Sticklebacks benefit from closer predator inspection: an experimental test of risk assessment

Abstract

Small fish often approach and “inspect” a detected predator from a short distance. This apparently paradoxical behaviour has been shown experimentally to incur a risk of death. To have a net benefit predator inspection behaviour must be advantageous. We tested experimentally whether a closer inspection distance per se provides inspecting three-spined sticklebacks (Gasterosteus aculeatus) with more information about a pike's (Esox lucius) hunger state. This prediction was confirmed: test fish that had been assigned to a close inspection distance started foraging earlier after they had inspected a satiated pike compared to those that had inspected a hungry pike. The fish could not assess the pike's hunger state as precisely from a far inspection distance. This was shown by a significant interaction between hunger state of the pike and inspection distance on the time to start foraging. The sticklebacks used the information gained from inspecting the predator because they attacked their own prey more frequently after they had inspected a satiated pike. This effect was most apparent when the fish attacked a high Daphnia density (which impairs vigilance) more frequently after they had seen a satiated pike from a close distance.