Our study was carried out with the resident community of bottlenose dolphins (Tursiops truncatus) near Sarasota Bay, Florida, USA. This dolphin community has been the focus of a long-term research program since 1970 (Wells et al. 1987; Scott et al. 1990). The community of about 160 resident dolphins spans up to five concurrent generations and includes individuals up to 67 years of age (Wells 2003, 2014; R. S. Wells, unpublished data). Since 1984, acoustic recordings of these dolphins have been made during occasional brief capture-release events, at which animals are assessed for various health and basic biological parameters (Wells and Scott 1990; Wells et al. 2004). During capture-release, a 500 × 4 m net is deployed from a small outboard vessel in water that is generally less than 2 m in depth. This creates a net corral that contains a small group (generally 1–4) of dolphins for short (1–4 h) periods of time. Throughout this time, while animals are either being held in the net corral or being examined out of the water, animals are recorded with suction cup hydrophones placed on the melon (forehead). This results in recordings that are generally high in signal-to-noise ratio. Whistles were recorded with hydrophones that were either custom built at the Woods Hole Oceanographic Institution or built by High Tech, Inc. (Gulfport, MI). Recordings were made onto a variety of different media over the years. From 1984 to 1989, Marantz or Sony stereo-cassette recorders were used (frequency response approximately 20–20,000 Hz), followed by Panasonic AG-6400 or AG-7400 videocassette recorders (frequency response approximately 20–32,000 Hz) through 2005. Since 2006, recordings have been made digitally, on a Sound Devices 744T recorder (sampled at 96 kHz). We now have a library of recordings of 272 dolphins, most of which have been recorded on multiple occasions (up to 18).

This recording library was used to select stimuli for the playback experiments. Typically dolphins produce large numbers of signature whistles during capture-release (e.g., Esch et al. 2009b), but non-signature whistles are occasionally produced as well. We selected a single non-signature whistle from each of 126 individual dolphins and used these to create 30-s playback sequences, with each containing 8–12 repetitions of the same non-signature whistle, depending on whistle length. The sequences thus contained approximately the same overall whistle content, as fewer exemplars were played of longer whistles. Overall stimulus durations (calculated by multiplying the number of stimuli presented by the length of each stimulus) were compared for related (mean = 6.3 s) versus unrelated stimuli (mean = 6.8 s) with a Wilcoxon signed-ranks test and were not found to be significantly different (N = 40, W = 362; Z = −0.38, P = 0.70).

A key aspect of this study is that we used the identical playback paradigm as in Sayigh et al. (1999) and Janik et al. (2006), so that results could be compared among the three sets of experiments. A target animal was presented with whistle stimuli from two familiar individuals, one related (as determined through long-term observations and confirmed through genetic testing) and one unrelated. Related individuals were usually mothers or independent offspring, but were occasionally siblings. The two stimulus animals had both associated with the target animal at similar levels over the previous two years, as calculated by coefficients of association (Cairns and Schwager 1987) derived from boat-based photographic identification survey data; these values were derived by dividing the number of sightings of two animals together by the total number of sightings of both individuals. When possible, the two stimulus animals were also matched for age and sex. Coefficients of association were compared for related (mean = 0.15) versus unrelated (mean = 0.05) pairings with a Wilcoxon signed-ranks test and were not found to be significantly different (N = 40, W = 220; Z = −1.56, P = 0.12).

As in the previous studies, the response variable measured was head turns toward the playback speaker. Playbacks were conducted during eight capture-release sessions from February 2004 through May 2014.

All of the following field and analysis methods are identical to those of Sayigh et al. (1999) and Janik et al. (2006), but will be described briefly here. We used a LL9162 underwater speaker (Lubell Labs, Columbus, OH) connected to a car power amplifier to play back sounds to the dolphins. Sound files were played from a Dell laptop computer. The frequency response for the combined system was 240–20,000 Hz ±3 dB. The source level was pre-set to produce a received level at a 2-m distance from the speaker (the location of our experimental animal) that approximated the received level of whistles produced by a nearby dolphin (as judged by the experimenters). Individual stimuli in each playback were normalized for average amplitude. Playbacks were monitored with a hydrophone next to the speaker, and vocalizations of the target dolphin were recorded with a suction cup hydrophone attached to the melon for the duration of the experiment. If there were other animals present during a playback, either in the water or on the deck of the boat, their whistles were also recorded with suction cup hydrophones. Recordings were made with either a Panasonic AG-7400 video recorder (2004–2005) or a Sound Devices 744T digital recorder (2006–2014), with frequency responses described above. Playback sessions were recorded on either a Sony DCRTRN 320 or a Canon Vixia HFR40 digital video camera from a platform on a boat approximately 2 m above the water surface at the speaker position (Fig. 2). The speaker was suspended from an anchored boat at approximately 1 m depth, and approximately 2 m to one side of the target animal.

Fig. 2 Playback experimental setup, showing the position of the videographer (sitting on top of the ladder on the boat), the playback speaker (held by the person at the foot of the ladder wearing a blue hat), and the target dolphin with a suction cup hydrophone on its melon. Photograph courtesy of Jim Schulz, Chicago Zoological Society, taken under National Marine Fisheries Service Scientific Research Permit No. 522-1785 Full size image

Dolphins were held loosely by about 4–5 handlers during the experiments but were able to turn their head freely in response to playbacks. All people holding the animal were blind to the playback sequence and could not hear in air when the stimuli were being played. Each target animal was held in position for a minimum of 5–10 min prior to an experiment so as to acclimate it to its surroundings. Each playback sequence lasted 30 s and was followed by 5 min of silence in order to document any continued responses by the target dolphin. We balanced the order of stimulus presentation, such that whistles from the related individual were played first in 20 trials and from the unrelated individual in 20 trials. We counted head turns greater than 20° toward the playback speaker within a 5.5-min period from the start of a playback as a response. Anything less than 20° was not counted because animals frequently moved back and forth within this range. Head turns were scored from video recordings of the playback sessions, without knowledge of the order of playback stimuli being presented. We compared the number of head turns toward non-signature whistles of related versus unrelated individuals. Each experiment was scored by at least two individuals, and scores were found to have a high level of agreement, with the overall statistical trends identical for both sets of scores (W scores from the Wilcoxon signed-ranks test comparing head turns to related versus unrelated stimuli were 290 and 292). Thus, again to be consistent with our earlier experiments, we used only one set of scores (those of author LS) for analyses. Whistle responses to playbacks were examined in Adobe Audition.

In addition to comparing responses to whistles of related and unrelated individuals, we examined effects of presentation order of the stimuli and sex of the target animal. As in our previous published studies (Sayigh et al. 1999; Janik et al. 2006), all data were tested with Wilcoxon signed-ranks tests. By keeping all aspects of this study identical to the previous studies, we were able to directly compare their results.

We also calculated effect size by dividing the Z score of the Wilcoxon signed-ranks test by the square root of the number of trials (Pallant 2007) and compared effect size in our published signature whistle playbacks to that observed in the current study. We compared the differences in number of head turns to related versus unrelated playbacks in our published studies to those observed in the current study with a Mann–Whitney U Test. We combined results from our natural (Sayigh et al. 1999) and synthetic (Janik et al. 2006) whistle experiments to obtain comparable sample sizes for these comparisons.