Island rodents are often larger and live at higher population densities than their mainland counterparts, characteristics that have been referred to as "island syndrome". Island syndrome has been well studied, but few studies have tested for island-mainland differences in stress physiology. We evaluated island syndrome within the context of stress physiology of white-footed mice (Peromyscus leucopus) captured from 11 islands and 5 mainland sites in Thousand Islands National Park, Ontario,

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... . Stress physiology was evaluated by quantifying corticosterone (a stress biomarker), the primary glucocorticoid in mice, from hair and its related metabolites from fecal samples. White-footed mice captured in this near-shore archipelago did not display characteristics of island syndrome, nor differences in levels of hair corticosterone or fecal corticosterone metabolites compared with mainland mice. We suggest that island white-footed mice experience similar degrees of stress in the Thousand Islands compared with the mainland. Although we did not find evidence of island syndrome or differences in glucocorticoid levels, we identified relationships between internal (sex, body mass) and external (season) factors and our hormonal indices of stress in white-footed mice. ABSTRACT 17 Island rodents are often larger and live at higher population densities than their mainland 18 counterparts, characteristics that have been referred to as "island syndrome". Island syndrome 19 has been well studied, but few studies have tested for island-mainland differences in stress 20 physiology. We evaluated island syndrome within the context of stress physiology of white-21 footed mice (Peromyscus leucopus) captured from 11 islands and 5 mainland sites in Thousand 22 Islands National Park, Ontario, Canada. Stress physiology was evaluated by quantifying 23 corticosterone (a stress biomarker), the primary glucocorticoid in mice, from hair and its related 24 metabolites from fecal samples. White-footed mice captured in this near-shore archipelago did 25 not display characteristics of island syndrome, nor differences in levels of hair corticosterone or 26 fecal corticosterone metabolites compared with mainland mice. We suggest that island white-27 footed mice experience similar degrees of stress in the Thousand Islands compared with the 28 mainland. Although we did not find evidence of island syndrome or differences in glucocorticoid 29 levels, we identified relationships between internal (sex, body mass) and external (season) 30 factors and our hormonal indices of stress in white-footed mice.