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First published online March 2, 2006
Journal of Experimental Biology 209, 1074-1084 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02104
Seismic signal production in a wolf spider: parallel versus serial multi-component signals
1 Division of Life Sciences, Integrative Behaviour and Neuroscience,
University of Toronto at Scarborough, Ontario, M1C 1A4, Canada
2 Departments of Zoology and Botany, University of British Columbia,
Vancouver, V6T 1Z4, Canada
3 School of Biological Sciences, University of Nebraska, Lincoln, NE 68588,
USA
* Author for correspondence (e-mail: elias{at}utsc.utoronto.ca)
Accepted 17 January 2006
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Summary |
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Key words: seismic signal, courtship display, Schizocosa stridulans, signal evolution, multiple signal, vibratory display
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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) are common in animal communication and there has been a
recent rise in studies examining the evolution and processing of multiple
signals (Candolin, 2003;
Hebets and Papaj, 2005;
Iwasa and Pomiankowski, 1994;
Johnstone, 1996;
Moller and Pomiankowski, 1993;
Partan and Marler, 1999;
Partan and Marler, 2005;
Pomiankowski and Iwasa, 1993;
Rowe, 1999;
Uetz and Roberts, 2002). One
category of multiple signals is multi-component signals – those
characterized as having multiple parts within a single sensory modality
(Candolin, 2003;
Hebets and Papaj, 2005;
Rowe, 1999). For example, many
song birds include a diversity of distinct syllables in their songs
(Catchpole and Slater, 1995),
and the seismic songs of some jumping spiders feature three distinct
components (Elias et al.,
2003). While the evolution and function of complex signaling has
been the focus of a growing body of work, there are currently few studies
examining the mechanisms underlying the production of multiple signals.
Acoustic multi-component signals can be produced using the same or
independent structures. Most work on multi-component song production has been
conducted in song birds and frogs, which, like many vertebrates, produce songs
using complex air expulsion apparatuses
(Gerhardt and Huber, 2002;
Goller and Suthers, 1995;
Suthers, 1990;
Suthers et al., 2004;
Suthers and Zollinger, 2004).
Studies of sound production in vertebrates tends to be focused on a small
number of structures and locations (e.g. syrinx in birds, vocal folds in
frogs) although there are some notable exceptions: for example, manikins
(Bostwick and Prum, 2005) and
some fish (Ladich, 2000;
Ladich and Bass, 1998). By
contrast, acoustic signal production in arthropods is not limited to specific
structures and song-producing devices and can be found on virtually any part
of their hard exoskeleton (Dumortier,
1963; Ewing, 1989;
Legendre, 1963). In addition,
arthropods can produce acoustic/vibratory signals using a myriad of mechanisms
(air expulsion, percussion, vibration/tremulation, stridulation, tymbal, and
`stick-and-slip' mechanisms), each of which can be found anywhere on their
body (Dumortier, 1963;
Ewing, 1989;
Gerhardt and Huber, 2002;
Huber et al., 1989;
Legendre, 1963;
Markl, 1983;
Patek, 2001;
Rovner, 1980;
Uetz and Stratton, 1982).
Although many arthropods can produce multi-component songs with different
mechanisms as well as different structures
(Cokl and Doberlet, 2003;
Cokl et al., 2004;
Gogala, 1985;
Kalmring, 1985;
Kalmring, 1997;
Moraes et al., 2005;
Popper et al., 2001;
Virant-Doberlet and Cokl,
2004), most work on sound-production mechanisms in arthropods has
focused on relatively simple calling signals of acoustic Orthoptera
(Bailey and Rentz, 1990;
Gerhardt and Huber, 2002;
Huber et al., 1989; but see
Kalmring, 1997).
Wolf spiders (Family Lycosidae) have been used as models to study the
evolution and function of communication, particularly the genus
Schizocosa (Ahtiainen et al.,
2003; Ahtiainen et al.,
2004; Ahtiainen et al.,
2005; Hebets,
2005; Hebets and Uetz,
1999; Hebets and Uetz,
2000; Kotiaho et al.,
1998; Miller et al.,
1998; Parri et al.,
2002; Rivero et al.,
2000; Scheffer et al.,
1996; Stratton and Uetz,
1983; Stratton and Uetz,
1986; Taylor et al.,
2005; Uetz and Roberts,
2002; Uetz and Stratton,
1982). Wolf spider males communicate to females using multimodal
displays often consisting of chemical, visual and seismic (vibratory)
components (Hebets, 2005;
Hebets and Uetz, 1999;
Hebets and Uetz, 2000;
Roberts and Uetz, 2004;
Roberts and Uetz, 2005;
Scheffer et al., 1996;
Taylor et al., 2005;
Uetz and Roberts, 2002). In
particular, seismic components have special relevance as the majority of
spiders use vibrations as the predominant modality guiding behavior
(Barth, 1985;
Barth, 1998;
Barth, 2002;
Foelix, 1996;
Uetz and Stratton, 1982).
Seismic signals produced by males during courtship displays have been shown to
be important in mate choice and species recognition
(Hebets, 2005;
Hebets and Uetz, 1999;
Parri et al., 2002;
Rivero et al., 2000;
Uetz and Stratton, 1982).
Despite this importance, seismic signals in Schizocosa remain poorly
understood and very few studies have examined the mechanisms used to produce
seismic signals (Rovner, 1967;
Rovner, 1975).
Using the novel technique of high-speed cinematography, Rovner showed that,
contrary to the widely held belief that wolf spiders produced sounds by
percussion, some wolf spiders produced seismic signals by stridulation
(Rovner, 1975). Subsequent
studies found evidence for stridulatory apparatuses in other wolf spiders
(Fernandez-Montraveta and Simo,
2002). Recent developments in non-contact vibration recording
techniques as well as synchronized high-speed videography provide powerful
tools to re-examine seismic signal production mechanisms in wolf spiders
(Elias et al., 2003;
Nieh and Tautz, 2000). In this
study, we examined the signal-production mechanisms of the wolf spider
Schizocosa stridulans Stratton.
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Materials and methods |
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Recording procedures
Courtship arenas were constructed by stretching nylon fabric on a circular
26.5 cm needlepoint frame. We used an artificial courting substrate in order
to facilitate synchronous high-speed video and laser vibrometer recordings. In
the field, males are found in deciduous forests on a substratum composed of
complex leaf litter, but no differences have been observed in male courtship
behavior on natural versus artificial substrates (E.A.H., personal
observation). We characterized the nylon fabric and determined that the nylon
substrate passed all frequencies in the animal's signaling bandwidth equally
(data not shown). A virgin female was confined overnight on the arena to
deposit silk. Silk has been shown to be an effective releaser of male
courtship in many Schizocosa species in both natural and artificial
substrates (Stratton, 1983;
Stratton, 1997;
Stratton and Uetz, 1983;
Stratton and Uetz, 1986). A
total of eight different virgin females was used. At the beginning of every
recording session, females were removed and, subsequently, males placed in the
arena. Recording started when males began courting. We recorded substrate
vibrations produced during courtship using a laser doppler vibrometer (LDV)
(Polytec OFV 3001 controller, OFV 511 sensor head; Walbronn, Germany)
(Michelsen et al., 1982). A
piece of reflective tape (approx. 1 mm2) was attached at the centre
of the arena to serve as a measurement point for the LDV. The LDV signal was
synchronized with two concurrent methods of video recording. (1) The LDV
signal was recorded on the audio track during standard videotaping of
courtship behavior (Navitar Zoom 7000 lens; Rochester, NY, USA; Panasonic
GP-KR222; Matsushita Electric Industrial Co., Osaka, Japan; Sony DVCAM DSR-20
digital VCR; Tokyo, Japan; 44.1 kHz audio sampling rate). (2) The LDV signal
was digitized (National Instruments PCI-6023E; Austin, TX, USA; 10 kHz
sampling rate) simultaneously with the capture of digital high-speed video
(500 frames s–1; RedLake Motionscope PCI 1000; San Diego, CA,
USA) using Midas software (v.2.0; Xcitex, Inc., Cambridge, MA, USA). All
recordings were made on a vibration-isolated table. At the conclusion of each
recording session, females were confined to arenas to deposit fresh silk.
Experimental manipulations
Recordings of seismic signals were made from each male prior to
experimental manipulation. We manipulated males by either (1) preventing
palpal movement by waxing the tibio–cymbial joint using a mixture of
beeswax and colliphonium, (2) preventing abdominal (opisthosoma) movements by
attaching the cephalothorax (prosoma) to the abdomen with wax or (3)
manipulating both the cephalothorax–abdominal joint and the palpal
tibio-cymbial joint. To ensure that these treatments did not affect normal
locomotory activities, we waited two days following these manipulations and
observed whether or not animals were able to successfully capture prey. We
used only males that were able to capture prey during this interval.
Sound and video analysis
Complete courtships of 30 different males were recorded. Seismic courtship
signal displays can be divided into two distinct categories: (1) `rev'
displays (`pulses' in Stratton,
1997) and (2) `idle' displays (`trills' in
Stratton, 1997). Examples were
selected for detailed analysis. Body movements for an individual were measured
from digital high-speed video using Midas software. Power spectra of vibratory
signals were calculated using Matlab software (v.7.1; The Mathworks, Natick,
MA, USA) and 2–3 signals averaged for each individual. Spectrograms were
made using Raven (Cornell University, Lab of Ornithology). Males frequently
changed position when producing seismic signals (see below), hence it was
difficult to maintain the laser at a constant distance from the courting male.
Although there were differences in the attenuation of signals depending on a
male's final courting position, attenuation characteristics were similar for
all frequencies, hence peak frequency measurements were not significantly
distorted and, furthermore, comparisons of different signal conditions were
based on normalized spectra (see below).
Power spectra analysis
Within a treatment set (control, experimental) from an individual animal,
individual signals (see below) were identified using videotaped data, and a
random selection of each seismic signal display acquired. Normalized power
spectra of rev displays were then calculated using the pwelch function in
Matlab. Peak intensities were measured for low- (0–500 Hz) and
high-frequency (500–3000 Hz) bands. Within treatment sets for each
individual, intensity was normalized to the highest intensity produced.
Differences between treatment sets were tested for significance
(P<0.05) using a paired t-test. Statistical tests were
conducted using the Systat statistical analysis package (SSI, Richmond, CA,
USA).
Scanning electron microscopy
Palps were dissected to separate the joint between the tarsus and cymbium.
Specimens were dehydrated by a series of increasing ethanol concentrations
(10, 30, 50, 60, 70, 80, 90 and 95%) for 20 min in each solution. Following
dehydration, they were mounted on a 12.7 mm-diameter aluminum stub
(Canemco-Marivac part # 700-1; Quebec, Canada) with doubled-sided adhesive
tape (3MTM Scotch No. 655) and gold sputter coated with a Polaron SEM
coating unit PS3 (Watford, UK). Specimens were viewed at 20 keV using Hitachi
SEM model S-530 (Tokyo, Japan) and photographed onto 35 mm Fujifilm Neopan 100
black-and-white film with a Nikon FG-20 camera.
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). Males were
recorded prior to treatment, and then with the tibio-cymbial joint of the palp
immobilized (Fig. 2B). We could
readily identify the occurrence of each signal type by the postures and
movement characteristics of each signal from videotapes
(Fig. 1Bi). When palpal flexion was prevented, higher frequency components of rev signals were attenuated while lower frequencies were not (Figs 2B, 3A). No significant differences were observed for low-frequency peaks between control and experimental treatments (low-frequency control treatment –7.089±2.692 dB, mean ± s.d.; low-frequency experimental treatment –4.975±1.604 dB, mean ± s.d.; N=5, t1,4=–1.734, P=0.158; Fig. 3A). Significant differences were observed for high-frequency peaks between control and experimental treatments (high-frequency control treatment –11.273± 5.179 dB, mean ± s.d.; high-frequency experimental treatment –38.507±3.812 dB, mean ± s.d.; N=5, t1,4=8.816, P<0.001; Fig. 3A). Low-frequency components of the signal remained, consisting of vibrations of 0.240±0.011 s (mean ± s.e.m., N=20 from 5 males) in duration and of low frequency (80.18±11.45 Hz, mean ± s.e.m.; range 23–244 Hz; N=17 from 5 males).
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Abdominal movement. Analysis of high-speed videos suggested that low-frequency components are produced by abdominal movements and not by flexions of the tibio-cymbial joint of the palp. Males were recorded both prior to and after abdomen immobilization (Fig. 2C). We were readily able to identify the attempted production of each seismic signal component by the postures and movement of males from the videotape (Fig. 1Bi).
When abdominal movements were prevented, lower frequency components of rev signals were attenuated while higher frequencies were not (Figs 2C, 3B). Significant differences were observed for low-frequency peaks between control and experimental treatments (low-frequency control treatment –4.553±1.303 dB, mean ± s.d.; low-frequency experimental treatment –14.927±6.223 dB, mean ± s.d.; N=4, t1,3=4.117, P=0.026; Fig. 3B). No significant differences were observed for high-frequency peaks between control and experimental treatments (high-frequency control treatment –14.651±8.069 dB, mean ± s.d.; high-frequency experimental treatment –18.650±6.177 dB, mean ± s.d.; N=4, t1,3=1.163, P=0.329; Fig. 3B). Examining the vibrations produced after manipulation showed that components produced during palpal flexion are a train of brief 0.262±0.012 s (mean ± s.e.m., N=17 from 5 males) pulses of relatively broadband, high-frequency vibrations (peak frequency 890±65.48 Hz, mean ± s.e.m.; range 644–2186 Hz; N=20 from 5 males).
While preventing abdominal movements attenuated low-frequency signal components, they were not eliminated completely. A low-frequency peak was still observed at a similar frequency to intact signals (80.17±11.45, mean ± s.e.m., N=17 from 5 males). This weaker low-frequency peak corresponded to vibrations produced by the gross movements of the palps – the rate of palpal flexions rather than the stridulation generated by these movements (see above).
Palpal and abdominal movement. In order to test whether abdominal and palpal movements were sufficient to explain all components of rev seismic signals in S. stridulans, we immobilized both the tibio-cymbial joint of the palp and the cephalothorax–abdomen joint (Fig. 2D). Males were recorded prior to treatment, then with palps and abdomen immobilized. We could readily identify the occurrence of each signal type by the postures and movement characteristic of each signal from videotapes (Fig. 1Bi).
All components of rev signals were attenuated following experimental manipulation (Figs 2D, 3C). Significant differences were observed for low-frequency peaks (low-frequency control treatment –4.444±1.590 dB, mean ± s.d.; low-frequency experimental treatment –24.536±5.339 dB, mean ± s.d.; N=5, t1,4=–8.574, P=0.001; Fig. 3C) and high-frequency peaks (high-frequency control treatment –8.218±1.488 dB, mean ± s.d.; high-frequency experimental treatment –39.093±7.960 dB, mean ± s.d.; N=5, t1,4=8.703, P=0.001; Fig. 3C) between control and experimental treatment. Hence, palpal and abdominal movements are sufficient to produce multi-component rev signals.
Idle courtship displays
Signal characteristics
The predominant seismic displays observed in this study were rev displays.
Males, however, also produced another type of signal, idle displays (`trill'
in Stratton, 1997). Males
assumed a courtship posture where all the legs were spread widely apart and
the palps were placed perpendicular to the arena surface. The male then
positioned its forelegs in an arched position above the arena and quickly
tapped its legs on the substrate (percussion;
Fig. 4i). Four to 11 individual
leg taps occurred in rapid succession. Leg taps are audible to the human ear
(Stratton, 1997). After the
leg taps, male spiders rapidly flexed the distal joint of the palp
(Fig. 4ii). Individual palps
were flexed out of phase to produce a sustained series of brief seismic pulses
(duration 3.32±0.757 s, mean ± s.e.m.; range 0.5–9.4 s;
N=16 from 8 males). This movement corresponded with high-frequency,
broadband seismic vibrations (peak frequency 950±127.29 Hz, mean
± s.e.m.; range 450–1250 Hz; N=16 from 8 males) similar
to rev displays. No abdominal vibrations were observed during idle displays.
Low-frequency components appeared to have been caused by the rate of palpal
flexion.
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Experimental manipulations
Idle displays did not occur in all the individuals recorded, hence it was
not possible to perform statistical analyses on control and experimental
treatments. We recorded idle displays whenever possible and present individual
examples of idle displays from all treatment groups. When the palpal
tibio-cymbial joint was immobilized, signals were greatly attenuated
(Fig. 5B). When abdomens were
immobilized, no differences were observed, although low-frequency components
were slightly attenuated (Fig.
5C). When both palps and abdomens were immobilized, signals were
greatly attenuated (Fig. 5D).
Idle displays are thus produced predominantly by the palps, and flexion of the
palpal tibio-cymbial joint is sufficient to explain idle displays.
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;
Rovner, 1975;
Stratton, 1997). Therefore, we
examined, using scanning electron microscopy (SEM), the palpal tibio-cymbial
joint (Fig. 6A) of males
(Rovner, 1975). SEMs revealed
a presence of a hardened scraper on the dorsal surface of male cymbium
(Fig. 6B). In the apposing
tibial area, we noted the presence of a file on the dorsal base of the tarsus
(Fig. 6C).
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;
Elias et al., 2004;
Elias et al., 2005), comprise
multiple seismic components that are produced by independent mechanisms. In
addition, in some cases multiple components are produced simultaneously
(parallel multi-component displays) while in other cases multiple components
are produced in a temporal series (serial multi-component displays).
Alternative modes of combining multiple signal components may be an important
factor in signal function, but also one that is constrained by the mechanisms
and structures involved in the production. The type and location of
sound-producing structures as well as the sound-production mechanism used can
give insight into the mechanisms driving signal evolution. Several basic
arrangements are possible for structures involved in the production of
multi-component signals. Individual components may be produced by (1) the same
structure, (2) bilaterally similar but independent structures or (3)
completely independent structures. In addition, signals using these structures
can be produced using (1) the same mechanism or (2) multiple mechanisms. A
typical example of signals using a single structure and mechanism is the
katydid Metrioptera sphagnorum, where multiple signal components are
all produced by a forewing file-and-scraper mechanism (stridulation) but
different signal components are produced by different parts of the file
(Morris and Pipher, 1972). An
example of multi-component signals produced using similar structures but
independent mechanisms is jumping spiders that produce seismic signal
components using abdominal movements alone (tremulation) and abdominal
movements coupled to a frequency multiplier (stridulation)
(Elias et al., 2003). Examples
of independent bilaterally symmetrical structures used to produce
multi-component signals include song birds that produce signals using a pair
of syringeal vibrators, each of which can potentially produce harmonically
unrelated sounds at once (Bradbury and
Vehrencamp, 1998; Goller and
Suthers, 1995; Suthers,
1990; Suthers et al.,
2004; Suthers and Zollinger,
2004), and grasshoppers that stridulate using structures found on
both hind legs (Elsner and Wasser,
1995; von Helversen et al.,
2004). Wolf spiders (present study) also stridulate using
bilaterally symmetrical palps. Independent bilaterally symmetrical structures
usually produce sounds using the same mechanism.
In this study, wolf spiders use independent structures including palps,
abdomen and forelegs to produce multi-component songs. In addition, each
structure produces components using a different mechanism (stridulation,
tremulation and percussion). Such signals are presumably costly to produce
since animals must develop and coordinate completely different structures and
neuromuscular systems. Male S. stridulans court females on natural
substrates of leaf litter (E.A.H., unpublished observation), and courtship
signals must often propagate through multiple leaves, where signals may become
distorted and attenuated due to filtering, reflection and scattering. There is
often no simple relationship between the amplitude of the signal and the
distance from the sender along single leaf surfaces
(Barth, 2002;
Cokl et al., 2004;
Magal et al., 2000;
Michelsen et al., 1982) and
this presumably becomes more complex as signals propagate through multiple
leaves. Given this signaling environment, knowledge of seismic
signal-production mechanisms can provide insights regarding multi-component
signal function.
A high-bandwidth signal may be a way to ensure that some part of a signal
is detected by the receiver in a complex signaling environment with
unpredictable filtering properties (redundant backups)
(Iwasa and Pomiankowski, 1994;
Johnstone, 1996;
Pomiankowski and Iwasa, 1993;
Partan and Marler, 2005).
Production of multiple components by the palps and abdomen may be a way to
expand the bandwidth of seismic signals. This does not, however, explain why
components are produced in parallel (see below). In addition, high bandwidths
are a property of percussive signals, and percussion does not require
coordination from multiple sources (i.e. less costly to produce). If selection
has acted to increase signal bandwidth, then one would predict that percussive
components should dominate signals. In this species of wolf spiders,
percussion only occurs in idle and not rev displays. This suggests that
back-up signals alone are not sufficient to explain S. stridulans
seismic signals.
The location of distinct, independent sound-production areas in S.
stridulans suggests that males may be transmitting non-redundant/multiple
messages (Iwasa and Pomiankowski,
1994; Johnstone,
1996; Pomiankowski and Iwasa,
1993; Partan and Marler,
2005). Multiple messages hypotheses propose that multiple signal
components relay different information to receivers. For example, the
stridulatory apparatus located on the palpal tibio-cymbial joint is
developmentally fixed at maturation and may provide more static information
about male quality such as overall size. By contrast, abdomen size is not
fixed but is dependent on the recent feeding history of the male, potentially
providing more dynamic information about male quality such as foraging
success. Since the rev displays of S. stridulans consist of both a
high- and a low-frequency component, males may be able to code for information
about both long-term (developmental) quality (high-frequency component) and
short-term quality (low-frequency component). Females attending to the rev
display would then be gaining information about multiple aspects of a male's
condition. Idle displays, on the other hand, are produced using leg percussion
and palpal stridulation but not abdominal tremulations. Idle displays are much
longer in duration but occur rarely. It is possible that idle displays provide
further information about long-term (developmental) quality. Leg taps (and
presumably idle displays) occur more frequently immediately prior to mounting
and copulation of a female (Stratton, 1998), hence idle displays may function
as short-distance signals since males producing long-duration signals may risk
interference from other male signals (see below). Leg taps may function as an
attention primer to the long-duration stridulatory component, as has been
suggested in other spiders (Elias et al.,
2003). Further experiments are necessary to test these hypotheses,
with special consideration of signal quality at different distances from
courting males, as courting substrates can heavily influence signal
characteristics (Cokl et al.,
2004; Cokl et al.,
2005; Elias et al.,
2004; Magal et al.,
2000; Michelsen et al.,
1982).
Alternatively, multi-component signal evolution may be driven by
inter-component interactions (Hebets and
Papaj, 2005). Many types of seismic signals (i.e. bending waves)
have the property of dispersive propagation – different frequencies
travel at different speeds (Aicher and
Tautz, 1990; Barth,
2002; Cremer et al.,
1973; Michelsen et al.,
1982). It is therefore theoretically possible that if wolf spiders
use bending waves to communicate, wolf spider females can locate potential
mates by measuring arrival-time differences of low- and high-frequency signal
components, as has been shown in scorpions
(Brownell and Farley, 1979;
Brownell, 1977;
Brownell and Van Hemmen,
2000). Further studies are necessary to demonstrate such a
function in S. stridulans.
While the above hypotheses focus mainly on signal-producing structures, we
can also generate hypotheses about signal function based on the timing of
multiple components (i.e. serial or parallel). Serial displays are very common
in the animal communication literature. For example, multiple syllable types
in birds (Catchpole and Slater,
1995), insect songs (Guerra
and Morris, 2002) and jumping spiders
(Elias et al., 2003). Serial
multi-component signals may evolve in systems that favor sequential assessment
of signals, for example birds, where repertoire size is a measure of male
quality (Catchpole and Slater,
1995), or when one signal component acts as an amplifier to
another (Hasson, 1991).
Parallel multi-component signals are less common in the literature and
refer to components that are produced concurrently resulting in an integrated
signal (in terms of timing). In this study, wolf spiders produce rev displays
using a combination of abdominal tremulations and palpal stridulation slightly
offset in time but produced concurrently. Some bugs, crabs and katydids also
appear to produce parallel signals (Cokl
and Doberlet, 2003; Cokl et
al., 2004; Gogala,
1985; Kalmring,
1985; Kalmring,
1997; Moraes et al.,
2005; Popper et al.,
2001; Virant-Doberlet and
Cokl, 2004). Identification of parallel multi-component signals
requires a detailed analysis of sound-production mechanisms and it is possible
that parallel multi-component signals are common.
The selective forces that may drive the evolution of serial versus parallel multi-component signals have not been addressed in the literature. Parallel signals may evolve under selection for increased information content in systems where communication occurs under time constraints. The threats of eavesdropping by predators or interference by competitors may limit available signaling time for male S. stridulans. Additionally, pre-copulatory sexual cannibalism is relatively common in S. stridulans (E.A.H., unpublished data) and males must rapidly identify themselves as potential mates to avoid a predatory response from females. Pre-copulatory sexual cannibalism may thus impose strong selection favoring rapid information transfer in male courtship thorough parallel multi-component signaling.
S. stridulans males also intermittently produce serial signals
(idle displays). The rarity of idle displays may reflect selection against
long-duration signals. In addition, idle displays occur more frequently when
males and females are in close proximity (Stratton, 1998). In these
circumstances, interference from rival male signals may be reduced.
Theoretical models of multiple signal evolution have shown that, with
increasing costs, signals should evolve to be simpler and unicomponent
(Iwasa and Pomiankowski, 1994;
Pomiankowski and Iwasa, 1993;
Pomiankowski and Iwasa, 1998).
The present study suggests an alternative where increasing costs of assessment
and/or signaling do not lead to simplification but instead to the
economization of signals by parallel multi-component signal production.
Alternatively, a parallel multi-component signal could arise through a co-evolutionary elaboration of a simple (unicomponent) signal. In S. stridulans, abdominal tremulations occur at the same frequency as palpal flexions and appear to amplify low-frequency components produced incidentally by stridulatory movements. It is possible that in an ancestral unicomponent stridulating species, low frequencies produced by body flexions were an informative cue to females since larger animals can induce larger vibrations. Males could then evolve abdominal tremulation as an exaggeration of the incidental body movement cue. Parallel multi-component signals could thus reflect an ancestral origin and could persist in wolf spiders since abdominal tremulations appear to function to increase the intensity of low-frequency components.
In summary, detailed knowledge of sound-production mechanisms is necessary in discussions of signal design and evolution. First, the location and types of sound-production mechanisms can offer insights into hypotheses on signal evolution and function. Second, the majority of work on multiple signals has focused on signals that are easily discriminated by humans, i.e. either multimodal signals or serial multi-component signals. As a result, little work has been conducted on parallel multi-component signals as they require detailed knowledge of sound-producing mechanisms. Parallel multi-component signals are a major category of multiple signals and more detailed knowledge is required to determine the frequency of such signals in nature as well as to illuminate the selective pressures that drive the evolution of different types of multiple signals (multimodal vs parallel multi-component vs serial multi-component).
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Acknowledgments |
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References |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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