A dual function of white coloration in a nocturnal spider Dolomedes raptor (Araneae: Pisauridae)

Abstract

Nocturnal animals frequently possess seemingly conspicuous colour patterns that can function in a variety of ways (e.g. prey attraction, camouflage, predator avoidance, etc.). The use of colour patterns in intraspecific signalling, especially reproductive activities, in nocturnal animals has received relatively little attention. This study tested for a dual function of colour in the nocturnal fishing spider, Dolomedes raptor (Araneae: Pisauridae), whose males develop dimorphic white stripes at sexual maturation. We tested for a role in foraging as well as mate assessment. First, quantifications of the natural variation of male stripes indicated a correlation between stripe area and male body size and weight. Subsequent diet experiments confirmed that the area of a male's white stripes are body size-dependent and thus could potentially convey information to choosey females about male quality. Field experiments used dummies resembling D. raptor in appearance to test a prey attraction function of the white stripes. We found that dummies with the white stripes present attracted significantly more prey than those without stripes. Finally, we used males with manipulated phenotypes in laboratory mating trials and found that males with intact white stripes were significantly more likely to be accepted by females than those with the white stripes eliminated. Together, our results document a nutrient-dependent trait that functions not only in strengthening foraging success, but also in a mating context, increasing male mating success. We suggest that the role of these male white stripes in reproduction has been facilitated by their function in foraging.

Full text

A dual function of white coloration in a nocturnal spider Dolomedes
raptor (Araneae: Pisauridae)
Tai-Shen Lin
a
, Shichang Zhang
a
, Chen-Pan Liao
a
, Eileen A. Hebets
b
, I-Min Tso
a
,
c
,
*
a
Department of Life Science, Tunghai University, Taichung, Taiwan
b
School of Biological Sciences, University of Nebraska, Lincoln, NE, U.S.A.
c
Center for Tropical Ecology &Biodiversity, Tunghai University, Taichung, Taiwan
article info
Article history:
Received 5 February 2015
Initial acceptance 5 March 2015
Final acceptance 8 June 2015
Published online
MS. number: 15-00096R
Keywords:
Dolomedes raptor
nocturnal animal
sexual selection
visual signal
wandering spider
Nocturnal animals frequently possess seemingly conspicuous colour patterns that can function in a va-
riety of ways (e.g. prey attraction, camouflage, predator avoidance, etc.). The use of colour patterns in
intraspecific signalling, especially reproductive activities, in nocturnal animals has received relatively
little attention. This study tested for a dual function of colour in the nocturnal fishing spider, Dolomedes
raptor (Araneae: Pisauridae), whose males develop dimorphic white stripes at sexual maturation. We
tested for a role in foraging as well as mate assessment. First, quantifications of the natural variation of
male stripes indicated a correlation between stripe area and male body size and weight. Subsequent diet
experiments confirmed that the area of a male's white stripes are body size-dependent and thus could
potentially convey information to choosey females about male quality. Field experiments used dummies
resembling D. raptor in appearance to test a prey attraction function of the white stripes. We found that
dummies with the white stripes present attracted significantly more prey than those without stripes.
Finally, we used males with manipulated phenotypes in laboratory mating trials and found that males
with intact white stripes were significantly more likely to be accepted by females than those with the
white stripes eliminated. Together, our results document a nutrient-dependent trait that functions not
only in strengthening foraging success, but also in a mating context, increasing male mating success. We
suggest that the role of these male white stripes in reproduction has been facilitated by their function in
foraging.
©2015 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
The elaborate coloration and intricate visual patterns found in
many animal groups have inspired artists and intrigued scientists
for centuries (Andersson, 1994). While aesthetically often utterly
captivating, these colours and patterns can have very distinct
functions. For example, aposematic coloration in some animals can
be used to warn off predators (Stevens &Ruxton, 2012). The ‘eye
spots’in the wings of some caterpillars can startle or intimidate
predators (Kodandaramaiah, 2011; Stevens, 2005)ordeflect attacks
by predators to less vulnerable body parts (Prudic, Stoehr, Wasik, &
Monteiro, 2015). Another example is aggressive mimicry, in which
the colour pattern closely mimics certain visual signals that can be
used by some animals to lure prey (O'Hanlon, Holwell, &
Herberstein, 2014; Pietsch &Grobecker, 1978). Alternatively, elab-
orate visual components can function in reproduction, such as for
attracting mates (G
omez et al., 2009; Lim, Land, &Li, 2007). The
visual signals used in mate attraction are often sexually dimorphic,
with the most striking of these secondary sexual traits typically
observed in diurnally active males (Maynard Smith &Harper,
2004).
While not as immediately conspicuous or obvious to us, due, in
part, to our diurnal nature, nocturnal animals also possess colora-
tion and/or other visually conspicuous traits (G
omez et al., 2009;
Penteriani, del Mar Delgado, Alonso-Alvarez, &Sergio, 2007). For
example, many nocturnal animals, or those living in consistently
dark environments, produce and emit bioluminescence (Broadley
&Stringer, 2001; Lloyd, 1971). Bioluminescence, a phenomenon
in which animals produce light by themselves rather than reflecting
light from external sources, is taxonomically widespread, as it is
seen in numerous marine vertebrates and invertebrates, fungi,
some bacteria and fireflies (Widder, 2010; Widder, Latz, Herring, &
Case, 1984). The many functions of bioluminescence include
mimicry, camouflage, distraction, warning and even attracting
mates (Haddock, Moline, &Case, 2010; Lloyd, 1971; Widder, 2010).
In addition to bioluminescence, recent studies have shown that
*Correspondence: I-Min Tso, Department of Life Science, Tunghai University,
Taichung 40704, Taichung, Taiwan.
E-mail address: spider@thu.edu.tw (I. -M. Tso).
Contents lists available at ScienceDirect
Animal Behaviour
journal homepage: www.elsevier.com/locate/anbehav
http://dx.doi.org/10.1016/j.anbehav.2015.07.001
0003-3472/©2015 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Animal Behaviour 108 (2015) 25e32

nocturnal animals may use colour signals in activities such as
claiming advantage in maleemale combat (e.g. eagle owl, Bubo
bubo,Penteriani et al., 2007; frog, Phyllomedusa boliviana,Jansen &
K€
ohler, 2008), defence (e.g. glow-worm, Lampyris noctiluca,De
Cock &Matthysen, 2003;Motyxia millipedes, Marek, Papaj,
Yeager, Molina, &Moore, 2011), prey attraction (e.g. glow-worm,
Arachnocampa luminosa,Broadley &Stringer, 2001) and predator
avoidance (e.g. leiognathid fish, McFall-Ngai &Morin, 1991).
However, colour signals used in the context of reproduction of
nocturnal organisms have not been widely reported (G
omez et al.,
2009; Lewis &Cratsley, 2008). Recently, it was reported that in the
Neotropical spider Paratrechalea ornata (Trechaleidae) males were
more attractive to females when the chelicerae of the males were
painted white, implying that conspicuous visual signals can be
detected and may also play roles in the mating of nocturnal ar-
thropods (Trillo, Melo-Gonz
alez, &Albo, 2014).
Perhaps surprising to some, spiders are among the many taxo-
nomic groups that possess extraordinary sexual dimorphism, with
males of some species exhibiting colourful bodies and leg orna-
ments in addition to elaborate dynamic motion displays (Framenau
&Hebets, 2007; Hebets &Uetz, 2000; Uhl &Elias, 2011). Salticids
(jumping spiders), for example, can use sex-specific visually
mediated displays to court females (Li et al., 2008; Lim et al., 2007;
Lim, Li, &Li, 2008). Members of the diurnally active and diverse
genus Habronattus (ca. 100 species) are well known for their elab-
orate male secondary sexual traits which include spectacular colour
patterns, morphological ‘ornaments’and complex courtship dis-
plays (Elias, Maddison, Peckmezian, Girard, &Mason, 2012; Girard,
Kasumovic, &Elias, 2011). It has been known in sexually dimorphic
animals that diet is closely related to the attractiveness of the
sexual traits that are preferred by females when choosing a mate
(Ferkin, Sorokin, Johnston, &Lee, 1997; Hill, 1992; McGlothlin,
Duffy, Henry-Freeman, &Ketterson, 2007). Related studies in spi-
ders, however, have rarely been reported. Hebets, Wesson, and
Shamble (2008) found in Schizocosa wolf spiders that males
receiving a high-quality diet matured more quickly and were
significantly larger as adults than those receiving a low-quality diet,
and high-quality diet males had larger sexual traits and were more
successful in courting females than low-quality diet males. Yet, it is
unknown whether, in nocturnal organisms, diet would influence
physical condition and potential sexual traits.
The conspicuous body coloration of spiders not only plays roles
in sexual selection, but also functions in foraging, for instance as a
prey lure. For example, the brightly coloured body parts of the giant
wood spider, Nephila pilipes, have been shown to function as a vi-
sual lure for attracting both diurnal and nocturnal prey (Chuang,
Yang, &Tso, 2007). The conspicuous yellow ventral spots in the
garden spider, Neoscona punctigera, can also lure prey at night
(Blamires et al., 2012). Aside from these examples, the role of
conspicuous coloration in luring prey at night has not been well
studied or known in nocturnal cursorial arthropods.
The fishing spider, Dolomedes raptor (Araneae: Pisauridae), like
most members of its genus, is a nocturnal wandering species
inhabiting low-altitude streams. These nocturnal predators prey on
aquatic and semiaquatic invertebrates and vertebrates (Bleckmann
&Lotz, 1987; Zimmermann &Spence, 1989). The species exhibits
dramatic sexual colour dimorphism, with mature males displaying
two white stripes at the two margins of the cephalothorax (Fig. 1a).
Females lack these white stripes, but they do possess white spots at
the tips of their legs, especially the anterior two pairs (Fig. 1b). The
presence of white coloration in both sexes suggests a potential
function in prey attraction, while the distinct placement of color-
ation in males in combination with their courtship advances hints
towards a role in courtship communication. Here, we hypothesized
that the bright white patch on the body of male D. raptor plays
important roles in both foraging (i.e. prey attraction) and sexual
selection (i.e. a sexual character in female mate assessment). To test
these two hypotheses, we first quantified the natural variation in
colour patterns of males that were collected from the field. We next
conducted diet experiments in the laboratory to test whether the
colour pattern was nutrition dependent and thus potentially
capable of conveying information about a male's past foraging
history. Then, we conducted field observations using different
dummies that resembled male D. raptor in size and colour. Finally,
we conducted mating trials using manipulated males. We predicted
that dummies with the colour pattern would attract more prey and
males with the colour pattern would be more successful in mating.
METHODS
Natural Variation in Male Coloration
The white stripes on the males are composed of tiny white hairs.
To determine the natural variation in the area of these white
stripes, we collected males (N¼32) at low-altitude streams near
Dongshi Forest Recreation Area in Dongshi, Taichung city, Taiwan
(120

52
0
03.96
0
E, 24

17
0
06.78
0
N) and brought them back to the
laboratory in Tunghai University. To measure the area of white
stripes on the carapace, we first anaesthetized the spiders with
carbon dioxide and then used a digital camera (Olympus
m
1030
SW) to take images of the spiders. We used GIMP2 software (www.
gimp.org) to measure the foreleg length, carapace length, carapace
width and the area of the white stripe on the cephalothorax. The
body weights were also measured. We then calculated the standard
scores (i.e. [variable emean variable]/standard deviation of vari-
able) for these variables. To check the relationship between white
stripe area and body condition (i.e. body size and weight), we first
applied a principal component analysis (PCA) based on the stan-
dard scores of four body characteristics (carapace length, carapace
Figure 1. (a) Male and (b) female Dolomedes raptor showing their dimorphic body
coloration patterns.
T.-S. Lin et al. / Animal Behaviour 108 (2015) 25e3226

width, foreleg length and spider weight). Then, we calculated the
score of ‘body size and weight’by extracting the first principal
components of the PCA analysis. Finally, we fitted the white stripe
area with the ‘body size and weight score’using a simple linear
regression. Using the same method, we also calculated the score of
‘body size’using variables such as carapace length, carapace width
and foreleg length. We then fitted the white stripe area with ‘body
size score’using a quadratic regression to calculate the residuals.
Finally, we fitted the white stripe area with these residuals by using
a simple linear regression.
To investigate whether the brightness of the white stripe
correlated with spider body size and weight, we first measured the
reflectance spectra of 32 mature males using the following
equation:
white stripe brightness ¼Z
700 nm
300 nm
Sdl;
where Sis the reflectance spectra (%) of the white stripe and
l
is
wavelength (nm). We fitted the white stripe brightness with ‘body
size and weight score’as well as ‘residual of spider weight against
body size score’by using simple linear regressions. The normality of
the residuals of the linear regression was checked by the Shapir-
oeWilk normality test, while the homoscedasticity of the variance
of the residuals was checked by the White's test.
Nutrition Dependence of Male Coloration
Sexual traits are often closely associated with body condition of
mate-searching individuals (Andersson,1994) and our aim here was
to determine whether the white stripes on male D. raptor were
nutrition dependent. To achieve this, subadult males weresubjected
to different levels of nutrient intake to see whether such treatments
would affect the area of white stripes when the spiders matured. We
collected juvenile males from the study site in Dongshi, Taichung
city, Taiwan and raised them individually to the subadult stage by
giving them one cricket (Acheta bimaculata; body length ca. 10 mm)
once a week. Before the nutrition manipulation, we measured the
body weight and white stripe area of subadults and randomly
assigned them to high-nutrition (fed one cricket every 2 days;
N¼19) and low-nutrition (fed one cricket every 7 days; N¼12)
treatments. When the spiders matured, we measured the body
weight, white stripe area and number of days taken to reach
maturity. The white stripe area of males in the two treatment groups
was compared by a general linear model in which the nutrition
treatment, spiders' pretreatment white stripe area and the interac-
tion term were considered. The homogeneity assumption of the
model was checked by a Bartlett's test, and we found that the vari-
ances among the four treatments were not (but very close to)
significantly different. ShapiroeWilk normality tests showed that
the variables in the two groups were normally distributed, so we
used a Welch two-sample ttest to compare the number of days taken
to reach maturity of males in the two treatment groups.
Prey Attraction Function of Male Coloration
To investigate the function of the white stripes on males' bodies,
we first tested the hypothesis that white stripes may function inprey
attraction. We conducted field observations using dummies that
resemble male D. raptor in size and colour. We measured the chro-
matic properties of selected brown and white dummy construction
paper across a 300e700 nm spectrum using a USB4000 spectro-
photometer (Ocean Optics, Dunedin, FL, U.S.A.; Chuang, Yang, &Tso,
2008). The spectral properties across a 300e70 0 nm spectrum were
also measured for the body (legs, palps, carapace and abdomen) and
the white stripe of live adult male D. raptor collected from Dongshi
(N¼5) so as to compare spectral properties between corresponding
dummy and live spider body parts.
The dummies were divided into two groups: control group (with
white stripes, N¼35) and experimental group (without the white
stripes, N¼46). In the control group, stripes made fromwhite paper
were pasted onto the cephalothorax region, while in the experi-
mental group, only the glue that was used to paste thewhite stripes
was applied on that region (Appendix Fig. A1). Field experiments
were conducted between 28 August and 1 September in 2009 and
between 17 and 24 July in 2010 at the study site where we collected
D. raptor. The habitat is characterized by the presence of many rocks
(20e40 cm in diameter) along the banks of calm streams with slow
water flow and relatively closed canopy. In the field, male D. raptor
were frequently observed perching on the rocks. Therefore, we
randomly chose rocks of similar size and placed two types of
dummies on them alternately. Video cameras with infrared night
view scopes (Sony SR-100 and SR-62, Tokyo, Japan) were placed
approximately 1 m from each dummy to monitor them from 2000 to
0400 hours each night. The video footages were viewed in the lab-
oratory in Tunghai University to check for events such as prey
attraction and predator attraction. An ‘attraction’event was defined
as an insect approaching directly within 5 cm of a dummy. We
defined prey attraction rate as the number of prey approaching
events per hour of footage. A negative binomial regressionwas used
to compare the prey attraction rates of two types of dummies.
Reproductive Function of Male Coloration
Finally, we tested the hypothesis that the white stripes of males
were involved in reproductive behaviour and thus would affect a
male's courting success. We collected subadult males and females
from the study site in Dongshi, Taichung city, Taiwanand each spider
was individually raised in a Plexiglas container (30 15 cm and
18 cm high). The bottom of the container was submerged in 2 cm of
water (water changed once a week) and a sponge was placed for the
spider to perch on. Opaque plastic sheets were placed between
containers to prevent visual contact between individual spiders. We
fed spiders one cricket (A. bimaculata, body length ca. 10 mm) once a
week until they reached maturity. The age of spiders used in the
experiment was no more than 1 month after sexual maturity.
We randomly assigned mature males into two groups. Males in
the experimental group were manipulated by covering the white
stripes with brown paint with a reflectance spectrum similar to that
of the spiders' brown body colour (Appendix Fig. A2). For males in
the control group, we applied the same amount of brown paint on
the brown part of the cephalothorax. All the spiders were used only
once to exclude the potential influence of prior experience. A glass
terrarium (30 30 cm and 15 cm high) with a cylindrical stage and
plastic board at the bottom was used as the arena for mate choice
trials. The arena was covered by a piece of glass to prevent the
spiders from escaping during the experiments. In each trial, a fe-
male was first introduced to the arena for 1 h to let it adapt to the
environment and produce dragline silks, which may contain sex
pheromones, and then a male was introduced to the arena. The
behaviours and interactions of the spiders were recorded by
infrared video cameras. Preliminary experiments showed that the
courtship of a male D. raptor includes the following four steps: (1)
following female's dragline; (2) waving forelegs; (3) tapping fe-
male's legs; and (4) climbing onto female's back. It has been re-
ported that vibratory signals (transmitted by water surface wave)
play roles in the courtship of Dolomedes spiders (Arnqvist, 1992;
Bleckmann &Bender, 1987; Roland &Rovner, 1983). Thus, to
T.-S. Lin et al. / Animal Behaviour 108 (2015) 25e32 27

differentiate the role of the visual signal, we did not provide water
in the arena for the spider to generate vibratory signals. We
considered courtship successful when a male was able to climb
onto a female's dorsum without being kicked off or cannibalized. A
permutation Pearson chi-square test of homogeneity was used to
compare the courtship success of male D. raptor receiving different
treatments. All statistical tests were carried out in the R program-
ming environment version 3.1.2 (R Core Team, 2014). A general
linear mixed model was fitted by using an R package lmerTest
version 2.0-20 (Kuznetsova, Brockhoff, &Christensen, 2014).
Ethical Note
To minimize adverse impacts on their welfare, we treated the
spiders gently during the experiment and released them afterwards
to their original habitat. The experimental procedures strictly
adhered to the ‘research ethics and animal treatment’legal re-
quirements of Tunghai University.
RESULTS
Natural Variation in Male Coloration
In the field, body size, body weight and white stripe area of
mature males varied considerably. The white stripe area was
significantly and positively correlated with body size and weight
score (Fig. 2a, Appendix Table A1). This result indicated that the
white stripe area can be used as an indicator of spider body size and
weight. Spider weight was highly correlated with spider size
(R
2
¼0.954, P<0.0001; Fig. 2b, Appendix Table A1). However, the
white stripe area was not significantly correlated with the residual
of spider weight against body size score (Fig. 2c, Appendix
Table A1). This result indicated that, after we controlled for the
spider body size, the spider body weight cannot predict the white
stripe area. The residuals of the simple linear regression were all
normally distributed, and the variances of these residuals were all
homogeneous.
Neither the ‘body size and weight score’nor the ‘residual of
spider weight against body size score’was significantly correlated
with white stripe brightness (Appendix Fig. A3,Table A2). These
results showed that the white stripe brightness cannot be predicted
by raw/standardized spider size.
Nutrition Dependence of Male Coloration
We found no significant difference in the stripe area of the two
groups of spiders before the nutrition manipulation (Fig. 3,
Appendix Table A3). However, the stripe area of spiders in the two
groups varied greatly after the treatments. Males receiving the
high-nutrition treatment (N¼19) moulted faster (t
14
¼6.36,
P<0.001; Appendix Fig. A4) and had larger white stripe areas than
those receiving the low-nutrition treatment (N¼12; Fig. 3,
Appendix Table A3). These results indicate that the white stripe
area can reflect the feeding history of adult males.
Prey Attraction Function of Male Coloration
From field experiments conducted in 2009 and 2010, more than
600 h of video footage were obtained. A preliminary analysis
showed no significant difference between results obtained from the
2 years, so we pooled the 2009 and 2010 data. A Pearson goodness-
of-fit chi-square test showed that the prey attraction data fitted
well with a negative binomial model (c
2
79
¼86:15, P¼0.273). Re-
sults of a negative binomial regression showed that dummies with
white stripes attracted significantly more prey than those without
(Fig. 4a, Appendix Table A4).
Reproductive Function of Male Coloration
In the laboratory, 29 mating trials were performed (N¼14 fo r
males without white stripes and N¼15 for males with white stripes).
Males in the two groups did not differ significantly in their body
–4 –2 0 2 4
0
0.05
0.1
0.15
Body size and weight score
White stripe area (cm²)
(a)
y = 0.0778 + 0.0120x, R² = 0.9285
–4 –2 0 2 4
0
0.2
0.4
Body size score
Spider weight (g)
(b)
y = 0.2230 + 0.0599x + 0.004x²
R² = 0.954
0
0.05
0.1
0.15
Residual of spider weight
a
g
ainst bod
y
size score
White stripe area (cm²)
(c)
y = 0.0778 + 0.0609x, R² = 0.0033
0.1
0.3
0.5
–0.06 –0.02 0.02 0.06
0 0.04–0.04
Figure 2. Relationships between (a) white stripe area and body size and weight score,
(b) spider weight and spider body size score, and (c) white stripe area and residual of
spider weight against body size score of adult males collected from the field.
NS
**
High nutrition
Low nutrition
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0Pretest Post-test
Area of white stripe (cm2)
Figure 3. White stripe area of male D. raptor before (pretest) and after (post-test)
receiving low-nutrition (N¼12) and high-nutrition (N¼19) treatments. **P<0.01.
T.-S. Lin et al. / Animal Behaviour 108 (2015) 25e3228

weight (ttest: t
27
¼0.366, P¼0.718). Males with white stripes were
more likely to be accepted by females (control group: 13 of 15) than
those in which the white stripes were covered up (experimental
group: 6 of 14; c
2
1
¼6:152, simulated P¼0.021, based on 5 0 00 000
replications; Fig. 4b). Males without white stripes had a high proba-
bility of being rejected or attacked by females (8 of 14, 57%).
DISCUSSION
This study empirically shows that colour signals in the nocturnal
predatory spider D. raptor have a dual function, foraging and
reproduction. The nutrition-dependent white stripes on male
D. raptor appear to function in both courtship success and prey
attraction. The increased prey attraction of spider dummies with
versus without white stripes suggests their role in prey attraction
while the finding that males with white stripes were more likely to
be accepted by females indicates that this conspicuous body colour
probably serves as a visual-based sexual signal, a rarely docu-
mented phenomenon in animals active at night. In this paper, we
used the term ‘conspicuous’to describe the colour pattern of the
body of the spiders, because the reflectance of the white part
(above 40%) is much higher than that of other parts of the body
(around 5%; Appendix Fig. A2). However, the visual systems of
humans and arthropods are very different (Endler, 1990), so the
conspicuousness of colour patterns under the two systems may be
different as well, especially in dim light conditions. However, the
results of our dummy experiments showed that the sharp contrast
between the white and brown body parts did attract the attention
of prey insects. Therefore, we may safely state in this case that the
colour pattern is highly visible to these organisms.
In many nocturnal animals, intraspecific communication under
dim light conditions relies on pheromones (e.g. moths and rodents,
Wyatt, 2014), acoustic signals (e.g. crickets and frogs, Searcy &
Andersson, 1986), ultrasound (e.g. bats, Pfalzer &Kusch, 2003)or
bioluminescence (e.g. fireflies, Lewis &Cratsley, 2008). Nocturnal
spiders have been previously reported to employ mainly acoustic
signals (e.g. percussion and stridulation in wolf spiders; Hebets &
Uetz, 2000) and pheromones (Gaskett, 2007) in intraspecific
communication. The documented dual function of male white
stripes in D. raptor suggests that this prey attraction signal might
have been co-opted for a communicative function in reproductive
behaviour.
The potential role of visual signals in the courtship of nocturnal
animals has not been extensively explored, except in fireflies (Lewis
&Cratsley, 2008). In addition to a prior study on a tree frog (G
omez
et al., 2009), our present study on the fishing spider D. raptor
suggests that colour signals may be more broadly employed in the
courtship of nocturnal animals than previously appreciated.
Numerous nocturnal animals have good colour vision and colour
cues have been demonstrated to be important in nonreproductive
aspects of their lives, such as foraging (Kelber, Balkenius, &
Warrant, 2002), navigation (Warrant &Dacke, 2011) and mutual-
istic interaction (Peng, Blamires, Agnarsson, Lin, &Tso, 2013).
However, so far it is unknown whether in nocturnal animals a
conspicuous body colour can serve as disruptive coloration under
dim night conditions. In theory, the contrasting colours can
enhance the disruptive effect (Cuthill et al., 2005), which may
enable these nocturnal species to avoid attack from their predators.
Therefore, we expect that, as more effort is invested in studying the
roles of body colour of nocturnal animals, the findings may
dramatically change our perspective regarding the nature of
interspecific and intraspecific interactions at night.
The results of our study suggest that the conspicuous white
stripes in male D. raptor may represent a reliable visual indicator of
feeding history and body size/weight, and may be a visual signal
used by females during mate assessment. Exaggerated sexual traits
of males may be favoured by females because they may reflect the
survival or competitive capacities of the owner (Emlen, 2008;
Emlen, Warren, Johns, Dworkin, &Lavine, 2012). In diurnal ani-
mals, the physical form of traits that may convey increased survival
or competitive capacities are very diverse, with visual signals
employed in many organisms (Andersson, 1994). In contrast to
diurnal animals, previously documented sexually selected traits
associated with male quality in nocturnal animals have typically
been acoustic or chemical in nature (Fisher, Swaisgood, &Fitch-
Snyder, 2003; M
arquez, Bosch, &Eekhout, 2008). Our study,
however, documented a clear example in which the white stripe
area of male D. raptor in the field varied considerably, yet was
closely associated with body size/weight. Indeed, our manipula-
tions showed that nutrition could influence a male's time to
maturation as well as the area of his white stripes, with high-
nutrition males displaying larger white stripe areas than low-
nutrition males. It is currently unknown whether females are in-
clined to choose males with larger white stripes when they are
given multiple options and research examining the preference of
females for males with different sizes of white stripe is needed.
Our field experiment using dummies showed that the white
stripes on male D. raptor can attract prey, implying that this con-
spicuous colour signal has a foraging function. Conspicuous body
coloration has recently been reported to function as a prey lure in
nocturnal web-building spiders, such as N. pilipes (Chuang et al.,
2007) and N. punctigera (Blamires et al., 2012). In these spiders,
coloration can apparently increase foraging success (i.e. increase
the interception rate of the web) by attracting prey such as moths
(Blamires et al., 2012; Chuang et al., 2008). The conspicuous body
0.5
0.4
0.3
0.2
0.1
0
Experimental
group
Control
group
Experimental
g
rou
p
Control
g
rou
p
Rejected by female
Accepted by female
100
80
60
40
20
0
Percentage
Prey attraction rate
(number / h)
(a)
(b)
***
*
Figure 4. (a) Mean þSE prey attraction rates (number of prey attracted per hour of
monitoring) of dummies in experimental (white stripe removed, N¼46) and control
(white stripe present, N¼35) groups. ***P<0.001. (b) Courting success rates of male
D. raptor in the experimental (white stripes covered by paint, N¼14) and control
(paint on brown part, N¼15) groups. *P<0.05.
T.-S. Lin et al. / Animal Behaviour 108 (2015) 25e32 29

colour of these nocturnal web-building spiders, however, is un-
likely to also have a sexual function because males of these species
are very myopic and dull in colour (Land, 1985). Therefore, the role
of conspicuous body coloration in orb-web spiders seems restricted
to prey attraction.
In Araneae, the true spiders, species using conspicuous body
colour in sexual activities have been mostly reported in the RTA
(retrolateral tibial apophysis) clade, such as spiders in the family
Lycosidae (Hebets &Uetz, 1999) and Salticidae (Lim et al., 2007),
taxonomic groups that also have good vision. We suspect that in
Pisauridae fishing spiders (also an RTA taxon), the prey-attracting
conspicuous body colour may be endowed with a sexual function
in accordance with the evolution of good nocturnal visual ability in
these spiders, a hypothesis that requires additional testing.
Regardless, the sexually dimorphic white stripes on mature males
appear to function in mate attraction, suggesting that females at
least have the perceptual capacity to assess this trait. While
comparative work would be necessary to test this, we propose that
the white coloration on female and male D. raptor evolved initially
in a prey attraction context and then this coloration was co-opted
for a role in male courtship and female assessment. Examples
such as this of dual-function signals can help us understand the
evolutionary history and current functions of animal displays.
Although the conspicuous visual signal of white stripes appears
important for courting males, nothing is yet known of any costs they
might incur. Bright signals, even in nocturnal animals, can be
exploited by potential predatorsand parasitoids (Zuk &Kolluru,1998).
For example, the aquatic wasp, Anoplius depressipes,canactively
search for Dolomedes spiders that hide inside the curled-up leaves of
water lilies (Roble, 1985) and vertebrate predators with relatively
good low-light vision such as fishes and frogs also pose a large threat
to fishing spiders (Suter, 2003). In this particular system, these trade-
offs would be especially complex as the presence/absence of white
stripesinfluences foraging success,which in turn influencesthe size of
the white stripes, which then influences a male's mating success.
Studies exploring the evolutionary and functional interplay between
all of these interactions would be illuminating.
Acknowledgments
The study was funded by the National Science Council, Taiwan
(NSC-99-2632-B-029-001-MY3, NSC-102-2311-B-029-001-MY3)
grants to I.M.T and the Ministry of Science and Technology, Taiwan
postdoctoral grant (MOST 103-2811-B029-001) to S.Z. We are
grateful to the two anonymous referees for their constructive
comments.
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Appendix
Table A1
Results of simple linear regression (SLR) between different spider body parameters
SLR analysis Coefficient Estimate SE tdfP
White stripe area against body size and weight score DV: white stripe area (cm
2
)
Intercept 0.0778 0.0012 65.04 30 <0.0001
Body size and weight score 0.0120 0.0006 19.74 30 <0.0001
Spider weight against body size score DV: spider weight (g)
Intercept 0.2227 0.0063 35.276 29 <0.0001
Body size score 0.0599 0.0025 23.930 29 <0.0001
(Body size score)
2
0.0041 0.0016 2.592 29 0.015
White stripe area against residual of spider weight against body size score DV: white stripe area (cm
2
)
Intercept 0.0778 0.0045 17.420 30 <0.0001
Residual of spider weight against body size score 0.0609 0.1945 0.313 30 0.756
Table A2
Results of simple linear regression (SLR) between the brightness of the white stripe area and the body size or the residual of spider weight against body size score
SLR analysis Coefficient Estimate SE tdfP
White stripe brightness against body size and weight score DV: white stripe brightness
Intercept 24 949.4 552.3 45.175 30 <0.0001
Body size and weight score 236.8 279.9 0.846 30 0.404
White stripe brightness against residual of spider weight against body size
score
DV: white stripe brightness
Intercept 24 949.4 540.6 46.150 30 <0.0001
Residual of spider weight against body size
score
33 757.9 23 540.9 1.434 30 0.162
Table A4
Results of negative binomial regression comparing the prey attraction rates of dummies of the experimental (white stripes removed) and control group (white stripes intact)
Coefficient Estimate of
b
SE ZP
Intercept 0.9913 0.1795 5.522 <0.0001
Treatment*(Experiment vs control) 1.0592 0.2691 3.9368 <0.0001
*
The ratio between probabilities of two certain events was e
b
.
Table A3
Results of a general linear mixed model comparing the white stripe area of males receiving different nutrition treatments
Coefficient Estimate SE df t P
Intercept 0.06680 0.004018 16.627 <0.0001 <0.0001
b
1
[posthigh vs postlow] 0.02331 0.008377 33.96 2.783 0.009
b
2
[prehigh vs prelow] 0.001273 0.008377 33.96 0.152 0.880
b
3
[(prehigh þprelow)/2 vs (posthigh þpostlow)/2] 0.04477 0.002367 28.63 18.91 <0.0001
The nutrition treatment, spiders' pretreatment white stripe area and the interaction term were considered in this model.
T.-S. Lin et al. / Animal Behaviour 108 (2015) 25e32 31

Figure A1. A schematic drawing of dummies used in (a) control (white stripes present)
and (b) experimental (white stripes absent) groups.
Brown colour paint
Brown colour paper
Brown region of spider
White colour paper
White region of spider
20
15
10
5
300 400 500 600 700
Wavelength (nm)
300 400 500 600 700
Wavelen
g
th (nm)
100
80
60
40
20
Reflectance (%)
(a)
(b)
Figure A2. (a) The reflectance spectra of the brown region of male D. raptor, the brown
paper used to construct dummies and the brown paint used to alter the colour signal of
white stripes. (b) The reflectance spectra of the white stripes of male D. raptor and the
white paper used to construct white stripes on dummies.
–4 –2 0 2 4
10 000
25 000
40 000
Body size and weight score
White stripe brightness (%-nm)
y = 24949 + 236x, R² = 0.0233
(a)
–0.06 –0.02 0.02 0.06
Residual of spider weight
a
g
ainst bod
y
size score
y = 24949 + 33758x ,R² = 0.0642
(b)
20 000
15 000
30 000
35 000
10 000
25 000
40 000
20 000
15 000
30 000
35 000
Figure A3. Relationships between the white stripe brightness and (a) ‘body size and
weight score’and (b) ‘residual of spider weight against body size score’.
***
80
60
40
20
0High Low
Nutrition treatment
Moulting time (day)
Figure A4. Mean þSE number of days taken to reach maturity of subadult males
receiving high-nutrition (N¼19) and low-nutrition (N¼14) treatments. ***P<0.001.
T.-S. Lin et al. / Animal Behaviour 108 (2015) 25e3232