Dolphin-Watching Boats Affect Whistle Frequency Modulation in Bottlenose Dolphins

Abstract

Bottlenose dolphins’ whistles are key in social communication, conveying information about conspecifics and the environment. Therefore, their study can help to infer habitat use and identify areas of concern due to human activities. Here we studied the whistles of bottlenose dolphins (Tursiops truncatus) in two sites of the archipelago of Bocas del Toro, Panama, that contrast in boat traffic. Almirante Bay is a site dominated by taxi-boats and Dolphin Bay is a major location for boat-based dolphin watching. Recordings were made using bottom-mounted hydrophones and from the research boat using an over-the-side hydrophone and a broadband recording system. A total recording effort time of 1,726 h was analyzed. Our results show significant differences in boat detection between sites, and a higher number of whistles detected per minute in the site with tour-boat traffic. Furthermore, whistle modulation accounted for most of the differences between sites, boat presence, and whistle types. Dolphin whistle modulation is thought to be a potential indicator of emotional states including danger, alertness, and stress. In this study, dolphin signature whistle modulation increased significantly with boat presence in both sites but changes in modulation were greater in Dolphin Bay where tour-boats directly and sometimes aggressively interact with the animals. These results support a potential association between whistle modulation and stress (or alertness). These findings indicate that if tour-boat captains behave more like taxi-boat captains by e.g., reducing the distance of approach and contact time during dolphin interactions, dolphin communication, and emotional state would be less disrupted. These measures are implemented in the national guidelines for whale-watching and are known to tour-boat operators. The key to protecting these dolphins is in finding ways to effectively enforce these operator guidelines.

Dolphin communication during widespread systematic noise reduction-a natural experiment amid COVID-19 lockdowns

“…We hypothesized that the shift from tour boats to transport boats during most of 2020 resulted in a quieter and subsequently a less stressful habitat for dolphins. Based on our previous work (Perez-Ortega et al, 2021) we predict that a quieter environment will result in an increase in dolphin acoustic detections, a richer and more diverse whistle repertoire, and a decrease in whistle modulation during lockdown.…”

“…Adjustments in signal frequency have been reported in several populations of bottlenose dolphins. Under conditions of elevated low and high frequency ambient noise, bottlenose dolphins produce high frequency signals (Churchill et al, 2016) and low frequency signals, respectively (Morisaka et al, 2005;Gospić & Picciulin 2016;Perez-Ortega et al, 2021). Those adjustments can be important to minimize auditory masking (Cunningham & Mountain, 2014), compensate for a reduced communication range (Clark et al, 2009;Erbe et al, 2019), and communicate stress and alertness to conspecifics (Esch et al, 2009;Perez-Ortega et al, 2021).…”

“…Signature whistles are stereotypic sounds that encode information about individual identity and, thus, are used as contact calls (Caldwell et al, 1990), whereas non-signature whistles are non-stereotypic sounds produced in a wide range of social contexts (Sayigh et al, 1990;Macfarlane et al, 2017;2017Rachinas-Lopes et al, 2017. However, most studies of the impact of anthropogenic noise on dolphin whistles do not take into account whistle function, and limit descriptions to overall changes in frequency, contour complexity, and temporal characteristics (Morisaka et al, 2005a;May-Collado & Wartzok 2008;Perez-Ortega et al, 2021). These changes include lengthening or shortening of whistle duration, increasing or decreasing whistle frequency, and simplification of whistles contours when ship noise increases (Fouda et al, 2018;Morisaka et al, 2005a).…”

“…When multiple boats are present, interactions with dolphins can be intense due to the lack of compliance with national regulations for approaching tactics and distance (Sitar et al, 2016). During these encounters, dolphins produce whistles with significantly higher modulation than in the absence of boats (Perez-Ortega et al, 2021), potentially indicating changes in their behavioral and 'emotional' states (Esch et al, 2009).…”

First Report of Epimeletic and Acoustic Behavior in Mediterranean Common Bottlenose Dolphins (Tursiops truncatus) Carrying Dead Calves

“…Bottlenose dolphins are a highly vocal species with a complex vocal repertoire [ 12 ]. They show strong vocal plasticity [ 13 ] and are known to adapt the emission rates and the acoustic features of their vocalizations depending on the contexts and emotional state [ 14 , 15 , 16 ]. However, despite the consistent number of reports on epimeletic behavior for this species, still little is known about their acoustic production in such contexts, and little evidence has been reported.…”