Predation cost of rapid growth: behavioural coupling and physiological decoupling

Summary1 Despite its prominent role in life‐history theory, there is no direct empirical evidence for a behaviourally mediated predation cost of rapid growth. Moreover, we know little about how digestive physiology may also influence the shape of the growth/predation risk trade‐off function. 2 We determined the role of behaviour and digestive physiology in experiments in which damselfly larvae were induced to grow slowly or rapidly by manipulating photoperiod (time stress), and exposure to a fish predator. 3 We showed that larvae under time stress grew more rapidly. Rapid‐growing larvae had a higher foraging activity and a higher growth efficiency. 4 Under predation risk, larvae not only had a lower foraging activity but also a lower growth efficiency. 5 Rapid‐growing larvae (i.e. those under time stress) balanced the growth/predation risk trade‐off differently and took more risk in the presence of a predator, which resulted in a behaviourally mediated higher predation cost compared to slow‐growing larvae. Their higher growth efficiency, however, made this cost smaller compared to a completely behaviourally mediated rapid‐growth strategy. 6 Our results provide the first explicit experimental proof of a behaviourally mediated predation cost of rapid growth. Besides a behavioural coupling of growth and predation risk, resulting in the well‐known trade‐off, we also found a partial decoupling of these two processes by digestive physiology.