Biological signalling as non-coercive manipulation: revisiting and finetuning the evolutionary definition of a signal

We show that definitions of animal signalling thus far failed to adequately distinguish signalling from coercion. We derive a general evolutionary definition of signalling that unambiguously distinguishes signalling from coercion, and that only requires signaller benefit and adaptedness but remains undetermined about receiver benefit and coadaptedness. As such, the scope of this definition is broader than receiver-beneficial (i.e., informative, full-coadaptive) signalling alone, and also includes receiver-costly or parasitic (i.e., mal-informative, not-full-coadaptive) signals. Such a 'broad-spectrum' definition of signalling – which is at the same time sharply delineated from coercion – is increasingly needed, since a signal’s reliability versus unreliability, and even its honesty versus deception, are increasingly being studied as spectrally graded properties of an animal signal, with mixed and variable strategies in a population to be expected. Overall, after decades of enduring ambiguity, crucial concepts, such as ‘coercion’, ‘manipulation’ and ‘signalling’, are elucidated. We conclude with some considerations on how far this evolutionary definition of animal signalling could be applied to ‘Darwinian individuals’ outside the animal kingdom, i.e., among non-animal species and also among within-organism Darwinian individuals like cancer cells and selfish genetic elements, and how this relates to signal transduction in cell signalling.