Table Of Content2007. The Journal of Arachnology 35:38-45
SEISMIC SEXUAL SIGNAL DESIGN OF TWO SYMPATRIC
BURROWING TARANTULA SPIDERS FROM MEADOWS OF
URUGUAY: EUPALAESTRUS WEIJENBERGHI AND
ACANTHOSCURRIA SUINA (ARANEAE, THERAPHOSIDAE)
Veronica Quirici' and Fernando G. Costa: Laboratorio de Etologia, Ecologia y
Evolucion, Institute de Investigaciones Biologicas Clemente Estable, Avenida Italia
3318, Montevideo, Uruguay.
ABSTRACT. Eupalestrus weijenberghi (Thorell 1894) and Acanthosciirria siiina Pocock 1903 are two
fossorial, sympatric, synchronic, and similar-sized species of tarantulas that inhabit the meadows ofUru-
guay. Both species use seismic signals for communicating during courtship (body vibrations in males, leg
tapping in females), but little is known eitherabout temporal and spectral signal characteristics oreffective
signal range. Males were elicited to court (body vibrations) by exposing them to sexual pheromones in
one end of a terrarium, whereas females were successively placed in burrows at different distances from
the courting male. Seismic male signals were registered by using a geophone placed near the females. We
found that E. weijenberghi male signals reach at least 135.75 cm, whereas A. sidna signals reach at least
110.5 cm. There was no correlation between latency of female response and distances at which the male
was courting for either of the spider species. ForA. sidna, a bout of vibration was characterized by two
syllables (tucks), followed by a series of syllables (buzz 1 and buzz 2). Eupalestrus weijenberghi males
show several tucks, followed by buzz 1 and buzz 2. Differences in sexual signals (length of syllables and
numberoftucks) are assumed to maintain the reproductive isolation between these similarsized, sympatric
and synchronic species. Similarities are seen in adaptations to the same environment, allowing effective
rates of sexual communication at long distances.
Keywords: Tarantula male signals, seismic signal reach, courtship behavior
m
Vibratory sexual signals (acoustic and seis- 1981; Barth et al. 1988) and 1.20 in ther-
mic) are frequently used by courting male spi- aphosids (using an artificial substrate Prentice
ders, including tarantulas (Uetz & Stratton 1997). The usual indicator ofeffective signals
&
1982, 1983; Prentice 1992, 1997; Quirici is the behavioral change of the receptive fe-
Costa 2005). As tarantulas are nearly blind, male. The most unambiguous receptive re-
vibratory signals are particularly useful as sponse is a female signal, usually leg waving,
these signals do not need light to be perceived leg tapping, or body vibrations. These behav-
and they are quite directional (Foelix 1982; iors have been observed in lycosids (Rovner
Redondo 1994). Long distance communica- 1968; Costa 1975), ctenids (Rovner & Barth
tion is particularly important for male spiders 1981) and theraphosids (Prentice 1992; Quir-
due to the cannibalistic lifestyle of females ici & Costa 2005).
&
(Krafft 1980; Uetz Stratton 1983), and, as Eupalestrus weijenberghi (Thorell 1894)
mentioned by Narins (1990) and by Endler and Acanthoscurria suina Pocock 1903 are
(2000), these signals propagate long distances two sympatric, synchronic, and similar-sized
without loss of effectiveness. Seismic signals tarantulas species that inhabit bun'ows in the
generated by male spiders can reach 1 m in meadows of Uruguay. Both species can be
ctenids (through leaves, Rovner & Barth found in high densities: E. weijenberghi
shows a random distribution while A. suina
' Present address: Center for Advanced Studies in shows aggregated distribution (Perez-Miles et
Ecology and Biodiversity (CASEB) and Departa- al. 2005). The mating season ofthese two spe-
mento de Ecologia, Pontiheia Universidad Catolica cies is briefand synchronic, starting at the end
de Chile, Casilla 1 14-D, Santigo, Chile. E-mail: of the summer (February-March) when a
[email protected] huge number of walking males appear on the
38
QUIRICI & COSTA—SEXUAL SIGNAL DESIGN OF TARANTULA SPIDERS 39
roads (“road tarantulas”) and ending two
month later when all males die (Costa &
Perez-Miles 2002; Perez-Miles et al. 2005).
Adult males live only 2 mo in the wild, but
persist in good body condition for 4-5 mo
underlaboratory conditions (Perez-Miles et al.
2005); females can live around 8-10 yr (Costa —
pers. obs.). Females continue molting Figure 1. Schematic representation of the ex-
perimental design used forthe two theraphosid spe-
throughout their lives, so in each molt they cies. Broken vertical lines represent the metallic
become “virgins” (without sperm in the grid separating the male from the female. Males
sperm receptacles). were placed on the soil, in the same place, while
These two species share the same repro- females remained inside the burrows that werecho-
ductive strategy and behavior. They can be sen by us at random.
distinguished by color (A. suina brown, E.
weijenberghi black, with pale bands in legs),
by conspicuous tibial spurs (only on E. wei- and 15 cm height, with 5 cm of soil as sub-
jenberghi forelegs), and by defensive displays strate and water provision, and were fed live
(A. suina raises forelegs and opens fangs, cockroaches (Blaptica dubia, Blattaria, Bla-
whereas E. weijenberghi raises the abdomen). beridae) provided ad libitum. Voucher speci-
When the walking male touches the female mens of both species were deposited in the
silk, which has been imbued with sexual pher- entomological collection at the School of Sci-
omone, he courts the female primarily by ences, Universidad de la Republica, Monte-
means of body vibration. Body vibrations video, Uruguay.
generate seismic signals eliciting receptive fe- Spiders were kept in captivity for at least
male behavior (leg tapping), which orients the two weeks before being tested. Experiments
male toward the burrow entrance (Quirici & were carried out in four long glass terraria of
Costa 2005). The seismic nature of this sys- 20 cm width and 20 cm height containing a 7
tem was elucidated by Quirici & Costa (2005) cm thick layer of soil as a substrate (collected
using a discontinuous substrate, but effective from their natural habitat). For E. weijenber-
signal range and details of their temporal and ghi, the length of the two terraria used was
spectral characteristics were unknown. Our 170 cm; for A. suina, the two terraria mea-
objectives in the present study were to find out sured 120 cm length because preliminary ob-
the temporal characteristics of the signals of servations had suggested that E. weijenberghi
both species, and also to find out how far sig- signals reached farther than those ofA. suina.
nals propagate and still elicit female leg tap- In each terrarium, five burrows were con-
ping displays. structed in a line against the glass wall, allow-
METHODS ing visual observations offemale behavior in-
side burrows. The distance between two
—
Materials. Spider males of both species consecutive burrows was 30 cm forE. weijen-
were collected during March 2004 in the prov- berghi and 25 cm for A. suina (Fig. 1). Each
inces of Canelones (Solymar Norte, 34°45'S, terrarium was placed on an anti-vibratory ta-
56°00'W, and Salinas Norte, 34°45'S, ble (weighty wood plate placed on four in-
55°50'W) and Montevideo (Melilla, 34°45'S, flated bicycle tires) in order to minimize
56°20'W), Uruguay, with the maximal dis- ground vibrations and to prevent seismic sig-
tance among captures being 40 km. For all nals from passing between teiTaria (in a pre-
experiments, females of known reproductive liminary study without the anti-vibratory table
history were used. They were collected from a female responded to a courting male in an
the same localities between 1996 and 1999 adjacent terrarium). A geophone with vertical
and raised under laboratory conditions. All fe- polarization (Geospace industries, model GS
males used in the study molted in the labo- 20-DX, damping resistor = 1000 ohms) was
ratory between December 2003 and January used to collect substratum-coupled vibratory
2004. A total of 10 females and 25 males for signals. To test if signals would pass to an
each species were used in this study. They adjacent ten*arium, the geophone was placed
were housed in glass jars of 9.5 cm diameter in one terrarium and a courting male in the
40 THE JOURNAL OF ARACHNOLOGY
—
Table L Experimental design for each of the posed to two different males. Only one ex-
four terraria used in this study (two for A. suina periment was conducted per day.
and two forE. weijenberghi). Each male group was Trials in each terrarium were carried out for
composed offive individuals, and each ofthemwas >24 h, allowing each female to adapt to the
used once with one female for each ofthe two ter- burrow (acclimatization period). This time pe-
raria. The five females (A, B, C, D, and E) were
riod also allowed another female to release
located in each burrow, avoiding repeated encoun-
ters with the same male. A total of 50 trials were sex pheromones at the end of the terrarium
performed for each spider species. where the males were located to help elicit
male courtship. At approximately 19:00 h this
Burrow Burrow Burrow Burrow Burrow last female was removed and a male was lo-
Males 1 2 3 4 5 cated in the same place. The time ofeach ex-
Group A B c D E periment was recorded from the start of male
1
Group 2 E A B C D courtship until completion of female sexual
Group 3 D E A B C response or until 30 min after courtship if
Group 4 C D E A B there was no female response. The geophone
Group 5 B C D E A was located each time near the entrance ofthe
occupied burrow. Distance from males to the
geophone vary from 21.5 cm, 50.5 cm, 75.5
other, and any courting signals could be reg- cm, 110 cm and 134.75 cm for E. weijenber-
istered by the geophone. Data were recorded ghi, and 15.25 cm, 37.25 cm, 60.25 cm, 84.75
on an analog recorder UHER 4200, at a speed and 110.5 cm for A. suina, which correspond
of 9.5 m/s (frequency band of 20 Hz to 16 to the burrow location. A total of50 trials was
kHz). Experiments were carried out from 16 recorded for each—species.
March to 16 April 2004, synchronously with Data analysis. The temporal characteris-
the reproductive period ofthese species under tics of male courtship signals were analyzed
natural conditions. During the experiments, using the software, Sound Ruler (0.9.4.0),
mean room temperature was 24.96° C (±2.05 which allows the visual and acoustic identifi-
SD). — cation of pulses and accompanying silences.
Experimental design* Preliminary exper- We analyzed a total of 47 male courtship sig-
iments analyzing the duration of bouts of vi- nals for E. weijenberghi and 67 for A. suina,
brations produced by males prompted the use belonging to 15 males from each species.
of the two step sample method in this study The Fisher two tailed exact probability test
(Snedecor & Cochran 1984). Primary sample (Siegel 1956) was used to compare the occur-
units were the species and the secondary sam- rence of female response among burrows for
ples units were the individuals within each each species. The latency of female response
species. Variance components were examined was used as an estimate of female receptivity
ANOVA
throughout one-sample two-step (hi- (defined as the time period from the end of
erarchical ANOVA), following Sokal & Rohlf the first male bout of vibration until the first
(1979). The experimental design is shown in female leg tapping, according to Quirici &
Table 1. The order of consecutive burrows, Costa 2005). Simple linear regression was
where females were located one a time, is in- used to examine a relationship between the
dicated in Arabic numbers (1 to 5). Individual latency of female response (dependent vari-
females are identified by capital letters (A to able) and the distance between male and fe-
E). All burrows were tested with a female in- male (as an independent variable). Previously,
side, perceiving male vibration bouts at all the regression analysis residues were plotted in
experimental distances. Each group (1 to 5) order to remove outliers and assess normality.
was composed of five different males. All the Analyses of variance were used for testing
males used, as well as the male-female pairs significance ofthe regressions (Daniels 1989).
and the female burrow locations, were ran- When assumptions ofnormality or variance
domly assigned but repetition was avoided. homogeneity were not confirmed, non-para-
Since two terraria were used for each species, metric tests (Mann-Whitney U-test and Krus-
each male courted two different females dur- kal-Wallis) were used. All statistical analyses
ing the entire experimental period, one in each were performed using the software program,
terrarium. Consequently, each female was ex- Rproject (http://www.r-project.org).
QUIRICI & COSTA—SEXUAL SIGNAL DESIGN OF TARANTULA SPIDERS 41
—
Table 2. Occurrence frequency of leg tapping weijenberghi = 0.479, ^
from 10 different females located in the five bur- 0.492, n — 44, P = 0.09) nor for A[c,riticsalu]ina
rows. Distances between vibrating male and each ~ 0.257, F ~ 0.614, n = 35,
female burrow were: 1 = 21.5 cm, 2 = 50.5 cm, 3 P [=calc0ul.a3te7d)]. Moreover,[cdriitiscatl]ance explained less
E=. w75e.i5jencbme,rg4hi,=an1d101c=m1a5n.d255cm=, 2134==.7357.c25mcfmo,r than 2% of variation in female behavior in
3 = 60.25 cm, 4 = 84.75 and 5 = 110.5 cm for both species.
Acanthoscurria suina: description of
A. suina. —
male courtship signal. The set of consecu-
Burrow tive male body vibrations separated by pauses,
or “bouts,” was characterized by two tem-
1 2 3 4 5 poral components. A first component included
E. weijenberghi 10 9 10 9 9 a single syllable, identified as “tuck,” where-
A. suina 8 8 6 7 8 as a second component involved a series of
syllables, referred to as “buzzes” (terms were
chosen to resemble the sounds made by the
RESULTS spiders) (Fig. 2). A typical male vibrationbout
Female responses.—In 94% of the trials was composed of two consecutive “tucks”
with E. weijenberghi and 74% of the trials (tuck 1 mean duration = 0.100 s, SD = 0.113,
with in A. suina, females responded to court- n ~ 22; tuck 2 mean duration — 0.064 s, SD
= 0.025, n =25) followed by two consecutive
ship at all distances, including cases at the
maximal possible distance forE. weijenberghi “buzzes.” (buzz 1 mean duration = 0.55 s,
(135.75 cm) and also for A. suina (110.5 cm) SD = 0.34, n = 63; buzz 2 mean duration =
(Table 2). No statistical differences were 0.745 s, SD = 0.237, n = 54). Periods of si-
found using the Fisher exact test when com- lence between tucks, between tuck 2 and buzz
paring the occurrence of female response of 1 and between buzz 1 and buzz 2, were called
each species in each of the five burrows {P = “interpulses”: II (mean duration = 0.282 s,
00a..n04d8764Pff~oorrIbbuufrrorrroobwwur13r,,oEPw =5=).10.f5o8r2buforrrobwur2r,oPw 4=, 0dSu.Dr4a1t4=iosn,0.S1=4D,0=n.580=9.38s1,26,),SnD12==(2m40e).a1n8a4n,ddunr13at=i(omn5e2a)=n,
Table 3 shows the recorded values oflaten- respectively.
cy of female calls for both species and for There were no significant differences be-
each burrow. The latency was highly variable tween the duration of tuck 1 and tuck 2 (U =
in both species, with all the obtained variation 148, P = 0.429, n — 14, 25). When comparing
coefficients higher than 60%. When compar- all interpulse periods, significant differences
ing latency values across all five burrows, we were found (Kruskal-Wallis test: 7/292 —
found no statistical differences either for E. 23.44, P < 0.001). Interpulse 3 (13) had the
weijenberghi (7/447 “ 1.192, P = 0.879) orfor longest mean value. Buzz 2 had a greater du-
A. suina (7/437 = 1.032, P = 0.905). ration than buzz 1 (two-tailed t-test, t = "4.5,
Simple linear regression analyses showed df= 115, P < 0.001).
no significant correlation between distance Eupalaestrus weijenb—erghi: description of
and latency of female response, neither for E. male courtship signal. As inA. suina, male
—
Table 3. Latency (s) for female response (leg tapping behavior) for both tarantula species and foreach
burrow. Values shown are means, standard deviations (SD) and coefficients of variation (CV).
Eupalaestrus weijenberghi Acanthoscurria suina
Burrow Mean SD CV Mean SD CV
1 172.33 245.27 142% 246.97 188.3 76%
2 80.5 76.1 95% 321.13 468.98 146%
3 104.96 68.78 66% 366.9 236.65 65%
4 79.82 47.5 60% 456.27 518.63 114%
5 127.63 193.89 152% 288.5 311.28 108%
42 THE JOURNAL OF ARACHNOLOGY
'luck 2
'rime (seconds)
0 60 s
—
Figure 2. Above; schematic drawing ofmaleA. siiina courtship through time. Oblique lines represent
a “bout” of vibration produced by the male. Entire sequence corresponded to one minute. Bellow: oscil-
logram of a single bout of vibration. All the components are indicated (tucks, inteipulse periods, and
buzz). Tuck 1 and tuck 2 were amplified.
—
bout vibrations of E. weijenherghi presented both spider species. When signal compo-
single syllables or tucks (tuck 1 mean duration nents were compared, buzz 1 ofE. weijenher-
= 0.066 s, SD = 0.034, n = 33; tuck 2 mean ghi showed a longer duration than A. suina (t
duration == 0.088 s, SD = 0.049, n =33) and = 1.067, P < 0.001, df = 107), whereas in-
sequences of syllables orbuzzes (buzz 1 mean terpulse duration between buzz 1 and buzz 2
duration = 0.871 s, SD = 0.238, n = 46; buzz was longer in A. suina (t = —3.667, P =
2 mean duration = 0.812, SD = 0.183, n = 0.004). No significant differences were de-
45). Separately by a serious of interpulses: II tected in any of the other parameters; dura-
(mean = 0.391 s, SD = 0.289, n = 33), 12 tions of tuck 1 (f/ = 339.5, P = 0.505, n =
(mean = 0.504 s, SD = 0.3, n = 33) and 13 33, 23), inteipulse {U = 176, P = 0.08, n =
(mean = 0.459 s, SD = 0.159, n = 45). How- 32, 16), tuck 2{U = 316, P = 0.129, n = 33,
ever, vibrations in E. weijenherghi differed 25), interpulse (f = 1.005, P = 0.319, df =
from A. suina signals in the number of tucks 55, n — 33, 24) and buzz 2 (r = 1.569, P =
preceding buzzes, from six to eight. Figure 3 0.1 19, df = 97, n = 45, 54). Considering all
shows a schematic representation of a male parameters measured, E. weijenherghi signal
vibration bout throughout time and an oscil- bouts were twice as long as A. suina ones
logram of a bout of E. weijenherghi. (3.82 s versus 1.902 s) (taking into account all
There were no statistical differences in the tucks that precede buzz 1 in E. weijenherghi)
duration of the tucks before buzz {U = and were statistically significantly different
\
398.5, P = 0.062, n = 33, 33), or among si- (Mann-Whitnney U-test U = 175, P < 0.001,
lences (Kruskal-Wallis test, //2ji() = 5.92, P = n = 46, 67). So, differences between species
0.06). Contrary to A. suina, no differences be- were in the duration of buzz 1 and in the du-
tween buzz 1 and buzz 2 were found {t = 1.3, ration of 13 {A. suina showing higher values)
df= 89, P = 0.194). and in the number oftucks that preceded buzz
Male sexual signal vibrations: comparing 1 in E. weijenherghi (from six to eight).
QUIRICI & COSTA—SEXUAL SIGNAL DESIGN OE TARANTULA SPIDERS 43
01s
Tuck 2
Time(seconds)
0 60 S
—
Eigure 3. Above: schematic drawing of male E. weijenberghi courtship through time. Oblique lines
represent a “bout” of vibration produced by the male. Entire sequence corresponded to one minute.
Bellow: oscillogram of a single bout of vibration. All the components are indicated (tucks, interpulse
periods, and buzz). Tuck 1 and tuck 2 were amplified.
DISCUSSION zhansky 1940; Greenfield 1997). Forexample,
—
Male signal characteristics. Since the the sympatric species of the ctenid spider Cu-
polarization plane of the geophone was ver- piennius diverge in their courtship signals,
&
tical, and seismic signals generated by the mainly in the temporal structure (Schuch
courting males were well perceived by the ap- Barth 1990). —
paratus, these signals would correspond to the Female latency and signal efficacy. Re-
kind of seismic signals called “Rayleigh gression analyses showed no differences in
waves” (Narins 1990). However, the co-oc- the latency of female response as a function
currence of signals called “Love” waves (Na- of male-female distances. We conclude that
rins 1990) can not be discounted in E. weijen- courtship signals were above the response
berghi and A. suina. A geophone with threshold of females, and that signal features
horizontal polarization would be required to that elicited female responses were unchanged
record them and this was not available for this (until 135.75 cm in E. weijenberghi (135.75
study. For example, the fiddler crab Uca pug- cm) and 110.5 cm in A. suina). However field
ilator use both kinds of waves for communi- experiments would be necessary to confirm
cating (Aicher & Tautz 1990). these data. For example Elias et al. (2004)
Three interspecific differences were found found that in the salticid spider, Habronattus
in the temporal structure of the signal: in the dossenus Griswold 1987 the effectiveness of
length of buzz 1, in the length of interpulse male courtship seismic signal appears to be
period between the two buzzes, and in the strongly constrained by the available substra-
number of tucks that precede buzz 1. Diver- tum resource. Seismic signals reaching long
gence in sexual signals among sympatric and distances were extensively reported in scor-
synchronous species would be an indicator pions (Brownell & Hemmen 2001), crickets
& &
that natural selection is acting on the signal (Hill Shadley 2001), amphibians (Lewis
design, favoring species recognition (see Dob- Narins 1985), fossorial rodents (Heth et al.
44 THE JOURNAL OF ARACHNOLOGY
1987, 1988; Randall 1989, 1993, 1995; Rado this meteorological condition that character-
et al. 1989; Narins et al. 1997; Mason & Na- izes the reproductive period ofboth these spe-
&
rins 2001), and spiders (Rovner Barth 1981; cies.
Barth et al. 1988; Prentice 1997). Therefore, From this present study we conclude that
we can make a prediction about what could there exists convergence and divergence in the
occur in open grasslands. Given that female temporal signal characteristics between these
burrows are in high densities (from 0.07 in- species and that the signals produced by males
dividuals/m^ to 0.65 individuals/m^) with dis- are propagated long distances. In addition
tances between burrows entrances ranges from males appear to be taking advantage of the
8.0 cm to 36 cm (Perez-Miles et al. 2005), climatological conditions of the reproductive
many females could respond to male seismic period when the signals transmission efficien-
signals and compete among themselves, per- cy is high.
forming leg tapping to orient the male towards ACKNOWLEDGMENTS
each burrow entrance. This scenario is plau-
sible principally inA. suina where female bur- We are very grateful to Peter Narins who
rows showed an aggregated dist—ribution. gave us the equipment to gather the data. We
The use of seismic signals. Sharing the also thank Mario Penna forhis help during the
same spatial and temporal characteristics, both different stages of the work and for his cor-
species would converge towards similar seis- rections and suggestions on the first draft of
mic signal characteristics. As mentioned by the manuscript. Angel Caputi taught V.Q. to
Greenfield (1994), the environmental and analyze signals and helped us with her sug-
physical conditions that determine the repro- gestions. Gabriel Francescoli gave us the tape
ductive period would be some ofthe pressures recorder and some literature. We also thank
that determine the sexual signal design Luis Ebensperger for suggestions on the first
through selection ofsignal efficacy. Due to the draft of the manuscript and for improving the
We
fact that Eupalaestrus weijenberghi andAcan- English. also appreciate the constructive
thoscurria suina inhabit the same habitat and criticisms of two anonymous reviewers on a
microhabitat, it was expected that they would previous version of this manuscript.
use the same communicatory channel. Sec- LITERATURE CITED
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s19o8m8e; Haiultlho2r0s01h;avMeaspoonst&ulaNtaerdin(sHe2t0h01)e,t tahle. nnailcaotifoCnoimnpatrheatfiivddelePrhycrsaibo,loUgcyaApug1i6l6a:t3o4r5.-J3o5u3r.-
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