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Abstract

Although predation is commonly thought to exert the strongest selective pressure on colouration in aposematic species, sexual selection may also influence colouration. Specifically, polymorphism in aposematic species cannot be explained by natural selection alone. 2.Males of the aposematic wood tiger moth (Arctia plantaginis) are polymorphic for hindwing colouration throughout most of their range. In Scandinavia, they display either white or yellow hindwings. Female hindwing colouration varies continuously from bright orange to red. Redder females and yellow males suffer least from bird predation. 3.White males often have higher mating success than yellow males. Therefore, we ask whether females can discriminate the two male morphs by colour. Males approach females by following pheromone plumes from a distance, but search visually at short range. This raises the questions whether males discriminate female colouration and, in turn, whether female colouration is also sexually selected. 4.Using electroretinograms, we found significantly larger retinal responses in male than female A. plantaginis, but similar spectral sensitivities in both sexes, with peaks in the UV (349 nm), blue (457 nm), and green (521 nm) wavelength range. 5.According to colour vision models, conspecifics can discriminate white and yellow males as separate morphs, but not orange and red females. For moths and birds (Cyanistes caeruleus), white males are more conspicuous against green and brown backgrounds, mostly due to UV reflectivity, and red females are slightly more conspicuous than orange females. 6.The costly red colouration among females is likely selected by predator pressure, not by conspecifics, whereas male colour polymorphism is probably maintained, at least partly, by a the opposing forces of predation pressure favouring yellow males, and female preference for white males. Whether or not the preference for white males is based on visual cues requires further testing. 7.The evolution of polymorphic aposematic animals can be better understood when the visual system of the species and their predators is taken into consideration. This article is protected by copyright. All rights reserved.
Function al Ecology. 20 18; 1–13.  wileyonlinelibrar y.com/journal/fec  
|
 1
© 2018 The Authors. Functional Ecology
© 2018 British Ecological Society
Received:11September2017 
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  Accepted:20Februa ry2018
DOI : 10.1111/1365 -2435.13100
RESEARCH ARTICLE
An aposematic colour- polymorphic moth seen through the
eyes of conspecifics and predators – Sensitivity and colour
discrimination in a tiger moth
Miriam J. Henze1| Olle Lind2| Johanna Mappes3| Bibiana Rojas3| Almut Kelber1
1Lund Vision Group, Depar tment of
Biology, Lund University, Lund, Sweden
2Department of Philosophy, Cognitive
Science, Lund University, Lund, Sweden
3Centre of E xcellence in Biological
Interactions, University of Jy väskylä,
Jyväskylä,Finland
Correspondence
AlmutKelber
Email: almut.kelber@biol.lu.se
Present address
Miriam J. Henze, Queensland Brain
Institute, University of Queensland, St Lucia,
Qld,Australia
Funding information
FinnishCentreofExcellenceinBiological
Interactions,Grant/AwardNumber:
2100000256; The Swedish Research
Council,Grant/AwardNumber:2012-02212
and 637-2013-388
Handling Editor: Christine Miller
Abstract
1. Althoughpredationiscommonlythoughttoexertthestrongestselectivepressure
on coloration in aposematic species, sexual selection may also influence colora-
tion. Specifically, polymorphism in aposematic species cannot be explained by
natural selection alone.
2. Males of the aposematic wood tiger moth (Arctia plantaginis) are polymorphic for
hindwing coloration throughout most of their range. In Scandinavia, they display
eitherwhiteoryellowhindwings.Femalehindwingcolorationvariescontinuously
from bright orange to red. Redder females and yellow males suffer least from bird
predation.
3. White males often have higher mating success than yellow males. Therefore, we
ask whether females can discriminate the two male morphs by colour. Males ap-
proach females by following pheromone plumes from a distance, but search visu-
ally at short range. This raises the questions whether males discriminate female
coloration and, in turn, whether female coloration is also sexually selected.
4. Using electroretinograms, we found significantly larger retinal responses in male
than female A. plantaginis, but similar spectral sensitivities in both sexes, with
peaks in the UV (349 nm), blue (457 nm) and green (521 nm) wavelength range.
5. Accordingtocolourvisionmodels,conspecificscandiscriminatewhiteandyellow
malesasseparatemorphs, butnotorangeandredfemales.Formothsandbirds
(Cyanistes caeruleus), white males are more conspicuous against green and brown
backgrounds, mostly due to UV reflectivity, and red females are slightly more con-
spicuous than orange females.
6. The costly red coloration among females is likely selected by predator pressure,
not by conspecifics, whereas male colour polymorphism is probably maintained, at
least partly, by the opposing forces of predation pressure favouring yellow males,
and female preference for white males. Whether or not the preference for white
males is based on visual cues requires further testing.
7. The evolution of polymorphic aposematic animals can be better understood when
the visual system of the species and their predators is taken into consideration.
KEYWORDS
Arctiidmoths,colourpolymorphism,colourvision,naturalselection,predatorpressure,
sexual selection, spectral sensitivity
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1 | INTRODUCTION
Predation is commonly advocated as the strongest selective pres-
sure on the evolution of conspicuous coloration in aposematic or-
ganisms (Poulton, 1887; Ruxton, Sherratt, & Speed, 20 04). However,
in aposematic species with variable coloration within a given popu-
lation (colour polymorphism), this principle is challenged by the diffi-
culty for predators to learn several colour morphs (Endler & Mappes,
2004; Lindström, Alatalo, Lyytinen, & Mappes, 2001; Mallet &
Joron, 1999). Previous studies have pointed at sexual selection as
an alternative or additional selective pressure influencing the main-
tenance of colour variation (Crothers & Cummings, 2013; Jiggins,
Naisbit, Coe, & Mallet, 2001; O’Donald & Majerus, 1984). This
could be particularly relevant for species displaying sexual dichro-
matism, colour polymorphism in one of the sexes or both (Maan &
Cummings,2009;Nokelainen,Hegna,Reudler,Lindstedt,&Mappes,
2012; Rojas & Endler, 2013), as natural selection may favour one
colour morph while sexual selection favours another (Crothers &
Cumming s, 2013; Nokelainen eta l., 2012). Moreover, each of t he
different morphs might exploit receiver biases or limitations in the
receivers’perceptualsystems, especially whilesearching for mates
(Limeri & Morehouse, 2014).
The wood tiger moth Arctia plantaginis (formerly Parasemia plan-
taginis; Rönkä,Mappes, Kaila,& Wahlberg,2016)is anaposematic,
colour- polymorphic and sexually dimorphic arctiid moth with a wide
distributionacrosstheHolarctic. AdultsofA. plantaginis are active
and mate from mid- June until the end of July in most of their range.
They are diurnal- crepuscular, meaning that females release pher-
omones towards the evening hours, whereas males already start
flying during daytime (Conner, 2009). Male flight activity is highest
between 18 and 22 hr, and female calling and mating activity peak
at 20hr (own obse rvations from t he field, Gordo n, Kokko, Rojas,
Nokelainen,&Mappes,2015;Rojas,Gordon,& Mappes, 2015). At
this time of the year, nights in central and northern Europe are short;
at the latitude of Helsinki (60°), the sun does not set before 22 hr,
and nocturnal light levels are never reached.
Adultwoodtigermothshaveaconspicuousblack-and-whitepattern
on the forewings and red, orange, yellow, white or black hindwings. In
some populations, males are monomorphic, but in large parts of Europe,
two male colour morphs with either white or yellow hindwings co- occur
(Hegna,Galarza,&Mappes,2015).InFinnishpopulations, frequencies
of yellow and white morphs vary from 60:40 to 25:75 (Nokelainen,
2013). The hindwing colour of females in Europe ranges continuously
from orange to red within populations, except for Scotland, where all
females are yellow, and Georgia, where all females are red (Lindstedt
et al., 2011; Hegna et al., 2015; B. Rojas, personal observation).
Both adult males (Nokelainen etal., 2012; Rojas etal., 2017)
and females (Brain, 2016; Lindstedt, Reudler Talsma, Ihalainen,
Lindström, & Mappes, 2010) are unpalatable to bird predators,
which learn the moths’ hindwing coloration as a warning signal
(Lindstedtetal.,2011;Nokelainen,Valkonen,Lindstedt, &Mappes,
2014;Nokelainenetal.,2012;Rönkä,DePasqual,Mappes,Gordon,
&Rojas,2018).AlthoughthecolorationofA. plantaginis has mostly
been studied within the context of aposematism and predator–prey
interactions (i.e. natural selection), considering birds as the main
signalreceivers(e.g.Nokelainenetal.,2012,2014),wingcoloration
may also have a function in intraspecific communication (e.g. sex-
ual selection). Experiments suggest that white males have a higher
matingsuccessthanyellowmales(Gordon etal.,2015; Nokelainen
et al., 2012), particularly if males experience stress. However, these
experiments did not reveal whether females based their choice on
colour or on other properties related to the colour morph.
While females attract males from afar by pheromones, ap-
proaching males visually search for females at short range in the
vegetation (B. Rojas & J. Mappes, personal observation). Males might
be choosy about which female(s) to approach, and females may or
may not mate with a male they have attracted (Gordon et al., 2015).
Altogether,thisraisesthequestionwhetherwingcolorationplaysa
roleinmatedetectionandchoice.Addressingthisquestionrequires
that we know how tiger moths perceive their own wing colours.
In this study, we investigated the eyes of A. plantaginis using his-
tology and electroretinograms (ERGs). We determined the spectral
sensitivity of the retina, measured the wing reflectance and used
a colour vision model to estimate how well the moths can detect
conspecifics against natural backgrounds, and discriminate between
their colour morphs. We also assessed how well a natural preda-
tor, the blue tit Cyanistes caeruleus, can detect the different colour
morphs of the wood tiger moth. Our results allow predictions on the
opposing selective forces that act on the colour morphs and main-
tain colour polymorphism within populations.
2 | MATERIALS AND METHODS
2.1 | Animals
We investigated animals from a stock of wood tiger moths from
southernand centralFinlandthathasbeenkeptatthe Universityof
Jyväskylä,Finland,since2013,withwildindividualsbeingaddedevery
year during the field season and crosses between brothers and sisters
beingsystematicallyavoided.Frequenciesofyellowandwhitemorphs
were kept to approximately 50:50. Larvae were raised under green-
house conditions and fed dandelion (Taraxacum sp.) leaves ad libitum
(for more details, see Lindstedt et al., 2010). Rearing containers were
checked and cleaned daily until the larvae pupated. Pupae were kept
at24°Cintransparentplasticboxeslinedwithpapertowels.Afterthe
moths had eclosed, straightened their wings and hardened, they were
subjected to experiments the same day or transferred to a dark cham-
ber and stored at 8°C for later investigation. Moths used for electro-
physiology originated from the 2nd, moths for anatomy from the 9th
and those for reflectance measurements from the 11th generation of
the laboratory stock.
2.2 | Eye histology
Three male and three female A. plantaginis were decapitated,
and their heads dissected and fixated in 2% glutaraldehyde, 2%
    
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HENZE Et al .
formaldehyde and 2% sucrose in 0.15 M sodium cacodylate buffer
for12–24hr.After repeated rinsingin buffer,theheads wereem-
bedded in epoxy resin. Vertical sections of 3 μm thickness were
taken with a Reichert Ultracut microtome using glass knives. The
sections were placed on a slide, dried on a hot plate, stained with
toluidine blue and photographed under a light microscope. Rhabdom
lengt hs were measure d from microgr aphs using Image J150 ( NIH,
Bethesda,MD,USA),followingthecourseoftherhabdomfromthe
base of the crystalline cone to the basement membrane.
2.3 | Electroretinogram (ERG) recordings
Forelectrophysiology,amale(n = 8) or female (n = 6) adult moth was
mounted onaholderandplacedinalight-tight,darkFaradaycage.
To explore regional differences in the retina, we inserted a tungsten
recording electrode either in the ventral (n = 9) or in the dorsal (n = 5)
half of one compound eye. The eye was illuminated via the central
fibre of a light guide, whose angular position and distance were ad-
justed to maximize responses to flashes of 40 ms duration separated
bypausesof5s.Afterdarkadaptationfor30min,thespectralsen-
sitivity was measured up to four times (see Supporting Information
Methods) presenting narrowband spectral flashes of equal photon
flux as described previously (Jakobsson, Henze, Svensson, Lind, &
Anderbrant,2017;Tellesetal.,2014).Precedingandfollowingeach
spectral series, we determined a voltage response–intensity (V- log
I) relationship using flashes of white light with increasing intensity
I. Response characteristics of the dark- adapted retina were investi-
gated 3–20 times for each individual, with 10 or more datasets for
most animals recorded at different times of the day.
To isolate the contribution of short- wavelength receptors to the
ERG, we repeated the recording protocol with continuous adapta-
tion light from one of four light emitting diodes (LEDs) presented
through the peripheral fibres of the light guide in the order red,
amber, green and blue (see Supporting Information Methods). Each
LED was switched on 5 min before a series of recordings started and
operated to produce increasing intensities at the position of the eye
in subsequent experiments. In between sets of experiments and in
the end, we monitored the state of the animal by repeatedly record-
ing spectral control series from the dark- adapted retina.
2.4 | ERG analyses
ERGs were analysed by custom- made Matlab scripts (R2013b or
R2015b, The MathWorks, 160 Natick, MA , USA). The response
amplitude V was calculated as the absolute value of the potential
change from the baseline at stimulus onset to maximal hyperpolari-
zation. To assess the stability of the recordings and to document the
dynamic r ange and satu ration level of re sponses, a Na ka-Rushton
function was fit ted to each V- log I dataset based on a nonlinear least-
squares solution.
Forthedark-adaptedretina,wecalculatedtheasymptoticmax-
imal response Vmax, the intensity K that elicited a half- maximal re-
sponse,andtheslopeoftheNaka-RushtonfunctionatK. We tested
the effect of sex and measurement number (repetitions) on these
three parameters using linear mixed- effects models with a nested
design (measurement number nested within individual). To test for
time dependence, we fitted linear regression models to the data,
with the effect of sex adjusted for the time of day in hours after mid-
night. All statisticalmodels were implementedin R v3.3.3. (RCore
Tea m 2017).
Based onthe Naka-Rushtonfunctionfitted to the V- log I data-
sets obtained before and after each spectral series, we converted
response amplitudes for spectral flashes into normalized sensitivi-
ties as described in Telles et al. (2014). Templates (Govardovskii,
Fyhrquist,Reuter,Kuzmin,& Donner,2000)wereusedtoestimate
the sensitivity maxima (λα) of receptor types contributing to the
averaged sensitivity curves (n = 4) by a nonlinear least- squares ap-
proach (see Supporting Information Methods). To get the best es-
timate for λα of a specific receptor type, we selected cur ves with
minimal contributions from other receptors.
2.5 | Reflectance measurements
Spectral reflectance s(λ) of 16 white and 10 yellow males, as well
as 8 females classified as orange and 10 females classified as red,
was measured in the range of 300 to 70 0 nm, with 1 nm resolu-
tion, usinganOcean OpticsUSB4000 spectrometer (Dunedin, FL,
USA) conn ected to a PX-2-pulsed Xen on lamp (for furt her details
seeLindstedtetal.,2011;Nokelainenetal.,2012).ASpectralonTM
whitereflectancestandard(Labsphere,Congleton,UK)wasusedfor
calibration.
2.6 | Model calculations
The quantum catch Qi of photoreceptor i (i = UV, blue, green recep-
tor) is given by:
where λ is wavelength, R is receptor sensitivity, s is the reflectance
spectrum of the specimen, I is the illumination spectrum, and k is a
scaling factor given by adaptation to the background spectrum sb:
We estimated visual contrast using a receptor noise- limited
(RNL)modelofcolourdiscrimination(Vorobyev&Osorio,1998).In
this model, we suppose that colour discrimination limits are set by
receptor noise that propagates into higher neuronal levels via retinal
opponent mechanisms. We assume a loglinear relationship between
receptor quantum catch and receptor signals, f = ln(Qi), so that the
contrast Δf between two stimuli s1 and s2 is
Each receptor mechanism is limited by a Weber fraction ω given
by receptor noise:
(1)
Q
i=ki
700
300
Ri(λ)s(λ)I(λ)dλ
,
(2)
k
i=
1
700
300
R
i
(λ)s
b
(λ)I(λ)dλ
.
(3)
Δ
f=ln
(Q
i
s
1
Qis2).
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Functional Ecology
HENZE Et al .
where e is the noise in each receptor type and η is the proportion
ofeachreceptorchannelintheretina.Asreceptornoiseandspatial
summation are unknown for A. plantaginis, we set noise in the green
receptor channel to 0.1, the level measured in honeybees (Vorobyev,
Brandt,Peitsch,Laughlin,&Menzel,2001).Assumingthateachom-
matidium has one UV, one blue and seven green receptors, as in
various other moths (Briscoe, 20 08; Jakobsson et al., 2017; Warrant,
Kelber,&Kristensen,2003;butseeBelušiç,Šporar,&Megliç,2017),
we set the receptor proportions to 1:1:7 (UV:blue:green receptors).
Given the uncertainty of noise in moth photoreceptors, we focus on
relative chromatic contrast and also test whether our conclusions
hold even for a flat receptor abundance ratio of 1:1:1.
Colour contrast S is given by:
The unit for colour contrast is jnds (just noticeable differences),
and1jndisdefinedasthecolourcontrastatdetectionthreshold.For
estimating the contrast in the visual system of the blue tit (Cyanistes
caeruleus), we used the model for tetrachromatic bird vision (given in
Supporting Information Methods).
We calculated colour contrast under standard daylight illumina-
tion (d65; Wyszecki & Stiles, 2000) using two background spectra
corresponding to green and brown vegetation. Our conclusions do
not change when assuming the illumination in a deciduous forest
(Håstad, Victorsson, & Ödeen, 2005), daylight d75, or light spectra
typicalforsunsetortwilight(Johnsenetal.,2006)instead(FigureS3).
We determine the position of colour loci (position of colours in the
colour space) in a chromaticity diagram according to the following axes:
where the coefficients (A, B, a, b) scale the chromaticity diagram ac-
cording to th e receptor noise- limited mod el such that the Euclid ean dis-
tance of 1 between two colour loci corresponds to 1 jnd (Equation 5):
AchromaticcontrastwascalculatedasMichelsoncontrast(see
Supporting Information Methods).
3 | RESULTS
3.1 | Eye anatomy
We inspected vertical sections of the compound eyes and optic lobes
of male and female Arctia plantaginis under the light microscope. The
light-adapted eyes(Figure1) are close to spherical and have a cor-
neaof≈18μm thickness, a crystalline cone of about twice this length,
and 76±8 μm long rhabdoms in females (n = 10), but 100±11 μm long
rhabdoms in males (n=10).Asnoclearzonecouldbedetected, the
eyes likely function as apposition eyes. The sections of the heads also
revealed that the second optic neuropil, the lamina, is spatially not di-
rectly adjacent to the retina in A. plantaginis(Figure1).
3.2 | Electroretinograms (ERGs)
ERGs from the compound eyes of A. plantaginis consisted of mono-
phasic hyperpolar izations (Figure2a), indicatin g that we recorded
(4)
ω
i=
e
i
η
i
,
(5)
S
2=
ω2
UV(Δfgreen −Δfblue)
2
2
blue(Δfgreen −ΔfUV)
2
2
green(Δfblue −ΔfUV)
2
(
ωUVωblue )2+
(
ωUVωgreen
)
2
+
(
ωblueωgreen
)
2
.
X
1=A
(
fgreen fblue
)
(6)
=B
f
af
+bf
(7)
A
=
1
ωblue2
+ωgreen2
,
(8)
B
=
ωblue2
+ωgreen2
ωUVωblue
2
+
ωUVωgreen
2
+
ωblueωgreen
2
,
(9)
a
=(ωblue)
2
(
ωblue
)
2
+
(
ωgreen
)
2
,
(10)
b
=(ωgreen)
2
(
ωblue
)
2
+
(
ωgreen
)
2
.
FIGURE1 Vertical sections of the compound eyes of female
(top) and male (bottom) Arctia plantaginis. Left, enlarged region
of the eye with c cornea; cc, crystalline cone; rh, rhabdom; b,
basement membrane; scale bar 50 μm. Right, eyes (R retina) and
first optic neuropil, lamina (L), scale bar 100 μm
    
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Functional Ecology
HENZE Et al .
photoreceptor responses only. The rather long distance between
retinaandlamina(seeFigure1)isprobablythereasonwhyourelec-
trode did not pick up signals of downstream neurons.
We observed striking differences in response strength between
males (n = 8) and females (n=6; Figu re2; data availabl e from the
Dryad Digital Repository: https://doi.org/10.5061/dryad.s46t627).
In the dark- adapted retina, the estimated maximal response Vmax to
flashes of white light was more than twice as large in males (mean
± standard error: 18.6 ± 1.0 mV) as in females (6.5 ± 1.1 mV; values
are based on a linear mixed- effects model accounting for repeated
measurements per individual). This difference of 12 mV was highly
significant (t(12) = 8.86, p < .001, Table 1). K, the intensity that elicits
a half- maximal response (Vmax/2) and marks the turning point of the
Naka-Rushtonfunctionfittedtotheresponse–intensity(V- log I) re-
lationship, did not differ significantly between the sexes. However,
the slope of the curve at K was significantly higher for males than
females (mean difference 3.1 mV/log I, t(12) = 7.69, p < .001, Table 1
andFigure2).Regardlessof thestatistical model,sex had a robust
effect on Vmax and on the Slope at K, but not on K itself (compare
Table 1 and Table S1). This was independent of the number of mea-
surements or the time of day, at least for the 16 hrs between 10
and 2 hr (the next day), for which we have recordings. Measurement
number had a significant effect on K, but the p- value was high and
the size of the effect minimal (mean difference −0.02mV/log I,
t(130)=−2.10,p = 0.04, Table 1).
The spectral sensitivity curves obtained under dark adaptation
and adaptation to different intensities of red, yellow, green or blue
light were all in agreement with the assumption of three photorecep-
tor types expressing visual pigments with peak sensitivities in the
UV,blueandgreenrange(e.g.seeFigure3;dataavailablefromthe
Dryad Digital Repository: https://doi.org/10.5061/dryad.s46t627).
Adding another spectral receptor type to themodels didnotcon-
siderably improve the fit. Differences in the wavelengths of maximal
sensitivity λα between males and females, and dorsal and ventral eye
regions were within the limits of experimental accuracy. We there-
fore averaged all results for each receptor type leading to a λα- value
FIGURE2 Sex differences in response strength of the dark- adapted retina in Arctia plantaginis. (a) Electroretinograms (ERGs) from the
compoundeyesofamaleandafemalemothinresponseto40msflashesofwhitelightwithincreasingintensity.Forclarity,onlyevery
secondERGisshown.(b)AmplitudesV (potential changes from the baseline at stimulus onset to maximal hyperpolarization) plotted against
intensity Iforthedatasetspartiallyshownin(a).WefittedaNaka-Rushtonfunctiontoeachdatasetandestimatedtheasymptoticmaximal
response Vmax, the intensity K that elicited a half- maximal response, and the Slope of the cur ve at K. The inset illustrates the range of data
from all eight males and six females. (c) Sex differences of the parameters Vmax, K and Slope at K (mean ± standard error) tested by linear
mixed- effects models
K [log I ]Slope at K
[mV / log I ]
Vmax [mV ]
20
10
0
6
4
0
0.0
0.5
1.0
2
*** n.s. ***
Vmax / 2
K
Vmax / 2
K
20
10
0
–6 –4 –2 0 2 4
Intensity [log
I ]
Female Male
Amplitude [mV
]
(a) (b) (c)
Flash
off
ERG
0.1 s
6 mV
on
V-log I relationship
20
0
0
-4
10
Range
TABLE1 Influence of sex and measurement number on response properties of the dark- adapted retina in the compound eyes of
A. plantaginistestedbylinearmixed-effectsmodels.ForadefinitionofVmax, K and Slope at K,seeFigure2
Mean difference Standard error
95% confidence
interval tdf p value
Vmax [mV]
Sex 12.0 1.4 9.1 , 1 5.0 8.87 12 <.001
Measurement no. 0.1 0.1 −0.2,0.3 0.54 13 0 .59
K [log I]
Sex −0.1 0.2 −0.5,0.3 −0.72 12 .48
Measurement no. −0.02 0.01 −0.1,−0.001 −2.10 130 .04
Slope at K [mV/ lo g I]
Sex 3.1 0.4 2.2, 4.0 7.6 9 12 <.0 01
Measurement no. 0.1 0.04 −0.03,0.1 1.23 130 .22
6 
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FIGURE3 Spectral sensitivities of male (left) and female (right) Arctia plantaginisderivedfromERGs.Foreachindividual,averaged
measurements (n = 4) from one compound eye under different adaptation states were selected to estimate the wavelength of maximal
sensitivity (λα) for three spectral types of photoreceptor. Example curves used to assess λα of the green receptors (a, b), λα of the UV receptors
with λα of the green receptors predetermined (c, d), and λα of the blue receptors with λα of the green and UV receptors predetermined (e, f).
(g, h) Examples, in which the blue receptors were suppressed by blue light, isolating the responses of the UV and green receptors
1.0
0.5
0.0
(a)
1.0
0.5
0.0
(c)
1.0
0.5
0.0
(e)
Relative sensitivit
yRelative sensitivity Relative sensitivity Relative sensitivity
1.0
0.5
0.0
(g)
Wavelength [nm]
(h)
(f)
(d)
(b)
Wavelength [nm]
Males Females
Green adaptation
1×1014 quanta cm–2 s–1
Green adaptation
1×1012 quanta cm–2 s–1
Blue adaptation
2×1013 quanta cm–2 s–1
Blue adaptation
2×1013 quanta cm–2 s–1
Red adaptation
1×1014 quanta cm–2 s–1
Amber adaptation
2×1014 quanta cm–2 s–1
Dark adaptation Dark adaptation
Mean ± standard deviation
UV receptors
Blue receptors
Sum
Green receptors
300 400 500 600 700 300400 500600 700
300 400 500 600 700 300400 500600 700
300 400 500 600 700 300400 500600 700
300 400 500 600 700 300400 500600 700
    
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of 521 ± 2 nm (mean ± standard deviation, n = 14) for the green re-
ceptor, 456 ± 7 nm (n = 14) for the blue receptor and 349 ± 2 nm
(n=13) for the UV re ceptor. Note that the peak s ensitivities (λα)
of the three receptors are not equally spaced across the spectrum.
Instead, the sensitivity peak of the blue receptor is shifted to longer
wavelengths (i.e. red-shifted)by 21nm. Forthe dark-adapted eye,
the sensitivity peak of the green receptor type was 11 to 12 times
as high as the sensitivity peak of the UV and the blue receptor type,
indicating a larger number of green receptors, likely also with bigger
rhabdoms.
3.3 | Wing coloration
Asinpreviousstudies on thespecies, we classified malesafter in-
spection by the human eye as belonging to the white or the yellow
morph, and females as either orange or red. While we measured very
similar reflectance curves from the forewings of all morphs, hind-
wings differed. In particular, the hindwings of white males reflected
more UV th an those of any other m orph (Figure4; da ta available
from the Dryad Digital Repository: https://doi.org/10.5061/dryad.
s46t627).
We used the receptor sensitivities determined by ERGs
(Figure5a)toestimate thecolour contrastofwhiteand coloured
wing patches of A. plantaginis in daylight illumination (Figure5c)
withtheRNLmodelofcolourdiscrimination(Vorobyev&Osorio,
1998). This model provides a quantitative measure of chromatic
contrast while achromatic (or luminance) contrast is ignored.
Because one of the input values, the noise level in each photo-
receptor channel, is unknown for A . plantaginis, we can only give
a rough estimate of absolute discrimination thresholds. Instead,
we focus on relative colour contrast, which is unaffected by this
uncertainty (TableS2, FigureS2).Wedo notpredict colourcon-
trasts for black wing regions, assuming that the contrasts between
black and either white or coloured areas are mostly detected by
the achromatic channel.
We first address the discrimination between wing colours of
different morphs, and plot all colours in the colour space described
by the RNL mo del. We assume that t he moths can discr iminate
distinct morphs by colour, if the average contrast between the co-
lours of different morphs exceeds the contrast between colours
within each morph such that morph colours form non- overlapping
clusters.
Thisisnotthecasefortheforewingcolours(Table2,Figure6a).
Hindwingcolorationismorevariable(Fig6b)differingmoststrongly
in the stimulation of the UV receptor, which is evident from the
oblique-verticaldistributionofcolourlociincolourspace(Figure6b).
The average contrast between hindwing colours of white and yel-
low males is higher than the contrast within each of these morphs
(Table 2), and—with the exception of two outliers—the hindwing
colour of the white morph forms a distinct cluster in colour space,
separatedfromallother morphs(Figure6b). Thus,weassumethat
A. plantaginis likely discriminates white and yellow males as two dif-
ferent colour morphs, just as we do, based on human vision. The
orange and red hindwing colours of females, in contrast, do not
formdistinctclusters,butlargelyoverlap(Table2,Fig6b)andbuild
a continuum. While conspecifics will be able to discriminate certain
orange and red individuals, they will not be able to categorize them
visually as belonging to two morphs.
FIGURE4 Reflectance of forewing (dashed lines) and hindwing
(solid lines) colours of male and female Arctia plantaginis. Lines give
averages and grey areas standard deviations. Data from 16 white
males, 10 yellow males, 8 orange females and 10 red females
Wavelength [nm]
White male
1.0
0.5
0.0
Yellow male
1.0
0.5
0.0
ReflectanceReflectance
Orange female
1.0
0.5
0.0
Reflectance
Red female
1.0
0.5
0.0
300400 500600
700
300400 500600
700
300400 500600
700
300400 500600
700
Reflectance
Forewings
Hindwings
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Next,weinvestigatedthecolourcontrastsofthewingsagainst
differentnaturalbackgrounds(Figure5c).Forthemothsthemselves,
wing colours have very similar chromatic contrasts against green and
brown backgrounds, and hindwings are more conspicuous than fore-
wings(redsymbolsinFigure7).Allachromaticcontrastsarehigher
than0.1andthuseasilydiscriminable(FigureS1).
To understand whether the red- shift of the blue receptor of
A. plantaginis enhances its ability to detect and differentiate conspe-
cifics, we compared colour discrimination of A. plantaginis with that
of Macroglossum stellatarum, a moth with equally spaced receptor
sensitivities(Figure5a,datafromTellesetal.,2014)(whitesymbols
in Figure7). Assuming the same noise levels, we found no signifi-
cant differences between these two visual systems, neither in the
discrimination of wing colours against green or brown backgrounds
(Figure7 ), nor in the discrimi nation betwee n wing patches (Table
S3).Allresultsholdtrueevenifwe assumedifferent relativenum-
bers of receptortypes(Figure S2) or different illumination spectra
(FigureS3).
Finally,weaskedwhetherthebluetit,apredatorwithtetrachro-
maticcolourvision(forreceptorsensitivities,seeFigure5b),cande-
tectspecificmorphsbetterthanothers.Forthisbird,colourcontrast
of all except the white hindwings is higher against the green than
the brown background, and all hindwing colours are more conspic-
uous than forewing colours on both backgrounds (blue symbols in
Figure7).Achromaticcontrastsarealsohigh(FigureS1),butbirdsare
generally thought to use chromatic rather than achromatic contrast.
4 | DISCUSSION
Classical approaches have invoked predation as the most represent-
ative selective pressure favouring aposematic coloration. However,
this approach falls short to explain the intrapopulation variation in
the coloration of some aposematic species, as this variation is ex-
pected to hinder predator learning and the subsequent avoidance
of aposematic prey. Sexual selection has been suggested as a pres-
sure capable of counterbalancing predation, thus allowing multiple
morphs of an aposematic species to coexist. This could be the case
in the polymorphic wood tiger moth, where one morph appears to
be favoured by predators while the other morph seems to be pre-
ferred by females. In the present study, we investigated whether the
moths can visually discriminate the colour morphs of conspecifics, a
precondition for sexual selection to play a role in the maintenance of
this polymorphism. Our results indicate that male and female wood
tiger moths have similar spectral sensitivities and, while both sexes
can discriminate the two male colour morphs, they are unable to dis-
tinguish orangefrom red femalemorphs.Furthermore,inthe eyes
of both conspecifics and predators, white males are more conspicu-
ous against green and brown backgrounds, whereas red females are
slightly more conspicuous than orange females. We discuss the im-
plications of our findings below.
4.1 | Eye anatomy and sensitivity
Wood tiger moths appear to have functional apposition compound
eyes(Figure1),astherhabdomsofthephotoreceptorsareindirect
contact with the cr ystalline cones, without the clear zone that is typ-
icalforsuperpositioneyes(Land&Nilsson,2002).Assuperposition
eyes are the general rule for moths (Warrant et al., 2003) including
FIGURE5 Data used in model calculations. (a) Spectral
sensitivities of photoreceptors in the compound eyes of the wood
tiger moth (Arctia plantaginis) and the hummingbird hawkmoth
(Macroglossum stellatarum; data from Telles et al., 2014). The
sensitivities of UV and green receptors based on the template
(Govardovskii et al., 20 00) are the same in both species, but
the sensitivity of the blue receptors in A. plantaginis is red-
shifted. (b) Spectral sensitivities of the four single cones in the
blue tit (Cyanistes caeruleus; data from Hart et al. 2000). UVS,
ultraviolet sensitive; SWS, short- wavelength sensitive; MWS,
middle- wavelength sensitive; LWS, long- wavelength sensitive. (c)
Reflectance spectra of green and brown vegetation and irradiance
of daylight (d65; Wyszecki & Stiles, 2000)
1.0
0.5
0.0
300 400 500 600 700
300 400 500 600 700
Relative sensitivity Relative sensitivity
1.0
0.5
0.0
300 400 500 600 700
1.0
0.5
0.0
Wavelength [nm]
Daylight d65
Brown vegetation
Green vegetation
Nornalized irradiance
Background reflectanc
e
(a)
(b)
(c)
UV Blue Green
UVSSWS MWS LWS
    
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 9
Functional Ecology
HENZE Et al .
Arctiidae(D.-E.Nilsson,personalcommunication),thisresultcomes
as a surprise, although similar cases have been reported (Warrant
et al., 2003). Males have slightly longer rhabdoms than females,
which should make their photoreceptors ≈20% more sensitive.
However, this can only partly explain why the signals recorded from
the dark- adapted retina were more than 100% larger in males than
females (Figure2). Further studies are required to understand the
physiological basis of the observed sex- specific differences.
Our analyses suggest that the eyes of both sexes are optimized
for similar light intensities, because the linear working range of
male and female eyes is centred on the same intensity K (Table 1,
Figure2).However,theslopeofthecurvesissteeperinmalesthan
females, indicating that intensity differences translate into larger re-
sponse differences, which might increase contrast sensitivity.
Higher (absolute and contrast) sensitivity of male eyes fits well
tothenaturalbehaviourofthemoths.Females,whichcarryaheavy
weight of developed eggs when they eclose, mostly sit on the vege-
tation and emit pheromones until they mate, lay eggs and die. Their
flight activity is limited and not important for survival or reproduc-
tion. Males instead fly to find a mate and have to detect them visu-
allyindensevegetation.Forflightcontrolandfordetectingfemales,
males need sensitive eyes.
Our results show sex differences in the response strength of the
photoreceptors to flashes of white light but not in their spectral tun-
ing(Figure3).We have foundevidence forthree spectral types of
photoreceptor, with sensitivity peaks for UV, blue and green light.
More than three receptor types have been described in many but-
terflies (Arikawa & Stavenga, 2014), yet rarely in moths (Belušiç
et al., 2017; Jakobsson et al., 2017; Langer, Hamann, & Meinecke,
1979; Warrant et al., 2003). We cannot exclude that we missed addi-
tional receptor types in A. plantaginis, but these receptors, if present,
either have a spectral sensitivity close to those characterized here or
do not contribute substantially to the ERG.
While the spectral sensitivities of the UV and green receptors of
A. plantaginis are similar to those found in sphingid moths (Manduca
sexta: Wh ite, Xu, Munch, Be nnett, & Gr able, 2003; Macroglossum
stellatarum: Telles et al., 2014), the sensitivity of the blue recep-
tor is red-shif ted (Figure5a). It is normally assumed that equally
spaced photoreceptor sensitivities allow for optimal discrimination
of general colour stimuli (e.g. Barlow, 1982). However, our models
comparing A. plantaginis with M. stellatarum revealed no significant
differences in discriminability of colours and thus no obvious effect
of the red- shift of the blue receptor in A. plantaginis(Figure7).
4.2 | Wing coloration in the eyes of conspecifics
Seen with the eyes of A. plantaginis, forewings do not differ between
morphs, but white and yellow males have clearly distinct hindwing
colorat ion (Table2, Figure6). Mos tly because of t he stronger UV
reflection of their hindwings, white males have a higher chromatic
contrast than yellow males against green and brown backgrounds
(Figure 7). Nokelainen etal.(2012) showed the opposite for a flat
white background spectrum. Thus, yellow males have a higher
TABLE2 Colourcontrast(jnds)betweenpatchesofforewingsandhindwingsinmaleandfemalewoodtigermoths.Foreachcomparison,
we give the median colour contrast with the 25th and 75th percentiles in brackets
Male forewing colour Female forewing colour
White Yel low Orange Red
White 0.9 (0.3–1.6) 1.0 (0.7–1.4) Orange 1.1 (0.5–1.8) 1.1 (0.6–1.4)
Yello w 0.7 (0.3–1.2) Red 0.5 (0.2–0.9)
Male hindwing colour Female hindwing colour
White Yel low Orange Red
White 0.1 (0.1–0.3) 2.6 (1.8–3.3) Orange 1.4 ( 0.6 –2.7 ) 1.7 (1.1–2.5)
Yello w 0.9 (0.5–1.5) Red 0.5 (0.2–0.8)
FIGURE6 The loci of forewing (a) and hindwing (b) colours
in the chromaticity diagram of Arctia plantaginis. Each data point
represents the colour locus given by a reflectance spectrum
illuminated by daylight d65. Receptors are adapted to a green
background (green triangle); the colour locus of the brown
background (brown triangle) is shown for comparison. The
Euclidean distance between loci equals their contrast expressed in
jnds (see Methods for details). Colour loci are visualized on specific
opponent axes in the chromaticity diagram, but the contrast
between colours is set by receptor noise and independent of
specific colour opponency (Vorobyev & Osorio, 1998)
−3
−2
3
5
−3
−2
3
5
X2
UV-(Blue+Green)
X1
Green – Blue
(a) Forewings (b)
Hindwings
White male
Yellow male
Orange female
Red female
10 
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HENZE Et al .
chromatic contrast than white males on the white bark of a birch
tree, but a lower contrast on green or brown vegetation. This could
have implications for sexual selection in the species (Gordon et al.,
2015; Nokelain en etal., 2012). Our r esults also s how that orange
and red females cannot be discriminated as distinct colour morphs
(Table2,Figure6).Therefore,wesuggestthatfemalehindwingcol-
oration is of little importance for mate selection.
It is interesting to note that the high contrast of the white hind-
wings against green and brown backgrounds and to the hindwing
colours of other morphs mostly results from high UV reflection.
Obviously, our anthropomorphic view and naming of colour morphs
do not always conform with their appearance to conspecifics and
can be misleading when searching for communication signals.
4.3 | Aposematic vs. intraspecific signals
To a bird, white and yellow males of A. plantaginis have a similar con-
trast ag ainst a green ba ckground (blu e symbols in Fig ure7). Blue
tits hesitate much longer before attacking yellow than white males
presentedonagreenbackground(Nokelainenetal.,2012),but the
predation on both morphs depends on the local bird community
(Nokelain en etal., 2014). We suggest that c olour per se, and per-
haps the black- and- yellow pattern, and not the contrast against the
background determines the aposematic function of the hindwings.
Alternatively,therecentlyobservedstrongpyrazineodourofyellow
males, which is more effective than that of white males (Rojas et al.,
2017), may explain the hesitation of birds when attacking yellow
moths. By contrast, the strong UV reflection of the hindwings of
white males, though making them conspicuous to both females and
birds, is unlikely to contribute to avoidance learning. UV is not an
efficient warning signal on it s own (Lyy tinen, Alatalo, Lindström,
& Mappes, 2001) and can even attract birds rather than stop them
from attacking (Lyytinen, Lindström, & Mappes, 2004; Olofsson,
Vallin,Jakobsson,&Wiklund,2010).Nevertheless,ourresultssup-
port the role of natural selection in the evolution of hindwing colour
in A. plantaginis males and suggest that sexual selection may play a
role, too.
Our model calculations indicate that the variation in female hind-
wing color ation is perceived as a continuum by conspecifics; it is thus
unlikely to contribute to mate choice. Predation, however, seems to
represent a selection pressure on female coloration, as birds avoid
red females more often than orange females (Lindstedt et al., 2011)
and learn the red colour faster (Rönkä et al., 2018). While the red
pigmentation warns birds about unprofitability (Lindstedt et al.,
2011), it is costly and depends on larval diet, such that larvae reared
on a diet high in iridoid glycosides become adult s with paler (less red)
wing pigmentation (Lindstedt et al., 2010).
Forewings,on the other hand,are highlyconspicuous because
of the high achromatic (luminance) contrast between white and
blackpatches(FigureS1).Naïve birds are more strongly attracted
to patterns combining yellow or red with black- and- white than with
grey patches. However, they learn to avoid the former much faster
(Zylinski & Osorio, 2013). The combination of black- and- white fore-
wings with red or yellow hindwings may thus be an efficient learning
FIGURE7 Colour contrast between
wing patches and (a) a green, or (b) a
brown background in Macroglossum
stellatarum (white), Arctia plantaginis
(red) and Cyanistes caeruleus (blue).
Contrast was calculated for forewing
and hindwing colours from 16 white and
10 yellow males, and 8 orange and 10
red females. Boxes show the 25th, 50th
and 75th percentiles of colour contrast
with whiskers spanning the range of the
distributions to a maximum of ± 1.5 times
the interquartile range (75th- 25th). Data
points outside this interval are counted as
outliers (crosses). The absolute threshold
of detection is set to 1 jnd; however, in
the moths, this limit is uncertain as the
receptor noise levels are not well known
(see Methods)
0
1
5
10
15
0
1
5
10
Colour contrast [jnds]
White Yellow Orange Red
Forewing Hindwing
White Yellow Orange Red
Contrast against a green background
White Yellow Orange Red
Forewing Hindwing
White Yellow Orange Red
Contrast against a brown background
Colour contrast [jnds]
(a)
(b) 15
    
|
 11
Functional Ecology
HENZE Et al .
cue for birds. In fact, birds readily avoid the forewing pattern of
A. plantaginis (Hegna & Mappes, 2014).
The interplay of natural and sexual selection in the evolution of
visual signals manifests in different ways across taxa. Brightly co-
loured models of red postman butterflies (Heliconius erato) attracted
more males and deterred predators more efficiently than achromatic
modelsandmodelsofnonlocalmorphs(Finkbeiner,Briscoe,&Reed,
2014). This suggests that predator deterrence (aposematism) and
mate choice work in the same direction, which may explain why only
one morph occurs in the studied population of this Heliconius species
(Finkbeineretal.,2014).Incontrast,in small cabbagewhitebutter-
flies (Pieris rapae), females prefer males that are more contrasting
against the background. These males are also most conspicuous to
birds, and thus in greater risk of predation (Morehouse & Rutowski,
2010), a situation resembling the balance between natural and sex-
ual selection in the well- known case of Trinidadian guppies (Endler,
1983).
Opposing sexual and natural selection alone cannot explain the
maintenance of local polymorphism, as this would require exactly
equal strength of both selective pressures. Thus, additional selec-
tive or genetic factors are usually involved. In A. plantaginis, where
yellow males, on average, have an advantage against predators while
white males tend to have a mating advantage, gene flow and vari-
able selection by predators seem to offer an explanation for poly-
morphis m (Galarza, No kelainen, As hrafi, Hegna , & Mappes, 2014;
Nokelainenetal.,2014).OurresultsonA. plantaginis vision confirm
that male selection for female coloration is unlikely, but a female may
use chromatic cues for mate choice once a male has approached her,
favouring the UV- reflecting white males. Whether females use vi-
sual cues in mate selection, or whether other traits explain mating
success of white males better, requires further investigation.
ACKNOWLEDGEMENTS
We are grateful to K. Suisto and the greenhouse staff at the
UniversityofJyväskyläforrearing the animals,toO.Nokelainen
and C. Lindstedt for kindly providing reflectance measurements of
A. plantaginis, to Carina Rasmussen for excellent help with histol-
ogy,toDan-E.Nilssonforinsightsintolepidopteraneyetypes,to
Thomas L abhart , Eric Warrant an d Dan-E. Nilsson for a ccess to
equipment, to Thomas Labhart for advice on data analyses and
toAlan Ho for diligentassistance with statistics. Wethank Alan
Goldizen and three anonymous reviewers for critical comments
on earlie r versions of the m anuscript. Fu nding from the F innish
Cent re of Excellen ce in Biological I nteractions (210 00002 56) to J. M.
and B.R. and fromthe SwedishResearchCouncil toA.K.(2012-
2212) and O. Lind (637- 2013- 388) is gratefully acknowledged.
AUTHOR CONTRIBUTIONS
J.M., B.R.,M.J.H.andA .K.planned the study.J.M.andB.R.pro-
vided animals, wing spectra and ecological knowledge on the
study sp ecies, A. K. studie d eye histology, M. J.H. measure d and
analysedERGs, and O.L.calculated colourvision models. All au-
thors discussed the results, contributed to writing text and pre-
paring f igures and app roved the publ ication. No a uthor has any
conflict of interest.
DATA ACCESSIBILITY
All origi nal data used in this s tudy that are not pr esented in the
main text or the supplements, can be found on the Dr yad Digital
Repositor y: https://doi.org/10.5061/dryad.s46t627 (Henze et al.,
2018).
ORCID
Almut Kelber http://orcid.org/0000-0003-3937-2808
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How to cite this article: Henze MJ, Lind O, Mappes J,
RojasB,KelberA.Anaposematiccolour-polymorphicmoth
seen through the eyes of conspecifics and predators –
Sensitivity and colour discrimination in a tiger moth. Funct
Ecol. 2018;00:1–13. https://doi.org/10.1111/1365-
2435.13100
... Lastly, we asked whether these genotypephenotype associations may have ecological relevance beyond the human-visible spectrum. Using vision modelling, Henze et al. (2018) investigated the differences in the discriminability of the wood tiger moth colour morphs by moth conspecifics and bird predators. Here, we used receptor-noise-limited vision modelling (Maia et al., 2013; to test pairwise genotype chromatic contrasts of hindwing colour using human, avian and moth vision models. ...
... For moth vision model, spectral sensitivities of cone cells (uv, sw, mw) were obtained from (Henze et al., 2018), and cone ratios 1:1:1, were used as the specific ratio is unknown. As we were interested in differences in chromatic contrast (dS), we excluded the achromatic contrast (dL) from the vision model analysis. ...
... In males, the linear discriminant analysis separated the three-group problem with high accuracy. Successful discrimination between the three groups of males was expected due to differences in short and long wavelength reflectance between the two white morphs (Henze et al., 2018;Nokelainen et al., 2012). Also, the white homozygotes have a lower thorax UV reflectance, smaller thorax by abdomen ratio (i.e. ...
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The definition of colour polymorphism is intuitive: genetic variants express discretely coloured phenotypes. This classification is, however, elusive as humans form subjective categories or ignore differences that cannot be seen by human eyes. We demonstrate an example of a 'cryptic morph' in a polymorphic wood tiger moth (Arctia plantaginis), a phenomenon that may be common among well-studied species. We used pedigree data from nearly 20,000 individuals to infer the inheritance of hindwing colouration. The evidence supports a single Mendelian locus with two alleles in males: WW and Wy produce the white and yy the yellow hindwing colour. The inheritance could not be resolved in females as their hindwing colour varies continuously with no clear link with male genotypes. Next, we investigated if the male genotype can be predicted from their phenotype by machine learning algorithms and by human observers. Linear discriminant analysis grouped male genotypes with 97% accuracy, whereas humans could only group the yy genotype. Using vision modelling, we also tested whether the genotypes have differential discriminability to humans, moth conspecifics and their bird predators. The human perception was poor separating the genotypes, but avian and moth vision models with ultraviolet sensitivity could separate white WW and Wy males. We emphasize the importance of objective methodology when studying colour polymorphism. Our findings indicate that by-eye categorization methods may be problematic, because humans fail to see differences that can be visible for relevant receivers. Ultimately, receivers equipped with different perception than ours may impose selection to morphs hidden from human sight.
... Lastly, we asked whether these genotypephenotype associations may have ecological relevance beyond the human-visible spectrum. Using vision modelling, Henze et al. (2018) investigated the differences in the discriminability of the wood tiger moth colour morphs by moth conspecifics and bird predators. Here, we used receptor-noise-limited vision modelling (Maia et al., 2013; to test pairwise genotype chromatic contrasts of hindwing colour using human, avian and moth vision models. ...
... For moth vision model, spectral sensitivities of cone cells (uv, sw, mw) were obtained from (Henze et al., 2018), and cone ratios 1:1:1, were used as the specific ratio is unknown. As we were interested in differences in chromatic contrast (dS), we excluded the achromatic contrast (dL) from the vision model analysis. ...
... In males, the linear discriminant analysis separated the three-group problem with high accuracy. Successful discrimination between the three groups of males was expected due to differences in short and long wavelength reflectance between the two white morphs (Henze et al., 2018;Nokelainen et al., 2012). Also, the white homozygotes have a lower thorax UV reflectance, smaller thorax by abdomen ratio (i.e. ...
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Phenotypic variation is suggested to facilitate the persistence of environmentally growing pathogens under environmental change. Here we hypothesized that the intensive farming environment induces higher phenotypic variation in microbial pathogens than natural environment, because of high stochasticity for growth and stronger survival selection compared to the natural environment. We tested the hypothesis with an opportunistic fish pathogen Flavobacterium columnare isolated either from fish farms or from natural waters. We measured growth parameters of two morphotypes from all isolates in different resource concentrations and two temperatures relevant for the occurrence of disease epidemics at farms and tested their virulence using a zebrafish (Danio rerio) infection model. According to our hypothesis, isolates originating from the fish farms had higher phenotypic variation in growth between the morphotypes than the isolates from natural waters. The difference was more pronounced in higher resource concentrations and the higher temperature, suggesting that phenotypic variation is driven by the exploitation of increased outside‐host resources at farms. Phenotypic variation of virulence was not observed based on isolate origin but only based on morphotype. However, when in contact with the larger fish, the less virulent morphotype of some of the isolates also had high virulence. As the less virulent morphotype also had higher growth rate in outside‐host resources, the results suggest that both morphotypes can contribute to F. columnare epidemics at fish farms, especially with current prospects of warming temperatures. Our results suggest that higher phenotypic variation per se does not lead to higher virulence, but that environmental conditions at fish farms could select isolates with high phenotypic variation in bacterial population and hence affect evolution in F. columnare at fish farms. Our results highlight the multifaceted effects of human‐induced environmental alterations in shaping epidemiology and evolution in microbial pathogens.
... The location of a color locus in the triangle represents the relative excitation of the three receptor types by that floral sample (Balkenius et al., 2004). The light spectrum typical for sunset was used as ambient illumination (Henze et al., 2018), because hawkmoth foraging begins from sunset to dusk in this biome (Moré et al., 2006), although it can continue through the night (Vesprini and Galetto, 2000). ...
... Chromatic contrast describes the color contrast that excludes luminance information while the achromatic contrast refers to the luminance difference between a flower color and its background. For chromatic contrast we used the following parameters: Weber fraction of 0.1 as empirically estimated for the tiger moth Arctia plantaginis (Henze et al., 2018); noise was set as "neural"; photoreceptor densities UV = 0.1, B = 0.23, L = 0.67 based on data from the ventral portion of the compound eye of M. sexta (White et al., 2003); and quantum catch was set to "Qi." Achromatic contrast was calculated as the contrast produced in the longwave photoreceptor (Henze et al., 2018). All calculations were performed using the pavo v.2.2.0 package (Maia et al., 2019) of R software (R Core Team, 2020). ...
... Chromatic contrast describes the color contrast that excludes luminance information while the achromatic contrast refers to the luminance difference between a flower color and its background. For chromatic contrast we used the following parameters: Weber fraction of 0.1 as empirically estimated for the tiger moth Arctia plantaginis (Henze et al., 2018); noise was set as "neural"; photoreceptor densities UV = 0.1, B = 0.23, L = 0.67 based on data from the ventral portion of the compound eye of M. sexta (White et al., 2003); and quantum catch was set to "Qi." Achromatic contrast was calculated as the contrast produced in the longwave photoreceptor (Henze et al., 2018). All calculations were performed using the pavo v.2.2.0 package (Maia et al., 2019) of R software (R Core Team, 2020). ...
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Flower phenotype may diverge within plant lineages when moving across pollination climates (geographic differences in pollinator abundance or preference). Here we explored the potential importance of pollinators as drivers of floral color diversification in the nightshade genus Jaborosa taking into account color perception capabilities of the actual pollinators (nocturnal hawkmoths vs saprophilous flies) under a geographic perspective. We analyzed the association between transitions across environments and perceptual color axes using comparative methods. Our results revealed two major evolutionary themes in Jaborosa: 1) a ‘warm subtropical sphingophilous clade’ composed of three hawkmoth-pollinated species found in humid lowland habitats, with large white flowers that clustered together in the visual space of a model hawkmoth (Manduca sexta) and a ‘cool-temperate brood-deceptive clade’ composed of largely fly-pollinated species with small dark flowers found at high altitudes (Andes) or latitudes (Patagonian Steppe), that clustered together in the visual space of a model blowfly (Lucilia sp.). Our findings suggest that the ability of plants to colonize newly formed environments during Andean orogeny and the ecological changes that followed were concomitant with transitions in flower color as perceived by different pollinator functional groups. Our findings suggest that habitat and pollination mode are inextricably linked in the history of this South American plant lineage.
... We used the Wood tiger moth (Arctia plantaginis, Erebidae: Arctiinae) as our model prey. This species has distinctive wing color morphs that its bird predators can detect (Henze et al. 2018), it produces defensive chemicals eliciting predator avoidance, which justifies its status as an aposematic organism (Nokelainen et al. 2012;Hegna et al. 2013;Burdfield-Steel et al. 2018;Rönkä et al. 2018a). We focused on males, which are polymorphic regarding their hind-wing coloration: their hind wings may be yellow (chroma-rich) or white (luminance-rich). ...
... Differences in luminance might be more readily detectable by predators than chromatic contrast differences under low light conditions, since discerning colors can be constrained by scarce luminosity (Penteriani and Delgado 2017). However, our vision modeling results did not support that light environment skews the detectability of the two morphs (Henze et al. 2018). Instead, the luminance and chromatic contrasts were relatively constant in the two light environments. ...
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A big question in behavioral ecology is what drives diversity of color signals. One possible explanation is that environmental conditions, such as light environment, may alter visual signaling of prey, which could affect predator decision-making. Here, we tested the context-dependent predator selection on prey coloration. In the first experiment, we tested detectability of artificial visual stimuli to blue tits (Cyanistes caeruleus) by manipulating stimulus luminance and chromatic context of the background. We expected the presence of the chromatic context to facilitate faster target detection. As expected, blue tits found targets on chromatic yellow background faster than on achromatic grey background whereas in the latter, targets were found with smaller contrast differences to the background. In the second experiment, we tested the effect of two light environments on the survival of aposematic, color polymorphic wood tiger moth (Arctia plantaginis). As luminance contrast should be more detectable than chromatic contrast in low light intensities, we expected birds, if they find the moths aversive, to avoid the white morph which is more conspicuous than the yellow morph in low light (and vice versa in bright light). Alternatively, birds may attack first moths that are more detectable. We found birds to attack yellow moths first in low light conditions, whereas white moths were attacked first more frequently in bright light conditions. Our results show that light environments affect predator foraging decisions, which may facilitate context-dependent selection on visual signals and diversity of prey phenotypes in the wild.
... The pierid butterfly Pieris rapae has both a duplicated blue opsin and spectrally tuned filtering pigments: photoreceptor modifications that may be crucial for mate recognition by males (Arikawa et al., 2005;Wakakuwa et al., 2010). Yet another study has found that while both sexes of the wood tiger moth, Arcia plantaginis can distinguish between white and yellow male morphs (and females prefer to mate with white males), variation in female orange and red coloration is indiscriminable by both sexes, suggesting the moths' visual system has evolved to facilitate female choice (Henze et al., 2018). ...
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In true color vision, animals discriminate between light wavelengths, regardless of intensity, using at least two photoreceptors with different spectral sensitivity peaks. Heliconius butterflies have duplicate UV opsin genes, which encode ultraviolet and violet photoreceptors, respectively. In Heliconius erato, only females express the ultraviolet photoreceptor, suggesting females (but not males) can discriminate between UV wavelengths. We tested the ability of H. erato, and two species lacking the violet receptor, Heliconius melpomene and Eueides isabella, to discriminate between 380 and 390 nm, and between 400 and 436 nm, after being trained to associate each stimulus with a sugar reward. We found that only H. erato females have color vision in the UV range. Across species, both sexes show color vision in the blue range. Models of H. erato color vision suggest that females have an advantage over males in discriminating the inner UV-yellow corollas of Psiguria flowers from their outer orange petals. Moreover, previous models ( McCulloch et al., 2017) suggested that H. erato males have an advantage over females in discriminating Heliconius 3-hydroxykynurenine (3-OHK) yellow wing coloration from non-3-OHK yellow wing coloration found in other heliconiines. These results provide some of the first behavioral evidence for female H. erato UV color discrimination in the context of foraging, lending support to the hypothesis ( Briscoe et al., 2010) that the duplicated UV opsin genes function together in UV color vision. Taken together, the sexually dimorphic visual system of H. erato appears to have been shaped by both sexual selection and sex-specific natural selection.
... if the fitness of a phenotype depends on its relative frequency in a population (Henze et al., 2018;Hughes et al., 2013;Pérez i de Lanuza et al., 2017;Surmacki et al., 2013;Svensson, 2017). ...
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Balancing selection is important for the maintenance of polymorphism as it can prevent either fixation of one morph through directional selection or genetic drift, or speciation by disruptive selection. Polychromatism, the presence of multiple genetically determined colour phenotypes, can be maintained if the fitness of alternative morphs depends on the relative frequency in a population. In aggressive species, negative frequency‐dependent antagonism can prevent an increase in the frequency of rare morphs as they would only benefit from increased fitness while they are rare. Heterospecific aggression is common in nature and has the potential to contribute to rare morph advantage. Here we carry out field observations and laboratory aggression experiments with mbuna cichlids from Lake Malawi, to investigate the role of con‐ and heterospecific aggression in the maintenance of polychromatism and identify benefits to rare morphs which are likely to result from reduced aggression. We hypothesise that rare morph individuals receive less aggression than common morph individuals and therefore have an ecological advantage. Within species we found that males and females bias aggression towards their own morph, adding to the evidence that inherent own‐morph aggression biases can contribute to balancing selection. Over‐representation of rare morph territory owners may be influenced by two factors; higher tolerance of different morph individuals as neighbours, and ability of rare morphs to spend more time feeding. Reduced aggression to rare morph individuals by heterospecifics may also contribute to rare morph advantage.
... Balancing selection is important for the maintenance of polymorphism as it can prevent either fixation of one morph through directional selection or genetic drift, or speciation by disruptive selection (Huxley 1955;Wellenreuther et al. 2014;Kim et al. 2019). Polychromatism (colour polymorphism) can be maintained if the fitness of alternative morphs differs in time or space in heterogeneous environments, or if the fitness of a phenotype depends on its relative frequency in a population (Hughes et al. 2013;Pérez i de Lanuza et al. 2017;Surmacki et al. 2013;Svensson 2017;Henze et al. 2018). ...
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Abstract Balancing selection is important for the maintenance of polymorphism as it can prevent either fixation of one morph through directional selection or genetic drift, or speciation by disruptive selection. Polychromatism can be maintained if the fitness of alternative morphs depends on the relative frequency in a population. In aggressive species, negative frequency-dependent antagonism can prevent an increase in the frequency of rare morphs as they would only benefit from increased fitness while they are rare. Heterospecific aggression is common in nature and has the potential to contribute to rare morph advantage. Here we carry out field observations and laboratory aggression experiments with mbuna cichlids from Lake Malawi, to investigate the role of con- and heterospecific aggression in the maintenance of polychromatism and identify benefits to rare mores which are likely to result from reduced aggression. Within species we found that males and females bias aggression towards their own morph, adding to the evidence that inherent own-morph aggression biases can contribute to balancing selection. Over-representation of rare morph territory owners may be influenced by two factors; higher tolerance of different morph individuals as neighbours, and ability of rare morphs to spend more time feeding. Reduced aggression to rare morph individuals by heterospecifics may also contribute to rare morph advantage. Key words: Malawi, cichlid, blotch polymorphism, aggression, rare morph advantage
... A locally common forewing type was used to reduce potential novelty effect caused by the forewing pattern . Resemblance of the artificial models to the real moths was verified by taking measurements of reflectance from the black and coloured areas of real moth wings and printed wings with a Maya2000 Pro spectrometer (Ocean Optics) using a PX-2 Pulsed Xenon Light Source (Ocean Optics) for illumination and adjusting the model colours with Gimp (2.8.16) to match the natural wing colour as closely as possible with a calibrated (HP Colour LaserJet CP2025) printer (spectral match between printed moths and real wings was inspected by visual comparison of reflectance curves as in R€ onk€ a et al., 2018, where identical models were used, and a detailed avian vision model with JNDs' of all three morphs on different backgrounds is reported in Henze et al., 2018). Thus, we can expect all avian predators to see the moth dummies similarly as they would see the real moths, regardless of birds' visual properties, which may vary among species. ...
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Warning signals are predicted to develop signal monomorphism via positive frequency‐dependent selection (+FDS) albeit many aposematic systems exhibit signal polymorphism. To understand this mismatch, we conducted a large‐scale predation experiment in four countries, among which the frequencies of hindwing warning coloration of the aposematic moth, Arctia plantaginis, differ. Here we show that selection by avian predators on warning colour is predicted by local morph frequency and predator community composition. We found +FDS to be the strongest in monomorphic Scotland and lowest in polymorphic Finland, where the attack risk of moth morphs depended on the local avian community. +FDS was also found where the predator community was the least diverse (Georgia), whereas in the most diverse avian community (Estonia), hardly any models were attacked. Our results support the idea that spatial variation in predator communities alters the strength or direction of selection on warning signals, thus facilitating a geographic mosaic of selection. A geographic mosaic of selection by predators could explain the paradoxical maintenance of warning signal variation, but direct ecological evidence is scarce and focused on tropical systems. We monitored local avian predators and attacks on 4000 + moth models representing red, yellow or white warning colour morphs in a temperate moth system with natural variation in local morph frequencies. We found positive frequency‐dependent selection to be strongest in monomorphic populations and the direction and strength of selection to be significantly associated with local predator community composition and diversification, which can explain not only geographic variation (polytypism) but also local polymorphism when coupled with gene flow.
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Predators efficiently learn to avoid one type of warning signal rather than several, making colour polymorphisms unexpected. Aposematic wood tiger moth males Parasemia plantaginis have either white or yellow hindwing coloration across Eu-rope. Previous studies indicate that yellow males are better defended from predators, while white males have a positively frequency dependent mating advantage. However, the potential frequency-dependent behavioural differences in flight between the morphs, as well as the role of male-male interactions in inducing flying activity, have not been previously considered. We ran an outdoor cage experiment where proportions of both male morphs were manipulated to test whether flying activity was frequency-dependent and differed between morphs. The white morph was significantly more active than the yellow one across all treatments, and sustained activity for longer. Overall activity for both morphs was considerably lower in the yellow-biased environment, suggesting that higher proportions of yellow males in a population may lead to overall reduced flying activity. The activity of the yellow morph also followed a steeper, narrower curve than that of the white morph during peak female calling activity. We suggest that white males, with their presumably less costly defences, have more resources to invest in flight for predator escape and finding mates. Yellow males, which are better protected but less sexually selected, may instead compensate their lower flight activity by 'flying smart' during the peak female-calling periods. Thus, both morphs may be able to behaviourally balance the trade-off between warning signal selection and sexual selection. Our results emphasize the greater need to investigate animal behaviour and colour polymorphisms in natural or semi-natural environments [Current Zoology 61 (4): 765–772, 2015].
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