Summary
The trait compensation and cospecialization hypotheses make contrasting predictions on how boldness is co-adapted with antipredator defences. If trait compensation occurs, then bold individuals should be equipped with better antipredator defences to compensate for their increased risk exposure. By contrast, if trait co-specialization occurs, shy individuals should be equipped with better antipredator defences to enhance overall protection from predation. Here, we test the two alternative hypotheses by evaluating the among- and within-individual relationships between boldness and chemical defences in the American giant millipede (Narceus americanus; order Spirobolida). After controlling for test sequence, body length, air temperature, and time of day, latency to conglobate (i.e. “curl up”) upon disturbance and duration of conglobation were both found to be repeatable (R = 0.28 and 0.35). Moreover, the latency and duration of conglobation were negatively correlated at both the among- and within-individual levels (r = − 0.46 and − 0.32). Hence, individuals displayed consistent differences in risk-taking along a “shy-bold” axis. Millipedes also displayed—albeit weaker—individual differences in their probability to secrete chemical defences (R = 0.12), but no significant relationship was found with conglobation latency or duration. Overall, these results suggest that chemical defences evolved separately from the shy-bold axis (as measured with conglobation behaviour) as two independent antipredator strategies in millipedes.