letters to nature ................................................................. colour variants within a species9), and can explain the observation Three-butter¯y system provides a that distasteful Heliconius14,15 species are generally monomorphic within a given geographic area9. ®eld test of muÈllerian mimicry This study tests the evolution of muÈllerian mimetic coloration (in Heliconius) by exploiting an unusual system of colour-pattern Durrell D. Kapan* polymorphism in one species of Heliconius butter¯y. In western Ecuador, two colour morphs of the unpalatable H. cydno butter¯y Centre for Biodiversity Research, Department of Zoology, University of British (yellow and white) resemble two different unpalatable species, Colombia, Vancouver, BC, V6T 1Z4, Canada, and Section of Integrative Biology, Heliconius eleuchia (yellow) and Heliconius sapho (white) (Fig. 1; Patterson Laboratories, University of Texas, Austin, Texas 78712-1064, USA the proposed co-models of H. cydno8). Where all three species occur * Present address: Departemento de Biologia, Universidad de Puerto RicoÐRio together, the two monomorphic co-models vary in relative abun- Piedras, San Juan 00931-3360, Puerto Rico dance (see Methods)8. .............................................................................................................................................. I used the distasteful polymorphic H. cydno to test two predic- In 1879, MuÈller proposed that two brightly coloured distasteful tions of muÈllerian mimicry theory: ®rst, that unfamiliar experi- butter¯y species (co-models) that share a single warning-colour mental H. cydno morphs whose wing colour does not match the pattern would bene®t by spreading the selective burden of locally dominant co-model suffer more attacks by predators com- educating predators1±5. The mutual bene®t of sharing warning pared with control morphs whose wing colour does match the co- signals among distasteful species, so-called muÈllerian mimicry, is model1; second, that differences in survival between experimental supported by comparative evidence2,3, theoretical studies5,6 and and control butter¯ies decreases with increasing release density. laboratory simulations7; however, to date, this key exemplar of Predators must attack a number of butter¯ies to learn to avoid an adaptive evolution has not been experimentally tested in the ®eld. unfamiliar experimental colour pattern15. These attacked butter¯ies To measure natural selection generated by muÈllerian mimicry, I will represent a lower fraction of the experimental butter¯ies when exploited the unusual polymorphism of Heliconius cydno more experimental butter¯ies are released3,10,11. In fact, predator (Lepidoptera: Nymphalidae)8. Here I show increased survival of education may have thwarted previous warning-colour experiments H. cydno morphs that match locally abundant monomorphic co- that failed to detect differences between experimental and control model species. This study demonstrates muÈllerian mimicry in treatments because, in an attempt to achieve signi®cant samples sizes, the ®eld. It also shows that muÈllerian mimicry with several co- they used high-density releases, long release times or re-released models generates geographically divergent selection, which butter¯ies at the same study site10,11,16±18. explains the existence of polymorphism in distasteful species To test the ®rst prediction, I captured pairs of yellow and white with warning coloration9. H. cydno butter¯ies from two source sites and released them at The importance of warning coloration in Heliconius butter¯ies four other sites that were dominated by yellow or white co-models has been examined by marking out wing colour-pattern elements or (Table 1, H. eleuchia or H. sapho). In contrast to previous experi- by transferring colour-pattern races across an inter-racial hybrid ments, this reciprocal transplant design measures the bene®t that a zone10,11. These studies showed that there is strong, positive rare unpalatable morph of H. cydno derives from muÈllerian mimicry frequency-dependent (aposematic) selection12 acting on colour with either of its more common co-model species (Fig. 1). I tested pattern in a species, but failed to measure directly the bene®t of simultaneously the second prediction by releasing these H. cydno muÈllerian mimicry. This is because they measured purifying pairs at different densities: low density (1 pair every 163±194 m of selection on colour pattern in a numerically dominant co-model trail) in three sites (Table 1, Manta Real, Agua Caliente and species Heliconius erato (W. W. Benson, personal commu- Tinalandia) and high density (1 pair every 40 m of trail) at one nication)10,11,13 rather than selection for colour-pattern convergence site (Table 1, Maquipucuna; also see Methods)8. between species (Fig. 1). The results of both studies demonstrate As predicted, experimental individuals had a lower frequency of how natural selection operates on intraspeci®c variation in warning being refound on subsequent visits (resighted) compared with coloration (uninitiated predators rapidly eliminate rare warning- controls for each study site (Fig. 2). From resight data, I used a b c H. sapho candidus H. erato peterivana H. erato emma H. erato favorinus Polymorphic H. eleuchia eleusinus forms of H. cydno alithea Figure 1 Experimental designs of aposematic selection and muÈllerian mimicry in hybrid zone11. In both studies abundant H. erato were manipulated at sites where they Heliconius butter¯ies. a, Experimental (dashed line) and control butter¯ies (solid lines) of a occurred with the rare co-model H. melpomene (W. W. Benson, personal communica- single Costa Rican population of H. erato peterivana. The red forewing patch is darkened tion)10,11,30. c, Reciprocal transfer of polymorphic H. cydno. White H. cydno are control for experimentals and an equivalent amount of ink is applied to dark parts of the forewing butter¯ies (solid arrow) when transferred to sites dominated by the white co-model for controls10. b, Experimental (dashed arrows) and control (solid arrows) butter¯ies of (H. sapho) and experimental butter¯ies (dashed arrow) when transferred to sites H. erato emma and H. erato favorinus. Experimental butter¯ies were transferred across dominated by the yellow co-model (H. eleuchia). The opposite is true for the yellow their common hybrid zone (grey line) and control butter¯ies were transferred parallel to the H. cydno butter¯y. 338 © 2001 Macmillan Magazines Ltd NATURE | VOL 409 | 18 JANUARY 2001 | www.nature.com letters to nature maximum likelihood methods19,20 to estimate survival differences output22, are equivalent to a selective coef®cient (s) of 0.64 against between experimental and control butter¯ies21. This involved par- experimentals (see Methods)11. titioning butter¯y disappearance into two phases: disappearance Release density affects survival differences. At high release during the ®rst 24 h (to estimate the probability of establishment, density, selection was not detected: life expectancies for experi- PE) and subsequent disappearance (to estimate the exponential mental and control butter¯ies were similar (16 and 17 days 11 mortality rate, l) (see Methods) . Disappearance rates of released respectively, G4 = 1.72, P = 0.79). In contrast, life expectancy was butter¯ies were in the predicted direction: experimental butter¯ies 2 days for experimental and 12 days for control butter¯ies at the had higher initial disappearance rates (lower PE) and higher sub- three sites of low-density release combined (Fig. 3a, G2 = 8.92, sequent disappearance rates (Fig. 3, l). P = 0.012). The greater disappearance rate of experimental butter¯ies is not Differences in life expectancy between experimental and control due to emigration from release sites, as the dispersal distances of butter¯ies vary predictably with the density of release, suggesting experimental and control butter¯ies were very similar. The mean that avian predators such as Jacamars (Galbulidae), motmots maximum distance moved by each resighted butter¯y did not differ (Motmotidae) and tyrant ¯ycatchers (Tyrannidae) are capable of between experimental (105 6 35.9 m; mean 6 s.e.m.) and control learning and can select against unfamiliar coloration of experi- 15 (118 6 37.4 m) butter¯ies (t41 = 0.41, P = 0.68; on ln-transformed mental butter¯ies in the ®eld . The lower life expectancy of distance data). Very few released butter¯ies ¯ew long distances. experimental relative to control butter¯ies at sites of low-density Only visually oriented predators are expected to affect experimental release indicates that looking like an abundant co-model increases butter¯ies while leaving the controls alone. It is therefore reasonable the probability of survival. In contrast, at high release density, life- to attribute differences in disappearance rate to differences in expectancy differences between treatments are negligible. Predators survival between morphs. attack a number of experimental colour morphs to learn avoidance Morphs differed in survival (life expectancy, PE/l): the maximum of an unfamiliar colour pattern, but at high release density this likelihood estimate of life expectancy across all sites was 5 days for number will be a lower fraction of the total experimentals released. experimental butter¯ies and 14 days for control butter¯ies (Fig. 3b, Even if predators are responsible for the high initial loss of experi- G4 = 10.72, P = 0.03). These differences in life expectancy for mental butter¯ies (low PE)