Behav Ecol Sociobiol (2013) 67:1483–1498 DOI 10.1007/s00265-013-1519-4 ORIGINAL PAPER The dominance of seismic signaling and selection for signal complexity in Schizocosa multimodal courtship displays Eileen A. Hebets & Cor J. Vink & Laura Sullivan-Beckers & Malcolm F. Rosenthal Received: 27 September 2012 /Revised: 7 March 2013 /Accepted: 9 March 2013 /Published online: 17 April 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract Schizocosa wolf spiders show tremendous diversity signaling is correlated with seismic signal complexity and (2) in courtship complexity, with different species employing vary- the importance of visual signaling is correlated with visual ing numbers of components within and across sensory modal- signal complexity. We find a significant relationship between ities. Using a comparative approach, we investigate the visual signal importance and visual signal complexity, but no importance of each signaling modality in the courtship display relationship between seismic signal importance and seismic of five Schizocosa species (three stridulating and two drum- signal complexity. Finally, we test the hypothesis that selection ming) by assessing mating success under manipulated signaling acts on complexity per se by determining whether seismic and environments. Irrespective of the degree of male ornamentation, visual signal complexity is correlated across species. We find the three stridulating species exhibit a dependence on the seis- support for this hypothesis in a significant relationship between mic, but not visual, signaling environment for mating success. seismic and visual signal complexity. Mating was independent of signaling environment for the two drumming species. We next ask whether the degree to which Keywords Communication . Female choice . Repertoire each species depends upon a signaling modality for mating (i.e., size . Sexual selection . Signal efficacy . Diversification modality importance) is correlated with the estimated modality- specific signal complexity. We first calculate effect sizes for the influence of seismic versus visual signaling environments on the Introduction likelihood to mate for ten Schizocosa species and then use an element-counting approach to calculate seismic and visual sig- Complexity, or the intricate combination of many parts, is a nal complexity scores. We use a phylogenetic regression anal- fundamental attribute of biological systems, and arguably no- ysis to test two predictions: (1) the importance of seismic where is it more conspicuous than in the courtship displays of certain animal groups. Yet the factors that drive this complexity remain unknown, as do the answers to such questions as Communicated by J. P. Higham whether courtship complexity (the combination of multiple This manuscript is part of the special issue Multimodal Communication— display components) functions differently across different ani- Guest Editors: James P. Higham and Eileen A. Hebets mal groups or in different behavioral contexts. Observed com- Electronic supplementary material The online version of this article plexity in animal courtship displays can be thought of in terms (doi:10.1007/s00265-013-1519-4) contains supplementary material, of courtship elaboration and can incorporate multiple visual which is available to authorized users. components in the form of distinct morphological structures, E. A. Hebets (*) : L. Sullivan-Beckers : M. F. Rosenthal conspicuous coloration or pigment patterns, and/or vigorous School of Biological Sciences, University of Nebraska, multifaceted movements. Additionally, courting animals may Lincoln, NE, USA incorporate the production and dissemination of chemical com- e-mail: [email protected] pounds, multicomponent acoustic signals, and/or air move- C. J. Vink ments, among others. Many such multifaceted courtship AgResearch, Lincoln Research Centre, Christchurch, New Zealand displays incorporate signals or components that are categorized by their physical properties into different sensory channels or C. J. Vink Entomology Research Museum, Lincoln University, Lincoln, modalities (see Hebets 2011). Such displays are considered to New Zealand be multimodal (Rowe 1999; Partan and Marler 1999). 1484 Behav Ecol Sociobiol (2013) 67:1483–1498 Perhaps more striking than the diversity and ubiquitous monophyletic North American Schizocosa clade, there exist nature of multimodal courtship displays is the frequency of extremes in terms of both courtship signal complexity and observable disparity in the degree of complexity among close- sexual dimorphism. Some species employ relatively simple, ly related animal species (Darwin 1871). In some instances, seismic-only courtship displays, while others incorporate not such as within the North American wolf spider genus only more complex multicomponent seismic signals, but also Schizocosa, this disparity is manifested as seemingly different visual displays that involve vigorous waving of ornamented investment in signaling modalities between closely related forelegs (reviewed in Stratton 2005; Framenau and Hebets species, even sister taxa (see Fig. 1). Such divergence in traits 2007; Vaccaro et al. 2010). Foreleg ornamentation, a sexually associated with courtship displays among close relatives has dimorphic trait, consists of some combination of pigmentation led scientists to hypothesize that sexual communication, and and/or brushes of setae (or hairs) on various segments of the likely sexual selection, plays a prominent role in morpholog- first pair of walking legs in mature males (reviewed in Stratton ical diversification and, ultimately, in speciation (Grant and 2005; Framenau and Hebets 2007). The first, and sometimes Grant 1997; Shaw and Parsons 2002;GerhardtandHuber second, pair of walking legs is also the pair frequently waved 2002). The evidence for this hypothesis, however, is during courtship dances. Among the 31 taxa included in a underwhelming (see Servedio 2012), and determining the recent morphological phylogenetic analysis (23 described extent to which sexual selection, versus natural selection, is species plus additional populations of select species), foreleg involved in diversification and speciation remains a central ornamentation was hypothesized to have evolved indepen- goal of evolutionary biology (Ritchie 2007; Kraaijeveld et al. dently five or six times, and to have been subsequently lost 2011). Species groups characterized by distinct and wide- two or three times (Stratton 2005). While all described spread variation in courtship displays and associated morpho- Schizocosa species incorporate a seismic signal in their court- logical traits provide ideal systems to tackle this goal. Here, ship displays, only some possess additional courtship compo- we use just such a group, Schizocosa wolf spiders, to explore nents in the form of foreleg ornaments and leg waves— the evolution and function of complex, multimodal courtship making the displays of these species multimodal in nature signaling across numerous species. (seismic plus visual). The genus Schizocosa consists of at least 63 species world- Numerous studies have demonstrated female choice asso- wide, 23 of which occur in North America (for discussion of ciated with male courtship displays across Schizocosa species relevance of non-Nearctic species which are likely to belong (Stratton and Uetz 1981, 1983; McClintock and Uetz 1996; in other genera, see Stratton 2005). Across the reciprocally Scheffer et al. 1996; Hebets and Uetz 1999; Hebets and Uetz Fig. 1 Bayesian consensus tree based on cytochrome c oxidase subunit 1 (COI) sequence data. Thickened bars represent species with tibial bristles present (from Stratton 2005). Waveforms are placed next to the species of focus and lines beneath each waveform depict one second of courtship. Although depicted as separate species here for clarity, COI has not revealed reciprocal monophyly for S. ocreata and S. rovneri. For a tree with posterior probabilities and branch lengths proportional to the expected number of substitutions per site see Supplementary Fig. 1 Behav Ecol Sociobiol (2013) 67:1483–1498 1485 2000; Uetz and Roberts 2002; Hebets 2003;Hebets2005; seismic signal and the visual signal for all ten species. Using Persons and Uetz 2005;UetzandNorton2007; Hebets 2007, our calculated effect sizes and our complexity scores, we test 2008; Hebets et al. 2008a;GibsonandUetz2008;Hebetsetal. the hypothesis that modality-specific signal complexity is 2011;Rundusetal.2011; among others). Several of these driven by modality-specific receiver responses. Specifically, studies have involved artificial manipulations of display com- we test the following predictions using phylogenetic compar- ponents and subsequent assessment of female responses. A ative methods: (1) the importance of seismic signaling for series of studies manipulated components of the visual signal mating (seismic signal effect size) is correlated with seismic using video playback and examined female receptivity dis- signal complexity and (2) the importance of visual signaling plays (for review see Uetz and Roberts 2002;Hebets2005; for mating (visual signal effect size) is correlated with visual Uetz and Norton 2007; Hebets 2008), while others have signal complexity. Finally, we ask whether there is a relation- studied female receptivity responses to isolated display com- ship between the degree of signal complexity across signaling ponents of live courting males (e.g., Scheffer et al. 1996; modalities and test the following prediction: