Three-dimensional barricading of a predatory trap reduces predation and enhances prey capture Sean J. Blamires, Chueh Hou, Lin-Fei Chen, Chen-Pan Liao & I-Min Tso Behavioral Ecology and Sociobiology ISSN 0340-5443 Behav Ecol Sociobiol DOI 10.1007/s00265-013-1493-x 1 23 Your article is protected by copyright and all rights are held exclusively by Springer- Verlag Berlin Heidelberg. This e-offprint is for personal use only and shall not be self- archived in electronic repositories. If you wish to self-archive your work, please use the accepted author’s version for posting to your own website or your institution’s repository. You may further deposit the accepted author’s version on a funder’s repository at a funder’s request, provided it is not made publicly available until 12 months after publication. 1 23 Author's personal copy Behav Ecol Sociobiol DOI 10.1007/s00265-013-1493-x ORIGINAL PAPER Three-dimensional barricading of a predatory trap reduces predation and enhances prey capture Sean J. Blamires & Chueh Hou & Lin-Fei Chen & Chen-Pan Liao & I-Min Tso Received: 31 July 2012 /Revised: 17 January 2013 /Accepted: 21 January 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract Animal structures come at material, energetic, Introduction time, and expression costs. Some orb-web spiders add three-dimensional barrier structures to their webs, but many Structures are constructed by animals at material, energetic, do not. Predator protection is considered to be the principal time, and expression costs (Hansell 2005; Blamires et al. benefit of adding these structures. Accordingly, it remains 2010; Tseng et al. 2011). Material costs constitute the lim- paradoxical why some orb-web spiders might construct the itations of the physical and chemical properties of the mate- barriers while others do not. Here, we experimentally deter- rials that the structure is made from. Energetic costs include mined whether the barrier structure added to the horizontal metabolic energy expended secreting or gathering the mate- orb web of the spider Cyrtophora moluccensis deters pred- rials and building the structure. Time costs include the time ators at the cost of reducing the amount of prey captured in diverted from foraging, mating, or avoiding predators. the field. We conducted experiments by day and night to Expression costs are encountered over and above the other assess whether the effects vary with the time of day. We costs and may include reduced prey capture success and found that the three-dimensional barriers not only offered increased exposure to predators (Hansell 2005). protection from predatory wasps by day but also enhanced Aerial web building was a key evolutionary innovation the amount of prey captured by day and night. Moreover, that enabled spiders to capture and consume flying insects the barrier structure appears particularly useful at catching (Nentwig and Heimer 1987; Craig 2003; Blackledge et al. moths, the largest and most energetically profitable prey that 2009). It, nonetheless, came at various costs including the it encounters. We, therefore, concluded that reducing the energetic cost of producing more and a wider range of silk, a energetic and time costs associated with producing and loss of time that could be devoted to foraging or reproduc- depositing extra silk threads is the principal reason why tion and increased exposure to predators (Craig 2003; barrier structures are used intermittently among orb-web Hansell 2005). Some araneid and nephilid orb-web spiders spiders. add three-dimensional barrier structures, made principally from stiff major ampullate silk, to their two-dimensional Keywords Costs–benefits . Barrier structure . Three- webs, while many other orb-web spiders do not dimensional orb webs . Cyrtophora moluccensis (Blackledge et al. 2009, 2011; Blamires et al. 2012a). In some taxa (e.g., members of the genera Cyrtophora and Mecynogea; Levi 1997), the structures are always present, Communicated by M. Elgar suggesting it is an obligate component of their web-building behavior. However, in other taxa (e.g., members of the S. J. Blamires : C. Hou : L.-F. Chen : C.-P. Liao : I.-M. Tso Department of Life Science, Tunghai University, genus Nephila), the webs are vertical and contain sticky Taichung 40704, Taiwan silk, and the building of a barrier structure depends on the species and/or ontogenetic stage (Higgins 1990, 1992; * I.-M. Tso ( ) Blamires et al. 2010). Barrier structures, henceforth, do not Center for Tropical Ecology and Biodiversity, Tunghai University, Taichung 40704, Taiwan seem to be of homologous origin among orb-web spiders. e-mail: [email protected] The mechanical properties of the silk threads from which the Author's personal copy Behav Ecol Sociobiol barrier structures are made, however, are homologous across captured, we experimentally removed the barrier structures the different users (Blamires et al. 2012a, b). from the webs of wild Cyrtophora moluccensis, a large- The barrier structures consist of non-sticky inelastic silk, bodied (adult female body length≈20 mm) orb-web spider so barricading the web to protect the spider against wasp that forages by both day and night (Lubin 1974; Baba et al. and bird predators seems to be the primary advantage of 2007; Blamires et al. 2012a). It adds a three-dimensional adding them to orb webs (Blackledge et al. 2003; Manicom silk barrier structure to its web that extends up to 1 m above et al. 2008; Blamires et al. 2010; Tseng et al. 2011). This, its approximately 0.3 m-wide horizontal orb (Fig. 1). Since however, suggests that orb-web spiders that do not add the C. moluccensis forages diurnally and nocturnally, we con- structures build and occupy webs under greater predation ducted the experiments during the day and night to ascertain risk. It accordingly seems perplexing why spiders might do whether foraging at either time affects the barrier structure’s this, unless there are considerable, as yet unmeasured, costs influence on predator avoidance or prey capture. If the associated with adding the structures. barrier structure does not come at a cost of a reduction in The genera Cyrtophora and Mecynogea are permanent prey capture, it implies that the material, energetic, or time users of barrier structures, with their orb webs reduced to a costs associated with producing and depositing the barrier horizontally aligned prey-catching orb that lacks the sticky silk silks deters many orb-web spiders from building barrier characteristic of most other orb webs (Eberhard 1990; structures. Blackledge et al. 2009, 2011;Blamiresetal.2012a). These spiders position themselves on the underside of the horizontal orb and catch prey that ricochet off the barrier structure and fall Materials and methods into the orb. Since the architecture of the webs of these spiders fundamentally differs from those of other orb-web spiders Field experiments which generally build vertically aligned two-dimensional orb webs (Eberhard 1990; Blackledge et al. 2011), it might be Our experiments were performed over eight consecutive supposed that the barrier structure is used to support the days in 2007 and 2008 in a secondary forest dominated by horizontal orb, or the orb, for some reason, cannot function mulberry and elephant’s ear shrubs at Ape Hill (22°38′19″ without the barrier structure present, or the presence of barrier N, 120°15′54″ E), near National Sun Yat-Sen University, silk induces prey to behave differently (Craig 1986; Rypstra Kaohsiung City, Taiwan. At 0800 hours each morning, we and Buddle 2013) and become predisposed to capture. Some randomly selected 18 adult female C. moluccensis webs and spiders, e.g., members of the genera Leucauge and either removed the web barrier structures using a stick of hot Gertschanapis (Eberhard 1990; Blackledge et al. 2009, incense (BW− treatment) or retained them (BW+ treatment). 2011), nevertheless, build horizontal orb webs without barrier Individual spiders generally live for less than a year, so it structures. Furthermore, other orb-web spiders, e.g., members was unlikely that individuals were repeatedly measured each of the genera Nephila and Thelacantha, add barrier structures year as we only used adults. As the orb is anchored to to their vertical two-dimensional webs (Blamires et al. 2010; vegetation by frame threads that are distinct from the barrier Tseng et al. 2011). Subsequently, the above proposition seems threads, removing the barrier structure did not affect web untenable, although it remains to be determined whether bar- rier structures predispose insects to alter their behavior. Flying insects that strike the barrier threads of Cyrtophora spp. webs bounce off them and are directed toward the prey-capturing orb (Lubin 1974). This is analo- gous to the prey capture enhancing “ricochet” or “shadow” effects that result from web aggregations (Uetz 1989; Rao 2009). Accordingly, by enhancing the amount of prey- deflecting silk, and/or the area over which prey are captured, the barrier may be implicit in the capture of significantly more prey compared to the orb alone. Nevertheless, if the preys can see or detect the barrier from a distance and identify it as a threat, they may be able to avoid interacting with it (Craig 1986). If this occurs, the barrier would actu- ally come at an expression cost of significantly reducing the amount of prey captured. To test the hypothesis that barrier structures deter preda- Fig. 1 Barrier structure and horizontal capture orb of the three-dimen- tors at the expense of a reduction in the amount of prey sional orb web built by C. moluccensis Author's personal copy Behav Ecol Sociobiol tension or damage the web. The experiments ran for 14 h The predator approach and attack data were not normally and the smell of the incense dissipated within 5–10 s, so it (Kolmogorov–Smirnov test; P<0.05) or Poisson (binomial had no effect on the experimental outcomes.