  Many biological and social systems show significant levels of collective
action. Several cooperation mechanisms have been proposed, yet they have been
mostly studied independently. Among these, direct reciprocity supports
cooperation on the basis of repeated interactions among individuals. Signals
and quorum dynamics may also drive cooperation. Here, we resort to an
evolutionary game theoretical model to jointly analyse these two mechanisms and
study the conditions in which evolution selects for direct reciprocity,
signalling, or their combination. We show that signalling alone leads to higher
levels of cooperation than when combined with reciprocity, while offering
additional robustness against errors. Specifically, successful strategies in
the realm of direct reciprocity are often not selected in the presence of
signalling, and memory of past interactions is only exploited opportunistically
in case of earlier coordination failure. Differently, signalling always
evolves, even when costly. In the light of these results, it may be easier to
understand why direct reciprocity has been observed only in a limited number of
cases among non-humans, whereas signalling is widespread at all levels of
complexity.
