenomenon, exhibited in about 0.06% of all spider species Aviles et al . 2007. It is confounding because these low occurrences, along with its multiple origins, imply that sociality is advantageous over th e short term, but over the long term the negative consequences of inbreeding may ultimately doom the lineage Agnarsson et al . 2007a. Social behavior can allow an organism to occupy a niche that it may not otherwise be able to access Wilson 1975; and i n the case of Anelosimus sp. , it could show benefits in saving on per capita investment in silk structures Riechert 1985, access to larger prey Jones and Parker 2000, predator defense, and access to mates Aviles and Bukowski 2006. The negative conseq uence of inbreeding depression that accompanies sociality in Anelosimus in all its origins Agnarsson et al . 2006 should have adverse effects on the evolution of sociality in a species. However, it has recently been shown that extrinsic demographic and ec ological factors selecting for group living and extreme philopatry may be much more powerful determinants of fitness Aviles and Bukowski 2006. These factors are what have allowed sociality to evolve and persist. That sociality generally decreases with i ncreased altitude Aviles et al . 2007 and other related biotic and abiotic factors has enabled research to analyze the determinants of sociality in an evolutionary context. A recent study found that the absence of a sufficient supply of large insects at h igher elevations and higher latitudes may restrict some or all social species to their lowland tropical habitat, and another lends support to this Â€prey sizeÂ hypothesis Powers and Aviles 2007; Aviles et al . 2007. Many other species of Anelosimus have b een described as subsocial living in the natal nest with the mother for the juvenile stage, and dispersal from the web before maturation at elevations around Monteverde, Costa Rica, a tropical premontane habitat Aviles et al . 2007. Research has sugges ted that the reason these are subsocial species is their size relation to their prey. As juveniles, these species have a much smaller body ratio to the body size of their prey. As they progress through instars, they grow larger, and by the time of maturi ty they are as larger or larger than most prey items. Because of this, when they are small juveniles they exist in the same web to maximize prey capturing in order to increase fitness and survive. When they reach maturity, they can easily capture most prey items on their own, and thus become territorial and disperse. If this new species of Anelosimus were subsocial as well, then it would exhibit similar characteristics. It was hypothesized that more spiders in a web acting together would be more effici ent at capturing prey, and that juveniles would be more efficient at capturing prey similar to their body size. After the prey was caught, I predicted that most of the spiders in the web would then feed communally, and no two mature spiders would be found in one web. Through analysis of prey capture and feeding, evidence of subsociality is herein provided, adding to the number of subsocial species of Anelosimus at higher elevations.
3 METHODS Natural history observations of Anelosimus sp. The study was c onducted in November, 2007 on the Sendero Cerro Amigo of the Monteverde Biological Reserve, Costa Rica, between elevations 1450m and 1550m. Data were collected from twenty different webs located along the edge of the road, where the population is ubiquito us. Twenty webs were selected, each containing a mother and juveniles, for data collection. Natural history observations were noted as a basis for analysis but were not included in data. These included females guarding egg sacks, sharing the web with oth er species, and the incorporation of dead plant matter into the web which was noted in each web. Webs ranged from 0.2m to 2.3m from the ground, were all found in open areas, and were constructed on the tips of branches of both living and dead plant matt er. Numbers of individuals in each web varied, ranging from 4 to 23 individuals, and no web contained more than one mature spider. Sampling. Fruit flies and small grasshoppers were collected in vials, and thrown into webs. In one five minute period, the following observations were recorded: capture time, number and identity of attackers, the individual that finally captured the prey, the number of feeders observed on the prey, and the number and identity of spiders in the web. Additional information was recorded for the physical properties of the web, including: web size, height off the ground, the type of substrate, and the dead leaf matter incorporated. Analysis. The relationship between capture time and number of spiders in the web was plotted and a linear correlation was performed to determine any trends. The frequency of capture by a juvenile was compared to whether or not the prey was larger than the juvenile; this was not statistically analyzed because of the small sample size, but a trend was e xhibited. Lastly, the number of spiders in the web was correlated using a Spearman Rank test with the number of spiders feeding on prey. RESULTS Twelve sets of data were recorded, and in total there were eight fruit flies captured, two grasshoppers c aptured, and two data sets of prey escape. There was no correlation between capture time and number of spiders in the web N=10; RÂ²=0.08; p=0.84; Fig. 1. There were only ten sets of data for this correlation, and capture times ranged from one second to 4 5 seconds. The frequency of capture by a juvenile showed a trend, that if the prey was larger than the juvenile, it was either rarely caught by the juveniles, caught by the mother, or it escaped the web Fig. 2. There was no analysis of this trend becaus e the low number of replications did not fit into statistical parameters 7 and 5 pieces of data, respectively. Table 1 shows a quick less than ten seconds capture time of prey smaller than juveniles. The number of spiders in the web was
4 strongly corr elated with the number of spiders feeding on prey captured N=8; R=0.97; p=.0001; Fig. 3. 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 Number of Spiders in the Web Capture Time seconds 0 1 2 3 4 5 6 7 Prey Captured by Juveniles Prey Not Captured by Juveniles Smaller Than Juveniles Larger Than Juveniles FIGURE 1. The ability of a colony to capture prey efficien tly in webs containing varying numbers of individuals N=10; RÂ²=0.08; p=0.84. FIGURE 2. Comparing the ability of juveniles to prey capture in relation to their body size. N=12
5 TABLE 1. Capture time of juveniles when prey is smaller than their body size. Individual to Capture Prey Larger Than the Juveniles? Capture Time seconds Juvenile N 2 Juvenile N 1 Juvenile N 4 Juvenile N 3 Juvenile N 7 Juvenile N 4 0 5 10 15 20 25 0 5 10 15 20 25 Number of Spiders Feeding on Prey Number of Spiders in the Web DISCUSSION The data presented here in the analysis of prey capture and communal feeding provide evidence that this species of Anelosimus could be classified as a subsocial species. After each prey capture, almost all of the spiders in the web converged to feed; this gives loose evidence that they not only tolerate each other, but that they are also participating in social activities. Females an d their offspring were observed in webs, solitary females were observed guarding their egg sacks, juveniles fed communally, and in one instance dispersal from the web were FIGURE 3. Observed number of spiders in the web partaking in communal feeding N=8; R=0.97; p=.0001.
6 witnessed. These are all traits characteristic of a subsocial spider species. In a si milar study, several other subsocial species were described at the same elevation of Monteverde, around 1500m: A. elegans, A. octavius, A. pallatanga, A. tosum, along with one newly discovered social species, A. guacamayos, which occurs just above or belo w 1500m Aviles et al. 2007. It is interesting to note that at lower elevations A. guacamayos exhibited quasisocial tendencies, and as elevation increased the species exhibited more subsocial tendencies around the elevation of Monteverde. The evidence on Anelosimus sp. and the elevation where it is found support this trend that species tend to be more subsocial as elevation increases. Capture time was not correlated to the number of individuals in the web Fig. 1. This was possibly due to the fact th at most prey items were of a ratio closer to the body size of the juveniles. Since the prey was small, and manageable as evidenced by the relatively fast capture time shown in Table 1, an individual juvenile did not need others to help capture prey, and thus the findings Fig. 1. The prey capture average was altered by one outlier, 45 seconds to prey capture. It is surprising that the web could retain a prey item for that long. This prey individual happened to be larger than the body size of the juv eniles, and so they attacked the prey slowly and simultaneously. This could partly explain the development of subsociality in this species, as the juveniles, possibly being smaller than most prey items, need to work together to capture prey. When they r each their final instars, they then disperse to become solitary as they may become larger than most prey they capture, and existing socially is then not beneficial. This has also been seen in the subsocial species, A. baeza and A. arizona, where size of c aptured prey relative to mean spider size in colonies was large in the early communal phase Powers and Aviles 2007. As spiders approached dispersal, the spiders grew to be as large as or larger than most prey items Powers and Aviles 2007. This seems to be consistent with the factors that define a subsocial species Â‚ absence of a sufficient supply of large insects at higher elevations and higher latitudes that may restrict social species to their lowland tropical habitat Powers and Aviles 2007. In other words, where size of prey generally decreases with altitude, the ratio of spider size to prey size grows in favor of the spider, making it easier for them to capture prey caught in the web, and selection for sociality beyond the juvenile stage decreases. There is a trend shown in Figure 2 that juveniles more frequently capture prey closer to or less than their own size. Whenever the prey item was smaller than the juveniles, they showed a quick reaction time Table 1, and successfully caught smaller prey w ith a higher frequency than larger prey Fig. 2. Typically the mother would catch prey that was larger than the juveniles. This also illustrates a benefit of early communal living, because the mother ensures her offspring can feed. In all instances but one, where the mother caught the prey, she bit, wrapped, and left the prey to be eaten by the juveniles. Subsequently, when a prey item was caught most of the juveniles in the web converged to feed communally Fig. 3. Such a strong relation was observed because almost all spiders in the web were found feeding, and those that were not were active in the web. It appeared that some spiders were also excluded from feeding just because there was not ample feeding space around the prey. In one instance, wher e there were only four spiders found in the web, the juveniles were larger and appeared in a later stage of development. When the prey was captured, only two spiders attacked; once the prey was caught, only one spider fed on the prey and did not allow the other to feed. This is evidence of the mechanism for dispersal driving competition between older or larger individuals. Here a previously not described species, Anelosimus sp., has been shown to be a subsocial species found around 1500m elevation. Th is research supports the trend that subsocial
7 Anelosimus spiders are found more commonly at higher altitudes, where quasisocial species are not. Further research on this species may aid in determining why this trend exists. A pertinent question for future research is whether or not sociality in spiders leads to higher fitness than that of solitary spiders. Knowing that subsocial spiders tend to be found at higher elevations, and that at higher elevations it has been found that available prey size is smalle r, this could explain why most species are subsocial. As individuals in these species reach maturity, it is easy for them to catch prey. Then they do not need to rely on group cooperation to catch prey, or having their mothers to catch prey to survive, a nd they disperse. The two trends in insect and prey size and availability with respect to altitude make a compelling argument for describing differences in sociality in one genus or even one species, but it has not yet been determined why ultimately soci ality has evolved if there exists a wide range of sizes of all kinds of solitary spiders, in all habitats where social spiders are extant. An excellent study aiming to analyze the differential fitness and success of social clades versus non social clades, in direct measurement of biomass and fitness, may finally explain the evolutionary origins of sociality. If they find that social clades do in fact have higher biomass and fitness, then this and subsequent inbreeding depression could explain why social ity has arisen so many times and is still extant, yet rarely occurring. Agnarsson et al. 2006. ACKNOWLEDGEMENTS Thank you Tania. Thank you to the Monteverde Biological Station for the area of study. Thanks Alan and Karen Masters as well for advising and help throughout the project. Thanks Taegan for helping with prey collection in that stinky compost pile. Thanks Pablo for his part in making this a worthwhile experience. LITERATURE CITED Agnarsson, I., L. Aviles, J. A. Coddington and W. P. Mad dison. 2006. Sociality in Theridiid spiders: repeated origins of an evolutionary dead end. The Society for the Study of Evolution. 60: 2342 2351. Agnarsson, I., G. Barrantes and L. J. May Collado. 2007a. Notes on the Biology of Anelosimus pacificus Levi, 1963 Theridiidae, Araneae Âƒ evidence for an evolutionary reversal to a less social stateÂ„. Journal of Natural History. 40: 2681 2687. Agnarsson, I., W. P. Maddison and Leticia Aviles. 2007b. The phylogeny of the social Anelosimus spiders Araneae: Ther idiidae inferred from six molecular loci and morphology. Mol. Phylo. and Evol. 43: 833 851. Aviles, L., I. Agnarsson, P. A. Salazar, J. Purcell, G. Iturralde, E. C. Yip, K. S. Powers and T. C. Bukowski. 2007. Natural History Miscellany: altitudinal patte rns of spider sociality and the biology of a new midelevation social Anelosimus species in Ecuador. 2007. The Amer. Nat. 170: 783 792. 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 J. Guevara. 2007. Multiple techniques confirm elevational differences in insect size that may influence spider sociality. Ecology. 88: 2015 2033. Aviles, L. and P. Tufino. 1998. Colony size and individual fitness in the social sp ider Anelosimus eximius . The Amer. Nat. 152: 403 418. Jones, T. C. and P. G. Parker. 2000. Costs and benefits of foraging associated with delayed dispersal in the spider Anelosimus studiosus Araneae, Theridiidae. J. Arachnol. 28: 61 Âƒ 69. Powers, Kimberl y S. and Laticia Aviles. 2007. The role of prey size and abundance in the geographical distribution of spider sociality. Journal of Animal Ecology. 76: 995 1003.
8 Purcell, J. and L. Aviles. 2007. Smaller colonies and more solitary living mark higher elevat ion populations of a social spider. J. of Anim. Ecol. 76: 590 597. Riechert, S. E. 1985. Why do some spiders cooperate? Agelena consociata : a case study. Behav. Ecol. Symp. Entomol. Soc. 68: 106 Âƒ 116. Wilson, E.O. and B. Holldobler. 2005. Eusociality: ori gin and consequences. Proc. Natl. Acad. Sci. 102: 16119.