Associations between metabolic rate and personality in a free-living small mammal are driven by date of birth.

Physiological states are often regarded as drivers of personality differences, although the causal link between them remains unclear. Studies focusing on energy metabolism often report inconsistent associations with personality traits. We studied this association in 75 free-living female bush Karoo rats (Otomys unisulcatus) by testing their personality traits, resting metabolic rate (RMR) and, furthermore, metabolic rate stress response. We found a proactive behavioural syndrome, repeatable RMR and metabolic responses to an acute acoustic stressor. Linear mixed models showed that more proactive individuals exhibited higher RMR. To further explore this relationship, we conducted path analysis incorporating life history, environmental and ecological factors (such as date of birth, age, temperature and food abundance). This analysis revealed that the observed link between personality and RMR was not direct but instead may be mediated by the date of birth: individuals born later in the season were more proactive, had higher RMR and showed lower metabolic responses to acute stress. Importantly, personality and metabolic rate were not directly associated after accounting for date of birth. This finding highlights the importance of considering broader ecological/life-history contexts when interpreting physiological-behavioural correlations and offers a possible explanation for previous contradictory results regarding the personality-RMR relationship.