A tropical horde of counterfeit predator eyesDaniel H. Janzena,1, Winnie Hallwachsa, and John M. BurnsbaDepartment of Biology, University of Pennsylvania, Philadelphia, PA 19104-6018; and bDepartment of Entomology, National Museum ofNatural 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)

Wepropose that themany different, but essentially similar, eye-like and face-like color patterns displayed by hundreds of species of tropicalcaterpillars and pupae—26 examples of which are displayed here from the dry, cloud, and rain forests of Area de Conservacion Guanacaste(ACG) in northwestern Costa Rica—constitute a huge and pervasive mimicry complex that is evolutionarily generated and sustained by thesurvival behavior of a large and multispecific array of potential predators: the insect-eating birds. We propose that these predators arevariously and innately programmed to flee when abruptly confronted, at close range, with what appears to be an eye of one of theirpredators. Such a mimetic complex differs from various classical Batesian and Müllerian mimicry complexes of adult butterflies 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 forthe 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 memberof this mimicry complex. Indeed, selection may not favor exact resemblance among these mimics at all. Such convergence throughselection could create a superabundance of one particular false eyespot or face pattern, thereby increasing the likelihood of a bird speciesor guild learning to associate that pattern with harmless prey.

caterpillar | mimicry | pupa | insectivorousbirds | innate behavior

You are a 12-gram, insectivorous,tropical rainforest bird, foragingin shady, tangled, dappled, rus-tling foliage where edible cater-

pillars and other insects are likely toshelter. You want to live 10–20 years.You are peering under leaves, poking intorolled ones, searching around stems, ex-ploring bark crevices and other insecthiding places. Abruptly an eye appears,1–5 centimeters from your bill. The eye ora portion of it is half seen, obstructed,shadowed, partly out of focus, more or lessround, multicolored, and perhaps moving.If you pause a millisecond to ask whetherthat eye belongs to acceptable prey or toa predator, you are likely to be—and ittakes only once—someone’s breakfast.Your innate reaction to the eye must beinstant flight, that is, a “startle” coupledwith distancing. The bird that must learnto avoid what appears to be a predator’seye is not long for this world. Now, a safefew meters away, are you going to goback to see whether that was food? No.You, like billions of other individuals

and hundreds of other species for tensof millions of years, have just been a playerin an act of natural selection favoringmutations that lead to the multitudes

Fig. 1. The 7-mm-wide pupa of Cephise nuspesez (23) (Hesperiidae), a Costa Rican skipper butterfly as itappears to a foraging bird that (Upper) has poked into the front of the rolled leaf shelter constructed bythe caterpillar or (Lower) has opened the roll from above. When disturbed, this pupa rotates to presentits 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 conflict of interest.

This article is a PNAS Direct Submission.1To whom correspondence should be addressed. E-mail:djanzen@sas.upenn.edu.

This article contains supporting information online atwww.pnas.org/lookup/suppl/doi:10.1073/pnas.0912122107/-/DCSupplemental.

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of “false eye” color patterns, “eyespot”patterns, or “facsimiles of eyes” and“faces” adorning tropical caterpillars andpupae (Figs. 1–4, and SI Appendix, Figs.S1 and S2). These eyespots are round oroval and mono- or polychromatic, withround or slit pupils. Associated body pat-terns often suggest other head and facialfeatures, which in turn enhance theeye-like nature of the spots. Depending onthe angle of observation and on how muchor which part shows, one pattern mayeven simulate different faces (Figs. 1and 2). None of these patterns exactlymatches the eyes or face of any particularspecies of predator; but, even whenquickly and partially glimpsed, all give theillusion of an eye or face. These false eyesare mimicking the eyes and faces of suchpredators of insect-eating birds as snakes,

lizards, other birds, and small mammals, asperceived at close range by the in-sectivorous birds in their natural world.These color patterns—long noticed byfield naturalists, evolutionary behaviorists(see especially refs. 1–3), ecologists, tax-onomists, ecotourists, and, no doubt, ourdistant ancestors—and the birds’ reactionsto them, are the evolutionary footprints ofpredator/prey encounters as shallow astoday and as deep as the first terrestrialvertebrate eyes. Such footprints are scat-tered across many diurnal vertebrate/preyinteractions (e.g., refs. 4 and 5), but herewe focus only on those of caterpillarsand pupae and the birds that eat them.

DiscussionWe 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, arepowerful indirect evidence that the avianreactions to false eyes are innate. Never-theless, we expect the reactions to varyinterspecifically in connection with theintensity of the bird’s selective regime, thebird’s learning ability and personal historyof predator avoidance, and the evoca-tiveness (e.g., ref. 5) of any particular falseeye(s) as perceived by the bird in thehabitats in which it characteristicallyforages. The response may also vary in-traspecifically with the bird’s microenvi-ronmental circumstances and withexperience—for example, light level,proximity, degree to which the false eye orthe whole insect is obstructed, what thebird’s neighbors and life have taught it,whether it has recently suffered a nearmiss, how hungry it is (e.g., refs. 3, 6), sizeof eyespot (e.g., refs. 5, 7), etc.The sum of these avian reactions across

many tropical circumstances, habitats, andecosystems is a diffuse selective pressureto which we suggest that innumerablespecies of caterpillars and pupae havevariously responded in the evolution oftheir color patterns. The diffuse natureof the syndrome highlighted here alsoapplies to mimicry based on aposematic(warning) colors, cryptic colors, flash col-ors, and behaviors associated with them.For example, the outcome of innateavoidance of coral snakes by birds (8) mayextend to other toxic snakes and to harm-less ones (4), to turtles (9), and to bothtoxic and harmless caterpillars (9). Thosespecies-rich tropical complexes (4) arenot our concern here but seem to displaythe same phenomenon with the same rootcause. The 36 species of caterpillars andpupae in Figs. 1–4 and SI Appendix, Figs.S1 and S2, are a small and partial repre-sentation of the hundreds of species withfalse eyes and faces encountered in thecourse of a 30-year, ongoing inventory ofca. 450,000 individuals and >5,000 speciesof 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

moths or butterflies has its own evolu-tionary pedigree. Each has its own degreeof retention of traits that have been in-tensively favored by natural selection inthe past and that may not be maintainedtoday by anything more complex thanphylogenetic inertia, the absence of anopposing selective force (11), and themultispecific array of insect-eating birdsscouring tropical vegetation every day,year in and year out. Once a species hasevolved false eyes (or any facial patternthat elicits a fear/flee reaction), thosefalse eyes may barely diminish crypticity

Fig. 2. The 50-mm-long last instar caterpillar of Costa Rican Ridens panche (Hesperiidae) at the momentwhen its leaf shelter is forced open (Upper) and a few seconds later (Lower), when it presents glowingred false eye spots directed at the invader and glowing lemon-yellow eye spots in the dark of the cavernbehind. Both kinds of false eyes are thrust at the leaf roll entrance until the invader leaves.

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at a distance, cost next to nothing physi-ologically, and greatly benefit their bearerin a close encounter with a predator.These color patterns differ from those of

classic mimic/model systems in that theirvalue to the mimic depends not on thecloseness of the match to a specific model,but rather on sufficient similarity to aneye and/or face to trigger the fear/fleereaction in an insect-eating bird. Howsimilar is necessary (5) will vary from birdspecies to bird species and among foragingsituations. The ubiquity of this multi-specific mimicry complex argues stronglyfor the widespread presence of the fear/flee reaction in many species of small birds(or a few quite common ones).Much contemporary mimicry theory and

popular commentary explicitly or implicitlystress the importance of experience,learning, and memory of the potentialpredator in predator/prey interactions.However, we believe that it generallyunderestimates the fact that potentialpredators also innately avoid various apo-sematic signals and similarities to attributesof their predators, as shown by both ex-perimental study (e.g., refs. 5, 8, 12–16),observation (see especially ref. 4), andour natural history observations, and assketched out by Blest (3) although sub-sequently largely ignored for the past halfcentury. When the dominant response ofthe predator is innate rather than (or aswell as) learned, there are major changesin mimicry theory and interpretation ofnatural history with respect to the relativeimportance of mimic/model ratios (4),scarcity of models (4, 17, 18), intensityof selection (5, 15), ability to remember(6), etc. Any color, pattern, motion, orsound of a caterpillar or pupa that elicitsinnate avoidance of a lethal outcome forthe bird selectively favors both the preda-tor and the prey. False eye and facemimicry need not “exactly” match the realeyes of any particular species of predatorin order to be selected for, much as highlyeffective cryptic behavior and color pat-terns often do not precisely match thepatterns and colors of any particularbackground. The eyespot and face pat-terns need only contain features thatstimulate predator recognition by smallpredators themselves (5). Although Blest(ref. 3 and references therein) built onthese concepts in detail, they have re-ceived little attention from the many bi-ologists dealing with mimic/modelsystems in the tropics, most of whom havefocused on the exactness of mimicryamong distasteful models and mimics thatdisplay diurnally in ostentatious flight.False-eye color patterns on butterfly

wings may serve to deflect a bird’s strikefrom the actual head of the butterfly(3, 19, 20) instead of startling a potentialpredator away. However, false-eye color

patterns on butterfly wings can also reducepredation attempts (refs. 5 and 16 andreviews therein). Both of these hypothe-sized and confirmed processes may beoperative at the same time with the samespecies of prey and different species ofpredators, but we are concerned here withfalse eyes and faces on relatively sessilecaterpillars and pupae. There is no selec-tive value in deflecting a bird’s strike to thesite of the false eye on these animals.Equally, we are not concerned here withthe question of what shape or intensity ofan eyespot (5) confers protection at anygiven moment with any particular bird.Some first and approximate general-

izations about the mimic/model complex oftropical caterpillars and pupae (Figs. 1–4and SI Appendix, Figs. S1–S14) emergefrom our observations of their naturalhistory. Taken as a whole, their traitssuggest to us the long-term and pervasiveoperation of natural selection by thespecies-rich and abundant guild of smallvertebrate diurnal predators on cater-pillars and pupae in tropical forest. Wedo not propose alternative hypotheses forthis multispecific display by sessile preybecause we cannot think of any that arecompatible with the collective naturalhistory of the hundreds of species of avianand lepidopteran actors.False eyes and faces:

(i) Are comparatively more commonon species of caterpillars and pupaethat live in great part concealed inmicrohabitats that are often of lowand variable light levels, and thatare searched by diurnally foragingbirds—rolled and silked leaves, silk/leaf/dead leaf tangles, dark shadowsunder large leaves, crevices in treebark, etc. Because many kinds ofinsects and spiders hide in such pla-ces, they are rich foraging groundsfor birds—but with hidden dangers.Consider the caterpillar of the hes-periid (skipper) butterfly in Fig. 2: itis hidden in its silk and leaf shelterduring daylight hours, emerging tofeed at dusk or night. Its false eyesare exposed when its shelter is tornopen; and at that time, it thrusts the“face” of its head out at the intru-der instead of retreating or turningaway (or simply starting to repairits shelter). This behavior is sharedwith more than 100 species of ACGskipper butterfly caterpillars and withmany species in other families.Again, the skipper butterfly pupa inFig. 1 spends 2 weeks hidden in asilked, rolled leaf and is visually ex-posed only when a diurnal predator

opens that shelter. Then the pupa,which is firmly anchored at its base,twists on this anchor so as to projectits “face” out of the entrance atthe 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 displayedby the caterpillar in reaction to theapproach or touch of a “large” ob-ject. The false eyes in Figs. 3 B, H,I, K, and L are visible as false eyesonly when the caterpillar expandsand displays the crucial body part.

(iii) Usually occur on caterpillars and pu-pae that are otherwise crypticallycolored and patterned (rather thanostentatious); and these mimeticfeatures are not visible at any signif-icant distance, even when the cater-pillar or pupa lives fully exposed.For example, the ground colors andpatterns of the caterpillars and pu-pae in Figs. 3 and 4 are generallygreen, gray, brown, or black, ratherthan bright red, yellow, or blue.

(iv) Are not of any one specific “eye”shape or color but rather rangefrom astonishingly detailed mimicsof snake eyes and scales (e.g., Fig.4H) tominimal suggestions of pairedapproximate circles or dots in sur-rounding face-like patterns (e.g.,Fig. 4 B, F, and G). Even whenapproximate, these patterns are suf-ficiently eye-like and face-like tostimulate visual receptors/mentalprocesses that vertebrate predatorshave evolved for rapidly recognizingwhat might be an eye, regardless ofhow imperfectly or fractionally seen(see refs. 5 and 15 for elaboration). Itis hard to be convinced that the falseeyes in Fig. 1 are not real, and wesuspect even harder for a small birdwhen foraging (e.g., see ref. 2 for anextratropical example).

(v) Are usually paired and evolution-arily derived from paired, more orless circular structures (e.g., pupalspiracles) or patterns. (On occasion,median circular patterns are theevolutionary precursors of one-eyedmimics, especially in caterpillars ofSphingidae and Notodontidae.)False eyes are not derived from realcaterpillar “eyes” (stemmata), whichare tiny light sensors on the lower“cheeks” of the head, or from the

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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 forlateral and dorsal views of the same species of caterpillars).

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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 forlateral and dorsal views of the same species of pupae).

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position of future real eyes insidethe pupa. The external surfaces ofthe paired, pupal thoracic spiracleshave frequently given rise to pupalfalse eyespots (e.g., all of the falseeyes in Figs. 1 and 4 are evolution-arily modified thoracic spiracles).However, we add that there may beselection to enhance almost anyshape and color that can give thehint of eyes (e.g., refs. 5, 15).

(vi) Are usually on the head end or therear end of the caterpillar, and onthe front end of the pupa. These arethe parts that a predator is most likelyto see when probing the site of cater-pillar or pupa concealment and thatresemble in position and shape themost dangerous part of a predator’spredator. Not emphasized in Fig. 3(but see Fig. 3A andF) is the fact thatthese caterpillars often strike a sinu-ous pose with the body at the sametime the head or front bearing falseeyespots is thrust at the intruder.

(vii) Are often combined with other col-ors and shapes that, when viewedfrom different directions, preserveor enhance the deception. This mayinclude Escher-like illusions andtransformations. For example, thesame false eyes and associated facialpatterns of the pupa in Fig. 1 give theillusion of two different faces, de-pending on whether they are viewedfrom above or from the front.

(viii) Are present in almost all ACGLepidoptera families with large cat-erpillars and pupae (2–10 cm inlength), and even in some fami-lies (e.g., Limacodidae, Crambidae,and Elachistidae) with quite smallcaterpillars (only 1–2 cm in length).Although a 1–2-cm, generally greento brown caterpillar inside a tangleof silk and leaves might seem impos-sibly small for a snakemimic, the fron-tal false eyes coupled with highlysinuous movements may well elicita flight reaction by a small bird op-erating largely on reflexes in closequarters.

(ix) Have independently evolved in nu-merous taxonomic lineages. However,there are also species-rich cladeswithin, for example, the Hesperiidaeand the genus Xylophanes of theSphingidae, in which the counterfeiteyes and faces of caterpillars and pu-pae (Figs. 1–4 and SI Appendix, Figs.S1 and S2) apparently stem phyloge-

netically from a single evolutionaryevent instead of through convergence.

(x) Are also encountered—although lessfrequently—on extratropical speciesof caterpillars (e.g., Pterourus, Pap-ilionidae; Xylophanes, Sphingidae).However, these caterpillars are oftensubject to predator pressure by insect-eating migrant birds that spend majorparts of their lives in the tropics (andoften evolutionarily originated there)and therefore may extend the syn-drome envisioned here far outside ofthe tropics and into habitats that areless rich in predators on small birdsthan are many tropical ecosystems.

(xi) May be overlooked by the casualobserver owing to the plethora ofadditional caterpillar and pupa col-ors and patterns (and the manyforms of crypsis) that the animalspresent in “standard” lateral or dor-sal views [e.g., see SI Appendix forthe lateral and dorsal views of thesame caterpillars (SI Appendix, Figs.S3–S7) and pupae (SI Appendix,Figs. S8–S14) as in face and rearviews in Figs. 3 and 4 and SI Appen-dix, Figs. S1 and S2].

The great abundance and speciesrichness of caterpillars and pupae intropical foliage suggest that the foraginginsectivorous bird may encounter tensto hundreds of false-eyed individuals perday (more at low to medium elevationsthan at elevations above 1,500 m, whichhave fewer species of large caterpillarsand leaf-rollers). There is no reason topostulate that the bird would learnabout each species individually andmentally compare it with other predator-mimicking species, or compare its falseeyes with those of any particular speciesof potential predator.

ConclusionWe postulate that all of these false-eyedspecies collectively constitute an enor-mous mimicry complex that is evolution-arily generated and sustained by thediverse actions and foraging traits ofa large and multispecific array of avianpredators that are innately programmedto instantly flee when in startlingly closeproximity to the eye of another species,or to something that resembles such aneye. As stated at the outset, the bird thatmust learn to avoid an eye is not longfor this world. In contrast to classicalBatesian mimicry—in which the mimicsare generally thought to be signifi-cantly rarer than the models—thereare 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 foragingbird that makes the mistake of pausingwhen encountering what might be aneye of one of its predators, coupled withthe 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 conclusionthrough the ongoing caterpillar survey ofACG is that essentially all tropical cater-pillars that live exposed, and many ofthose that do not, are visually mimetic ofsomething—an inedible background orobject (22), some other aposematic ormimetic caterpillar, a dangerous predator,or some combination of these.The multispecific diversity of caterpillar

and pupal false eyes is evolutionarily gen-erated and maintained by the activitiesof a heterogeneous array of species ofbirds (and perhaps some small primates).These range from fixed-behavior (“stupid”)birds to ones that are “smart” and plasticlearners. Only a moderate number of in-dividuals and species of fixedly (innately)dupable birds may be required to main-tain a large array of false eye and facepatterns on many species of caterpillarsand pupae. These species of birds mayevolutionarily drive each of the eye-likeand face-like patterns to be somethingmore similar to an eye and/or face asthey perceive it, without any reference tothe false eyes and faces of other co-occurring caterpillar species. We suggestthat each bird is responding to an eye-likeor face-like stimulus, even though thatstimulus is only an approximation of thereal eye or face of any particular species ofpredator, or the false eye or face of any co-occurring species of caterpillar or pupa.The generally great advantage of false eyesand faces is not seriously diminished bythe existence of some species of birds thatcan quickly determine that the mimeticcaterpillar or pupa is edible (see ref. 6).Indeed, it can be postulated that selectionmay even work against exact resemblanceamong mimics because that could leadto a superabundance of one particularfalse eye and/or face pattern, thereby in-creasing the likelihood of a bird species orguild learning to associate that patternwith a harmless meal at the momentof encounter.We wish to emphasize that, in high-

lighting the role of innate avoidance ofthreats by potential predators in thisanalysis and discussion of mimicry, wedo not intend to diminish the one-to-oneand one-on-one approaches inherent inmany Batesian and Müllerian experimen-tal mimicry studies. Rather, we wish tobroaden our understanding by recognizingthat when the avoidance is innate, various

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assumptions, hypotheses, tests, and in-terpretations of mimicry may need to bemodified. Equally, we emphasize that whatis a mimic in the eyes or mind of onepredator may not be to another. There aremodels and mimics, and actions that arelearned and innate, and they do not map

perfectly on one another across the speciesand situations in which they occur.

ACKNOWLEDGMENTS.We thank the Area de Con-servacion Guanacaste (ACG) team of parataxono-mists (10) for their support in finding and rearingthe caterpillars and pupae discussed here; the

ACG administration for saving the forest wherethese 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 andDEB 9306296, 9400829, 9705072, 0072730, and0515699 and grants from Guanacaste Dry ForestConservation Fund and ACG (D.H.J.).

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