Does predation risk outweigh the costs of lost feeding opportunities or does it generate a behavioural trade-off? A case study with Iberian ribbed newt larvae

As an evolutionary force, predation increases predator fitness, whereas prey future fitness is reduced to zero. Mesopredators are subjected to pressures as both predator and prey, which could generate trade-offs between predatory and anti-predatory behaviours. We subjected larvae of the mesopredator Pleurodeles waltl newt to a potential conflict of interest between reducing predation risk and fulfilling nutritional requirements. For this, we performed an experiment with a factorial design: (1) to induce different urges as predators, individuals underwent either restricted or unrestricted feeding; and (2) to induce different urges as prey, individuals were exposed to environments with and without predator snake kairomones. We recorded activity rate, swim rate, flight initiation distance (FID) and escape speed. FID increased in the presence of kairomones, as predicted by escape theory when the costs of not fleeing are greater than those of fleeing. Escape speed increased in the presence of kairomones, but only in restricted feeding larvae, probably due to lower body weight because of reduced food intake. We detected no effect of treatment on activity or swim rates. Predation risk apparently outweighed the costs of feeding opportunities lost rather than generated a behavioural trade-off. Collectively, the data suggested a greater effect of predator avoidance than of prey acquisition.