Why are embodied social signals concentrated towards the rostral region? — The rostrum concentration hypothesis

Shun Satoh , Hiroshi Matsui

Although frequently embodied, the relationship of animal social communication with body layout has rarely been investigated from a unified cognitive perspective. Across animal taxa, socially relevant signals, ranging from facial expressions and gaze to colouration and morphology, are strikingly concentrated towards the anterior region of the body. Here, we propose the Rostrum Concentration Hypothesis (RCH), a conceptual framework positing that social signals preferentially evolve and converge along the rostral body axis across bilaterian animals. We argue that this pattern does not reflect shared anatomical homology, but rather emerged from convergent interactions among body layouts, sensory organ concentrations, attentional biases and socio-ecological demands. Drawing on evidence from various taxa spanning from primates to insects, we suggest that the rostral concentration of social signals reflects the coevolution of signal production and receiver cognitive mechanisms. By reframing ‘facial’ communication as a broader organizational principle of the body layout, the RCH reduces the risk of anthropomorphic interpretation and bridges research across behavioural ecology, cognitive science, sensory ecology and evolutionary biology.