![]() ![]() A third vocalization category, the low-frequency `pop', has been recently documented during foraging bouts over habitats that may be less amenable to high-frequency echolocation clicks, such as seagrass beds( Fig. 2A) ( Tyack and Clark,2000 Janik et al.,2006) and echolocation clicks (20-100 kHz) are emitted during navigation and foraging ( Au,1993 Johnson et al.,2004). High-frequency whistles (5-20 kHz) are used during social communication with conspecifics ( Fig. Bottlenose dolphins employ a variety of vocalizations during social communication and foraging. ![]() In this study, we test whether exposure to playback of vocalizations of bottlenose dolphins elicits rapid changes in the vocal behavior and/or stress hormone levels in a primary prey species, the Gulf toadfish. Furthermore, glucocorticoids are known to cause rapid increases in the output of a hindbrain-spinal vocal pattern generator that establishes the temporal properties of natural calls in toadfishes( Remage-Healey and Bass, 2004 Remage-Healey and Bass, 2006),but the relationship of this observation to natural behavior is largely unexplored. Although elevated circulating stress hormones are linked to anti-predator behaviors such as defensive posture( Blanchard et al., 1998), it is unknown whether reductions in acoustic behavior and conspicuousness are also linked to changes in glucocorticoids. Across vertebrates,including teleosts, exposure to predator cues in a wide-variety of contexts elicits robust increases in circulating stress hormones( Blanchard et al., 1998 Kagawa and Mugiya, 2000 Kavaliers et al., 2001 Cockrem and Silverin, 2002 Clinchy et al., 2004). The mechanisms of behavioral adjustment during predation events are unclear, but may include brief changes in hormone levels. These findings lend strong support to the hypothesis that individuals of a prey species modulate communication behavior in the presence of a predator, and also suggest that short-term glucocorticoid elevation is associated with anti-predator behavior. Predator sound playbacks also had consequences for circulating stress hormones, as cortisol levels were significantly elevated in male toadfish exposed to dolphin pops compared with snapping shrimp pops. Highfrequency dolphin sounds (whistles) and low-frequency snapping shrimp pops (ambient control sounds) each had no effect on toadfish calling rates. ![]() Using underwater playbacks to toadfish in their natural environment, we found that low-frequency dolphin sounds (`pops') within the toadfish's range of hearing dramatically reduce toadfish calling rates by 50%. Here, we used both behavioral (vocalizations) and physiological (plasma cortisol levels) parameters to determine if male Gulf toadfish can, in turn, detect the acoustic signals of bottlenose dolphins. In addition, soniferous fishes constitute over 80% of Tursiops diet, and toadfishes alone account for approximately 13% of the stomach contents of adult bottlenose dolphins. Previous work showed that bottlenose dolphins ( Tursiops truncatus) behaviorally orient toward the sounds of prey, including the advertisement calls of male Gulf toadfish ( Opsanus beta). The passive listening hypothesis proposes that dolphins and whales detect acoustic signals emitted by prey, including sound-producing (soniferous)fishes. ![]()
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