stabilimenta function

Stabilimenta - doi 10.1098/rspb.2001.1709 Stabilimenta attract unwelcome predators to orb-webs Wee Khee Seah and Daiqin Li Department of Biological

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Unformatted text preview: doi 10.1098/rspb.2001.1709 Stabilimenta attract unwelcome predators to orb-webs Wee Khee Seah and Daiqin Li* Department of Biological Sciences, National University of Singapore, Singapore 119260 Conspicuous behaviour exposes animals to predation; prey-attraction thus often con£icts with antipredator behaviour. The fact that a conspicuous ultraviolet-light re£ecting silken structure in the orb-webs of certain spider species, known as a stabilimentum, makes the webs obvious to both prey and predators has been used to argue that spiders bene¢t from building stabilimenta by attracting prey and/or defending against visually hunting predators. Here, we provide experimental evidence that stabilimenta can act as visual signals that attract web-invading spider-eating predators with acute vision to the webs. We also show that the predators can learn to remember a particular type of stabilimentum. Thus, stabilimentum-building spiders risk a high level of predation by attracting visually hunting predators. Keywords: Argiope versicolor ; silk; stabilimenta; spider; Portia labiata ; predator attraction 1. INTRODUCTION Biologists have always been fascinated by the spider's web in all its various forms (see Foelix 1996) and long been puzzled by the function of the so-called stabilimenta in the webs of certain orb-weaving spiders (Robinson & Robinson 1970). The orb-web is well known as a device used in foraging as well as in defending against predators (e.g. T olbert 1975; Edmunds & Edmunds 1986; Eberhard 1990; Blackledge & Wenzel 1999). The stabilimentum, however, is an extra silk structure that is added by certain species of spider to the central portion of the orb-web after web construction. Why these spiders, but not others, include stabilimenta in their webs has been debated for more than a century (Simon 1895). Although many functional hypotheses have been proposed for silk stabilimenta, the most recent and plausible, but also contentious, hypotheses are the prey-attraction hypothesis (Craig & Bernard 1990; Elgar et al. 1996; Tso 1996, 1998; Watanabe 1999) and the predator-defence hypothesis (Horton 1980; Eisner & Nowicki 1983; Schoener & Spiller 1992; Kerr 1993; Blackledge 1998a ; Blackledge & Wenzel 1999). Interestingly, both hypotheses have used the fact that stabilimenta have a bright re£ectance across wavelengths of light visible to some animals, including ultraviolet (UV) light (Craig & Bernard 1990), to argue that stabilimenta act as visual signals that make webs more conspicuous either to prey or to predators. Thus, spiders bene¢t from building conspicuous stabilimenta by attracting prey to the webs, by advertising the presence of sticky webs to predators or by distracting predators. However, building stabilimenta may be costly because stabilimenta provide visual cues that insect prey use to avoid webs, thus reducing the web builder's foraging e¤ciency (Blackledge 1998a ; Blackledge & Wenzel 2000, 2001). However, predators can also potentially perceive these conspicuous visual signals. Therefore, one may hypothesize that the stabilimenta could also operate to the detriment of a spider by attracting unwelcome visual predators to the orb-web, if UV-re£ecting stabilimenta * Author for correspondence ([email protected]). Proc. R. Soc. Lond. B (2001) 268, 1553^1558 Received 20 December 2000 Accepted 12 April 2001 make webs more conspicuous. Surprisingly, no e¡ort has been made to test this hypothesis. Here, we experimentally test this hypothesis using a species of orb-web weaving spider, Argiope versicolor (Doleschall) (Araneae: Araneidae), and a species of predator, Portia labiata (Thorell), a web-invading and spider-eating jumping spider (Araneae: Salticidae) from Singapore. Like other Argiope species, A. versicolor build di¡erent types of stabilimenta, including disc-like stabilimenta built only by juveniles and cruciforms created by adults. A total of ¢ve di¡erent types of stabilimenta have been described in A. versicolor (¢gure 1; W. K. Seah and D. Li, unpublished data). It is known that webs with stabilimenta are able to attract prey (Craig & Bernard 1990; Tso 1996, 1998; Watanabe 1999; but see Blackledge & Wenzel 1999). As mentioned above, conspicuous signals may also attract the attention of predators (Eisner & Nowicki 1983). P. labiata is sympatric with A. versicolor and is known to enter the webs of A. versicolor to prey on it, both in nature and in the laboratory (D. Li, unpublished data). Like other salticids, Portia spp. di¡er from most spiders in having remarkably acute vision (Blest et al. 1990) that permits them to discriminate visually between di¡erent prey, predators and conspeci¢cs (Jackson & Blest 1982; Li & Jackson 1996). Jumping spiders are also known to be dichromatic, having the physiological capacity to distinguish colours using the UV-sensitive and greensensitive cells in their principal (anterior median) eyes (Land 1969; DeVoe 1975; Nakamura & Yamashita 2000). On the basis of visual cues alone, Portia spp. can distinguish a web from an empty cage, and can also distinguish between quiescent spiders, insects and egg sacs (Jackson 1995; Jackson & Pollard 1996; Li & Jackson 1996). Therefore, we assume that P. labiata is able to distinguish between decorated and undecorated webs and that it can make a choice as to which web it wants to enter. In this study, we use P. labiata as a predator of A. versicolor to test three hypotheses: ¢rst, that P. labiata can use conspicuous UV-light re£ecting stabilimenta to locate and catch A. versicolor ; second, that the presence of A. versicolor has no e¡ect on the attraction of P. labiata by 1553 & 2001 The Royal Society 1554 W. K. Seah and D. Li Stabilimenta attract predators experience of A. versicolor prior to testing. They were kept individually in plastic cylindrical cages (diameter  height: 100 mm  80 mm) and provided with a diet of fruit £ies (Drosophila melanogaster Meigen) and house£ies (Musca domestica L.) ad libitum before testing. No individual P. labiata was used in more than one test of any one type. (b) Predator-attraction test Figure 1. Webs and forms of stabilimenta of Argiope versicolor. The spiders either spin a new decoration each day, which may vary in design, or do not decorate their webs at all. (a) Disk-like (discoid) stabilimentum built by juveniles, (b^e) cruciform stabilimenta with four, three, two and one arms, respectively, and ( f ) a web without a stabilimentum. Scale bar: ca. 1 cm. stabilimenta; and third, like stingless bees that can learn to avoid webs (Craig 1994), P. labiata is able to remember a particular type of decorated web. 2. METHODS (a) General Maintenance procedures, cage design, basic testing methods and terminology were as reported in earlier spider studies (see Jackson & Hallas 1986) and only essential details are given here. All experiments were carried out in a controlled laboratory (relative humidity, 80^90%; temperature, 24 Æ 1 8C; 12 L:12 D photoperiod; laboratory light intensity, 332.8 Æ 4.3 lux). We collected A. versicolor juveniles and females from Bukit Timah Nature Reserve in Singapore in 1999. They were kept individually in plastic-framed cages with removable glass sides (200 mm  200 mm  50 mm). Wooden sticks were connected to the sides of the cages to provide attachment sites for the webs. A. versicolor fastens web lines primarily to the wooden sticks and the plastic frame instead of to the glass, and so the sides could be removed from the cages with minimal damage to the webs. Except when otherwise stated, the webs were always exposed during the tests to ensure that the predator, P. labiata, could see the web without obstruction or re£ection from the cage. A. versicolor juveniles were used to build discoid stabilimenta and A. versicolor adult females were used to build cruciform stabilimenta with di¡erent numbers of arms (see ¢gure 1). Both vacant webs (i.e. A. versicolor was not on its web) and webs with A. versicolor were used. Vacant webs were obtained by removing the host spiders 24 h before tests began. We used laboratory cultures of P. labiata that originated from specimens collected in the habitat of A. versicolor. All P. labiata used were females (body length, 6^8 mm) that did not have any Proc. R. Soc. Lond. B (2001) We designed a choice test to determine whether P. labiata could be attracted to a web by the presence of a stabilimentum. A web without a stabilimentum (¢gure 1f ) was compared with one with a stabilimentum (¢gure 1a ^ e), making an e¡ort to pair similarly sized webs. Each P. labiata was starved for 1 week before the test. We conducted two series of tests: in one series, P. labiata were tested using webs with A. versicolor left in them; in the other series, the same P. labiata were tested using webs without A. versicolor. T esting began by putting P. labiata between two cages that bore webs. The two webs were set 20 cm apart and P. labiata was placed 10 cm from each web. Each test ended when P. labiata chose one of the two webs in the vacant-web tests, ate one A. versicolor in the occupied-web tests or when 10 min had elapsed, whichever came ¢rst. However, observations were continued until the sequence had terminated if P. labiata responded to a web, even after the allocated time. All tests were conducted in a plastic box with a glass bottom. After each test the glass panel was wiped with 70% ethanol and with distilled water to ensure that all secretions and pheromones deposited by the previous P. labiata were removed. We ran all tests under the same light conditions (light intensity, 430.2 Æ 17.40 lux) and a sheet of black cardboard was provided as a background for each web. We analysed the data using a w2 test of goodness of ¢t (Zar 1996). (c) E¡ect of A. versicolor on attraction of predators to webs A. versicolor may be attractive to predators, so the presence of a spider may a¡ect the attraction of predators to the web. T test o the in£uence of A. versicolor, a naive P. labiata was given a choice between webs with and without A. versicolor, making an e¡ort to pair similarly sized webs with the same type of stabilimentum. T esting began by putting a P. labiata between two web-bearing cages as in the predator-attraction tests. T ests ended when P. labiata chose one of the two webs or when 10 min had elapsed, whichever came ¢rst. Comparisons were made using the w2 test of goodness of ¢t (Zar 1996). (d) Learning experiments Portia spp. are known to exhibit trial-and-error learning (Jackson 1992; Jackson & Wilcox 1993). T examine the ability o of P. labiata to detect and remember webs with stabilimenta, we individually trained P. labiata to associate decorations with foraging success. P. labiata was ¢rst trained to associate the cruciform stabilimentum with one arm in the web with food, A. versicolor. Then, a web with a one-arm cruciform stabilimentum was used in combination with a web with any other type of stabilimentum. The two webs were presented to P. labiata simultaneously, but A. versicolor was only present in the hub of the web with the one-arm cruciform stabilimentum. In the other web, a small piece of cotton ball was attached on the hub to replace the absent prey. P. labiata was starved for 3 days before the experiments. Each P. labiata was allowed to take its time to ¢nd the web containing the food. Those that went to the web with the cotton ball were gently removed back to the starting point, and the training continued until they got onto the web with the Stabilimenta attract predators W. K. Seah and D. Li 1555 Table 1. Results from predator-attraction tests in which one web with stabilimentum and one web without a stabilimentum were presented to Portia labiata simultaneously in bright conditions (430.2 Æ 17.4 lux). web A web B presence/absence of A. versicolor number choosing web A first number choosing web B first test of goodness of fit present 13 27 χ 2 =4.90; p <0.05 absent 12 28 χ 2 =6.40; p <0.01 present 13 27 χ 2 =4.90; p <0.05 absent 12 28 χ 2 =6.40; p <0.01 present 8 32 χ 2 =14.4; p <0.001 absent 10 30 χ 2 =10.0; p <0.005 present 11 29 χ 2 =8.10; p <0.005 absent 8 32 χ 2 =14.4; p <0.001 present 9 31 χ 2 =12.1; p <0.001 absent 7 33 χ 2 =15.7; p <0.001 one-arm cruciform stabilimentum. The training exercise was performed eight times, twice for each pair of web types, with each P. labiata. When P. labiata chose the right web, it was allowed to feed on A. versicolor for 5 min as a reward. Subsequently, the trained P. labiata were kept without food for 3 days after the date of learning. They were then used in the learning experiments to determine whether they could associate a stabilimentum with food. We also used a choice test to examine whether a predator remembered the familiar stabilimentum. In these tests, two webs, one with a familiar stabilimentum (onearm cruciform) and the other with an unfamiliar stabilimentum, had A. versicolor present in the centre of the hub. Each test began by putting a trained P. labiata in between the two webs, as in the predator-attraction tests. T ests ended when P. labiata chose one of the two cages and entered the web or when 10 min had elapsed, whichever came ¢rst. 3. RESULTS (a) Predator-attraction test When given a choice between webs with and without stabilimenta, regardless of whether A. versicolor was in the web or not, P. labiata preferred the webs with stabilimenta to those without stabilimenta in all tests on all ¢ve combinations of webs (table 1). (b) E¡ect of the presence of A. versicolor on the attractiveness of webs to predators When given a choice between webs with and without A. versicolor, P. labiata chose the webs with A. versicolor as often as they chose the webs without A. versicolor (table 2). Proc. R. Soc. Lond. B (2001) (c) Learning experiments When P. labiata were trained to associate a one-arm cruciform stabilimentum with the presence of A. versicolor, P. labiata chose the web with the one-arm cruciform (familiar) stabilimentum more often than they chose other types of decorated webs (table 3). 4. DISCUSSION When a predator (receiver) encounters a spider (sender) in a web with a conspicuous UV-re£ecting stabilimentum (visual signal), there are three possible responses: ¢rst, being trapped if the predator is unable to determine the presence of the stabilimentum in the web; second, avoiding the web passively or actively (e.g. some arthropod predators and birds); and third, being attracted to the web if it is able to see the stabilimentum. Surprisingly, almost all studies investigating the adaptive signi¢cance of stabilimenta as defensive structures have tried to examine how a predator avoids a web with a stabilimentum (e.g. Blackledge 1998a ; Blackledge & Wenzel 1999, 2000). Horton (1980) stated that the web is a negative stimulus to a bird because sticky silk adheres to the bird's feathers and clumps them; Horton found that the cross (cruciform) stabilimentum advertises the presence of the noxious web, enabling birds to avoid webs with stabilimenta. Eisner & Nowicki (1983) measured the durability of 30 webs with stabilimenta and 30 webs without stabilimenta in the ¢eld, and found that 8% of the stabilimentum-free webs persisted undamaged as compared with 60% of the webs with 1556 W. K. Seah and D. Li Stabilimenta attract predators Table 2. Results from tests of the e¡ect of the presence or absence of Argiope versicolor on the attraction of predators to webs. A naive Portia labiata was given a choice between a web with A. versicolor left in it and a web without A. versicolor; both webs had with the same type of stabilimentum. number choosing web (B) first test of goodness of fit 14 χ 2 =0.133, not significant 17 13 χ 2 =0.533, not significant 18 12 χ 2 =1.2, not significant 17 13 χ 2 =0.533, not significant 18 web (B), without A. versicolor number choosing web (A) first 16 web (A), with A. versicolor 12 χ 2 =1.2, not significant Table 3. Results from learning experiments in which a Portia labiata trained with a one-arm cruciform stabilimentum plus food as a reward was given a choice between a web with a familiar stabilimentum (one-arm cruciform) and a web with an unfamiliar stabilimentum; both webs contained Argiope versicolor. (Data in the third and fourth columns only are used in these tests. Null hypothesis: if P. labiata chose a web, then the probability of choosing web A, with a familiar stabilimentum, is equal to the probability of choosing web B, with an unfamiliar stabilimentum.) number choosing web B first test of goodness of fit 12 χ 2 =5.16; p <0.05 27 14 χ 2 =4.10; p <0.05 29 11 χ 2 =8.10; p <0.005 30 web B number choosing web A first 26 web A 9 χ 2 =11.3; p <0.001 stabilimenta. Thus, `web durability was therefore due to the presence of the visual markers' and `stabilimenta serve as visual advertisements of webs' (Eisner & Nowicki 1983, p.186). Blackledge & Wenzel (1999) conducted experiments to determine the e¡ect of stabilimenta on web damage by birds, and their data support the hypothesis that stabilimenta can function as a defence against birds. In this study, we have demonstrated experimentally that predators are attracted to webs by the stabilimenta and feed on the resident spiders: the web-invading spidereating jumping spider P. labiata consistently chose the webs with stabilimenta more often than the webs without Proc. R. Soc. Lond. B (2001) stabilimenta (table 1). We also found that the presence of A. versicolor in the web had no e¡ect on P. labiata's attraction to the stabilimenta (table 2). Our results suggest that the stabilimenta alone is a su¤cient visual signal to attract predators to the web. Not surprisingly, stabilimenta acting as highly visual signals to prey and predators create trade-o¡s in signal design between foraging, avoiding attack by predators and defence when actually attacked. Like other visual signals that are designed for communicating with conspeci¢cs that carry a greater risk of predator attraction (Guilford 1990; Endler 1993; Ryan & Rand 1993), conspicuous stabilimenta built by the orb owners to attract visual prey or to warn some visual Stabilimenta attract predators W. K. Seah and D. Li 1557 predators of the presence of noxious sticky silk in the orbwebs may also increase the risk of predation by attracting particular types of predators. However, whether or not the silk is conspicuous to insects is controversial and has important implications for the function of stabilimenta (in terms of the preyattraction hypothesis versus the predator-defence hypothesis) (Craig & Bernard 1990; Blackledge 1998a; Blackledge & Wenzel 1999, 2000). The prey-attraction hypothesis argues that UV re£ection makes stabilimenta conspicuous to insects and attracts insects to webs (Craig & Bernard 1990; Tso 1996, 1998; Watanabe 1999; Herberstein et al. 2000). However, the predator-defence hypothesis suggests that insects avoid webs, based upon the presence of stabilimenta, and that re£ection of UV light is crucial to make the stabilimenta cryptic to insects, thereby reducing their ability to avoid webs (Blackledge 1998a ; Blackledge & Wenzel 2000, 2001). There are several explanations for the incompatible di¡erences between these two hypotheses. Many of the data relating to the attraction of prey to stabilimenta are correlational (Craig & Bernard 1990; Tso 1996, 1998; Watanabe 1999; Herberstein et al. 2000), while the data supporting insect avoidance of stabilimenta come from a manipulative experiment (Blackledge & Wenzel 1999) that controlled for a strong in£uence of foraging success on the presence of stabilimenta in webs. Several studies have shown that spiders capturing more prey are more likely to build stabilimenta, which can produce a correlation between the capture of high numbers of prey and the presence of stabilimenta in webs (Blackledge 1998b ; Herberstein et al. 2000). Furthermore, choice experiments demonstrating attraction of insects to stabilimenta have been performed in laboratory settings (Craig & Bernard 1990; Watanabe 1999), while the choice experiment showing the cryptic nature of the silk to bees was performed under natural conditions (Blackledge & Wenzel 2000). The background against which a signal is viewed is crucial to its conspicuousness (Blackledge 1998a). Animals can easily ¢nd a signal conspicuous in the laboratory when that same signal is inconspicuous outside. We have demonstrated that P. labiata are capable of using stabilimenta to locate A. versicolor prey in the laboratory, but it remains to be determined how important this is for P. labiata in the ¢eld. If a web-building spider consistently builds the same form of stabilimentum in its web, it may increase the risk of predation because predators may remember the form of the stabilimentum. In the learning experiments, we demonstrated that P. labiata can learn to associate the form of the stabilimentum with food: when given a choice between a web with a familiar form of stabilimentum and a web with an unfamiliar form of stabilimentum, P. labiata chose to enter the web with the familiar form of stabilimentum more often than the web with the unfamiliar form of stabilimentum. Building stabilimenta may attract more prey, thus increasing foraging e¤ciency (Craig & Bernard 1990; Elgar et al. 1996; Tso 1996, 1998; Watanabe 1999), or reduce the risk of predation (Robinson & Robinson 1970; Eisner & Nowicki 1983; Blackledge 1998a,b ; Blackledge & Wenzel 1999). T increase their foraging success o by building stabilimenta to attract prey and, at the Proc. R. Soc. Lond. B (2001) same time, reduce the risk of predation, spiders must have strategies to balance the trade-o¡. Variation in stabilimentum-building behaviour seems to be a strategy that has evolved in these spiders to balance this trade-o¡. In nature, when Argiope argentata spins a new web each morning the spider produces a decoration whose pattern or pattern orientation di¡ers from that spun the previous day, and on some days spiders spin no decoration at all (Craig 1994). By varying their web decorations randomly and daily, A. argentata seem to have evolved a foraging behaviour that inhibits bees both from learning to avoid webs and from remembering their location (Craig 1994) because learning is constrained by anatomical and physiological limits to the animal's perceptual capabilities and by the complexity of its neural-processing system ¨ (Dukas 1993; Belisle & Cresswell 1997; Endler & Basolo 1998). Whether, by varying their web decorations randomly and daily, Argiope spiders also prevent some predators, such as Portia, from learning to be attracted to their webs needs further study. We are grateful to Koh T eck Hui for helping us collect spiders, Goh Poh Moi for rearing house£ies and Lim Lek Min for drawing the stabilimenta in tables 1, 2 and 3. Comments and suggestions from T odd A. Blackledge, Koh T ech Hui and an anonymous reviewer greatly improved the manuscript. This work was supported by generous grants (R-154-000-060-112 and R-154-000-072-112) to D. Li from the National University of Singapore Academic Research Fund. REFERENCES ¨ Belisle, C. & Cresswell, J. 1997 The e¡ects of limited memory capacity on foraging behavior. Theor. Popul. Biol. 52, 78^90. Blackledge, T. A. 1998a Signal con£ict in spider webs driven by predators and prey. Proc. R. Soc. Lond. B 265, 1991^1996. Blackledge, T. 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