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The evolution of warning signals

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Abstract

Warning coloration signals are a familiar and conspicuous phenomenon in nature. However, the fundamental question of how warning signals initially evolved remains unanswered. For an unpalatable prey to evolve a signal to indicate its unprofitability, a rare and conspicuous mutant in a population of unpalatable cryptic prey must overcome a double disadvantage: a greater risk of being detected (as a result of being more conspicuous) and of being attacked (because its rarity results in a decreased association with aversion) by a predator1,2. Although the prior evolution of prey gregariousness may help warning signals to evolve3,4,5,6,7,8, such an evolutionary order may not always be the case4,9,10,11. Here we present a theoretical model that describes a mechanism for the evolution of warning signals without having to invoke gregariousness. Specifically, a predator's generalization of stimulus in associative learning, with a ‘peak shift’ towards greater conspicuousness5,12,13,14,15, allows a warning signal to evolve when the prey population density exceeds a threshold. Once a warning signal starts to evolve, it continues to grow; the resulting, evolutionarily stable16 conspicuousness of prey is discontinuously greater than that of the original cryptic prey, drawing an unambiguous distinction in their appearance.

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Figure 1: The stochastic change in the level of associative memories of a predator who randomly searches a prey population consisting of two prey types.
Figure 2: The evolutionary dynamics of the degree x of conspicuousness oftheprey population for the case in which a( y,x,u) = u(0.1 + x)/(1 + x) × exp(−( x − y)2)(1 − 0.5 exp(− y)), u = 1 and p( x) = x/(1 + x).

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Acknowledgements

We thank N. Yamamura for discussions; O. Takeyama for technical advice; and J.Lawton and the NERC Centre for Population Biology for hospitality and discussions offered to M.H. in finalizing the manuscript. This work was supported by an MESSC grant to M.H.

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Authors and Affiliations

  1. Laboratoire d'Ecologie, UMR 7625, Ecole Normale Supérieure, 46 rue d'Ulm, Paris, F-75230 Cedex 05, France

    Shigeo Yachi

  2. Center for Ecological Research, Kyoto University, 606-01, Kyoto, Japan

    Masahiko Higashi

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  1. Shigeo Yachi
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  2. Masahiko Higashi
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Correspondence to Masahiko Higashi.

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Yachi, S., Higashi, M. The evolution of warning signals. Nature 394, 882–884 (1998). https://doi.org/10.1038/29751

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