The Social Help Hypothesis of Vertebrate Pain Systems: A Two-Threshold Model Linking Nociceptor Density to Social Aid Feasibility

Pain systems in vertebrates exhibit remarkable diversity, ranging from <1% C-fibers in elasmobranchs to 83% in humans. This paper proposes a novel two-threshold model explaining this variation through the lens of social help behavior. We hypothesize that nociceptor presence and density in vertebrates are determined by a hierarchical framework: (1) sociality, (2) presence of social aid behaviors (parental care, intragroup assistance to injured individuals), (3) feasibility of such aid, and (4) degree of need for external assistance. In species lacking both sociality and social aid behaviors, nociceptors are maintained at basal levels sufficient only for internal danger signaling. However, when sociality and social aid behaviors are present, nociceptor density scales with the physical feasibility of receiving help and the species' reliance on such assistance.We present comparative evidence across vertebrate taxa supporting this framework, including an extended C-fiber spectrum from sharks (<1%) through amphibians (44%), reptiles (23%), birds (50-65%), cats (40-90%), to social mammals (70-83%). The eusocial naked mole-rat provides a crucial test case: despite high sociality, they lack pain sensitivity and notably exhibit no injured-individual care behavior. Furthermore, we present novel ontogenetic evidence showing that pain sensitivity decreases from juvenile to adult stages as parental care period ends—validated in humans and supported by preliminary evidence in domestic cats. This ontogenetic pattern represents a key prediction distinguishing species with time-limited parental care from those with lifelong social support. Keywords: nociception, C-fibers, social behavior, parental care, vertebrate evolution, help-seeking behavior, naked mole-rat, ontogenetic development, domestic cat, comparative neurobiology