Ann. Zool. Fennici 39: 307–315 ISSN 0003-455X Helsinki 9 December 2002 © Finnish Zoological and Botanical Publishing Board 2002

The adaptive signifi cance of stabilimenta in orb-webs: a hierarchical approach

Philip T. Starks

University of California at Berkeley, Environmental Science, Policy and Man- agement, Division of Biology, 201 Wellman Hall #3112, Berkeley, CA 94720-3112, USA; Present address: 310 Dana Labs, Department of Biology, Tufts University, Medford, MA 02155, USA (e-mail: [email protected])

Received 7 December 2001, accepted 11 March 2002

Starks, P. T. 2002: The adaptive signifi cance of stabilimenta in orb-webs: a hierarchical approach. — Ann. Zool. Fennici 39: 307–315.

Conditional strategy theory provides a theoretical and experimental framework from which to predict both population differences in the expression of alternative behav- ioral phenotypes and the condition above and below which individuals will perform different tactics. Here I briefl y review the current functional explanations for sta- bilimentum production in orb-web and reinterpret previous results within a conditional strategy framework. I argue that using this framework and incorporating a hierarchical approach to the multiple possible selective benefi ts of the structure is suffi cient to explain some of the seemingly contradictory results in the literature.

Introduction to vary both across the animalʼs lifetime and in response to its immediate surroundings (Herber- Stabilimenta are conspicuous silk structures stein et al. 2000a). that are often attached to the webs of some spe- Because these structures are attached to the cies of orb-web constructing spiders (Araneae: web — an external and, due to its prey capturing Araneidae, Tetragnathidae, Uloboridae; Fig. 1). function, essential component of fi tness In addition to attracting predators (Bruce et al. — stabilimenta offer a unique opportunity to 2001, Seah & Li 2001) and prey (Craig & Ber- study the adaptive signifi cance of an extended nard 1990), these structures have long attracted phenotype (Dawkins 1983). Although non- the attention of naturalists (McCook 1889, adaptive explanations for the production of sta- Simon 1895). The stabilimenta are categorized bilimenta have been proposed (e.g., a response by their shape, location with the web, and/or to stress or a result of silk regulation), these number (Table 1). In general, these structures explanations are not well supported by empiri- are located at the center of the web and may cal data (Herberstein et al. 2000a). Conversely, extend toward its periphery. The specifi c form many studies have shown an adaptive benefi t of that a spider may construct, however, is likely having stabilimenta in the web (see Herberstein 308 Starks • ANN. ZOOL. FENNICI Vol. 39

hypotheses has been challenging, and a dearth of experimental studies has left these questions open. Recent empirical studies have tried to discriminate between the foraging enhancement and predator avoidance hypotheses (Blackledge 1998a, 1998b, Blackledge & Wenzel 1999, 2000, 2001, Bruce et al. 2001, Craig et al. 2001). These adaptive hypotheses, however, may not be mutu- ally exclusive. Thus, focusing on “disproving” one to support another may be more likely to inhibit, than to enhance, progress. Much correlational evidence has been gath- ered suggesting a relationship between stabili- menta and elevated foraging success (Herber- stein et al. 2000a, but see Blackledge & Wenzel 1999). This relationship has been presented as causal, that is, stabilimentum presence results in higher foraging success. Blackledge (1998a) critically tested this assumption by providing different amounts of food to captive spiders and recording stabilimentum production and length. Because well-fed spiders produced more and Fig. 1. Orb-web spider with stabilimenta. The pho- larger stabilimenta than did “starved” spiders, tograph was taken in Ithaca, NY, USA (42°27´N, the author concluded that the causal relationship 76°29´W) and is provided courtesy of Dr. C.A. was in the other direction, that is, stabilimentum Blackie. presence results from higher foraging success. Consequently, further research (Blackledge & et al. 2000a). A single unifying adaptive function Wenzel 1999, 2000, 2001) was conducted to for stabilimenta, however, has been diffi cult to determine the non-foraging related adaptive support due to within and across population (and benefi ts of stabilimentum production. ) variation in its production (Herberstein Given the importance of causation in the et al. 2000a). I argue that, within the theoretical relationship between foraging and stabilimen- framework of a conditional strategy, taking a tum production, the data presented in Blackledge hierarchical approach to the selective benefi ts of (1998a) deserve a closer examination. Arguing stabilimenta may explain some of the seemingly against a foraging benefi t, the author states, “it is contradictory results previously published. diffi cult to believe that a structure which attracts Several adaptive functions for stabilimenta prey and is energetically inexpensive would be have been suggested; these adaptive functions less common and smaller in the webs of starved include mechanical augmentation, thermoregu- spiders” (Blackledge 1998a: p. 26). This may lation, web protection, predator avoidance, and be an overstatement for three reasons. First, it foraging enhancement (Table 2; Herberstein et al. is unclear that a structure accounting for 10% of 2000a). The mechanical augmentation hypoth- the total web weight (Blackledge 1998a: p. 25) esis, suggesting that stabilimenta strengthen or is energetically inexpensive. Second, only two fi ne-tune the web, has not been well supported of the four results presented show a signifi cant either theoretically or by empirical data (Her- difference in stabilimentum production between berstein et al. 2000a; but see Wanatabe 2000). well-fed and starved spiders, and this signifi cant Indeed, stabilimenta often appear so loosely result is confi ned to one of the two studied spe- attached to the web that any mechanical advan- cies (in aurantia and not A. trifasiata). tage would likely be slight (but see Wanatabe Finally, and most importantly, the spiders do not 2000). Discriminating between the remaining appear to have been effectively starved: weight ANN. ZOOL. FENNICI Vol. 39 • Stabilimenta in orb-webs 309 changes during the experimental period were in greater detail. not signifi cantly different between well-fed and With current interpretations in confl ict, there starved spiders of either species (Blackledge remain at least four possible adaptive functions 1998a). Indeed, starved A. trifasiata more than for stabilimenta: thermoregulation, web protec- doubled their weight during this period. Still, tion, predator avoidance, and foraging enhance- signifi cant differences between well-fed and ment (Table 2). Indeed, confl ict in interpretation less well-fed spiders in the length of stabili- is expected because there is some support for menta remain for all comparisons, which sug- all of these functions. When one seeks a single gests that this relationship should be examined explanation of the production of the structure,

Table 1. Schematic representations of webs containing stabilimenta (after Herberstein et al. 2000a).

Basic forms Schematic representation

Discoid: Discoid stabilimenta are found at the hub of webs and are common in juvenile spiders of the genus Argiope.

Spiral: Spiral stabilimenta are found at the hub of webs and are common to spiders within the family Uloboridae.

Linear: A linear pattern is represented by two zigzag shaped structures which radiate out from the center of the web (e.g., see Fig. 1)

Cruciate: A cruciate pattern is represented by up to four zigzag shaped structures that radiate out from the center of the web and form a cross.

Table 2. Adaptive explanations for stabilimenta. (See text for references and for additional, more in-depth explanations.)

Hypothesis Explanation

Mechanical augmentation Stabilimenta may be used to strengthen the web or otherwise increase its relative stability. Thermoregulation Stabilimenta may be used to shield the sun and thus prevent overheating. Web protection Stabilimenta may signal web presence to larger to induce avoidance, thereby decreasing web damage. Predator avoidance Stabilimenta may inhibit predators from successful attacks by shielding or altering the appearance of the spider. Foraging enhancement Stabilimenta may attract prey items by mimicking fl ower appearance or open space for movement. 310 Starks • ANN. ZOOL. FENNICI Vol. 39 problems arise for four major related reasons. ruled out: individuals within populations dis- First, stabilimentum production has evolved at playing alternative strategies cannot perform least nine times within the Araneidae (Scharff more than one tactic. Since individual spiders & Coddington 1997) and there is no guarantee sometimes build stabilimenta (one tactic), and that it evolved each time due to the same selec- other times do not build stabilimenta (another tion pressure. Second, the current utility of sta- tactic), these animals do not fi t the restrictions bilimenta may differ from the original benefi t, of alternative strategy models. The remaining that is, stabilimenta may have evolved due to two classes of strategies are mixed strategies and selection pressures different from the ones that conditional strategies. It is possible that presence currently maintain the trait. Third, because cur- and absence of stabilimenta are different tactics rent selective pressures may differ greatly across in a mixed strategy, and that both tactics have the the diversity of environments these animals requisite negatively frequency dependent fi tness are found, the current primary adaptive benefi t that equalize at a point of average fi tnesses (see (e.g., foraging success, predator avoidance) is Hauber 1998). However, I use the framework of also likely to differ across populations. Finally, a conditional strategy due to its overwhelming the primary benefi t of stabilimenta may differ representation in nature (Gross 1996, Kain 1999) greatly during the life history of the spider. As and because stabilimentum production shows such, a single explanation that can be extended to context-dependent changes based on ontogeny all stabilimentum-producing spiders is unlikely. (e.g., see Nentwig & Heimer 1987) and envi- ronmental conditions (e.g., see Herberstein et al. 2000b, Craig et al. 2001). Transferring between Conditional strategies a conditional strategy and a mixed strategy is easily done if empirical data suggest the need. Taking a hierarchical approach to the selective Tactics in a conditional strategy differ in their benefi ts of stabilimenta may provide a suitable expected fi tness payoffs. These payoffs are also framework for understanding within and between predicted to differ as a function of the condition species differences in the observed frequencies of the . Natural selection should favor ani- of stabilimentum production (and related phe- mals that maximize their potential fi tness payoff, nomena), as well as changes in web design over thus we expect that an animal will perform the the course of a spiderʼs lifetime. A hierarchical tactic that maximizes its fi tness given its cur- approach simply means that all adaptive func- rent condition. For the system examined here, tions are considered for a single population, and these fi tness curves will vary with environmental then ranked in order of importance. The ranking conditions (e.g., the relative predator population is not essential for the model (see below), but size). The assumption that the animal is capable may be useful in determining a series of experi- of altering its web and stabilimentum building mental tests to examine any given population. behavior in response to environmental conditions The function-specifi c difference between fi tness is well met by these spiders (e.g., see Blackledge costs and benefi ts can then be summed, and the 1998a, Chmiel et al. 2000, Herberstein et al. resulting condition-dependent values can be 2000b, Craig et al. 2001). This basic conditional envisaged within the framework of a conditional strategy framework can be used to make sense of strategy containing two tactics: stabilimentum some of the seemingly contradictory results pre- presence and stabilimentum absence (Fig. 2). viously published. I have assumed that stabilimentum attach- ment behavior is a single genetically monomor- phic conditional strategy with at least two result- Adaptive functions ant tactics. In general, there are three descriptors used in models of the selective maintenance of Web protection alternative behavioral phenotypes (Gross 1996). One of the three — alternative strategies or Stabilimenta have been suggested to be honest genetic polymorphisms — can be immediately indicators to animals (predominantly ) of ANN. ZOOL. FENNICI Vol. 39 • Stabilimenta in orb-webs 311

A. Conditional strategy: baseline fitness Tactic 1 Fig. 2. Heuristic diagram Tactic 2 of a conditional strategy defi ned by two tactics: Tactic 1 Tactic 2 stabilimentum absence (Tactic 1) and stabilimen- tum presence (Tactic 2). The point of intersection between the tactic-specifi c Fitness Population frequency fi tness curves is the condi- tion where the fi tness for Tactic 1 both tactics is equivalent. At points above or below this point of intersection, Tactic 2 individuals are predicted to perform the tactic with Condition (e.g., body size) the highest expected fi t- ness payoff. Circles rep- B. Conditional strategy: increased fitness for tactic 2 resent the population and differently shaded regions represent the observed proportion of the different Tactic 1 Tactic 2 tactics. A represents the baseline fi tnesses in a pop- ulation and B represents the same population after the fi tness of Tactic 2 has Fitness Population frequency increased. The fi tness of Tactic 2 could increase, Tactic 1 for example, if the popula- tion of non-predatory web damaging birds increases. Tactic 2 (Adapted from Gross 1996, Starks & Reeve 1999). Condition (e.g., body size) the presence of noxious sticky webs (Eisner & tory populations, a larger proportion of Nowicki 1983, Kerr 1993, Blackledge & Wenzel webs containing stabilimenta would be observed 1999). As such, animals tend to avoid the sta- in the environment with the smaller predatory bilimenta, which protects both the spider and bird population. its web. Using the framework of a conditional strategy, we expect the fi tness of the stabilimen- tum present tactic to rise with an increase in the Predator avoidance non-predatory bird population (see Fig. 2). This increase in tactic fi tness will result in a greater While stabilimenta may be an honest signal proportion of webs containing stabilimenta. to birds, they may also be used for deception. Empirical evidence supports this hypothesis: Disk-shaped stabilimenta may shield a spider the frequency of webs containing stabilimenta is from a predatorʼs view (Ewer 1972, Eberhard greater in environments with larger bird popula- 1973), while linear stabilimenta may disguise tions than in environments with smaller or no a spiderʼs silhouette (Edmunds 1986) or falsely bird populations (Kerr 1993). Because stabili- suggest that the spider is a good deal larger than menta may attract predators (Seah & Li 2001, it is (Schoener & Spiller 1992). Blackledge Blackledge & Wenzel 2001, Bruce et al. 2001) & Wenzel (2001) present data that suggests and because some birds prey on orb-web spiders that stabilimenta can block or visually impede (e.g., hummingbirds), one would predict that attacks, and that this impediment may given two environments with equal non-preda- offer a time delay suffi cient for escape. Using 312 Starks • ANN. ZOOL. FENNICI Vol. 39

Conditional strategy: fitness for three tactics

Discoid None Cruciate

T3

T1 T2 T2 T3

T2 Fitness

T1

Condition

Fig. 3. Heuristic diagram of a conditional strategy defi ned by three tactics. Tactics represent different stabilimentum patterns. The point of intersection between any two tactic-specifi c fi tness curves is the condition where the fi tness for both tactics is equivalent. Individuals are predicted to perform the tactic with the highest expected fi tness payoff for their given condition. Constraints have been assumed such that not all tactics are available for individuals of different conditions. For example, not all tactics may be available to juvenile spiders. (Adapted from Gross 1996, Starks & Reeve 1999). an elegant design incorporating fi eld cages and population is a strong predictor of stabilimentum captive , the researchers recorded and presence: A. keyserlingi decrease stabilimentum compared the web characteristics of spiders that production in dense vegetation environments were and were not preyed upon. A larger propor- likely to have a high proportion of praying tion of spiders without stabilimenta were preyed mantids, a predator of orb-web spiders (Bruce upon than those with stabilimenta. et al. 2001). Interestingly, Blackledge & Wenzel The form of stabilimentum (disk-shaped or (2001) report a different trend: A. trifasciata linear) used for protection is likely to vary with increase stabilimentum production in exposed the predator population. For example, a cruciate environments. The primary difference between pattern (four linear structures; Craig & Bernard systems may not be the spider species, but rather 1990) may be more effective at increasing the the predator species. Whereas Bruce and col- spiderʼs apparent size and thus is more benefi - leagues (2001) examined an ambush predator, cial in environments with a large proportion of Blackledge & Wenzel (2001) examined an air- gape-limited predators such as lizards (Schoener borne predator (the mud-dauber wasp). As such, & Spiller 1992). Conversely, linear stabilimenta the relative proportions of predator types may (Horton 1980; Fig. 1) may be optimal for alter- be a strong predictor of the frequency of stabili- ing the silhouette or visually impeding wasp mentum production. attacks and thus may be more benefi cial in envi- ronments with a large proportion of invertebrate predators (Blackledge & Wenzel 2001). Consid- Foraging enhancement ering different stabilimentum patterns as sepa- rate tactics in a conditional strategy provides a Another deceptive role for stabilimenta has framework for examining questions related to been suggested: stabilimenta may enhance predator-dependent population differences in the primary function of webs — prey capture. the frequencies of stabilimentum pattern and/or The spectral properties of stabilimenta may presence (see Fig. 3). deceive prey by altering the appearance of the Recent research has shown that the predator web to make it resemble a fl ower or a gap in ANN. ZOOL. FENNICI Vol. 39 • Stabilimenta in orb-webs 313 vegetation (Craig & Bernard 1990). As such, of producing stabilimenta in the same web posi- a web with a stabilimentum may attract more tion. Altering the fi tness payoff of each tactic pollinating or simply more insects in should result in stabilimentum production dif- transit. In accordance with this hypothesis, a ferences across different experimental systems. number of research projects have shown that The foraging success of orb-web spiders is prey interception rates are higher in webs with going to be a direct refl ection of population-spe- stabilimenta than in webs without stabilimenta cifi c prey and predator characteristics (Black- (e.g., Craig & Bernard 1990, Tso 1996, 1998a, ledge & Wenzel 2001, Bruce et al. 2001, Craig 1998b, Herberstein 2000, Bruce et al. 2001, et al. 2001). A recent experimental study by Craig et al. 2001; but see Blackledge & Wenzel Bruce and colleagues (2001) has taken advan- 1999). tage of the confl ict between foraging success The question one may ask is, if stabilimenta and predator avoidance. These researchers found increase foraging success, why do some spiders that, although stabilimenta increase foraging produce webs without stabilimenta? Besides two success, they are not as frequently constructed obvious answers — spiders do not hunt continu- if the likelihood of predator presence is high. ously and spider predators may use stabilimenta These results are analogous to the hypothesized in their search image (Seah & Li 2001, Black- results in Fig. 2: when the cost of stabilimentum ledge & Wenzel 2001, Bruce et al. 2001) — there production increases, its observed frequency in seems to be a cost to predictable stabilimentum the population decreases. production (Herberstein et al. 2000a). Some prey learn to avoid stabilimenta (Craig 1994a, 1994b), thus forcing spiders to either relocate webs or Thermoregulation alter the pattern of stabilimenta. Not only may there be a cost to repetitious Although not all forms of stabilimenta are web patterns, prey may differ in their suscep- likely to have any thermoregulatory effects, tibility to stabilimentum-adorned webs. Black- stabilimenta in a disk-shaped pattern may ledge & Wenzel (1999) show that dipterans are provide shade and thus prevent overheating much more frequently captured in webs without (Humphreys 1992). Disk-shaped stabilimenta stabilimenta than in those with stabilimenta. are more common in juvenile spiders than in Using fl ies in the feeding protocol, Watan- adults (Clyne 1969, Ewer 1972, Nentwig & abe (1999) showed that Octonoba sybotides Heimer 1987, Herberstein et al. 2000a) and thus decreased its production of linear stabilimenta in may refl ect different selection pressures across response to high levels of (dipteran) prey avail- an animalʼs life history. Given the size differ- ability. As such, population-specifi c prey differ- ences between juvenile and adults, and given ences in stabilimentum susceptibility will likely that smaller individuals are more susceptible infl uence the frequency of both stabilimentum to fl uctuating temperatures, use of stabilimenta production and form (see Craig et al. 2001). for thermoregulation may be a tactic benefi cial Accordingly, experiments using a single prey to small individuals of a given developmental species that show differences in stabilimentum stage but not to large ones (see Fig. 3). Assum- production between high and low feeding pro- ing this to be the case, one would expect to see tocol spiders (e.g., Blackledge 1998a, Watanabe small individuals of a given species construct 1999) may provide valuable information specifi c more disk-shaped stabilimenta in environments to a single prey species, but not information gen- where high temperatures occur frequently than eralizable to the overall foraging biology of the in environments where temperatures are more animal. Once again viewing behavioral differ- stable. In addition, if disk-shaped patterns of sta- ences as alternative tactics in a conditional strat- bilimenta are thermoregulatory structures whose egy, experimental manipulation (e.g. altering production is dependent on condition, a condi- prey type and/or diversity) can be used to alter tion threshold above which animals no longer the relative success of producing stabilimenta, construct such structures should be identifi able of producing specifi c forms of stabilimenta, and experimentally. 314 Starks • ANN. ZOOL. FENNICI Vol. 39

Conclusions tifactorial framework. The added complexity of this approach, however, is unlikely to augment I have discussed each of the four current adap- the intuitive explanatory power of the condi- tive explanations of stabilimenta separately and tional strategy framework. have placed them within the framework of a The goal of this brief report was to promote a conditional strategy. I have shown that this potential framework for understanding the vari- framework can be used to make sense of pub- ability in stabilimentum production observed in lished differences in stabilimentum production, nature. In order to accomplish this goal, I have specifi cally those results based on differing (1) provided a brief review of some of the possible bird populations, (2) predator populations, (3) adaptive functions of stabilimenta. In no way prey populations, and (4) spider developmental should this review be considered exhaustive: stages. The true benefi t of this approach, how- many fi ne projects have not been discussed (see ever, will not be reached until the combined Herberstein et al. 2000a for a comprehensive effect of the separate adaptive functions are review). I hope that enough has been discussed, calculated and compared across populations. A however, to provide a foundation for understand- holistic approach to stabilimenta is likely to ing the basic questions and, more importantly, to show great fl uctuations in population-specifi c provide a holistic framework from which to con- selection pressures. Uncovering these fl uctua- tinue to study this fascinating behavior. tions will allow for species-specifi c examina- tions of behavioral plasticity as well as fi ne- tuned experimental manipulations of the given Acknowledgements systems. It may be that the predominant factor lead- PTS is supported by a Miller Fellowship from the Miller ing to stabilimentum production in one popula- Institute for Basic Research in Science. I thank Todd Black- tion (e.g., web protection) will be of secondary ledge for helpful conversations leading to the production of this manuscript. An early version of this manuscript ben- importance in other populations. Conversely, efi ted from the careful and critical reviews by CA Blackie, different levels of confl icting selection pres- WM Getz, ME Hauber, K Rankin & CC Wilmers. sures may lead to similar stabilimenta preva- lence across different populations. For example, populations containing high proportions of air- References borne invertebrate predators and stabilimentum avoiding prey may have the same frequency Blackledge, T. A. 1998a: Stabilimentum variation and for- of stabilimentum production as populations con- aging success in Argiope aurantia and Argiope trifas- taining low proportions of airborne invertebrate ciata (Araneae: Araneidae). — J. Zool. 246: 21–27. predators and stabilimentum avoiding prey. With- Blackledge, T. A. 1998b: Signal confl ict in spider webs driven by predators and prey. — P. Roy. Soc. Lond. B out adopting a hierarchical approach to the 265: 1991–1996. system, however, such subtle differences will Blackledge, T. A. & Wenzel, J. W. 1999: Do stabilimenta remain unidentifi ed. in orb webs attract prey or defend spiders? — Behav. I have used the term hierarchy because the Ecol. 10: 372–376. specifi c adaptive benefi ts and functions of stabil- Blackledge, T. A. & Wenzel, J. W. 2000: The evolution of imenta are likely to vary across populations. As cryptic : a behavioral test. — Behav. Ecol. 11: 142–145. such, the primary fi tness benefi t in one environ- Blackledge, T. A. & Wenzel, J. W. 2001: Silk mediated ment (e.g., prey capture) may be of secondary defense by an orb web spider against predatory mud- importance (or even detrimental) in other envi- dauber wasps. — Behaviour 138: 155–171. ronments. Identifying the environment-specifi c Bruce, M. J., Herberstein, M. E. & Elgar, M. A. 2001: Sig- primary adaptive functions of stabilimenta will nalling confl ict between prey and predator attraction. enable more fi ne-tuned experimental analysis of — J. Evol. Biol. 14: 786–794. Chmiel, K., Herberstein, M. E. & Elgar, M. A. 2000: Web the systems in question. The hierarchical view damage and feeding experience infl uence web site presented here represents a multicomponent tenacity in the orb-web spider Argiope keyserlingi approach that could be enveloped within a mul- Karsch. — Anim. Behav. 60: 821–826. ANN. ZOOL. FENNICI Vol. 39 • Stabilimenta in orb-webs 315

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