Relationships between Spatial Biology and Physiological Ecology in the Gopher Tortoise, Gopherus polyphemus

AbstractThe overlap between spatial and physiological ecology is generally understudied, yet both fields are fundamentally related in assessing how individuals balance limited resources. Herein, we quantified the relationships between spatial ecology using two parameters of home range (annual home range area and number of burrows used in 1 yr) and four measures of physiology that integrate stress and immunity (baseline plasma corticosterone [CORT] concentration, plasma lactate concentration, heterophil-to-lymphocyte [H∶L] ratio, and bactericidal ability [BA]) in a wild free-ranging population of the gopher tortoise (Gopherus polyphemus) to test the hypothesis that space usage is correlated with physiological state. We also used structural equation models (SEMs) to test for causative relationships between the spatial and physiological parameters. We predicted that larger home ranges would be negatively correlated with traditional biomarkers of stress and positively correlated with immunity, consistent with our hypothesis that home ranges are determined based on individual condition. Males had larger home ranges, used more burrows, and had higher baseline CORT than females. We found significant negative correlations between lactate and home range ($r=-0.456$, $\mathrm{df}=21$, $P=0.029$). CORT was negatively correlated with the number of burrows used in both sexes ($F=7.322$, $\mathrm{df}=\mathrm{2,}20$, $P=0.003$, adjusted $R^2=0.383$). No correlations were observed between space use and BA or, notably, H∶L ratio. SEMs suggested that variation in the number of burrows used was a result of variation in baseline CORT. The lack of a relationship between H∶L ratio and home range suggests that home range differences are not associated with differences in chronic stress, despite the pattern between baseline CORT and number of burrows used. Instead, this study indicates that animals balance trade-offs in energetics, likely by way of baseline corticosteroid, in such a way as to maintain function across continuously variable home range strategies.