Abstract and Figures

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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LETTER Geographic mosaic of selection by avian predators on
hindwing warning colour in a polymorphic aposematic moth
Katja R
onk
a,
1,2,3
*
Janne K. Valkonen,
1
Ossi Nokelainen,
1
Bibiana Rojas,
1
Swanne Gordon,
1,4
Emily Burdfield-Steel
1,5
and
Johanna Mappes
1,3
Abstract
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 commu-
nities alters the strength or direction of selection on warning signals, thus facilitating a geographic
mosaic of selection.
Keywords
Aposematism, Arctia plantaginis, colour polymorphism, frequency-dependent selection, predator
prey interactions, predators, signal convergence, signal variation, wood tiger moth.
Ecology Letters (2020) 23: 1654–1663
INTRODUCTION
The survival strategy of aposematism, wherein prey use
warning signals that predators learn to associate with their
unprofitability and subsequently avoid, has stimulated bio-
logical studies for centuries (Wallace, 1867; Poulton, 1890;
Cott, 1940; Mappes et al., 2005; Merrill et al., 2015; Skel-
horn et al., 2016; Ruxton et al., 2018). In aposematism, prey
benefit from lowered costs of predator education by carrying
a common signal, whereas predators reduce risks by not
attacking defended prey. This results in selection for local
similarity in warning signals, a view that has been corrobo-
rated by theoretical approaches (e.g. M
uller, 1878; Mallet
and Joron, 1999; Sherratt, 2008; Aubier and Sherratt, 2015),
laboratory experiments (e.g. Greenwood et al., 1989; Lind-
str
om et al., 2001; Rowland et al., 2007) and field studies
(e.g. Mallet and Barton, 1989; Kapan, 2001; Borer et al.,
2010; Dell’aglio et al., 2016; Chouteau et al., 2016). Never-
theless, phenotypic variation and polymorphism in apose-
matic organisms are widespread in nature (e.g. frogs: Rojas,
2017; Siddiqi et al., 2004; newts: Beukema et al., 2016;
Mochida, 2011; butterflies: Merrill et al., 2015; moths:
Brakefield and Liebert, 1985; bumblebees: Plowright and
Owen, 1980; beetles: Bocek and Bocak, 2016; Brakefield,
1985; locusts: Nabours, 1929; myriapods: Marek and Bond,
2009; nudibranchs: Winters et al., 2017), which requires an
evolutionary explanation.
Given that the association between prey warning signal and
defence should be learned by each generation of predators
(Mappes et al., 2014), the benefit of signal sharing depends on
how often predators encounter the signal. The encounter rate
then depends on both the frequency (M
uller, 1879; Heino
et al., 1998) and density (M
uller, 1879; Sword, 1999; Rowland
et al., 2007; Endler and Rojas, 2009) of prey carrying the sig-
nal. Thus, it is expected that selection on aposematism is posi-
tively frequency-dependent (+FDS), with predators avoiding
the most common warning signal in a locality (Sherratt, 2008;
Comeault and Noonan, 2011; Chouteau and Angers, 2011;
Chouteau et al., 2016; Ruxton et al., 2018).
On the other hand, several mechanisms have been proposed
to counterbalance selection for signal monomorphism and
facilitate warning colour polymorphism (reviewed in Briolat
et al., 2018). For example temporally and spatially varying
interspecific interactions can result in geographically variable
patterns of polymorphism (McLean & Stuart-Fox 2014), par-
ticularly when coupled with limited amounts of gene flow
between differentially selected populations (e.g. Merilaita,
2001; Gordon et al., 2015; Aubier and Sherratt, 2015). Often
these mechanisms are thought to act simultaneously, or alter-
nate in time or space (Mallet and Joron, 1999; Gray and
McKinnon, 2007; Stevens and Ruxton, 2012) creating a geo-
graphic mosaic of selection (Thompson, 2005). Although both
theoretical (e.g. Gordon et al., 2015; Aubier and Sherratt,
2015; Holmes et al., 2017) and experimental work (e.g.
1
Department of Biological and Environmental Science, University of Jyv
askyl
a,
Jyv
askyl
a, Finland
2
Helsinki Institute of Life Sciences, University of Helsinki, Helsinki, Finland
3
Organismal and Evolutionary Biology Research Programme, Faculty of Bio-
logical and Environmental Sciences, University of Helsinki, Helsinki, Finland
4
Department of Biology, Washington University in St. Louis, St. Louis, MO,
USA
5
Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam,
Amsterdam, The Netherlands
*Correspondence: E-mail: katja.ronka@helsinki.fi
©2020 The Authors. Ecology Letters published by John Wiley & Sons Ltd.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use,
distribution and reproduction in any medium, provided the original work is properly cited.
Ecology Letters, (2020) 23: 1654–1663 doi: 10.1111/ele.13597
Willink et al., 2014; Aluthwattha et al., 2017) have identified
several mechanisms that allow multiple morphs to persist,
there is no conclusive evidence from the field and the relative
importance of different selective agents is not well understood
(Stevens and Ruxton, 2012; Chouteau et al., 2016). Alas, there
is little empirical evidence as to the role of predator communi-
ties on local or global morph frequencies of aposematic prey.
The variation in the degree of warning colour polymor-
phism shown by the wood tiger moth (Arctia plantaginis)
across the Western Palaearctic provides an excellent system to
study how warning signal variation is maintained in the wild
(Hegna et al., 2015). At a local scale, predator community
structure (Nokelainen et al., 2014) and sexual selection (Noke-
lainen et al., 2012; Gordon et al., 2015) have been found to
alter the direction of selection on white and yellow male
morphs, but no previous studies have addressed selection on a
wide geographical scale and including A. plantaginis females,
which are red or yellow. We exposed artificial moths repre-
senting the three hindwing colour morphs (white, yellow, red),
to local predators in a field experiment spanning across four
countries, while monitoring the abundance and community
structure of local predator species. We tested whether (1)
selection by predators favours the locally common morph; (2)
the community structure of avian predators is associated with
the predation pressure on different morphs; and (3) there is
variation in the direction or strength of selection among pop-
ulations, matching the local morph frequencies. Variable selec-
tion pressure is one of the main candidate mechanisms for the
maintenance of polymorphism. By our work, we provide the
best-documented case to date of a geographic mosaic of selec-
tion on warning signals at broad spatial scales.
MATERIAL AND METHODS
Study system
Adult wood tiger moths, Arctia plantaginis (Erebidae: Arcti-
inae; formerly Parasemia; see R
onk
aet al., 2016 for classifica-
tion), show conspicuous warning colours and possess a
chemical defence fluid, which contains pyrazines (Rojas et al.,
2017; Burdfield-Steel et al., 2018b) and is a deterrent to avian
predators (Rojas et al., 2017; Burdfield-Steel et al., 2018a).
Their warning coloration varies throughout their Holarctic
distribution, but local polymorphism is common too (Hegna
et al., 2015). In the Western Palaearctic male hindwing colour
is either white or yellow, or varies more continuously between
yellow and red as seen in females. We selected four study
locations that represent the colour variation continuum from
monomorphic to polymorphic Arctia plantaginis populations
in the Western Palaearctic (Fig. 1). For the purposes of this
study, we assigned both sexes to belong to the white, yellow
or red morph based on their hindwing colour, and simplify
the continuous hindwing coloration of females and Georgian
males into two classes: yellow and red (categorised by human
eye in 6 grades as in Lindstedt et al., 2011), here grades 1-2
are determined yellow and 3-6 red). Accordingly, Scotland is
monomorphic with yellow males and females, Georgia is
mostly red with 4.3% of males being yellow and Estonia and
Finland are polymorphic with all females caught between
2013 and 2015 classified as red, and males as either white or
yellow (Fig. 1).
Wood tiger moths are widespread but often low in numbers.
Therefore, colour morph frequencies were calculated by popu-
lation based on annual surveys using pheromone traps and
netting between 2009 and 2014 in Scotland, and 20132015 in
Georgia, Estonia and Finland. Morph frequencies for white
and yellow males, and yellow or red males in Georgia, were
calculated as the average frequencies from all data available.
Morph frequencies for yellow and red females were based on
netting data, as the pheromone traps only lure male moths.
Because our data set was thus biased towards male moths, we
corrected the morph frequencies according to a sex ratio of 45
females to 156 males, based on a markreleaserecapture
study spanning two years in Central Finland (Gordon et al.
unpublished data). This sex ratio was used, as it is likely to
depict the detectability of each morph more accurately than
an even 1:1 sex ratio. The higher frequency of males to
females is supported by two observations: male wood tiger
moths live longer and fly more actively than females, and the
adult sex ratio immediately after eclosion is slightly biased
towards males even in laboratory conditions (K. Suisto, per-
sonal communication). The concluding morph frequencies
(Fig. 1a and 2a) are consistent with museum samples (Hegna
et al., 2015) and laboratory stocks originating from the four
study populations (Central-Finland, Estonia, Scotland and
Georgia).
Predation experiment
To estimate the attack risk of white, yellow and red hindwing
colour morphs by local predators in the wild we used artificial
moth models, resembling real A. plantaginis morphs. Models
with plasticine (Caran D’Ache Modela 0259.009 Black) bodies
attached to printed waterproof (Rite in the Rain ©, JL Dar-
ling Corporation, Tacoma, WA, USA) paper wings were pre-
pared following methods described in Nokelainen et al.
(2014). Models were constructed using pictures of one white
moth hindwing and two forewings, one with a typical Euro-
pean pattern and another with a typical Caucasian (Georgian)
pattern, which were copied and assembled in GIMP 2.8.16
SOFTWARE (GNU Image manipulation program) to create
six models representing the white, yellow and red morphs in
Europe and Georgia (Fig. 1b). A locally common forewing
type was used to reduce potential novelty effect caused by the
forewing pattern (Hegna and Mappes, 2014). 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 Col-
our LaserJet CP2025) printer (spectral match between printed
moths and real wings was inspected by visual comparison of
reflectance curves as in R
onk
aet 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
©2020 The Authors. Ecology Letters published by John Wiley & Sons Ltd.
Letter Geographic mosaic of selection on warning colour 1655
predators to see the moth dummies similarly as they would
see the real moths, regardless of birds’ visual properties, which
may vary among species. As our study focused on the hind-
wing coloration, all other variables such as wing size and pat-
tern were kept constant.
We set up 60 predation transects across the four study pop-
ulations (15 in each country) in open, semi-open and closed
natural habitats where the wood tiger moth and its potential
avian predators were known or presumed to occur. The pre-
dation transects were set at least 500 m apart to avoid birds
having overlapping territories between the transects. Along
each 900 m transect 20 white, 20 yellow and 20 red artificial
moth models were set individually every 15 meters using a
randomised block design, so that two models of the same col-
our would never be next to each other. Models were pinned
directly on natural vegetation, either to green leaves large
enough to support their weight, or to tree trunks, as visibly as
possible. All models were left in the field for a maximum of
6 days (26 days, 4 days on average), during the A. plan-
taginis flight season in 2014 (May 31st July 6th in Estonia,
May 26thJuly 6th in Finland, June 15thJuly 30th in Scot-
land and July 12thAugust 3rd in Georgia). Predation events
were recorded every 24 hours except for days of heavy rain
(as birds were likely not active). For practical reasons (i.e.
accessibility of mountain roads and weather conditions) the
protocol was modified in Georgia. The 20 white, 20 yellow
and 20 red models were set every 10 m totalling up to 600 m,
left in the field for three consecutive days (72 h), and checked
only once.
Attacks were recorded based on imprints on the plasticine
body and fractures in the wings (see Supplemental Experimen-
tal Procedures). Only clear avian attacks were included in the
analyses (Tables S1 and S2). Missing and attacked models
were replaced with a new model of the same colour to ensure
constant morph frequency during the experiment. Excluding
or keeping consecutive attacks on the replaced models in the
analyses did not markedly change the outcome, reported here
(Table 1) for the data set including replaced models (4004
observations) and for the data set including original models
only (3600 observations; in Supplemental Table S3). There-
fore, we kept the replaced models in for all of the analyses, as
it increased the sample size.
Measures of predator community
To estimate the abundances of different insect-feeding birds,
which are the most likely predators of wood tiger moths, we
counted birds belonging to the orders Passeriformes and Pici-
formes (Supplemental Table S3). These counts were done
once, either before or during the predation experiment, along
(a)
(b)
Figure 1 (a) Study populations and local morph frequencies and (b) moth models used in Europe and Georgia representing a local forewing pattern and
white, yellow or red hindwing morph. At the monomorphic end of the monomorphic-polymorphic continuum in Scotland, both sexes have yellow
hindwing coloration. In Georgia red is the dominating hindwing colour, but male coloration varies continuously towards yellow. In Estonia, white
hindwing colour dominates, as the males are almost exclusively white and females red. In Southern Finland all three colour morphs are present (white and
yellow male morphs and females vary continuously from yellow to red).
©2020 The Authors. Ecology Letters published by John Wiley & Sons Ltd.
1656 K. R
onk
aet al. Letter
the predation lines using a modified transect count method
(see Nokelainen et al., 2014). Bird species observed only in
one transect (out of 60), or clearly not adapted to prey on
moths (e.g. crossbills), were excluded from analysis. Observa-
tions were done within 25 m from the middle of the transect
in calm weather between 6 am and 1 pm, when birds were
most active. ShannonWiener diversity index (Fig. 3) was cal-
culated using R package ‘vegan’ 2.5-6 (Oksanen et al. 2013).
Statistical analyses
To investigate how local predator community affects the
direction and strength of selection on wood tiger moth
morphs, we constructed generalised linear mixed models.
Because the artificial moths were presented to predators over
a different number of days in each transect, the attack risk
(attacked or not) within a day exposed was used as the
response variable for all analyses, modelled with a binomial
distribution and a logit link function. First, we tested whether
predators select for wood tiger moth warning colours in a fre-
quency-dependent manner across populations (Fig. 2). For
this, we used local morph frequency calculated from field
monitoring data and its interaction with colour morph as the
explanatory variables in Model 1 (Table 1). Transect ID,
nested within country, was set as the random factor to
account for the nested spatial structure of the study design.
To test for predator community composition effects, the
dimensions of the bird count data, consisting of 12 genera,
was first reduced with a principal component analysis using
the R function ‘princomp’. To avoid overparameterisation,
the main effects of the first three resulting components (ex-
plaining 44.7 %, 33.7 % and 8.5 % of the variation in preda-
tor community), and their three-way interactions with morph
colour and country, were included one by one as explanatory
variables in three separate GLMMs (Table 2). Country was
included as an explanatory variable to test for local
differences in selection and thus transect ID alone was set as
a random effect to each model.
Statistical models were simplified using a backward stepwise
deletion method based on significance of terms in the models.
Variables were excluded one by one from the full models,
until only main effects or significant interactions were left in
each model. All analyses were performed with R (RCore-
Team, 2013) in RStudio 0.99.491 (RStudio Team, 2015), using
the package lme4 (Bates et al. 2015).
RESULTS
Positive frequency-dependent selection
Altogether, we observed a total of 718 bird attacks on the
4004 artificial moths (Table S1). The relative attack risk of
each colour morph was lower when the natural frequencies of
the respective morph were higher in relation to the others
(Table 1, Fig. 2). Also, the morphs with intermediate local
frequencies show corresponding levels of attack risk (Fig. 2).
This effect did not depend on colour morph itself (Table 1,
Table S2), as expected if the local predator avoidance depends
more on local morph frequency than on morph colour.
Predator community
The attacks were not evenly distributed across countries or
transects (Fig. 2c). Predation pressure varied between and
within countries, being highest in Scotland and lowest in Esto-
nia (Fig. 2c). Georgia had the lowest amount of insect feeding
birds observed (2.1 per 100 meters) compared to Finland
(2.6), Scotland (4.0) and Estonia (4.4) respectively. Georgia
also had the least diverse predator community measured with
ShannonWiener diversity index, whereas Estonia was most
diverse, followed by Scotland (Fig. 3). Across countries, the
three most commonly observed potential predators included
the common chaffinch, the willow warbler (replaced by green
warbler in Georgia) and the great tit (Supplemental Table S4),
the latter of which was observed to attack the artificial moths.
The first three principal components (PC1, PC2 and PC3) that
explained 44.7 %, 33.7 % and 8.5 %, respectively, captured
87.0 % of variance in the predator community data. PC1 was
dominated by Sylvidae (warblers), Fringillidae (finches) and
Muscicapidae (flycatchers), which loaded in the negative end,
whereas the positive end of the axis was loaded with Paridae
(tits) (Fig. 3). PC2 was dominated by Paridae and PC3 with
Fringillidae, Muscicapidae and Troglodytidae (the Eurasian
wren) (see Supplemental Table S5 for factor loadings).
Significant association between predator community structure and
selection
A consecutive analysis, where the effect of predator commu-
nity on the attack risk of each moth colour morph was
addressed, revealed a significant three-way-interaction between
moth colour, country and PC1 (Model 2, Table 2a, Fig. 3).
This significant interaction means that the variation in preda-
tor community structure captured by PC1 is associated with
predation pressure on different colour morphs, but the
Table 1 Positive frequency-dependency of the estimated attack risk
(a) Model selection Dd.f. LRT Pr(Chi) model AIC
colour *morph frequency 3957.0
colour +morph frequency 2 0.1949 0.907 3953.2
(b) Model 1a
Random effects Variance SD
transect within country 0.3315 0.5758
country 0.1633 0.4041
Fixed effects Estimate SE Z-value P-value
(Intercept): colour[w] 3.0433 0.2275 13.376 <0.001
colour[y] 0.0923 0.0940 0.982 0.3259
colour[r] 0.0841 0.0925 0.909 0.3633
morph frequency 0.3728 0.1071 3.481 0.0005
The asterisk (
*
) denotes both main effects and interaction terms used.
(a) The model including only main effects of morph frequency and morph
colour (underlined) was selected because we did not find a significant
interaction. (b) Estimates of Model 1a. Values of significance level <0.05
are bolded. Dd.f. denotes change in model degrees of freedom.
©2020 The Authors. Ecology Letters published by John Wiley & Sons Ltd.
Letter Geographic mosaic of selection on warning colour 1657
direction of the association is different between countries (i.e.
between local communities). PC2 and PC3 were not signifi-
cantly associated with predation pressure (Table 2c and 2d).
DISCUSSION
Our experiment is among the few experimental approaches
integrating community-level interactions into the study of
selection on warning signals (Mochida, 2011; Valkonen et al.,
2012; Nokelainen et al., 2014; Aluthwattha et al., 2017), and
the first to do so on such a large geographical scale. With a
wide-ranging field experiment spanning populations varying in
their degree of polymorphism, we demonstrate that local bird
predators avoid locally common morphs, but also that both
the strength and direction of selection on warning colour
varies geographically. We found that changes in local preda-
tor communities drive geographical variation in selection
despite positive frequency-dependence. Local predatorprey
interactions are thus contributing to the maintenance of both
geographical variation and local polymorphism in warning
signals.
Local avian predators appear as key in driving warning col-
our evolution, which can take different evolutionary trajecto-
ries over a geographical scale. Here, the effect of predator
community on attack risk towards each morph varied signifi-
cantly across countries. The first component from the princi-
pal component analysis, explaining 44.7 % of the variation in
the abundances of insectivorous birds in different families, sig-
nificantly affected estimated risk of attack. However, it did so
differently towards each morph in the different countries.
Interpreting the component loadings and model estimates
(Table 2, Fig. 3), the Paridae (e.g. tits) and Prunellidae (con-
sisting of only one species, the dunnock, Prunella modularis)
selected for different morphs in different countries. Our results
corroborate the predator community effects found by Noke-
lainen et al. (2014), as we also found that in Finland the
(a)
(b)
(c)
(d)
Figure 2 Wood tiger moth morph frequencies compared to expected and observed predation risk and selection differential by country. (a) Local morph
frequencies calculated from annual monitoring data, (b) expected attack risk according to the +FDS hypothesis, where each morph is attacked according
to its local frequency, (c) observed predation illustrated as GLMM estimates of daily attack risk for each morph by country and (d) observed differencein
attacks per morph compared to a situation where all morphs would be attacked equally. Morph colours (white, yellow, red) as in Figure 1.
©2020 The Authors. Ecology Letters published by John Wiley & Sons Ltd.
1658 K. R
onk
aet al. Letter
yellow morph is better protected in communities characterised
by Paridae, whereas the white morph is favoured in communi-
ties characterised by Prunellidae. In contrast, an opposite
effect was found in Scotland where the yellow morph domi-
nates, suggesting that local predators can select for different
colours depending on the communities they are exposed to.
Our experiment showed that across countries locally domi-
nating colour morphs were attacked least, as predicted
by +FDS. Thus, warning signal efficacy is enhanced with
increasing frequency of similarly signalling individuals as pre-
dicted due to the number-dependence of predator learning
and memorisation. Nonetheless, we found geographical varia-
tion in the strength of predator-induced selection. Comparison
with previous experiments in those study areas that overlap
(Nokelainen et al., 2014) also reveal temporal differences. We
found high overall predation pressure in Scotland where the
yellow morph was in favour compared to other study loca-
tions. Although Nokelainen et al. (2014) did not detect posi-
tive frequency dependency, they also found much higher
overall attack rates in Scotland compared to Southern Fin-
land and Estonia. On the other hand, Nokelainen et al. (2014)
found that yellow males were significantly less attacked than
white males in Southern Finland, whereas in our study the
yellow morph tended to have more attacks than the other
morphs. Interestingly, the frequency of yellow and white
morphs varies in Southern Finland in a biannual cycle
(Galarza et al., 2014), and the yellow morph was more com-
mon during Nokelainen et al.’s (2014) study, whereas in con-
trast the white was more common during our experiment,
suggesting again that the locally most common morphs have
an advantage. Temporal fluctuations in local predatorprey
interactions could therefore plausibly explain why estimates of
predation pressure on different colour morphs conducted in
different years have varied.
All morphs were attacked at equally low levels in Estonia,
which implies spatial variation in the strength of selection or
even locally relaxed natural selection on the warning signal.
The low predation pressure is not explained by a low number
of predators, as there were more insectivorous birds in Esto-
nia than in any other study site (Table S4). The bird commu-
nity composition in Estonia differed from the other countries
though, being most diverse and characterised by Sylvidae,
Fringillidae, Muscicapidae, Turdidae, Troglodytidae and Ori-
olidae, suggesting that the strength of selection was lower in
Table 2 The interaction effect of predator community and location (coun-
try, C) on the attack risk towards the wood tiger moth colour morphs
(a) Model selection with PC1
D
d.f. LRT
Pr
(Chi)
model
AIC
PC1*colour*C 3956.1
PC1 +colour+C+PC1:colour +PC1:
C+colour:C
6 14.35 0.026 3958.4
(b) Model 2
Random
effects Variance SD
Transect 0.2779 0.5272
Fixed effects Estimate SE Z-value P-value
(Intercept): colour[w], C[Finland] 3.556 0.438 8.126 <0.001
colour[y] 0.869 0.407 2.134 0.033
colour[r] 0.370 0.440 0.840 0.401
PC1 0.139 0.088 1.590 0.112
C[Estonia] 0.309 0.621 0.498 0.619
C[Georgia] 0.618 0.647 0.955 0.340
C[Scotland] 0.843 0.477 1.766 0.077
PC1: colour[y] 0.164 0.083 1.989 0.047
PC1: colour[r] 0.115 0.089 1.299 0.194
C[Estonia]: colour[y] 0.463 0.606 0.765 0.444
C[Estonia]: colour[r] 0.297 0.618 0.481 0.631
C[Georgia]: colour[y] 0.773 0.592 1.306 0.192
C[Georgia]: colour[r] 0.041 0.616 0.067 0.947
C[Scotland]: colour[y] 1.169 0.445 2.624 0.009
C[Scotland]: colour[r] 0.345 0.472 0.730 0.466
PC1: C[Estonia] 0.123 0.097 1.273 0.203
PC1: C[Georgia] 0.143 0.111 1.280 0.201
PC1: C[Scotland] 0.159 0.103 0.551 0.121
C[Estonia]: PC1: colour[y] 0.197 0.093 2.115 0.035
C[Estonia]: PC1: colour[r] 0.176 0.099 1.781 0.075
C[Georgia]: PC1: colour[y] 0.105 0.105 0.991 0.322
C[Georgia]: PC1: colour[r] 0.033 0.113 0.294 0.769
C[Scotland]: PC1: colour[y] 0.256 0.100 2.550 0.011
C[Scotland]: PC1: colour[r] 0.091 0.102 0.892 0.372
(c) Model selection with PC2
D
d.f. LRT
Pr
(Chi)
Model
AIC
PC2*colour*country 3962.6
PC2 +colour+C+PC2:colour +PC2:
C+colour:C
6 4.413 0.621 3955.0
PC2 +colour+C+PC2:colour +colour:
C
3 3.334 0.343 3952.4
PC2 +colour+C+colour:C 2 3.923 0.141 3952.3
PC2 +colour+C 6 16.737 0.010 3957.0
Colour*country 1 0.545 0.460 3950.8
(d) Model selection with PC3
D
d.f. LRT
Pr
(Chi)
Model
AIC
PC3*colour*country 3963.6
PC3 +colour+C+PC3:colour +PC3:
C+colour:C
6 5.190 0.520 3956.8
PC3 +colour+C+PC3:colour +colour:
C
3 1.400 0.706 3952.2
PC3 +colour+C+colour:C 2 4.599 0.100 3952.8
PC3 +colour+C 6 16.726 0.010 3957.5
Colour*country 1 0.034 0.854 3950.8
(a) Model selection starting from the main effects, interactions and a
three-way interaction between principal component 1 (PC1), country (C)
and colour morph (colour) as the explanatory variables, with the best-fit
model underlined. (b) Estimates from the selected model (Model 2) with a
significant three-way interaction of principal component 1, colour morph
and country. (c) Model selection for principal component 2. Principal
component 2 had no significant effects on attack risk, and thus estimates
are not shown. (d) Model selection for principal component 3. Principal
component 3 had no significant effects on attack risk, and thus estimates
are not shown. Values of significance level <0.05 are bolded. Dd.f.
denotes change in model degrees of freedom.
The asterisk (*) denotes both main effects and interaction terms used.
(continues)
©2020 The Authors. Ecology Letters published by John Wiley & Sons Ltd.
Letter Geographic mosaic of selection on warning colour 1659
diverse communities as opposed to when Paridae (e.g. tits)
characterised the community.
Other properties of the predator community that can affect
the strength of selection on warning signals include the rela-
tive abundance of na
ıve vs. experienced predators (Mappes
et al., 2014), predators’ capacity to learn many different sig-
nals (Beatty et al., 2004), broad generalisation between the
morphs (Balogh and Leimar, 2005; Sherratt, 2008), conflicting
selection by different predators (Valkonen et al., 2012; Noke-
lainen et al., 2014), and the spatial arrangement of predators
in relation to prey (Endler and Rojas, 2009). It is also possible
that moth behaviour (e.g. flight activity) varies between habi-
tats, sexes or colour morphs, leading to differences in expo-
sure to potential predators (see e.g. Rojas et al., 2015). While
long distance mate attraction is achieved through female pher-
omone signalling, there is evidence of differential mating suc-
cess between the white and yellow male morphs in Finland
(Nokelainen et al., 2012). Thus, sexual selection may well play
a role in maintaining polymorphism in the Finnish population
(Gordon et al., 2015), but its role in other populations
remains unknown.
In temperate regions, most insectivorous birds are migratory
and prey population sizes are highly variable due to intersea-
sonal weather variability. This is likely to cause variation in
the relative abundances of na
ıve predators across the breeding
season. Furthermore, local seasonal communities are continu-
ously changing, altering the direction and/or strength of selec-
tion on warning signals (Mappes et al., 2014). Siepielski et al.
(2013) reviewed directional selection on phenotypes, and
found that selection tends to vary more in strength than in
direction between populations, with the majority of their
examples coming from mid-latitudes in the northern hemi-
sphere. Most experimental evidence of +FDS in the wild,
however, comes from tropical systems (Mallet and Barton,
1989; Comeault and Noonan, 2011; Chouteau and Angers,
2011), where the prey and predator community composition is
temporally less variable (Mittelbach et al., 2007). In such com-
munities, strong +FDS can lead to very accurate mimicry
between warning coloured prey, whereas in more variable
conditions, higher levels of variation and polymorphism can
be maintained.
The paradoxical maintenance of local polymorphism
despite +FDS could thus be explained by spatial and tempo-
ral variation in morph survival combined with individuals
migrating between the subpopulations (Gordon et al., 2015;
McLean & Stuart-Fox 2014; Joron et al., 1999). Differences
in the level of population isolation, and thus gene flow
between them, could explain part of the geographical varia-
tion in wood tiger moth warning colours. Population genetic
evidence supports this theory: the red-dominated Georgian
subspecies A. p. caucasica occurring in the mountains of Cau-
casus is genetically differentiated from all other sampled
European populations, having a distinctive genomic composi-
tion from the rest of Western Palaearctic samples (Hegna
et al., 2015; R
onk
aet al., 2016; Yen et al., 2020). The Finnish
and Estonian populations clustered together away from the
Scottish population, as would be predicted by effects of isola-
tion by distance (Yen et al., 2020). Thus, predator selection
against rarity and genetic isolation of Georgian and Scottish
populations can explain the monomorphism in those popula-
tions. Furthermore, the long-term co-existence of multiple
morphs and the low genetic differentiation among
Figure 3 Community composition and diversity of insectivorous birds per population. Factor scores and loading of the first principal component describing
44.7% of the total variation in bird communities across countries. Panel (a) family level component loadings, panel (b) the three-way interaction effect of
predator community on estimated attack risk of different colour morphs (black line corresponds to the white morph) at each transect illustrated by
population, panel (c) ShannonWiener diversity indexes calculated per transect and plotted by population.
©2020 The Authors. Ecology Letters published by John Wiley & Sons Ltd.
1660 K. R
onk
aet al. Letter
polymorphic populations in Finland with yearly variation in
genetic structure (Galarza et al., 2014) indicate a role for gene
flow along with varying predation pressure in maintaining
local populations at different frequencies. Despite the genetic
similarity and gene flow between Estonian and Finnish popu-
lations, Estonian moth populations are practically monomor-
phic white, whereas Finnish populations are polymorphic.
Although the populations are connected, bird communities
are remarkably different between them. Thus, we suggest that
relaxed predator selection in Estonia together with sexual
selection that seems to favour white morphs, especially when
common, (see Nokelainen et al., 2012; Gordon et al., 2015)
may explain the white dominance of this population.
As recently noted by several authors (e.g. Nokelainen et al.,
2014; Skelhorn et al., 2016; Chouteau et al., 2016), more
experimental work is needed to clarify predatorprey interac-
tions at the community level in order to understand how
selection is driving the evolution of warning signals in diverse
natural ecosystems. Our experiment is so far the most compre-
hensive analysis showing how spatio-temporal variation in
predatorprey communities affects the maintenance of within-
species variation and evolutionary pathways to biodiversity. It
shows that, while +FDS is acting in most populations, spatial
variation in predator and prey communities alters the strength
or direction of selection on warning signals, thus facilitating a
geographical mosaic of selection which can maintain polymor-
phism.
ACKNOWLEDGEMENTS
We thank T~
onis Tasane, Sally Rigg, Heli Kinnunen, Diana
Abondano and Gocha Golubiani for help with the predation
experiments; Otso H
a
ar
a, for counting birds in Finland; peo-
ple at JYU who helped cutting out moth models; Kaisa
Suisto, D. Abondano, Liisa H
am
al
ainen, Morgan Brain and
Sini Burdillat, for rearing moths; and Jimi Kirvesoja for
managing databases. The Agency of Protected Areas from the
Georgian Ministry of Environment and Natural Resources
Protection, provided the collection permits for Georgia; we
are indebted to Khatuna Tsiklauri for help, and to the Okro-
melidze family for hospitality. Members of the ‘Plantaginis
Journal Club’, Chris Jiggins, Mathieu Joron, Brice Noonan,
Rose Thorogood, Øysten Opedal and three anonymous
reviewers provided comments that improved an earlier version
of the manuscript. This study was funded by the Academy of
Finland (Projects 284666 and 320438 to JM). The authors
have no conflict of interest to declare.
AUTHORSHIP
JM, KR, ON, JV, BR and SG designed the experiment. All
authors contributed to the fieldwork. KR, JV and ON did the
statistical analysis. KR led the writing and all authors con-
tributed to it and accepted the final version.
PEER REVIEW
The peer review history for this article is available at https://
publons.com/publon/10.1111/ele.13597.
DATA ACCESSIBILITY STATEMENT
Data available from the Figshare Repository: https://doi.org/
10.6084/m9.figshare.12750797.
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SUPPORTING INFORMATION
Additional supporting information may be found online in
the Supporting Information section at the end of the article.
Editor, Greg Grether
Manuscript received 9 April 2020
First decision made 16 May 2020
Manuscript accepted 28 July 2020
©2020 The Authors. Ecology Letters published by John Wiley & Sons Ltd.
Letter Geographic mosaic of selection on warning colour 1663
... From an evolutionary standpoint, it is plausible that interplay between natural and sexual selection facilitates polymorphism in this species (Gordon et al., 2015Nokelainen et al., 2012;Rönkä et al., 2020). Since male wood tiger moths, which are actively searching for females in the vegetation, have limited ability to see differences in yellow-orange-red hues (Henze et al., 2018), it is unlikely that sexual selection alone would be responsible for the colouration of females, but we cannot exclude the possibility that male colouration could be used in intraspecific commu- For tetrachromatic avian vision uv, sw and lw sensitivities were used for blue, green and red channels respectively. ...
... Also, avian predators may distinguish between the nuances in colouration among the genotypes as they, and wood tiger moths, perceive UV wavelengths that are beyond human perception (Henze et al., 2018). Thus, ecologically relevant receivers, predators and conspecifics, may exert different selection pressures on visual signals beyond our perception (Endler, 1978) and maintain colour polymorphism in natural conditions (Galarza et al., 2014;Mochida, 2011;Nokelainen et al., 2014;Rönkä et al., 2020). ...
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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.
... From an evolutionary standpoint, it is plausible that interplay between natural and sexual selection facilitates polymorphism in this species (Gordon et al., 2015Nokelainen et al., 2012;Rönkä et al., 2020). Since male wood tiger moths, which are actively searching for females in the vegetation, have limited ability to see differences in yellow-orange-red hues (Henze et al., 2018), it is unlikely that sexual selection alone would be responsible for the colouration of females, but we cannot exclude the possibility that male colouration could be used in intraspecific commu- For tetrachromatic avian vision uv, sw and lw sensitivities were used for blue, green and red channels respectively. ...
... Also, avian predators may distinguish between the nuances in colouration among the genotypes as they, and wood tiger moths, perceive UV wavelengths that are beyond human perception (Henze et al., 2018). Thus, ecologically relevant receivers, predators and conspecifics, may exert different selection pressures on visual signals beyond our perception (Endler, 1978) and maintain colour polymorphism in natural conditions (Galarza et al., 2014;Mochida, 2011;Nokelainen et al., 2014;Rönkä et al., 2020). ...
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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.
... Additional experiments are needed to determine the interactive effects of predator's generalization, larval group size, larval defensive displays, and color. To assess whether predation risk for yellow-and white-bodied larvae varies among N. lecontei populations, future research should also test for differences in the survival of white-and yellow-bodied larvae across their geographical range and across different visual backgrounds (Rojas et al. 2014;Rönkä et al. 2020). ...
... Thus, rather than strong top-down selection by predators and pathogens, bottom-up selection via host-plant characteristics may have driven shifts in warning color among N. lecontei populations. This finding contrasts with strong top-down selection pressures that may explain endogenously produced polymorphic and polytypic warning color in Arctia plantaginis (Rönkä et al. 2020) and polytypic warning color in Heliconius melpomene. Historical demographic analyses suggest that H. melpomene radiated into areas already occupied by another, more abundant aposematic species, H. erato (e.g., Turner et al. 1996;Mallet and Joron 1999;Kronforst and Gilbert 2008;Quek et al. 2010). ...
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Our understanding of how novel warning color traits evolve in natural populations is largely based on studies of reproductive stages and organisms with endogenously produced pigmentation. In these systems, genetic drift is often required for novel alleles to overcome strong purifying selection stemming from frequency‐dependent predation and positive assortative mating. Here, we integrate data from field surveys, predation experiments, population genomics, and phenotypic correlations to explain the origin and maintenance of geographic variation in a diet‐based larval pigmentation trait in the redheaded pine sawfly (Neodiprion lecontei), a pine‐feeding hymenopteran. Although our experiments confirm that N. lecontei larvae are indeed aposematic—and therefore likely to experience frequency‐dependent predation—our genomic data do not support a historical demographic scenario that would have facilitated the spread of an initially deleterious allele via drift. Additionally, significantly elevated differentiation at a known color locus suggests that geographic variation in larval color is currently maintained by selection. Together, these data suggest that the novel white morph likely spread via selection. However, white body color does not enhance aposematic displays, nor is it correlated with enhanced chemical defense or immune function. Instead, the derived white‐bodied morph is disproportionately abundant on a pine species with a reduced carotenoid content relative to other pine hosts, suggesting that bottom‐up selection via host plants may have driven divergence among populations. Overall, our results suggest that life stage and pigment source can have a substantial impact the evolution of novel warning signals, highlighting the need to investigate diverse aposematic taxa to develop a comprehensive understanding of color variation in nature. This article is protected by copyright. All rights reserved
... Yellow morphs have stronger chemical defences (Rojas et al., 2017), 71 but show reduced flight activity compared to white males, although yellows may fly at more 72 selective times, i.e. at peak female calling periods (Rojas, Gordon and Mappes, 2015). In 73 summary, there is a trade-off between natural selection through predation and reproductive 74 success, which contributes to the maintenance of this polymorphism (Rönkä et al., 2020). 75 ...
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Colour is often used as an aposematic warning signal, with predator learning expected to lead to a single colour pattern within a population. However, there are many puzzling cases where aposematic signals are also polymorphic. The wood tiger moth, Arctia plantaginis , uses bright hindwing colours as a signal of unpalatability, and males have discrete colour morphs which vary in frequency geographically. In Finland, both white and yellow morphs can be found, and these colour morphs also differ in behavioural and life-history traits. Complex polymorphisms such as these are often explained by supergenes. Here, we show that male colour is linked to an extra copy of a yellow family gene that is only present in the white morphs. This white-specific duplication, which we name valkea , is highly upregulated during wing development, and could act to reduce recombination, thus potentially representing a supergene. We also characterise the pigments responsible for yellow, white and black colouration, showing that yellow is partly produced by pheomelanins, while black is dopamine-derived eumelanin. The yellow family genes have been linked to melanin synthesis and behavioural traits in other insect species. Our results add to only a few examples of seemingly paradoxical and complex polymorphisms which are associated with single genes.
... A geographic mosaic theory of coevolution (Thompson 1999) can predict that heterogeneous predator selection may facilitate multiple prey appearances (Rönkä et al. 2020). With this respect, variable light conditions have been identified as a potential source of heterogeneous selection that could result in the emergence of such phenotype variation (Rojas et al. 2014;Tate et al. 2016;Passarotto et al. 2018;Kranz et al. 2018). ...
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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.
... Therea are also differences between genotypes in the white hue of the forewings, which is perceptible to birds (Nokelainen et al., in prep a). The color polymorphism is under selection by bird predators in the wild (Rönkä et al., 2020). In predation experiments, birds respond differently toward the hindwing morphs, avoiding either yellow (Nokelainen et al., 2012(Nokelainen et al., , 2014 or white , but see Rönkä et al. (2018). ...
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Aposematic organisms warn predators of their unprofitability using a combination of defenses, including visual warning signals, startling sounds, noxious odors, or aversive tastes. Using multiple lines of defense can help prey avoid predators by stimulating multiple senses and/or by acting at different stages of predation. We tested the efficacy of three lines of defense (color, smell, taste) during the predation sequence of aposematic wood tiger moths ( Arctia plantaginis ) using blue tit ( Cyanistes caeruleus ) predators. Moths with two hindwing phenotypes (genotypes: WW/Wy = white, yy = yellow) were manipulated to have defense fluid with aversive smell (methoxypyrazines), body tissues with aversive taste (pyrrolizidine alkaloids) or both. In early predation stages, moth color and smell had additive effects on bird approach latency and dropping the prey, with the strongest effect for moths of the white morph with defense fluids. Pyrrolizidine alkaloid sequestration was detrimental in early attack stages, suggesting a trade-off between pyrrolizidine alkaloid sequestration and investment in other defenses. In addition, pyrrolizidine alkaloid taste alone did not deter bird predators. Birds could only effectively discriminate toxic moths from non-toxic moths when neck fluids containing methoxypyrazines were present, at which point they abandoned attack at the consumption stage. As a result, moths of the white morph with an aversive methoxypyrazine smell and moths in the treatment with both chemical defenses had the greatest chance of survival. We suggest that methoxypyrazines act as context setting signals for warning colors and as attention alerting or “go-slow” signals for distasteful toxins, thereby mediating the relationship between warning signal and toxicity. Furthermore, we found that moths that were heterozygous for hindwing coloration had more effective defense fluids compared to other genotypes in terms of delaying approach and reducing the latency to drop the moth, suggesting a genetic link between coloration and defense that could help to explain the color polymorphism. Conclusively, these results indicate that color, smell, and taste constitute a multimodal warning signal that impedes predator attack and improves prey survival. This work highlights the importance of understanding the separate roles of color, smell and taste through the predation sequence and also within-species variation in chemical defenses.
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Many dendrobatid frogs are known to be aposematic: brightly coloured and unpalatable to predators. To deceive predators, frog models used to test for predatory colour bias must be similar in size, colour, shape, and movement to frogs. We carried out an experiment with moving models of the species Adelphobates galactonotus , in two localities. A. galactonotus is a polytypic frog and each population of the species has a distinct colour. Birds and mammals were the vertebrates responsible for the marks on the models, but there was no difference in frequency of attacks on local-, non-local- and cryptic-colour models. Only invertebrates avoided cryptic models. Different populations of the species seem to be under different predation pressure, but colour differentiation in this species is probably related to other mechanisms, such as sexual selection.
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In Western Europe, omnipresent human activities have considerable impacts on habitats at several spatial scales resulting in direct shifts in habitat characteristics. These modifications in habitat features can disrupt biotic interactions such as predation. Surprisingly, although snake species are facing a worldwide decline, relationships between habitat characteristics and predation pressure in snakes remain poorly understood. The main goal of this study was to assess predation pressure on a snake species (the common adder; Vipera berus) in relation to two habitat characteristics: fine-scale (microhabitat) vegetation complexity and habitat structure (linear/non-linear). Using 2400 artificial plasticine models of adder as lures in 12 sites in Wallonia (Belgium), we quantified and compared the relative predation risk with respect to these two habitat features. We showed that, all predators combined (mammals and birds), increasing vegetation complexity had a positive impact by decreasing predation pressure, while habitat linearity increased attack risk on adders. However, for mammalian predators, increasing structural complexity reduced predation risk in non-linear habitats while this risk remained constant and substantial in linear habitats. This suggests that the abiotic benefits of linear strips or edges may be balanced by high predation risks. For bird predators, habitat linearity had no effect on attack rates while an increase in structural complexity reduced attack probabilities. In the light of these results, we suggest applying management practices that ensure a high degree of structural complexity in semi-natural habitats concerned with snake conservation. Moreover, we recommend creating non-linear, highly structured habitat elements to hamper predation pressure by mammals.
Preprint
Chemical defences often vary within and between populations both in quantity and quality, which is puzzling if prey survival is dependent on the strength of the defence. We investigated the within- and between-population variability in chemical defence of the wood tiger moth (Arctia plantaginis). The major components of its defences, SBMP (2secbutyl3methoxypyrazine) and IBMP (2isobutyl3methoxypyrazine) are volatiles that deter bird attacks. We expected the variation to reflect populations predation pressures and early-life conditions. To understand the role of the methoxypyrazines, we experimentally manipulated synthetic SBMP and IBMP and tested the birds reactions. We found a considerable variation in methoxypyrazine amounts and composition, both from wild-caught and laboratory-raised male moths. In agreement with the cost of defence hypothesis, the moths raised in the laboratory had a higher amount of pyrazines. We found that SBMP is more effective at higher concentrations and that IBMP is more effective only in combination with SBMP and at lower concentrations. Our results fit findings from the wild: the amount of SBMP was higher in the populations with higher predation pressure. Altogether, this suggests that, regarding pyrazine concentration, more is not always better, and highlights the importance of testing the efficacy of chemical defence and its components with relevant predators, rather than relying only on results from chemical analyses
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Mimicry rings are communities of mimetic organisms that are excellent models for ecological and evolutionary studies because the community composition, the nature of the species interactions, the phenotypes under selection, and the selective agents are well characterized. Here, we review how regional and ecological filtering, density- and frequency-dependent selection, toxicity of prey, and age of mimicry rings shape their assembly. We synthesize findings from theoretical and empirical studies to generate the following hypotheses: ( a) the degree of unpalatability and age of mimicry rings increase mimicry ring size and ( b) the degree of unpalatability, generalization of the aposematic signal, and availability of alternative prey are positively related to the breadth of the protection umbrella for an aposematic signal and negatively related to the degree of mimetic resemblance. We also provide a phylogenetic framework in which key aspects of mimicry ring diversification may be studied. Expected final online publication date for the Annual Review of Ecology, Evolution, and Systematics, Volume 52 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
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Background Diploid genome assembly is typically impeded by heterozygosity because it introduces errors when haplotypes are collapsed into a consensus sequence. Trio binning offers an innovative solution that exploits heterozygosity for assembly. Short, parental reads are used to assign parental origin to long reads from their F1 offspring before assembly, enabling complete haplotype resolution. Trio binning could therefore provide an effective strategy for assembling highly heterozygous genomes, which are traditionally problematic, such as insect genomes. This includes the wood tiger moth (Arctia plantaginis), which is an evolutionary study system for warning colour polymorphism. Findings We produced a high-quality, haplotype-resolved assembly for Arctia plantaginis through trio binning. We sequenced a same-species family (F1 heterozygosity ∼1.9%) and used parental Illumina reads to bin 99.98% of offspring Pacific Biosciences reads by parental origin, before assembling each haplotype separately and scaffolding with 10X linked reads. Both assemblies are contiguous (mean scaffold N50: 8.2 Mb) and complete (mean BUSCO completeness: 97.3%), with annotations and 31 chromosomes identified through karyotyping. We used the assembly to analyse genome-wide population structure and relationships between 40 wild resequenced individuals from 5 populations across Europe, revealing the Georgian population as the most genetically differentiated with the lowest genetic diversity. Conclusions We present the first invertebrate genome to be assembled via trio binning. This assembly is one of the highest quality genomes available for Lepidoptera, supporting trio binning as a potent strategy for assembling heterozygous genomes. Using our assembly, we provide genomic insights into the geographic population structure of A. plantaginis.
Preprint
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Background Diploid genome assembly is typically impeded by heterozygosity, as it introduces errors when haplotypes are collapsed into a consensus sequence. Trio binning offers an innovative solution which exploits heterozygosity for assembly. Short, parental reads are used to assign parental origin to long reads from their F1 offspring before assembly, enabling complete haplotype resolution. Trio binning could therefore provide an effective strategy for assembling highly heterozygous genomes which are traditionally problematic, such as insect genomes. This includes the wood tiger moth ( Arctia plantaginis ), which is an evolutionary study system for warning colour polymorphism. Findings We produced a high-quality, haplotype-resolved assembly for Arctia plantaginis through trio binning. We sequenced a same-species family (F1 heterozygosity ∼1.9%) and used parental Illumina reads to bin 99.98% of offspring Pacific Biosciences reads by parental origin, before assembling each haplotype separately and scaffolding with 10X linked-reads. Both assemblies are highly contiguous (mean scaffold N50: 8.2Mb) and complete (mean BUSCO completeness: 97.3%), with complete annotations and 31 chromosomes identified through karyotyping. We employed the assembly to analyse genome-wide population structure and relationships between 40 wild resequenced individuals from five populations across Europe, revealing the Georgian population as the most genetically differentiated with the lowest genetic diversity. Conclusions We present the first invertebrate genome to be assembled via trio binning. This assembly is one of the highest quality genomes available for Lepidoptera, supporting trio binning as a potent strategy for assembling highly heterozygous genomes. Using this assembly, we provide genomic insights into geographic population structure of Arctia plantaginis.
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Aposematic theory has historically predicted that predators should select for warning signals to converge on a single form, as a result of frequency‐dependent learning. However, widespread variation in warning signals is observed across closely related species, populations and, most problematically for evolutionary biologists, among individuals in the same population. Recent research has yielded an increased awareness of this diversity, challenging the paradigm of signal monomorphy in aposematic animals. Here we provide a comprehensive synthesis of these disparate lines of investigation, identifying within them three broad classes of explanation for variation in aposematic warning signals: genetic mechanisms, differences among predators and predator behaviour, and alternative selection pressures upon the signal. The mechanisms producing warning coloration are also important. Detailed studies of the genetic basis of warning signals in some species, most notably Heliconius butterflies, are beginning to shed light on the genetic architecture facilitating or limiting key processes such as the evolution and maintenance of polymorphisms, hybridisation, and speciation. Work on predator behaviour is changing our perception of the predator community as a single homogenous selective agent, emphasising the dynamic nature of predator–prey interactions. Predator variability in a range of factors (e.g. perceptual abilities, tolerance to chemical defences, and individual motivation), suggests that the role of predators is more complicated than previously appreciated. With complex selection regimes at work, polytypisms and polymorphisms may even occur in Müllerian mimicry systems. Meanwhile, phenotypes are often multifunctional, and thus subject to additional biotic and abiotic selection pressures. Some of these selective pressures, primarily sexual selection and thermoregulation, have received considerable attention, while others, such as disease risk and parental effects, offer promising avenues to explore. As well as reviewing the existing evidence from both empirical studies and theoretical modelling, we highlight hypotheses that could benefit from further investigation in aposematic species. Finally by collating known instances of variation in warning signals, we provide a valuable resource for understanding the taxonomic spread of diversity in aposematic signalling and with which to direct future research. A greater appreciation of the extent of variation in aposematic species, and of the selective pressures and constraints which contribute to this once‐paradoxical phenomenon, yields a new perspective for the field of aposematic signalling.
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Many animals protect themselves from predation with chemicals, both self-made or sequestered from their diet. The potential drivers of the diversity of these chemicals have been long studied, but our knowledge of these chemicals and their acquisition mode is heavily based on specialist herbivores that sequester their defenses. The wood tiger moth (Arctia plantaginis, Linnaeus, 1758) is a well-studied aposematic species, but the nature of its chemical defenses has not been fully described. Here, we report the presence of two methoxypyrazines, 2-sec-butyl-3-methoxypyrazine and 2-isobutyl-3-methoxypyrazine, in the moths' defensive secretions. By raising larvae on an artificial diet, we confirm, for the first time, that their defensive compounds are produced de novo rather than sequestered from their diet. Pyrazines are known for their defensive function in invertebrates due to their distinctive odor, inducing aversion and facilitating predator learning. While their synthesis has been suspected, it has never previously been experimentally confirmed. Our results highlight the importance of considering de novo synthesis, in addition to sequestration, when studying the defensive capabilities of insects and other invertebrates.
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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.
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Local warning colour polymorphism, frequently observed in aposematic organisms, is evolutionarily puzzling. This is because variation in aposematic signals is expected to be selected against due to predators' difficulties associating several signals with a given unprofitable prey. One possible explanation for the existence of such variation is predator generalization, which occurs when predators learn to avoid one form and consequently avoid other sufficiently similar forms, relaxing selection for monomorphic signals. We tested this hypothesis by exposing the three different colour morphs of the aposematic wood tiger moth, Arctia plantaginis, existing in Finland to local wild-caught predators (blue tits, Cyanistes caeruleus). We designed artificial moths that varied only in their hindwing coloration (white, yellow and red) keeping other traits (e.g. wing pattern and size) constant. Thus, if the birds transferred their aversion of one morph to the other two we could infer that their visual appearances are sufficiently similar for predator generalization to take place. We found that, surprisingly, birds showed no preference or aversion for any of the three morphs presented. During the avoidance learning trials, birds learned to avoid the red morph considerably faster than the white or yellow morphs, confirming previous findings on the efficacy of red as a warning signal that facilitates predator learning. Birds did not generalize their learned avoidance of one colour morph to the other two morphs, suggesting that they pay more attention to conspicuous wing coloration than other traits. Our results are in accordance with previous findings that coloration plays a key role during avoidance learning and generalization, which has important implications for the evolution of mimicry. We conclude that, in the case of wood tiger moths, predator generalization is unlikely to explain the unexpected coexistence of different morphs.
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Animals have evolved different defensive strategies to survive predation, among which chemical defences are particularly widespread and diverse. Here we investigate the function of chemical defence diversity, hypothesizing that such diversity has evolved as a response to multiple enemies. The aposematic wood tiger moth (Arctia plantaginis) displays conspicuous hindwing coloration and secretes distinct defensive fluids from its thoracic glands and abdomen.We presented the two defensive fluids from laboratoryreared moths to two biologically relevant predators, birds and ants, and measured their reaction in controlled bioassays (no information on colour was provided). We found that defensive fluids are target-specific: thoracic fluids, and particularly 2-sec-butyl-3-methoxypyrazine, which they contain, deterred birds, but caused no aversive response in ants. By contrast, abdominal fluids were particularly deterrent to ants, while birds did not find them repellent. Our study, to our knowledge, is the first to show evidence of a single species producing separate chemical defences targeted to different predator types, highlighting the importance of taking into account complex predator communities in studies on the evolution of prey defence diversity.
Book
Avoiding Attack discusses the diversity of mechanisms by which prey avoid predator attacks and explores how such defensive mechanisms have evolved through natural selection. It considers how potential prey avoid detection, how they make themselves unprofitable to attack, how they communicate this status, and how other species have exploited these signals. Using carefully selected examples of camouflage, mimicry, and warning signals drawn from a wide range of species and ecosystems, the authors summarise the latest research into these fascinating adaptations, developing mathematical models where appropriate and making recommendations for future study. This second edition has been extensively rewritten, particularly in the application of modern genetic research techniques which have transformed our recent understanding of adaptations in evolutionary genomics and phylogenetics. The book also employs a more integrated and systematic approach, ensuring that each chapter has a broader focus on the evolutionary and ecological consequences of anti-predator adaptation. The field has grown and developed considerably over the last decade with an explosion of new research literature, making this new edition timely.