Divergent learning responses to a spatially consistent olfactory stimulus in two reef shark species

There is growing evidence of the important role learning plays in shark foraging, but few studies have examined the relationship between learning and foraging behavior in free-living settings. We addressed this knowledge gap by experimentally contrasting responses of blacktip reef Carcharhinus melanopterus and sicklefin lemon Negaprion acutidens sharks to an olfactory-only feeding stimulus—baited remote underwater video stations (BRUVS)—that was either spatially randomized (as a control) or offered repeatedly at the same location in the lagoon of Tetiaroa, French Polynesia. Relative to their response to the randomized BRUVS, blacktip reef sharks appeared to sensitize to the repeated treatment, exhibiting increasing relative abundance upon introduction of the cue (maximum number of individuals of a species observed on any frame of a video MaxN at deployment) and decreasing arrival times as the experiment progressed. By contrast, sicklefin lemon shark responses were either consistent across control and treatment BRUVS over time or suggested habituation (as evidenced by declining MaxN in response to the spatially repeated exposure). Accordingly, our findings advance our understanding of shark cognition by highlighting that sensitized learning responses to stable feeding cues can develop even when the olfactory attractant is not accompanied by a reward, while also indicating that shark responses to these cues can be species-specific. They also suggest that, for at least some shark species, olfactory cues alone could lead to learned responses that confound non-invasive efforts to monitor shark populations and communities (e.g. with BRUVS) and drive spatial behavior with the potential to affect both ecotourism and negative human-shark interactions.