The role of relatedness, age, and origin in shaping social networks for two bison (Bison bison) herds in north-central Montana

Species with fission-fusion social organization, where groups break apart and merge over time, show variable subgroup stability. Plains bison (Bison bison bison (Linnaeus, 1758)), a keystone species in North American grasslands, exhibit fission- fusion dynamics. However, it is unclear whether subgroups are stable over time nor whether they are composed of related individuals. We used fine-scale behavioral observations and movement data from GPS ear tags to construct social networks for two plains bison herds over multiple years at American Prairie in north-central Montana. These herds are semi-free roaming and graze year-round in 32.4 and 111.6 km2 fenced pastures. While the bison in our study did exhibit fission-fusion behavior, we did not observe stable subgroups in time-aggregated social networks constructed over single growing seasons (eigenvector modularity ranged from -0.008 to 0.027). We used Mantel tests to assess the relationships between association strength and relatedness, age, and place of origin. We found that only first-order relatives were more likely to associate, and shared age and place of origin had no significant impact. The observed lack of stable subgroups challenges prevailing assumptions and highlights the need for future research into the mechanisms of fission-fusion dynamics in plains bison and other managed social species.