Signals, cues and the nature of mimicry.

'Mimicry' is used in the evolutionary and ecological literature to describe diverse phenomena. Many are textbook examples of natural selection's power to produce stunning adaptations. However, there remains a lack of clarity over how mimetic resemblances are conceptually related to each other. The result is that categories denoting the traditional subdivisions of mimicry are applied inconsistently across studies, hindering attempts at conceptual unification. This review critically examines the logic by which mimicry can be conceptually organized and analysed. It highlights the following three evolutionarily relevant distinctions. (i) Are the model's traits being mimicked signals or cues? (ii) Does the mimic signal a fitness benefit or fitness cost in order to manipulate the receiver's behaviour? (iii) Is the mimic's signal deceptive? The first distinction divides mimicry into two broad categories: 'signal mimicry' and 'cue mimicry'. 'Signal mimicry' occurs when mimic and model share the same receiver, and 'cue mimicry' when mimic and model have different receivers or when there is no receiver for the model's trait. 'Masquerade' fits conceptually within cue mimicry. The second and third distinctions divide both signal and cue mimicry into four types each. These are the three traditional mimicry categories (aggressive, Batesian and Müllerian) and a fourth, often overlooked category for which the term 'rewarding mimicry' is suggested. Rewarding mimicry occurs when the mimic's signal is non-deceptive (as in Müllerian mimicry) but where the mimic signals a fitness benefit to the receiver (as in aggressive mimicry). The existence of rewarding mimicry is a logical extension of the criteria used to differentiate the three well-recognized forms of mimicry. These four forms of mimicry are not discrete, immutable types, but rather help to define important axes along which mimicry can vary.