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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
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, 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 evid ence for a subsocial classification In 12 webs of a female and her offspring, juveniles coordina ted efforts to capture prey. Al most 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, includ ing guarding of egg sacks, web heights of 0.2m2.3m, 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. RESUMEN El comportamiento social en las araas es poco frecuente y estudiado, as mismo parece ser ligado a un nico rasgo en la biologa de las araas. Recientemente, en el genero Anelosimus donde el comportamiento social se ha origin ado ocho o nueve veces, las investigaciones han demostrado que mas especies cuasisociales se encuentran a elevaciones bajas y mas especies subsociales a mayores elevaciones. El propsito de este estudio fue determinar el grado de socializad en una nueva especie de Anelosimus y aadir mas conocimiento a la base de socialidad en este genero de araa. El anlisis en la captura de presas y el comportamiento de alimentacin provee evidencia de comportamiento subsocial en esta especie. En las 12 telas analizadas una hembra y sus cras, los juveniles coordinan esfuerzos para capturar la presa. Casi todas los individuos en una tela se alimen tan comunalmente(R=0.97; p=.0001). Lo juveniles captura mayormente la presas mas pequeas que ellos y el tiempo de captura es mucho mayor con las presas de mayor tamao que ellos. Otro s aspectos de la historia natural que se describieron en este estudio incl uyen el cuido de las bolsas de hue vos, 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 aade conocimiento a los aspectos sociales en Anelosimus y con ellos talvez se provee informacin a los mecanismos de evolucin y sus orgenes. 1
INTRODUCTION Social spiders represent a remarkable evoluti onary phenomenon in that it has originated multiple times in only a few species. In the family Theridiidae, sociality has originated independently 18 or 19 times among only 23 species (Agnarsson et al. 2006). Repeated origins allow clear observation of how sociality evolved, and how it expl ains the evolution of some confounding traits, i.e. th ose selected at the gr oup level (Agnarsson et al 2006; Aviles and Tufino 1998). Spider sociality is a rare phenomenon, exhibi ted in about 0.06% of all spider species (Aviles et al 2007). It is confounding because this lo w occurrence, along with its multiple origins, imply that sociality is advantageous over the short term, but over the long term the negative consequences of inbreeding may u ltimately 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 in the case of Anelosimus sp., it could show benefits in saving on per capita investment in silk structures (Riechert 1985), acce ss to larger prey (Jones and Parker 2000), predator defense, and access to mates (Aviles and Bukowski 2006). The negative consequence of inbreeding depression that accompanies sociality in Anelosimus in all its origins (Agnarsson et al 2006) should have adverse affects on the evolution of sociality in a species. However, it has recently been s hown that extrinsic demographic and ecological factors selecting for group living and extreme philopatry may be mu ch more powerful determinants of fitness (Aviles and Bukowski 2006). These factors are wh at have allowed sociality to evolve and persist. That sociality generally decreases wi th increased altitude (Aviles et al 2007) and other related biotic and abiotic factors has enabled research to analyze th e determinants of sociality in an evolutionary context. A recent study found th at the absence of a sufficient supply of large insects at higher elevations and hi gher latitudes may restrict some or all social species to their lowland tropical habitat, and another lends support to this prey size hypothesis (Powers and Aviles 2007; Aviles et al 2007). Many other species of Anelosimus have been described as subs ocial (living in the natal nest with the mother for the juvenile stage, a nd dispersal from the web before maturation) at elevations around Monteverde, Costa Rica, a tropical premontane habitat (Aviles et al 2007). Research has suggested that the reason these are subsocial species is thei r size relation to their prey. As juveniles, these speci es 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 maturity they are as larger or larger than most prey items. Because of this, when they are small (juven iles) 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 efficient at capturing prey, and th at juveniles would be more effici ent at capturing prey similar to their body size. After the prey wa s 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 feed ing, evidence of subsociality is herein 2
provided, adding to the number of subsocial species of Anelosimus at higher elevations. METHODS Natural history observations of Anelosimus sp. The study was conducted in November, 2007 on th e 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, wh ere the population is ubiquitous. Twenty webs were selected, each containing a mother and juveniles, for data collection. Natural history observa tions were noted as a basis for analysis but were not included in data. These included females guarding egg sack s, sharing the web with other 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 f ound in open areas, and were constructed on the tips of branches of both living and dead plant matter. Numbers of individual s 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 re corded: capture time, number and identity of attackers, the individual that finally captured the prey, the nu mber 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 ti me and number of spiders in the web was plotted and a linear correlation was performed to determ ine 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, bu t a trend was exhibited. 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 r ecorded, and in total there were eight fruit flies captured, two grasshoppers captured, and two da ta sets of prey escape. There was no correlation between capture time and number of spiders in the web (N=10; R=0.08; p=0.84; Fig. 1). There were only ten sets of data for this corre lation, and capture times ranged from one second to 45 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 th e mother, or it escaped the web 3
(Fig. 2). There was no analysis of this trend b ecause 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 juvenile s. The number of spiders in the web was strongly correlated with the number of spider s 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 510152025 Number of Spiders in the Web FIGURE 1. The ability of a colony to capture prey efficiently in webs containing varying numbers of individuals (N=10; R=0.08; p=0.84). 4
0 1 2 3 4 5 6 7 Prey Captured by Juveniles Prey Not Captured by Juveniles Smaller Than Juveniles Larger Than Juveniles FIGURE 2. Comparing the ability of juveniles to prey capture in relation to their body size. (N=12) 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 5
0 5 10 15 20 25 0 5 10 15 20 25 Number of Spiders Feeding on Prey FIGURE 3. Observed number of spiders in the web partaking in communal feeding (N=8; R=0.97; p=.0001). DISCUSSION The data presented here in the analysis of prey capture and comm unal feeding provide evidence that this species of Anelosimus could be classified as a s ubsocial 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 th ey are also participating in social activities. Females and 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 was witnessed. These are all traits ch aracteristic of a subsocial spid er species. In a similar 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 below 1500m (Aviles et al. 2007). It is interesting to note that at lower elevations A. guacamayos exhibited quasisocial tendencies, and as elevation increased the speci es exhibited more subsocial te ndencies (around the elevation of Monteverde). The evidence on Anelosimus sp. and the elevation wher e 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 that most prey items were of a ratio closer to the body size of the juveniles. Since the prey was small, and manage able (as evidenced by the relatively fast capture time shown in Table 1), an individua l 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 th e web could retain a prey item for that long. This prey individual happened to be larger than the body size of the juveniles, and so they attacked the prey slowly 6
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 reach their final instars, they then disperse to become solitary as they may become larger than most prey they capture, and existing so cially is then not beneficial. This has also been seen in the subsocial species, A. baeza and A. arizona, where size of captured prey relative to mean spider size in colonies was large in the early communal phase (Powers and Aviles 2007). As spiders approached disper sal, the spiders grew to be as large or larger than most prey items (Powers and Aviles 2007). This seems to be consistent with the factors that define a subsocial speciesabsence of a sufficient supply of large insects at higher elevations and higher latitudes that may restrict soci al species to their lowland trop ical habitat (Powers and Aviles 2007). In other words, where size of prey genera lly decreases with altit ude, 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 juvenile s 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 with 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 moth er 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 almo st all spiders in the web were found feeding, and those th at 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, where there were only four spiders found in the web, the juveniles were larger and appeared in a later stage of development. Wh en 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 mechanis m 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 ar ound 1500m elevation. This research supports the trend that subsocial Anelosimus spiders are found more commonly at higher al titudes, where quas isocial species are not. Further research on this species may aid in determining why this trend exists. A pertinent question for future research is wh ether or not sociality in spiders leads to higher fitness than that of solitary spiders. K nowing that subsocial spiders tend to be found at higher elevations, and that at hi gher elevations it has been found that available prey size is smaller, this could explain why mo st 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, and they disperse. The two trends in insect and prey size and av ailability with respect to altitude make a compelling argument for describing differences in so ciality in one genus or even one species, but it has not yet been determined why ultimately sociality 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 succe ss of social clades versus nonsocial clades, in direct measurement of biomass and fitness, may finally explain the evolutionary origins of sociality. If they fi nd that social clades do in fact have higher biomass and fitness, 7
8 then this (and subsequent inbr eeding depression) could explain w hy sociality 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. Ma ddison. 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) evidence for an evolutionary re versal to a less social state. Journal of Natural History. 40: 2681-2687. Agnarsson, I., W. P. Maddison and Leticia Aviles. 2007b. The phylogeny of the social Anelosimus spiders (Araneae: Theridiidae) inferred from six molecular lo ci and morphology. Mol. Phylo. and Evol. 43: 833851. 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 patterns of spid er 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 conf irm elevational differences in insect size that may influence spider sociality. Ecology. 88: 2015-2033. Aviles, L. and P. Tufino. 1998. Colony size an d individual fitness in the social spider 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: 6169. Powers, Kimberly S. and Laticia Aviles. 2007. The ro le of prey size and abundance in the geographical distribution of spider sociality. Journal of Animal Ecology. 76: 995-1003. Purcell, J. and L. Aviles. 2007. Smaller colonies and more solitary living mark higher elevation 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: 10616. Wilson, E.O. and B. Holldobler. 2005. Eusociality: orig in and consequences. Proc. Natl. Acad. Sci. 102: 16119.