Behavioral Ecology
doi:10.1093/beheco/arl015
Advance Access publication 19 June 2006
The effects of pattern symmetry on detection
of disruptive and background-matching
coloration
Innes C. Cuthill, Martin Stevens, Amy M.M. Windsor, and Hannah J. Walker
School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK
Two, logically distinct but sometimes compatible, mechanisms of camouflage are background-matching and disruptive coloration. In the former, an animal’s coloration comprises a random sample of the background, and so target–background discrimination is impeded. In the latter, object or feature recognition is compromised by placing bold, high-contrast colors so that they
break up the prey’s body into apparently unconnected objects. Recent experimental evidence for the utility of disruptive colors,
above and beyond that conferred by background matching, has been based on artificial prey with patterns lacking a plane of
symmetry. However, it is plausible that the bilateral symmetry present in natural prey may compromise the efficiency of disruptive
coloration, on account of the potency of symmetry as a cue in visual search. In this study, we tested this prediction in the field, by
tracking the ‘‘survival’’ under bird predation of artificial mothlike targets placed on oak trees. These had background-matching
color patches placed either disruptively or nondisruptively and with or without bilateral symmetry. We found that symmetry
reduced the effectiveness of both nondisruptive and disruptive background-matching coloration to a similar degree so that the
negative effects of symmetry on concealment are no greater for disruptive than nondisruptive patterns. Key words: bird vision,
camouflage, crypsis, disruptive coloration, symmetry, visual search. [Behav Ecol 17:828–832 (2006)]
amouflage is one of the commonest forms of defensive
coloration (Cott 1940; Edmunds 1974; Endler 1991;
Ruxton et al. 2004). As visual segmentation of a potential prey
item from irrelevant background stimuli is aided by a mismatch in color or texture, it has been argued that background
matching (termed crypsis by Endler 1981) is perhaps the most
widespread form of camouflage. However, as recognized by
some of the pioneers of the evolutionary study of animal coloration (Cott 1940; Thayer 1909), even perfect background
matching can be compromised by a discontinuity in pattern at
the body’s edge, potentially exacerbated by shadow. Thayer
(1909) instead advanced a theory of ‘‘ruptive’’ coloration,
now termed disruptive coloration after Cott’s (1940) more comprehensive treatment of the theory (see Stevens, Cuthill,
Párraga, and Troscianko, forthcoming). Cott (1940; Merilaita
1998) proposed 3 key principles of disruptive camouflage:
‘‘differential blending,’’ where different color patches match
different components of the background, ‘‘maximum disruptive contrast,’’ where adjacent pattern elements are highly
contrasting in tone, and third, placement of some patches
near the body’s edge (or across distinctive features such as
eyes). These 3 principles reduce the probability that the prey’s
outline is detected and its features are grouped (perceptually)
as a coherent object of interest (Stevens and Cuthill forthcoming). Cott’s principles of disruptive coloration have only recently been tested, receiving both indirect (Merilaita 1998)
and direct support (Cuthill et al. 2005; Merilaita and Lind
2005; Stevens and Cuthill forthcoming).
In previous field experiments involving avian predators
(Cuthill et al. 2005), we showed that artificial mothlike targets
with differentially blending disruptive patterns survived significantly better than targets with nondisruptive background-
C
matching patterns. Furthermore, targets with highly contrasting disruptive patterns survived significantly better than equivalent low-contrast patterns, supporting the disruptive contrast
hypothesis. However, this work left at least one important
question unanswered (Sherratt et al. 2005). Is disruptive coloration effective when patterns are bilaterally symmetrical, as
they often are in nature but not in Cuthill et al. (2005)?
Many animals are bilaterally symmetrical, and because symmetry is a potent cue in visual search, symmetrical patterning
is likely to reduce the effectiveness of camouflage (Osorio
1994; Cuthill et al. 2006; Merilaita and Lind 2006). In fact,
Thayer (1909) recognized that ‘‘dual symmetry in pattern’’ is
one potential major flaw in camouflaged species. However, in
an experiment in which great tits (Parus major) foraged for
patterned artificial prey, Merilaita and Lind (2006) found that
not all symmetrical patterns were similarly disadvantaged.
While symmetrical pattern elements might be particularly
conspicuous, certain arrangements of patterns might reduce
the costs of symmetry. As this conclusion was based on the
relative success of just one pattern type, selected subjectively,
this hypothesis deserves systematic investigation. We therefore
compared the survival rates of artificial targets with disruptive
or nondisruptive background-matching patterns, which were
either symmetrical or asymmetrical about the vertical midline.
We predicted that disruptive coloration should reduce the
costs of bilateral pattern symmetry. The experiment also
serves to test the counterproposal that disruptive coloration
may be rendered ineffective as a camouflage strategy when
bilateral symmetry is present (i.e., unlike in Cuthill et al.
2005), shedding doubt on the explanatory power of the
theory of disruptive coloration in nature.
METHODS
Address correspondence to I.C. Cuthill. E-mail Address: i.cuthill@
bristol.ac.uk.
Received 23 February 2006; revised 17 May 2006; accepted 17 May
2006.
The Author 2006. Published by Oxford University Press on behalf of
the International Society for Behavioral Ecology. All rights reserved.
For permissions, please e-mail: [email protected]
The experiment aimed to determine the effects of symmetry
on both disruptive and nondisruptive background-matching
patterns and followed the same general procedure as in Cuthill
et al. (2005); a more detailed treatment of the protocol and
Cuthill et al.
•
Symmetry and camouflage
the rationale for various details of experimental design and
analysis can be found in Cuthill et al. (2006). We created
cardboard triangular artificial ‘‘moth’’ targets, approximately
50 mm wide by 25 mm high. These were not designed to
mimic any real species but were covered with patterns consisting of black and brown markings designed to match the background with respect to avian vision. These markings were
chosen to mimic the ridges on mature oak bark. Patterns were
samples of digital photos of oak tree trunks at 1:1 reproduction, taken with Nikon Coolpix 5700 camera, calibrated to
linearize the relationship between radiance and the grayscale in each color channel (Párraga 2003; Stevens, Párraga,
et al. forthcoming), and saved as uncompressed TIFF files.
Images were converted using Image J (Abràmoff et al. 2004;
Rasband 2005) to grayscale and thresholded at 50% to binary
(black/white) images to provide, when printed onto brown
card, barklike brown/black spatial variation. One or more
pattern elements were selected, using the ‘‘copy’’ tool, from
the thresholded photographs according to the criterion that
they should be small enough to fit within the final triangular
targets (Figure 1). The selection was then pasted into a triangular frame, placed either to touch the triangle’s edge (disruptive treatments) or not (nondisruptive treatments; see
below). Whether a given selection was placed disruptively or
not (and the treatments within those categories) was selected
using random numbers. Different samples, from different
trees, were used for each replicate target. Color matches of
treatments to natural bark were verified using spectrophotometry of stimuli and bark as described in Cuthill et al.
(2006). This was followed by modeling of predicted photon
catches (Maddocks et al. 2001) of a typical passerine bird,
the blue tit’s (Parus caeruleus) single-cone photoreceptors
(Hart et al. 2000), using irradiance spectra from woodland
shade under overcast skies, collected at our field site using
an Ocean Optics USB2000 spectrometer fitted with a cosine
corrector. Our acceptance criterion was simply that cone captures for the experimental stimuli fell within 1 SD. of the
means of those for oak bark (for the reflectance spectra on
which the calculations were based, both bark and paper, see
Cuthill et al. 2006).
There were 6 treatments in all (Figure 1): 1) disruptive
asymmetrical, with the black markings touching the edge of
the body, 2) disruptive symmetrical, where the markings on
one side of the body were a mirror reflection about the body
midline, 3) nondisruptive asymmetrical, where the black
markings were randomly placed but did not touch the edge
of the targets’ body, 4) nondisruptive symmetrical about the
midline, 5) disruptive midline symmetrical, where the pattern
elements were disruptive but also had to touch the midline of
the body, and 6) nondisruptive midline symmetrical, where
the markings were randomly placed but had to touch the
midline of the body and not touch the edge of the target.
Treatments 5 and 6 control for the fact that, on average, disruptive markings will be found further away from the midline
of the body because they are placed near the target edges, and
evidence from human studies indicates that object recognition of symmetrical patterns is facilitated when the patterns
are closer to the midline (Swaddle 1999b). Therefore, to determine if this is the case with our study, we can compare the
survival of the symmetrical midline to the symmetrical nonmidline treatments. Each target was different in its patterning,
and so no 2 targets were the same, but as indicated above,
because assignment of patterns to treatments was random,
there should have been no systematic differences in features
such as the area of black/brown. Analysis of a subset of the
stimuli, using the measurement tools in Image J, indicated
no obvious treatment differences (mean %black 6 SD in
treatments 1–6, respectively: 32.60 6 12.24, 29.55 6 12.97,
829
Figure 1
Sample targets: 1) disruptive asymmetrical, 2) disruptive symmetrical, 3) nondisruptive asymmetrical, 4) nondisruptive symmetrical,
5) disruptive midline symmetrical, and 6) nondisruptive midline
symmetrical. In the experiment, each replicate target had a
unique pattern.
25.55 6 8.02, 29.50 6 11.67, 29.40 6 8.80, and 31.70 6
10.26; N ¼ 20 in each treatment; F5,114 ¼ 1.02, P ¼ 0.410;
no Tukey pairwise comparison went below P ¼ 0.315, and
the directions of differences between symmetric and asymmetric treatments were not consistent).
Targets were pinned onto oak trees in the mixed deciduous
Leigh Woods National Nature Reserve, North Somerset, UK
(238.6#W, 5127.8#N) and their survival checked at approximately 2, 4, 24, and 48 h. Ten targets of each treatment were
pinned to oak tree trunks in each of 10 trials, in a randomized
block design, giving a total sample size of 600. Targets were
placed along nonlinear transects of approximately 1.5 km by
20 m (targets were placed on ,5% of available trees), and
were randomly allocated to trees, subject to the constraints
that no lichen covered the trunk and no young trees of trunk
circumference less than 0.9 m were used. Each block was undertaken on different days and in different locations of the
field site. The experiment was conduced between November
2004 and May 2005.
An edible component for birds was attached to each target,
consisting of a dead (frozen overnight at 80 C, then
thawed) mealworm (Tenebrio molitor larvae) pinned onto each
target. Avian predation was revealed by all or complete disappearance of the mealworm. Species observed taking the prey
in this and similar experiments in other field seasons included
blue tits (P. caeruleus), great tits (P. major), European robins
(Erithacus rubecula), chaffinches (Fringilla coelebs), and house
sparrows (Passer domesticus). Other insectivorous species such
as woodpeckers were observed in the area but were not actually seen taking prey. However, no systematic program of observation was carried out, this being incompatible with the
quantity of data required. Other forms of predation could
also be identified; spiders sucked fluids out leaving a hollow exoskeleton, and slugs left slime trails. Nonavian predation, the disappearance of the whole target, or survival to 48 h
were treated as ‘‘censored’’ values in survival analysis with
Cox proportional hazards regression (Cox 1972; Klein and
Moeschberger 2003; Cuthill et al. 2006). Significance was
tested with the Wald statistic (W), and after the establishment
of an overall treatment effect, differences were investigated
using linear contrasts (Rosenthal et al. 2000). The latter
830
Behavioral Ecology
consisted of main effects of pattern placement (disruptive vs.
nondisruptive treatments; 1, 2, 5 vs. 3, 4, 6), symmetry (1, 3 vs.
2, 4, 5, 6), proximity to the midline of symmetrical pattern
elements (near vs. far; 2, 4 vs. 5, 6), and the interaction between pattern placement and symmetry. The latter term tests
directly for a disproportionate cost of symmetry in disruptively
patterned targets. Simple pairwise comparison of all treatments yields the same conclusions; we present the contrast
analysis as it more fairly reflects our hypotheses (Rosenthal
et al. 2000) and represents a more parsimonious account of
the observed effects (a minimal adequate model). Effect sizes
are quoted as odds ratios (OR), which should be interpreted
as the probability of ‘‘death’’ in one treatment divided by that
in the other, such that an OR of 1 represents no difference
and an OR of 2 represents double the mortality per unit time
in one treatment than the other.
RESULTS
There was a significant difference in survival between the
treatments (W ¼ 37.918, P , 0.001, df ¼ 5; Figure 2). Linear
contrasts indicated that those treatments with the disruptive
patterns survived significantly better than the nondisruptive
treatments (pattern W ¼ 15.028, P , 0.001, df ¼ 1; OR ¼
1.307). Treatments with asymmetrical patterns survived better
than those with symmetrical patterns (symmetry W ¼ 24.513,
P , 0.001, df ¼ 1; OR ¼ 1.407). Contrary to expectation,
symmetrical patterns with markings touching the midline
did not suffer higher predation than those treatments with
markings placed away from the midline (proximity W ¼
0.368, P ¼ 0.544, df ¼ 1; OR ¼ 1.041). There was no interaction between pattern placement and symmetry (W ¼ 1.102,
P ¼ 0.294, df ¼ 1; OR ¼ 0.930), and removal of this interaction term from the model made little difference to the parameter estimates and P values of the main effects. Although not
an a priori hypothesis tested in our contrast analysis, we note
that those targets with symmetrical patterns placed disruptively survived similarly to those targets with asymmetrical patterns placed nondisruptively (Figure 2).
There was a significant effect of block (W ¼ 25.739, P ,
0.001, df ¼ 9), meaning that there were differences in average
predation rates in different parts of the woods on different
dates. As this does not affect our conclusions about the treatment effects and we have no way of testing whether this was
due to different weather conditions, bird densities, light environments, or levels of human disturbance, these average
block differences are not discussed further.
DISCUSSION
The results support the findings of Cuthill et al. (2005) that
disruptive markings provide a significant survival advantage
against avian predators, compared with targets with nondisruptive background-matching patterns. Also, as in Cuthill et al.
(2006), symmetry is a significant disadvantage in terms of predation risk for both disruptive and nondisruptive backgroundmatching patterns. These effects are additive, so there is no
evidence that disruptive markings bear a higher cost of conspicuousness when arranged symmetrically. Indeed, symmetrical disruptive patterns survive at least as well as asymmetrical
nondisruptive patterns (Figure 2), indicating that disruptive
patterning reduces the costs of symmetry for camouflaged
organisms.
The result that the symmetrical patterns placed near the
axis of symmetry survived as well as the nonmidline symmetrical patterns indicates that the detection of the targets was
not enhanced by symmetry lying closer to the midline or the
Figure 2
Survival curves of the patterned targets in the experiment: 1) disruptive asymmetrical, 2) disruptive symmetrical, 3) nondisruptive
asymmetrical, 4) nondisruptive symmetrical, 5) disruptive midline
symmetrical, and 6) nondisruptive midline symmetrical. Curves are
the probability of surviving bird predation as a function of time,
based on Kaplan–Meier estimates to account for censoring due to
nonavian predation and survival to the end of the study period
(SPSS Inc. 2003). The 2 long gaps without mortality correspond to
overnight periods when targets were not checked. The disruptive
symmetrical and nondisruptive asymmetrical treatment lines are
indistinguishable between days 1 and 2 (24 and 48 h).
mealworm. This is important because it indicates that the
superior survival of the disruptive symmetrical treatments over
the nondisruptive symmetrical treatments was not because of
midline symmetry effects, where the disruptive patterns are on
average further from the midline, but rather due to disruptive
coloration itself. The lack of midline symmetry effects is not
entirely surprising given that in our experiment target detection by birds probably occurred at a distance of several meters
(IC Cuthill, M Stevens, personal observation) and the stimuli
were relatively small, so minor differences in the placement of
symmetrical patterns will have only a limited impact on detection rates. We note that midline-proximity effects in human
search experiments are usually based on protocols where subjects view computer screens filling a large portion of the visual
field (e.g., Barlow and Reeves 1979; e.g., Herbert et al. 1994).
Also, the presence of the mealworm down the midline of the
target was itself an indicator of the axis of symmetry and may
have rendered any additional effects of the positions of the
patterns negligible. It is clear that our results do not diminish
the likelihood that symmetry close to the midline may be a revealing cue for larger prey, or when predators are closer to the
substrate, as was the case in the experiments of Merilaita and
Lind (2006). Furthermore, as they point out, mirror symmetry
about the midline may create novel pattern elements that are
themselves rare in the background and so distinctive.
The design of our experimental stimuli, with the mealworm
on top of the patterns, means that mealworm–wing contrast
must be considered as well as the relationship between wing
and background colors that was central to our study. Mealworms are yellow-brown and are lighter than either the brown
or black color patches on which they were pinned. Therefore,
in treatments where the black color patches were more likely
to be placed centrally, the mealworm would have been more
conspicuous than in those treatments where it was more likely
to lie on brown patches. This is potentially a seriously confound because the edge treatments may owe their reduced
conspicuousness to this effect rather than disruption of the
Cuthill et al.
•
Symmetry and camouflage
body outline. Fortunately, the manipulations of pattern positioning relative to the midline address this question as well as
that of symmetry detection. As treatments with dark patterns
far from the midline did not survive better than those positioned near the midline, mealworm-wing contrast seems to
have had a minor effect compared with wing-background contrast. This is not surprising because the area of the wings was
far greater than that of an average mealworm (wing area
625 mm2, mean mealworm area 65 mm2. See Stevens, Cuthill,
Windsor, and Walker forthcoming).
Disruptive coloration may help to reduce some of the negative survival implications of symmetrical patterns and enable
animals to exploit backgrounds and environments toward
which they have only a partial resemblance. However, symmetrical patterns are often still easier to detect. Also, observations
that some insects adopt behavioral strategies apparently to
introduce asymmetry, such as folding one wing over the other,
coupled with our results suggest that there can be clear disadvantages to symmetry in camouflage patterns. This poses
the obvious question of why so few animals with camouflage
patterns are asymmetrical and, in those species such as cuttlefish (Sepia spp.) that can generate asymmetrical patterns, symmetrical camouflage is nevertheless adopted (Langridge
2005). No doubt, in many species, such as butterflies, the
posture and wing-folding pattern means that both sides of
the body will not be observed simultaneously, but this is not
true of moths, many of which have a classic reliance on camouflage to reduce predation risk.
A plausible argument for symmetry in camouflaged Lepidoptera is that there are genetic or developmental restrictions
on pattern formation, if the latter is closely tied to morphology, as seems to be the case in butterflies (Monteiro et al.
1997). If wing coloration cannot readily be decoupled from
the underlying morphology, then selection for symmetry of
wing shape for flight performance is likely to exceed the camouflage benefit obtained from asymmetrical wing coloration
(see discussion in Cuthill et al. 2006). However, there are
documented cases of complete asymmetry in insect wing coloration (Barabás and Hancock 1999), indicating morphology
and color can be developmentally uncoupled, so ascribing insect bilateral symmetry to an absolute genetic/developmental
constraint seems unsatisfactory.
A modification of the constraint hypothesis is therefore that
only large levels of asymmetry bring a benefit in terms of camouflage, and for the usually accepted reasons (e.g., Dawkins
1976, 1996), such large mutations and developmental changes
are statistically unlikely. In our experiment, the stimuli were
either perfectly symmetrical or entirely asymmetrical. Real
animals that appear at a glance to be highly symmetrical will
have low levels of fluctuating asymmetry (Møller and Swaddle
1997), which selection could potentially enhance through relaxation of developmental canalization. To the best of our
knowledge, only one study has quantified the level of asymmetry found in potentially concealing patterns in Lepidoptera
(Forsman and Merilaita 2003), and this found only limited
differences in asymmetry compared with signaling patterns.
Crucially, it is unknown whether low levels of asymmetry in
camouflage reduce detection chances, or are even detectable
(see particularly Swaddle 1999a), so maybe, from a starting
point of high symmetry, the strength of selection for asymmetrical cryptic coloration is negligible. Clearly, current theories
of camouflage would be greatly strengthened by a firmer understanding of the mechanisms of symmetry perception and
genetic investigations into the origin and plasticity of the genetic basis of pattern symmetry.
The research was supported by a Biotechnology and Biological Sciences Research Council (BBSRC) grant to I.C.C., T. S. Troscianko, and
831
J. C. Partridge and by a BBSRC studentship to M.S. Our experiment
was conceived and completed prior to the publication of Merilaita and
Lind (2006), and we are grateful to Sami and Johan for freely discussing their research with us. Great minds think alike? Thanks to Sami
Merilaita and an anonymous referee for their perceptive comments
on the manuscript. I.C.C. and M.S. conceived the experiment, designed the stimuli, and wrote the manuscript; all authors took an
equal role in the field experiment.
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