Agnieszka Sergiel, Craig Whitman, Mark Haroldson, Matthew Gould, Elise Loggers, Cecily Costello, Kerry Gunther, Stephen Smith, Courtney Cass, Frank van Manen

Understanding how individual traits and environmental factors influence physiological state is essential for predicting species responses to changing environments and long-term persistence. Haematological parameters provide integrative measures of health and physiological condition and can reflect variation in both intrinsic traits and environmental context. In the Greater Yellowstone Ecosystem of North America brown bears have experienced substantial ecosystem-level changes in recent decades, including density-dependent demographic processes, dietary shifts, and increased use of human-modified landscapes. We used long-term research and monitoring data (2000–2024) to examine sources of variation in haematological parameters in free-ranging brown bears. Using an extensive dataset comprising 378 complete blood counts from 257 adults (86 females, 171 males), we evaluated the relative associations of individual traits (sex, age, percent body fat, diet) and external factors (density, human modification) with 14 haematological parameters. Parameter-specific linear and generalized mixed-effects models were fitted to assess patterns across different physiological axes. We found contrasting patterns in the correlates of haematological parameters. Oxygen-transport–related parameters were more strongly associated with intrinsic traits, whereas immune-related parameters showed greater associations with environmental variables, particularly elevation and population density, which were significant predictors of leukocyte profiles. Together, these findings suggest that different components of haematological variation reflect distinct physiological and ecological correlates. The study findings support the use of haematology as an integrative indicator of physiological variation in wildlife and provides important insight into how intrinsic condition and environmental context are reflected in blood-based metrics within complex, changing ecosystems.