Signalling turns out to be as complex as the problems a species has to solve

The argument

A second case for complexity reasons from function. Signalling exists to solve problems, and proponents argue that many species face problems that cannot be solved with a small fixed set of signals. Animals living in large groups with stable relationships must keep track of who is related to whom, who outranks whom and who has reciprocated, and they coordinate movement, defence, foraging and the care of young across those relationships. Some recognise individuals by voice and address them differently; some recruit particular others to particular tasks; some withhold or alter signals depending on who is listening. A system able to do that, on this account, is doing considerably more than announcing an internal state. The second strand is methodological. Proponents argue that estimates of complexity have risen steadily as the means of measuring improved. Signals below and above human hearing, in chemical traces, in vibration through soil and water, and in movement too fast for the eye, were simply not recorded until instruments could capture them, so the apparent simplicity of many species was an artefact of what observers could detect. Where recording has become dense enough to analyse statistically, repeated structure has turned up in signal sequences that had previously been described as undifferentiated. Proponents draw a cautious conclusion with a clear direction. The honest summary, on this view, is that animal communication has been found to be more complex every time the question has been asked more carefully, and that a position resting on the absence of evidence is weak when the evidence has been accumulating steadily in one direction.

Premises

Counter-arguments

Sceptics argue that social complexity does not entail communicative complexity. Much coordination in animal groups is achieved by simple rules applied locally, by reading others' postures and movements rather than by signalling, and by individual recognition, which requires only a signature and not a vocabulary. On the methodological point they agree that more signals have been recorded while denying the inference: detecting signals in new channels lengthens the inventory without showing that the system can combine, refer, or express anything new. A longer list of calls, they argue, is still a list.

Rejecting the premises

[Rejecting P1] Coordination of movement, defence and foraging is achieved in many species by simple local rules and by watching what neighbours do, so the problems named here do not require a complex signalling system. [Rejecting P2] Individual recognition needs only a distinctive signature per animal, and varying a signal with the audience needs only sensitivity to who is present; neither amounts to saying anything about the world. [Rejecting P3] A longer inventory of detected signals is not greater complexity in the sense at issue, which concerns whether the system can combine and refer, and new recording channels add items without adding that capacity. [Rejecting C] An appeal to the direction of the evidence is not an argument about its current state, and the premises leave the kind of system unchanged.