Neuroecological selection for musical features throughspatial reciprocity in long-term partnerships

Chemical sensing via olfaction constitutes a most ancient form of inter-organism communication. But acoustical signaling via tonal and rhythmic patterning is also common among higher vertebrates. Animals that live in well ventilated habitats and move in diasporic ways have further evolved more spectrally varied and discretized call structure. But unlike song in birds, researchers have struggled to locate isolated nucleii specialized for music cognition. The brain stem, midbrain, hindbrain, and forebrain, however, all largely associate with aspects of musical performance, perception, memory, and emotion. I hypothesized that spectral features of musical display evolved as honest signals of spatial cognition for precarious locomotor tasks associated with nurturing and protecting vulnerable offspring. I investigated possible connections between motor, visual, and spatial cognitive areas in relation to both signaler production and receiver processing of acoustical features of musical output. Brain component volume fractions of 42 parts from 48 primates were compiled, from a single source, and compared against a vocal complexity index (ARDI) as well as individual musical feature scores: including tone, interval, repetition, transposition rhythm, and unique syllable count. Structures for spatial and visual perception as well as motor control and emotional processing associated moderately with areas used by species who produce calls with both temporal and spectral musical features. These findings are consistent with a dual (both receiver- and signaler-side) function of musical signals. Associations with spatio-social areas (e.g. schizocortex and insula) support direct selection for a paralimbic-based neighbor orienting [PIANO] sensory modality for mapping and anticipating movement of fellow arboreal cohabitants. Associations with motor areas (e.g. LGN, mid-brain, and thalamus) support the complementary model that signaler capacities for spatio-motive emplacement [ME] are indirectly selected by conspecific receivers. This dual manifestation in low-parity species that locomote in diasporic ways through (arboreally) diffuse habitats, is compatible with musicality serving as courtship signals by long-term mates with consistent and reliable spatial capacities directly relevant to care of vulnerable (arboreal) offspring.