• bonobo
• communication
• facial expressions
• gestures
• sexuality
• signals

Communication can be defined, in behavioral ecology, as the process through sender individuals called actor uses specifically structured signals or display to modify behavior of receiver (Krebs & Davies, 2002) . This definition includes only those signals and displays that specifically evolved for having a communicative function and therefore, explains their ultimate causation (Mayr 1961; Tinbergen 1963).
Animals can use different sensorial channels to exchange information: chemical, acoustic, tactile and visual, moreover egalitarian species living in complex social systems, show the highest communicative complexity.
Natural selection had facilitate, and continue to encourage, communicative signals with high efficiency in term of costs-benefits, within the environmental opportunities scenario and considering that signal’s effectiveness depends by the answer of receivers, we can glimpse the essential role played by receiver in the evolutionary origin of signals and in the evolution of communication and its complexity.
Despite the unquestionable importance of establishing the ultimate causation of primate communicative signals, the current debate on primate communication is mainly focused on their proximate causation. More in detail, scientists applying a more psychological approach are particularly interested in establishing whether a communicative signal is the outcome of intentional or emotional processes (Liebal et al. 2013).