Geometric Descriptors Reveal Social Niche Dynamics in Japanese Macaques

Quantifying how group living shapes resource use requires linking within-group differentiation to between-group segregation, but existing isotopic niche metrics rarely capture both within a single geometric framework. We present two descriptors of multivariate isotopic niche structure: individual Mahalanobis distances (D_i), measuring within-group peripherality, and a directional containment index (Asym), measuring how niche regions of coexisting groups are nested. We applied these metrics to Japanese macaques (Macaca fuscata) on Koshima Island using bulk hair δ¹³C, δ¹⁵N, and δ³⁴S from a dominant group, a subordinate group, and solitary males across 2022-2024. δ³⁴S consistently captured ecological differentiation along a coastal-inland gradient. Within the spatially constrained Koshima Island population, which has a long history of provisioning, both niche size and directional containment differed markedly among the three study years, demonstrating that the observed inter-unit isotopic configuration was not fixed over the study period. Within-unit peripherality also varied among years in the two social groups but was comparatively stable in solitary males. These isotopic patterns are consistent with differences in social and spatial resource use, but the underlying behavioral mechanisms were not measured directly and require validation using independent ranging and feeding data. Our approach provides a framework for quantifying multilevel isotopic niche geometry in socially structured populations.