Fig. 4. Predator vocalizations (dolphin pops) induce a significant increase in baseline plasma cortisol levels, relative to control stimuli (snapping shrimp pops). Plasma was immediately sampled following the end of the playback periods. Sample sizes are indicated at the bottom of each bar. The dolphin pops group represents both animals receiving playbacks of dolphin pops alone (N=4) and a subset of the animals that received dolphin whistles and pops (N=4; also see Fig. 3). Bars indicate mean ± s.e.m. ^*P=0.03.