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Co-mimics have a mutualistic relationship despite unequal defences


In the first clear mathematical treatment of natural selection, Müller1 proposed that a shared warning signal (mimicry) would benefit defended prey species by sharing out the per capita mortality incurred during predator education. Although mimicry is a mainstay of adaptationist thinking, there has been repeated debate on whether there is a mutualistic or a parasitic relationship between unequally defended co-mimic species2,3,4,5. Here we show that the relationship between unequally defended species is mutualistic. We examined this in a ‘novel world’6 of artificial prey with wild predators (great tit, Parus major). We kept the abundance of a highly defended prey (‘model’) constant and increased the density of a moderately defended prey (‘defended mimic’) of either perfect or imperfect mimetic resemblance to the model. Both model and defended mimic showed a net benefit from a density-dependent decrease in their per capita mortality. Even when the effect of dilution through density was controlled for, defended mimics did not induce additional attacks on the model, but we found selection for accurate signal mimicry. In comparison, the addition of fully edible (batesian) mimics did increase additional attacks on the model, but as a result of dilution this resulted in no overall increase in per capita mortality. By ignoring the effects of density, current theories may have overestimated the parasitic costs imposed by less defended mimics on highly defended models.

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Figure 1: Per capita mortality of models and defended mimics, and relative predation risk of models.
Figure 2: Per capita mortality of models and edible mimics, total numbers of models and edible mimics killed, and relative predation risk for models.


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We thank H. Nisu for taking care of the birds, and G. Ruxton and I. Harvey for commenting on the manuscript. The Academy of Finland (CoE programme) and the University of Jyväskylä provided funding, and the Konnevesi Research Station provided facilities. H.M.R. is funded by NERC, and M.P.S. is funded by Leverhulme trust.

Author Contributions All authors contributed equally to the planning of the work and writing the paper. H.M.R. and E.I. were the primary data collectors. First authorship was decided between H.M.R. and E.I.; authors after the first are listed in alphabetical order.

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Correspondence to Hannah M. Rowland.

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Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Notes which include Supplementary Figure S1 illustrating the artificial prey items presented on the black-and-white aviary background; Supplementary Time census analyses investigating the mortality consequences to the prey when predators forage for a fixed time clarified by Supplementary Figure S2; Supplementary analysis of risk of predation during learning studies the relative risk of the Model in more detail during the learning process of the birds pictured in Supplementary Figure S3; and Supplementary analysis presented in Supplementary Tables S1-4 of the match between our experimental results and Müller’s original equation based on two estimates of the quantity of prey needed to reach learning. (PDF 256 kb)

Supplementary Figure S1 was inadvertently omitted from Supplementary Information document in original publication. This was corrected on 5th October 2007.

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Rowland, H., Ihalainen, E., Lindström, L. et al. Co-mimics have a mutualistic relationship despite unequal defences. Nature 448, 64–67 (2007).

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