Assortative mating by aggressiveness type in orb weaving spiders

Abstract

Animals within a population differ consistently in behavior over time and/or across conditions. A general question is how such differences referred to as personalities are maintained through evolution. One suggested mechanism is a nonrandom mate choice, which has been supported in species in which mate choice associates with direct material benefits. Much less is known about mating patterns and personality in species where males provide only sperm and in which the benefits of female choice are based only on good and/or compatible genes. The bridge spider Larinioides sclopetarius Clerck (Araneidae) exhibits heritable between-individual differences in intrasex aggressiveness. We studied mating probabilities by aggressiveness type of both sexes, and success in sperm competition of aggressive versus nonaggressive males. We staged trials that resemble field conditions: 4 males (2 aggressive and 2 nonaggressive) had simultaneous choice between an aggressive and a nonaggressive female. Although there were no differences in initial approaches of male types toward female types, aggressive males mainly mated with aggressive females, and nonaggressive males more likely mated with nonaggressive females. Female aggressiveness type was not related to fecundity, which may be a consequence of equal food supply in the laboratory. However, in double-mating trials using the sterile-male technique to measure paternity of aggressive versus nonaggressive males, we found that sons of aggressive parents fathered relatively more offspring. We conclude that assortative mating by aggressiveness type might maintain between-individual differences in aggressiveness in L. sclopetarius.

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Behavioral
Ecology
The ofcial journal of the
ISBE
International Society for Behavioral Ecology
Original Article
Assortative mating by aggressiveness type in
orb weaving spiders
SimonaKralj-Fišer,a,b Graciela A.Sanguino Mostajo,b OnnoPreik,b StanoPekár,c and Jutta
M.Schneiderb
aInstitute of Biology, Scientific Research Centre, Slovenian Academy of Sciences and Arts, Novi trg
2, PO Box 306, SI-1001 Ljubljana, Slovenia, bBiozentrum Grindel, Zoological Institute & Museum,
University of Hamburg, Martin-Luther-King Platz 3, D-20146 Hamburg, Germany, and cDepartment of
Botany and Zoology, Faculty of Sciences, Masaryk University, Kotlá
ř
ská 2, 611 37 Brno, Czech Republic
Animals within a population differ consistently in behavior over time and/or across conditions. Ageneral question is how such differ-
ences referred to as personalities are maintained through evolution. One suggested mechanism is a nonrandom mate choice, which
has been supported in species in which mate choice associates with direct material benefits. Much less is known about mating pat-
terns and personality in species where males provide only sperm and in which the benefits of female choice are based only on good
and/or compatible genes. The bridge spider Larinioides sclopetarius Clerck (Araneidae) exhibits heritable between-individual dif-
ferences in intrasex aggressiveness. We studied mating probabilities by aggressiveness type of both sexes, and success in sperm
competition of aggressive versus nonaggressive males. We staged trials that resemble field conditions: 4 males (2 aggressive and 2
nonaggressive) had simultaneous choice between an aggressive and a nonaggressive female. Although there were no differences in
initial approaches of male types toward female types, aggressive males mainly mated with aggressive females, and nonaggressive
males more likely mated with nonaggressive females. Female aggressiveness type was not related to fecundity, which may be a conse-
quence of equal food supply in the laboratory. However, in double-mating trials using the sterile-male technique to measure paternity of
aggressive versus nonaggressive males, we found that sons of aggressive parents fathered relatively more offspring. We conclude that
assortative mating by aggressiveness type might maintain between-individual differences in aggressiveness in L. sclopetarius.
Key words: aggression, competition, female choice, personality, reproductive success, sexual selection. [Behav Ecol]
INTRODUCTION
Optimality approaches in behavioral ecology assume that selec-
tion favors the convergence of behavioral types/strategies toward
a single one with the highest fitness (Dall et al. 2004). Individual
dierences in behavior have been traditionally considered as noise
around an adaptive population mean. Variation in behavior was
interpreted as plasticity or alternative tactics (Gross 1996). Contrary
to this view, evidence is accumulating that individuals of the same
sex and age within a given population consistently dier from each
other in their behavioral characteristics referred to as personalities
(Zuckerman 1991).
This raises questions about why individual personality dierences
exist, that is, how they are generated and maintained (Sih et al.
2004; Schuett et al. 2010). A number of functional explanations
for the evolutionary maintenance of personalities have been sug-
gested. Between-individual behavioral dierences were proposed to
exist due to life-history trade-os (Stamps 2007; Wolf et al. 2007),
condition-dependent selection (Gross 1996), fluctuating selection,
or negative frequency–dependent selection (Dingemanse and Reale
2005; Smith and Blumstein 2008; Kralj-Fišer and Schneider 2012).
Within-individual behavioral consistency may be explained by
physiological constraints (Sih etal. 2004), benefits of predictabil-
ity (Dall etal. 2004; McNamara etal. 2009), and positive feedback
loops between state and behavior (Dall etal. 2004; Sih etal. 2004).
Recently, Schuett et al. (2010) suggested that personalities might
have evolved due to sexual selection probably acting together with
the other proposed evolutionary processes.
Sexual selection may maintain personalities through nonrandom
mate choice and male–male competition (Schuett et al. 2010).
Choosy individuals may gain benefits if potential mates vary in
fecundity or fertility, parental abilities, resources and status, “good
genes”, and/or genetic compatibility (Bateson 1983; Andersson
1994; Kokko et al. 2003; Ne and Pitcher 2005). A certain
personality type may be associated with quality as a mate while this
quality could be additive or nonadditive. In the former case, certain
personalities would be favored over others, whereas in the latter,
Address correspondence to S.Kralj-Fišer. E-mail: simonakf@gmail.com.
Received 7 October 2012; revised 5 March 2013; accepted 16 March
2013.
Behavioral Ecology
doi:10.1093/beheco/art030
Behavioral Ecology Advance Access published April 18, 2013
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Behavioral Ecology
the match of the 2 partners matters. Depending on the connection
between personality type, and male and female fitness, the resulting
mating pattern could be assortative or disassortative.
Assortative mating by behavioral type may occur if the same
behavioral type indicates quality as a mate in both sexes. High-
quality individuals will then mate with each other, leaving individu-
als scoring low in this trait to pair up with one another (Johnson
1988; Davies 1989; McNamara and Collins 1990). If both extremes
on a behavioral axis, for example, shy and bold individuals, have
similar fitness, between-individual variability could be maintained
in such a way (Schuett et al. 2010). Assortative mating patterns
could also be produced if mates with compatible behavioral types
are preferred (Tregenza and Wedell 2000; Ne and Pitcher 2005)
although depending on the nature of the best match, compatibility
as a driving force of sexual selection can also produce disassorta-
tive mating patterns by behavioral type. Selecting a mate with the
same behavioral type can be favored by selection if this improves
coordination and cooperation within the pair and increases repro-
ductive success (Budaev etal. 1999; Both et al. 2005; Schuett and
Dall 2009; Royle et al. 2010; Schuett, Dall, etal. 2011; Schuett,
Godin, etal. 2011; Gabriel and Black 2012). On the other hand,
choosing a mate with another behavioral type could be adaptive if
this reduces the probability of genetic or behavioral incompatibili-
ties (Ens etal. 1993; Dingemanse etal. 2004; van Oers etal. 2008).
Studies that consider the role of personalities in sexual selection
have mostly been done with vertebrate systems in which males pro-
vide direct benefits to the female or ospring (reviewed in Schuett
etal. 2010); much less is known about the influence of sexual selec-
tion on personality in species where males provide only sperm (but
see Sinn etal. 2006) and in which benefits of female choice are sup-
posed to be based on good and/or compatible genes that increase
ospring fitness (Zeh and Zeh 1997; Kokko et al. 2003). Among
invertebrates, mate choice and personality have been studied in
the socially polymorphic spider Anelosimus studiosus (Hentz) (Pruitt
etal. 2008, 2011; Pruitt and Riechert 2009a, 2009b). In this spe-
cies, females are larger and the more dangerous sex, and aggres-
sive males preferentially choose to mate with social (docile) females,
whereas docile males more often mate with aggressive ones pro-
ducing a disassortative mating pattern (Pruitt and Riechert 2009a,
2009b).
However, the reproductive success of a male with a particular
behavioral type may depend on the behavioral type of his mate.
In mating experiments with A.studiosus, aggressive males had lower
reproductive success when mated with aggressive (asocial) females
relative to docile males (Pruitt etal. 2011). It is possible that aggres-
sive females exert some downstream regulatory mechanism that
bias male insemination success; perhaps they do so in accordance
to genetic (in)compatibility, which seems to be associated with
aggressiveness type (Pruitt etal. 2011). Very few studies have dealt
with thisissue.
We aimed to study mating patterns in relation to behavioral
types in the bridge spider Larinioides sclopetarius Clerck (Araneidae),
an orb-weaver inhabiting areas across the Holarctic. Recent
experiments revealed heritable between-individual dierences in
aggressiveness toward a conspecific in both sexes, which were inde-
pendent of spider size (Kralj-Fišer and Schneider 2012). A simu-
lation of high-density conditions suggested that groups consisting
of a mixture of aggressive and nonaggressive types did better than
groups that consisted of nonaggressive types supporting the notion
that negative frequency–dependent selection could be relevant
(Kralj-Fišer and Schneider 2012). A male-biased operational sex
ratio and male–male competition for access to mates may select for
high male aggressiveness. We expected that aggressive males would
outcompete less aggressive rivals in access to preferred female phe-
notypes. Females mate multiply and vary in body mass (Kleinteich
and Schneider 2011), which is a good predictor of fecundity in spi-
ders in general (Bristowe 1958). Females are rarely sexually can-
nibalistic, and body size and aggressiveness of laboratory-reared
females are not correlated, so that we cannot make straightforward
predictions concerning male preference for female behavioraltypes.
Furthermore, we conducted a sperm competition study to test
the prediction that aggressive males may not only win precopu-
latory competition but inseminate more eggs as well. We sequen-
tially mated females with 2 males taken from the extremes of a
distribution of aggression scores. We measured paternity using a
sterile-male method and expected higher numbers of fertilized
eggs by the aggressive male regardless of mating order.
MATERIALS AND METHODS
Rearing
We used subadult spiders, kept in 200-mL plastic cups and fed
with Drosophila sp. until adulthood. We reared Drosophila larva
on a high-quality medium so that the adult flies contain all the
nutrients required for the spiders (Mayntz and Toft 2001). Spiders
grew and survived very well on this diet (Kleinteich and Schneider
2011). When adult, males remained in the 200-mL cups under
the same feeding treatment, whereas females were transferred into
plastic frames (h= 36, d =6, w = 36 cm). Adult female spiders
were fed with larger flies (Calliphora sp.). Spiders (26 males and 30
females) were fed twice per week, watered 5 days per week, and
kept at room temperature under light:dark cycle 14:10 h (early
autumn, breeding) conditions throughout the duration of the
experiments.
Experimental protocol
Aggressiveness toward same sex conspecific
We tested both male and female aggressiveness by placing 2 indi-
viduals of the same sex close to each other. Generally the female
resident was found in 1 corner of the frame, the intruder was then
carefully placed with the paintbrush approximately 5 cm from the
resident. Females were tested twice, once as residents in their own
web and once as intruders on an unfamiliar web, and the order of
acting as intruder or resident was randomized.
Males give up web-building when adult. To observe males in
a competitive situation, 2 males were placed on a haphazardly
chosen female web. Males were placed in the center of the female
web, approximately 5 cm from each other (ca. 15 cm from a
female). In intrasex contests that lasted 20 min, aggressiveness was
measured as a sum of scores based on the frequency of aggressive
behavior such as approaching (score = 1), web-shaking (score =
1), attacking (score = 2), chasing (score = 3), and biting (score =
3) (Kralj-Fišer, Gregorič, etal. 2011; Kralj-Fišer, Schneider, et al.
2011). “Approach” was defined as a movement by one spider with
the result of shortening the distance to the other individual and
“chasing” as a quick move in the direction of the other individual
resulting in a successful attack or its escape.
According to the mean aggressiveness (average of sum of the
aggression scores from the 2 contests), test individuals were ranked
and divided into 2 groups: aggressive (higher than median) and
nonaggressive (lower than median). Aggressiveness scores showed a
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Kralj-Fišer etal. • Nonrandom mating by personality type
continuous distribution but we chose to simplify the variation into a
dichotomous treatment to facilitate data analysis.
Matingtrials
Spiders were allocated into 15 groups of 6 individuals each for the
mating experiments. Each group consisted of 1 aggressive and 1
nonaggressive female (both virgin) and of 2 aggressive and 2 non-
aggressive males. We did not have enough males to use each one
only once (see Appendix 1). To account for a potential correlated
response due to repeated use of the same individuals, male ID was
included in the covariance structure of each linear model used to
analyze data.
Initially, the 2 females were housed in 2 frames separated by a
solid Perspex division (Figure1A). In the field, webs occur in close
proximity and are often not strictly separated from one another
but use common frame threads (Kleinteich 2009). In order to
simulate a realistic choice condition for the males, we simply
removed the division between the 2 frames several days before the
mating experiment so that we had a single large frame (h= 36,
d = 12, w = 36 cm) with 2 females and their webs (Figure 1B).
To minimize the risk of aggressive behavior or even cannibalism
due to hunger, we fed females a day prior to experiment. Prior
to each experiment, we weighed all spiders using an electronic
balance (Mettler Toledo XS105 Dual Range) to an accuracy of
0.01 mg. We marked all spider males and females individually
using nontoxic paint and released the 4 marked males into the
frame (Figure1C), making sure they touched both webs, so they
perceived the presence of the 2 females.
All male–female interactions including aggressiveness, court-
ship, and copulation were recorded for 2 h. Female aggression
toward males was quantified based on the same scoring system as
the aggressiveness test (see above). The exception was biting the
male’s tarsus, which seems to be a part of the courtship or an indi-
cator of the female willingness to mate as it generally occurs during
courtship and never induces termination of courtship or aggressive
behavior (Kralj-Fišer S, personal observation). Therefore, it was not
considered as indicator of mating aggressiveness.
Courtship and copulation were observed and timed with a stop-
watch. To measure the duration of copulation, the stopwatch was
started at genital contact and stopped when genitalia were discon-
nected. Courtship starts with the male entering the web, signaling
his presence to the female. The female either stays passive or comes
out of her retreat (unless she acts aggressively), and the male will
start touching her body with his legs. Courtship ends with copula-
tion or the male leaving the web. We recorded the number (N) of
approaches (meaning the male moved toward the female on the web
without touching her) and touches (the male touched the female
with his legs) from each male toward each female. Copulation was
observed for frequency and order for each male. Very short appar-
ent copulations (less than 1 s) were recorded; however, their validity
as true copulations with sperm transfer were only later confirmed
when we were able to assess whether they resulted in fertilization of
eggs. This was determined by inspecting the incubated egg sacs for
hatching success.
After mating, females were kept individually and fed as before
until they had produced 2 egg sacs. Each egg sac was placed in a
controlled climate chamber at 25°C and light:dark cycle of 14:10
h and was misted with water on 5days per week. One week after
the first spiderlings had hatched, the clutches were frozen at −80°C
and then transferred into 70% ethanol. Numbers of hatched o-
spring were counted under the microscope.
After the experiments had ended, the tibia + patella length of
the first leg from all the tested individuals was measured to an accu-
racy of 0.01 mm using a LEICA MZ 16 stereomicroscope and the
corresponding computer software.
Paternitystudy
We used unrelated F1 spiders derived from a heritability study,
hence with known heritage of aggressiveness type (Kralj-Fišer and
Schneider 2012). To standardize the contribution of the female
aggression level to paternity, females were daughters of parents
taken from the middle of the distribution of aggressiveness scores,
whereas males were sons of parents with high or low aggressiveness
scores, hence taken from the extremes only. We refer to sons of
aggressive parents as aggressive males (Agg) and to nonaggressive
males (Non) for sons of nonaggressive parents. Half of the males
from each group were randomly picked and irradiated for 50 min
with a dosage of 40 Gray (0.8 Gy/min). The irradiation causes
DNA damage, and although the sperm is still capable of fertilizing
eggs, these eggs will not develop (Schneider and Andrade 2011).
Fifteen females received an irradiated nonaggressive males first
and an untreated aggressive male second (Non(I)/Agg(N)) and
15 females received an irradiated aggressive male first and an
untreated nonaggressive male second (Agg(I)/Non(N)). Hence,
all first males were irradiated. We did not vary the order of the
irradiation as we were interested in the relative paternity; the
absolute fertilization success depending on the irradiation was
irrelevant. We avoided systematic errors due to the irradiation
treatment by treating both groups of males. Two cases had to be
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Figure1
Experimental setup to observe for mating pattern between aggressive and
nonaggressive females and between 2 aggressive and 2 nonaggressive males.
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Behavioral Ecology
excluded: one because the first male was not prevented from getting
2 insertions and one extreme outlier where the female was very
close to oviposition so that the sperm of the 2nd male could not
be used. The exclusions reduced sample sizes to 14 in the Agg(I)/
Non(N) and 15 in the Non(I)/Agg(N) treatment. The 2 groups of
females did not dier in weight (Anova, F1,27 = 0.02, P=0.88) or
in the clutch size of their 1st (Anova, F1,27 = 1.64, P=0.21) and
2nd egg sacs (Anova, F1,25=0.31, P=0.58). Average clutch size in
the Agg(I)/Non(N) was 115.6 (standard error [SE]= 9.35) in the
1st and 128.5 (SE=8.04) in the 2nd clutch. In the Non(I)/Agg(N)
treatment, 1st clutches contained 99.0 (SE=7.72) and 2nd clutches
contained 109.3 (SE = 12.01) eggs. First and second males did not
dier in body mass or in body size measured as tibia–patella length
(Anovas, all P values >0.68).
The 2 males that mated with the same female were carefully
size matched and a correlation between the weight of the 2 males
and their size was highly significant (weight: Pearson correlation
r=0.95, df=25, P=0.0001; size: r=0.71, df=25, P=0.0001).
However, although the weight match was very good, a dierence
in size was still present as well as a large variation between trials
so that male body size was entered as a covariate into the analysis.
In addition to the above treatments, an irradiation control
treatment was performed in which 5 females mated with 2 irra-
diated males. None of these females produced hatchlings show-
ing that the irradiation was successful. We know from numerous
breeding experiments with this species that natural hatching
success is very high (Kleinteich 2009).
Each male was allowed a single intromission and was then
removed. Copulation duration was measured using a stopwatch.
Females were then kept until they had produced 2 egg sacs. Each
egg sac was left to hatch in a controlled climate chamber (as
above). One week after the first spiderlings had hatched, the egg
sacs were frozen at −80°C and then transferred into 70% ethanol.
Hatched and unhatched eggs were counted under the micro-
scope. Each clutch was counted twice on 2 dierent days without
knowing from which treatment they had derived.
Statistical analyses
The eect of 6 predictors, female patella + tibia I length, male
patella + tibia I length, female age, male age, female behavioral
type, and male behavioral type, on the mating probability, num-
ber of mates, first male choice, copulation frequencies, durations
of copulation, number of approaches, number of touches, dura-
tions of courtship, female aggression scores toward males, egg sac
numbers, egg sac weight, and hatchling numbers was studied using
generalized estimating equations (GEE) or generalized linear mixed
models (GLMM) because we used a block design with repeated use
of the same individuals. GEE were used when only 1 “random”
variable (group ID) was in the model and because we were not
interested in the estimation of variance of the random eects. GEE
models the correlation among subjects, resulting from repeated
use of the same individuals, in the covariance matrix of residuals
and provides correct marginal or population average models even
when the correlation structure is not perfectly specified (Hardin
and Hilbe 2003). GLMM was used in situations when 2 crossed
variables considered “random” (group and male ID) were in the
model, though we were not particularly interested in the estima-
tion of variation for the random eects. The random eects of the
male ID were significant (P< 0.05) in analyses when the follow-
ing response variables were used: copulation frequencies, number
of approaches, number of touches, and female aggression scores
toward males. GLMM models the correlation among subjects in
the covariance matrix of random eects (Zuur etal. 2009). We kept
both “random” variables in the model and simplified only the fixed
part of the linear predictor using stepwise deletion of nonsignifi-
cant eects, which included all predictors in an additive way. The
Poisson error structure (GEE-p, GLMM-p) was chosen when the
response variables were counts; binomial errors were used (GEE-b,
GLMM-b) when the response was of binary character. Alog-nor-
mal model (GEE, GLMM) or gamma errors (GEE-g, GLMM-g)
were assumed when the response variable was time due to increase
of variance with themean.
Generalized linear models with binomial error structure (GLM-
b) were used to study the relationship between several predictors
(female weight, copulation duration, male weight, male aggres-
sion, and male size) and hatching success. Only eggs fathered by
the untreated of the 2 males hatched, whereas the eggs fathered by
the irradiated male died during early development. Hence hatching
success equals paternity success of the untreated male. Data for 2
consecutive egg sacs were pooled and used as a measure of hatch-
ing success. The linear predictor included main eects and their
2-way interactions.
All analyses were performed within R (R Development Core
Team 2010). For the GEE, we used the geepack package (Yan
and Fine 2004) and for the GLMM, the lme4 package (Bates and
Maechler 2009).
RESULTS
Courtship
Aggressive and nonaggressive males did not dier significantly in
the frequencies of approaches to females in general, regardless
of female aggression type (GLMM-p, X1
201=
.,
P=0.73), in the
number of touches of females (GLMM-p, X1
213=
.,
P= 0.26), or
in courtship durations (GLMM, X1
211=
.,
P=0.29). The approach
frequency (GLMM-p, X1
213 8=
.,
P = 0.0002) and number of
touches (GLMM-p, X1
242=
.,
P = 0.04) decreased with female
age, whereas the eects of all other explanatory variables (female
size, male size, and male age) were not significant (GLMM-p,
P >0.1).
Aggression levels of the male and the female were not important
for the first approach as evidenced by the nonsignificant interaction
(GEE-b, X1
201<
.,
P=1.00). Including all approaches, aggressive
males approached aggressive females almost twice as often as
nonaggressive females, whereas nonaggressive males slightly more
often approached nonaggressive females (GLMM-p, X1
237 1=
.,
P < 0.0001, Figure 2A). Furthermore, aggressive males touched
aggressive females significantly (almost 3 times) more often
(GLMM-p, X1
219 8=
.,
P < 0.0001, Figure2B) and courted them
significantly (3 times) longer (GLMM, X1
279=
.,
P = 0.005,
Figure 2C) than nonaggressive males. The dierence was less
pronounced but reversed in nonaggressive females, which were
more often touched and courted for longer by nonaggressivemales.
Aggressive females were 1.5 times more aggressive toward males
than nonaggressive females. Female aggressiveness decreased with
her size (GLMM-p, X1
245=
.,
P=0.03). Aggressive females were
twice as aggressive toward aggressive males than toward nonaggres-
sive males, whereas nonaggressive females had higher aggression
frequencies toward nonaggressive males (GLMM-p, X1
241 6=
.,
P < 0.0001, Figure 3). Aggressive and nonaggressive males over-
all did not receive significantly dierent levels of aggression from
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Kralj-Fišer etal. • Nonrandom mating by personality type
females (GLMM-p, X1
209=
.,
P = 0.36). Independent of female
and male behavioral types, female aggressiveness toward males
increased with female age (GLMM-p, X1
251=
.,
P=0.03).
Mating
Aggressive and nonaggressive females did not dier significantly
in the frequencies of mating (GLMM-g, X1
2001=
.,
P = 0.92,
Table1), in total number of males they copulated with (GLMM-p,
X1
2008=
.,
P = 0.78), or in durations of mating (GLMM-p,
X1
207=
.,
P = 0.4). Aggressive and nonaggressive males did
not dier significantly in the occurrences of mating (GLMM-b,
X1
204=
.,
P=0.54, Table1), in the durations of mating (GLMM,
X1
201<
.,
P = 0.82), in the frequencies of mating (GLMM-b,
X1
207=
.,
P=0.41), and in the number of females they copulate
with (GLMM-p, X1
205=
.,
P=0.48).
However, we detected a significant interaction between female
and male aggressiveness type on the probability of mating
(GLMM-b, X1
248=
.,
P = 0.03). Indeed, the mating probability
of aggressive females with aggressive males was more than 3 times
higher than the probability of mating with nonaggressive males
(Figure4A). Accordingly, nonaggressive females significantly more
likely mated (1.45 times) with nonaggressive males (GLMM-p,
X1
219 5=
.,
P < 0.0001, Figure4B). Mating durations were not sig-
nificantly aected by the aggressiveness of either males or females
(GLMM, X1
202=
.,
P = 0.67). Mating frequency decreased with
female age (GLMM-p, X1
219 1=
.,
P < 0.0001).
Reproductive success
Nonaggressive and aggressive females did not dier significantly in
the number of egg sacs (GEE-p, X1
205=
.,
P=0.47), average egg
sac mass (GEE, X1
211=
.,
P=0.29) and average number of hatch-
lings (GEE-p, X1
2014=
.,
P = 0.71, Table 1). Number of hatch-
lings decreased with female age (GEE-p, X1
259=
.,
P=0.02) and
increased with female size (GEE-p, X1
227 4=
.,
P<0.0001).
Paternity success
The proportion of eggs sired by the untreated male (hatched
eggs) was not related to female weight (GLM-b, F1,24 = 0.1,
P = 0.96), copulation duration (GLM-b, F1,24 = 2.7, P = 0.11),
and male weight (GLM-b, F1,24= 3.4, P= 0.08). Nevertheless, it
was positively related to body size of males (GLM-b, F1,24= 7.5,
P = 0.011). Aggressive males, however, sired significantly higher
proportions of eggs than nonaggressive males (GLM-b, F1,25=6.4,
P=0.02, Figure5).
DISCUSSION
Our study shows that mating between male and female bridge spi-
ders is not random in respect to their behavioral types: aggressive
males more likely mated with aggressive females, whereas non-
aggressive males more likely mated with nonaggressive females
producing a pattern of assortative mating. We propose that non-
random mating by behavioral type perhaps in combination with
negative frequency–dependent selection could maintain between-
individual variability in aggressiveness in L.sclopetarius.
This is the second study to investigate mating patterns by aggres-
siveness type in a spider. In the cooperative breeding spider A. stu-
diosus, docile females more often mate with aggressive males and
vice versa; hence, in contrast to the assortative pattern in the bridge
spiders, these spiders mate disassortatively (Pruitt and Riechert
Page 5 of 8
0
2
4
6
8
10
12
No.ofapproaches
Aggressive M
Aggressive
Nonaggressive M
Nonaggressive
A
0
1
2
3
4
5
6
7
No.oftouches
B
0
100
200
300
400
500
600
Female
Duration [s]
C
Aggressive M
Nonaggressive M
Aggressive M
Nonaggressive M
Aggressive Nonaggressive
Aggressive Nonaggressive
Figure2
Comparison of courtship behavior between aggressive and nonaggressive
males toward aggressive and nonaggressive females. Comparison of (A)
the number of approaches, (B) number of touches, and (C) duration of
courtship. Bars are means and whiskers are SEs.
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Behavioral Ecology
2009a, 2009b; Pruitt etal. 2011). There are several possible expla-
nations for the dierences. Anelosimus studiosus spiders exhibit high
sexual size dimorphism, where larger females readily consume a
male prior to copulation; hence males might be selected to avoid
mating with aggressive females. In this context, docile male spiders
perhaps have better dispositions to perform sneaking behavior or
may invest more in reducing female incentive to attack and are
more likely to succeed in mating with aggressive females. In our
study species, selection to avoid aggressive females may be relatively
weak, as female aggression hardly ever results in sexual cannibalism
and males are not much smaller than females and perfectly able
to resist female attacks. Apositive correlation between sexual size
dimorphism and sexual cannibalism is a general pattern among spi-
ders (Wilder and Rypstra 2008). Furthermore, A.studiosus are social
spiders and males in general prefer to mate with social females
(Pruitt and Riechert 2009a). However, in our study species, it is less
clear which female behavioral type (if only one) is preferred. We
speculated that males prefer aggressive females in the field because
these are likely more fecund (see below).
Although female aggressiveness types did not dier in their
reproductive success when mating with the male of the same
behavioral type, aggressive males sired more ospring than non-
aggressive ones under sperm competition when females were of
medium behavioral type. Because we used sons of parents with
known aggressiveness type, which is heritable (Kralj-Fišer and
Schneider 2012), aggressive males appear to father sons that are
better in sperm competition at least if mated to aggressive females.
If this is a heritable male property, all females should preferentially
copulate with aggressive males. However, nonaggressive females
mated more frequently with nonaggressive males. There are 2
nonmutually exclusive explanations. First, nonaggressive females
do not produce better sperm competitors with aggressive sires,
for example, because the combination of maternal and paternal
behavioral type is relevant for the inheritance of male aggressive-
ness type. In fact, L. sclopetarius and A. studiosus ospring exhibit
aggression levels similar to average parental aggression; hence
high aggressiveness of both parents is likely needed for inheritance
of high aggressiveness (Kralj-Fišer and Schneider 2012). Also, it
would be important to test if aggressive males also win in sperm
competition with nonaggressive males when mated with a docile
female.
Page 6 of 8
0
2
4
6
8
10
12
14
16
18
Female
Aggression score
Aggressive M
Nonaggressive M
Aggressive Nonaggressive
Figure3
Comparison of the aggressive scores of aggressive and nonaggressive
females toward aggressive and nonaggressive males. Bars are means and
whiskers are SEs.
Table1
Mean and SE of mating and reproductive parameters of
aggressive and nonaggressive females
Aggressive Nonaggressive
No. of matings 2.7 (0.9) 3.7 (1.1)
No. of males mated with 1.0 (0.3) 1.1 (0.2)
Total duration of copulation (s) 22.39 (8.8) 20.69 (4.9)
Egg sac mass 0.11 (0.03) 0.10 (0.03)
No. of hatchlings 46.7 (19.2) 19.7 (17.7)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Female
Frequencyofmatings
B
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Mating probability
A
Aggressive M
Nonaggressive M
Aggressive Nonaggressive
Aggressive M
Nonaggressive M
Aggressive Nonaggressive
Figure4
Comparison between aggressive and nonaggressive females and between
aggressive and nonaggressive males for (A) the mating probability and (B)
frequency of matings. Bars are means and whiskers are SEs.
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Kralj-Fišer etal. • Nonrandom mating by personality type
The second possible explanation is that aggressive males
actively avoid mating with nonaggressive females as long as there
are alternatives. The reasons would be the same as above namely
that assortative matings create phenotypes with the highest
quality so that they father better sons only in combination with
aggressive females. Male choosiness could be facilitated if they
were sperm limited and were unable to mate with many females.
We have not specifically studied potential mating frequencies
in bridge spiders but past mating trials provided no evidence
for such a limitation. The presence of male mating preferences
would also explain their increased investment in courtship and
their persistence with aggressive females. These options have to
be further explored.
Aggressive and nonaggressive females did not dier signifi-
cantly in their reproductive success in our study, although there is
a marked (but not significant) dierence in the number of hatch-
lings (Table 1). There was a large variation and small sample
size so that we cannot base any firm conclusion on these data.
Dierences in female reproductive success between aggressive-
ness types may have been masked by the rearing conditions in
the laboratory, where females were kept and fed individually and
with the same quantity and quality of prey. Aggressive females in
predatory species are often reported to be more vigorous hunt-
ers resulting in higher prey capture success, improved growth
rates, and larger body size at a better condition (Bristowe 1958;
Arnqvist and Henriksson 1997). All these parameters are directly
linked to increased fecundity in invertebrates, including spiders
and our study species (this study; Kleinteich 2009). It should be
further explored if female reproductive success between aggres-
sive types remains similar under more natural conditions or
whether aggressiveness is favored through increased hunting suc-
cess. Bridge spiders are nocturnal hunters of small prey insects
that emerge from the water. In urban habitats, they gather around
light sources and compete for the best web sites (Heiling 1999).
Being aggressive may on the one hand improve chances of gain-
ing a rich site but may also be disadvantageous if foraging is
compromised by frequent territorial encounters. Arecent experi-
ment with high densities of females and competition over prey
suggests that groups of only aggressive females have lower mor-
tality than mixed groups (Kralj-Fišer and Schneider 2012). Field
studies are currently under way to address the natural composi-
tion of behavioral types in high-density groups.
The maintenance of alternative phenotypes is often promoted
by frequency-dependent advantages or by trade-os such as high
costs of aggressiveness through injuries or cannibalism. In A. stu-
diosus, docile females have an advantage in high densities and meek
males profit when frequencies of aggressive males are high (Pruitt
and Riechert 2011). In L.sclopetarious, there is evidence that nonag-
gressive behavioral types have advantages if aggressive individuals
are too frequent at profitable web sites (Kralj-Fišer and Schneider
2012) but more studies are required to assess whether such fre-
quency-dependent mechanisms in combination with assortative
mating patterns are sucient to explain the maintenance of consis-
tent dierences in aggressivenesstypes.
In summary, L. sclopetarius spiders mated assortatively by their
aggressive types. Male aggressiveness level was positively related to
the number of sired ospring and likely signaled high quality in
L. sclopetarius males. Female reproductive success was not directly
related to aggressiveness type in this study, but positively correlated
to female size. Our studies suggest that assortative mating pattern
by aggressiveness type might be relevant for maintaining between-
individual dierences in aggressiveness at least in L.sclopetarius (this
study, Kralj-Fišer and Schneider 2012).
FUNDING
S.K.-F. was granted by Humboldt Postdoctoral Fellowship,
Humboldt Return Fellowship, and ARRS Postdoctoral Fellowship
Z1-4194.
We thank Tomma Dirks and Angelika Tabel-Hellwig for excellent spider
husbandry, and Wiebke Schuett for their comments on the manuscript. We
thank Cene Fišer for drawing Figure 1.
Handling editor: Ben Hatchwell
Page 7 of 8
6.06.5 7.0
0.0 0.2 0.4 0.6 0.8 1.0
Male size [mm]
Hatching success
Aggressive M
Non-aggressive M
Figure5
Relationship between the male size (patella + tibia length of the first leg)
and the hatching success (i.e., the proportion of eggs hatched) of 2 egg sacs
(pooled) for aggressive and nonaggressive males. Logit models are displayed.
Appendix 1
Males from the 15 mating trials
Group ID
Male ID
Male 1 Male 2 Male 3 Male 4
1 17 23 21 22
2 4 30 28 13
3 6 16 12 26
4 7 10 26 12
5 15 19 29 14
6 15 19 14 29
7 6 16 3 11
8 2 20 25 18
9 23 9 28 13
10 19 15 29 3
11 2 10 22 11
12 2 20 21 22
13 24 4 28 25
14 7 30 26 12
15 9 20 11 3
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Behavioral Ecology
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