A tropical horde of counterfeit predator eyes
Daniel H. Janzen
a,1
, Winnie Hallwachs
a
, and John M. Burns
b
a
Department of Biology, University of Pennsylvania, Philadelphia, PA 19104-6018; and
b
Department of Entomology, National Museum of
Natural History, Smithsonian Institution, Washington, DC 20013-7012
Edited by May R. Berenbaum, University of Illinois at Urbana?Champaign, Urbana, IL, and approved May 14, 2010 (received for review January 26, 2010)
Weproposethatthemanydifferent,butessentiallysimilar,eye-likeandface-likecolorpatternsdisplayedbyhundredsofspeciesoftropical
caterpillars and pupae?26 examples of which are displayed here from the dry, cloud, and rain forests of Area de ConservacionGuanacaste
(ACG) in northwestern Costa Rica?constitute a huge and pervasive mimicry complex that is evolutionarily generated and sustained by the
survival behavior of a large and multispeci?c array of potential predators: the insect-eating birds. We propose that these predators are
variously and innately programmed to ?ee when abruptly confronted, at close range, with what appears to be an eye of one of their
predators. Such a mimetic complex differs from various classical Batesian and M?llerian mimicry complexes of adult butter?ies in that
(i) the predators sustain it for the most part by innate traits rather than by avoidance behavior learned through disagreeable experiences,
(ii) the more or less harmless, sessile, and largely edible mimics vastly outnumber the models, and (iii) there is no particular selection for
the eye-like color pattern to closely mimic the eye or face of any particular predator of the insect-eating birds or that of any other member
of this mimicry complex. Indeed, selection may not favor exact resemblance among these mimics at all. Such convergence through
selection could create a superabundance of one particular false eyespot or face pattern, thereby increasing the likelihood of a bird species
or guild learning to associate that pattern with harmless prey.
caterpillar
|
mimicry
|
pupa
|
insectivorous
birds
|
innate behavior
Y
ou are a 12-gram, insectivorous,
tropical rainforest bird, foraging
in shady, tangled, dappled, rus-
tling foliage where edible cater-
pillars and other insects are likely to
shelter. You want to live 10?20 years.
You are peering under leaves, poking into
rolled ones, searching around stems, ex-
ploring bark crevices and other insect
hiding places. Abruptly an eye appears,
1?5 centimeters from your bill. The eye or
a portion of it is half seen, obstructed,
shadowed, partly out of focus, more or less
round, multicolored, and perhaps moving.
If you pause a millisecond to ask whether
that eye belongs to acceptable prey or to
a predator, you are likely to be?and it
takes only once?someone?s breakfast.
Your innate reaction to the eye must be
instant ?ight, that is, a ?startle? coupled
with distancing. The bird that must learn
to avoid what appears to be a predator?s
eye is not long for this world. Now, a safe
few meters away, are you going to go
back to see whether that was food? No.
You, like billions of other individuals
and hundreds of other species for tens
of millions of years, have just been a player
in an act of natural selection favoring
mutations that lead to the multitudes
Fig. 1. The 7-mm-wide pupa of Cephise nuspesez (23) (Hesperiidae), a Costa Rican skipper butter?yasit
appears to a foraging bird that (Upper) has poked into the front of the rolled leaf shelter constructed by
the caterpillar or (Lower) has opened the roll from above. When disturbed, this pupa rotates to present
its face to the open end of the leaf roll.
Author contributions: D.H.J., W.H., and J.M.B. designed re-
search; D.H.J., W.H., and J.M.B. performed research; D.H.J.,
W.H., and J.M.B. contributed new reagents/analytic tools;
D.H.J., W.H., and J.M.B. analyzed data; and D.H.J., W.H.,
and J.M.B. wrote the paper.
The authors declare no con?ict of interest.
This article is a PNAS Direct Submission.
1
To whom correspondence should be addressed. E-mail:
djanzen@sas.upenn.edu.
This article contains supporting information online at
www.pnas.org/lookup/suppl/doi:10.1073/pnas.
0912122107/-/DCSupplemental.
www.pnas.org/cgi/doi/10.1073/pnas.0912122107 PNAS Early Edition | 1of7
PERSPECTIVE
of ?false eye? color patterns, ?eyespot?
patterns, or ?facsimiles of eyes? and
?faces? adorning tropical caterpillars and
pupae (Figs. 1?4, and SI Appendix, Figs.
S1 and S2). These eyespots are round or
oval and mono- or polychromatic, with
round or slit pupils. Associated body pat-
terns often suggest other head and facial
features, which in turn enhance the
eye-like nature of the spots. Depending on
the angle of observation and on how much
or which part shows, one pattern may
even simulate different faces (Figs. 1
and 2). None of these patterns exactly
matches the eyes or face of any particular
species of predator; but, even when
quickly and partially glimpsed, all give the
illusion of an eye or face. These false eyes
are mimicking the eyes and faces of such
predators of insect-eating birds as snakes,
lizards, other birds, and small mammals, as
perceived at close range by the in-
sectivorous birds in their natural world.
These color patterns?long noticed by
?eld naturalists, evolutionary behaviorists
(see especially refs. 1?3), ecologists, tax-
onomists, ecotourists, and, no doubt, our
distant ancestors?and the birds? reactions
to them, are the evolutionary footprints of
predator/prey encounters as shallow as
today and as deep as the ?rst terrestrial
vertebrate eyes. Such footprints are scat-
tered across many diurnal vertebrate/prey
interactions (e.g., refs. 4 and 5), but here
we focus only on those of caterpillars
and pupae and the birds that eat them.
Discussion
We postulate that both the frequent oc-
currence of false eyespots on tropical cat-
erpillars and pupae, and the great tax-
onomic diversity of their bearers, are
powerful indirect evidence that the avian
reactions to false eyes are innate. Never-
theless, we expect the reactions to vary
interspeci?cally in connection with the
intensity of the bird?s selective regime, the
bird?s learning ability and personal history
of predator avoidance, and the evoca-
tiveness (e.g., ref. 5) of any particular false
eye(s) as perceived by the bird in the
habitats in which it characteristically
forages. The response may also vary in-
traspeci?cally with the bird?s microenvi-
ronmental circumstances and with
experience?for example, light level,
proximity, degree to which the false eye or
the whole insect is obstructed, what the
bird?s neighbors and life have taught it,
whether it has recently suffered a near
miss, how hungry it is (e.g., refs. 3, 6), size
of eyespot (e.g., refs. 5, 7), etc.
The sum of these avian reactions across
many tropical circumstances, habitats, and
ecosystems is a diffuse selective pressure
to which we suggest that innumerable
species of caterpillars and pupae have
variously responded in the evolution of
their color patterns. The diffuse nature
of the syndrome highlighted here also
applies to mimicry based on aposematic
(warning) colors, cryptic colors, ?ash col-
ors, and behaviors associated with them.
For example, the outcome of innate
avoidance of coral snakes by birds (8) may
extend to other toxic snakes and to harm-
less ones (4), to turtles (9), and to both
toxic and harmless caterpillars (9). Those
species-rich tropical complexes (4) are
not our concern here but seem to display
the same phenomenon with the same root
cause. The 36 species of caterpillars and
pupae in Figs. 1?4 and SI Appendix, Figs.
S1 and S2, are a small and partial repre-
sentation of the hundreds of species with
false eyes and faces encountered in the
course of a 30-year, ongoing inventory of
ca. 450,000 individuals and >5,000 species
of caterpillars and pupae in the dry,
cloud, and rain forest of Area de Con-
servacion Guanacaste (ACG) in north-
western Costa Rica (http://janzen.sas.
upenn.edu; ref. 10).
Each of these species of immature
mothsorbutter?ies has its own evolu-
tionary pedigree. Each has its own degree
of retention of traits that have been in-
tensively favored by natural selection in
the past and that may not be maintained
today by anything more complex than
phylogenetic inertia, the absence of an
opposing selective force (11), and the
multispeci?c array of insect-eating birds
scouring tropical vegetation every day,
year in and year out. Once a species has
evolved false eyes (or any facial pattern
that elicits a fear/?ee reaction), those
false eyes may barely diminish crypticity
Fig. 2. The 50-mm-long last instar caterpillar of Costa Rican Ridens panche (Hesperiidae) at the moment
when its leaf shelter is forced open (Upper) and a few seconds later (Lower), when it presents glowing
red false eye spots directed at the invader and glowing lemon-yellow eye spots in the dark of the cavern
behind. Both kinds of false eyes are thrust at the leaf roll entrance until the invader leaves.
2of7 | www.pnas.org/cgi/doi/10.1073/pnas.0912122107 Janzen et al.
at a distance, cost next to nothing physi-
ologically, and greatly bene?t their bearer
in a close encounter with a predator.
These color patterns differ from those of
classic mimic/model systems in that their
value to the mimic depends not on the
closeness of the match to a speci?c model,
but rather on suf?cient similarity to an
eye and/or face to trigger the fear/?ee
reaction in an insect-eating bird. How
similar is necessary (5) will vary from bird
species to bird species and among foraging
situations. The ubiquity of this multi-
speci?c mimicry complex argues strongly
for the widespread presence of the fear/
?ee reaction in many species of small birds
(or a few quite common ones).
Much contemporary mimicry theory and
popular commentary explicitly or implicitly
stress the importance of experience,
learning, and memory of the potential
predator in predator/prey interactions.
However, we believe that it generally
underestimates the fact that potential
predators also innately avoid various apo-
sematicsignalsandsimilaritiestoattributes
of their predators, as shown by both ex-
perimental study (e.g., refs. 5, 8, 12?16),
observation (see especially ref. 4), and
our natural history observations, and as
sketched out by Blest (3) although sub-
sequently largely ignored for the past half
century. When the dominant response of
the predator is innate rather than (or as
well as) learned, there are major changes
in mimicry theory and interpretation of
natural history with respect to the relative
importance of mimic/model ratios (4),
scarcity of models (4, 17, 18), intensity
of selection (5, 15), ability to remember
(6), etc. Any color, pattern, motion, or
sound of a caterpillar or pupa that elicits
innate avoidance of a lethal outcome for
the bird selectively favors both the preda-
tor and the prey. False eye and face
mimicry need not ?exactly? match the real
eyes of any particular species of predator
in order to be selected for, much as highly
effective cryptic behavior and color pat-
terns often do not precisely match the
patterns and colors of any particular
background. The eyespot and face pat-
terns need only contain features that
stimulate predator recognition by small
predators themselves (5). Although Blest
(ref. 3 and references therein) built on
these concepts in detail, they have re-
ceived little attention from the many bi-
ologists dealing with mimic/model
systems in the tropics, most of whom have
focused on the exactness of mimicry
among distasteful models and mimics that
display diurnally in ostentatious ?ight.
False-eye color patterns on butter?y
wings may serve to de?ect a bird?s strike
from the actual head of the butter?y
(3, 19, 20) instead of startling a potential
predator away. However, false-eye color
patterns on butter?y wings can also reduce
predation attempts (refs. 5 and 16 and
reviews therein). Both of these hypothe-
sized and con?rmed processes may be
operative at the same time with the same
species of prey and different species of
predators, but we are concerned here with
false eyes and faces on relatively sessile
caterpillars and pupae. There is no selec-
tive value in de?ecting a bird?s strike to the
site of the false eye on these animals.
Equally, we are not concerned here with
the question of what shape or intensity of
an eyespot (5) confers protection at any
given moment with any particular bird.
Some ?rst and approximate general-
izations about the mimic/model complex of
tropical caterpillars and pupae (Figs. 1?4
and SI Appendix, Figs. S1?S14) emerge
from our observations of their natural
history. Taken as a whole, their traits
suggest to us the long-term and pervasive
operation of natural selection by the
species-rich and abundant guild of small
vertebrate diurnal predators on cater-
pillars and pupae in tropical forest. We
do not propose alternative hypotheses for
this multispeci?c display by sessile prey
because we cannot think of any that are
compatible with the collective natural
history of the hundreds of species of avian
and lepidopteran actors.
False eyes and faces:
(i) Are comparatively more common
on species of caterpillars and pupae
that live in great part concealed in
microhabitats that are often of low
and variable light levels, and that
are searched by diurnally foraging
birds?rolled and silked leaves, silk/
leaf/dead leaf tangles, dark shadows
under large leaves, crevices in tree
bark, etc. Because many kinds of in-
sects and spiders hide in such pla-
ces, they are rich foraging grounds
for birds?but with hidden dangers.
Consider the caterpillar of the hes-
periid (skipper) butter?y in Fig. 2: it
is hidden in its silk and leaf shelter
during daylight hours, emerging to
feed at dusk or night. Its false eyes
are exposed when its shelter is torn
open; and at that time, it thrusts the
?face? of its head out at the intru-
der instead of retreating or turning
away (or simply starting to repair
its shelter). This behavior is shared
with more than 100 species of ACG
skipperbutter?ycaterpillarsandwith
manyspeciesinotherfamilies.Again,
the skipper butter?ypupainFig.1
spends 2 weeks hidden in a silked,
rolled leaf and is visually exposed
only when a diurnal predator opens
that shelter. Then the pupa, which is
?rmly anchored at its base, twists on
this anchor so as to project its ?face?
out of the entrance at the forager.
(ii) Also occur on (often large) caterpil-
lars or pupae that live fully exposed,
but with their false eye(s) often hid-
den in folds of cuticle until explic-
itly and ostentatiously displayed
by the caterpillar in reaction to the
approach or touch of a ?large? ob-
ject. The false eyes in Figs. 3 B, H,
I, K, and L are visible as false eyes
only when the caterpillar expands and
displays the crucial body part.
(iii) Usually occur on caterpillars and pu-
pae that are otherwise cryptically
colored and patterned (rather than
ostentatious); and these mimetic
features are not visible at any signif-
icant distance, even when the cater-
pillar or pupa lives fully exposed.
For example, the ground colors and
patterns of the caterpillars and pu-
pae in Figs. 3 and 4 are generally
green, gray, brown, or black, rather
than bright red, yellow, or blue.
(iv) Are not of any one speci?c ?eye?
shape or color but rather range
from astonishingly detailed mimics
of snake eyes and scales (e.g., Fig.
4H)tominimalsuggestionsofpaired
approximate circles or dots in sur-
rounding face-like patterns (e.g.,
Fig. 4 B, F, and G). Even when
approximate, these patterns are suf-
?ciently eye-like and face-like to
stimulate visual receptors/mental
processes that vertebrate predators
have evolved for rapidly recognizing
what might be an eye, regardless of
how imperfectly or fractionally seen
(seerefs.5and15forelaboration).It
ishardtobeconvincedthatthefalse
eyes in Fig. 1 are not real, and we
suspect even harder for a small bird
when foraging (e.g., see ref. 2 for an
extratropical example).
(v) Are usually paired and evolution-
arily derived from paired, more or
less circular structures (e.g., pupal
spiracles) or patterns. (On occasion,
median circular patterns are the
evolutionary precursors of one-eyed
mimics, especially in caterpillars of
Sphingidae and Notodontidae.) False
eyes are not derived from real cat-
erpillar ?eyes? (stemmata), which
are tiny light sensors on the lower
?cheeks? of the head, or from the
position of future real eyes inside
Janzen et al. PNAS Early Edition | 3of7
Fig. 3. Representative ACG caterpillar false eyes and faces (see SI Appendix, Table S1 for names and voucher codes and SI Appendix, Figs. S3?S8 and ref. 24 for
lateral and dorsal views of the same species of caterpillars).
4of7 | www.pnas.org/cgi/doi/10.1073/pnas.0912122107 Janzen et al.
Fig. 4. Representative ACG pupa false eyes and faces (see SI Appendix, Table S1 for names and voucher codes and SI Appendix, Figs. S9?S14 and ref. 24 for
lateral and dorsal views of the same species of pupae).
Janzen et al. PNAS Early Edition | 5of7
the pupa. The external surfaces of
the paired, pupal thoracic spiracles
have frequently given rise to pupal
false eyespots (e.g., all of the false
eyes in Figs. 1 and 4 are evolution-
arily modi?ed thoracic spiracles).
However, we add that there may be
selection to enhance almost any
shape and color that can give the
hint of eyes (e.g., refs. 5, 15).
(vi) Are usually on the head end or the
rear end of the caterpillar, and on
the front end of the pupa. These are
thepartsthatapredatorismostlikely
toseewhenprobingthesiteofcater-
pillar or pupa concealment and that
resemble in position and shape the
most dangerous part of a predator?s
predator. Not emphasized in Fig. 3
(butseeFig.3AandF)isthefactthat
these caterpillars often strike a sinu-
ous pose with the body at the same
time the head or front bearing false
eyespots is thrust at the intruder.
(vii) Are often combined with other col-
ors and shapes that, when viewed
from different directions, preserve
or enhance the deception. This may
include Escher-like illusions and
transformations. For example, the
same falseeyes andassociatedfacial
patternsofthepupainFig.1givethe
illusion of two different faces, de-
pending on whether they are viewed
from above or from the front.
(viii) Are present in almost all ACG
Lepidoptera families with large
caterpillars and pupae (2?10 cm in
length), and even in some fami-
lies (e.g., Limacodidae, Crambi-
dae, and Elachistidae) with quite
small caterpillars (only 1?2cmin
length). Although a 1?2-cm, gen-
erally green to brown caterpillar
inside a tangle of silk and leaves
might seem impossibly small for
a snake mimic, the frontal false
eyes coupled with highly sinuous
movements may well elicit a ?ight
reaction by a small bird operating
largelyonre?exesinclosequarters.
(ix) Have independently evolved in nu-
meroustaxonomiclineages.However,
there are also species-rich clades
within, for example, the Hesperiidae
and the genus Xylophanes of the
Sphingidae, in which the counterfeit
eyes and faces of caterpillars and pu-
pae (Figs. 1?4andSI Appendix, Figs.
S1 and S2) apparently stem
phylogenetically from a single evo-
lutionary event instead of through
convergence.
(x) Are also encountered?although less
frequently?on extratropical species
of caterpillars (e.g., Pterourus,Pap-
ilionidae; Xylophanes, Sphingidae).
However, these caterpillars are often
subjecttopredatorpressurebyinsect-
eating migrant birds that spend major
parts of their lives in the tropics (and
often evolutionarily originated there)
and therefore may extend the syn-
drome envisioned here far outside of
the tropics and into habitats that are
less rich in predators on small birds
than are many tropical ecosystems.
(xi) May be overlooked by the casual
observer owing to the plethora of
additional caterpillar and pupa col-
ors and patterns (and the many
forms of crypsis) that the animals
present in ?standard? lateral or dor-
sal views [e.g., see SI Appendix for
the lateral and dorsal views of the
same caterpillars (SI Appendix, Figs.
S3?S7)andpupae(SI Appendix,
Figs. S8?S14) as in face and rear
views in Figs. 3 and 4 and SI Ap-
pendix, Figs. S1 and S2].
The great abundance and species rich-
ness of caterpillars and pupae in tropical
foliage suggest that the foraging insec-
tivorous bird may encounter tens to hun-
dreds of false-eyed individuals per day
(more at low to medium elevations than
at elevations above 1,500 m, which have
fewer species of large caterpillars and leaf-
rollers). There is no reason to postulate
that the bird would learn about each spe-
cies individually and mentally compare it
with other predator-mimicking species,
or compare its false eyes with those of any
particular species of potential predator.
Conclusion
We postulate that all of these false-eyed
species collectively constitute an enor-
mous mimicry complex that is evolution-
arily generated and sustained by the
diverse actions and foraging traits of
a large and multispeci?c array of avian
predators that are innately programmed
to instantly ?ee when in startlingly close
proximity to the eye of another species,
or to something that resembles such an
eye. As stated at the outset, the bird that
must learn to avoid an eye is not long
for this world. In contrast to classical
Batesian mimicry?in which the mimics
are generally thought to be signi?-
cantly rarer than the models?there
are many hundreds of false-eyed cater-
pillars and pupae for every vertebrate
predator per hectare of tropical forest.
This proportion is maintained by the ex-
tremely high cost paid by the foraging
bird that makes the mistake of pausing
when encountering what might be an
eye of one of its predators, coupled with
the low price paid by passing up a poten-
tial morsel.
There have been arguments as to the
existence of mimicry among caterpillars
(see review in ref. 21). Our conclusion
through the ongoing caterpillar survey of
ACG is that essentially all tropical cater-
pillars that live exposed, and many of
those that do not, are visually mimetic of
something?an inedible background or
object (22), some other aposematic or
mimetic caterpillar, a dangerous predator,
or some combination of these.
The multispeci?c diversity of caterpillar
and pupal false eyes is evolutionarily gen-
erated and maintained by the activities
of a heterogeneous array of species of
birds (and perhaps some small primates).
Theserangefrom?xed-behavior(?stupid?)
birds to ones that are ?smart? and plastic
learners. Only a moderate number of in-
dividuals and species of ?xedly (innately)
dupable birds may be required to main-
tain a large array of false eye and face
patterns on many species of caterpillars
and pupae. These species of birds may
evolutionarily drive each of the eye-like
and face-like patterns to be something
more similar to an eye and/or face as
they perceive it, without any reference to
the false eyes and faces of other co-
occurring caterpillar species. We suggest
that each bird is responding to an eye-like
or face-like stimulus, even though that
stimulus is only an approximation of the
real eye or face of any particular species of
predator, or the false eye or face of any co-
occurring species of caterpillar or pupa.
The generally great advantage of false eyes
and faces is not seriously diminished by
the existence of some species of birds that
can quickly determine that the mimetic
caterpillar or pupa is edible (see ref. 6).
Indeed, it can be postulated that selection
may even work against exact resemblance
among mimics because that could lead
to a superabundance of one particular
false eye and/or face pattern, thereby in-
creasing the likelihood of a bird species or
guild learning to associate that pattern
with a harmless meal at the moment
of encounter.
We wish to emphasize that, in high-
lighting the role of innate avoidance of
threats by potential predators in this
analysis and discussion of mimicry, we
do not intend to diminish the one-to-one
and one-on-one approaches inherent in
many Batesian and M?llerian experimen-
tal mimicry studies. Rather, we wish to
broaden our understanding by recognizing
that when the avoidance is innate, various
assumptions, hypotheses, tests, and
6of7 | www.pnas.org/cgi/doi/10.1073/pnas.0912122107 Janzen et al.
interpretations of mimicry may need to be
modi?ed. Equally, we emphasize that what
is a mimic in the eyes or mind of one
predator may not be to another. There are
models and mimics, and actions that are
learned and innate, and they do not map
perfectly on one another across the species
and situations in which they occur.
ACKNOWLEDGMENTS.Wethank theAreadeCon-
servacion Guanacaste (ACG) team of parataxono-
mists (10) for their support in ?nding and rearing
the caterpillars and pupae discussed here; the
ACG administration for saving the forest where
these caterpillars and pupae live; and four anon-
ymous reviewers for their comments on the man-
uscript. This study was supported by US Nati-
onal Science Foundation Grants BSR 9024770 and
DEB 9306296, 9400829, 9705072, 0072730, and
0515699 and grants from Guanacaste Dry Forest
Conservation Fund and ACG (D.H.J.).
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